THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
HISTORY OF ASTRONOMY 
 
A POPULAR 
 
 HISTORY OF ASTRONOMY 
 
 DURING 
 
 THE NINETEENTH CENTUEY 
 
 BY 
 
 AGNES M. 
 
 JUPITER 1879 
 
 SATURN 1885 
 
 THIRD EDITION 
 
 LONDON: ADAM & CHARLES BLACK 
 1893 
 
CU 
 
 PREFACE TO THE THIRD EDITION. 
 
 THE interval of six years since the publication of the Second 
 Edition of the present work has been marvellously fruitful of 
 astronomical discoveries. Hence a searching revision was called 
 for, and has been executed without stint of care or pains. 
 Additions and substitutions have been freely made ; and it has 
 been the aim of the writer, not only to furnish the large amount 
 of requisite new information, but to incorporate it so completely 
 with the pre-existing text as to leave no gaps in the narrative 
 suggesting " interpolations " to the refined critical sense. The 
 book has thus grown, and been brought down to date, by a 
 process of assimilation, rather than of mere accretion. Its 
 adjuncts, too, have been renovated. The foot-note references 
 have been multiplied ; the Index has been rendered more copious ; 
 the Chronological Table has been considerably extended; and 
 several new Tables of data, likely to be useful, while not every- 
 where accessible to students, have been appended. Besides the 
 Frontispiece and Vignette, retained from the Second Edition, for 
 which the writer renews her thanks to Dr. Common and the 
 MM. Henry, five Plates have been prepared, with the most 
 kind assistance of Mr. Kanyard, especially for the Third. Their 
 availability is due to the courtesy of the Council of the Eoyal 
 Astronomical Society, of Professor George E. Hale, of Professor 
 
 M368106 
 
vi PREFACE TO THE THIRD EDITION. 
 
 Barnard, and of Dr. and Mrs. Huggins, to all of whom the 
 writer offers grateful acknowledgments. She ventures to hope 
 that the remodelled work will enjoy no less favour with the 
 public than was shown to : lts earlier issues. 
 
 LONDON, May 1893. 
 
PREFACE TO THE FIRST EDITION. 
 
 THE progress of astronomy during the last hundred years has 
 been rapid and extraordinary. In its distinctive features, 
 moreover, the nature of that progress has been such as to lend 
 itself with facility to untechnical treatment. To this circum- 
 stance the present volume owes its origin. It embodies an 
 attempt to enable the ordinary reader to follow, with intelligent 
 interest, the course of modern astronomical inquiries and to 
 realise (so far as it can at present be realised) the full effect of 
 the comprehensive change in the whole aspect, purposes, and 
 methods of celestial science introduced by the momentous disco- 
 very of spectrum analysis. 
 
 Since Professor Grant's invaluable work on the History of 
 Physical Astronomy was published, a third of a century has 
 elapsed. During the interval, a so-called " new astronomy " 
 has grown up by the side of the old. One effect of its advent 
 has been to render the science of the heavenly bodies more 
 popular, both in its needs and in its nature, than formerly. 
 More popular in its needs, since its progress now primarily 
 depends upon the interest in, and consequent efforts towards 
 its advancement of the general public; more popular in its 
 nature, because the kind of knowledge it now chiefly tends to 
 accumulate is more easily intelligible less remote from ordi- 
 
viii PREFACE TO THE FIRST EDITION. 
 
 nary experience than that evolved by the aid of the calculus 
 from materials collected by the use of the transit-instrument 
 and chronograph. 
 
 It has thus become practicable to describe in simple language 
 the most essential parts of recent astronomical discoveries; 
 and being practicable, it could not be otherwise than desirable 
 to do so. The service to astronomy itself would be not in- 
 considerable of enlisting wider sympathies on its behalf ; while 
 to help one single mind towards a fuller understanding of the 
 manifold works which have, in all ages, irresistibly spoken to 
 man of the glory of God, might well be an object of no ignoble 
 ambition. 
 
 The present volume does not profess to be a complete or 
 exhaustive History of Astronomy during the period covered 
 by it. Its design is to present a view of the progress of 
 celestial science, on its most characteristic side, since the time 
 of Herschel. Abstruse mathematical theories, unless in some 
 of their more striking results, are excluded from consideration. 
 These, during the eighteenth century, constituted the sum and 
 substance of astronomy; and their fundamental importance 
 can never be diminished, and should never be ignored. But, 
 as the outcome of the enormous development given to the 
 powers of the telescope in recent times, together with the 
 swift advance of physical science, and the inclusion, by means 
 of the spectroscope, of the heavenly bodies within the domain 
 of its inquiries, much knowledge has been acquired regarding 
 the nature and condition of those bodies, forming, it might be 
 said, a science apart, and disembarrassed from immediate de- 
 pendence upon intricate, and except to the initiated, unin- 
 telligible formulse. This kind of knowledge forms the main 
 subject of the book now offered to the public. 
 
 There are many reasons for preferring a history to a formal 
 treatise on astronomy. In a treatise, what we know is set 
 
PREFACE TO THE FIRST EDITION. ix 
 
 forth. A history tells us, in addition, how we came to know 
 it. It thus places facts before us in the natural order of their 
 ascertainment, and narrates instead of enumerating. The story 
 to be told leaves the marvels of imagination far behind, and 
 requires no embellishment from literary art or high-flown 
 phrases. Its best ornament is unvarnished truthfulness, and 
 this at least may confidently be claimed to be bestowed upon 
 it in the ensuing pages. 
 
 In them unity of treatment is sought to be combined with 
 a due regard to chronological sequence by grouping in sepa- 
 rate chapters the various events relating to the several depart- 
 ments of descriptive astronomy. The whole is divided into 
 two parts, the line between which is roughly drawn at the 
 middle of the present century. Herschel's inquiries into the 
 construction of the heavens strike the keynote of the first 
 part; the discovery of sun-spot and magnetic periodicity and 
 of spectrum analysis determine the character of the second. 
 Where the nature of the subject required it, however, this 
 arrangement has been disregarded. Clearness and consist- 
 ency should obviously take precedence of method. Thus, in 
 treating of the telescopic scrutiny of the various planets, the 
 whole of the related facts have been collected, into an un- 
 interrupted narrative. A division, elsewhere natural and helpful, 
 would here have been purely artificial, and therefore confusing. 
 The interests of students have been consulted by a full and 
 authentic system of references to the sources of information 
 relied upon. Materials have been derived, as a rule with 
 very few exceptions, from the original authorities. The system 
 adopted has been to take as little as possible at second-hand. 
 Much pains have been taken to trace the origin of ideas, 
 often obscurely enunciated long before they came to resound 
 through the scientific world, and to give to each individual 
 discoverer, strictly and impartially, his due. Prominence has 
 
x PREFACE TO THE FIRST EDITION. 
 
 also been assigned to the biographical element, as underlying 
 and determining the whole course of human endeavour. The 
 advance of knowledge may be called a vital process. The lives 
 of men are absorbed into ancl assimilated by it. Inquiries into 
 the kind and mode of the surrender in each separate case must 
 always possess a strong interest whether for study or for 
 example. 
 
 The acknowledgments of the writer are due to Professor 
 Edward S. Holden, director of the Washburn Observatory, 
 Wisconsin, and to Dr. Copeland, chief astronomer of Lord 
 Crawford's Observatory at Dunecht, for many valuable com- 
 munications. 
 
 LONDON, September 1885. 
 
CONTENTS. 
 
 INTRODUCTION. 
 
 Three Kinds of Astronomy Progress of the Science during the 
 Eighteenth Century Popularity and Rapid Advance during the 
 Nineteenth Century . 
 
 PART I. 
 
 PROGRESS OF ASTRONOMY DURING THE FIRST HALF 
 OF THE NINETEENTH CENTURY. 
 
 CHAPTER I. 
 
 FOUNDATION OF SIDEEEAL ASTBONOMY. 
 
 State of Knowledge regarding the Stars in the Eighteenth Century 
 Career of Sir William Herschel Constitution of the Stellar System 
 Double Stars Herschel's Discovery of their Revolutions His 
 Method of Star-Gauging Discoveries of Nebulae Theory of their 
 Condensation into Stars Summary of Results . . . Page 10 
 
 CHAPTER II. 
 
 PKOGEESS OF SIDEREAL ASTRONOMY. 
 
 Exact Astronomy in Germany Career of Bessel His Fundamenta 
 Astronomice Career of Fraunhofer Parallaxes of Fixed Stars 
 Translation of the Solar System Astronomy of the Invisible 
 Struve's Researches in Double Stars Sir John Herschel's Exploration 
 of the Heavens Character of Fifty Years' Progress . . Page 32 
 
xii CONTENTS. 
 
 CHAPTER III. 
 
 PEOGEESS OF KNOWLEDGE EEGAEDING THE SUN. 
 
 Early Views as to the Nature of Sun-Spots Wilson's Observations and 
 Reasonings Herschel's Theory,f the Solar Constitution Sir John 
 Herschel's Trade-Wind Hypothesis Baily's Beads Total Solar 
 Eclipse of 1842 Corona and Prominences Eclipse of 1851 Page 62 
 
 CHAPTER IV. 
 PLANETAEY DISCOVEEIES. 
 
 Bode's Law Search for a Missing Planet Its Discovery by Piazzi 
 Further Discoveries of Minor Planets Unexplained Disturbance of 
 Uranus Discovery of Neptune Its Satellite An Eighth Saturnian 
 Moon Saturn's Dusky Ring The Uranian System . . Page 87 
 
 CHAPTER V. 
 COMETS. 
 
 Predicted Return of Halley's Comet Career of Olbers Acceleration of 
 Encke's Comet Biela's Comet Its Duplication Faye's Comet 
 Comet of 1811 Electrical Theory of Cometary Emanations The 
 Earth in a Comet's Tail Second Return of Halley's Comet Great 
 Comet of 1843 Results to Knowledge .... Page 109 
 
 CHAPTER VI. 
 INSTEUMENTAL ADVANCES. 
 
 Two Principles of Telescopic Construction Early Reflectors Three 
 Varieties Herschel's Specula High Magnifying Powers Invention 
 of the Achromatic Lens Guinand's Optical Glass The Great Rosse 
 Reflector Its Disclosures Mounting of Telescopes Astronomical 
 Circles Personal Equation Page 134 
 
 PART II. 
 
 RECENT PROGRESS OF ASTRONOMY. 
 CHAPTER I. 
 
 FOUNDATION OF ASTEONOMICAL PHYSICS. 
 
 Schwabe's Discovery of a Decennial Sun-Spot Period Coincidence with 
 Period of Magnetic Disturbance Sun-Spots and Weather Spectrum 
 Analysis Preliminary Inquiries Fraunhof er Lines Kirchhoff 's Prin- 
 ciple Anticipations Elementary Principles of Spectrum Analysis 
 Unity of Nature p a ge 155 
 
CONTENTS. xiii 
 
 CHAPTER II. 
 
 SOLAR OBSERVATIONS AND THEORIES. 
 
 Black Openings in Spots Carrington's Observations Rotation of the 
 Sun KirchhofE 's Theory of the Solar Constitution Faye's Views 
 Solar Photography Kew Observations Spectroscopic Method 
 Cyclonic Theory of Sun-Spots Volcanic Hypothesis A Solar Out- 
 burst Sun-Spot Periodicity Planetary Influence Nasmyth's Willow 
 Leaves Page 178 
 
 CHAPTER III. 
 
 RECENT SOLAR ECLIPSES. 
 
 Expeditions to Spain Great Indian Eclipse New Method of Viewing 
 Prominences Total Eclipse Visible in North America Spectrum of 
 the Corona Eclipse of 1870 Young's Reversing Layer Eclipse of 
 1871 Corona of 1878 Varying Types Egyptian Eclipse Extra- 
 Eclipse Coronal Photography Observations at Caroline Island 
 Diffraction Theory of Corona Photographs of Corona in 1886 
 and 1889 Mechanical Theory Mathematical Theory Nature of 
 Corona Page 207 
 
 CHAPTER IV. 
 
 SPECTROSCOPIC WORK ON THE SUN. 
 
 Chemistry of Prominences Study of their Forms Two Classes Distri- 
 bution of Prominences Structure of the Chromosphere Spectro- 
 scopic Measurement of Movements in Line of Sight Spectroscopic 
 Determination of Solar Rotation Velocities of Transport in the Sun 
 Lockyer's Theory of Dissociation Solar Constituents Absence of 
 Oxygen Page 241 
 
 CHAPTER V. 
 
 TEMPERATURE OF THE SUN. 
 
 Thermal Power of the Sun Radiation and Temperature Estimates of 
 Solar Temperature Rosetti's Result Zollner's Method Larigley's 
 Experiment at Pittsburg The Sun's Atmosphere Selective Absorp- 
 tion by our Air The Sun Blue The Solar Constant . . Page 268 
 
xiv CONTENTS. 
 
 CHAPTEE VI. 
 
 THE SUN'S DISTANCE. 
 
 Difficulty of the Problem Oppositions of Mars Transits of Venus 
 Lunar Disturbance Velocity of Light Transit of 1874 Inconclusive 
 Result Opposition of Mars in 1877 Measurements of Minor Planets 
 Transit of 1882 Newcomb's Determination of the Velocity of Light 
 Combined Eesult . . . . . J^ * . Page 280 
 
 CHAPTER VII. 
 
 PLANETS AND SATELLITES. 
 
 Schroter's Life and Work Luminous Appearances during Transits of 
 Mercury Mountains of Mercury Rotation Intra-Mercurian Planets 
 Schiaparelli's Conclusion as to the Rotation of Venus Mountains 
 and Atmosphere of Venus Ashen Light Illusory Satellite 
 Solidity of the Earth Secular Changes of Climate Figure of the 
 Globe Study of the Moon's Surface Lunar Atmosphere New 
 Craters Thermal Effects of Moonlight Tidal Friction . Page 299 
 
 CHAPTER VIII. 
 
 PLANETS AND SATELLITES 
 
 Analogy between Mars and the Earth Martian Snowcaps, Seas, and 
 Continents Climate and Atmosphere Schiaparelli's Canals Dis- 
 covery of Two Martian Satellites Distribution of the Minor Planets 
 Their Collective Mass and Estimated Diameters Condition of 
 Jupiter His Spectrum Transits of his Satellites Their Mode of 
 Rotation Discovery of an Inner Satellite The Great Red Spot 
 Constitution of Saturn's Rings Period of Rotation of the Planet 
 Variability of Japetus Equatorial Markings on Uranus His 
 Spectrum Rotation of Neptune Trans-Neptunian Planets Page 336 
 
 CHAPTER IX. 
 THEOEIES OP PLANETAEY EVOLUTION. 
 
 Origin of the World according to Kant Laplace's Nebular Hypothesis- 
 Maintenance of the Sun's Heat Meteoric Hypothesis Radiation the 
 Result of Contraction Regenerative Theory Faye's Scheme of 
 Planetary Development Origin of the Moon Effects of Tidal 
 Friction . ' ' . . p a( j e 374 
 
CONTENTS. xv 
 
 CHAPTER X. 
 
 RECENT COMETS. 
 
 Donati's Comet The Earth again Involved in a Comet's Tail Comets of 
 the August and November Meteors Star Showers Comets and 
 Meteors Biela's Comet and the Andromedes Orbits of Meteorites 
 Meteors with Stationary Radiants Spectroscopic Analysis of 
 Cometary Light Page 392 
 
 CHAPTER XI. 
 
 EECENT COMETS (continued). 
 
 Forms of Comets' Tails Electrical Repulsion Bredichin's Three Types 
 Great Southern Comet Supposed Previous Appearances Tebbutt's 
 Comet and the Comet of 1807 Successful Photographs Schaeberle's 
 Comet Comet Wells Sodium Blaze in Spectrum Great Comet of 
 1882 Transit Across the Sun Relation to Comets of 1843 and *88o 
 Cometary Systems Origin of Comets Rediscovery of Lexell's 
 Comet Swift's Comet Holmes's Comet .... Page 417 
 
 CHAPTER XII. 
 STAES AND NEBULA. 
 
 Stellar Chemistry Four Orders of Stars Their Relative Ages Gaseous 
 Stars Spectroscopic Star-Catalogues Stellar Chemistry The Draper 
 Catalogue Velocities of Stars in Line of Sight Spectroscopic 
 Binaries Eclipses of Algol New Stars Outbursts in Nebula3 
 Nova Aurigas Gaseous Nebulae Variable Nebulas Movements of 
 Nebulas Stellar and Nebular Photography Nebulas in the Pleiades 
 Photographic Star-Charting Stellar Parallax Double Stars 
 Stellar Photometry Status of Nebulas Photographs and Drawings 
 of the Milky Way Star Drift Page 450 
 
 CHAPTER XIII. 
 METHODS OF BESEAECH. 
 
 Development of Telescopic Power Silvered Glass Reflectors Giant 
 Refractors Comparison with Reflectors The Lick Telescope Atmo- 
 spheric Disturbance Mechanical Difficulties The Equatoreal Coude 
 The Photographic Camera Retrospect and Conclusion . Page 512 
 
 APPENDIX 531 
 
 INDEX 555 
 
LIST OF ILLUSTRATIONS. 
 
 Photograph of the Great Nebula in Orion (1883) . . Frontispiece 
 
 Photographs of Jupiter (1879) and of Saturn (1885) . Vignette 
 Plate I. Photographs of the Solar Chromosphere and 
 
 Prominences To face p. 246 
 
 ,, IT. The Great Comet of September 1882 photo- 
 graphed at the Cape .... 434 
 
 III. Photographs of Swift's Comet (1892) . . ., 446 
 
 IV. Photographic and Visual Spectrum of Nova 
 
 Aurigae 478 
 
 ,, V. Photograph of the Milky Way in Sagittarius 508 
 
HISTORY OF ASTRONOMY 
 
 DURDsG THE NINETEENTH CENTURY. 
 
 
 
 INTRODUCTION. 
 
 WE can distinguish three kinds of astronomy, each with a 
 different origin and history, but all mutually dependent, and 
 composing, in their fundamental unity, one science. First in 
 order of time came the art of observing the returns and 
 measuring the places of the heavenly bodies. This was the sole 
 astronomy of the Chinese and Chaldeans ; but to it the vigorous 
 Greek mind added a highly complex geometrical plan of their 
 movements, for which Copernicus substituted a more harmonious 
 system, without as yet any idea of a compelling cause. The 
 planets revolved in circles because it was their nature to do so, 
 just as laudanum sets to sleep because it possesses a virtus 
 dormittta. This first and oldest branch is known as " observa- 
 tional.^ or " practical astronomy. " Its business is to note facts 
 as accurately as possible ; and it is essentially unconcerned with 
 schemes for connecting those facts in a manner satisfactory to 
 the reason. 
 
 The second kind of astronomy was founded by Newton. Its 
 nature is best indicated by the term " gravitational " ; but it is 
 also called " theoretical astronomy." l It is based on the idea of 
 cause ; and the whole of its elaborate structure is reared accord- 
 ing to the dictates of a single law, simple in itself, but the 
 
 1 The denomination "physical astronomy," first used by Kepler, and long 
 appropriated to this branch of the science, has of late been otherwise applied. 
 
2 HISTORY OF ASTRONOMY. 
 
 tangled web of whose consequences can be unravelled only by the 
 subtle agency of an elaborate calculus. 
 
 The third and last division of celestial science may properly be 
 termed "physical and descriptive astronomy." It seeks to know 
 what the heavenly bodies are in themselves, leaving the How ? 
 and the Wherefore? of their movements to be otherwise 
 answered. Now such inquiries became possible only with the 
 invention of the telescope, so that Galileo was, in point of fact, 
 their originator. But Herschel was the first to give them a 
 prominence which the whole progress of science during the 
 nineteenth century has served to confirm and render more 
 exclusive. Inquisitions begun with the telescope have been 
 extended and made effective in unhoped-for directions by the aid 
 of the spectroscope and photographic camera ; and a large part 
 of our attention in the present volume will be occupied with the 
 brilliant results thus achieved. 
 
 The unexpected development of this new physical-celestial 
 science is the leading fact in recent astronomical history. It was 
 out of the regular course of events. In the degree in which it 
 has actually occurred it could certainly not have been foreseen. 
 It was a seizing of the prize by a competitor who had hardly 
 been thought qualified to enter the lists. Orthodox astronomers 
 of the old school looked with a certain contempt upon observers 
 who spent their nights in scrutinising the faces of the moon and 
 planets rather than in timing their transits, or devoted daylight 
 energies, not to reductions and computations, but to counting 
 and measuring spots on the sun. They were regarded as 
 irregular practitioners, to be tolerated perhaps, but certainly not 
 encouraged. 
 
 The advance of astronomy in the eighteenth century ran in 
 general an even and logical course. The age succeeding Newton's 
 had for its special task to demonstrate the universal validity, and 
 trace the complex results of the law of gravitation. The accom- 
 plishment of that task occupied just one hundred years. It was 
 virtually brought to a close when Laplace explained to the 
 French Academy, November 19, 1787, the cause of the moon's 
 accelerated motion. As a mere machine, the solar system, so far 
 
INTRODUCTION. 3 
 
 as it was then known, was found to be complete and intelligible 
 in all its parts ; and in the Mtfcanique Celeste its mechanical per- 
 fections were displayed under a form of majestic unity which 
 fitly commemorated the successive triumphs of analytical genius 
 over problems among the most arduous ever dealt with by the 
 mind of man. 
 
 Theory, however, demands a practical test. All its data are 
 derived from observation; and their insecurity becomes less 
 tolerable as it advances nearer to perfection. Observation, on 
 the other hand, is the pitiless critic of theory ; it detects weak 
 points, and provokes reforms which may be the beginnings of 
 discovery. Thus, theory and observation mutually act and react, 
 each alternately taking the lead in the endless race of improve- 
 ment. 
 
 Now, while in France Lagrange and Laplace were bringing 
 the gravitational theory of the solar system to completion, work 
 of a very different kind, yet not less indispensable to the future 
 welfare of astronomy, was being done in England. The Royal 
 Observatory at Greenwich is one of the few useful institutions 
 which date their origin from the reign of Charles II. The lead- 
 ing position which it still occupies in the science of celestial 
 observation was, for near a century and a half after its foundation, 
 an exclusive one. Delambre remarked that, had all other materials 
 of the kind been destroyed, the Greenwich records alone would 
 suffice for the restoration of astronomy ; 1 and the establishment 
 was indeed absolutely without a rival. Systematic observations 
 of sun, moon, stars, and planets were during the whole of the 
 eighteenth century made only at Greenwich. Here materials 
 were accumulated for the secure correction of theory, and here 
 refinements were introduced by which the exquisite accuracy of 
 modern practice in astronomy was eventually attained. 
 
 The chief promoter of these improvements was James Bradley. 
 Few men have possessed in an equal degree with him the power 
 of seeing accurately, and reasoning on what they see. He let 
 nothing pass. The slightest inconsistency between what appeared 
 and what was to be expected roused his keenest attention ; 
 
 1 Histoire de V Astronomic au xviii 6 Stide, p. 267. 
 
4 HISTORY OF ASTRONOMY. 
 
 and he never relaxed his mental grip of a subject until it had 
 yielded to his persistent inquisition. It was to these qualities 
 that he owed his discoveries of the aberration of light and the 
 nutation of the earth's axis. ..The first was announced in 1729. 
 What is meant by it is that, owing to the circumstance of light 
 not being instantaneously transmitted, the heavenly bodies 
 appear shifted from their true places by an , amount depending 
 upon the ratio which the velocity of light bears to the speed of 
 the earth in its orbit. Because light travels with enormous 
 rapidity, the shifting is very slight ; and each star returns to its 
 original position at the end of a year. 
 
 Bradley's second great discovery was finally ascertained in 
 1748. Nutation is a real " nodding " of the terrestrial axis pro- 
 duced by the dragging of the moon at the terrestrial equatorial 
 protuberance. From it results an apparent displacement of the 
 stars, each of them describing a little ellipse about its true or 
 " mean " position, in a period of eighteen years and about seven 
 months. 
 
 Now an acquaintance with the fact and the laws of each of 
 these minute irregularities is vital to the progress of observational 
 astronomy; for without it the places of the heavenly bodies 
 could never be accurately known or compared. So that Bradley, 
 by their detection, at once raised the science to a higher grade 
 of precision. Nor was this the whole of his work. Appointed 
 Astronomer-Koyal in 1742, he executed during the years 1750- 
 62 a series of observations which formed the real beginning of 
 exact astronomy. Part of their superiority must, indeed, be 
 attributed to the co-operation of John Bird, who provided Brad- 
 ley in 1750 with a measuring instrument of till then unequalled 
 excellence. For not only was the art of observing in the 
 eighteenth century a peculiarly English art, but the means of 
 observing were furnished almost exclusively by British artists. 
 John Dollond, the son of a Spitalfields weaver, invented the 
 achromatic lens in 1758, removing thereby the chief obstacle to 
 the development of the powers of refracting telescopes ; James 
 Short, of Edinburgh, was without a rival in the construction of 
 reflectors; the sectors, quadrants, and circles of Graham, Bird, 
 
INTRODUCTION. 5 
 
 Kamsden, and Gary were inimitable by Continental workman- 
 ship. 
 
 Thus practical and theoretical astronomy advanced on parallel 
 lines in England and France respectively, the improvement of 
 their several tools the telescope and the quadrant on the one 
 side, and the calculus on the other keeping pace. The whole 
 future of the science seemed to be theirs. The cessation of 
 interest through a too speedy attainment of the perfection 
 towards which each spurred the other, appeared to be the only 
 danger it held in store for them. When, all at once a rival 
 stood by their side not, indeed, menacing their progress, but 
 threatening to absorb their popularity. 
 
 The rise of Herschel was the one conspicuous anomaly in the 
 astronomical history of the eighteenth century. It proved 
 decisive of the course of events in the nineteenth. It was 
 unexplained by anything that had gone before ; yet all that 
 came after hinged upon it. It gave a new direction to effort ; 
 it lent a fresh impulse to thought. It opened a channel for 
 the widespread public interest which was gathering towards 
 astronomical subjects, to flow in. 
 
 Much of this interest was due to the occurrence of events 
 calculated to arrest the attention and excite the wonder of the 
 uninitiated. The predicted return of Halley's comet in 1759 
 verified, after an unprecedented fashion, the computations of 
 astronomers. It deprived such bodies for ever of their porten- 
 tous character ; it ranked them as denizens of the solar system. 
 Again, the transits of Venus in 1761 and 1769 were the first 
 occurrences of the kind since the awakening of science to their 
 consequence. Imposing preparations, journeys to remote and 
 hardly accessible regions, official expeditions, international 
 communications, all for the purpose of observing them to the 
 best advantage, brought their high significance vividly to the 
 public consciousness ; a result aided by the facile pen of Lalande, 
 in rendering intelligible the means by which these elaborate 
 arrangements were to issue in an accurate knowledge of the 
 sun's distance. Lastly, Herschel's discovery of Uranus, March 13, 
 1781, had the surprising effect of utter novelty. Since the 
 
6 HISTORY OF ASTRONOMY. 
 
 human race had become acquainted with the company of the 
 planets, no addition had been made to their number. The event 
 thus broke with immemorial traditions, and seemed to show 
 astronomy as still young and fujl of unlooked-for possibilities. 
 
 Further popularity accrued to the science from the sequel of 
 a career so strikingly opened. Herschel's huge telescopes, his 
 detection by their means of two Saturnian and as many "[Iranian 
 moons, his piercing scrutiny of the sun, picturesque theory of its 
 constitution, and sagacious indication of the route pursued by it 
 through space ; his discovery of stellar revolving systems, his 
 bold soundings of the universe, his grandiose ideas, and the 
 elevated yet simple language in which they were conveyed 
 formed a combination powerfully effective to those least suscep- 
 tible of new impressions. Nor was the evoked enthusiasm 
 limited to the British Isles. In Germany, Schroter followed 
 longo intervallo in Herschel's track. Von Zach set on foot 
 from Gotha that general communication of ideas which gives life 
 to a forward movement. Bode wrote much and well for un- 
 learned readers. Lalande, by his popular lectures and treatises, 
 helped to form an audience which Laplace himself did not 
 disdain to address in the Exposition du Syst&me du Monde. 
 
 This great accession of popularity gave the impulse to the 
 extraordinarily rapid progress of astronomy in the nineteenth 
 century. Official patronage combined with individual zeal 
 sufficed for the elder branches of the science. A few well- 
 endowed institutions could accumulate the materials needed by 
 a few isolated thinkers for the construction of theories of 
 wonderful beauty and elaboration, yet precluded, by their 
 abstract nature, from winning general applause. But the new 
 physical astronomy depends for its prosperity upon the favour 
 of the multitude whom its striking results are well fitted to 
 attract. It is, in a special manner, the science of amateurs. It 
 welcomes the most unpretending co-operation. There is no one 
 " with a true eye and a faithful hand " but can do good work in 
 watching the heavens. And not unfrequently prizes of discovery 
 which the most perfect appliances failed to grasp have fallen to 
 the share of ignorant or ill-provided assiduity. 
 
INTRODUCTION. 7 
 
 Observers, accordingly, have multiplied; observatories have 
 been founded in all parts of the world ; associations have been 
 constituted for mutual help and counsel. A formal astronomical 
 congress met in 1789 at Gotha then, under Duke Ernest II. 
 and Von Zach, the focus of German astronomy and instituted 
 a combined search for the planet suspected to revolve undis- 
 covered between the orbits of Mars and Jupiter. The Astrono- 
 mical Society of London was established in 1820, and the 
 similar German institution in 1863. Both have been highly 
 influential in promoting the interests, local and general, of the 
 science they were organised to serve ; while functions corre- 
 sponding to theirs have been discharged elsewhere by older or 
 less specially constituted bodies, and new ones of a more popular 
 character are springing up on all sides. 
 
 Modern facilities of communication have helped to impress 
 more deeply upon modern astronomy its associative character. 
 The electric telegraph gives a certain ubiquity which is invalu- 
 able to an observer of the skies. With the help of a wire, a 
 battery, and a code of signals, he sees whatever is visible from 
 any portion of our globe, depending, however, upon other eyes 
 than his own, and so entering as a unit into a widespread com- 
 bination of intelligence. The press, again, has been a potent 
 agent of co-operation. It has mainly contributed to unite 
 astronomers all over the world into a body animated by the 
 single aim of collecting " particulars " in their special branch for 
 what Bacon termed a History of Nature, eventually to be 
 interpreted according to the sagacious insight of some one 
 among them gifted above his fellows. The first really effective 
 astronomical periodical was the Monatliclie Correspondent, started 
 by Von Zach in the year 1800. It was followed in 1822 by the 
 Astronomische Nachricliten, later by the Memoirs and Monthh 
 Notices of the Astronomical Society, and by the host of varied 
 publications which now, in every civilised country, communicate 
 the discoveries made in astronomy to divers classes of readers, 
 and so incalculably quicken the current of its onward flow. 
 
 Public favour brings in its train material resources. It is 
 represented by individual enterprise, and finds expression in an 
 
8 HISTORY OF ASTRONOMY. 
 
 ample liberality. The first regular observatory in the Southern 
 Hemisphere was founded at Paramatta by Sir Thomas Mak- 
 dougall Brisbane in 1821. The Royal Observatory at the Cape 
 of Good Hope was completed in 1829. Similar establishments 
 were set to work by the East India Company at Madras, 
 Bombay, and St. Helena, during the first third of the nineteenth 
 century. The organisation of astronomy in the United States 
 of America was due to a strong wave of popular enthusiasm. 
 In 1825 John Quincy Adams vainly urged upon Congress the 
 foundation of a National Observatory ; but in 1 843 the lectures 
 of Ormsby MacKnight Mitchel on celestial phenomena stirred an 
 impressionable audience to the pitch of providing him with the 
 means of erecting at Cincinnati the first astronomical establish- 
 ment worthy the name in that great country. On the 1st of 
 January 1882 no less than one hundred and forty-four were 
 active within its boundaries. 
 
 The apparition of the great comet of 1843 g ave an additional 
 fillip to the movement. To the excitement caused by it the 
 Harvard College Observatory called the "American Pulkowa " 
 directly owed its origin ; and the example was not ineffective 
 elsewhere. Corporations, universities, municipalities, vied with 
 each other in the creation of such institutions ; private subscrip- 
 tions poured in ; emissaries were sent to Europe to purchase 
 instruments and to procure instruction in their use. In a few 
 years the young Eepublic was, in point of astronomical efficiency, 
 at least on a level with countries where the science had been 
 fostered since the dawn of civilisation. 
 
 A vast widening of the scope of astronomy has accompanied, 
 and in part occasioned, the great extension of its area of cultiva- 
 tion which our age has witnessed. In the last century its 
 purview was a comparatively narrow one. Problems lying 
 beyond the range of the solar system were almost unheeded, 
 because they seemed inscrutable. Herschel first showed the 
 sidereal universe as accessible to investigation, and thereby 
 offered to science new worlds majestic, manifold, " infinitely 
 infinite " to our apprehension in number, variety, and extent 
 for future conquest. Their gradual appropriation has absorbed, 
 
INTRODUCTION. 9 
 
 and will long continue to absorb, the powers wliicli it has served 
 to develop. 
 
 But this is not the only direction in which astronomy has 
 enlarged, or rather has levelled, its boundaries. The unification 
 of the physical sciences is perhaps the greatest intellectual feat 
 of recent times. The process has included astronomy ; so that, 
 like Bacon, she may now be said to have "taken all knowledge " 
 (of that kind) "for her province." In return, she proffers 
 potent aid for its increase. Every comet that approaches the 
 sun is the scene of experiments in the electrical illumination of 
 rarefied matter, performed on a huge scale for our benefit. The 
 sun, stars, and nebulae form so many celestial laboratories, where 
 the nature and mutual relations of the chemical " elements " 
 may be tried by more stringent tests than sublunary conditions 
 afford. The laws of terrestrial magnetism can be completely 
 investigated only with the aid of a concurrent study of the 
 face of the sun. The positions of the planets will perhaps one 
 day tell us something of impending droughts, famines, and 
 cyclones. 
 
 Astronomy generalises the results of other sciences. She 
 exhibits the laws of Nature working over a wider area, and 
 under more varied conditions, than ordinary experience presents. 
 Ordinary experience, on the other hand, has become indispensable 
 to her progress. She takes in at one view the indefinitely great 
 and the indefinitely little. The mutual revolutions of the stellar 
 multitude during tracts of time which seem to lengthen out to 
 eternity as the mind attempts to traverse them, she does not 
 admit to be beyond her ken ; nor is she indifferent to the con- 
 stitution of the minutest atom of matter that thrills the ether 
 into light. How she entered upon this vastly expanded inherit- 
 ance, and how, so far, she has dealt with it, is attempted to be 
 set forth in the ensuing chapters. 
 
PART I. 
 
 PROGRESS OF ASTRONOMY DURING THE FIRST HALF 
 OF THE NINETEENTH CENTURY. 
 
 CHAPTER I. 
 FOUNDATION OF SIDEREAL ASTRONOMY. 
 
 UNTIL nearly a hundred years ago the stars were regarded by 
 practical astronomers mainly as a number of convenient fixed 
 points by which the motions of the various members of the solar 
 system could be determined and compared. Their recognised 
 function, in fact, was that of milestones on the great celestial 
 highway traversed by the planets, as well as on the byways of 
 space occasionally pursued by comets. Not that curiosity as to 
 their nature, and even conjecture as to their origin, were at any 
 period absent. Both were from time to time powerfully 
 stimulated by the appearance of startling novelties in a region 
 described by philosophers as "incorruptible," or exempt from 
 change. The catalogue of Hipparchus probably, and certainly 
 that of Tycho Brahe, some seventeen centuries later, owed each 
 its origin to the temporary blaze of a new star. The general 
 aspect of the skies was thus (however imperfectly) recorded from 
 age to age, and with improved appliances the enumeration was 
 rendered more and more accurate and complete ; but the secrets 
 of the stellar sphere remained inviolate. 
 
 In a qualified, though very real sense, Sir William Herschel 
 may be called the Founder of Sidereal Astronomy. Before his 
 time some curious facts had been noted, and some ingenious 
 
CHAP. i. SIDEREAL ASTRONOMY. u 
 
 speculations hazarded, regarding the condition of the stars, but 
 not even the rudiments of systematic knowledge had been 
 acquired. The facts ascertained can be summed up in a very few 
 sentences. 
 
 Giordano Bruno was the first to set the suns of space in 
 motion, but in imagination only. His daring surmise was, 
 however, confirmed in 1718, when Halley announced 1 that 
 Sirius, Aldebaran, Betelgeux, and Arcturus had unmistakably 
 shifted their quarters in the sky since Ptolemy assigned their 
 places in his catalogue. A similar conclusion was reached by 
 J. Cassini in 1738, from a comparison of his own observations 
 with those made at Cayenne by Richer in 1672; and Tobias 
 Mayer drew up in 1756 a list showing the direction and amount 
 of about fifty-seven proper motions, 2 founded on star-places 
 determined by Olaus Homer fifty years previously. Thus the 
 stars were 110 longer regarded as "fixed," but the question 
 remained whether the movements perceived were real or only 
 apparent; and this it was not yet found possible to answer. 
 Already, in the previous century, the ingenious Robert Hooke 
 had suggested an "alteration of the very system of the sun," 3 
 to account for certain suspected changes in stellar positions ; 
 Bradley in 1748, and Lambert in 1761, pointed out that such 
 apparent displacements (by that time well ascertained) were in all 
 probability a combined effect of motions both of sun and stars ; 
 and Mayer actually attempted the analysis, but without result. 
 
 On the 1 3th of August 1596, David Fabricius, an unpro- 
 fessional astronomer in East Friesland, saw in the neck of the 
 Whale a star of the third magnitude, which by October had 
 disappeared. It was, however, visible in 1603, when Bayer 
 marked it in his catalogue with the Greek letter o, and was 
 watched through its phases of brightening and apparent extinc- 
 tion by a Dutch professor named Holwarda in I638-39. 4 From 
 
 1 Phil Trans., vol. xxx., p. 737. 2 Out of eighty stars compared, fifty- 
 seven were found to have changed their places by more than 10". Lesser 
 discrepancies were at that time regarded as falling within the limits of obser- 
 vational error. Tobice Mayeri Up. Inedita, t. i., pp. 80-81, and Herschel in 
 Phil. Trans., vol. Ixxiii., pp. 275-278. 3 Posthumous Works, p. 701. 4 Arago 
 in Annuaire du Bureau des Longitudes, 1842, p. 313. 
 
12 HISTORY OF ASTRONOMY. PARTI. 
 
 Hevelius this first-known periodical star received the name of 
 "Mira," or the Wonderful, and Boulliaud in 1667 fixed the 
 length of its cycle of change at 334 days. It was not a solitary 
 instance. A star in the Swan was perceived by Janson in 1600 
 to show fluctuations of light, and Montanari found in 1669 that 
 Algol in Perseus shared the same peculiarity to a marked degree. 
 Altogether the class embraced in 1782 half-a-dozen members. 
 When it is added that a few star-couples had been noted in singu- 
 larly, but it was supposed accidentally, close juxtaposition, and 
 that the failure of repeated attempts to find an annual parallax 
 pointed to distances for the stars at least 400,000 times that of the 
 earth from the sun, 1 the picture of sidereal science, when the last 
 quarter of the eighteenth century began, is practically complete. 
 It included three items of information : that the stars have 
 motions, real or apparent ; that they are immeasurably remote ; 
 and that a few shine with a periodically variable light. Nor 
 were the facts thus scantily collected ordered into any promise of 
 further development. They lay at once isolated and confused 
 before the inquirer. They needed to be both multiplied and 
 marshalled, and it seemed as if centuries of patient toil must 
 elapse before any reliable conclusions could be derived from 
 them. The sidereal world was thus the recognised domain of 
 far-reaching speculations, which remained wholly uncramped by 
 systematic research until Herschel entered upon his career as 
 an observer of the heavens. 
 
 The greatest of modern astronomers was born at Hanover, 
 NoverrJber 15, 1738. He was the fourth child of Isaac Herschel, 
 a hautboy-player in the band of the Hanoverian Guard, and was 
 early trained to follow his father's profession. On the termina- 
 tion, however, of the disastrous campaign of 1757, his parents 
 removed him from the regiment, there is reason to believe, in a 
 somewhat unceremonious manner. Technically, indeed, he in- 
 curred the penalties of desertion, remitted according to the 
 Duke of Sussex's statement to Sir George Airy by a formal 
 pardon handed to him personally by George III. on his presenta- 
 
 1 Bradley to Halley, Phil. Trans., vol. xxxv. (1728), p. 660. His observa- 
 tions were directly applicable to only two stars, y Draconis and t] Ursas 
 Majoris, but i5ome lesser ones were included in the same result. 
 
CHAP. i. SIDEREAL ASTRONOMY. 13 
 
 tion in I782. 1 At the age of nineteen, then, his military service 
 having lasted four years, he came to England to seek his fortune. 
 Of the life of struggle and privation which ensued little is known 
 beyond the circumstances that in 1760 he was engaged in training 
 the regimental band of the Durham Militia, and that in 1765 he 
 was appointed organist at Halifax. This post he exchanged a 
 year later for the more distinguished one of organist at the 
 Octagon Chapel in Bath. The tide of prosperity now began to 
 flow for him. The most brilliant and modish society in England 
 was at that time to be met at Bath, and the young Hanoverian 
 quickly found himself a favourite and the fashion in it. Engage- 
 ments multiplied upon him. He became director of the public 
 concerts ; he conducted oratorios, engaged singers, organised 
 rehearsals, composed anthems, chants, choral services, besides 
 undertaking private tuitions, at times amounting to thirty-five or 
 even thirty-eight lessons a week. He in fact personified the musical 
 activity of a place then eminently and energetically musical. 
 
 But these multifarious avocations did not take up the whole 
 of his thoughts. His education, notwithstanding the poverty of 
 his family, had not been neglected, and he had always greedily 
 assimilated every kind of knowledge that came in his way. 
 Now that he was a busy and a prosperous man, it might have been 
 expected that he would run on in the deep professional groove 
 laid down for him. On .the contrary, his passion for learning 
 seemed to increase with the diminution of the time available 
 for its gratification. He studied Italian, Greek, mathematics ; 
 Maclaurin's Fluxions served to " unbend his mind " ; Smith's 
 Harmonics and Optics and Ferguson's Astronomy were the 
 nightly companions of his pillow. What he read stimulated 
 without satisfying his intellect. He desired not only to know, 
 but to discover. In 1773 he hired a small telescope, and through 
 it caught a preliminary glimpse of the rich and varied fields in 
 which for so many years he was to expatiate. Henceforward 
 the purpose of his life was fixed : it was to obtain " a knowledge 
 of the construction of the heavens;" 2 and to this sublime am- 
 bition he remained true until the end. 
 
 1 Holden, Sir William Herschel, his Life and Works, p. 17. 
 
 2 Phil. Trans., vol. ci., p. 269. 
 
I 4 HISTORY OF ASTRONOMY. PARTI. 
 
 A more powerful instrument was the first desideratum ; and 
 here his mechanical genius came to his aid. Having purchased 
 the apparatus of a Quaker optician, he set about the manu- 
 facture of specula with a zeal ^which seemed to anticipate the 
 wonders they were to disclose to him. It was not until fifteen 
 years later that his grinding and polishing machines were 
 invented, so the work had at that time to be entirely done by 
 hand. During this tedious and laborious process (which could 
 not be interrupted without injury, and lasted on one occasion 
 sixteen hours), his strength was supported by morsels of food 
 put into his mouth by his sister, 1 and his mind amused by her 
 reading aloud to him the Arabian Nights, Don Quixote, or other 
 light works. At length, after repeated failures, he found him- 
 self provided with a reflecting telescope a 5 J-foot Gregorian 
 of his own construction. A copy of his first observation with 
 it, on the great Nebula in Orion an object of continual amaze- 
 ment and assiduous inquiry to him is preserved by the Royal 
 Society. It bears the date March 4, I774- 2 
 
 In the following year he executed his first " review of the 
 heavens," memorable chiefly as an evidence of the grand and 
 novel conceptions which already inspired him, and of the enthu- 
 siasm with which he delivered himself up to their guidance. 
 Overwhelmed with professional engagements, he still contrived 
 to snatch some moments for the stars ; and between the acts at 
 the theatre was often seen running from the harpsichord to his 
 telescope, no doubt with that " uncommon precipitancy which 
 accompanied all his actions." 3 He now rapidly increased the 
 power and perfection of his telescopes. Mirrors of seven, ten, 
 even twenty feet focal length, were successively completed, and 
 unprecedented magnifying powers employed. His energy was 
 unceasing, his perseverance indomitable. In the course of 
 twenty-one years no less than 430 parabolic specula left his 
 
 1 Caroline Lucretia Herschel, born at Hanover, March 16, 1750, died in the 
 same place, January 9, 1848. She came to England in 1772, and was her 
 brother's devoted assistant, first in his musical undertakings, and afterwards 
 down to the end of his.life, in his astronomical labours. 2 Holden, op. cit. 
 p. 39. 3 Memoir of Caroline fferschel, p. 37. 
 
CHAP. i. SIDEREAL ASTRONOMY. 15 
 
 hands. He had entered upon his forty-second year when he 
 sent his first paper to the Philosophical Transactions; yet 
 during the ensuing thirty-nine years his contributions many of 
 them elaborate treatises numbered sixty-nine, forming a series 
 of extraordinary importance to the history of astronomy. As a 
 mere explorer of the heavens his labours were prodigious. He 
 discovered 2500 nebulae, 806 double stars, passed the whole 
 firmament in review four several times, counted the stars in 
 3400 "gauge-fields," and executed a photometric classification 
 of the principal stars, founded on an elaborate (and the first 
 systematically conducted) investigation of their relative bright- 
 ness. He was as careful and patient as he was rapid ; spared 
 no time and omitted no precaution to secure accuracy in his 
 observations ; yet in one night he would examine, singly and 
 attentively, up to 400 separate objects. 
 
 The discovery of Uranus was a mere incident of the scheme 
 he had marked out for himself a fruit, gathered as it were 
 by the way. It formed, nevertheless, the turning-point in his 
 career. From a star-gazing musician he was at once trans- 
 formed into an eminent astronomer; he was relieved from 
 the drudgery of a toilsome profession, and installed as Eoyal 
 Astronomer, with a modest salary of 200 a year ; funds were 
 provided for the construction of the forty-foot reflector, from 
 the great space-penetrating power of which he expected as yet 
 unheard-of revelations ; in fine, his future work was not only 
 rendered possible, but it was stamped as authoritative. 1 On 
 Whit-Sunday 1782, William and Caroline Herschel played and 
 sang in public for the last time in St. Margaret's Chapel, Bath ; 
 in August of the same year the household was moved to 
 Datchet, near Windsor, and on April 3, 1786, to Slough. Here 
 happiness and honours crowded on the fortunate discoverer. In 
 1788 he married Mary, only child of James Baldwin, a merchant 
 of the city of London, and widow of Mr. John Pitt a lady 
 whose domestic virtues were enhanced by the possession of a 
 large jointure. The fruit of their union was one son, of whose 
 work the worthy sequel of his father's we shall have to 
 
 1 See Holden's Sir William Herschel, p. 54. 
 
16 HISTORY OF ASTRONOMY. PARTI. 
 
 speak further on. Herschel was created a Knight of the 
 Hanoverian Guelphic Order in 1816, and in 1821 he became the 
 first President of the Eoyal Astronomical Society, his son being 
 its first Foreign Secretary. Bjut his health had now for some 
 years been failing, and on August 25, 1822, he died at Slough, 
 in the eighty-fourth year of his age, and was buried in Upton 
 churchyard. 
 
 His epitaph claims for him the lofty praise of having " burst 
 the barriers of heaven." Let us see in what sense this is 
 true. 
 
 The first to form any definite idea as to the constitution of 
 the stellar system was Thomas Wright, the son of a carpenter 
 living at Byer's Green, near Durham. With him originated 
 what has been called the " Grindstone Theory " of the universe, 
 which regarded the Milky Way as the projection on the sphere 
 of a stratum or disc of stars (our sun occupying a position near 
 the centre), similar in magnitude and distribution to the lucid 
 orbs of the constellations. 1 He was followed by Kant, 2 who 
 transcended the views of his predecessor by assigning to nebulae 
 the position they long continued to occupy, rather on imagina- 
 tive than scientific grounds, of "island universes," external to, 
 and co-equal with the Galaxy. Johann Heinrich Lambert, 3 a 
 tailor's apprentice from Miihlhausen, followed, but independently. 
 The conceptions of this remarkable man were grandiose, his 
 intuitions bold, his views on some points a singular anticipation 
 of subsequent discoveries. The sidereal world presented itself to 
 him as a hierarchy of systems, starting from the planetary 
 scheme, rising to throngs of suns within the circuit of the 
 Milky Way the "ecliptic of the stars," as he phrased it 
 expanding to include groups of many Milky Ways ; these again 
 combining to form the unit of a higher order of assemblage, and 
 so onwards and upwards until the mind reels and sinks before 
 the immensity of the contemplated creations. 
 
 1 An Original Theory or New Hypothesis of the Universe, London, 1750. 
 See also De Morgan's summary of his views in Philosophical Magazine, April 
 1848. 2 Allgemeine Naturgeschichte und Theorie des Himmels, 1755. U Cosmo- 
 logische Brief e, Augsburg, 1761. 
 
CHAP. i. SIDEREAL ASTRONOMY. 17 
 
 " Thus everything revolves the earth around the sun ; the 
 sun around the centre of his system ; this system round a centre 
 common to it with other systems ; this group, this assemblage of 
 systems, round a centre which is common to it with other groups 
 of the same kind ; and where shall we have done ? ' ?1 
 
 The stupendous problem thus speculatively attempted, Herschel 
 undertook to grapple with experimentally. The upshot of this 
 memorable inquiry was the inclusion, for the first time, within 
 the sphere of human knowledge, of a connected body of facts, 
 and inferences from facts, regarding the sidereal universe ; in 
 other words, the foundation of what may properly be called a 
 science of the stars. 
 
 Tobias Mayer had illustrated the perspective effects which 
 must ensue in the stellar sphere from a translation of the solar 
 system, by comparing them to the separating in front and 
 closing up behind of trees in a forest to the eye of an advancing 
 spectator; 2 but the appearances which he thus correctly 
 described he was unable to detect. By a more searching 
 analysis of a smaller collection of proper motions, Herschel 
 succeeded in rendering apparent the very consequences foreseen 
 by Mayer. He showed, for example, that Arcturus and Vega 
 did, in fact, appear to recede from, and Sirius and Aldebaran to 
 approach, each other by very minute amounts ; and, with a 
 striking effort of divinatory genius, placed the " apex," or point 
 of direction of the sun's motion, close to the star X in the con- 
 stellation Hercules, 3 within a few degrees of the spot indicated 
 by later and indefinitely more refined methods of research. He 
 resumed the subject in i8c>5, 4 but though employing a more 
 vigorous method was scarcely so happy in his result. In i8o6, 5 
 
 1 The System of the World, p. 125, London, 1800 (a translation of the last- 
 mentioned work). Lambert regarded nebulae as composed of stars crowded 
 together, but not as external universes. In the case of the Orion nebula, in- 
 deed, he throws out such a conjecture, but afterwards suggests that it may 
 form a centre for that one of the subordinate systems composing the Milky 
 Way to which our sun belongs. - Opera Inedita, t. i., p. 79. 3 Phil Trans., 
 vol. Ixxiii. (i783),p. 273. It is worthy of remark that Prevost, almost 
 simultaneously with Herschel, executed an investigation similar to his with 
 very considerable success. Klugel confirmed Herschel's result by an analytical 
 inquiry in 1789. 4 Phil. Trans., vol. xcv., p. 233. 5 Ibid., vol. xcvi., p. 205. 
 
i8 HISTORY OF ASTRONOMY. PARTI. 
 
 lie made a preliminary attempt to ascertain the speed of the 
 sun's journey, which he no doubt greatly under-estimated in 
 fixing it at about three miles a second. Yet the validity of his 
 general conclusion as to the line of solar travel, though long 
 doubted, has been triumphantly confirmed. The question as to 
 the " secular parallax " of the fixed stars was in effect answered. 
 
 With their annual parallax, however, the case was very 
 different. The search for it had already led Bradley to the 
 important discoveries of the aberration of light and the nutation 
 of the earth's axis ; it was now about to lead Herschel to a 
 discovery of a different, but even more elevated character. Yet 
 in neither case was the object primarily sought attained. 
 
 From the very first promulgation of the Copernican theory the 
 seeming immobility of the stars had been urged as an argument 
 against its truth ; for if the earth really travelled in a vast orbit 
 round the sun, objects in surrounding space should appear to 
 change their positions, unless their distances were on a scale 
 which, to the narrow ideas of the universe then prevailing, 
 seemed altogether extravagant. 1 The existence of such apparent 
 or " parallactic " displacements was accordingly regarded as the 
 touchstone of the new views, and their detection became an 
 object of earnest desire to those interested in maintaining them. 
 Copernicus himself made the attempt; but with his "Trique- 
 trum," a jointed wooden rule with the divisions marked in ink, 
 constructed by himself, he was hardly able to measure angles of 
 ten minutes, far less fractions of a second. Galileo, a more 
 impassioned defender of the system, strained his ears, as it were, 
 from Arcetri, in his blind and sorrowful old age, for news of a 
 discovery which two more centuries had still to wait for. Hooke 
 believed he had found a parallax for the bright star in the Head 
 of the Dragon ; but was deceived. Bradley convinced himself 
 that such effects were too minute for his instruments to 
 measure. Herschel made a fresh attempt by a practically 
 untried method. 
 
 It is a matter of daily experience that two objects situated at 
 
 1 "Ingens bolus devorandus est," Kepler admitted to Herwart in May 
 1603. 
 
CHAP. i. SIDEREAL ASTRONOMY. 19 
 
 different distances seem to a beholder in motion to move 
 relatively to each other. This principle Galileo, in the third of 
 his Dialogues on the Systems of the World, 1 proposed to apply 
 to the determination of stellar parallax; for two stars, lying 
 apparently close together, but in reality separated by a great 
 gulf of space, must shift their mutual positions when observed 
 from opposite points of the earth's orbit ; or rather, the remoter 
 forms a virtually fixed point, to which the movements of the 
 other can be conveniently referred. By this means complications 
 were abolished more numerous and perplexing than Galileo 
 himself was aware of, and the problem was reduced to one of 
 simple micrometrical measurement. The "double-star method" 
 was also suggested by James Gregory in 1675, and again by 
 Wallis in i693; 2 Huygens first, and afterwards Dr. Long of 
 Cambridge (about 1750), made futile experiments with it; and 
 it eventually led, in the hands of Bessel, to the successful deter- 
 mination of the parallax of 6 1 Cygni. 
 
 Its advantages were not lost upon Herschel. His attempt to 
 assign definite distances to the nearest stars was no isolated 
 effort, but part of the settled plan upon which his observations 
 were conducted. He proposed to sound the heavens, and the 
 first requisite was a knowledge of the length of his sounding-line. 
 Thus it came about that his special attention was early directed 
 to double stars. 
 
 " I resolved," he writes, 3 " to examine every star in the heavens 
 with the utmost attention and a very high power, that I might 
 collect such materials for this research as would enable me to fix 
 my observations upon those that would best answer my end. 
 The subject has already proved so extensive, and still promises 
 so rich a harvest to those who are inclined to be diligent in the 
 pursuit, that I cannot help inviting every lover of astronomy to 
 join with me in observations that must inevitably lead to new 
 discoveries." 
 
 The first result of these inquiries was a classed catalogue of 
 269 double stars presented to the Royal Society in 1782, followed, 
 
 1 Opere, t. i., p. 415. 2 Phil Trans., vol. xvii., p. 848. 
 3 Phil. Trans., vol. Ixxii., p. 97. 
 
20 HISTORY OF ASTRONOMY. PARTI. 
 
 after three years, by an additional list of 434. In both these 
 collections the distances separating the individuals of each pair 
 were carefully measured, and (with a few exceptions) the direc- 
 tions with reference to a celestial meridian, of the lines joining 
 their centres (technically called "angles of position") were 
 determined with the aid of a " revolving- wire micrometer," 
 specially devised for the purpose. Moreover, an important 
 novelty was introduced by the observation of the various colours 
 visible in the star-couples, the singular and vivid contrasts of 
 which were now for the first time described. 
 
 Double stars were at that time supposed to be a purely optical 
 phenomenon. Their components, it was thought, while in reality 
 indefinitely remote from each other, were brought into fortuitous 
 contiguity by the chance of lying nearly in the same line of sight 
 from the earth. Yet Bradley had noticed a change of 30, be- 
 tween 1718 and 1759, in the position-angle of the two stars 
 forming Castor, and was thus within a hair's breadth of the dis- 
 covery of their physical connection. 1 While the Rev. John 
 Michell, arguing by the doctrine of probabilities, wrote as follows 
 in 1767: "It is highly probable in particular, and next to a 
 certainty in general, that such double stars as appear to consist 
 of two or more stars placed very near together, do really consist 
 of stars placed near together, and under the influence of some 
 general law." 2 And in 1784 : 3 " It is not improbable that a few 
 years may inform us that some of the great number of double, 
 triple stars, &c., which have been observed by Mr. Herschel, are 
 systems of bodies revolving about each other." 
 
 This remarkable speculative anticipation had a practical 
 counterpart in Germany. Father Christia.n Mayer, a Jesuit 
 astronomer at Mannheim, set himself, in January 1776, to col- 
 lect examples of stellar pairs, and shortly afterwards published 
 the supposed discovery of " satellites " to many of the princi- 
 pal stars. 4 His observations, however, were neither exact nor 
 prolonged enough to lead to useful results in such an inquiry. 
 
 1 Doberck, Observatory, vol. ii., p. no. - Phil. Trans., vol. Ivii., p. 249. 
 3 Ibid., vol. Ixxiv., p. 56. 4 Beobachtungen von Fixsterntrdbanten, 1778; 
 and De Novis in Ccelo tiidereo Phcenomenis, 1779. 
 
CHAP. i. SIDEREAL ASTRONOMY. 21 
 
 His disclosures were derided ; his planet-stars treated as results 
 of hallucination. On ria point cru a des choses aussi extraordi- 
 naires, wrote Lalande 1 within one year of a better-grounded 
 announcement to the same effect. 
 
 Herschel at first shared the general opinion as to the merely 
 optical connection of double stars. Of this the purpose for 
 which he made his colllection is in itself sufficient evidence, 
 since what may be called the differential method of parallaxes 
 depends, as we have seen, for its efficacy upon disparity of 
 distance. It was "much too soon," he declared in 1782, 2 "to 
 form any theories of small stars revolving round large ones ; " 
 while in the year following, 3 he remarked that the identical, 
 proper motions of the two stars forming, to the naked eye, the 
 single bright orb of Castor could only be explained as both 
 equally due to the " systematic parallax " caused by the sun's 
 movement in space. Plainly showing that the notion of a 
 physical tie, compelling the two bodies to travel together, had 
 not as yet entered into his speculations. But he was eminently 
 open to conviction, and had, moreover, by observations un- 
 paralleled in amount as well as in kind, prepared ample 
 materials for convincing himself and others. In 1802 he was 
 able to announce the fact of his discovery, and in the two 
 ensuing years to lay in detail before the Eoyal Society proofs, 
 gathered from the labours of a quarter of a century, of orbital 
 revolution in the case of as many as fifty double stars, hence- 
 forth, he declared, to be held as real binary combinations, 
 " intimately held together by the bond of mutual attraction." 4 
 The fortunate preservation in Dr. Maskelyne's note-book of a 
 remark made by Bradley about 1759, to the effect that the line 
 joining the components of Castor was an exact prolongation of 
 that joining Castor with Pollux, added eighteen years to the 
 time during which the pair were under scrutiny, and confirmed 
 the evidence of change afforded by more recent observations. 
 Approximate periods were fixed for many of the revolving suns 
 for Castor, 342 years; for y Leonis, 1200, 8 Serpentis, 375, 
 
 1 Bibliographic, p. 569. - Phil. Trans., vol. Ixxii., p. 162. 3 Ibid., vol. 
 Ixxiii., p. 272. 4 Phil. Trans., vol. xciii., p 340. 
 
22 HISTORY OF ASTRONOMY. PART i. 
 
 g Bootis, 1 68 1 years ; c Lyrae was noted as a " double-double-star," 
 a change of relative situation having been detected in each of 
 the two pairs composing the group ; and the occultation of 
 one star by another in the course of their mutual revolutions, 
 was described as having been exemplified in 1795 by the rapidly 
 circulating system of Herculis. 
 
 Thus, by the sagacity and perseverance of a single observer, a 
 firm basis was at last provided upon which to raise the edifice of 
 sidereal science. The analogy long presumed to exist between 
 the mighty star of our system and the bright points of light 
 spangling the firmament was shown to be no fiction of the 
 imagination, but a physical reality ; the fundamental quality of 
 attractive power was proved to be common to matter so far as 
 the telescope was capable of exploring, and law, subordination, 
 and regularity to give testimony of supreme and intelligent 
 design no less in those limitless regions of space than in our 
 narrow terrestrial home. The discovery was emphatically (in 
 Arago's phrase) "one with a future," since it introduced the 
 element of precise knowledge where more or less probable con- 
 jecture had previously held almost undivided sway ; and precise 
 knowledge tends to propagate itself and advance from point to 
 point. 
 
 We have now to speak of Herschel's pioneering work in the 
 skies. To explore with line and plummet the shining zone of the 
 Milky Way, to delineate its form, measure its dimensions, and 
 search out the intricacies of its construction, was the primary 
 task of his life, which he never lost sight of, and to which all 
 his other investigations were subordinate. He was absolutely 
 alone in this bold endeavour. Unaided, he had to devise methods, 
 accumulate materials, and sift out results. Yet it may safely be 
 asserted that all the knowledge we possess on this sublime 
 subject was prepared, and the greater part of it anticipated, 
 by him. 
 
 The ingenious method of " star-gauging," and its issue in the 
 delineation of the sidereal system as an irregular stratum of 
 evenly-scattered suns, is the best-known part of his work. But 
 it was, in truth, only a first rude approximation, the principle of 
 
CHAP. i. SIDEREAL ASTRONOMY. 23 
 
 which maintained its credit in the literature of astronomy a full 
 half-century after its abandonment by its author. This principle 
 was the general equality of star distribution. If equal portions 
 of space really held equal numbers of stars, it is obvious that the 
 number of stars visible in any particular direction would be 
 strictly proportional to the range of the system in that direction, 
 apparent accumulation being produced by real extent. The pro- 
 cess of " gauging the heavens," accordingly, consisted in counting 
 the stars in successive telescopic fields, and calculating thence the 
 depths of space necessary to contain them. The result of 3400 
 such operations was the plan of the Galaxy familiar to every 
 reader of an astronomical text-book. Widely-varying evidence 
 was, as might have been expected, derived from an examination 
 of different portions of the sky. Some fields of view were almost 
 blank, while others, in or near the Milky Way, blazed with the 
 radiance of many hundred stars compressed into an area about 
 one-fourth that of the full-moon. In the most crowded parts 
 116,000 were stated to have been passed in review within a 
 quarter of an hour. Here the "length of his sounding-line" 
 was estimated by Herschel at about 497 times the distance of 
 Sirius in other words, the bounding orb, or farthest sun of the 
 system in that direction, so far as was revealed by the 2O-foot 
 reflector, was thus inconceivably remote. But since the distance 
 of Sirius, no less than of every other fixed star, was as yet an 
 unknown quantity, the dimensions inferred for the Galaxy were 
 of course purely relative ; a knowledge of its form and structure 
 might (admitting the truth of the fundamental hypothesis) be 
 obtained, but its real or absolute size remained altogether 
 undetermined. 
 
 Even as early as 1785, however, Herschel perceived traces of 
 a tendency which completely invalidated the supposition of any 
 approach to an average uniformity of distribution. This was 
 the action of what he called a " clustering power " in the Milky 
 Way. " Many gathering clusters " x were already discernible to 
 him even while he endeavoured to obtain a "true mean result " 
 on the assumption that each star in space was separated from 
 
 1 Phil. Trans., vol. Ixxv., p. 255. 
 
24 HISTORY OF ASTRONOMY. PARTI. 
 
 its neighbours as widely as the sun from Sirius. "It appears." 
 he wrote in 1789, '''that the heavens consist of regions where 
 suns are gathered into separate systems " ; and in certain 
 assemblages he was able to trace " a course or tide of stars 
 setting towards a centre," denoting, not doubtfully, the presence 
 of attractive forces. 1 Thirteen years later, he described our sun 
 and his constellated companions as surrounded by "a magni- 
 ficent collection of innumerable stars, called the Milky Way, 
 which must occasion a very powerful balance of opposite attrac- 
 tions to hold the intermediate stars at rest. For though our sun, 
 and all the stars we see, may truly be said to be in the plane of 
 the Milky Way, yet I am now convinced, by a long inspection 
 and continued examination of it, that the Milky Way itself 
 consists of stars very differently scattered from those which are 
 immediately about us." " This immense aggregation," he added, 
 "is by no means uniform. Its component stars show evident 
 signs of clustering together into many separate allotments." ' 2 
 
 The following sentences, written in 1811, contain a definite 
 retractation of the view frequently attributed to him : 
 
 "I must freely confess," he says, " that by continuing my 
 sweeps of the heavens my opinion of the arrangement of the 
 stars and their magnitudes, and of some other particulars, has 
 undergone a gradual change ; and indeed, when the novelty of 
 the subject is considered, we cannot be surprised that many 
 things formerly taken for granted should on examination prove 
 to be different from what they were generally but incautiously 
 supposed to be. For instance, an equal scattering of the stars 
 may be admitted in certain calculations ; but when we examine 
 the Milky Way, or the closely compressed clusters of stars of 
 which my catalogues have recorded so many instances, this 
 supposed equality of scattering must be given up." 3 
 
 Another assumption, the fallacy of which he had not the 
 means of detecting since become available, was retained by him 
 to the end of his life. It was that the brightness of a star 
 afforded an approximate measure of its distance. Upon this 
 
 1 Phil. Trans., vol. Ixxix., pp. 214, 222. - Hid., vol. xcii., pp. 479, 495. 
 a Ibid., vol. ci., p. 269. 
 
CHAP. i. SIDEREAL ASTRONOMY. 25 
 
 principle he founded in 1817 his method of "limiting aper- 
 tures," 1 by which two stars, brought into view in two precisely 
 similar telescopes were "equalised" by covering a certain 
 portion of the object-glass collecting the more brilliant rays. 
 The distances of the orbs compared were then taken to be in the 
 ratio of the reduced to the original apertures of the instruments 
 with which they were examined. If indeed the absolute lustre 
 of each were the same, the result might be accepted with con- 
 fidence ; but since we have no warrant for assuming a " standard 
 star " to facilitate our computations, but much reason to suppose 
 an indefinite range, not only of size but of intrinsic brilliancy, in 
 the suns of our firmament, conclusions drawn from such a 
 comparison are entirely worthless. 
 
 In another branch of sidereal science besides that of stellar 
 aggregation, Herschel may justly be styled a pioneer. He was 
 the first to bestow serious study on the enigmatical objects 
 known as "nebulae." The history of the acquaintance of our 
 race with them is comparatively short. The only one recognised 
 before the invention of the telescope was that in the girdle of 
 Andromeda, certainly familiar in the middle of the tenth century 
 to the Persian astronomer Abdurrahman Al-Sufi ; and marked 
 with dots on an old Dutch chart of the constellation, presumably 
 about 1500 A.D. 2 Yet so little was it noticed that it might 
 practically be said as far as Europe is concerned to have been 
 discovered in 1612 by Simon Marius (Mayer of Genzenhausen), 
 who aptly described its appearance as that of a "candle shining 
 through horn." The first mention of the great Orion nebula is 
 by a Swiss Jesuit named Cysatus, who succeeded Father 
 Scheiner in the chair of mathematics at Ingolstadt. He used it, 
 apparently without any suspicion of its novelty, as a term of 
 comparison for the comet of December i6i8. 3 A novelty, 
 nevertheless, to astronomers it still remained in 1656, when 
 Huygens discerned, "as it were, an hiatus in the sky, affording 
 
 1 Phil. Trans., vol. cvii., p. 311. 2 Bullialdus, De Nebulosd Stella in Cingulo 
 Andromedce (1667) ; see also G. P. Bond, Mem. Am. Ac., vol. iii., p. 75, and 
 Holden's Monograph on the Orion Nebula, Washington Observations, vol. xxv., 
 1878 (pub. 1882). s Mathemata Astronomica, p. 75. 
 
26 HISTORY OF ASTRONOMY. PARTI. 
 
 a glimpse of a more luminous region beyond." * Halley in 1716 
 knew of six nebulae, which he believed to be composed of a " lucid 
 medium " diffused through the ether of space. 2 He appears, 
 however, to have been unacquainted with some previously noticed 
 by Hevelius. Lacaille brought Iback with him from the Cape a list 
 of forty-two the first-fruits of observation in Southern skies 
 arranged in three numerically equal classes ; 3 and Messier 
 (nicknamed by Louis XV. the "ferret of comets"), finding- 
 such objects a source of extreme perplexity in the pursuit of 
 his chosen game, attempted to eliminate by methodising them, 
 and drew up a catalogue comprising, in 1781, 103 entries. 4 
 
 These preliminary attempts shrank into insignificance when 
 Herschel began to "sweep the heavens" with his giant tele- 
 scopes. In 1786 he presented to the lloyal Society a descriptive 
 catalogue of 1000 nebulae and clusters, followed, three years 
 later, by a second of an equal number ; to which he added in 
 1802 a further gleaning of 500. On the subject of their nature 
 his views underwent a remarkable change. Finding that his 
 potent instruments resolved into stars many nebulous patches in 
 which no signs of such a structure had previously been dis- 
 cernible, he naturally concluded that " resolvability " was merely 
 a question of distance and telescopic power. He was (as he said 
 himself) led on by almost imperceptible degrees from evident 
 clusters, such as the Pleiades, to spots without a trace of stellar 
 formation, the gradations being so well connected as to leave no 
 doubt that all these phenomena were equally stellar. The 
 singular variety of their appearance was thus described by 
 him: 
 
 "I have seen," he says "double and treble nebulae variously 
 arranged; large ones with small, seeming attendants; narrow, 
 but much extended lucid nebulae or bright dashes ; some of the 
 shape of a fan, resembling an electric brush, issuing from a lucid 
 point ; others of the cometic shape, with a seeming nucleus in 
 the centre, or like cloudy stars surrounded with a nebulous 
 
 1 8y 'sterna Saturnium, p. 9. " Phil. Trans., vol. xxix., p. 390. 3 Mem. 
 Ac. des Sciences, 1755. 4 Conn, des Temps, 1784 (pub. 1781), p. 227. A 
 previous list of forty-five had appeared in Mem. Ac. d. Sc. 1771. 
 
CHAP. i. SIDEREAL ASTRONOMY. 27 
 
 atmosphere ; a different sort, again, contain a nebulosity of the 
 milky kind, like that wonderful, inexplicable phenomenon about 
 6 Orionis ; while others shine with a fainter, mottled kind of 
 light, which denotes their being resolvable into stars." * 
 
 "These curious objects" he considered to be "no less than 
 whole sidereal systems," 2 some of which might "well outvie our 
 Milky Way in grandeur." He admitted, however, a wide 
 diversity in condition as well as compass. The system to which 
 our sun belongs he described as "a very extensive branching 
 congeries of many millions of stars, which probably owes its 
 origin to many remarkably large as well as pretty closely 
 scattered small stars, that may have drawn together the rest." 3 
 But the continued action of this same " clustering power " would, 
 he supposed, eventually lead to the breaking up of the original 
 majestic Galaxy into two or three hundred separate groups, 
 already visibly gathering. Such minor nebulas, due to the 
 "decay "of other "branching nebulas" similar to our own, he 
 recognised by the score, lying, as it were, stratified in certain 
 quarters of the sky. "One of these nebulous beds," he informs 
 us, "is so rich that in passing through a section of it, in the 
 time of only thirty-six minutes, I detected no less than thirty- 
 one nebulas, all distinctly visible upon a fine blue sky." The 
 stratum of Coma Berenices he judged to be the nearest to 
 our system of such layers ; nor did the marked aggregation of 
 nebulas towards both poles of the circle of the Milky Way escape 
 his notice. 
 
 By a continuation of the same process of reasoning, he was 
 enabled (as he thought) to trace the life-history of nebulas from 
 a primitive loose and extended formation, through clusters of 
 gradually increasing compression, down to the kind named by 
 him " Planetary " because of the defined and uniform discs which 
 they present. These he regarded as " very aged, and drawing on 
 towards a period of change or dissolution." 4 
 
 " This method of vie wing the heavens," he concluded, " seems 
 to throw them into a new kind of light. They now are seen to 
 
 1 Phil. Trans., vol. Ixxiv., p. 442. 2 Ibid., vol. Ixxix., p. 213. 3 Ibid., 
 vol. Ixxv., p. 254. * Ibid., vol. Ixxix., p. 225. 
 
28 HISTORY OF ASTRONOMY. PART r. 
 
 resemble a luxuriant garden which contains the greatest variety 
 of productions in different nourishing beds ; and one advantage 
 we may at least reap from it is, that we can, as it were, extend 
 the range of our experience to an immense duration. For, to 
 continue the simile which I have borrowed from the vegetable 
 kingdom, is it not almost the same thing whether we live 
 successively to witness the germination, blooming, foliage, 
 fecundity, fading, withering, and corruption of a plant, or 
 whether a vast number of specimens, selected from every stage 
 through which the plant passes in the course of its existence, be 
 brought at once to our view ? " x 
 
 But already this supposed continuity was broken. After 
 mature deliberation on the phenomena presented by nebulous 
 stars, Herschel was induced, in 1791, to modify essentially his 
 original opinion. 
 
 " When I pursued these researches," he says, " I was in the 
 situation of a natural philosopher who follows the various species 
 of animals and insects from the height of their perfection down 
 to the lowest ebb of life ; when, arriving at the vegetable king- 
 dom, he can scarcely point out to us the precise boundary where 
 the animal ceases and the plant begins ; and may even go so far 
 as to suspect them not to be essentially different. But, recollect- 
 ing himself, he compares, for instance, one of the human species 
 to a tree, and all doubt upon the subject vanishes before him. 
 In the same manner we pass through gentle steps from a coarse 
 cluster of stars, such as the Pleiades . . . till we find ourselves 
 brought to an object such as the nebula in Orion, where we are 
 still inclined to remain in the once adopted idea of stars exceed- 
 ingly remote and inconceivably crowded, as being the occasion of 
 that remarkable appearance. It seems, therefore, to require a 
 more dissimilar object to set us right again, A glance like that 
 of the naturalist, who casts his eye from the perfect animal to 
 the perfect vegetable, is wanting to remove the veil from the 
 mind of the astronomer. The object I have mentioned above is 
 the phenomenon that was wanting for this purpose. View, for 
 instance, the ipth cluster of my 6th class, and afterwards cast 
 
 1 Pldl. Trans., vol. Ixxix., p. 226. 
 
CHAP. i. SIDEREAL ASTRONOMY. 29 
 
 your eye on this cloudy star, and the result will be no less decisive 
 than that of the naturalist we have alluded to. Our judgment, 
 I may venture to say, will be, that the nebulosity about the star 
 is not of a starry nature." x 
 
 The conviction thus arrived at of the existence in space of a 
 widely diffused "shining fluid" (a conviction long afterwards 
 fully justified by the spectroscope) led him into a field of endless 
 speculation. What was its nature? Should it "be compared 
 to the coruscation of the electric fluid in the aurora borealis ? or 
 to the more magnificent cone of the zodiacal light ? " Above 
 all, what was its function in the cosmos ? And on this point he 
 already gave a hint of the direction in which his mind was mov- 
 ing by the remark that this self-luminous matter seemed " more 
 fit to produce a star by its condensation, than to depend on the 
 star for its existence." 2 
 
 This was not a novel idea. Tycho Brahe had tried to explain 
 the blaze of the star of 1572 as due to a sudden concentration 
 of nebulous material in the Milky Way, even pointing out the 
 space left dark and void by the withdrawal of the luminous 
 stuff ; and Kepler, theorising on a similar stellar apparition in 
 1604, followed nearly in the same track. But under Herschel's 
 treatment the nebular origin of stars first acquired the consist- 
 ency of a formal theory. He meditated upon it long and 
 earnestly, and in two elaborate treatises, published respect- 
 ively in 1811 and 1814, he at length set forth the arguments 
 in its favour. These rested entirely upon the " principle of con- 
 tinuity." Between the successive classes of his progressive 
 assortment of objects there was, as he said, " perhaps not so 
 much difference as would be in an annual description of the 
 human figure, were it given from the birth of a child till he 
 comes to be a man in his prime." 3 From diffused nebulosity, 
 barely visible in the most powerful light-gathering instruments, 
 but which he estimated to cover nearly 152 square degrees of 
 the heavens, 4 to planetary nebulae, supposed to be already 
 centrally solid, instances were alleged by him of every stage and 
 
 1 Phil. Trans., vol. Ixxxi., p. 72. - Ibid., p. 85. 3 Ibid., vol. ci., p. 271. 
 i Ibid., p. 277. 
 
30 HISTORY OF ASTRONOMY. PARTI. 
 
 phase of condensation. The validity of his reasoning, however, 
 was evidently impaired by his confessed inability to distinguish 
 between the dim rays of remote clusters and the milky light of 
 true gaseous nebulae. ., 
 
 It may be said that such speculations are futile in themselves, 
 and necessarily barren of results. But they gratify an inherent 
 tendency of the human mind, and, if pursued in a becoming 
 spirit, should be neither reproved nor disdained. Herschel's 
 theory still holds the field, the testimony of recent discoveries 
 with regard to it having proved strongly confirmatory of its 
 principle, although not of its details. Strangely enough, it 
 seems to have been propounded in complete independence of 
 Laplace's nebular hypothesis as to the origin of the solar 
 system. Indeed, it dated, as we have seen, in its first incep- 
 tion, from 1791, while the French geometrician's view was not 
 advanced until 1796. 
 
 We may now briefly sum up the chief results of Herschel's 
 long years of "watching the heavens." The apparent motions 
 of the stars had been disentangled ; one portion being clearly 
 shown to be due to a translation towards a point in the constel- 
 lation Hercules of the sun and his attendant planets ; while a 
 large balance of displacement was left to be accounted for 
 by real movements, various in extent and direction, of the stars 
 themselves. By the action of a central force similar to, if not 
 identical with, gravity, suns of every degree of size and splen- 
 dour, and sometimes brilliantly contrasted in colour, were seen 
 to be held together in systems, consisting of two, three, four, 
 even six members, whose revolutions exhibited a wide range of 
 variety both in period and in orbital form. A new department 
 of physical astronomy was thus created, 1 and rigid calculation 
 for the first time made possible within the astral region. The 
 vast problem of the arrangement and relations of the millions 
 of stars forming the Milky Way was shown to be capable 
 of experimental treatment, and of at least partial solution, 
 notwithstanding the variety and complexity seen to prevail, to 
 an extent previously undreamt of, in the arrangement of that 
 
 1 Sir J. Herschel, Phil. Trans., vol. cxvi., part iii., p. i. 
 
CHAP. i. SIDEREAL ASTRONOMY. 31 
 
 majestic system. The existence of a luminous fluid, diffused 
 through enormous tracts of space, and intimately associated 
 with stellar bodies, was virtually demonstrated, and its place 
 and use in creation attempted to be divined by a bold but 
 plausible conjecture. Change on a stupendous scale was 
 inferred or observed to be everywhere in progress. Periodical 
 stars shone out and again decayed ; progressive ebbings or 
 flowings of light were indicated as probable in many stars under 
 no formal suspicion of variability ; forces were everywhere 
 perceived to be at work, by which the very structure of the 
 heavens themselves must be slowly but fundamentally modified. 
 In all directions groups were seen to be formed or forming ; tides 
 and streams of suns to be setting towards powerful centres of 
 attraction ; new systems to be in process of formation, while 
 effete ones hastened to decay or regeneration when the course 
 appointed for them by Infinite Wisdom was run. And thus, 
 to quote the words of the observer who " had looked farther into 
 space than ever human being did before him," 1 "the state into 
 which the incessant action of the clustering power has brought 
 the Milky Way at present, is a kind of chronometer that may be 
 used to measure the time of its past and future existence ; and 
 although we do not know the rate of going of this mysterious 
 chronometer, it is nevertheless certain that, since the breaking-up 
 of the parts of the Milky Way affords a proof that it cannot last 
 for ever, it equally bears witness that its past duration cannot be 
 admitted to be infinite." 2 
 
 1 His own words to the poet Campbell, cited by Holden, Life and Works, 
 p. 109. 2 ' Phil. Tram., vol. civ., p. 283. 
 
CHAPTER II. 
 PROGRESS OF SIDEREAL ASTRONOMY. 
 
 WE have now to consider labours of a totally different character 
 from those of Sir William Herschel. Exploration and discovery 
 do not constitute the whole business of astronomy; the less 
 adventurous, though not less arduous, task of gaining a more 
 and more complete mastery over the problems immemorially 
 presented to her, may, on the contrary, be said to form her 
 primary duty. A knowledge of the movements of the heavenly 
 bodies has, from the earliest times, been demanded by the urgent 
 needs of mankind ; and science finds its advantage, as in many 
 cases it has taken its origin, in condescension to practical claims. 
 Indeed, to bring such knowledge as near as possible to absolute 
 precision has been denned by no mean authority 1 as the true end 
 of astronomy. 
 
 Several causes concurred about the beginning of the present 
 century to give a fresh and powerful impulse to investigations 
 having this end in view. The rapid progress of theory almost 
 compelled a corresponding advance in observation ; instrumental 
 improvements rendered such an advance possible; Herschel's 
 discoveries quickened public interest in celestial inquiries ; royal, 
 imperial, and grand-ducal patronage widened the scope of 
 individual effort. The heart of the new movement was in 
 Germany. Hitherto the observatory of Elamsteed and Bradley 
 had been the acknowledged centre of practical astronomy; 
 Greenwich observations were the standard of reference all over 
 Europe ; and the art of observing prospered in direct proportion 
 
 1 Bessel, Populdre Vorlesungen, pp. 6, 408. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 33 
 
 to the fidelity with which. Greenwich methods were imitated. 
 Dr. Maskelyne, who held the post of Astronomer Royal during 
 forty-six years (from 1765 to 1811), was no unworthy successor 
 to the eminent men who had gone before him. His foundation 
 of the Nautical Almanac (in 1767) alone constitutes a valid title 
 of fame ; he introduced at the Observatory the important inno- 
 vation of the systematic publication of results ; and the careful 
 and prolonged series of observations executed by him formed the 
 basis of the improved theories, and corrected tables of the 
 celestial movements, which were rapidly being brought to 
 completion abroad. His catalogue of thirty-six " fundamental" 
 stars was besides excellent in its way, and most serviceable. Yet 
 he was devoid of Bradley's instinct for divining the needs of the 
 future. He was fitted rather to continue a tradition than to 
 found a school. The old ways were dear to him; and, inde- 
 fatigable as he was, a definite purpose was wanting to compel him, 
 by its exigencies, along the path of progress. Thus, for almost 
 fifty years after Bradley's death, the acquisition of a small 
 achromatic 1 was the only notable change made in the instru- 
 mental equipment of the Observatory. The transit, the zenith 
 sector, and the mural quadrant, with which Bradley had done his 
 incomparable work, retained their places long after they had 
 become deteriorated by time and obsolete by the progress of 
 invention; and it was not until the very close of his career 
 that Maskelyne, compelled by Pond's detection of serious 
 errors, ordered a Troughton's circle, which he did not live to 
 employ. 
 
 Meanwhile, the heavy national disasters with which Germany 
 was overwhelmed in the early part of the present century, seemed 
 to stimulate rather than impede the intellectual revival already 
 for some years in progress there. Astronomy was amongst the 
 first of the sciences to feel the new impulse. By the efforts of 
 Bode, Olbers, Schroter, and Von Zach, just and elevated ideas on 
 the subject were propagated, intelligence was diffused, and a firm 
 ground prepared for common action in mutual sympathy and 
 disinterested zeal. They were powerfully seconded by the 
 
 1 Fitted to the old transit instrument, July 11, 1772. 
 3 
 
34 HISTORY OF ASTRONOMY. PARTI. 
 
 foundation, in 1804, by a young artillery officer named Von 
 Keichenbach, of an Optical and Mechanical Institute at Munich. 
 Here the work of English instrumental artists was for the first 
 time rivalled, and that of English opticians when Fraunhofer 
 entered the new establishment far surpassed. The development 
 given to the refracting telescope by this extraordinary man was 
 indispensable to the progress of that fundamental part of 
 astronomy which consists in the exact determination of the 
 places of the heavenly bodies. Reflectors are brilliant engines of 
 discovery, but they lend themselves with difficulty to the prosaic 
 work of measuring right ascensions and polar distances. A signal 
 improvement in the art of making and working flint-glass thus 
 most opportunely coincided with the rise of a German school of 
 scientific mechanicians, to furnish the instrumental means needed 
 for the reform which was at hand. Of the leader of that reform 
 it is now time to speak. 
 
 Friedrich Wilhelm Bessel was born at Minden, in Westphalia, 
 July 22, 1784. A certain taste for figures, coupled with a still 
 stronger distaste for the Latin accidence, directed his inclination 
 and his father's choice towards a mercantile career. In his 
 fifteenth year, accordingly, he entered the house of Kuhlenkamp 
 & Sons, in Bremen, as an apprenticed clerk. He was now 
 thrown completely upon his own resources. From his father, a 
 struggling Government official, heavily weighted with a large 
 family, he was well aware that he had nothing to expect ; his 
 dormant faculties were roused by the necessity for self-depend- 
 ence, and he set himself to push manfully forward along the path 
 that lay before him. The post of supercargo on one of the 
 trading expeditions sent out from the Hanseatic towns to China 
 and the East Indies was the aim of his boyish ambition, for the 
 attainment of which he sought to qualify himself by the indus- 
 trious acquisition of suitable and useful knowledge. He learned 
 English in two or three months ; picked up Spanish with the 
 casual aid of a gunsmith's apprentice ; studied the geography of 
 the distant lands which he hoped to visit ; collected information 
 as to their climates, inhabitants, products, and the courses of 
 trade. He desired to add some acquaintance with the art (then 
 
CHAP. ii. SIDEREAL ASTRONOMY. 35 
 
 much neglected) of taking observations at sea ; and thus, led on 
 from navigation to astronomy, and from astronomy to mathe- 
 matics, he groped his way into a new world. 
 
 It was characteristic of him that the practical problems of 
 science should have attracted him before his mind was as yet 
 sufficiently matured to feel the charm of its abstract beauties. 
 His first attempt at observation was made with a sextant, 
 rudely constructed under his own directions, and a common 
 clock. Its object was the determination of the longitude of 
 Bremen, and its success, he tells us himself, 1 filled him with a 
 rapture of delight, which, by confirming his tastes, decided his 
 destiny. He now eagerly studied Bode's Jahrbuch and Von 
 Zach's Monatliclie Correspondents, overcoming each difficulty as it 
 arose with the aid of Lalande's Traitt d' Astronomic, and supply- 
 ing, with amazing rapidity, his early deficiency in mathematical 
 training. In two years he was able to attack a problem which 
 would have tasked the patience, if not the skill, of the most 
 experienced astronomer. Amongst the Earl of Egremont's 
 papers Von Zach had discovered Harriot's observations on 
 Halley's comet at its appearance in 1607, an ^ published them as 
 a supplement to Bode's Annual. With an elaborate care 
 inspired by his youthful ardour, though hardly merited by their 
 loose nature, Bessel deduced from them an orbit for that 
 celebrated body, and presented the work to Olbers, whose repu- 
 tation in cometary researches gave a special fitness to the 
 proffered homage. The benevolent physician-astronomer of 
 Bremen welcomed with surprised delight such a performance 
 emanating from such a source. Fifteen years before, the 
 French Academy had crowned a similar performance ; now its 
 equal was produced by a youth of twenty, busily engaged in 
 commercial pursuits, self-taught, and obliged to snatch from 
 sleep the hours devoted to study. The paper was immediately 
 sent to Von Zach for publication, with a note from Olbers 
 explaining the circumstances of its author, and the name of 
 Bessel became the common property of learned Europe. 
 
 He had, however, as yet no intention of adopting astronomy 
 1 Brieficechsel mit Olbers, p. xvi. 
 
36 HISTORY OF ASTRONOMY. PART i. 
 
 as his profession. For two years lie continued to work in the 
 counting-house by day, and to pore over the Mteanique Cdestc 
 and the Differential Calculus by night. But the post of 
 assistant in Schroter's observatory at Lilienthal having become 
 vacant by the removal of Harding to Gottingen in 1805, Olbers 
 procured for him the offer of it. It was not without a struggle 
 that he resolved to exchange the desk for the telescope. His 
 reputation with his employers was of the highest ; he had 
 thoroughly mastered the details of the business, which his keen 
 practical intelligence followed with lively interest ; his years 
 of apprenticeship were on the point of expiring, and an imme- 
 diate, and not unwelcome prospect of comparative affluence lay 
 before him. The love of science, however, prevailed ; he chose 
 poverty and the stars, and went to Lilienthal with a salary of 
 a hundred thalers yearly. Looking back over his life's work, 
 Olbers long afterwards declared that the greatest service which 
 he had rendered to astronomy was that of having discerned, 
 directed, and promoted the genius of Bessel. 1 
 
 For four years he continued in Schroter's employment. At 
 the end of that time the Prussian Government chose him to 
 superintend the erection of a new observatory at Konigsberg. 
 which after many vexatious delays caused by the prostrate 
 condition of the country, was finished towards the end of 1813. 
 Konigsberg was the first really efficient German observatory. 
 It became, moreover, a centre of improvement, not for Germany 
 alone, but for the whole astronomical world. During two-and- 
 thirty years it was the scene of Bessel's labours, and Bessel's 
 labours had for their aim the reconstruction, on an amended 
 and uniform plan, of the entire science of observation. 
 
 A knowledge of the places of the stars is the foundation of 
 astronomy. 2 Their configuration lends to the skies their dis- 
 tinctive features, and marks out the shifting tracks of more 
 mobile objects with relatively fixed, and generally unvarying 
 points of light. A more detailed and accurate acquaintance 
 with the stellar multitude, regarded from a purely uranogra- 
 phical point of view, has accordingly formed at all times a 
 
 1 R. Wolf, Gescli. der Astron., p. 518. - Bessel, Pop. For/., p. 22. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 37 
 
 primary object of celestial science, and has, during the present 
 century, been cultivated with a zeal and success by which all 
 previous efforts are dwarfed into insignificance. In Lalande's 
 Histoirc Cdlestc, published in 1801, the places of no less than 
 47.390 stars were given, but. in the rough, as it were, and con- 
 sequently needing laborious processes of calculation to render 
 them available for exact purposes. Piazzi set an example of 
 improved methods of observation, resulting in the publication, 
 in 1803 and 1814, of two catalogues of about 7600 stars the 
 second being a revision and enlargement of the first which for 
 their time were models of what such works should be. Stephen 
 Groombridge at Blackheath was similarly and most beneficially 
 active. But something more was needed than the diligence of 
 individual observers. A systematic reform was called for; and 
 it was this which Bessel undertook and carried through. 
 
 Direct observation furnishes only what has been called the 
 " raw material " of the positions of the heavenly bodies. 1 A 
 number of highly complex corrections have to be applied before 
 their mean can be disengaged from their apparent places on the 
 sphere. Of these, the most considerable and familiar is atmo- 
 spheric refraction, by which objects seem to stand higher in the 
 sky than they in reality do, the effect being evanescent at the 
 zenith, and attaining, by gradations varying with conditions of 
 pressure and temperature, a maximum at the horizon. Moreover, 
 the points to which measurements are referred are themselves in 
 motion, either continually in one direction, or periodically to 
 and fro. The precession of the equinoxes is slowly progressive, 
 or rather retrogressive ; the nutation of the pole oscillatory in a 
 period of about eighteen years. Added to which, the non- 
 instantaneous transmission of light, combined with the move- 
 ment of the earth in its orbit, causes a minute displacement 
 known as aberration. 
 
 Now it is easy to see that any uncertainty in the application 
 of these corrections saps the very foundations of exact astronomy. 
 Extremely minute quantities, it is true, are concerned ; but the 
 life and progress of modern celestial science depends upon the 
 
 ] Bessel, Pop. Vorl., p. 440. 
 
38 HISTORY OF ASTRONOMY. PART i. 
 
 sure recognition of extremely minute quantities. In the early 
 years of this century, however, no uniform system of " reduc- 
 tion " (so the complete correction of observational results is 
 termed) had been established. .. Much was left to the individual 
 caprice of observers, who selected for the several " elements " 
 of reduction such values as seemed best to themselves. 
 Hence arose much hurtful confusion, tending to hinder united 
 action and mar the usefulness of laborious researches. For this 
 state of things, Bessel, by the exercise of consummate diligence, 
 sagacity, and patience, provided an entirely satisfactory remedy. 
 His first step was an elaborate investigation of the precious 
 series of observations made by Bradley at Greenwich from 1750 
 until his death in 1762. The catalogue of 3222 stars which he 
 extracted from them, gave the earliest example of the systematic 
 reduction on a uniform plan of such a body of work. It is 
 difficult, without entering into details out of place in a volume 
 like the present, to convey an idea of the arduous nature of this 
 task. It involved the formation of a theory of the errors of each 
 of Bradley's instruments, and a difficult and delicate inquiry into 
 the true value of each correction to be applied before the entries 
 in the Greenwich journals could be developed into a finished and 
 authentic catalogue. Although completed in 1813, it was not 
 until five years later that the results appeared with the proud, 
 but not inappropriate title of Fundamenta Astronomic^. The 
 eminent value of the work consisted in this, that by providing a 
 mass of entirely reliable information as to the state of the 
 heavens at the epoch 1755, it threw back the beginning of exact 
 astronomy almost half a century. By comparison with Piazzi's 
 catalogues the amount of precession was more accurately deter- 
 mined, the proper motions of a considerable number of stars 
 became known with certainty, and definite prediction the 
 certificate of initiation into the secrets of Nature at last became 
 possible as regards the places of the stars. Bessel's final 
 improvements in the methods of reduction were published in 
 1830 in his Tabulce Regiomontance. They not only constituted 
 an advance in accuracy, but afforded a vast increase of facility in 
 application, and were at once and everywhere adopted. Thus 
 
CHAP. ii. SIDEREAL ASTRONOMY. 39 
 
 astronomy became a truly universal science ; uncertainties and 
 disparities were banished, and observations made at all times and 
 places rendered mutually comparable. 1 
 
 More, however, yet remained to be done. In order to verify 
 with greater strictness the results drawn from the Bradley and 
 Piazzi catalogues, a third term of comparison was wanted, and 
 this Bessel undertook to supply. By a course of 75,011 
 observations, executed during the years 182133, with the 
 utmost nicety of care, the number of accurately known stars 
 was brought up to above $0,000, and an ample store of trust- 
 worthy facts laid up for the use of future astronomers. In 
 this department Argelander, whom he attracted from finance 
 to astronomy, and trained in his own methods, was his assistant 
 and successor. The great " Bonn Durchmusterung," 2 in which 
 324,198 stars visible in the northern hemisphere are enumerated, 
 and the corresponding "Atlas" published in 1857-63, con- 
 stituting a picture of our sidereal surroundings of heretofore 
 unapproached completeness, may be justly said to owe their 
 origin to Bessel's initiative, and to form a sequel to what he 
 commenced. 
 
 But his activity was not solely occupied with the promotion 
 of a comprehensive reform in astronomy ; it embraced special 
 problems as well. The long-baffled search for a parallax of 
 the fixed stars was resumed with fresh zeal as each mechanical 
 or optical improvement held out fresh hopes of a successful issue. 
 Illusory results had for some time abounded. Piazzi in. 1 805 per- 
 ceived, as he supposed, considerable annual displacements in Vega, 
 Aldebaran, Sirius, and Procyon ; the truth being that his instru- 
 ments were worn out with constant use, and could no longer be de- 
 pended upon. 3 His countryman, Calandrelli, was similarly de- 
 luded. The celebrated controversy between the Astronomer Royal 
 and Dr. Brinkley, director of the Dublin College Observatory, 
 turned on the same subject. Brinkley, who was in possession of a 
 first-rate meridian circle, believed himself to have discovered rela- 
 tively large parallaxes for four of the brightest stars ; Pond, rely- 
 
 1 Durege, HesseVs Leben und Wirken, p. 28. '- Banner JBeobachtungen, 
 Bd. iii.-v., 1859-62. s Bessel, Pop. Vorl., p. 238. 
 
40 HISTORY OF ASTRONOMY. PART i. 
 
 ing on the testimony of the Greenwich instruments, asserted their 
 nullity. The dispute was protracted for fourteen years, from 
 iSiOto 1824, and was brought to no definite conclusion; but 
 the strong presumption on the negative side was abundantly 
 justified in the event. 
 
 There was good reason for incredulity in the matter of par- 
 allaxes. Announcements of their detection had become so 
 frequent as to be discredited before they were disproved ; and 
 Struve, who investigated the subject at Dorpat in 1818-21. had 
 clearly shown that the quantities concerned were so small as to 
 lie beyond the reliable measuring powers of any instrument then 
 in use. Already, however, the means were being prepared of 
 giving to those powers a large increase. 
 
 On the 2 1st July 1801, two old houses in an alley of Munich 
 tumbled down, burying in their ruins the occupants, of whom 
 one alone was extricated alive, though seriously injured. This 
 was an orphan lad of fourteen named Joseph Fraunhofer. The 
 Elector Maximilian Joseph was witness of the scene, became 
 interested in the survivor, and consoled his misfortune with a 
 present of eighteen ducats. Seldom was money better bestowed. 
 Part of it went to buy books and a glass-polishing machine, 
 with the help of which young Fraunhofer studied mathematics 
 and optics, and secretly exercised himself in the shaping and 
 finishing of lenses ; the remainder purchased his release from the 
 tyranny of one Weichselberger, a looking-glass maker by trade, 
 to whom he had been bound apprentice on the death of his 
 parents. A period of struggle and privation followed, during 
 which, however, he rapidly extended his acquirement ; and was 
 thus eminently fitted for the task awaiting him, when, in 1806, 
 he entered the optical department of the establishment founded 
 two years previously by Yon Reichenbach and Utzschneider. He 
 now zealously devoted himself to the improvement of the 
 achromatic telescope ; and, after a prolonged study of the theory 
 of lenses, and many toilsome experiments in the manufacture of 
 flint-glass, he succeeded in perfecting, December 12, 1817, an 
 object-glass of exquisite quality and finish, g\ inches in diameter 
 and of fourteen feet focal length. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 41 
 
 This (as it was then considered) gigantic lens was secured by 
 Struve for the Russian Government, and the " great Dorpat 
 refractor" the first of the large achromatics which have 
 played such an important part in modern astronomy was, 
 late in 1824, set up in the place which it still occupies. By 
 ingenious improvements in mounting and fitting, it was adapted 
 to the finest micrometrical work, and thus offered unprecedented 
 facilities both for tlie examination of double stars (in which 
 Struve chiefly employed it), and for such subtle measurements 
 as might serve to reveal or disprove the existence of a sensible 
 stellar parallax. Fraunhofer, moreover, constructed for the 
 observatory at Konigsberg the first really available heliometer. 
 The principle of this instrument (termed with more propriety a 
 " divided object-glass micrometer ") is the separation, by a 
 strictly measurable amount, of two distinct images of the 
 same object. If a double star, for instance, be under examina- 
 tion, the two half-lenses into which the object-glass is divided 
 are shifted until the upper star (say) in one image is brought 
 into coincidence with the lower star in the other, when their 
 distance apart becomes known by the amount of motion em- 
 ployed. 1 
 
 This virtually new engine of research was delivered and 
 mounted in 1829, three years after the termination of the life of 
 its deviser. The Dorpat lens had brought to Fraunhofer a title 
 of nobility and the sole management of the Munich Optical 
 Institute (completely separated since 1814 from the mechanical 
 department). What he had achieved, however, was but a small 
 part of what he meant to achieve. He saw before him the 
 possibility of nearly quadrupling the light-gathering capacity of 
 the great achromatic acquired by Struve ; he meditated improve- 
 ments in reflectors as important as those he had already effected 
 in refractors ; and was besides eagerly occupied with investiga- 
 tions into the nature of light, the momentous character of 
 
 1 The heads of the screws applied to move the halves of the object-glass in 
 the Konigsberg heliometer are of so considerable a size that a thousandth 
 part of a revolution, equivalent to ^ ff of a second of arc, can be measured with 
 the utmost accuracy. Main, in li. A. #. Mem., vol. xii., p. 53. 
 
42 HISTORY OF ASTRONOMY. PARTI. 
 
 which we shall by-and-by have an opportunity of estimating. 
 But his health was impaired, it is said, from the weakening 
 effects of his early accident, combined with excessive and un- 
 wholesome toil, and, still hoping for its restoration from a pro- 
 jected journey to Italy, he died of consumption, June 7, 1826, 
 aged thirty-nine years. His tomb in Munich bears the concise 
 eulogy, Approximamt sidera. 
 
 Bessel had no sooner made himself acquainted with the 
 exquisite defining powers of the Konigsberg heliometer, than he 
 resolved to employ them in an attack upon the now secular 
 problem of star-distances. But it was not until 1837 that he 
 found leisure to pursue the inquiry. In choosing his test-star 
 he adopted a new principle. It had hitherto been assumed that 
 our nearest neighbour in space must be found amongst the 
 brightest ornaments of our skies. The knowledge of stellar 
 proper motions afforded by the critical comparison of recent with 
 earlier star-places, suggested a different criterion of distance. 
 It is impossible to escape from the conclusion that the apparently 
 swiftest-moving stars are, on the whole, also the nearest to us, 
 however numerous the individual exceptions to the rule. Now, 
 as early as I792, 1 Piazzi had noted, as an indication of relative 
 vicinity to the earth, the unusually large proper motion (5.2" 
 annually) of a double star of the fifth magnitude in the constella- 
 tion of the Swan. Still more emphatically in i8i2 2 Bessel drew 
 the attention of astronomers to the fact, and 61 Cygni became 
 known as the "flying star." The seeming rate of its flight, 
 indeed, is of so leisurely a kind, that in a thousand years it will 
 have shifted its place by less than 3^ lunar diameters, and that 
 a quarter of a million would be required to carry it round the 
 entire circuit of the visible heavens. Nevertheless, it has few 
 rivals in rapidity of movement, the apparent displacement of the 
 vast majority of stars being, by comparison, almost insensible. 
 
 This interesting, though inconspicuous object, then, was chosen 
 by Bessel to be put to the question with his heliometer, while 
 Struve made a similar and somewhat earlier trial with the bright 
 
 1 Specola Astronomica di Palermo, lib. vi., p. 10, note. ' 2 Monatlictie Corre- 
 spondent, vol. xxvi., p. 162. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 43 
 
 gem of the Lyre, whose Arabic title of the " Falling Eagle" 
 survives as a time-worn remnant in " Vega." Both astronomers 
 agreed to use the "differential" method, for which their instru- 
 ments and the vicinity to their selected stars of minute, physi- 
 cally detached companions offered special facilities. In the last 
 month of 1838 Bessel made known the result of one year's 
 observations, showing for 61 Cygni a parallax of about a third 
 of a second (O.3I36"). 1 He then had his heliometer taken down 
 and repaired, after which he resumed the inquiry, and finally 
 terminated a series of 402 measures in March i84O. 2 The 
 resulting parallax of 0.3483" (corresponding to a distance about 
 600,000 times that of the earth from the sun), seemed to be 
 ascertained beyond the possibility of cavil, and is memorable as 
 the first published instance of the fathom-line, so industriously 
 thrown into celestial space, having really and indubitably touched 
 bottom. It was confirmed in 1842-43 with curious exactness by 
 C. A. F. Peters at Pulkowa ; but later researches showed that it 
 required increase to nearly half a second. 3 
 
 Struve's measurements inspired less confidence. They ex- 
 tended over three years (1835-38), but were comparatively few, 
 and were frequently interrupted. The parallax, accordingly, of 
 about a quarter of a second (0.26 13") which he derived from 
 them for a Lyrge, and announced in 1840,* has proved consider- 
 ably too large. 5 
 
 Meanwhile a result of the same kind, but of a more striking 
 character than either Bessel's or Struve's, had been obtained, one 
 might almost say casually, by a different method and in a dis- 
 tant region. Thomas Henderson, originally an attorney's clerk 
 in his native town of Dundee, had become known for his 
 astronomical attainments, and was appointed in 1831 to direct 
 
 1 Astronomische NachricJiten, Nos. 365-366. It should be explained that 
 which is called the "annual parallax " of a star is only half its apparent dis- 
 placement. In other words, it is the angle subtended at the distance of that 
 particular star by the radius of the earth's orbit. 2 Astr. Nach., Nos. 401- 
 402. 3 Sir E. Ball's measurements at Dun sink give to 61 Cygni a parallax 
 of 0.47" ; Professor Pritchard has obtained, by photographic determinations, 
 one of 0.43". * Additamentum in Mensuras Micrometricas, p. 28. 5 Elkin's 
 corrected result (in 1892) for the parallax of Vega is 0.092". 
 
44 HISTORY OF ASTRONOMY. PARTI. 
 
 the recently completed observatory at the Cape of Good Hope. 
 He began observing in April 1832, and, the serious shortcomings 
 of his instrument notwithstanding, executed during the thirteen 
 months of his tenure- of office a, surprising amount of first-rate 
 work. With a view to correcting the declination of the lustrous 
 double star a Centauri (which ranks after Sirius and Canopus 
 as the third brightest orb in the heavens), he, effected a number 
 of successive determinations of its position, and on being in- 
 formed of its very considerable proper motion (3.6" annually), he 
 resolved to examine the observations already made for possible 
 traces of paral lactic displacement. This was done on his return 
 to Scotland, where he filled the office of Astronomer Koyal from 
 1834 until his premature death in 1844. The result justified his 
 expectations. From the declination measurements made at the 
 Cape and duly reduced, a parallax of about one second of arc 
 clearly emerged (diminished by Gill's and Elkin's observations, 
 1882-1883, to 0.75"), but, by perhaps an excess of caution, was 
 withheld from publication until fuller certainty was afforded by 
 the concurrent testimony of Lieutenant Meadows' determinations 
 of the same star's right ascension. 1 When at last, January 9, 1 839, 
 Henderson communicated his discovery to the Astronomical 
 Society, he could no longer claim the priority which was his due. 
 Bessel had anticipated him with the parallax of 61 Cygni by just 
 two months. 
 
 Thus from three different quarters, three successful and 
 almost simultaneous assaults were delivered upon a long- 
 beleagured citadel of celestial secrets. The same work has since 
 been steadily pursued, with the general result of showing that, 
 as regards their overwhelming majority, the stars are far too 
 remote to show even the slightest trace of optical shifting from 
 the revolution of the earth in its orbit. In more than fifty cases, 
 however, small parallaxes have been determined, some certainly 
 (that is, within moderate limits of error), others more or less 
 precariously. The list is an instructive one, in its omissions no 
 less than in its contents. It includes stars of many degrees of 
 brightness, from Sirius down to a nameless telescopic star in the 
 
 ] Mem. Roy. Astr. Soc., vol. xi., p. 61. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 45 
 
 Great Bear ; l yet the vicinity of this minute object is so much 
 greater than that of the brilliant Vega, that the latter trans- 
 ported to its place would increase in lustre thirty times. 
 Moreover, many of the brightest stars are found to have no 
 sensible parallax, while the majority of those ascertained to be 
 nearest to the earth are of fifth, sixth, even ninth magnitudes. 
 The obvious conclusions follow that the range of variety in the 
 sidereal system is enormously greater than had been supposed, 
 and that estimates of distance based upon apparent magnitude 
 must be wholly futile. Thus, the splendid Canopus, Arcturus, 
 and Rigel, can be inferred, from their indefinite remoteness, to 
 exceed our sun thousands of times in size and lustre ; while 
 many inconspicuous objects, which prove to be in our relative 
 vicinity, must be notably his inferiors. The limits of real stellar 
 magnitude are then set very widely apart. At the same time, 
 both the so-called " optical" and '-geometrical" methods of 
 relatively estimating star-distances are seen to have a foundation 
 of fact, although so disguised by complicated relations as to be 
 of very doubtful individual application. On the whole, the 
 chances are in favour of the superior vicinity of a bright star 
 over a faint one ; and, on the whole, the stars in swiftest 
 apparent motion are amongst those whose actual remoteness is 
 least. Indeed, there is no escape from either conclusion, unless 
 on the supposition of special arrangements in themselves highly 
 improbable, and, we may confidently say, non-existent. 
 
 The distances even of the few stars found to have measurable 
 parallaxes are on a scale entirely beyond the powers of the 
 human mind to conceive. In the attempt both to realise them 
 distinctly, and to express them conveniently, a new unit of 
 length, itself of bewildering magnitude, has originated. This is 
 what we may call the light-journey of one year. The subtle 
 vibrations of the ether, propagated on all sides from the surface 
 of luminous bodies, travel at the rate of 186,300 miles a second, 
 or (in round numbers) six billions of miles a year. Four and 
 
 1 That numbered 21,185 in Lalande's Hist. Gel., found by Argelander to 
 have a proper motion of 4.734", and by Winnecke a parallax of 0.51 1". Month. 
 Not., vol. xviii., p. 289. 
 
46 HISTORY OF ASTRONOMY. PARTI. 
 
 a third such measures are needed to span the abyss that 
 separates us from the nearest fixed star. In other words, light 
 takes four years and four months to reach the earth from a 
 Centauri ; yet a Centauri lies some ten billions of miles nearer 
 to us (so far as is yet known) -than any other member of the 
 sidereal system ! 
 
 The determination of parallax leads, in the case of binary 
 systems, to the determination of mass ; for the distance from 
 the earth of the two bodies forming such a system being 
 ascertained, the seconds of arc apparently separating them from 
 each other can be translated into millions of miles ; and we 
 only need to add a knowledge of their period to enable us, 
 by an easy sum in proportion, to find their combined mass in 
 terms of that of the sun. Thus, since according to Dr. 
 Doberck's elements the two stars forming a Centauri revolve 
 round their common centre of gravity at a mean distance nearly 
 25 times the radius of the earth's orbit, in a period of 88 years, 
 the attractive force of the two together must be just twice the 
 solar. We may gather some idea of their relations by placing 
 in imagination a second luminary like our sun in circulation 
 between the orbits of Neptune and Uranus. But systems of 
 still more majestic proportions are reduced by extreme remote- 
 ness to apparent insignificance. A double star of the fourth 
 magnitude in Cassiopeia (Eta), to which a small parallax is 
 ascribed on the authority of 0. Struve, appears to be more than 
 nine times as massive as the central orb of our world ; while a 
 much less conspicuous pair 85 Pegasi exerts, if the available 
 data can be depended upon, fully thirteen times the solar 
 gravitating power. 
 
 Further, the actual rate of proper motions, so far as regards 
 that part of them which is projected upon the sphere, can be 
 ascertained for stars at known distances. The annual journey, 
 for instance, of 61 Cygni across the line of sight amounts to 
 IOOO, and that of a Centauri to 446 millions of miles. A small 
 star, numbered 1830 in Groombridge's Circumpolar Catalogue, 
 " devours the way" at the rate of 230 miles a second a speed, 
 in Newcomb's opinion, beyond the gravitating power of the 
 
CHAP. ii. SIDEREAL ASTRONOMY. 47 
 
 entire sidereal system to control; and Toucange possesses, 
 according to Dr. Gill, nearly half that amazing velocity 
 besides whatever movement each may have towards or from 
 the earth, of which the spectroscope may eventually give an 
 account. 
 
 Herschel's conclusion as to the movement of the sun among 
 the stars was not admitted as valid by the most eminent of his 
 successors. Bessel maintained that there was absolutely no 
 preponderating evidence in favour of its supposed direction 
 towards a point in the constellation Hercules. 1 Biot, Burck- 
 hardt, even Herschel's own son, shared his incredulity. But 
 the appearance of Argelander's prize-essay in i837 2 changed the 
 aspect of the question. Herschel's first memorable solution in 
 1783 was based upon the motions of thirteen stars, imperfectly 
 known; his second, in 1805, upon those of no more than six. 
 Argelander now obtained an entirely concordant result from the 
 large number of 390, determined with the scrupulous accuracy 
 characteristic of Bessel's work and his own. The reality of the 
 fact thus persistently disclosed could no longer be doubted ; it 
 was confirmed five years later by the younger Struve, and still 
 more strikingly in i847 3 by Galloway's investigation, founded 
 exclusively on the apparent displacements of southern stars. 
 In 1859 and 1863, the late Sir George Airy and Mr. Dunkin, 4 
 employing all the resources of modern science, and commanding 
 the wealth of material furnished by 1167 proper motions care- 
 fully determined by Mr. Main, reached conclusions closely 
 similar to that indicated nearly eighty years previously by the 
 first great sidereal astronomer ; which Mr. Plummer's reinvesti- 
 gation of the subject in i883 5 served but slightly to modify. 
 Yet astronomers were not satisfied. Dr. Auwers of Berlin 
 finished in 1886 a splendid piece of work, 6 for which he received 
 in 1888 the Gold Medal of the Koyal Astronomical Society. 
 It consisted in reducing afresh, with the aid of the most 
 refined modern data, Bradley's original stars, and comparing 
 
 1 Fund. Astr., p. 309. - Mem. Pres. a ?Ac. de St. Peter si., t. iii. 3 Phil. 
 Trans., vol. cxxxvii., p. 79. 4 Mem. Roy. Astr. Soc., vols. xxviii. and xxxii. 
 "' Ibid., vol. xlvii., p. 327. 6 Not yet completely published (1893). 
 
48 HISTORY OF ASTRONOMY. PARTI. 
 
 their places thus obtained for the year 1755 with those assigned 
 to them from observations made at Greenwich after the lapse of 
 ninety years,. In the interval, as was to be anticipated, most of 
 them were found to have travelled over some small span of the 
 heavens, and there resulted a stock of nearly three thousand 
 highly authentic proper motions. These ample materials were 
 turned to account by M. Ludwig Struve 1 for a discussion of the 
 sun's motion, of which the upshot was to shift its point of aim 
 to the bordering region of the constellations Hercules and Lyra. 
 Still later, Professor Lewis Boss of Albany, N.Y., 2 and M. Oscar 
 Stumpe of Bonn, 3 made similar experiments with variously 
 assorted lists of stars, by which the more easterly position of 
 the solar apex was fully confirmed. The brilliant Vega may 
 indeed be said to mark a centre round which newly determined 
 apexes tend loosely to group themselves. 4 The general direc- 
 tion of the solar movement may nevertheless be regarded as 
 fairly well known ; but as to its rate, the grounds of inference 
 long remained unsatisfactory. Otto Struve's estimate of 154 
 million miles a year is based upon the assumption of an average 
 annual parallax, for stars of the first magnitude, of about a 
 quarter of a second ; an assumption negatived by actual experi- 
 ence. Fortunately, however, as will be seen further on, a method 
 of determining the sun's velocity independently of any knowledge 
 of star-distances is now rapidly becoming available. 
 
 As might have been expected, speculation has not been idle 
 regarding the purpose and goal of the strange voyage of 
 discovery through space upon which our system is embarked ; 
 but altogether fruitlessly. The variety of the conjectures 
 hazarded in the matter is in itself a measure of their futility. 
 Long ago, before the construction of the heavens had as yet 
 been made the subject of methodical enquiry, Kant was disposed 
 to regard Sirius as the '-'central sun" of the Milky Way ; while 
 Lambert surmised that the vast Orion nebula might serve as the 
 regulating power of a subordinate group including our sun. 
 
 1 Memoires de St. Petersburg, t. xxxv., No. 3, 1887. ~ Astronomical 
 Journal, No. 213. 3 Astr. Nach., Nos. 2999, 3000. 4 Cf. Month. Notices + 
 vol. li., p. 243 ; Nature, vol. xliv., p. 572. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 49 
 
 Herschel threw out the hint that the great cluster in Hercules 
 (estimated to include 14,000 stars) might prove to be the 
 supreme seat of attractive force - 1 Argelander placed his central 
 body in the constellation Perseus ; 2 Fomalhaut, the brilliant of 
 the Southern Fish, was set in the post of honour by Bogus- 
 lawski of Breslau. Madler (who succeeded Struve at Dorpat in 
 1839) concluded from a more formal inquiry that the ruling 
 power in the sidereal system resided, not in any single prepon- 
 derating mass, but in the centre of gravity of the self-controlled 
 revolving multitude. 3 In the former case (as we know from the 
 example of the planetary scheme), the stellar motions would be 
 most rapid near the centre ; in the latter they would become 
 accelerated with remoteness from it. 4 Madler showed that no part 
 of the heavens could be indicated as a region of exceptionally 
 swift movements, such as would result from the presence of a 
 gigantic (though possibly obscure) ruling body ; but that a 
 community of extremely sluggish movements undoubtedly 
 existed in and near the group of the Pleiades, where, accord- 
 ingly, he placed the centre of gravity of the Milky Way. 5 The 
 bright star Alcyone thus became the " central sun," but in a 
 purely passive sense, its headship being determined by its situa- 
 tion at the point of neutralisation of opposing tendencies, and of 
 consequent rest. The solar period of revolution round this point 
 was, by an avowedly conjectural method, fixed at 18,200,000 
 years, implying, on the extremely hazardous supposition that 
 the distance of Alcyone is thirty-four million times that of the 
 earth from the sun, a velocity for our system of about thirty 
 miles a second. 
 
 The scheme of sidereal government framed by the Dorpat 
 astronomer was, it may be observed, of the most approved con- 
 stitutional type ; deprivation, rather than increase of influence 
 
 1 Phil. Trans., vol. xcvi., p. 230. 2 Mem. Pris. a I' Ac. de St. Petersboura, 
 t. iii., p. 603 (read Feb. 5, 1837). 3 Die Centralsonne, Astr. Nach., Nos. 
 566-567, 1846. 4 Sir J.' Herschel, note to Treatise on Astronomy, and Phil. 
 Trans., vol. cxxiii., part ii., p. 502. 5 The position is (as Sir J. Herschel 
 pointed out, Outlines of Astronomy, p. 631, loth ed.) placed beyond the range 
 of reasonable probability by its remoteness (fully 26) from the galactic 
 plane. 
 
 4 
 
50 HISTORY OF ASTRONOMY PARTI. 
 
 accompanying the office of chief dignitary. But while we are 
 still ignorant, and shall perhaps ever remain so, of the funda- 
 mental plan upon which the Galaxy is organised, recent investi- 
 gations tend more and more to exhibit it, not as monarchical 
 (so to speak), but as federative. The community of proper 
 motions detected by Madler in the vicinity of the Pleiades may 
 accordingly possess a significance altogether different from what 
 he imagined. 
 
 Bessel's so-called " foundation of an Astronomy of the 
 Invisible " now claims attention. 1 His prediction regarding the 
 planet Neptune does not belong to the present division of our 
 subject ; a strictly analogous discovery in the sidereal system 
 was, however, also very clearly foreshadowed by him. His 
 earliest suspicions of non-uniformity in the proper motion of 
 Sirius dated from 1834 ; they extended to Procyon in 1840; and 
 after a series of refined measurements with the new Repsold 
 circle, he announced in 1844 his conclusion that these irregu- 
 larities were due to the presence of obscure bodies round which 
 the two bright Dog-stars revolved as they pursued their way 
 across the sphere. 2 He even assigned to each an approximate 
 period of half a century. " I adhere to the conviction," he 
 wrote later to Humboldt, " that Procyon and Sirius form real 
 binary systems, consisting of a visible and an invisible star. 
 There is no reason to suppose luminosity an essential quality 
 of cosmical bodies. The visibility of countless stars is no 
 argument against the invisibility of countless others." 3 
 
 An inference so contradictory to received ideas obtained little 
 credit, until Peters found, in 1851,* that the apparent anomalies 
 in the movements of Sirius could be completely explained by an 
 orbital revolution in a period of fifty years. Bessel's prevision 
 was destined to be still more triumphantly vindicated. On the 
 3 1st of January 1862, while in the act of trying a new 1 8-inch 
 refractor, Mr. Alvan G. Clark (one of the celebrated firm, of 
 American opticians) actually discovered the hypothetical Sirian 
 
 1 Madler in Westmnann's Jahrbuch, 1867, p. 615. ~ Letter from Bessel to 
 Sir J. Herschel, Month. Xot., vol. vi., p. 139. 3 Wolf, Gesch. d, Astr. 
 p. 743, note. 4 Astr. Nach., Nos. 745-748. 
 
-CHAP. ii. SIDEREAL ASTRONOMY. 51 
 
 companion in the precise position required by theory. It has 
 now been watched through three-fifths of a revolution (period 
 49.4 years), and proves to be very slightly luminous in proportion 
 to its mass. Its attractive power, in fact, is nearly half that of 
 its primary, while it emits only 1 1 p-th of its light. Sirius itself, 
 on the other hand, possesses a far higher radiative intensity than 
 our sun. It gravitates admitting Dr. Gill's parallax of 0.38" to 
 be exact like two suns, but shines like sixty-three. Possibly it 
 is enormously distended by heat, and undoubtedly its atmosphere 
 intercepts a very much smaller proportion of its light than in 
 stars of the solar class. As regards Procyon, visual verification 
 is still wanting, but to the mental eye the presence of a con- 
 siderable disturbing mass was fully assured through the inquiry 
 instituted by Auwers in I862. 1 The period of forty years then 
 .assigned to the system appears confirmed by recent observations. 
 
 But Bessel was not destined to witness the recognition of 
 "the in visible "as a legitimate and profitable field for astro- 
 nomical research. He died March 17, 1846, just six months 
 before the discovery of Neptune, of an obscure disease^ 
 eventually found to be occasioned by an extensive fungus- 
 growth in the stomach. The place which he left vacant was 
 not one easy to fill. His life's work might be truly described 
 as ''epoch-making." Rarely indeed shall we find one who 
 reconciled with the same success the claims of theoretical and 
 practical astronomy, or surveyed the science which he had made 
 his own with a glance equally comprehensive, practical, and 
 profound. 
 
 The career of Friedrich Georg Wilhelm Struve illustrates the 
 maxim that science differentiates as it develops. He was, while 
 much besides, a specialist in double stars. His earliest recorded 
 use of the telescope was to verify Herschel's conclusion as to the 
 revolving movement of Castor, and he never varied from the 
 predilection which this first observation at once indicated and 
 determined. He was born at Altona, of a respectable yeoman 
 family, April 15, 1793, and in 1811 took a degree in philology 
 .at the new Russian University of Dorpat. He then turned to 
 
 1 Astr. Nach., Nos. 1371-1373. 
 
52 HISTORY OF ASTRONOMY. PARTI. 
 
 science, was appointed in 1813 to a professorship of astronomy 
 and mathematics, and began regular work in the Dorpat 
 Observatory just erected by Parrot for Alexander I. It was not, 
 however, until 1819 that the acquisition of a 5 -foot refractor by 
 Troughton enabled him to take the position-angles of double 
 stars with regularity and tolerable precision. The resulting 
 catalogue of 795 stellar systems gave the signal for a general 
 resumption of the Herschelian labours in this branch. Moreover, 
 the extraordinary facilities for observation afforded by the Fraun- 
 hof er achromatic encouraged him to undertake, February 1 1 , 
 1825, a review of the entire heavens down to 15 south of the 
 celestial equator, which occupied more than two years, and 
 yielded, from an examination of above I2O.OOO stars, a harvest 
 of about 2200 previously unnoticed composite objects. The 
 ensuing ten years were devoted to delicate and patient mea- 
 surements, the results of which were embodied in Mensurw 
 Micrometricce, published at St. Petersburg in 1836. This monu- 
 mental work gives the places, angles of position, distances, 
 colours, and relative brightness of 3112 double and multiple 
 stars, all determined with the utmost skill and care. The record 
 is one which gains in value with the process of time, and will for 
 ages serve as a standard of reference by which to detect change 
 or confirm discovery. 
 
 It appears from Struve's researches that about one in forty of 
 all stars down to the ninth magnitude is composite, but that the 
 proportion is doubled in the brighter orders. 1 This he attributed 
 to the difficulty of detecting the faint companions of very 
 remote orbs. It was also noticed, both by him and Bessel, that 
 double stars are in general remarkable for large proper motions. 
 Struve's catalogue included no star of which the components 
 were more than 32" apart, because beyond that distance the 
 chances of merely optical juxtaposition become considerable ; 
 but the immense preponderance of extremely close over (as it 
 were) loosely yoked bodies is such as to demonstrate their 
 physical connection, even if no other proof were forthcoming. 
 Many stars previously believed to be single divided under the 
 
 1 Ueber die Doppelsterne, Bericftt. 1827, p. 22. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 53 
 
 scrutiny of the Dorpat refractor; while in some cases, one 
 member of a (supposed) binary system revealed itself as double, 
 thus placing the surprised observer in the unexpected presence of 
 a triple group of suns. Five instances were noted of two pairs 
 lying so close together as to induce a conviction of their mutual 
 dependence; 1 besides which, 124 examples occurred of triple, 
 quadruple, and multiple combinations, the reality of which was 
 open to no reasonable doubt. 2 
 
 It was first pointed out by Bessel that the fact of stars exhi- 
 biting a common proper motion might serve as an unfailing test 
 of their real association into systems. This was, accordingly, 
 one of the chief criteria employed by Struve to distinguish true 
 binaries from merely optical couples. On this ground alone, 6 1 
 Cygni was admitted to be a genuine double star ; and it was 
 shown that, although its components appeared to follow almost 
 strictly rectilinear paths, yet the probability of their forming a 
 connected pair is actually greater than that of the sun rising 
 to-morrow morning. 3 Moreover, this tie of an identical move- 
 ment was discovered to unite bodies 4 far beyond the range of 
 distance ordinarily separating the members of binary systems, 
 and to prevail so extensively as to lead to the conclusion 
 that single do not outnumber conjoined stars more than twice 
 or thrice. 5 
 
 In 1835 Struve was summoned by the Emperor Nicholas to 
 superintend the erection of a new observatory at Pulkowa, near 
 St. Petersburg, destined for the special cultivation of sidereal 
 astronomy. Boundless resources were placed at his disposal, 
 and the institution created by him was acknowledged to surpass 
 all others of its kind in splendour, efficiency, and completeness. 
 Its chief instrumental glory was a refractor of fifteen inches 
 aperture by Merz and Mahler (Fraunhofer's successors), which 
 left the famous Dorpat telescope far behind, and remained long 
 without a rival. On the completion of this model establish- 
 
 1 Ueber die Doppelsterne, Bericht, 1827, p. 25. - Mensurce Micr., p. xcix. 
 3 Stellarum Fixarum imprimis Duplicium et Multiplicium Posi.tiones Media 1 , pp. 
 cxc., cciii. 4 For instance, the southern stars, 36A Ophiuchi (itself double) 
 and 30 Scorpii, which are 12' 10" apart. Ibid., p. cciii. 5 Stellarum Fixa- 
 rum, <&c., p. ccliii. 
 
54 HISTORY OF ASTRONOMY. PARTI, 
 
 ment, August 19, 1839, Struve was installed as its director, 
 and continued to fulfil the important duties of the post with his 
 accustomed vigour until 1858, when illness compelled his virtual 
 resignation in favour of his son Otto Struve, born at Dorpat in 
 1819. He died November 23, i"864. 
 
 An inquiry into the laws of stellar distribution, undertaken 
 during the early years of his residence at Pulkowa, led Struve 
 to confirm in the main the inferences arrived at by Herschel 
 as to the construction of the heavens. According to his view, 
 the appearance known as the Milky Way is produced by a 
 collection of (for the most part) irregularly condensed star- 
 clusters, within which the sun is somewhat eccentrically placed. 
 The nebulous ring which thus integrates the light of countless 
 worlds was supposed by him to be made up of stars scattered 
 over a bent or " broken plane," or to lie in two planes slightly 
 inclined to each other, our system occupying a position near 
 their intersection. 1 He further attempted to show that the- 
 limits of this vast assemblage must remain for ever shrouded 
 from hn man discernment, owing to the gradual extinction of 
 light in its passage through space, 2 and sought to confer upon 
 this celebrated hypothesis a definiteness and certainty far 
 beyond the aspirations of its earlier advocates, Cheseaux and 
 Olbers ; but arbitrary assumptions vitiated his reasonings on 
 this, as well as on some other points. 3 
 
 In his special line as a celestial explorer of the most com- 
 prehensive type, Sir William Herschel had but one legitimate 
 successor, and that successor was his son. John Frederick 
 William Herschel was born at Slough, March 17, 1792, 
 graduated with the highest honours from St. John's College, 
 Cambridge, in 1813. and entered upon legal studies with a view 
 to being called to the Bar. But his share in an early compact 
 with Peacock and Babbage, "to do their best to leave the world 
 wiser than they found it," was not thus to be fulfilled. The 
 acquaintance of Dr. Wollaston decided his scientific vocation- 
 Already, in 1816, we find him reviewing some of his father's 
 
 1 Etudes d" Astronomic Stellaire, 1847, p. 82. - Ibid., p. 86. s See Encke's- 
 criticism in Astr. Nach., No. 622. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 55 
 
 double stars; and he completed in 1820 the 1 8-inch speculum 
 which was to be the chief instrument of his investigations. Soon 
 afterwards, he undertook, in conjunction with Mr. (afterwards 
 Sir James) South, a series of observations, issuing in the presen- 
 tation to the Royal Society of a paper 1 containing micrometrical 
 measurements of 380 binary stars, by which the elder Herschel's 
 inferences of orbital motion were, in many cases, strikingly 
 confirmed. A star in the Northern Crown, for instance (j 
 Coronas), had completed more than one entire circuit since its 
 first discovery ; another, r Serpentarii, had closed up into appa- 
 rent singleness ; while the motion of a third, Ursae Majoris, in 
 an obviously eccentric orbit, was so rapid as to admit of being 
 traced and measured from month to month. 
 
 It was from the first confidently believed that the force retain- 
 ing double stars in curvilinear paths was identical with that 
 governing the planetary revolutions. But that identity was not 
 ascertained until Savary of Paris showed, in i82/, 2 that the 
 movements of the above-named binary in the Great Bear, could 
 be represented with all attainable accuracy by an ellipse calculated 
 on orthodox gravitational principles with a period of 58 J years. 
 Eiicke followed at Berlin with a still more elegant method ; and 
 Sir John Herschel, pointing out the uselessness of analytical 
 refinements where the data were necessarily so imperfect, 
 described in 1831 a graphical process by which "the aid of the 
 eye and hand" was brought in "to guide the judgment in a case 
 where judgment only, and not calculation, could be of any 
 avail." 3 The subject has since been cultivated with diligence, 
 and not without success ; but our acquaintance with stellar 
 orbits can hardly yet be said to have emerged from the tentative 
 stage. 
 
 In 1825 Herschel undertook, and executed with great assiduity 
 during the ensuing eight years, a general survey of the northern 
 heavens, directed chiefly towards the verification of his father's 
 nebular discoveries. The outcome was a catalogue of 2306 
 nebulae and clusters, of which 525 were observed for the first 
 
 1 Phil. Trans., vol. cxiv., part iii., 1824. 2 Conn. d. Temps, 1830. 
 3 It. A. tf. Mem., vol. v., 1833, p. 178. 
 
56 HISTORY OF ASTRONOMY. PARTI. 
 
 time, besides 3347 double stars discovered almost incidentally. 1 
 ''Strongly invited," as he tells us himself, "by the peculiar 
 interest of the subject, and the wonderful nature of the objects 
 which presented themselves," he resolved to attempt the comple- 
 tion of the survey in the southern hemisphere. With this noble 
 object in view, he embarked his family and instruments on board 
 the Mount Stewart Mphinstone, and, after a prosperous voyage, 
 landed at Cape Town on the i6th of January 1834. Choosing 
 as the scene of his observations a rural spot under the shelter of 
 Table Mountain, he began regular "sweeping" on the 5th of 
 March. The site of his great reflector is now marked with an 
 obelisk, and the name of Feldhausen has become memorable in 
 the history of science ; for the four years' work done there may 
 truly be said to open the chapter of our knowledge as regards the 
 southern skies. 
 
 The full results of Herschel's journey to the Cape were not 
 made public until 1847, when a splendid volume 2 embodying 
 them was brought out at the expense of the Duke of North- 
 umberland. They form a sequel to his father's labours such as 
 the investigations of one man have rarely received from those of 
 another. What the elder observer did for the northern heavens, 
 the younger did for the southern, and with generally concordant 
 results. Reviving the paternal method of " star-gauging," he 
 showed, from a count of 2299 fields, that the Milky Way sur- 
 rounds the solar system as a complete annulus of minute stars ; 
 not, however, quite symmetrically, since the sun appears to lie 
 somewhat nearer to those portions visible in the southern hemi- 
 sphere, which accordingly display a brighter lustre and a more 
 complicated structure than the northern branches. The singular 
 cosmical agglomerations known as the " Magellanic Clouds " were 
 now, for the first time, submitted to a detailed, though admit- 
 tedly incomplete, examination, the almost inconceivable richness 
 and variety of their contents being such that a lifetime might 
 with great profit be devoted to their study. In the Greater 
 Nubecula, within a compass of forty-two square degrees, 
 
 1 Phil. Trans., vol. cxxiii., and Results, (Oc., Introd. - .Results of Astro- 
 nomical Observations made during the years 1834-8 at tfie Cape of Good Hope. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 57 
 
 Herschel reckoned 278 distinct nebulae and clusters, besides 
 fifty or sixty outliers, and a large number of stars intermixed 
 with diffused nebulosity in all, 919 catalogued objects, and, 
 for the Lesser Cloud, 244. Yet this was only the most con- 
 spicuous part of what his twenty-foot revealed. Such an 
 extraordinary concentration of bodies so various led him to the 
 inevitable conclusion that "the Nubeculae are to be regarded as 
 systems sui generis, and which have no analogues in our hemi- 
 sphere." l He noted also the blankness of surrounding space, 
 especially in the case of Nubecula Minor, "the access to which 
 on all sides," he remarked, " is through a desert ; " as if the 
 cosmical material in the neighbourhood had been swept up and 
 garnered in these mighty groups. 2 
 
 Of southern double stars, he discovered and gave careful 
 measurements of 2102, and described 1708 nebulae, of which at 
 least 300 were new. The list was illustrated with a number of 
 drawings, some of them extremely beautiful and elaborate. 
 
 Sir John Herschel's views as to the nature of nebulae were 
 considerably modified by Lord Eosse's success in "resolving" 
 with his great reflectors a crowd of these objects into stars. His 
 former somewhat hesitating belief in the existence of phos- 
 phorescent matter, " disseminated through extensive regions of 
 space in the manner of a cloud or fog," 3 was changed into a 
 conviction that no valid distinction could be established between 
 the faintest wisp of cosmical vapour just discernible with a 
 powerful telescope, and the most brilliant and obvious cluster. 
 He admitted, however, an immense range of possible variety in 
 the size and mode of aggregation of the stellar constituents of 
 various nebulas. Some might appear nebulous from the close- 
 ness of their parts ; some from their smallness. Others, he 
 suggested, might be formed of " discrete luminous bodies float- 
 ing in a non-luminous medium ; " 4 while the annular kind 
 probably consisted of " hollow shells of stars." 5 That a 
 physical, and not merely an optical, connection unites nebulae 
 
 1 Itesults, &c., p. 147. - See Proctor's Universe of Stars, p. 92. 3 A 
 Treatise on Astronomy, 1833, p. 406. 4 Results, &c., p. 139. 5 Ibid., pp. 
 24, 142. 
 
58 HISTORY OF ASTRONOMY. PARTI. 
 
 with the embroidery (so to speak) of small stars with which they 
 are in many instances profusely decorated, was evident to him, 
 as it must be to all who look as closely and see as clearly as he 
 did. His description of No. 2093 in his northern catalogue as 
 " a network of tracery of nebula following the lines of a similar 
 network of stars," 1 would alone suffice to dispel the idea of 
 accidental scattering ; and many other examples of a like import 
 might be quoted. The remarkably frequent occurrence of one 
 or more minute stars in the close vicinity of " planetary " 
 nebulge led him to infer their dependent condition ; and he 
 advised the maintenance of a strict watch for evidences of 
 circulatory movements, not only over these supposed stellar 
 satellites, but also over the numerous " double nebulae," in 
 which, as he pointed out, " all the varieties of double stars as to 
 distance, position, and relative brightness, have their counter- 
 parts." He, moreover, investigated the subject of nebular 
 distribution by the simple and effectual method of graphic 
 delineation or " charting," and succeeded in showing that while 
 a much greater uniformity of scattering prevails in the southern 
 heavens than in the northern, a condensation is nevertheless 
 perceptible about the constellations Pisces and Cetus, roughly 
 corresponding to the " nebular region " in Virgo by its vicinity 
 (within 20 or 30) to the opposite pole of the Milky Way. He 
 concluded "that the nebulous system is distinct from the 
 sidereal, though involving, and perhaps to a certain extent 
 intermixed with, the latter." 2 
 
 Towards the close of his residence at Feldhausen, Herschel 
 was fortunate enough to witness one of those singular changes 
 in the aspect of the firmament which occasionally challenge the 
 attention even of the incurious, and excite the deepest wonder 
 of the philosophical observer. Immersed apparently in the Argo 
 nebula is a large star denominated rj Argus. When Halley 
 visited St. Helena in 1677. it seemed of the fourth magnitude ; 
 but Lacaille in the middle of the following century, and others 
 after him, classed it as of the second. In 1827 the traveller 
 Burchell, being then at St. Paul, near Rio Janeiro, remarked 
 
 1 Phil. Trans., vol. cxxiii., p. 503. - Itesults, &c., p. 136. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 59 
 
 that it had unexpectedly assumed the first rank a circum- 
 stance the more surprising to him because he had frequently, 
 when in Africa during the years 1811 to 1815, noted it as of only 
 fourth magnitude. This observation, however, did not become 
 generally known until later. Herschel, on his arrival at 
 Feldhausen, registered the star as a bright second, and had no 
 suspicion of its unusual character until December 1 6, 1837, 
 when he suddenly perceived it with its light almost tripled. It 
 then far outshone Rigel in Orion, and on the 2nd of January 
 following, it very nearly matched a Centauri. From that date it 
 declined ; but a second and even brighter maximum occurred in 
 April 1843, when Maclear, then director of the Cape Observa- 
 tory, saw it blaze out with a splendour approaching that of 
 Sirius. Its waxings and wanings were marked by curious 
 'trepidations" of brightness extremely perplexing in theory. 
 In 1863 it had sunk below the fifth magnitude, and in 1869 
 was barely visible to the naked eye ; yet it was not until 
 eighteen years later that it touched a minimum of 7.6 magni- 
 tude. Soon afterwards a recovery of brightness set in, but was 
 not carried very far ; and the star now shines steadily as of the 
 seventh magnitude, its reddish light contrasting effectively with 
 the silvery rays of the surrounding nebula. An attempt to 
 include its fluctuations within a cycle of seventy years l has 
 signally failed ; the extent and character of the vicissitudes to 
 which it is subject stamping it rather as a species of connecting 
 link between periodical and temporary stars. 2 
 
 Among the numerous topics which engaged Herschel's atten- 
 tion at the Cape was that of relative stellar brightness. Having 
 contrived an "astrometer" in which an "artificial star," formed 
 by the total reflection of moonlight from the base of a prism, 
 served as a standard of comparison, he was able to estimate the 
 lustre of the natural stars examined by the distances at which 
 the artificial object appeared equal respectively to each. He thus 
 constructed a table of 191 of the principal stars, 3 both in the 
 northern and southern hemispheres, setting forth the numerical 
 
 1 Loomis in Month. Not., vol. xxix., p. 298. - See the Author's tiysttm of 
 the /Stars, pp. 116-120. 3 Outlines of Astr , App. 1. 
 
60 HISTORY OF ASTRONOMY. PARTI. 
 
 values of their apparent brightness relatively to that of a Cen- 
 tauri, which he selected as a unit of measurement. Further, 
 the light of the full moon being found by him to exceed that of 
 his standard star 27,498 times, and Dr. Wollaston having shown 
 that the light of the full moon is to that of the sun as 
 i: 80 1, 072 1 (Zollner made the ratio I : 618,000), it became 
 possible to compare stellar with solar radiance. Hence was 
 derived, in the case of the few stars at ascertained distances, a 
 knowledge of real lustre. Alpha Centauri, for example, emits 
 about four times, Capella 250 times as much light as our sun; 
 while Arcturus, judging from its indefinite distance, must display 
 the splendour of a couple of thousand such luminaries. 
 
 Herschel returned to England in the spring of 1838, bringing 
 with him a wealth of observation and discovery such as had 
 perhaps never before been amassed in so short a time. Deserved 
 honours awaited him. He was created a baronet on the occasion 
 of the Queen's coronation (he had been knighted in 1831); 
 universities and learned societies vied with each other in shower- 
 ing distinctions upon him ; and the success of an enterprise in 
 which scientific zeal was tinctured with an attractive flavour of 
 adventurous romance, was justly regarded as a matter of national 
 pride. His career as an observing astronomer was now virtually 
 closed, and he devoted his leisure to the collection and arrange- 
 ment of the abundant trophies of his father's and his own 
 activity. The resulting great catalogue of 5079 nebulae (includ- 
 ing all then certainly known), published in the Philosophical 
 Transactions for 1864, is, and will probably long remain, the 
 fundamental source of information on the subject; 2 but he 
 unfortunately did not live to finish the companion work on 
 double stars, for which he had accumulated a vast store of 
 materials. 3 He died at Collingwood in Kent, May II, 1871, in 
 
 1 Phil. Trans., vol. cxix.. p. 27. 2 Dr. Dreyer's New General Catalogue, 
 published in 1888 as vol. xlix. of the Royal Astronomical Society's Memoirs, 
 is an enlargement of Herschel's work. It includes 7,840 entries. 3 A list 
 of 10.320 composite stars was drawn out by him in order of right ascension, 
 and has been published in vol. xl. of Mem. R. A. S. ; but the data requisite 
 for their formation into a catalogue were not forthcoming. See Main's and 
 Pritchard's Preface to above, and Dunkin's Obituary Notices, p. 73. 
 
CHAP. ii. SIDEREAL ASTRONOMY. 61 
 
 the eightieth year of his age, and was buried in Westminster 
 Abbey, close beside the grave of Sir Isaac Newton. 
 
 The consideration of Sir John Herschel's Cape observations 
 brings us to the close of the period we are just now engaged in 
 studying. They were given to the world, as already stated, three 
 years before the middle of the century, and accurately represent 
 the condition of sidereal science at that date. Looking back 
 over the fifty years traversed, we can see at a glance how great 
 was the stride made in the interval. Not alone was acquaintance 
 with individual members of the cosmos vastly extended, but 
 their mutual relations, the laws governing their movements, 
 their distances from the earth, masses, and intrinsic lustre, had 
 begun to be successfully investigated. Begun to be ; for only 
 regarding a scarcely perceptible minority had even approximate 
 conclusions been arrived at. Nevertheless the whole progress of 
 the future lay in that beginning ; it was the thin end of the 
 wedge of exact knowledge. The principle of measurement had 
 been substituted for that of probability ; a basis had been found 
 large and strong enough to enable calculation to ascend from it 
 to the sidereal heavens ; and refinements had been introduced, 
 fruitful in performance, but still more in promise. Thus, rather 
 the kind than the amount of information collected was signifi- 
 cant for the time to come rather the methods employed than 
 the results actually secured rendered the first half of the nine- 
 teenth century of epochal importance in the history of our 
 knowledge of the stars. 
 
CHAPTER III. 
 PROGRESS OF KNOWLEDGE REGARDING THE SUN. 
 
 THE discovery of sun-spots in 1610 by Fabricius and Galileo 
 first opened a way for inquiry into the solar constitution ; but it 
 was long before that way was followed with system or profit. 
 The seeming irregularity of the phenomena discouraged con- 
 tinuous attention ; casual observations were made the basis of" 
 arbitrary conjectures, and real knowledge received little or no 
 increase. In 1 620 we find Jean Tarde, canon of Sarlat, arguing 
 that because the sun is "the eye of the world." and the eye of 
 the world cannot suffer from ophthalmia, therefore the appear- 
 ances in question must be due, not to actual specks or stains on 
 the bright solar disc, but to the transits of a number of small 
 planets across it ! To this new group of heavenly bodies he 
 gave the name of " Borbonia Sidera," and they were claimed in 
 1633 for the House of Hapsburg, under the title of " Austriaca 
 Sidera " by Father Malapert ius, a Belgian Jesuit. 1 A similar 
 view was temporarily maintained against Galileo, by the justly 
 celebrated Father Scheiner of Ingolstadt. and later by William 
 Gascoigne, the inventor of the micrometer ; but most of those 
 who were capable of thinking at all on such subjects (and they 
 were but few) adhered either to the cloud theory or to the slag 
 theory of sun-spots. The first was championed by Galileo, the 
 second by Simon Marius, " astronomer and physician " to the 
 brother Margraves of Brandenburg. The latter opinion received 
 a further notable development from the fact that in 1618, a year 
 remarkable for the appearance of three bright comets, the sun 
 
 1 Kosmos, Bd. iii., p. 409; Lalande, Bibliographie Astronomique, pp. 179, 202. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 63 
 
 was almost free from spots ; whence it was inferred that the 
 cindery refuse from the great solar conflagration, which usually 
 appeared as dark blotches on its surface, was occasionally thrown 
 off in the form of comets, leaving the sun, like a snuffed taper, 
 to blaze with renewed brilliancy. 1 
 
 In the following century, Derham gathered from observations 
 carried on during the years 1703-11, "That the spots on the 
 sun are caused by the eruption of some new volcano therein, 
 which at first pouring out a prodigious quantity of smoke and 
 other opacous matter, causeth the spots ; and as that fuliginous 
 matter decayeth and spendeth itself, and the volcano at last 
 becomes more torrid and flaming, so the spots decay, and grow 
 to umbrae, and at last to faculae." 2 
 
 The view, confidently upheld by Lalande, 3 that spots were 
 rocky elevations uncovered by the casual ebbing of a luminous 
 ocean, the surrounding penumbrse representing shoals or sand- 
 banks, had even less to recommend it than Derham's volcanic 
 theory. Both were, however, significant of a growing tendency 
 to bring solar phenomena within the compass of terrestrial 
 analogies. 
 
 For 164 years, then, after Galileo first levelled his telescope 
 at the setting sun, next to nothing was learned as to its nature ; 
 and the facts immediately ascertained of its rotation on an axis 
 nearly erect on the plane of the ecliptic, in a period of between 
 twenty-five and twenty-six days, and of the virtual limitation of 
 the spots to a so-called "royal" zone extending some thirty 
 degrees north and south of the solar equator, gained little 
 either in precision or development from five generations of 
 astronomers. 
 
 1 K. Wolf, Die Sonne und ihre F/ecken, p. y. Marius himself, however, seems 
 to have held the Aristotelian terrestrial-exhalation theory of cometary origin. 
 See his curious little tract, Astronomische und Astroloyische Beschreibung des 
 Cometen, Niirnberg, 1619. 2 Phil. Trans., vol. xxvii., p. 274. Umbras (now 
 called penumbrce) are spaces of half-shadow which usually encircle spots. 
 Faculce (" little torches," so named by Scheiner) are bright streaks or patches 
 closely associated with spots. 3 Mem. Ac. Sc., 1776 (pub. 1779), p. 507. The 
 merit, however (if merit it be), of having first put forward (about 1671) the 
 hypothesis alluded to in the text belongs to D. Cassini. See Delambre, Hist, 
 de VAstr. Mod., t. ii., p. 694 ; and Kosmos, Bd. iii., p. 410. 
 
64 HISTORY OF ASTRONOMY. PARTI. 
 
 But in November 1769 a spot of extraordinary size engaged 
 the attention of Alexander Wilson, professor of astronomy in 
 the University of Glasgow. He watched it day by day, and to 
 good purpose. As the. great globe slowly revolved, carrying the 
 spot towards its western edge, he was struck with the gradual 
 contraction and final disappearance of the penumbra on the side 
 next the centre of the disc ; and when on the 6th of December 
 the same spot re-emerged on the eastern limb, he perceived, as 
 he had anticipated, that the shady zone was now deficient on 
 the opposite side, and resumed its original completeness as it 
 returned to a central position. Similar perspective effects were 
 visible in numerous other spots subsequently examined by him, 
 and he was thus in I774 1 able to prove by strict geometrical 
 reasoning that such appearances were, as a matter of fact, 
 produced by vast excavations in the sun's substance. It was 
 not, indeed, the first time that such a view had been suggested. 
 Father Schemer's later observations plainly foreshadowed it ; 2 
 a conjecture to the same effect was emitted by Leonard Host of 
 Nuremberg early in the eighteenth century ; 3 both by Lahire in 
 1703 and by J. Cassini in 1719 spots had been seen to form actual 
 notches on the solar limb ; while Pastor Schiilen of Essingen 
 convinced himself in 1770, by the careful study of appearances 
 similar to those noted by Wilson, of the fact detected by him. 4 
 Nevertheless, Wilson's demonstration came with all the surprise 
 of novelty, as well as with all the force of truth. 
 
 The general theory by which it was accompanied rested on a 
 very different footing. It was avowedly tentative, and was set 
 forth in the modest shape of an interrogatory. "Is it not 
 reasonable to think," he asked, " that the great and stupendous 
 body of the sun is made up of two kinds of matter, very 
 different in their qualities ; that by far the greater part is solid 
 and dark, and that this immense and dark globe is encompassed 
 with a thin covering of that resplendent substance from which 
 the sun would seem to derive the whole of his vivifying heat 
 
 1 Phil. Trans., vol. Ixiv., part i., pp. 7-11. 2 Rosa Ursina, lib. iv., p. 507. 
 s R. Wolf, Die Sonne und ihre Flecken, p. 12. 4 Scbellen, Die Spectralanalysc* 
 Bd. ii., p. 56 (3rd ed.) 
 
CHAP. in. KNOWLEDGE OF THE SUN. 65 
 
 and energy ? " 1 He further suggested that the excavations or 
 spots might be occasioned " by the working of some sort of 
 elastic vapour which is generated within the dark globe," and 
 that the luminous matter being in some degree fluid, and being 
 acted upon by gravity, tended to flow down and cover the 
 nucleus. From these hints, supplemented by his own diligent 
 observations and sagacious reasonings, Herschel elaborated a 
 scheme of solar constitution which held its ground until the 
 physics of the sun were revolutionised by the spectroscope. 
 
 A cool, dark, solid globe, its surface diversified with moun- 
 tains and valleys, clothed in luxuriant vegetation, and " richly 
 stored with inhabitants," protected by a heavy cloud-canopy 
 from the intolerable glare of the upper luminous region, where 
 the dazzling coruscations of a solar aurora some thousands of 
 miles in depth evolved the stores of light and heat which vivify 
 our world such was the central luminary which Herschel 
 constructed with his wonted ingenuity, and described with his 
 wonted eloquence. 
 
 " This way of considering the sun and its atmosphere," he 
 says, 2 " removes the great dissimilarity we have hitherto been 
 used to find between its condition and that of the rest of the 
 great bodies of the solar system. The sun, viewed in this light, 
 appears to be nothing else than a very eminent, large, and lucid 
 planet, evidently the first, or, in strictness of speaking, the only 
 primary one of our system ; all others being truly secondary to 
 it. Its similarity to the other globes of the solar system with 
 regard to its solidity, its atmosphere, and its diversified surface, 
 the rotation upon its axis, and the fall of heavy bodies, leads us 
 on to suppose that it is most probably also inhabited, like the 
 rest of the planets, by beings whose organs are adapted to the 
 peculiar circumstances of that vast globe." 
 
 We smile at conclusions which our present knowledge con- 
 demns as extravagant and impossible, but such incidental flights 
 of fancy in no way derogate from the high value of Herschel's 
 contributions to solar science. The cloud-like character which 
 he attributed to the radiant shell of the sun (first named by 
 
 1 Phil. Trans., vol. Ixiv., p. 20. ~ Ibid., vol. Ixxxv., 1795, p. 63. 
 
 5 
 
66 HISTORY OF ASTRONOMY. PARTI. 
 
 Schroter the "photosphere ") is borne out by all recent investi- 
 gations ; he observed its mottled or corrugated aspect, resem- 
 bling, as he described it, the roughness on the rind of an 
 orange ; showed that " faculae " are elevations or heaped-up 
 ridges of the disturbed photo&pheric matter ; and threw out the 
 idea that spots may ensue from an excess of the ordinary 
 luminous emissions. A certain " empyreal " gas was, he sup- 
 posed (very much as Wilson had done), generated in the body 
 of the sun, and rising everywhere by reason of its lightness, 
 made for itself, when in moderate quantities, small openings or 
 "pores," 1 abundantly visible as dark points on the solar disc. 
 But should an uncommon quantity be formed, "it will," he 
 maintained, "burst through the planetary 2 regions of clouds, 
 and thus will produce great openings ; then, spreading itself 
 above them, it will occasion large shallows (penumbrse), and 
 mixing afterwards gradually with other superior gases, it will 
 promote the. increase, and assist in the maintenance of the 
 general luminous phenomena." 3 
 
 This partial anticipation of the modern view that the solar 
 radiations are maintained by some process of circulation within 
 the solar mass, was reached by Herschel through prolonged 
 study of the phenomena in question. The novel and important 
 idea contained in it, however, it was at that time premature 
 to attempt to develop. But though many of the subtler 
 suggestions of Herschel's genius passed unnoticed by his 
 contemporaries, the main result of his solar researches was an 
 unmistakable one. It was nothing less than the definitive 
 introduction into astronomy of the paradoxical conception of 
 the central fire and hearth of our system as a cold, dark, 
 terrestrial mass, wrapt in a mantle of innocuous radiance an 
 earth, so to speak, within a sun without. 
 
 Let us pause for a moment to consider the value of this 
 remarkable innovation. It certainly was not a step in the 
 direction of truth. On the contrary, the crude notions of 
 
 1 Phil. Trans., vol. xci., 1801, p. 303. - The supposed opaque or protective 
 stratum beneath the photosphere was named by him " planetary," from the 
 analogy of terrestrial clouds. 3 Ibid., p. 305. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 67 
 
 Anaxagoras and Xeno approached more nearly to what we now 
 know of the sun, than the complicated structure devised for the 
 happiness of a nobler race of beings than our own by the 
 benevolence of eighteenth-century astronomers. And yet it 
 undoubtedly constituted a very important advance in science. 
 It was the first earnest attempt to bring solar phenomena within 
 the compass of a rational system ; to put together into a consist- 
 ent whole the facts ascertained ; to fabricate, in short, a solar 
 machine that would in some fashion work. It is true that the 
 materials were inadequate and the design faulty. The resulting* 
 construction has not proved strong enough to stand the wear and 
 tear of time and discovery, but has had to be taken to pieces and 
 remodelled on a totally different plan. But the work was not 
 therefore done in vain. None of Bacon's aphorisms show a 
 clearer insight into the relations between the human mind and 
 the external world than that which declares " Truth to emerge 
 sooner from error than from confusion." 1 A definite theory 
 (even if a false one) gives holding-ground to thought. Facts 
 acquire a meaning with reference to it. It affords a motive for 
 accumulating them and a means of co-ordinating them; it 
 provides a framework for their arrangement, and a receptacle for 
 their preservation, until they become too strong and numerous to 
 be any longer included within arbitrary limits, and shatter the 
 vessel originally framed to contain them. 
 
 Such was the purpose subserved by Herschel's theory of the 
 sun. It helped to clarify ideas on the subject. The turbid 
 sense of groping and viewless ignorance gave place to the lucidity 
 of a plausible scheme. The persuasion of knowledge is a keen 
 incentive to its increase. Few men care to investigate what they 
 are obliged to admit themselves entirely ignorant of ; but once 
 started on the road of knowledge, real or supposed, they are 
 eager to pursue it. By the promulgation of a confident and 
 consistent view regarding the nature of the sun, accordingly, 
 research was encouraged, because it was rendered hopeful, and 
 inquirers were shown a path leading indefinitely onwards where 
 an impassable thicket had before seemed to bar the way. 
 
 1 Novum Oryanum, lib. ii., aph. 20. 
 
68 HISTORY OF ASTRONOMY. PARTI. 
 
 We have called the " terrestrial " theory of the sun's nature an 
 innovation, and so, as far as its general acceptance is concerned, 
 it may justly be termed ; but, like all successful innovations, it 
 was a long time brewing. It is extremely curious to find that 
 Herschel had a predecessor in its advocacy who never looked 
 through a telescope (nor, indeed, imagined the possibility of such 
 an instrument), who knew nothing of sunspots, was still (mis- 
 taken assertions to the contrary notwithstanding) in the bondage 
 of the geocentric system, and regarded nature from the lofty 
 standpoint of an idealist philosophy. This was the learned and 
 enlightened Cardinal Cusa, a fisherman's son from the banks of 
 the Moselle, whose distinguished career in the Church and in 
 literature extended over a considerable part of the fifteenth 
 century (1401-64). In. his singular treatise De Docttt Ignorantid, 
 one of the most notable literary monuments of the early Renais- 
 sance, the following passage occurs: "To a spectator on the 
 surface of the sun, the splendour which appears to us would be 
 invisible, since it contains, as it were, an earth for its central 
 mass, with a circumferential envelope of light and heat, and 
 between the two an atmosphere of water and clouds and trans- 
 lucent air." The luminary of Herschel's fancy could scarcely be 
 more clearly portrayed ; some added words, however, betray the 
 origin of the Cardinal's idea. "The earth also," he says, "would 
 appear as a shining star to any one outside the fiery element." 
 It was, in fact, an extension to the sun of the ancient elemental 
 doctrine; but an extension remarkable at that period, as pre- 
 monitory of the tendency, so powerfully developed by subsequent 
 discoveries, to assimilate the orbs of heaven to the model of our 
 insignificant planet, and to extend the brotherhood of our 
 system and our species to the farthest limit of the visible or 
 imaginable universe. 
 
 In later times we find Flamsteed communicating to Newton, 
 March 7, 1681, his opinion "that the substance of the sun is 
 terrestrial matter, his light but the liquid menstruum encom- 
 passing him." 1 Bode in 1776 arrived independently at the con- 
 clusion that "the sun is neither burning nor glowing, but in its 
 1 Brewster's Life of Xewton, vol. ii., p. 103. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 69 
 
 essence a dark planetary body, composed like our earth of land 
 and water, varied by mountains and valleys, and enveloped in a 
 vaporous atmosphere " ; l and the learned in general applauded 
 and acquiesced. The view, however, was in 1787 still so far 
 from popular, that the holding of it was alleged as a proof of 
 insanity in Dr. Elliot when accused of a murderous assault on 
 Miss Boydell. His friend Dr. Simmons stated on his behalf that 
 he had received from him in the preceding January a letter 
 giving evidence of a deranged mind, wherein he asserted '' that 
 the sun is not a body of fire, as hath been hitherto supposed, but 
 that its light proceeds from a dense and universal aurora, which 
 may afford ample light to the inhabitants of the surface beneath, 
 and yet be at such a distance aloft as not to annoy them. No 
 objection, he saith, ariseth to that great luminary's being in- 
 habited ; vegetation may obtain there as well as with us. There 
 may be water and dry land, hills and dales, rain and fair 
 weather ; and as the light, so the season must be eternal, con- 
 sequently it may easily be conceived to be by far the most bliss- 
 ful habitation of the whole system ! " The Eecorder, however, 
 we are told, objected that if an extravagant hypothesis were to 
 be adduced as proof of insanity, the same might hold good 
 with regard to some other speculators, and desired Dr. Simmons 
 to tell the court what he thought of the theories of Burnet and 
 Buffon. 2 
 
 Eight years later, this same " extravagant hypothesis," backed 
 by the powerful recommendation of Sir William Herschel, 
 obtained admittance to the venerable halls of science, there to 
 abide undisturbed for nearly seven decades. It is true there 
 were individual objectors, but their arguments made little im- 
 pression on the general body of opinion. Kuder blows were 
 required to shatter an hypothesis flattering to human pride of 
 invention in its completeness, in the plausible detail of observa- 
 tions by which it seemed to be supported, and in its condescension 
 to the natural pleasure in discovering resemblance under all but 
 total dissimilarity. 
 
 1 Besclidftigungen d. Berl. Ges. Naturforscliender Freunde, Bd. ii., p. 233. 
 2 Gentleman'' s Magazine, 1787, vol. ii., p. 636. 
 
70 HISTORY OF ASTRONOMY. PARTI. 
 
 Sir John Herschel included among the results of his multi- 
 farious labours at the Cape of Good Hope a careful study of the 
 sun-spots conspicuously visible towards the end of the year 1836 
 and in the early part of 1837. They were remarkable, he tells 
 us, for their forms and arrangement, as well as for their number 
 and size; one group, measured on the 29th of March in the 
 latter year, covering (apart from what may be called its outlying 
 dependencies) the vast area of five square minutes or 3780 
 million square miles. 1 We have at present to consider, however, 
 not so much these observations in themselves, as the chain of 
 theoretical suggestions by which they were connected. The 
 distribution of spots, it was pointed out, on two zones parallel to 
 the equator, showed plainly their intimate connection with the 
 solar rotation, and indicated as their cause fluid circulations 
 analogous to those producing the terrestrial trade and anti-trade 
 winds. 
 
 "The spots, in this view of the subject," he went on to say, 2 
 "would come to be assimilated to those regions on the earth's 
 surface where, for the moment, hurricanes and tornadoes prevail ; 
 the upper stratum being temporarily carried downwards, displacing 
 by its impetus the two strata of luminous matter beneath, the 
 upper of course to a greater extent than the lower, and thus 
 wholly or partially denuding the opaque surface of the sun 
 below. Such processes cannot be unaccompanied by vorticose 
 motions, which, left to themselves, die away by degrees and 
 dissipate, with the peculiarity that their lower portions come to 
 rest more speedily than their upper, by reason of the greater 
 resistance below, as well as the remoteness from the point of 
 action, which lies in a higher region, so that their centres (as 
 seen in our waterspouts, which are nothing but small tornadoes) 
 appear to retreat upwards. Now this agrees perfectly with what 
 is observed during the obliteration of the solar spots, which appear 
 as if filled in by the collapse of their sides, the penumbra closing 
 in upon the spot and disappearing after it." 
 
 When, however, it comes to be asked whether a cause can be 
 found by which a diversity of solar temperature might be pro- 
 1 Results, cOc., p. 432. - Jbid., &c., 434. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 71 
 
 duced corresponding with that which sets the currents of the 
 terrestrial atmosphere in motion, we are forced to reply that 
 we know of no such cause. For Sir John Herschel's hypo- 
 thesis of an increased retention of heat at the sun's equator, 
 due to the slightly spheroidal or bulging form of its outer 
 atmospheric envelope, assuredly gives no sufficient account of 
 such circulatory movements as he supposed to exist. Neverthe- 
 less, the view that the sun's rotation is intimately connected 
 with the formation of spots is so obviously correct, that we 
 can only wonder it was not thought of sooner, while we are 
 even now unable to explain with any certainty how it is so 
 connected. 
 
 Mere scrutiny of the solar surface, however, is not the only 
 means of solar observation. We have a satellite, and that 
 satellite from time to time acts most opportunely as a screen, 
 cutting off a part or the whole of those dazzling rays in which 
 the master-orb of our system veils himself from over-curious 
 regards. The importance of eclipses to the study of the solar 
 surroundings is of comparatively recent recognition ; neverthe- 
 less, much of what we know concerning them has been snatched, 
 as it were, by surprise under favour of the moon. In former 
 times, the sole astronomical use of such incidents was the cor- 
 rection of the received theories of the solar and lunar move- 
 ments ; the precise time of their occurrence was the main fact 
 to be noted, and subsidiary phenomena received but casual 
 attention. Now, their significance as a geometrical test of 
 tabular accuracy is altogether overshadowed by the interest 
 attaching to the physical observations for which they afford 
 propitious occasions. This change may be said to date, in its 
 pronounced form, from the great eclipse of 1842. Although a 
 necessary consequence of the general direction taken by scientific 
 progress, it remains associated in a special manner with the 
 name of Francis Baily. 
 
 The "philosopher of Newbury " was by profession a London 
 stockbroker, and a highly successful one. Nevertheless, his 
 services to science were numerous and invaluable, though not 
 of the brilliant kind which attract popular notice. Born at 
 
72 HISTORY OF ASTRONOMY. PARTI. 
 
 Newbery in Berkshire. April 28, 1774, and placed in the City 
 at the age of fourteen, he derived from the acquaintance of 
 Dr. Priestley a love of science which never afterwards left him. 
 It was, however, no passion such as flames up in the brain of 
 the destined discoverer, but a regulated inclination, kept well 
 within the bounds of an actively pursued commercial career. 
 After travelling for a year or two in what were then the wilds of 
 North America, he went on the Stock Exchange in 1799, and 
 earned during twenty-four years of assiduous application to 
 affairs a high reputation for integrity and ability, to which 
 corresponded an ample fortune. In the meantime the Astro- 
 nomical Society (largely through his co-operation) had been 
 founded ; he had for three years acted as its secretary, and he 
 now felt entitled to devote himself exclusively to a subject 
 which had long occupied his leisure hours. He accordingly in 
 1825 retired from business, purchased a house in Tavistock 
 Place, and fitted up there a small observatory. He was. how- 
 ever, by preference a computator rather than an observer. 
 What Sir John Herschel calls the "archaeology of practical 
 astronomy" found in him an especially zealous student. He 
 re-edited the star-catalogues of Ptolemy, Ulugh Beigh, Tycho 
 Brahe, Hevelius, Halley. Flamsteed, Lacaille, and Mayer; cal- 
 culated the eclipse of Thales and the eclipse of Agathocles, and 
 vindicated the memory of the first Astronomer Koyal. But 
 he was no less active in meeting present needs than in revising 
 past performances. The subject of the reduction of observa- 
 tions, then, as we have already explained, 1 in a state of deplor- 
 able confusion, attracted his most earnest attention, and he was 
 close on the track of Bessel when made acquainted with the 
 method of simplification devised at Konigsberg. Anticipated 
 as an inventor, he could still be of eminent use as a promoter 
 of these valuable improvements ; and. carrying them out on a 
 large scale in the star-catalogue of the Astronomical Society 
 (published in 1827). '-he put" (in the words of Herschel) "the 
 astronomical world in possession of a power which may be said, 
 
 1 See ante, p. 38. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 73 
 
 without exaggeration, to have changed the face of sidereal 
 astronomy." 1 
 
 His reputation was still further enhanced by his renewal, with 
 vastly improved apparatus, of the method, first used by Henry 
 Cavendish in 1797-98, for determining the density of the earth. 
 From a series of no less than 2153 delicate and difficult experi- 
 ments, conducted at Tavistock Place during the years 1838-42, 
 he concluded our planet to weigh 5.66 as much as a globe of 
 water of the same bulk ; and this result (slightly corrected) is 
 still accepted as a very close approximation to the truth. 
 
 What we have thus glanced at is but a fragment of the truly 
 surprising mass of work accomplished by Baily in the course of 
 a variously occupied life. A. rare combination of qualities fitted 
 him for his task. Unvarying health, undisturbed equanimity, 
 methodical habits, the power of directed and sustained thought, 
 combined to form in him an intellectual toiler of the surest, 
 though not perhaps of the very highest quality. He was in 
 harness almost to the end. He was destined scarcely to know 
 the miseries of enforced idleness or of consciously failing powers. 
 In 1 842 he completed the laborious reduction of Lalande's great 
 catalogue, undertaken at the request of the British Association, 
 and was still engaged in seeing it through the press when he 
 was attacked with what proved his last, as it was probably his 
 first serious illness. He, however, recovered sufficiently to attend 
 the Oxford Commemoration of July 2, 1844, where an honorary 
 degree of D.C.L. was conferred upon him in company with Airy 
 and Struve ; but sank rapidly after the effort, and died on the 
 3Oth of August following, at the age of seventy, lamented and 
 esteemed by all who knew him. 
 
 It is now time to consider his share in the promotion of solar 
 research. Eclipses of the sun, both ancient and modern, were a 
 speciality with him, and he was fortunate in those which came 
 under his observation. Such phenomena are of three kinds 
 partial, annular, and total. In a partial eclipse, the moon, 
 instead of passing directly between us and the sun, slips by, as it 
 
 1 Memoir of Francis Baily, Mtm. R. A. &, vol. xv., p. 524. 
 
74 HISTORY OF ASTRONOMY. PARTI. 
 
 were, a little on one side, thus cutting off from our sight only a 
 portion of his surface. An annular eclipse, on the other hand, 
 takes place when the moon is indeed centrally interposed, but 
 falls short of the apparent size required for the entire conceal- 
 ment of the solar disc, which consequently remains visible as a 
 bright ring or annulus, even when the obscuration is at its 
 height. In a total eclipse, on the contrary, the sun completely 
 disappears behind the dark body of the moon. The difference 
 of the two latter varieties is due to the fact that the apparent 
 diameters of the sun and moon are so nearly equal as to gain 
 alternate preponderance one over the other through the slight 
 periodical changes in their respective distances from the earth. 
 
 Now on the I5th of May 1836, an annular eclipse was visible 
 in the northern parts of Great Britain, and was observed by 
 Baily at Inch Bonney, near Jedburgh. It was here that he saw 
 the phenomenon which obtained the name of " Baily's Beads," 
 from the notoriety conferred upon it by his vivid description. 
 
 " When the cusps of the sun," he writes, " were about 40 
 asunder, a row of lucid points, like a string of bright beads, 
 irregular in size and distance from each other, suddenly formed 
 round that part of the circumference of the moon that was about 
 to enter, or which might be considered as having just entered 
 on the sun's disc. Its formation, indeed, was so rapid that it 
 presented the appearance of having been caused by the ignition 
 of a fine train of gunpowder. Finally, as the moon pursued her 
 course, the dark intervening spaces (which, at their origin, had 
 the appearance of lunar mountains in high relief, and which 
 still continued attached to the sun's border), were stretched out 
 into long, black, thick, parallel lines, joining the limbs of the 
 sun and moon ; when all at once they suddenly gave way, and 
 left the circumference of the sun and moon in those points, as 
 in the rest, comparatively smooth and circular, and the moon 
 perceptibly advanced on the face of the sun." 1 
 
 These curious appearances were not an absolute novelty. 
 Weber in 1791, and Von Zach in 1820, had seen the "beads"; 
 Van Swinden had described the " belts" or " threads." 2 These 
 
 1 Mem. B. A. S. t vol. x., pp. 5-6. - Ibid., pp. 14-17. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 75 
 
 last were, moreover (as Baily clearly perceived), completely 
 analogous to the " black ligament " which, formed so troublesome 
 a feature in the transits of Venus in 1764 and 1769, and which, 
 to the regret and confusion, though no longer to the surprise of 
 observers, was renewed in that of 1874. The phenomenon is 
 largely an effect of what is called irradiation, by which a bright 
 object seems to encroach upon a dark one; but under good 
 atmospheric and instrumental conditions it becomes incon- 
 spicuous. The " Beads " must always appear when the projected 
 lunar edge is serrated with mountains. In Baily's observation, 
 they were exaggerated and distorted by an irradiative clinging 
 together of the limbs of sun and moon. 
 
 The immediate result, however, was powerfully to stimulate 
 attention to solar eclipses in their physical aspect. Never before 
 had an occurrence of the kind been expected so eagerly or 
 prepared for so actively as that which was total over Central and 
 Southern Europe on the 8th of July 1842. Astronomers 
 hastened from all quarters to the favoured region. The Astro- 
 nomer Royal (Airy) repaired to Turin; Baily to Pavia; Otto 
 Struve threw aside his work amidst the stars at Pulkowa, and 
 went south as far as Lipeszk ; Schumacher travelled from Altona 
 to Vienna; Arago from Paris to Perpignan. Nor did their 
 trouble go unrewarded. The expectations of the most sanguine 
 were outdone by the wonders disclosed. 
 
 Baily (to whose narrative we recur) had set up his Dollond's 
 achromatic (3^ feet focal length) in an upper room of the 
 University of Pavia, and was eagerly engaged in noting a 
 partial repetition of the singular appearances seen by him in 
 1836, when he was "astounded by a tremendous burst of 
 applause from the streets below, and at the same moment was 
 electrified at the sight of one of the most brilliant and splendid 
 phenomena that can well be imagined. For at that instant the 
 dark body of the moon was suddenly surrounded with a corona, 
 or kind of bright glory similar in shape and relative magnitude 
 to that which painters draw round the heads of saints, and which 
 by the French is designated an aure'ole. Pavia contains many 
 thousand inhabitants, the major part of whom were, at this early 
 
76 HISTORY OF ASTRONOMY. PARTI. 
 
 liour, walking about the streets and squares or looking out of 
 windows, in order to witness this long-talked-of phenomenon ; 
 and when the total obscuration took place, which was instant- 
 aneous, there was an universal shout from every observer, which 
 ' made the welkin ring,' and;" for the moment, withdrew my 
 attention from the object with which I was immediately occupied. 
 I had indeed anticipated the appearance of a luminous circle 
 round the moon during the time of total obscurity; but I did not 
 expect, from any of the accounts of preceding eclipses that I 
 had read, to witness so magnificent an exhibition as that which 
 took place. . . . The breadth of the corona, measured from the 
 circumference of the moon, appeared to me to be nearly equal to 
 half the moon's diameter. It .had the appearance of brilliant 
 rays. The light was most dense (indeed I may say quite dense) 
 close to the border of the moon, and became gradually and 
 uniformly more attenuate as its distance therefrom increased, 
 assuming the form of diverging rays in a rectilinear line, which 
 at the extremity were more divided, and of an unequal length ; 
 so that in no part of the corona could I discover the regular and 
 well-defined shape of a ring at its outer margin. It appeared to 
 me to have the sun for its centre, but I had no means of taking 
 any accurate measures for determining this point. Its colour was 
 quite white, not pearl-colour, nor yellow, nor red, and the rays 
 had a vivid and flickering appearance, somewhat like that which 
 a gaslight illumination might be supposed to assume if formed 
 into a similar shape. . . . Splendid and astonishing, however, 
 as this remarkable phenomenon really was, and although it 
 could not fail to call forth the admiration and applause of every 
 beholder, yet I must confess that there was at the same time 
 something in its singular and wonderful appearance that was 
 appalling; and I can readily imagine that uncivilised nations 
 may occasionally have become alarmed and terrified at such an 
 object, more especially at times when the true cause of the 
 occurrence may have been but faintly understood, and the 
 phenomenon itself wholly unexpected. 
 
 " But the most remarkable circumstance attending the 
 phenomenon was the appearance of three large protuberances 
 
CHAP. in. KNOWLEDGE OF THE SUN. 77 
 
 apparently emanating from the circumference of the moon, but 
 evidently forming a portion of the corona. They had the 
 appearance of mountains of a prodigious elevation ; their colour 
 was red tinged with lilac or purple ; perhaps the colour of the 
 peach-blossom would more nearly represent it. They somewhat 
 resembled the snowy tops of the Alpine mountains when 
 coloured by the rising or setting sun. They resembled the 
 Alpine mountains also in another respect, inasmuch as their 
 light was perfectly steady, and had none of that nickering or 
 sparkling motion so visible in other parts of the corona. All 
 the three projections were of the same roseate cast of colour, 
 and very different from the brilliant vivid white light that 
 formed the corona ; but they differed from each other in 
 magnitude. . . . The whole of these three protuberances were 
 visible even to the last moment of total obscuration ; at least, 
 I never lost sight of them when looking in that direction ; and 
 when the first ray of light was admitted from the sun, they 
 vanished, with the corona, altogether, and daylight was instant- 
 aneously restored." 1 
 
 Notwithstanding unfavourable weather, the "red flames" 
 were perceived with little less clearness and no less amazement 
 from the Superga than at Pavia, and were even discerned by 
 Mr. Airy with the naked eye. " Their form " (the Astronomer 
 Royal wrote) "was nearly that of saw-teeth in the position 
 proper for a circular saw turned round in the same direction in 
 which the hands of a watch turn ; . . . their colour was a full 
 lake-red, and their brilliancy greater than that of any other part 
 of the ring." 2 
 
 The height of these extraordinary objects was estimated by 
 Arago at two minutes of arc, representing, at the sun's distance, 
 an actual elevation of 54,000 miles. When carefully watched, 
 the rose-flush of their illumination was perceived to fade 
 through violet to white as the light returned ; the same changes 
 in a reversed order having accompanied their first appearance. 
 Their forms, however, during about three minutes of visibility, 
 showed no change, although of so apparently unstable a 
 
 1 Mem. R. A. #., vol. xv., pp. 4-6. ' 2 Ibid., vol. xv., p. 16. 
 
78 HISTORY OF ASTRONOMY. PART i. 
 
 character as to suggest to Arago "mountains on the point of 
 crumbling into ruins " through topheaviness. 1 
 
 The corona, both as to figure and extent, presented very 
 different appearances at different stations. This was no doubt 
 due to varieties in atmospheric conditions. At the Superga, 
 for instance, all details of structure seem to have been effaced 
 by the murky air, only a comparatively feeble ring of light being 
 seen to encircle the moon. Elsewhere, a brilliant radiated 
 formation was conspicuous, spreading at four opposite points 
 into four vast luminous expansions, compared to feather-plumes 
 or aigrettes. 2 Arago at Perpignan noticed considerable irregu- 
 larities in the divergent rays : some appeared curved and 
 twisted ; a few lay across the others, in a direction almost 
 tangential to the moon's limb ; the general effect being 
 described as that of a "hank of thread in disorder." 3 At 
 Lipeszk, where the sun stood much higher above the horizon 
 than in Italy or France, the corona showed with surprising 
 splendour. Its apparent extent was judged by Struve to be no 
 less than twenty-five minutes (more than six times Airy's 
 estimate), while the great plumes spread their radiance to three 
 or four degrees from the dark lunar edge. So dazzling was the 
 light, that many well-instructed persons denied the totality of 
 the eclipse. Nor was the error without precedent, although the 
 appearances attending respectively a total and an annular eclipse 
 are in reality wholly dissimilar. In the latter case, the surviving 
 ring of sunlight becomes so much enlarged by irradiation, that 
 the interposed dark lunar body is reduced to comparative in- 
 significance, or even invisibility. Maclaurin tells us, 4 that during 
 an eclipse of this character which he observed at Edinburgh in 
 1737, " gentlemen by no means shortsighted declared themselves 
 unable to discern the moon upon the sun without the aid of a 
 smoked glass ; " and Baily (who, however, was shortsighted) 
 could distinguish, in 1836, with the naked eye, no trace of " the 
 globe of purple velvet " which the telescope revealed as projected 
 upon the face of the sun. 5 Moreover, the diminution of light was 
 
 1 Annuaire, 1846, p. 409. 2 Ibid., p. 317. 3 Ibid., p. 322. 4 Phil. 
 Trans., vol. xl., p. 192. 5 Mem. R. A. 8., vol. x., p. 17. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 79 
 
 described by him as " little more than might be caused by a 
 temporary cloud passing over the sun " ; the birds continued in 
 full song ; and " one cock in particular was crowing with all his 
 might while the annulus was forming." 
 
 Very different were the effects of the eclipse of 1 842, as to 
 which some interesting particulars were collected by Arago. 1 
 Beasts of burthen, he tells us, paused in their labour, and could 
 by no amount of punishment be induced to move until the sun 
 reappeared. Birds and beasts abandoned their food ; linnets 
 were found dead in their cages ; even ants suspended their toil. 
 Diligence-horses, on the other hand, seemed as insensible to the 
 phenomenon as locomotives. The convolvulus and some other 
 plants closed their leaves ; but those of the mimosa remained 
 open. The little light that remained was of a livid hue. One 
 observer described the general coloration as resembling the 
 lees of wine, but human faces showed pale olive or greenish. 
 We may, then, rest assured that none of the remarkable 
 obscurations recorded in history were due to eclipses of the 
 annular kind. 
 
 The existence of the corona is no modern discovery. Indeed, 
 it is too conspicuous an apparition to escape notice from the 
 least attentive or least practised observer of a total eclipse. 
 Nevertheless, explicit references to it are rare in early times. 
 Plutarch, however, speaks of a " certain splendour " compassing 
 round the hidden edge of the sun as a regular feature of total 
 eclipses ; 2 and the corona is expressly mentioned in a description 
 of an eclipse visible at Corfu in 968 A.D. 3 The first to take the 
 phenomenon into scientific consideration was Kepler. He 
 showed, from the positions in their orbits at the time, of the sun 
 and moon, that an eclipse observed by Clavius at Rome in 1567, 
 could not have been annular, 4 as the dazzling coronal radiance 
 visible during the obscuration had caused it to be believed. 
 
 1 Ann. du Bureau des Long. 1846, p. 309. " De Facie in Orbe Lunce, xix. 
 10. Cf. Grant, Astr. Nach., No. 1838. As to the phenomenon mentioned by 
 Philostratus in his Life of Apollonius (viii. 23) see W. T. Lynn, Observatory, 
 vol. ix., p. 128. 3 Schmidt, Astr. Nach., No. 1832. 4 Astronomic Pars 
 Optica, Op. omnia, t. ii., p. 317. 
 
8o HISTORY OF ASTRONOMY. PARTI. 
 
 Although he himself never witnessed a total eclipse of the sun, 
 he carefully collected and compared the remarks of those more 
 fortunate, and concluded that the ring of " flame-like splendour " 
 seen on such occasions was caused by the reflection of the solar 
 rays from matter condensed in/the neighbourhood either of the 
 sun or moon. 1 To the solar explanation he gave his own decided 
 preference, but, with one of those curious flashes of half-prophetic 
 insight characteristic of his genius, declared that " it should be 
 laid by ready for use, not brought into immediate requisition/' 2 
 So literally was his advice acted upon, that the theory, which we 
 now know to be (broadly speaking) the correct one, only emerged 
 from the repository of anticipated truths after 236 years of 
 almost complete retirement, and even then timorously and with 
 hesitation. 
 
 The first eclipse of which the attendant phenomena were 
 observed with tolerable exactness was that which was central in 
 the South of France, May 12, 1706. Cassini then put forward 
 the view that the "crown of pale light" seen round the lunar 
 disc was caused by the illumination of the zodiacal light ; 3 but 
 it failed to receive the attention which, as a step in the right 
 direction, it undoubtedly merited. Nine years later we meet 
 with Halley's comments on a similar event, the first which had 
 occurred in London since March 20, 1 140. By nine in the 
 morning of April 22 (o.s.), 1715, the obscuration, he tells us, 
 "was about ten digits, 4 when the face and colour of the t sky 
 began to change from perfect serene azure blue to a more dusky 
 livid colour, having an eye of purple interniixt. ... A few 
 seconds before the sun was all hid there discovered itself round 
 the moon a luminous ring, about a digit or perhaps a tenth part 
 of the moon's diameter in breadth. It was of a pale whiteness 
 or rather pearl colour, seeming to me a little tinged with the 
 colours of the iris, and to be concentric with the moon, whence 
 I concluded it the moon's atmosphere. But the great height 
 thereof, far exceeding our earth's atmosphere, and the observa- 
 
 1 De Stella Xovd, Op., t. ii., pp. 696-697. * Astr. Pars Optica, p. 320. 
 :; Jlem. de T Ac. des Sciences, 1715, p. 119. 4 A digit = r^th of the solar 
 diameter. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 81 
 
 tion of some, who found the breadth of the ring to increase on 
 the west side of the moon as emersion approached, together with 
 the contrary sentiments of those whose judgment I shall always 
 revere " (Newton is most probably referred to), " makes me less 
 confident, especially in a matter whereto I confess I gave not all 
 the attention requisite." He concludes by declining to decide 
 whether the <; enlightened atmosphere," which the appearance 
 ; in all respects resembled," " belonged to sun or moon." l 
 
 A French Academician, who happened to be in London at the 
 time, was less guarded in expressing an opinion. The Chevalier 
 de Louville declared emphatically for the lunar atmospheric 
 theory of the corona, 2 and his authority carried great weight. 
 It was, however, much discredited by an observation made by 
 Maraldi in 1724, to the effect that the luminous ring, instead of 
 travelling with the moon, was traversed by it. 3 This was in 
 reality decisive, though, as usual, belief lagged far behind 
 demonstration. Moreover, the advantage accruing from this 
 fresh testimony was adjudged to the wrong claimant. In 1715 
 n novel explanation had been offered by Delisle and Lahire, 4 
 supported by experiments regarded at the time as perfectly 
 satisfactory. The aureola round the eclipsed sun, they argued, 
 is simply a result of the diffraction or apparent bending of the 
 sunbeams that graze the surface of the lunar globe an effect of 
 the same kind as the coloured fringes of shadows. And this 
 view prevailed amongst men of science until (and even after) 
 Brewster showed, with clear and simple decisiveness, that such 
 an effect could by no possibility be appreciable at our distance 
 from the moon. 5 Don Jose Joaquim de Ferrer, who observed a 
 total eclipse of the sun at Kinderhook, in the State of New York, 
 on June 16, 1806, seems to have been ignorant that such a re- 
 fined optical rationale of the phenomenon was current in the 
 learned world. Two alternative explanations alone presented 
 themselves to his mind as possible. The bright ring round the 
 moon must be due to the illumination either of a lunar or of a 
 
 1 Phil. Trans., vol. xxix., pp. 247-249. - Mem. de VAc. des Sciences, 1715 ; 
 Histoire, p. 49 ; Memoires, pp. 93-98. 3 Hid., 1724, p. 178. 4 Mtm. de VAc. 
 des Sciences, 1715, pp. 161, 166-169. 5 -Ed- Ency., art. Astronomy, p. 635. 
 
 6 
 
82 HISTORY OF ASTRONOMY. PART i. 
 
 solar atmosphere. If the former, he calculated that it should 
 have a height fifty times that of the earth's gaseous envelope. 
 " Such an atmosphere," he rightly concluded, "cannot belong to 
 the moon, but must without any doubt belong to the sun." 1 He. 
 however, stood alone in this unhesitating assertion. 
 
 The importance of the problem was first brought fully home to 
 astronomers by the eclipse of 1842. The brilliant and complex 
 appearance which, on that occasion, challenged the attention of 
 so many observers, demanded and received, no longer the casual 
 attention hitherto bestowed upon it, but the most earnest study 
 of those interested in the progress of science. Nevertheless, it was 
 only by degrees and through a process of ''exclusions" (to use a 
 Baconian phrase) that the corona was put in its right place as a 
 solar appendage. As every other available explanation proved 
 inadmissible and dropped out of sight, the broad presentation of 
 fact remained, which, though of sufficiently obvious interpreta- 
 tion, was long and persistently misconstrued. Nor was it until 
 1 869 that absolutely decisive evidence on the subject was forth- 
 coming, as we shall see further on. 
 
 Sir John Herschel, writing to his venerable aunt, relates that 
 when the brilliant red flames burst into view behind the dark 
 moon on the morning of the 8th July 1842, the populace of 
 Milan, with the usual inconsequence of a crowd, raised the 
 shout, "Us leben die Astronomen ! " 2 In reality, none were 
 less prepared for their apparition than the class to whom the 
 applause due to the magnificent spectacle was thus adjudged. 
 And in some measure through their own fault ; for many partial 
 hints and some distinct statements from earlier observers had 
 given unheeded notice that some such phenomenon might be 
 expected to attend a solar eclipse. 
 
 What we now call the "chromosphere" is an envelope of 
 glowing gases, principally hydrogen, by which the sun is 
 completely covered, and from which the "prominences" are 
 emanations, eruptive or otherwise. Now, continual indications 
 of the presence of this fire-ocean had been detected during' 
 
 1 Trans. Am. Phil. /Sec., vol. vi., p. 274. - Memoir of Caroline Herscliel, 
 P- 327. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 83 
 
 eclipses in the eighteenth and nineteenth centuries. Captain 
 Stannyan, describing in a letter to Flamsteed an occurrence of 
 the kind witnessed by him at Berne on May I (o.s.), 1706, 
 says that the sun's " getting out of the eclipse was preceded by 
 a blood-red streak of light from its left limb." 1 A precisely 
 similar appearance was noted by both Halley and De Louville in 
 1715 ; during annular eclipses by Lord Aberdour in I737, 2 and 
 by Short in I748, 3 the tint of the ruby border being, however, 
 subdued to " brown or "dusky red" by the surviving sunlight ; 
 while observations identical in character were made at Amsterdam 
 in 1820,* at Edinburgh (by Henderson) in 1836, and at New 
 York in i838. 5 
 
 " Flames " or " prominences," if more conspicuous, are less 
 constant in their presence than the glowing stratum from which 
 they spring. The first to describe them was a Swedish professor 
 named Vassenius, who observed a total eclipse at Gothenburg. 
 May 2 (o.s.), I733. 6 His astonishment equalled his admira- 
 tion when he perceived, just outside the edge of the lunar disc r 
 and suspended, as it seemed, in the coronal atmosphere, three 
 or four reddish spots or clouds, one of which was so large as to 
 be detected with the naked eye. As to their nature, he did not- 
 even offer a speculation, further than by tacitly referring them 
 to the moon, in which position they appear to have remained so 
 long as the observation was held in mind. It was repeated in 
 1778 by a Spanish admiral, but with no better success in 
 directing efficacious attention to the phenomenon. Don Antonio 
 Ulloa was on board his ship the Espagne in passage from the 
 Azores to Cape St. Vincent on the 24th of June in that year 
 when a total eclipse of the sun occurred, of which he has left a 
 valuable description. His notices of the corona are full of 
 
 1 Phil. Trans., vol. xxv., p. 2240. - Ibid., vol. xl., p. 182. 3 Ibid., vol. 
 xlv., p. 586. 4 Mem. R. A. 8., vol. i., pp. 145, 148. 5 American Journal of 
 Science, vol. xlii., p. 396. " Phil. Trans., vol. xxxviii., p. 134. Father Secchi 
 has, however, pointed out a tolerably distinct mention of a prominence so far 
 back as 1239 A.D. In a description of a total eclipse of that date it is added, 
 " Et quoddam foramen erat ignitum in circulo solis ex parte inferiore " (Mura- 
 tori, Her. It. Scriptores, t. xiv., col. 1097). The " circulus solis" of course 
 signifies the corona." 
 
84 HISTORY OF ASTRONOMY. PARTI. 
 
 interest ; but what just now concerns us is the appearance of " a 
 red luminous point" "near the edge of the moon," which 
 gradually increased in size as the moon moved away from it, 
 and was visible during about a minute and a quarter. 1 He was 
 satisfied that it belonged to the sun because of its fiery colour 
 and growth in magnitude, and supposed that it was occasioned 
 by some crevice or inequality in the moon's limb, through which 
 the solar light penetrated. 
 
 Allusions less precise, both prior and subsequent, which it is 
 now easy to refer to similar objects (such as the "slender 
 columns of smoke " seen by Ferrer), 2 might be detailed ; but the 
 evidence already adduced suffices to show that the prominences 
 viewed with such amazement in 1842 were no unprecedented or 
 even unusual phenomenon. 
 
 It was more important, however, to decide what was their 
 nature than whether their appearance might have been anti- 
 cipated. They were generally, and not very incorrectly, set 
 down as solar clouds. Arago believed them to shine by reflected 
 light, 3 but the Abbe Peytal rightly considered them to be self- 
 luminous. Writing in a Montpellier paper of July 16, 1842, 
 he declared that we had now become assured of the existence of 
 a third or solar envelope, composed of a glowing substance of a 
 bright rose tint, forming mountains of prodigious elevation, 
 analogous in character to the clouds piled above our horizons. 4 
 This first distinct recognition of a very important feature of our 
 great luminary was probably founded on an observation made 
 by Berard at Toulon during the then recent eclipse, "of a very 
 fine red band, irregularly dentelated, or, as it were, crevassed 
 here and there," 5 encircling a large arc of the moon's circum- 
 ference. It can hardly, however, be said to have attracted 
 general notice until the 28th of July 1851. On that day 
 a total eclipse took place, which was observed with considerable 
 success in various parts of Sweden and Norway by a number 
 of English astronomers. Mr. Hind saw, on the south limb of 
 
 1 Phil. Trans., vol. Ixix., p. 114. - Trans. Am. Phil. Soc., vol. vi., 1809, 
 p. 267. 3 Annuaire, 1846, p. 460. 4 Ibid., p. 439, note. 5 Ibid., 1846 
 p. 416. 
 
CHAP. in. KNOWLEDGE OF THE SUN. 85 
 
 the moon, " a long range of rose-coloured flames," 1 described by 
 Dawes as " a low ridge of red prominences, resembling in outline 
 the tops of a very irregular range of hills." 2 Airy termed 
 the portion of this " rugged line of projections " visible to him 
 the sierra, and was struck with its brilliant light and "nearly 
 scarlet" colour. 3 Its true character of a continuous solar 
 envelope was inferred from these data by Grant, Swan, and 
 Littrow ; and was by Father Secchi formally accepted as estab- 
 lished after the great eclipse of 1 86o. 4 
 
 Several prominences of remarkable forms, especially one 
 variously compared to a Turkish scimitar, a sickle, and a boome- 
 rang, were seen in 1851. In connection with them two highly 
 significant circumstances were pointed out. First, that of the 
 approximate coincidence between their positions and those of 
 sun-spots previously observed. 5 Next, that "the moon passed 
 over them, leaving them behind, and revealing successive 
 portions as she advanced." 6 This latter fact (as to which there 
 could be no doubt, since it was separately noted by at least four 
 first-rate observers), was justly considered by the Astronomer 
 Koyal and others as affording absolute certainty of the solar 
 dependence of these singular objects. Nevertheless sceptics 
 were still found. M. Faye, of the French Academy, inclined to 
 a lunar origin for them ; 7 Professor von Feilitsch of Greifswald 
 published in 1852 a treatise for the express purpose of proving 
 all the luminous phenomena attendant on solar eclipses corona, 
 prominences and "sierra" to be purely optical appearances. 8 
 Happily, however, the unanswerable arguments of the photo- 
 graphic camera were soon to be made available against such 
 hardy incredulity. 
 
 Thus, the virtual discovery of the solar appendages, both 
 coronal and chromospheric, may be said to have been begun in 
 
 1 Mem. R. A. 8., vol. xxi., p. 82. 2 Ibid., p. 90. 3 Ibid., pp. 7-8. 4 Le 
 Soleil, t. i., p. 386. 5 By Williams and Stanistreet, Mem. R A. S., vol. xxi., 
 pp. 54, 56. Santini had made a similar observation at Padua in 1842. Grant, 
 Hist. Astr., p. 401. 6 Lassell in Month. Not., vol. xii., p. 53. 7 Comptes 
 jRendus, t. xxxiv., p. 155. 8 Optische (Inter sucliun gen, and Zeitschrift fur 
 popular 'e Mittheilungen, Bd. i., 1860, p. 201. 
 
86 HISTORY OF ASTRONOMY. PARTI. 
 
 1842, and completed in 1851. The current Herschelian theory 
 of the solar constitution remained, however, for the time, intact. 
 Difficulties, indeed, were thickening around it ; but their 
 discussion was perhaps felt to be premature, and they were 
 permitted to accumulate without debate, until fortified by 
 fresh testimony into unexpected and overwhelming prepon- 
 derance. 
 
CHAPTER IV. 
 PLANETARY DISCOVERIES. 
 
 Ix the course of his early gropings towards a law of the planetary 
 distances, Kepler tried the experiment of setting a planet, 
 invisible by reason of its smallness, to revolve in the vast region 
 of seemingly desert space separating Mars from Jupiter. 1 The 
 disproportionate magnitude of the same interval was explained 
 by Kant as due to the overweening size of Jupiter. The zone in 
 which each planet moved was, according to the philosopher of 
 Konigsberg, to be regarded as the empty storehouse from which 
 its materials had been derived. A definite relation should thus 
 exist between the planetary masses and the planetary intervals. 2 
 Lambert, on the other hand, sportively suggested that the body 
 or bodies (for it is noticeable that he speaks of them in the 
 plural) which once bridged this portentous gap in the solar 
 system, might, in some remote age, have been swept away by a 
 great comet, and forced to attend its wanderings through space. 3 
 These speculations were destined before long to assume a more 
 definite form. Johann Daniel Titius, a professor at Wittenberg 
 (where he died in 1796), pointed out in 1772, in a note to a 
 translation of Bonnet's Contemplation de la Nature? the existence 
 of a remarkable symmetry in the disposition of the bodies 
 constituting the solar system. By a certain series of numbers, 
 increasing in regular progression, 5 he showed that the distances 
 
 1 Op., t. i., p. 107. He interposed, but tentatively only, another similar 
 body between Mercury and Venus. - Allyemeine Naturgeschichte (ed. 1798), 
 pp. 118-119. :! Cosmolofjisclic, Briefe, No. i (quoted by Von Zach, Monat. 
 f'orr., vol. iii., p. 592). 4 Second ed., p. 7. See Bode, Von dem neMen 
 Hauptplaneten, p. 43, note. r> The representative numbers are obtained by 
 adding 4 to the following series (irregular, it will be observed, in its first 
 
88 HISTORY OF ASTRONOMY. PART i. 
 
 of the six known planets from the sun might be represented with 
 a close approach to accuracy. But with one striking interrup- 
 tion. The term of the series succeeding that which corresponded 
 to the orbit of Mars was without a celestial representative. The 
 orderly flow of the sequence was thus singularly broken. The 
 space where a planet should in fulfilment of the "Law" have 
 revolved, was, it appeared, untenanted. Johann Elert Bode, 
 then just about to begin his long career as leader of astronomical 
 thought and work at Berlin, marked at once the anomaly, and 
 filled the vacant interval with an hypothetical planet. The dis- 
 covery of Uranus at a distance falling but slightly short of 
 perfect conformity with the law of Titius, lent weight to a 
 seemingly hazardous prediction, and Von Zach was actually at 
 the pains, in 1785, to calculate what he termed "analogical" 
 elements 1 for this unseen and (by any effect or influence) unfelt 
 body. The search for it, though confessedly scarcely less 
 chimerical than that of alchemists for the philosopher's stone, he 
 kept steadily in view for fifteen years, and at length (September 
 21, 1800) succeeded in organising, in combination with five other 
 German astronomers assembled at Lilienthal, a force of what he 
 jocularly termed celestial police, for the express purpose of 
 tracking and intercepting the fugitive subject of the sun. The 
 zodiac was accordingly divided for purposes of scrutiny into 
 twenty-four zones; their apportionment to separate observers 
 was in part effected, and the association was rapidly getting into 
 working order, when news arrived that the missing planet had 
 been found, through no systematic plan of search, but by the 
 diligent, though otherwise directed labours of a distant watcher 
 of the skies. 
 
 Giuseppe Piazzi was born at Ponte in the Valtelline, July 16, 
 1746. He studied at various places and times under Tiraboschi, 
 Beccaria, Jacquier, and Le Sueur; and having entered the 
 Theatine order of monks at the age of eighteen, he taught 
 philosophy, science, and theology in several of the Italian cities, 
 
 member, which should be \ instead of o) : o, 3, 6, 12, 24, 48, &c. The formula 
 is a purely empirical one, and is, moreover, completely at fault as regards 
 the distance of Neptune. ] Monat. Corr., vol. iii., p. 596. 
 
CHAP. iv. PLANETARY DISCOVERIES. 89 
 
 as well as in Malta, until 1780, when the chair of mathematics in 
 the University of Palermo was offered to and accepted by him. 
 Prince Caramanico, then viceroy of Sicily, had scientific leanings, 
 and was easily won over to the project of building an observatory, 
 a commodious foundation for which was afforded by one of the 
 towers of the viceregal palace. This architecturally incongruous 
 addition to an ancient Saracenic edifice once the abode of 
 Kelbite and Zirite Emirs was completed in February 1791. 
 Piazzi, meanwhile, had devoted nearly three years to the 
 assiduous study of his new profession, acquiring a practical 
 knowledge of Lalande's methods at the ficole Militaire, and 
 of Maskelyne's at the Royal Observatory; and returned to 
 Palermo in 1789, bringing with him, in the great five-foot 
 circle which he had prevailed upon Eamsden to construct, 
 the most perfect measuring instrument hitherto employed by an 
 astronomer. 
 
 He had been above nine years at work on his star-catalogue, 
 and was still profoundly unconscious that a place amongst the 
 Lilienthal band 1 of astronomical detectives was being held in 
 reserve for him, when, on the first evening of the nineteenth 
 century, January I, 1801, he noted the position of an eighth- 
 magnitude star in a part of the constellation Taurus, to which an 
 error of Wollastoii's had directed his special attention. Re- 
 observing, according to his custom, the same set of fifty stars on 
 four consecutive nights, it seemed to him, on the 2nd, that the 
 one in question had slightly shifted its position to the west ; 
 on the 3rd he assured himself of the fact, and believed that he 
 had chanced upon a new kind of comet without tail or coma. 
 The wandering body (whatever its nature) exchanged retrograde 
 for direct motion on January 14 2 and was carefully watched by 
 Piazzi until February 1 1 , when a dangerous illness interrupted 
 his observations. He had, however, not omitted to give notice 
 of his discovery; but so precarious were communications in 
 those unpeaceful times, that his letter to Oriani of January 23 
 
 1 Wolf, Geschichte tier Astronomie, p. 648. - Such reversals of direction in 
 the apparent movements of the planets are a consequence o-f the earth's revo- 
 lution in its orbit. 
 
90 HISTORY OF ASTRONOMY. PART i. 
 
 did not reach Milan until April 5, while a missive of one day 
 later addressed to Bode came to hand at Berlin, March 20. 
 The delay just afforded time for the publication, by a young 
 philosopher of Jena named Hegel, of a vi Dissertation ?;1 showing, 
 by the clearest light of reason,- tJiat the number of the planets 
 could not exceed seven, and exposing the folly of certain 
 devotees of induction who souo'ht a new celestial bodv merely to 
 
 O */ t/ 
 
 fill a gap in a numerical series. 
 
 Unabashed by speculative scorn, Bode had scarcely read 
 Piazzi's letter when he concluded that it referred to the precise 
 body in question. The news spread rapidly, and created a 
 profound sensation not unmixed with alarm lest this latest 
 addition to the solar family should have been found only to be 
 again lost. For by that time Piazzi's moving star was too near 
 the sun to be any longer visible, and in order to rediscover it 
 after conjunction a tolerably accurate knowledge of its path was 
 indispensable. But a planetary orbit had never before been 
 calculated from such scanty data as Piazzi's observations 
 afforded ; ^ and the attempts made by nearly every astronomer 
 of note in Germany to compass the problem were manifestly 
 inadequate, failing even to account for the positions in which the 
 body had been actually seen, and a fortiori serving only to 
 mislead as to the places where, from September 1801, it ought 
 once more to have become discernible. It was in this extremity 
 that the celebrated mathematician Gauss came to the rescue. 
 He was then in his twenty-fifth year, and was earning his bread 
 by tuition at Brunswick, with many possibilities, but 110 settled 
 career before him. The news from Palermo may be said to 
 have converted him from an arithmetician into an astronomer. 
 He was already in possession of a new and more general method 
 of computing elliptical orbits ; and the system of " least squares," 
 which he had devised though not published, enabled him to 
 extract the most probable result from a given set of observa- 
 tions. Armed with these novel powers, he set to work, and the 
 
 1 Dissertatio PhlhsopMca de Orb/tis Phinetarum, 1801. See Wolf, Gescli. 
 d. Astr., p. 685. - Observations on Uranus, as a supposed fixed star, reached 
 back to 1690. 
 
CHAP. iv. PLANETARY DISCOVERIES. 91 
 
 communication in November of his elements and ephemeris for 
 the lost object revived the drooping hopes of the little band of 
 eager searchers. Their patience, however, was to be still further 
 tried. Clouds, mist, and sleet seemed to have conspired to cover 
 the retreat of the fugitive ; but on the last night of the year the 
 sky cleared unexpectedly with the setting in of a hard frost, and 
 there, in the north-western part of Virgo, nearly in the position 
 assigned by Gauss to the runaway planet, a strange star was 
 discerned by Von Zach l at Gotha. and on the subsequent even- 
 ing the anniversary of the original discovery by Olbers at 
 Bremen. The name of Ceres (as the tutelary goddess of Sicily) 
 was, by Piazzi's request, bestowed upon this first known of the 
 numerous and probably all but innumerable family of the minor 
 planets. 
 
 The recognition of the second followed as the immediate 
 consequence of the detection of the first. Olbers had made 
 himself so familiar with the positions of the small stars along 
 the track of the long-missing body, that he was at once struck 
 (March 28, 1802) with the presence of an intruder near the 
 spot where he had recently identified Ceres. He at first 
 believed the newcomer to be a variable star usually incon- 
 spicuous, but just then at its maximum of brightness ; but 
 within two hours he had convinced himself that it was no fixed 
 star, but a rapidly moving object. The aid of Gauss was 
 again invoked, and his prompt calculations showed that this 
 fresh celestial acquaintance (named " Pallas " by Olbers) re- 
 volved round the sun at nearly the same mean distance as 
 Ceres, and was beyond question of a strictly analogous 
 character. 
 
 This result was perplexing in the extreme. The symmetry 
 and simplicity of the planetary scheme appeared fatally com- 
 promised by the admission of many, where room could, 
 according to old-fashioned rules, only be found for one. A 
 daring hypothesis of Olbers's invention provided an exit from 
 the difficulty. He supposed that both Ceres and Pallas were 
 
 1 He had caught a glimpse of it on December 7, but was prevented by bad 
 weather from verifying his suspicion. Monat. Corr., vol. v., p. 171. 
 
92 HISTORY OF ASTRONOMY. PARTI. 
 
 fragments of a primitive trans-Martian planet, blown to pieces 
 in the remote past, either by the action of internal forces or by 
 the impact of a comet ; and predicted that many more such 
 fragments would be found to circulate in the same region. He, 
 moreover, pointed out that these numerous orbits, however 
 much they might differ in other respects, must all have a 
 common line of intersection, 1 and that the bodies moving in 
 them must consequently pass, at each revolution, through two 
 opposite points of the heavens, one situated in the Whale, the 
 other in the constellation of the Virgin, where already Pallas 
 had been found and Ceres recaptured. The intimation that 
 fresh discoveries might be expected in those particular regions 
 was singularly justified by the detection of two bodies now 
 known respectively as Juno and Vesta. The first was found near 
 the predicted spot in Cetus by Harding, Schroter's assistant 
 at Lilienthal, September 2, 1804; the second by Olbers him- 
 self in Virgo v after three years of patient scrutiny, March 29, 
 1807. 
 
 The theory of an exploded planet now seemed to have every- 
 thing in its favour. It required that the mean or average 
 distances of the newly-discovered bodies should be nearly the 
 same, but admitted a wide range of variety in the shapes and 
 positions of their orbits, provided always that they preserved 
 common points of intersection. These conditions were fulfilled 
 with a striking approach to exactness. Three of the four 
 " asteroids " (a designation introduced by Sir W. Herschel 2 ) 
 conformed with very approximate precision to " Bode's law " 
 of distances ; they all traversed, in their circuits round the 
 sun, nearly the same parts of Cetus and Virgo; while the 
 eccentricities and inclinations of their paths departed widely 
 from the planetary type that of Pallas, for example, making 
 with the ecliptic an angle of nearly 35. The minuteness of 
 these bodies appeared further to strengthen the imputation of a 
 
 1 Planetary fragments, hurled hi any direction, and with any velocity short of 
 that which would for ever release them from the solar sway, would continue 
 to describe elliptic orbits round the sun, all passing through the scene of the 
 explosion, and thus possessing a common line of intersection. '* Phil. Trans., 
 vol. xcii., part ii., p. 228. 
 
CHAP. iv. PLANETARY DISCOVERIES. 93 
 
 fragmentary character. Herschel estimated the diameter of 
 Ceres at 162, that of Pallas at 147 miles. 1 Juno is smaller than 
 either ; and even Vesta, which surpasses all the minor planets in 
 size, and may, under favourable circumstances, be seen with the 
 naked eye, has a diameter probably under 350 miles. A sus- 
 pected variability of brightness in some of the asteroids, some- 
 what hazardously explained as due to the irregularities of figure 
 to be expected in cosmical potsherds (so to speak), was added to 
 the confirmatory evidence. 2 The strong point of the theory, 
 however, lay not in what it explained, but in what it had pre- 
 dicted. It had been twice confirmed by actual exploration of the 
 skies, and had produced, in the recognition of Vesta, the first 
 recorded instance of the premeditated discovery of a heavenly 
 body. 
 
 The view not only commended itself to the facile imagination 
 of the unlearned, but received the sanction of the highest 
 scientific authority. The great Lagrange bestowed upon it his 
 analytical imprimatur, showing that the explosive forces required 
 to produce the supposed catastrophe came well within the bounds 
 of possibility a velocity of less than twenty times that of a 
 cannon-ball leaving the gun's mouth sufficing, according to its 
 calculation, to have launched the asteroidal fragments on their 
 respective paths. Indeed, he was disposed to regard the hypo- 
 thesis of disruption as more generally available than its author 
 had designed it to be, and proposed to supplement with it, as 
 explanatory of the eccentric orbits of comets, the nebular theory 
 of Laplace, thereby obtaining, as he said, " a complete view of 
 the origin of the planetary system more conformable to Nature 
 and mechanical laws than any yet proposed." 3 
 
 Nevertheless, the hypothesis of Olbers has not held its ground. 
 It seemed as if all the evidence available for its support 
 had been produced at once and spontaneously, while the un- 
 favourable items were elicited slowly, and, as it were, by cross- 
 
 1 Phil. Trans., vol. xcii., part ii., p. 218. In a letter to Von Zach of June 24, 
 1802, bespeaks of Pallas as "almost incredibly small," and makes it only 
 seventy English miles in diameter. Monat. Corr.,vol. vi.,pp. 89-90. ~ Olbers 
 Jfonat. Corr., vol. vi., p. 88. 3 Conn. d. Terns, for 1814, p. 218. 
 
94 HISTORY OF ASTRONOMY. PARTI. 
 
 examination. A more extended acquaintance with the group of 
 bodies whose peculiarities it was framed to explain has shown 
 them, after all, as recalcitrant to any such explanation. Coinci- 
 dences at the first view significant and striking have been 
 swamped by contrary examples ; : and a hasty general conclusion 
 has, by a not uncommon destiny, at last perished under the 
 accumulation of particulars. Moreover, as has been remarked by 
 Professor Newcomb, 1 mutual perturbations would rapidly efface 
 all traces of a common disruptive origin, and the catastrophe, to 
 be perceptible in its effects, should have been comparatively 
 recent. 
 
 A new generation of astronomers had arisen before any 
 additions were made to the little family of the minor planets. 
 Piazzi died in 1826, Harding in 1834. Olbers in 1840; all those 
 who had prepared or participated in the first discoveries passed 
 away without witnessing their resumption. In 1830, however, a 
 certain Hencke,- ex-postmaster in the Prussian town of Driessen. 
 set himself to watch for new planets, and after fifteen long 
 years his patience was rewarded. The asteroid found by him. 
 December 8, 1845, received the name of Astrsea, and his further 
 prosecution of the search resulted, July I, 1847, i n tne discovery 
 of Hebe. A few weeks later (August 1 3), Mr. Hind, after many 
 months' exploration from Mr. Bishop's observatory in the 
 Regent's Park, picked up Iris, and October 18, Flora. 2 The 
 next on the list was Metis, found by Mr. Graham, April 25, 1848, 
 at Markee in Ireland. 3 At the close of the period to which our 
 attention is at present limited, the number of these small bodies 
 known to astronomy was thirteen ; and the course of discovery 
 has since proceeded far more rapidly and with less interruption. 
 
 Both in itself and in its consequences the recognition of the 
 minor planets was of the highest importance to science. The 
 traditional ideas regarding'' the constitution of the solar system 
 were enlarged by the admission of a new class of bodies, 
 strongly contrasted, yet strictly co-ordinate with the old- 
 established planetary order ; the profusion of resource, so 
 
 1 Popular Astronomy, p. 327. - Month. Not. vol. vii., p. 299; vol. viii., 
 p. i. :5 Ibid., vol. viii., p. 146. 
 
CHAP. iv. PLANETARY DISCOVERIES. 95 
 
 conspicuous in the living kingdoms of Nature, was seen to 
 prevail no less in the celestial spaces ; and some faint prelimi- 
 nary notion was afforded of the indefinite complexity of rela- 
 tions underlying the apparent simplicity of the majestic scheme 
 to which our world belongs. Theoretical and practical astro- 
 nomy both derived profit from the admission of these apparently 
 insignificant strangers to the rights of citizenship of the solar 
 system. The disturbance of their motions by their giant 
 neighbour afforded a more accurate knowledge of the Jovian 
 mass, which Laplace had taken about Jyth too small ; the 
 anomalous character of their orbits presented geometers with 
 highly stimulating problems in the theory of perturbations ; 
 while the exigencies of the first discovery had produced the 
 Theoria Motus, and won Gauss over to the ranks of calculating 
 astronomy. Moreover, the sure prospect of further detections 
 powerfully incited to the exploration of the skies ; observers 
 became more numerous and more zealous in view of the prizes 
 held out to them ; star-maps were diligently constructed, and 
 the sidereal multitude strewn along the great zodiacal belt 
 acquired a fresh interest when it was perceived that its least 
 conspicuous member might be a planetary shred or projectile 
 in the majestic disguise of a distant sun. Harding's " Celestial 
 Atlas," designed for the special purpose of facilitating aste- 
 roidal research, was the first systematic attempt to represent- 
 to the eye the telescopic aspect of the heavens. It was while 
 engaged on its construction that the Lilienthal observer success- 
 fully intercepted Juno 011 her passage through the Whale in 
 1804; whereupon promoted to Gottingen, he there completed, 
 in 1822, the arduous task so opportunely entered upon a score 
 of years previously. Still more important were the great star- 
 maps of the Berlin Academy, undertaken at Bessel's suggestion, 
 with the same object of distinguishing errant from fixed stars, 
 and executed, under Encke's supervision, during the years 
 1830-59. They have played a noteworthy part in the history 
 of planetary discovery, nor of the minor kind alone. 
 
 We have now to recount an event unique in scientific history. 
 The discovery of Neptune has been characterised as the result 
 
96 HISTORY OF ASTRONOMY. PARTI. 
 
 of a " movement of the age," l and with some justice. It had 
 become necessaiy to the integrity of planetary theory. Until 
 it was accomplished, the phantom of an unexplained anomaly 
 in the orderly movements of the solar system must have con- 
 tinued to haunt astronomical consciousness. Moreover, it was 
 prepared by many, suggested as possible by not a few, and 
 actually achieved, simultaneously, independently, and com- 
 pletely, by two investigators. 
 
 The position of the planet Uranus was recorded as that of 
 a fixed star no less than twenty times between 1690 and the 
 epoch of its final detection by Herschel. But these early 
 observations, far from affording the expected facilities for the 
 calculation of its orbit, proved a source of grievous perplexity. 
 The utmost ingenuity of geometers failed to combine them 
 satisfactorily with the later Uraniaii places, and it became 
 evident, either that they were widely erroneous, or that the 
 revolving body was wandering from its ancient track. The 
 simplest course was to reject them altogether, and this was 
 done in the new Tables published in 1821 by Alexis Bouvard, 
 the indefatigable computating partner of Laplace. But the 
 trouble was not thus to be got rid of. After a few years fresh 
 irregularities began to appear, and continued to increase until 
 absolutely "intolerable." It may be stated as illustrative of 
 the perfection to which astronomy had been brought, that 
 divergencies regarded as menacing the very foundation of its 
 theories never entered the range of unaided vision. In other 
 words, if the theoretical and the real Uranus had been placed 
 side by side in the sky, they would have seemed, to the sharpest 
 eye, to form a single body. 2 
 
 The idea that these enigmatical disturbances were due to the 
 attraction of an unknown exterior body was a tolerably obvious 
 one ; and we accordingly find it suggested in many different 
 
 1 Airy, Mem. R. A. S. t vol. xvi., p. 386. - See Newcomb's Pop. Astr., p. 359. 
 The error of Uranus amounted, in 1844, to 2' ; but even the tailor of Breslau, 
 whose extraordinary powers of vision Humboldt commemorates (Kosmos, 
 Bd. ii., p. 112), could only see Jupiter's first satellite at its greatest elonga- 
 tion, 2' 15". He might, however, possibly have distinguished two objects of 
 equal lustre at a lesser interval. 
 
CHAP. iv. PLANETARY DISCOVERIES. 97 
 
 quarters. Bouvard himself was perhaps the first to conceive it. 
 He kept the possibility continually in view, and bequeathed to 
 his nephew's diligence the inquiry into its reality when he felt 
 that his own span was drawing to a close ; but before any pro- 
 gress had been made with it, he had already (June 7, 1843) 
 '' ceased to breathe and to calculate." The Rev. T. J. Hussey 
 actually entertained in 1834 the notion, but found his powers 
 inadequate to the task, of assigning an approximate place to the 
 disturbing body ; and Bessel, in 1 840, laid his plans for an 
 assault in form upon the Uranian difficulty, the triumphant 
 exit from which fatal illness frustrated his hopes of effecting or 
 even witnessing. 
 
 The problem was practically untouched when, in 1841, an 
 undergraduate of St. John's College, Cambridge, formed the 
 resolution of grappling with it. The projected task was an 
 arduous one. There were no guiding precedents for its conduct. 
 Analytical obstacles had to be encountered so formidable as to 
 appear invincible even to such a mathematician as Airy. John 
 Couch Adams, however, had no sooner taken his degree, which 
 he did as senior wrangler in January 1843, than he set resolutely 
 to work, and on October 21, 1845, was a ^ e to communicate to 
 the Astronomer Royal numerical estimates of the elements and 
 mass of the unknown planet, together with an indication of its 
 actual place in the heavens. These results, it has been well 
 said, 1 gave "the final and inexorable proof" of the validity of 
 Newton's Law. The date October 21, 1845, " may therefore be 
 regarded as marking a distinct epoch in the history of gravita- 
 tional astronomy." 
 
 Sir George Biddell Airy had begun in 1835 n ^ s l n g and 
 energetic administration of Greenwich Observatory, 2 and was 
 already in possession of data vitally important to the momen- 
 tous inquiry then on foot. At his suggestion, and under his 
 superintendence, the reduction of all the planetary observations 
 made at Greenwich from 1750 downwards had been under- 
 taken in 1833. The results, published in 1846, constituted a 
 
 1 J. W. L. Glaisher, Observatory, vol. xv., p. 177. - He resigned the post 
 of Astronomer Royal, August 15, 1881, and died, in his ninety-first year, 
 January 2, 1892. 
 
 7 
 
98 HISTORY OF ASTRONOMY. PARTI. 
 
 permanent and universal stock of materials for the correction 
 of planetary theory. But in the meantime, investigators, both 
 native and foreign, were freely supplied with the "places and 
 errors," which, clearly exhibiting the discrepancies between 
 observation and calculation between what was and what was 
 expected formed the very groundwork of future improvements. 
 Mr. Adams had no reason to complain of official discourtesy. 
 His labours received due and indispensable aid ; but their pur- 
 pose was regarded as chimerical. " I have always," Sir George 
 Airy wrote, 1 "considered the correctness of a distant mathemati- 
 cal result to be a subject rather of moral than of mathematical 
 evidence." But that actually before him seemed, from its very 
 novelty, to incur a suspicion of unlikelihood. No problem in 
 planetary disturbance had heretofore been attacked, so to speak, 
 from the rear. The inverse method was untried, and might 
 well be deemed impracticable. For the difficulty of determining 
 the perturbations produced by a given planet is small compared 
 with the difficulty of finding a planet by its resulting perturba- 
 tions. Laplace might have quailed before it ; yet it was now 
 grappled with as a first essay in celestial dynamics. Moreover, 
 Mr. Adams unaccountably neglected to answer until too late a 
 question regarded by Sir George Airy in the light of an experi- 
 mentum crucis as to the soundness of the new theory. Nor did 
 he himself take any steps to obtain a publicity which he was 
 more anxious to merit than to secure. The investigation con- 
 sequently remained buried in obscurity. It is now known that 
 had a search been instituted in the autumn of 1845 ^ or the 
 remote body whose existence had been so marvellously foretold, it 
 would have been found within three and a half lunar diameters 
 (i 49') of the spot assigned to it by Mr. Adams. Official zeal, 
 however, rarely oversteps the beaten track ; and the problemati- 
 cal needle was left to lie in its bundle of hay, from which, indeed, 
 but for an unlucky accident, it would certainly have been extri- 
 cated by an unofficial explorer. Mr. Dawes saw Adams's papers 
 at Greenwich ; 2 was struck with their purport, and wrote to his 
 
 1 Mem. B. A. 8., vol. xvi., p. 399. - Holden, Publications Astr. Society of 
 the Pacific, vol. iv., p. 21. 
 
CHAP. iv. PLANETARY DISCOVERIES. 99 
 
 friend Mr. Lassell, giving him the calculated place of, and 
 begging him to search for, the unknown body. With his 
 powerful reflector he could at once have identified it by its 
 disc, as Herschel did Uranus ; but a sprained ankle intervened. 
 By the time he had recovered, Mr. Dawes's letter was found to 
 have been, by mischance, destroyed ; and the needful particulars 
 contained in it were not easily recoverable. So the English 
 geometer's theory remained debarred from its proper experimental 
 verification. 
 
 A competitor, however, equally daring and more fortunate 
 audax fortund adjutus, as Gauss said of him was even then 
 entering the field. Urbain Jean Joseph Leverrier, the son of a 
 small Government employ 6 in Normandy, was born at Saint-Lo 
 March 1 1 , 1 8 1 1 . He studied with brilliant success at the ficole 
 Polytechnique, accepted the post of astronomical teacher there 
 in 1837, and, "docile to circumstance," immediately concen- 
 trated the whole of his vast, though as yet undeveloped powers 
 upon the formidable problems of celestial mechanics. He lost 
 no time in proving to the mathematical world that the race of 
 giants was not extinct. Two papers on the stability of the solar 
 system, presented to the Academy of Sciences, September 16, 
 and October 14, 1839, showed him to be the worthy successor of 
 Lagrange and Laplace, and encouraged hopes destined to be 
 abundantly realised. His attention was directed by Arago to 
 the Uranian difficulty in 1845, when he cheerfully put aside 
 certain intricate cometary researches upon which he happened 
 to be engaged, in order to obey with dutiful promptitude the 
 summons of the astronomical chief of France. In his first 
 memoir on the subject (communicated to the Academy, Novem- 
 ber 10, 1845), he proved the inadequacy of all known causes of 
 disturbance to account for the vagaries of Uranus ; in a second 
 (June i, 1846), he demonstrated that only an exterior body, 
 occupying at a certain date a determinate position in the zodiac, 
 could produce the observed effects ; in a third (August 3 1 , 
 1846), he assigned the orbit of the disturbing body, and an- 
 nounced its visibility as an object with a sensible disc about as 
 bright as a star of the eighth magnitude. 
 
ioo HISTORY OF ASTRONOMY. PARTI. 
 
 The question was now visibly approaching an issue. On 
 September 10, Sir John Herschel declared to the British Asso- 
 ciation respecting the hypothetical new planet : " We see it as 
 Columbus saw America from the coast of Spain. Its movements 
 have been felt, trembling along the far-reaching line of our 
 analysis with a certainty hardly inferior to that of occular 
 demonstration." Less than a fortnight later, September 23, 
 Professor Galle, of the Berlin Observatory,'' received a letter 
 from Leverrier requesting his aid in the telescopic part of the 
 inquiry already analytically completed. He directed his re- 
 fractor to the heavens that same night, and perceived, within 
 less than a degree of the spot indicated, an object with a 
 measurable disc nearly three seconds in diameter. Its absence 
 from Bremiker's recently-completed map of that region of the 
 sky showed it to be no star, and its movement in the predicted 
 direction confirmed without delay the strong persuasion of its 
 planetary nature. 1 
 
 In this remarkable manner the existence of the remote 
 member of our system known as " Neptune " was ascertained. 
 But the discovery, which faithfully reflected the duplicate char- 
 acter of ' the investigation which led to it, had been already 
 secured at Cambridge before it was announced from Berlin. 
 Sir George Airy's incredulity vanished in the face of the strik- 
 ing coincidence between the position assigned by Leverrier to 
 the unknown planet in June, and that laid down by Mr. Adams 
 in the previous October ; and on the pth of July he wrote to 
 Professor Challis, director of the Cambridge Observatory, re- 
 commending a search with the Northumberland equatoreal. 
 Had a good star-map been at hand, the process would have been 
 a simple one ; but of Bremiker's " Hora XXI." no news had yet 
 reached England, and there was no other chart sufficiently com- 
 prehensive to be available for an inquiry which, in the absence of 
 such aid. promised to be both long and laborious. As the event 
 proved, it might have been neither. " After four days of 
 observing," Professor Challis wrote, October 12, 1846, to Sir 
 
 1 For an account of D'Arrest's share in the detection see Copernicus, vol. ii., 
 PP' 63. 96. 
 
CHAP. iv. PLANETARY DISCOVERIES. 101 
 
 George Airy. " The planet was in my grasp if only I had 
 examined or mapped the observations." l Had he done so, the 
 first honours in the discovery, both theoretical and optical, would 
 have fallen to the University of Cambridge. But Professor 
 Cballis had other astronomical avocations to attend to, and, 
 moreover, his faith in the precision of the indications furnished 
 to him was, by his own confession, a very feeble one. For both 
 reasons he postponed to a later stage of the proceedings the 
 discussion and comparison of the data nightly furnished to him 
 by his telescope, and thus allowed to lie, as it were, latent in 
 his observations the momentous result which his diligence had 
 ensured, but which his delay suffered to be anticipated. 2 
 
 Nevertheless it should not be forgotten that the Berlin as- 
 tronomer had two circumstances in his favour apart from which 
 his swift success could hardly have been achieved. The first was 
 the possession of a good star-map ; the second was the clear and 
 confident nature of Leverrier's instructions. " Look where I tell 
 you," he seemed authoritatively to say, "and you will see an 
 object such as I describe." 3 And in fact, not only Galle on the 
 23rd of September, but also Challis on the 29th, immediately 
 after reading the French geometer's lucid and impressive treatise, 
 picked out from among the stellar points strewing the zodiac, a 
 small planetary disc, which eventually proved to be that of the 
 precise body he had been in search of during two months. 
 
 The controversy that ensued had its unworthy side ; but it 
 was entered into by neither of the parties principally concerned. 
 Mr. Adams bore the disappointment, which the laggard proceed- 
 ings at Greenwich and Cambridge had inflicted upon him, with 
 quiet heroism. His silence on the subject of what another man 
 would have called his wrongs remained unbroken to the end 
 of his life ; 4 and he took every opportunity of testifying his 
 admiration for the genius of Leverrier. 
 
 1 Mem. R. A. 8., vol. xvi.. p. 412. - He had recorded the places of 3150 
 stars (three of which were different positions of the planet), and was pre- 
 paring to map them, when, October i, news of the discovery arrived from 
 Berlin. Prof. Challis's Report, quoted in Obituary Notice, Month. Not., Feb. 
 1883, p. 170. 3 Se- Airy in Mem. R. A. 8., vol. xvi., p. 411. 4 He died 
 January 21, 1892, in his yist year. 
 
102 HISTORY OF ASTRONOMY. PARTI. 
 
 Personal questions, however, vanish in the magnitude of the 
 event they relate to. By it the last lingering doubts as to the 
 absolute exactness of the Newtonian Law were dissipated. 
 Eecondite analytical methods received a confirmation brilliant 
 and intelligible even to the minds of the vulgar, and emerged 
 from the patient solitude of the study to enjoy an hour of 
 clamorous triumph. For ever invisible to the unaided eye of 
 man, a sister-globe to our earth was shown to circulate, in 
 perpetual frozen exile, at thirty times its distance from the sun. 
 Nay, the possibility was made apparent that the limits of our 
 system were not even thus reached, but that yet profounder 
 abysses of space might shelter obedient, though little favoured 
 members of the solar family, by future astronomers to be recog- 
 nised through the sympathetic thrillings of Neptune, even as 
 Neptune himself was recognised through the tell-tale deviations 
 of Uranus. 
 
 It is curious to find that the fruit of Adams's and Leverrier's 
 laborious investigations had been accidentally all but snatched 
 half a century before it was ripe to be gathered. On the 8th, 
 and again on the loth of May 1795, Lalande noted the position 
 of Neptune as that of a fixed star, but perceiving that the two 
 observations did not agree, he suppressed the first as erroneous, 
 and pursued the inquiry no further. An immortality which he 
 would have been the last to despise hung in the balance ; the 
 feather-weight of his carelessness, however, kicked the beam, 
 and the discovery was reserved to be more hardly won by later 
 comers. 
 
 Bode's Law did good service in the quest for a trans- Uranian 
 planet by affording ground for a probable assumption as to its 
 distance. A starting-point for approximation was provided by 
 it; but it was soon found to be considerably at fault. Even 
 Uranus is about 36 millions of miles nearer to the sun than the 
 order of progression requires ; and Neptune's vast distance of 
 2800 million should be increased by no less than 800 million 
 miles, and its period of 165 lengthened out to 225 years, 1 in 
 order to bring it into conformity with the curious and un- 
 
 1 Ledger, The /Sun, its Planets and their /Satellites, p. 414. 
 
CHAP. iv. PLANETARY DISCOVERIES. 103 
 
 explained rule which planetary discoveries have alternately 
 tended to confirm and to invalidate. 
 
 Within seventeen days of its identification with the Berlin 
 achromatic, Neptune was found to be attended by a satellite. 
 This discovery was the first notable performance of the celebrated 
 two-foot reflector 1 erected by Mr. Lassell at his suggestively 
 named residence of Starfield, near Liverpool. William Lassell 
 was a brewer by profession, but by inclination an astronomer. 
 Born at Bolton in Lancashire, June 1 8, 1799, he closed a life of 
 eminent usefulness to science, October 5, 1880, thus spanning 
 with his well-spent years almost the entire of the momentous 
 period which we have undertaken to traverse. At the age of 
 twenty-one, being without the means to purchase, he undertook 
 to construct telescopes, and naturally turned his attention to the 
 reflecting sort, as favouring amateur efforts by the comparative 
 simplicity of its structure. His native ingenuity was remarkable, 
 and was developed by the hourly exigencies of his successive 
 enterprises. Their uniform success encouraged him to enlarge 
 his aims, and in 1844 he visited Birr Castle for the purpose of 
 inspecting the machine used in polishing the giant speculum of 
 Parsonstown. In the construction of his new instrument, how- 
 ever, he eventually discarded the model there obtained, and 
 worked on a method of his own, assisted by the supreme 
 mechanical skill of James Nasmyth. The result was a New- 
 tonian of exquisite definition, w r ith an aperture of two and a 
 focal length of twenty feet, provided by a novel artifice with the 
 equatoreal mounting, previously regarded as available only for 
 refractors. 
 
 This beautiful instrument afforded to its maker, October 10, 
 1846, a cursory view of a Neptunian attendant. But the planet 
 was then approaching the sun, and it was not until the 
 following July that the observation could be verified, which it 
 was completely, first by Lassell himself, and somewhat later 
 by Otto Struve and Bond of Cambridge (U.S.). When it is 
 considered that this remote object shines by reflecting sunlight 
 
 1 Presented by the Misses Lassell, after their father s death, to the Royal 
 Observatory. 
 
104 HISTORY OF ASTRONOMY. PART i. 
 
 reduced by distance to yjtfth f t' ne intensity with which it 
 illuminates our moon, the fact of its visibility, even in the most- 
 perfect telescopes, is a somewhat surprising one. It can only, 
 indeed, be accounted for by attributing to it dimensions very 
 considerable for a body of the Secondary order. It shares with 
 the moons of Uranus the peculiarity of retrograde motion ; that 
 is to say, its revolutions, running counter to the grand current 
 of movement in the solar system, are performed from east to 
 west, in a plane inclined at an angle of 35 to that of the 
 ecliptic. Their swiftness serves to measure the mass of the 
 globe round which they are performed. For while our moon 
 takes twenty-seven days and nearly eight hours to complete its 
 circuit of the earth, the satellite of Neptune, at a distance not 
 greatly inferior, sweeps round its primary in five days and 
 twenty-one hours, showing (according to a very simple principle, 
 of computation) that it is urged by a force seventeen times 
 greater than the terrestrial pull upon the lunar orb. Combining 
 this result with that of measurements of the small telescopic 
 disc of this farthest known planet, it is found that while in m ass 
 Neptune equals seventeen earths, in bulk it is equivalent to 
 eighty-five. This is as much as to say that it is composed of 
 relatively very light materials, or more probably of materials 
 distended by internal heat, as yet unwasted by radiation into 
 space, to about five times the volume they would occupy in the 
 interior of our globe. The fact, at any rate, is fairly well 
 ascertained, that the average density of Neptune differs little 
 from that of water. 
 
 We must now turn from this late-recognised member of our 
 system to bestow some brief attention upon the still fruitful field 
 of discovery offered by one of the immemorial five. The family 
 of Saturn. has been more gradually introduced to the notice of 
 astronomers than that of its brilliant neighbour. Titan, the 
 sixth Saturnian moon in order of distance, led the way, being- 
 detected by Huygens, March 25, 1655 ; Cassini made the 
 acquaintance of four more between 1671 and 1684; while 
 Mimas and Enceladus, the two innermost, were caught by 
 Herschel in 1789, as they threaded their lucid way along the 
 
CHAP. iv. PLANETARY DISCOVERIES. 105 
 
 edge of the almost vanished ring. In the distances of these 
 seven revolving bodies from their primary, an order of progres- 
 sion analogous to that pointed out by Titius in the planetary 
 intervals was found to prevail ; but with one conspicuous inter- 
 ruption, similar to that which had first suggested the search 
 for new members of the solar system. Between Titan and 
 Japetus the sixth and seventh reckoning outwards there 
 was obviously room for another satellite. It was discovered 
 on both sides of the Atlantic simultaneously, on the ipth of 
 September 1848. Mr. W. C. Bond, employing the splendid 
 .15-inch refractor of the Harvard Observatory, noticed, Septem- 
 ber 1 6, a minute star situated in the plane of Saturn's rings. 
 The same object was discerned by Mr. Lassell on the i8th. 
 On the following evening, both observers perceived that the 
 problematical speck of light kept up with, instead of being left 
 behind by, the planet as it moved, and hence inferred its true 
 character. 1 Hyperion, the seventh by distance and eighth by 
 recognition of Saturn's attendant train, is of so insignificant a 
 size when compared with some of its fellow-moons (Titan is but 
 little inferior to the planet Mars), as to have suggested to Sir 
 John Herschel 2 the idea that it might be only one of several 
 bodies revolving very close together in fact, an asteroidal 
 satellite ; but the conjecture has, so far, not been verified. 
 
 The coincidence of its duplicate discovery was singularly 
 paralleled two years later. Galileo's amazement when his " optic 
 glass " revealed to him the " triple form of Saturn planeta 
 tergeminus has proved to be, like the laughter of the gods, 
 " inextinguishable." It must revive in every one who contem- 
 plates anew the unique arrangements of that world apart known 
 to us as the Saturnian system. The resolution of the so-called 
 ansce, or "handles," into one encircling ring by Huygens in 
 1655 ; the discovery by Cassini in 1675 of the division of that 
 ring into two concentric ones ; together with Laplace's investiga- 
 tion of the conditions of stability of such a formation, constituted, 
 with some minor observations, the sum of the knowledge ob- 
 tained, up to the middle of the present century, on the subject 
 
 1 Grant, Hist, of Astr., p. 271. - Month. Not., vol. ix., p. 91. 
 
io6 HISTORY OF ASTRONOMY. PART i. 
 
 of this remarkable formation. The first place in the discovery 
 now about to be related belongs to an American astronomer. 
 
 William Cranch Bond, born in 1789 at Falmouth (now Port- 
 land), in the State of Maine, was a watchmaker whom the solar 
 eclipse of 1 806 attracted to study the wonders of the heavens. 
 When, in 1815, the erection of an observatory in connection with 
 Harvard College, Cambridge, was first contemplated, he under- 
 took a mission to England for the purpose of studying the 
 working of similar institutions there, and on his return erected 
 a private observatory at Dorchester, where he worked diligently 
 for many years. Meanwhile, the time was approaching for the 
 resumption of the long-postponed design of the Harvard 
 authorities ; and on the completion of the new establishment in 
 1844, Bond, who had for some time been officially connected 
 with the College and had carried on his scientific labours within 
 its precincts, was offered, and accepted the post of its director. 
 Placed in 1 847 in possession of one of the finest instruments in 
 the world a masterpiece of Merz and Mahler he headed the 
 now long list of distinguished Transatlantic observers. Like 
 the elder Struve, he left an heir to his office and to his 
 eminence; but George Bond unfortunately died in 1865, at 
 the early age of thirty-nine, having survived his father but six 
 years. 
 
 On the night of November 15, 1850 the air, remarkably 
 enough, being so hazy that only the brightest stars could be 
 perceived with the naked eye William Bond discovered a third 
 dusky ring, extending about halfway between the inner brighter 
 one and the globe of Saturn. A fortnight later, but before the 
 observation had been announced in England, the same appear- 
 ance was seen by the Kev. W. K. Dawes with the comparatively 
 small refractor of his observatory at Wateringbury, and on 
 December 3 was described by Mr. Lassell (then on a visit to 
 him) as " something like a crape veil covering a part of the sky 
 within the inner ring/' l Next morning the Times containing 
 the report of Bond's discovery reached Wateringbury. The 
 most surprising circumstance in the matter was that the novel 
 
 1 Month. Not., vol. xi., p. 21. 
 
CHAP. iv. PLANETARY DISCOVERIES. 107 
 
 appendage had remained so long unrecognised. As the rings 
 opened out to their full extent, it became obvious with very 
 moderate optical assistance ; yet some of the most acute observers 
 who have ever lived, using instruments of vast power, had here- 
 tofore failed to detect its presence. It soon appeared, however, 
 that Galle of Berlin 1 had noticed, June 10, 1838, a veil-like 
 extension of the lucid ring across half the dark space separating 
 it from the planet ; but the observation, although communicated 
 at the time to the Berlin Academy of Sciences, had remained 
 barren. Traces of the dark ring, moreover, were found in a 
 drawing executed by Campani in 1664; 2 and Picard (June 15, 
 i673), 3 Hadley (spring of 1720),* and Herschel, 5 had all un- 
 doubtedly seen it under the aspect of a dark bar or belt crossing 
 the Saturnian globe. It was, then, of no recent origin ; but there 
 seemed reason to think that it had lately gained considerably in 
 brightness. The full meaning of this remarkable fact it was re- 
 served for later investigations to develop. 
 
 What we may, in a certain sense, call the closing result of 
 the race for discovery, in which several observers seemed at that 
 time to be engaged, was the establishment, on a satisfactory 
 footing, of our acquaintance with the dependent system of 
 Uranus. Sir William Herschel, whose researches formed, in so 
 many distinct lines of astronomical inquiry, the starting-points 
 of future knowledge, detected, January n. I/S/, 6 two Uranian 
 moons, since called Oberon and Titania, and ascertained the 
 curious circumstance of their motion in a plane almost at right 
 angles to the ecliptic, in a direction contrary to that of all 
 previously known denizens (other than cometary) of the solar 
 kingdom. He believed that he caught occasional glimpses of 
 four more, but never succeeded in assuring himself of their 
 substantial existence. Even the two first remained unseen save 
 by himself until 1828, when bis son re-observed them with a 
 2O-foot reflector, similar to that with which they had been 
 
 1 Astr. Nach., No. 756 (May 2. 1851). - F. Secchi, Month. Not., vol. xiii., 
 
 p. 248. a Hind, Ibid., vol. xv., p. 32. 4 Lynn, Observatory, Oct. I, 1883 ; 
 
 Hadley, Phil. Trans., vol. xxxii., p. 385. 5 Proctor, /Saturn, and its /System, 
 p. 64. 6 Phil Trans., vol. Ixxvii., p. 125. 
 
io8 HISTORY OF ASTRONOMY. PARTI. 
 
 originally discovered. Thenceforward they were kept fairly 
 within view, but their four questionable companions, in spite of 
 some false alarms of detection, remained in the dubious condi- 
 tion in which Herschel had left them. At last, on October 24. 
 1 85 1, 1 after some years of fruitless watching, Mr. Lassell espied 
 " Ariel " and " Umbriel," two Uranian attendants, interior to 
 Oberou and Titania, and of about half their brightness ; so that 
 their disclosure is still reckoned amongst the very highest proofs 
 of instrumental power and perfection. In all probability they 
 were then for the first time seen; for although Professor 
 Holden, 2 director of the Lick Observatory, has made out a 
 plausible case in favour of the fitful visibility to Herschel of 
 each of them in turn, Mr. Lassell's argument, 3 that the glare of 
 the planet in Herschel's great specula must have rendered 
 almost impossible the perception of objects so minute and so 
 close to its disc, appears tolerably decisive to the contrary. 
 Uranus is thus attended by four moons, and, so far as present 
 knowledge extends, by no more. Among the most important of 
 the " negative results " 4 secured by Mr. Lassell's observations at 
 Malta during the years 1852-53 and 1861-65, were the con ~ 
 vincing evidence afforded by them that, without great increase 
 of optical power, no further Neptunian or Uranian satellites 
 can be perceived, and the consequent relegation of Herschel's 
 baffling quartette, notwithstanding the unquestioned place long 
 assigned to them in astronomical text-books, to the repose of 
 unreality. 
 
 1 Month. Not., vol. xi., p. 248. - Ibid., vol. xxxv . pp. 16-22. 3 lbid. r 
 p. 26. Jl Ibid., vol. xli., p. 190. 
 
CHAPTER V. 
 COMETS. 
 
 NEWTON showed that the bodies known as " comets," or hirsute 
 stars, obey the law of gravitation; but it was by no means 
 certain that the individual of the species observed by him in 
 1680 formed a permanent member of the solar system. The 
 velocity, in fact, of its rush round the sun was quite possibly 
 sufficient to carry it off for ever into the depths of space, there 
 to wander, a celestial casual, from star to star. With another 
 comet, however, which appeared two years later, the case was 
 different. Edmund Halley, who afterwards succeeded Flamsteed 
 as Astronomer Eoyal, calculated the elements of its orbit on 
 Newton's principles, and found them to resemble so closely those 
 similarly arrived at for comets observed by Peter Apian in I53 1 ' 
 and by Kepler in 1607, as almost to compel the inference that 
 all three apparitions were of a single body. This implied its 
 revolution in a period of about seventy-six years, and Halley 
 accordingly fixed its return for 1758-9. So conscious was he of 
 the importance of the announcement that he appealed to a 
 "candid posterity," in the event of its verification, to acknow- 
 ledge that the discovery was due to an Englishman. The 
 prediction was one of the test-questions put by Science to 
 Nature, on the replies to which largely depend both the develop- 
 ment of knowledge and the conviction of its reality. In the 
 present instance, the answer afforded may be said to have laid 
 the foundation of this branch of astronomy. Halley's comet 
 punctually reappeared on Christmas Day, 1758, and effected its 
 perihelion passage on the 1 2th of March following, thus proving 
 beyond dispute that some at least of these erratic bodies are 
 
no HISTORY OF ASTRONOMY. PARTI. 
 
 domesticated within our system, and strictly conform, if not to 
 its unwritten customs (so to speak), at any rate to its funda- 
 mental laws. Their movements, in short, were demonstrated 
 by the most unanswerable of all arguments that of verified 
 calculation to be calculable, and their investigation was erected 
 into a legitimate department of astronomical science. 
 
 This notable advance was the chief result obtained in the field 
 of inquiry just now under consideration during the eighteenth 
 century. But before it closed, its cultivation had received a 
 powerful stimulus through the invention of an improved method. 
 The name of Olbers has already been brought prominently before 
 our readers in connection with asteroidal discoveries; these, 
 however, were but chance excursions from the path of cometary 
 research which he steadily pursued through life. An early pre- 
 dilection for the stars was fixed in this particular direction by 
 one of the happy inspirations of genius. As he was watching, 
 one night in the year 1779, by the sick-bed of a fellow-student 
 in medicine at Gottingen, an important simplification in the 
 mode of computing the paths of comets occurred to him. 
 Although not made public until 1797, "Olbers's method "was 
 then universally adopted, and is still regarded as the most 
 expeditious and convenient in cases where absolute rigour is not 
 required. By its introduction, not only many a toilsome and 
 thankless hour was spared, but workers were multiplied, and 
 encouraged in the prosecution of labours more useful than 
 attractive. 
 
 The career of Heinrich Olbers is a brilliant example of what 
 may be done by an amateur in astronomy. He at no time did 
 regular work in an observatory ; he was never the possessor of a 
 transit or any other fixed instrument; moreover, all the best 
 years of his life were absorbed in the assiduous exercise of a 
 toilsome profession. In 1781 he settled as a physician in his 
 native town of Bremen (he was born in 1758 at Arbergen, a 
 neighbouring village, of which his father was pastor), and 
 continued in active practice for over forty years. It was thus 
 only the hours which his robust constitution enabled him to 
 spare from sleep that were available for his intellectual pleasures. 
 
CHAP. v. COMETS. 1 1 1 
 
 Yet his recreation was, as Von Zacli remarked, 1 no less prolific 
 of useful results than the severest work of other men. The 
 upper part of his house in the Sandgasse was fitted up with such 
 instruments and appliances as restrictions of space permitted, 
 and there, night after night during half a century and upwards, 
 he discovered, calculated, or observed the cometary visitants of 
 northern skies. Almost as effective in promoting the interests 
 of science as the valuable work actually done by him, was the 
 influence of his genial personality. He engaged confidence by 
 his ready and discerning sympathy ; he inspired affection by his 
 benevolent disinterestedness ; he quickened thought and awakened 
 zeal by the suggestions of a lively and inventive spirit, animated 
 with the warmest enthusiasm for the advancement of knowledge. 
 Nearly every astronomer in Germany enjoyed the benefits of a 
 frequently active correspondence with him, and his communica- 
 tions to the scientific periodicals of the time were numerous and 
 striking. The motive power of his mind was thus widely felt 
 and continually in action. Nor did it wholly cease to be exerted 
 even when the advance of age and the progress of infirmity 
 rendered him incapable of active occupation. He was, in fact. 
 alive even to the last day of his long life of eighty-one years ,- 
 and his death, which occurred March 2, 1840, left vacant a 
 position which a rare combination of moral and intellectual 
 qualities had conspired to render unique. 
 
 Amongst the many younger men who were attracted and 
 stimulated by intercourse with him was Johann Franz Encke. 
 But while Olbers became a mathematician because he was an 
 astronomer, Encke became an astronomer because he was a 
 mathematician. A born geometer, he was naturally sent to 
 Gottingen and placed under the tuition of Gauss. But geo- 
 meters are men ; and the contagion of patriotic fervour which 
 swept over Germany after the battle of Leipsic did not spare 
 Gauss's promising pupil. He took up arms in the Hanseatic 
 Legion, and marched and fought until the oppressor of his 
 country was safely ensconced behind the ocean-walls of St. 
 Helena. In the course of his campaigning he met Lindenau, 
 
 1 Allgemeine Geograpldsclie Epliemeriden , vol. iv., p. 287. 
 
ii2 HISTORY OF ASTRONOMY. PARTI. 
 
 the militant director of the Seeberg Observatory, and by his 
 influence was appointed his assistant, and eventually, in 1822, 
 became his successor. Thence he was promoted in 1825 to 
 Berlin, where he superintended the building of the new obser- 
 vatory, so actively promoted" by Humboldt, and remained at 
 its head until within some eighteen months of his death in 
 August 1865. 
 
 On the 26th of November 1818, Pons of Marseilles discovered 
 a comet, whose inconspicuous appearance gave little promise of 
 its becoming one of the most interesting objects in our system. 
 Encke at once took the calculation of its elements in hand, and 
 brought out the unexpected result that it revolved round the 
 sun in a period of about 3^ years. 1 He, moreover, detected its 
 identity with comets seen by Mechain in 1786, by Caroline 
 Herschel in 1795, by Pons, Huth, and Bouvard in 1805, and 
 after six laborious weeks of research into the disturbances 
 experienced by it from the planets during the entire interval 
 since its first ascertained appearance, he fixed May 24, 1822, 
 as the date of its next return to perihelion. Although on that 
 occasion, owing to the position of the earth, invisible in the 
 northern hemisphere, Sir Thomas Brisbane's observatory at 
 Paramatta was fortunately ready equipped for its recapture, 
 which Riimker effected quite close to the spot indicated by 
 Encke's ephemeris. 
 
 The importance of this event can be better understood when 
 it is remembered that it was only the second instance of the 
 recognised return of a comet (that of Halley's, sixty-three years 
 previously, having, as already stated, been the first); and that 
 it, moreover, established the existence of a new class of celestial 
 objects, somewhat loosely distinguished as " comets of short 
 period." These bodies (of which a couple of dozen have been 
 found to circulate within the orbit of Saturn) are remarkable as 
 showing certain planetary affinities in the manner of their motions 
 not at all perceptible in the wider travelling members of their 
 order. They revolve, without exception, in the same direction 
 
 1 Astr. Jakrbuch, 1823, p. 217. The period (1208 days) of this body is 
 considerably shorter than that of any other known comet. 
 
CHAP. v. COMETS. 113 
 
 as the planets from west to east; they exhibit a marked 
 tendency to conform to the zodiacal track which limits planetary 
 excursions north and south ; and their paths round the sun, 
 although much more eccentric than the approximately circular 
 planetary orbits, are far less so than the extravagantly long 
 ellipses in which comets comparatively untrained (as it were) 
 in the habits of the solar system ordinarily perform their 
 revolutions. 
 
 No great comet is of the " planetary " kind. These are, 
 indeed, only by exception visible to the naked eye ; they display 
 extremely feeble tail-producing powers, and give small signs of 
 central condensation. Thin wisps of cosmical cloud, they flit 
 across the telescopic field of view without sensibly obscuring the 
 smallest star. Their appearance, in short, suggests what some 
 notable facts in their history will presently be shown to confirm 
 that they are bodies already effete, and verging towards disso- 
 lution. If it be asked what possible connection can be shown to 
 exist between the shortness of period by which they are essen- 
 tially characterised, and what we may call their superannuated 
 condition, we are not altogether at a loss for an answer. Kep- 
 ler's remark, 1 that comets are consumed by their own emissions, 
 has undoubtedly a measure of truth in it. The substance 
 ejected into the tail must, in overwhelmingly large proportion, 
 be for ever lost to the central mass from which it issues. True, 
 it is of a nature inconceivably tenuous ; but unrepaired waste, 
 however small in amount, cannot be persisted in with impunity. 
 The incitement to such self-spoliation proceeds from the sun ; 
 it accordingly progresses more rapidly the more numerous 
 are the returns to the solar vicinity. Comets of short period 
 may thus reasonably be expected to wear out quickly. 
 
 They are, moreover, bodies subject to many adventures and 
 vicissitudes. Their aphelia or the farthest points of their 
 orbits from the sun are usually, if not invariably, situated so 
 near to the path either of Jupiter or of Saturn, as to permit 
 these giant planets to act as secondary rulers of their destinies. 
 
 1 " Sicut bombyces filo fundendo, sic cometas cauda exspiranda consumi 
 *et denique mori." De Cometis, Op., vol. vii.. p. no. 
 
H4 HISTORY OF ASTRONOMY. PARTI. 
 
 By their influence they were, in all likelihood, originally fixed in 
 their present tracks ; and by their influence, exerted in an 
 opposite sense, they may, in some cases, be eventually ejected 
 from them. Careers so varied, as can easily be imagined, are 
 apt to prove instructive, and astronomers have not been back- 
 ward in extracting from them the lessons they are fitted to 
 convey. Encke's comet, above all, has served as an index to 
 much curious information, and it may be hoped that its function 
 in that respect is by no means at an end. The great extent of 
 the solar system traversed by its eccentric path makes it 
 peculiarly useful for the determination of the planetary masses. 
 At perihelion it penetrates within the orbit of Mercury ; it 
 considerably transcends at aphelion the farthest excursion of 
 Pallas. Its vicinity to the first-named planet in August 1835 
 offered the first convenient opportunity of placing that body 
 in the astronomical balance. Its weight or mass had pre- 
 viously been assumed, not ascertained ; and the comparatively 
 slight deviation from its regular course impressed upon the- 
 comet by its attractive power showed that it had been assumed 
 nearly twice too great. 1 That fundamental datum of planetary 
 astronomy the mass of Jupiter was corrected by similar 
 means ; and it was reassuring to find the correction in satisfac- 
 tory accord with that already introduced from observation of the 
 asteroidal movements. 
 
 The fact that comets contract in approaching the sun had 
 been noticed by Hevelius ; Pingre admitted it with hesitating 
 perplexity ; 2 the example of Encke's comet rendered it con- 
 spicuous and undeniable. On the 28th of October 1828, the 
 diameter of the nebulous matter composing this body was 
 estimated at 312,000 miles". It was then about one and a half 
 times as remote from the sun as the earth is at the time of the 
 
 1 Considerable uncertainty, however, still prevails on the point. The inverse 
 relation assumed by Lagrange to exist between distance from the sun and 
 density brought out the Mercurian mass ^nrhmr that of the sun (Laplace,. 
 Exposition du Sijst. du Monde, t. ii., p. 50, ed. 1824). Von Asten deduced from 
 the movements of Encke's comet. 1818-48. a value of 7-TrzrffTTv ; while Backlund 
 derives -^^^^ from the close approach of the same body to the planet in 
 August 1878 (Bull Astr., t. iii., p. 473). - Arago, Annuairc (1832), p. 218. 
 
CHAP. v. COMETS. 115 
 
 equinox. On the 24th of December following, its distance 
 being reduced by nearly two-thirds, it was found to be only 
 14,000 miles across. That is to say, it had shrunk during those 
 two months of approach to TTWo^li part of its original volume ! 
 Yet it had still seventeen days' journey to make before reaching 
 perihelion. The same curious circumstance was even more 
 markedly apparent at its return in 1838. Its bulk, or the actual 
 space occupied by it, was reduced, as it drew near the hearth of 
 our system (so far at least as could be inferred from optical 
 evidence), in the enormous proportion of 800.000 to I. A 
 corresponding expansion on each occasion accompanied its 
 retirement from the sphere of observation. Similar changes 
 of volume, though rarely to the same astounding extent, 
 have been perceived in other comets. They still remain unex- 
 plained ; but it can scarcely be doubted that they are due to 
 the action of the same energetic internal forces which reveal 
 themselves in so many splendid and surprising cometary pheno- 
 mena. 
 
 Another question of singular interest was raised by Encke's 
 acute inquiries into the movements and disturbances of the first 
 known " comet of short period." He found from the first that 
 its revolutions were subject to some influence besides that of 
 gravity. After every possible allowance had been made for the 
 pulls, now backward, now forward, exerted upon it by the several 
 planets, there was still a surplus of acceleration left unaccounted 
 for. Each return to perihelion took place about two and a half 
 hours sooner than received theories warranted. Here, then, was 
 a "residual phenomenon" of the utmost promise for the dis- 
 closure of novel truths. Encke (in accordance with the opinion 
 of Olbers) explained it as due to the presence in space of some 
 such "subtle matter" as was long ago invoked by Euler 2 to be 
 the agent of eventual destruction for the fair scheme of planetary 
 creation. The apparent anomaly of accounting for an accelera- 
 tive effect by a retarding cause disappears when it is considered 
 that any check to the motion of bodies revolving round a centre 
 of attraction causes them to draw closer to it, thus shortening 
 
 1 Hind, The Comets, p. 20. - Phil. Trans., vol. xlvi., p. 204. 
 
n6 HISTORY OF ASTRONOMY. PARTI. 
 
 their periods and quickening their circulation. If space were 
 filled with a resisting medium capable of impeding, even in the 
 most infinitesimal degree, the swift course of the planets, their 
 orbits should necessarily be, npt ellipses, but very close elliptical 
 spirals, along which they would slowly, but inevitably, descend 
 into the burning lap of the sun. The circumstance that no such 
 tendency can be traced in their revolutions by no means sets the 
 question at rest. For it might well be that an effect totally 
 imperceptible until after the lapse of countless ages, as regards 
 the solid orbs of our system, might be obvious in the movements 
 of bodies like comets of small mass and great bulk ; just as a 
 feather or a gauze veil at once yields its motion to the resistance 
 of the air, while a cannon-ball cuts its way through with com- 
 paratively slight loss of velocity. 
 
 It will thus be seen that issues of the most momentous 
 character hang on the time-keeping of comets ; for plainly all 
 must in some degree suffer the same kind of hindrance as 
 Encke's, if the cause of that hindrance be the one suggested. 
 None of its congeners, however, show any trace of similar 
 symptoms. True, the late Professor Oppolzer announced, 1 in 
 1880, that a comet, first seen by Pons in 1819, and rediscovered 
 by Winnecke in 1858, having a period of 2052 days (5.6 years), 
 was accelerated at each revolution precisely in the manner re- 
 quired by Encke's theory. But M. von HaerdtPs subsequent in- 
 vestigation, the materials for which included numerous observa- 
 tions of the body in question at its return to the sun in 1886, 
 decisively negatived the presence of any such effect. 2 Moreover, 
 the researches of Von Asten and Backlund 3 into the movements 
 of Encke's comet revealed a perplexing circumstance. They 
 confirmed Encke's results for the period covered by them, but 
 exhibited the acceleration as having suddenly diminished by 
 nearly one-half in 1868. The reality and permanence of this 
 change were fully established by observations of the return in 
 March 1885. Some physical alteration of the retarded body 
 seems indicated ; but visual evidence countenances no such 
 
 1 Astr. Nach., No. 2314. 2 Comptes Rendus, t. cvii., p. 588. 3 Mem. de 
 St. Petersburg, t. xxxii., No. 3, 1884; Astr. Nach., No. 2727. 
 
CHAP. v. COMETS. 117 
 
 assumption. In aspect the comet is no less thin and diffuse than 
 in 1795 or in 1848. 
 
 The character of the supposed resistance in inter-planetary 
 space has, it may be remarked, been often misapprehended. 
 What Encke stipulated for was not a medium equally diffused 
 throughout the visible universe, such as the ethereal vehicle 
 of the vibrations of light, but a rare fluid, rapidly increasing 
 in density towards the sun. 1 This cannot be a solar atmosphere, 
 since it is mathematically certain, as Laplace has shown, 2 that 
 no envelope partaking of the sun's axial rotation can extend 
 farther from his surface than nine-tenths of the mean distance 
 of Mercury ; while physical evidence assures us that the actual 
 depth of the solar atmosphere bears a very minute proportion to 
 the possible depth theoretically assigned to it. That matter, 
 however, not atmospheric in its nature that is, neither forming 
 one body with the sun nor altogether aeriform exists in its 
 neighbourhood, can admit of no reasonable doubt. The great 
 lens-shaped mass of the zodiacal light, stretching out at times 
 far beyond the earth's orbit, may indeed be regarded as an 
 extension of the corona, the streamers of which themselves 
 mark the wide diffusion, all round the solar globe, of granular 
 or gaseous materials. Yet comets have been known to pene- 
 trate the sphere occupied by them without perceptible loss of 
 velocity. The hypothesis, then, of a resisting medium receives 
 at present no countenance from the movements of comets, 
 whether of short or of long periods. 
 
 Although Encke's comet has made thirty-two complete rounds 
 of its orbit since its first detection in 1786, it shows no certain 
 signs of decay. Variations in its brightness are, it is true, 
 conspicuous, but they do not proceed continuously. 3 
 
 The history of the next known planet-like comet has proved 
 of even more curious interest than that of the first. It was 
 discovered by an Austrian officer named Wilhelm von Biela at 
 Josephstadt in Bohemia, February 27, 1826, and ten days later by 
 the French astronomer Gambart at Marseilles. Both observers 
 
 1 Month. Not., vol. xix., p. 72. - Mecanique Celeste, t. ii., p. 197. s See 
 Berberich, Astr. Nach., Nos. 2836-7, 3125 ; Deichmiiller, Ibid., No. 3123. 
 
ii8 HISTORY OF ASTRONOMY. PARTI. 
 
 computed its orbit, showed its remarkable similarity to that 
 traversed by comets visible in 1772 and 1805, and connected 
 them together as previous appearances of the body just detected 
 by assigning to it's revolutipns a period of between six and 
 seven years. The two brief letters conveying these strikingly 
 similar inferences were printed side by side in the same number 
 of the Astronomische Nachrichten (No. 94) ; but Biela's priority 
 in the discovery of the comet was justly recognised by the 
 bestowal upon it of his name. 
 
 The object in question was at no time (subsequently to its 
 appearance in 1805) visible to the naked eye. Its aspect in 
 Sir John Herschel's great reflector on the 23rd of September 
 1832, was described by him as that of a "conspicuous nebula," 
 about 2\ to 3 minutes in diameter. No trace of a tail was 
 discernible. While he was engaged in watching it, a small 
 knot of minute stars (i6th or I7th magnitude) was directly 
 traversed by it. " and when on the cluster," he tells us, 1 it 
 "presented the appearance of a nebula resolvable and partly 
 resolved into stars, the stars of the cluster being visible through 
 the comet." Yet the depth of cometary matter through which 
 such faint stellar rays penetrated undimmed, was, near the 
 central parts of the globe, not less than 50,000 miles. 
 
 It is curious to find that this seemingly harmless, and we may 
 perhaps add effete body, gave occasion to the first (and not the 
 last) cometary "scare" of this enlightened century. Its orbit, 
 at the descending node, may be said to have intersected that of 
 the earth ; since, according as it bulged in or out under the 
 disturbing influence of the planets, the passage of the comet 
 was effected inside or outside the terrestrial track. Now certain 
 calculations published by Olbers in i828 2 showed that, on 
 October 29, 1832, a considerable portion of its nebulous sur- 
 roundings would actually sweep over the spot which, a month 
 later, would be occupied by our planet. It needed no more to 
 sst the popular imagination in a ferment. Astronomers, after 
 all, could not, by an alarmed public, be held to be infallible. 
 Their computations, it was averred, which a trifling oversight 
 
 1 Month. Not., vol. ii., p. ir;. * Astr. Xach., No. 128. 
 
CHAP. v. COMETS. 119 
 
 would suffice to vitiate, exhibited clearly enough the danger, but 
 afforded no guarantee of safety from a collision, with all the 
 terrific consequences frigidly enumerated by Laplace. Nor did 
 the panic subside until Arago formally demonstrated that the 
 earth and comet could by no possibility approach within less 
 than fifty millions of miles. 1 
 
 The return of the same body in 184546 was marked by an 
 extraordinary circumstance. When first seen, November 28, it 
 wore its usual aspect of a faint round patch of cosmical fog ; 
 but on December 19, Mr. Hind noticed that it had become 
 distorted somewhat into the form of a pear ; and ten days 
 later, it had divided into two separate objects. This singular 
 duplication was first perceived at New Haven in America, 
 December 29, 2 by Messrs. Herrick and Bradley, and by Lieut- 
 enant Maury at Washington, January 13, 1846. The earliest 
 British observer of the phenomenon (noticed by Wichmann the 
 same evening at Konigsberg) was Professor Challis. " I see tivo 
 comets ! " he exclaimed, putting his eye to the great equatoreal 
 of the Cambridge Observatory on the night of January 1 5 ; 
 then, distrustful of what his senses had told him. he called in 
 his judgment to correct their improbable report by resolving 
 one of the dubious objects into a hazy star. 3 On the 23rd, how- 
 ever, both were again seen by him in unmistakable cometary 
 shape, and until far on in March (Otto Struve caught a final 
 glimpse of the pair on the i6th of April), 4 continued to be 
 watched with equal curiosity and amazement by astronomers in 
 every part of the northern hemisphere. What Seneca reproved 
 Ephorus for supposing to have taken place in 373 B.C. what 
 Pingre blamed Kepler for conjecturing in 1618, had then 
 
 1 Annuaire (1832), p. 186. ~ Am. Journ. of Science, vol. i. (2nd series), 
 p. 293. Prof. Hubbard's calculations indicated a probability that the definitive 
 separation of the two nuclei occurred as early as September 30, 1844, Astro- 
 nomical Journal (Gould's), vol. iv., p. 5. See also, on the subject of this 
 comet, W. T. Lynn, Intellectual Observer, vol. xi., p. 208 ; E. Ledger, Observa- 
 tory, August, 1883, p. 244 ; and H. A. Newton, Am. Journ. of /Science, vol. 
 xxxi., p. 8 1, February 1886. 3 Month. Not., vol. vii., p. 73. 4 Bulletin Ac. 
 Imp. de St. Petersbourg, t. vi., col. 77. The latest observation of the parent 
 nucleus was that of Argelander, April 27, at Bonn. 
 
120 HISTORY OF ASTRONOMY. PART i. 
 
 actually occurred under tlie attentive eyes of science in the 
 middle of the nineteenth century ! 
 
 At a distance from each other of about two-thirds the dis- 
 tance of the moon .from the earth, the twin comets meantime 
 moved on tranquilly, so far, at least, as their course through the 
 heavens was concerned. Their extreme lightness, or the small 
 amount of matter contained in each, could not have received a 
 more signal illustration than by the fact that their revolutions 
 round the sun were performed independently ; that is to say,, 
 they travelled side by side without experiencing any appreciable 
 mutual disturbance, thus plainly showing that at an interval of 
 only 157,250 miles their attractive power was virtually inoper- 
 ative. Signs of internal agitation, however, were not wanting. 
 Each fragment threw out a short tail in a direction perpen- 
 dicular to the line joining their centres, and each developed a 
 bright nucleus, although the original comet had exhibited neither 
 of these signs of cometary vitality. A singular interchange of 
 brilliancy was, besides, observed to take place between these 
 small objects, each of which alternately outshone and was out- 
 shone by the other, while an arc of light, apparently proceeding 
 from the more lustrous, at times bridged the intervening space. 
 Obviously, the gravitational tie, rendered powerless by exiguity 
 of matter, was here replaced by some other form of mutual 
 action, the nature of which can as yet be dealt with only by 
 conjecture. 
 
 Once more, in August 1852, the double comet returned to the 
 neighbourhood of the sun, but under circumstances not the most 
 advantageous for observation. Indeed, the companion was not 
 detected until September 16, by Father Secchi at Home, and 
 was then perceived to have increased its distance from the 
 originating body to a million and a quarter of miles, or about 
 eight times the average interval at the former appearance. 
 Both vanished shortly afterwards, and have never since been 
 seen, notwithstanding the eager watch kept for objects of such 
 singular interest, and the accurate knowledge of their track 
 supplied by Santini's investigations. We can scarcely doubt- 
 that the fate has overtaken them which Newton assigned as the 
 
CHAP. v. COMETS. 121 
 
 end of all cometary existence. Diffundi tandem et spargi per 
 ccelos universos. 1 
 
 Biela's is not the only vanished comet. Brorsen's, discovered 
 at Kiel in 1846, and observed at several subsequent returns, 
 failed unaccountably to become visible in iSpo. 2 Yet numerous 
 sentinels were on the alert to surprise its approach along a well- 
 ascertained track, traversed in five and a half years. The object 
 presented from the first a somewhat time-worn aspect. It was 
 devoid of tail, or any other kind of appendage ; and the rapid 
 loss of the light acquired during perihelion passage was accom- 
 panied by the inordinate expansion of an already tenuous globular 
 mass. Another lost or mislaid comet is one found by De Vico 
 at Kome, August 22, 1844. It ought to have returned early in 
 1850, but failed, then and subsequently, to keep its appoint- 
 ments; for its supposed identity with Finlay's comet of 1886 
 has not been ratified by closer inquiry. 3 
 
 A telescopic comet with a period of 7^ years, discovered 
 November 22, 1843, by M. Faye of the Paris Observatory, formed 
 the subject of a characteristically patient and profound inquiry 
 on the part of Leverrier, designed to test its suggested identity 
 with LexelFs comet of 1770. The result was decisive against 
 the hypothesis of Valz, the divergences between the orbits of 
 the two bodies being found to increase instead of to diminish, as 
 the history of the new-comer was traced backwards into the last 
 century. 4 Faye's comet pursues a more nearly circular path than 
 any similar known object, except Holmes's comet of 1892 ; even 
 at its nearest approach to the sun it remains farther off than 
 Mars when he is most distant from it ; and it was proved by 
 the admirable researches of Professor Axel Moller, 5 of Lund, to 
 exhibit no trace of the action of a resisting medium. 
 
 Periodical comets are evidently bodies which have lived, each 
 through a chapter of accidents ; and a significant hint as to the 
 nature of their adventures can be gathered from the fact that 
 their aphelia are pretty closely grouped about the tracks of the 
 
 1 D'Arrest, Aatr. NacJt., No. 1624. - Der Brorserische Comet. Von Dr. 
 E. Lamp, Kiel, 1892. 3 Schulhof, Bull. Astr., t. vi., p. 515. 4 Comptes 
 Jtendus. t. xxv. , p. 570. 5 Month. Xot. , vol. xii., p. 248. 
 
122 HISTORY OF ASTRONOMY. PARTI. 
 
 major planets. Halley's, and four other comets are thus related 
 to Neptune ; eight connect themselves with Uranus, nine with 
 Saturn, twenty-five at least with Jupiter. Some form of depend- 
 ence is plainly indicated, and the recent researches of MM. 
 Tissandier 1 and Callandreau, 2 and of Professor Newton, 3 of Yale 
 College, leave scarcely a doubt that the " capture-theory " repre- 
 sents the essential truth in the matter. The original parabolic 
 paths of these comets were then changed into ellipses by the 
 backward pull of a planet, whose sphere of influence they 
 chanced to enter when approaching the sun from outer space. 
 Moreover, since a body thus affected should necessarily return at 
 each revolution to the scene of encounter, the same process of 
 retardation may, in some cases, have been repeated many times, 
 until the more restricted comet ary orbits were reduced to their 
 present dimensions. The prevalence, too, among periodical 
 comets, of direct motion, is shown to be inevitable by M. Callan- 
 dreau's demonstration that those travelling in a retrograde 
 direction would, by planetary action, be thrown outside the 
 probable range of terrestrial observation. The scarcity of hyper- 
 bolic comets can be similarly explained. They would be created 
 whenever the attractive influence of the disturbing planet was 
 exerted in a forward or accelerative sense, but could come only 
 by a rare exception to our notice. The inner planets, including 
 the earth, have also unquestionably played their parts in modify- 
 ing cometary orbits; and Mr. Plummer suggests, with some 
 show of reason, that the capture of Encke's comet may be a feat 
 due to Mercury. 4 
 
 No great comet appeared between the " star " which presided 
 at the birth of Napoleon and the "vintage" comet of 1811. 
 The latter was first descried by Flaugergues at Viviers, March 
 26, 1811; Wisniewski, at Neu-Tscherkask in Southern Russia, 
 caught the last glimpse of it, August 17, 1812. Two dis- 
 appearances in the solar rays as the earth moved round in its 
 orbit, and two reappearances after conjunction, were included in 
 
 1 Bull. Astr., t. vi., pp. 241, 289. -Etudes sur la Theorie des Cometes 
 periodiques, Paris, 1891. 3 Amer. Journ. of Science, vol. xlii.. pp. 183,482, 
 1891. 4 Observatory, vol. xiv., p. 194. 
 
CHAP. v. COMETS. 123 
 
 this unprecedentedly long period of visibility of 510 days. This 
 relative permanence (so far as the inhabitants of Europe were 
 concerned) was due to the high northern latitude attained near 
 perihelion, combined with a certain leisureliness of movement 
 along a path everywhere external to that of the earth. The 
 magnificent luminous train of this body, on October 15, the day 
 of its nearest terrestrial approach, covered an arc of the heavens 
 23 J degrees in length, corresponding to a real extension of one 
 hundred millions of miles. Its form was described by Sir 
 William Herschel as that of "an inverted hollow cone." and its 
 colour as yellowish, strongly contrasting with the bluish-green 
 tint of the " head," round which it was flung like a transparent 
 veil. The planetary disc of the head, 127,000 miles across, 
 appeared to be composed of strongly condensed nebulous matter ; 
 but somewhat eccentrically situated within it was a star-like 
 nucleus of a reddish tinge, which Herschel presumed to be solid, 
 and ascertained, with his usual care, to have a diameter of 428 
 miles. From the total absence of phases, as well as from the 
 vivacity of its radiance, he confidently inferred that its light was 
 not borrowed, but inherent. 1 
 
 This remarkable apparition formed the subject of a memoir 
 by Olbers, the striking yet steadily reasoned-out suggestions 
 contained in which there was at that time no means of follow- 
 ing up with profit. Only of late has the " electrical theory," of 
 which Zollner 3 regarded Olbers as the founder, assumed a definite 
 and measurable form, capable of being tested more and more 
 surely by the touchstone of fact, as knowledge makes its slow 
 inroads on the fundamental mystery of the physical universe. 
 
 The paraboloidal shape of the bright envelope separated by 
 a dark interval from the head of the great comet of 1 8 1 1 , and 
 constituting, as it were, the root of its tail, seemed to the 
 astronomer of Bremen to reveal the presence of a double repul- 
 sion ; the expelled vapours accumulating where the two forces, 
 solar and comet ary, balanced each other, and being then swept 
 
 1 Phil. Trans., vol. cii., pp. 118-124. - Ueber den Scliweif dcs grossen 
 Cometen von 1811, Monat. Corr., vol. xxv., pp. 3-22. Keprinted by Zollner, 
 Ueber die Natur der Cometen, pp. 3-15. ;J Natur der Cometen, p. 148. 
 
124 HISTORY OF ASTRONOMY. PARTI. 
 
 backward in a huge train. He accordingly distinguished three 
 classes of these bodies : First, comets which develop no matter 
 subject to solar repulsion. These have no tails, and are pro- 
 bably mere nebulosities, without solid nuclei. Secondly, comets 
 which are acted upon by solar repulsion only, and consequently 
 throw out no emanations towards the sun. Of this kind was a 
 bright comet visible in iSo/. 1 Thirdly, cornets like that of 
 1 8 1 1 , giving evidence of action of both kinds. These are 
 distinguished by a dark hoop encompassing the head and divid- 
 ing it from the luminous envelope, as well as by an obscure 
 caudal axis, resulting from the hollow, cone-like structure of the 
 tail. 
 
 Again, the ingenious view recently put forward by M. Bre- 
 dichin as to the connection between the form of these ap- 
 pendages and the kind of matter composing them, was very 
 clearly anticipated by Olbers. The amount of tail-curvature, 
 he pointed out, depends in each case upon the proportion 
 borne by the velocity of the ascending particles to that of the 
 comet in its orbit ; the swifter the outrush, the straighter the 
 resulting tail. But the velocity of the ascending particles 
 varies with the energy of their repulsion by the sun, and this 
 again, it may be presumed, with their quality. Thus multiple 
 tails are developed when the same comet throws off, as it 
 approaches perihelion, specifically distinct substances. The 
 long, straight ray which proceeded from the comet of 1807, 
 for example, was doubtless made up of particles subject to a 
 much more vigorous solar repulsion than those formed into 
 the shorter curved emanation issuing from it nearly in the 
 same direction. In the comet of 1 8 1 1 , he calculated that the 
 particles expelled from the head travelled to the remote 
 extremity of the tail in eleven minutes, indicating by this 
 enormous rapidity of movement (comparable to that of the 
 transmission of light) the action of a force greatly more power- 
 ful than the opposing one of gravity. The not uncommon 
 phenomena of multiple envelopes, on the other hand, he ex- 
 
 1 The subject of a classical memoir by Bessel, published in 1810. 
 
CHAP. v. COMETS. 125 
 
 plained as due to the varying amounts of repulsion exercised 
 by the nucleus itself on the different kinds of matter developed 
 from it. 
 
 The movements and perturbations of the comet of 1 8 1 1 were 
 110 less profoundly studied by Argelander than its physical 
 constitution by Olbers. The orbit which he assigned to it is of 
 such vast dimensions as to require no less than 3065 years for 
 the completion of its circuit ; and to carry the body describing 
 it at each revolution to fourteen times the distance from the 
 sun of the frigid Neptune. Thus, when it last visited our 
 neighbourhood, Achilles may have gazed on its imposing train 
 as he lay on the sands all night bewailing the loss of Patroclus ; 
 and when it returns, it will perhaps be to shine upon the ruins 
 of empires and civilisations still deep buried among the secrets 
 of the coming time. 1 
 
 On the 26th of June 1819, while the head of a comet passed 
 across the face of the sun, the earth was in all probability in- 
 volved in its tail. But of this remarkable double event nothing 
 was known until more than a month later, when the fact of its 
 past occurrence emerged from the calculations of Olbers. Nor 
 had the comet itself been generally visible previous to the first 
 days of July. Several observers, however, on the publication of 
 these results, brought forward accounts of singular spots per- 
 ceived by them upon the sun at the time of the transit, and the 
 original drawing of one of them, by Pastorff of Buchholtz, has 
 been preserved. This undoubtedly authentic delineation 3 repre- 
 sents a round nebulous object with a bright spot in the centre, 
 of decidedly cometary aspect, and not in the least like an 
 ordinary solar " macula." Mr. Hind, 4 nevertheless, has shown 
 that its position on the sun is irreconcilable with that which the 
 comet must have occupied; and Mr. Eanyard's discovery of a 
 similar smaller drawing by the same author, dated May 26, 
 i828, 5 reduces to evanescence the probability of its connection 
 
 1 A fresh investigation of its orbit has been published by N. Herz of Vienna, 
 See Bull. Astr., t. ix., p. 427. ~ Astr. Jahrbuch (Bode's), 1823, p. 134. 
 3 Reproduced in Webb's Celestial Objects, 4th. ed. 4 Month. Not., vol. xxxvi., 
 p. 309. 5 Celestial Objects, p. 40, note. 
 
126 HISTORY OF ASTRONOMY. PARTI. 
 
 with that body. Indeed, recent experience renders very doubt- 
 ful the possibility of such an observation. 
 
 The return of Halley's comet in 1835 was looked forward to 
 as an opportunity for testing the truth of floating cometary 
 theories, and did not altogether disappoint expectation. As 
 early as 1817, its movements and disturbances since 1759 were 
 proposed by the Turin Academy of Sciences as the subject of a 
 prize awarded to Baron Damoiseau. Pontecoitlant was adjudged 
 a similar distinction by the Paris Academy in 1829; while 
 Rosenberger's calculations were rewarded with the gold medal 
 of the Royal Astronomical Society. 1 The result entirely dis- 
 proved the hypothesis (designed to explain the invariability of 
 the planetary periods) of what may be described as a vortex of 
 attenuated matter moving with the planets, and offering, conse- 
 quently, no resistance to their motion. For since Halley's 
 comet revolves in the opposite direction in other words, has a 
 ' ; retrograde " movement it is plain that if compelled to make 
 head against an ethereal current, it would rapidly be deprived of 
 the tangential velocity which enables it to keep at its proper 
 distance from the sun, and would thus gradually but con- 
 spicuously approach, and eventually be precipitated upon it. 
 No such effect, however, has in this crucial instance been 
 detected. 
 
 On the 6th of August 1835, a nearly circular misty object 
 was seen at Rome not far from the predicted place of the 
 comet. It was not, however, until the middle of September 
 that it began to throw out a tail, which by the i$th of October 
 had attained a length of about 24 degrees (on the I9th, at 
 Madras, it extended to fully 3o), 2 the head showing to the naked 
 eye as a reddish star rather brighter than Aldebaran or 
 Antares. 3 Some curious phenomena accompanied the process 
 of tail formation. An outrush of luminous matter, resembling 
 in shape a partially opened fan, issued from the nucleus towards 
 the sun, and at a certain point, like smoke driven before a high 
 wind, was vehemently swept backward in a prolonged train. 
 
 1 See Airy's Address, Mem. B. A. 8., vol. x., p. 376. - Hind, The Comets,. 
 p. 47. :! Arago, Annuaire, 1836, p. 228. 
 
CHAP. v. COMETS. 127 
 
 The appearance of the comet at this time was compared by 
 Bessel, 1 who watched it with minute attention, to that of a 
 blazing rocket. He made the singular observation that this 
 fan of light, which seemed the source of supply for the tail, 
 oscillated like a pendulum to and fro across a line joining the 
 sun and nucleus, in a period of 4?- days ; and he was unable to 
 escape from the conclusion - that a repulsive force, about twice 
 as powerful as the attractive force of gravity, was concerned in 
 the production of these remarkable effects. Nor did he hesitate 
 to recur to the analogy of magnetic polarity, or to declare, still 
 more emphatically than Olbers, the emission of the tail to be a 
 purely electrical phenomenon." 3 
 
 The transformations undergone by this body were almost as 
 strange and complete as those which affected the brigands in 
 Dante's Inferno. When first seen, it wore the aspect of a 
 nebula ; later it put on the distinctive garb of a comet ; it next 
 appeared as a star ; finally, it dilated, first in a spherical, then in 
 a paraboloidal form, until May 5, 1836, when it vanished from 
 Herschel's observation at Feldhausen as if by melting into ad- 
 jacent space through the excessive diffusion of its light. A very 
 uncommon circumstance in its development was that it lost (it 
 would appear) all trace of tail previous to its arrival at peri- 
 helion on the 1 6th of November. Nor did it. begin to recover 
 its elongated shape for more than two months afterwards. On 
 the 23rd of January, Boguslawski perceived it as a star of the 
 sixth magnitude, without measurable disc. 4 " Only two nights 
 later, Maclear, director of the Cape Observatory, found the head 
 to be 131 seconds across. 5 And so rapidly did the augmenta- 
 tion of size progress, that Sir John Herschel estimated the 
 actual bulk of this singular object to have increased forty-fold 
 in the ensuing week. " I can hardly doubt," he remarks, " that 
 the comet was fairly evaporated in perihelio by the heat, and 
 
 1 Astr. Nach., No. 300. - It deserves to be recorded that Kobert Hooke 
 drew a very similar inference from his observations of the comets of 1680 
 and 1682. Month. Not., vol. xiv., pp. 77-83. 3 JJrifftrechsel z/cischen Olbers 
 und JBessd, Bd. ii., p. 390. 4 Herschel, Results, p. 405. 5 Mem. It. A. S., 
 vol. x., p. 92. 
 
128 HISTORY OF ASTRONOMY. PARTI. 
 
 resolved into transparent vapour, and is now in process of rapid 
 condensation and re-precipitation on the nucleus." l A plausible, 
 but no longer admissible, interpretation of a still unexplained 
 phenomenon. The next return of this body, which will be con- 
 siderably accelerated by Jupiter's influence, is expected to take 
 place in I9io. 2 
 
 By means of an instrument devised by himself for testing the 
 quality of light, Arago obtained decisive evidence that some at 
 least of the radiance proceeding from Halley's comet was derived 
 by reflection from the sun. 3 Indications of the same kind had 
 been afforded 4 by the comet which suddenly appeared above the 
 north-western horizon of Paris, July 3, 1819, after having 
 enveloped (as already stated) our terrestrial abode in its filmy 
 appendages; but the " polariscope " had not then reached the 
 perfection subsequently given to it, and its testimony was 
 accordingly far less reliable than in 1835. "Such experiments, 
 however, are in reality more beautiful and ingenious than 
 instructive, since ignited as well as obscure bodies possess the 
 power of throwing back light incident upon them, and must 
 consequently transmit to us from the neighbourhood of the sun 
 rays partly direct, partly reflected, of which a certain proportion 
 will exhibit the peculiarity known as polarisation. 
 
 The most brilliant comets of the century were suddenly 
 rivalled if not surpassed by the extraordinary object which 
 blazed out beside the sun, February 28, 1843. It was simul- 
 taneously perceived in Mexico and the United States, in 
 Southern Europe, and at sea off the Cape of Good Hope, 
 where the passengers on board the Owen Glendower were 
 amazed by the sight of a "short, dagger-like object," closely 
 following the sun towards the western horizon. 5 At Florence, 
 Amici found its distance from the sun's centre at noon to be 
 only i 23' ; and spectators at Parma were able, when sheltered 
 from the direct glare of midday, to trace the tail to a length of 
 four or five degrees. The full dimensions of this astonishing 
 
 1 Results, p. 401. 2 Pontecoulant, Comptes llendus, t. Iviii., p. 825. 
 3 Annuaire, 1836, p. 233. 4 Cosmos, vol. i., p. 90, note (Otte's trans.) 
 s HerscheL Outlines, p. 399, 9th ed. 
 
CHAP. v. COMETS. 129 
 
 appurtenance began to be disclosed a few days later. On the 
 3rd of March it measured 25, and on the nth, at Calcutta, 
 Mr. Clerihew observed a second streamer, nearly twice as long 
 as the first, and making an angle with it of 1 8, to have been 
 emitted in a single day. This rapidity of projection, Sir John 
 Herschel remarks, "conveys an astounding impression of the 
 intensity of the forces at work." "It is clear," he continues, 
 "that if we have to deed here with matter, such as we conceive it 
 viz., possessing inertia at all, it must be under the dominion of 
 forces incomparably more energetic than gravitation, and quite 
 of a different nature." 1 
 
 On the I /th of March a silvery ray, some 40 degrees long 
 :and slightly curved at its extremity, shone out above the sun- 
 set clouds in this country. No previous intimation had been 
 received of the possibility of such an apparition, and even 
 -astronomers no lightning messages across the seas being as 
 yet possible were perplexed. The nature of the phenomenon, 
 indeed, soon became evident, but the wonder of it did not 
 diminish with the study of its attendant circumstances. Never 
 before, within astronomical memory, had our system been tra- 
 versed by a body pursuing such an adventurous career. The 
 closest analogy was offered by the great comet of 1680 (New- 
 ton's), which rushed past the sun at a distance of only 144,000 
 miles ; but even this on the cosmical scale scarcely per- 
 ceptible interval was reduced nearly one-half in the case we 
 are now concerned with. The centre of the comet of 1843 
 approached the formidable luminary within 78,000 miles, leav- 
 ing, it is estimated, a clear space of not more than 32,000 
 between the surfaces of the bodies thus brought into such 
 perilous proximity. The escape of the wanderer was, however, 
 secured by the extraordinary rapidity of its flight. It swept past 
 perihelion at a rate 366 miles a second which, if continued, 
 would have carried it right round the sun in two hours ; and in 
 only eleven minutes more than that short period it actually de- 
 scribed half the curvature of its orbit an arc of 180 although 
 
 1 Herschel, Outlines, p. 398 (9th ed.). 
 ' 9 
 
130 HISTORY OF ASTRONOMY. PARTI. 
 
 in travelling over the remaining half many hundreds of sluggish 
 years will doubtless be consumed. 
 
 The behaviour of this comet may be regarded as an experi- 
 mentum crucis as to the nature of tails. For clearly no fixed 
 appendage many millions of miles in length could be whirled 
 like a brandished sabre from one side of the sun to the other 
 in 131 minutes. Cometary trains are then, as Olbers rightly 
 conceived them to be, emanations, not appendages incon- 
 ceivably rapid outflows of highly rarefied matter, the greater 
 part, if not all, of which becomes permanently detached from 
 the nucleus. 
 
 That of the comet of 1843 reached, about the time that it 
 became visible in this country, the extravagant length of 200 
 millions of miles. 1 It was narrow, and bounded by nearly 
 parallel and nearly rectilinear lines, resembling to borrow a 
 comparison of Aristotle's a "road" through the constellations ; 
 and after the 3rd of March showed no trace of hollowness, the 
 axis being, in fact, rather brighter than the edges. Distinctly 
 perceptible in it were those singular aurora-like coruscations 
 which gave to the "tresses" of Charles V.'s comet the appear- 
 ance as Cardan described it of "a torch agitated by the 
 wind," and have not unfrequently been observed to characterise 
 other similar objects. A consideration first adverted to by 
 Olbers proves these to originate in our own atmosphere. For 
 owing to the great difference in the distances from the earth of 
 the origin and extremity of such vast effluxes, the light proceed- 
 ing from their various parts is transmitted to our eyes in notably 
 different intervals of time. Consequently a luminous undula- 
 tion, even though propagated instantaneously from end to end 
 of a comet's tail, would appear to us to occupy many minutes in 
 its progress. But the coruscations in question pass as swiftly as 
 a falling star. They are, then, of terrestrial production. 
 
 Periods of the utmost variety were by different computators 
 assigned to the body, which arrived at perihelion, February 27, 
 1843, at 9.47 P.M. Professor Hubbard of Washington found 
 
 1 Boguslawski calculated that it extended on the 2ist of March to 581 
 million miles. Report Brit. Ass., 1845, p. 89. 
 
CHAP. v. COMETS. 131 
 
 that it required 533 years to complete a revolution; MM. 
 Laugier and Mauvais of Paris considered the true term to be 
 35 ;* Clausen looked for its return at the end of between six and 
 seven years. All these estimates were indeed admittedly uncertain, 
 the available data affording no sure means of determining the 
 value of this element ; yet there seems no doubt that they fitted 
 in more naturally with a period counted by centuries than with 
 one reckoned by decades. Nor could any previous appearance 
 be satisfactorily made out, although the similarity of the course 
 pursued by a brilliant comet in 1668, known as the " Spina" of 
 Cassini, made an identification not impossible. This would 
 imply a period of 175 years, and it was somewhat hastily 
 assumed that a number of earlier celestial visitants might thus 
 be connected as returns of the same body. 
 
 It may now be asked what were the conclusions regarding the 
 nature of comets drawn by astronomers from the considerable 
 mass of novel experience accumulated during the first half of this 
 century ? The first and best assured was that the matter com- 
 posing them is in a state of extreme tenuity. Numerous and 
 trustworthy observations showed that the feeblest rays of light 
 might traverse some hundreds of thousands of miles of their 
 substance, even where it was apparently most condensed, with- 
 out being perceptibly weakened. Nay, instances were recorded 
 in which stars were said to have gained in brightness from the 
 process! 2 On the 24th of June 1825, Olbers 3 saw the comet 
 then visible all but obliterated by the central passage of a star 
 too small to be distinguished with the naked eye, its own light 
 remaining wholly unchanged. A similar effect was noted De- 
 cember i , 1 8 1 1 , when the great comet of that year approached 
 so close to Altair, the lucida of the Eagle, that the star seemed 
 to be transformed into the nucleus of the comet. 4 Even the 
 central blaze of Halley's comet in 1835 was powerless to impede 
 the passage of stellar rays. Struve 5 observed at Dorpat, on 
 
 1 Comptes Bendus, t. xvi., p. 919. ~ Piazzi noticed a considerable increase 
 of lustre in a very faint star of the twelfth magnitude viewed through a 
 comet. Madler, Beden, &c., p. 248, note. 3 Astr. Jahrbuch, 1828, p. 151, 
 4 Madler, Gesch. d. Astr., Bd. ii., p. 412. 5 Becueil de I' Ac. Imp. de tit. 
 Petersbourg, 1835, p. 143. 
 
132 HISTORY OF ASTRONOMY. PARTI. 
 
 September 17, an all but central occupation ; Glaisher 1 one (so 
 far as he could ascertain) absolutely so eight days later at 
 Cambridge. In neither case was there any appreciable diminu- 
 tion of the star's light. Again, on the nth of October 1847, 
 Mr. Dawes, 2 an exceptionally keen observer, distinctly saw a 
 star of the tenth magnitude through the exact centre of a comet 
 discovered on the first of that month by Maria Mitchell of 
 Nantucket. 
 
 Examples, on the other hand, are not wanting of the diminu- 
 tion of stellar light under similar circumstances ; but probably 
 in general not more than would be accounted for by the 
 illumination of the background with diffused nebulous radiance. 3 
 In one solitary instance, however, on the 28th of November 
 1828, a star was alleged to have actually vanished behind a 
 comet. 4 The observer of this unique phenomenon was Wart- 
 mann of Geneva ; but his instrument was so defective as to 
 leave its reality open to grave doubt, especially when it is con- 
 sidered that the eclipsing body was Encke's comet, which better 
 equipped astronomers have, on various occasions, found to be 
 perfectly translucent. 
 
 From the failure to detect any effects of refraction in the 
 light of stars occulted by comets, it was inferred (though, as 
 we know now, erroneously) that their composition is rather that 
 of dust than that of vapour ; that they consist not of any con- 
 tinuous substance, but of discrete solid particles, very finely 
 divided and widely scattered. In conformity with this view 
 was the known smallness of their masses. Laplace had shown 
 that if the amount of matter forming Lexell's comet had been 
 as much as -5-^-$ of that contained in our globe, the effect of 
 its attraction, on the occasion of its approach with 1,438,000 
 miles of the earth, July I 5 1770, must have been apparent in 
 the lengthening of the year. And that some comets, at any 
 
 1 Guillemin's World of Comets, trans, by J. Glaisher, p, 294, note. - Month. 
 Not., vol. viii., p. 9. :J A real, though only partial, stoppage of light seems 
 indicated by Herschel's observations on the comet of 1807. Stars seen 
 through the tail, October 18, lost much of their lustre. One near the head was 
 only faintly -visible by glimpses. Phil. Trans., vol. xcvii., p. 153. 4 Arago, 
 Annuaire, 1832, p. 205. 
 
CHAP. v. COMETS. 133 
 
 rate, possess masses immeasurably below this maximum value, 
 was clearly proved by the undisturbed parallel march of the two 
 fragments of Biela in 1846. 
 
 But the discovery in this branch most distinctive of the period 
 under review is that of "short period" comets, of which four 1 
 were known in 1850. These, by the character of their move- 
 ments, serve as a link between the planetary and cometary 
 worlds, and by the nature of their construction, seem to mark 
 a stage in cometary decay. For that comets are rather transi- 
 tory agglomerations, than permanent products of cosmical 
 manufacture, appeared to be demonstrated by the division and 
 disappearance of one amongst their number, as well as by the 
 singular and rapid changes in appearance undergone by many, 
 and the seemingly irrevocable diffusion of their substance 
 visible in nearly all. They might then be defined, according to 
 the ideas respecting them prevalent forty-five years ago, as 
 bodies unconnected by origin with the solar system, but en- 
 countered, and to some extent appropriated by it in its progress 
 through space, owing their visibility in great part, if not 
 altogether, to light reflected from the sun, and their singular 
 and striking forms to the action of repulsive forces emanating 
 from him, the penalty of their evanescent splendour being paid 
 in gradual waste and final dissipation and extinction. 
 
 1 Viz., Encke's, Biela's, Faye's, and Brorsen's. 
 

 CHAPTER VI. 
 
 INSTRUMENTAL ADVANCES. 
 
 IT is impossible to follow with intelligent interest the course of 
 astronomical discovery without feeling some curiosity as to the 
 means by which such surpassing results have been secured. 
 Indeed, the bare acquaintance with what has been achieved, 
 without any corresponding knowledge of how it has been 
 achieved, supplies food for barren wonder rather than for fruit- 
 ful and profitable thought. Ideas advance most readily along 
 the solid ground of practical reality, and often find true sublimity 
 while laying aside empty marvels. Progress is the result, not 
 so much of sudden nights of genius, as of sustained, patient, 
 often commonplace endeavour ; and the true lesson of scientific 
 history lies in the close connection which it discloses between 
 the most brilliant developments of knowledge and the faithful 
 accomplishment of his daily task by each individual thinker and 
 worker. 
 
 It would be easy to fill a volume with the detailed account of 
 the long succession of optical and mechanical improvements by 
 means of which the observation of the heavens has been brought 
 to its present degree of perfection; but we must here content 
 ourselves with a summary sketch of the chief amongst them. 
 The first place in our consideration is naturally claimed by the 
 telescope. 
 
 This potent instrument, we need hardly remind our readers, 
 is of two distinct kinds that in which light is gathered 
 together into a focus by refraction, and that in which the same 
 end is attained by reflection. The image formed is in each case 
 viewed through a magnifying lens, or combination of lenses, 
 
CHAP. vi. INSTRUMENTAL ADVANCES. 135 
 
 called the eye-piece. Not for above a century after the "optic 
 glasses " invented or stumbled upon by the spectacle-maker of 
 Middleburg (1608) had become diffused over Europe, did the 
 reflecting telescope come, even in England, the place of its 
 birth, into general use. Its principle (a sufficiently obvious one) 
 had indeed been suggested by Mersenne as early as 1639 j 1 
 James Gregory in i663 2 described in detail a mode of embodying 
 that principle in a practical shape ; and Newton, adopting an 
 original system of construction, actually produced in 1668 a tiny 
 speculum, one inch across, by means of which the effective 
 distance of objects was reduced thirty-nine times. Neverthe- 
 less, the exorbitantly long tubeless refractors, introduced by 
 Huygens, maintained their reputation until Hadley exhibited to 
 the Royal Society in I723 3 a reflector sixty-two inches in focal 
 length, which rivalled in performance, and of course indefinitely 
 surpassed in manageability, one of the "aerial" kind of 123 
 feet. 
 
 The concave-mirror system now gained a decided ascendant, 
 and was brought to unexampled perfection by James Short of 
 Edinburgh during the years 1732-68. Its capabilities were, 
 however, first fully developed by William Herschel. The energy 
 and inventiveness of this extraordinary man marked an epoch 
 wherever they were applied. His ardent desire to measure and 
 gauge the stupendous array of worlds which his specula revealed 
 to him, made him continually intent upon adding to their 
 <; space-penetrating power" by increasing their light-gathering 
 surface. These, as he was the first to explain, 4 are in a constant 
 proportion one to the other. For a telescope with twice the 
 linear aperture of another will collect four times as much light, 
 and will consequently disclose an object four times as faint as 
 could be seen with the first, or, what comes to the same, an 
 object equally bright at twice the distance. In other words, it 
 will possess double the space-penetrating power of the smaller 
 instrument. Herschel's great mirrors the first examples of the 
 giant telescopes of modern times were then primarily engines 
 
 1 Grant, Hist. Astr., p. 527. 2 Optica Promota, p. 93. 3 Phil. Trans., 
 vol. xxxii., p. 383. 4 Ibid., vol. xc., p. 65. 
 
136 HISTORY OF ASTRONOMY. PARTI, 
 
 for extending the bounds of the visible universe ; and from the 
 sublimity of this final cause was derived the vivid enthusiasm 
 which animated his efforts towards success. 
 
 It seems probable that the seven-foot telescope constructed by 
 him in 1775 that is, within little more than a year after his 
 experiments in shaping and polishing metal had begun already 
 exceeded in genuine power all the productions of earlier opticians ; 
 and both his skill and his ambition rapidly developed. His en- 
 deavours culminated, after mirrors of ten, twenty, and thirty feet 
 focal length had successively left his hands, in the gigantic 
 forty-foot, completed August 28, 1789. It was the first re- 
 flector in which only a single mirror was employed. In the 
 "Gregorian" form, the focussed rays are, by a second reflection 
 from a small concave 1 mirror, thrown straight back through a. 
 central aperture in the larger one, behind which the eye-piece is 
 fixed. The object under examination is thus seen in the natural 
 direction. The " Newtonian," on the other hand, shows the 
 object in a line of sight at right angles to the true one, the 
 light collected by the speculum being diverted to one side of the 
 tube by the interposition of a small plane mirror, situated at an 
 angle of 45 to the axis of the instrument. Upon these two 
 systems Herschel worked until 1787, when, becoming convinced 
 of the supreme importance of economising light (necessarily 
 wasted by the second reflection), he laid aside the small mirror 
 of his forty-foot then in course of construction, and turned it 
 into a "front-view" reflector. This was done according to a 
 plan proposed by Lemaire in 1732 by slightly inclining the 
 speculum so as to enable the image formed by it to be viewed 
 with an eye-glass fixed at the upper margin of the tube. The 
 observer thus stood with his back turned to the object he was 
 engaged in scrutinising. 
 
 The advantages of the increased brilliancy afforded by this mo- 
 dification were strikingly illustrated by the discovery, August 2& 
 
 1 Cassegrain, a Frenchman, substituted in 1672 a convex fora concave secon- 
 dary speculum. The tube was thereby enabled to be shortened by twice the 
 focal length of the mirror in question. The great Melbourne reflector (four 
 feet aperture, by Grubb) is constructed upon this plan. 
 
CHAP. vi. INSTRUMENTAL ADVANCES. 137 
 
 and September 17, 1789, of the two Saturnian satellites nearest 
 the ring. Nevertheless, the monster telescope of Slough can- 
 not be said to have realised the sanguine expectations of its 
 constructor. The occasions on which it could be usefully em- 
 ployed were found to be extremely rare. It was injuriously 
 affected by every change of temperature. The great weight 
 (25 cwt.) of a speculum four feet in diameter rendered it 
 peculiarly liable to distortion. With all imaginable care, the 
 delicate lustre of its surface could not be preserved longer than 
 two years, 1 when the difficult process of repolishing had to be 
 undertaken. It was accordingly never used after 1811, when 
 having gone blind from damp, it lapsed by degrees into the 
 condition of a museum inmate. 
 
 The exceedingly high magnifying power s employed by 
 Herschel constituted a novelty in optical astronomy, to which he 
 attached great importance. Yet the work of ordinary observa- 
 tion would be hindered rather than helped by them. The 
 attempt to increase in this manner the efficacy of the telescope 
 is speedily checked by atmospheric, to say nothing of other 
 difficulties. Precisely in the same proportion as an object is 
 magnified, the disturbances of the medium through which it is 
 seen are magnified also. Even on the clearest and most tranquil 
 nights, the air is never for a moment really still. The rays of 
 light traversing it are continually broken by minute fluctuations 
 of refractive power caused by changes of temperature and 
 pressure, and the currents which these engender. With such 
 luminous quiverings and waverings the astronomer has always 
 more or less to reckon ; their absence is simply a question of 
 degree ; if sufficiently magnified, they are at all times capable of 
 rendering observation impossible. 
 
 Thus, such vast powers as 3000, 4000, 5000, even 6652,2 
 which Herschel now and again applied to his great telescopes, 
 would, save on the rarest occasions, prove an impediment rather 
 
 1 Phil. Trans., vol. civ., p. 275, note. - Phil. Trans., vol. xc., p. 70. With 
 the forty-foot, however, only very moderate powers seemed to have been 
 employed, whence Dr. Kobinson argued a deficiency of defining power. Proc. 
 Roy. Irish Ac., vol. ii.,p. II. 
 
138 HISTORY OF ASTRONOMY. PARTI. 
 
 than an aid to vision. They were, however, used by him only 
 for special purposes experimentally, not systematically, and 
 with the clearest discrimination of their advantages and draw- 
 backs. It is obvious that perfectly different ends are subserved 
 by increasing the aperture and by increasing the power of a 
 telescope. In the one case, a larger quantity of light is cap- 
 tured and concentrated ; in the other, the same amount is 
 distributed over a wider area. A diminution of brilliancy 
 accordingly attends, cceteris paribus, upon each augmentation of 
 apparent size. For this reason, such faint objects as nebulas are 
 most successfully observed with moderate powers applied to 
 instruments of a great capacity for light, the details of their 
 structure actually disappearing when highly magnified. With 
 stellar groups the reverse is the case. Stars cannot be magni- 
 fied, simply because they are too remote to have any sensible 
 dimensions ; but the space between them can. It was thus for 
 the purpose of dividing very close double stars that Herschel 
 increased to such an unprecedented extent the magnifying 
 capabilities of his instruments ; and to this improvement inci- 
 dentally the discovery of Uranus, March 13, I78I, 1 was due. 
 For by the examination with strong lenses of an object which, 
 even with a power of 227, presented a suspicious appearance, he 
 was able at once to pronounce its disc to be real, and not merely 
 " spurious," and so to distinguish it unerringly from the crowd 
 of stars amidst which it was moving. 
 
 While the reflecting telescope was astonishing the world by 
 its rapid development in the hands of Herschel, its unpretending 
 rival was slowly making its way towards the position which the 
 future had in store for it. The great obstacle which long stood 
 in the way of the improvement of refractors was the defect 
 known as " chromatic aberration." This is due to no other 
 cause than that which produces the rainbow and the spectrum 
 the separation, or " dispersion" in their passage through a 
 refracting medium, of the variously coloured rays composing a 
 beam of white light. In an ordinary lens there is no common 
 
 1 Phil. Trans., vol. Ixxi., p. 492. 
 
CHAP. vi. INSTRUMENTAL ADVANCES. 139 
 
 point of concentration ; each colour has its own separate focus ; 
 and the resulting image, formed by the super-position of as 
 many images as there are hues in the spectrum, is indistinctly 
 terminated with a tinted border, eminently baffling to exactness 
 of observation. 
 
 The extravagantly long telescopes of the seventeenth century 
 were designed to avoid this evil (as well as another source of 
 indistinct vision in the spherical shape of lenses) ; but no 
 attempt to remedy it was made until an Essex gentleman 
 succeeded, in 1733, in so combining lenses of flint and crown 
 glass as to produce refraction without colour. 1 Mr. Chester 
 More Hall was, however, equally indifferent to fame and profit, 
 and took no pains to make his invention public. The effective 
 discovery of the achromatic telescope was, accordingly, reserved 
 for John Dollond, whose method of correcting at the same time 
 chromatic and spherical aberration was laid before the Koyal 
 Society in 1758. Modern astronomy may be said to have been 
 thereby rendered possible. Refractors have always been found 
 better suited than reflectors to the ordinary work of observa- 
 tories. They are, so to speak, of a more robust, as well as of a 
 more plastic nature. They suffer less from vicissitudes of 
 temperature and climate. They retain their efficiency with 
 fewer precautions and under more trying circumstances. Above 
 all, they co-operate more readily with mechanical appliances, 
 and lend themselves with far greater facility to purposes of exact 
 measurement. 
 
 A practical difficulty, however, impeded the realisation of the 
 brilliant prospects held out by Dollond's invention. It was 
 found impossible to procure flint-glass, such as was needed for 
 optical use that is, of perfectly homogeneous quality except 
 in fragments of insignificant size. Discs of more than two or 
 three inches in diameter were of extreme rarity ; and the crush- 
 ing excise duty imposed upon the article by the financial 
 
 1 It is remarkable that, as early as 1695, the possibility of an achromatic 
 combination was inferred by David Gregory from the structure of the human 
 eye. See his Catoptricce et Dioptricce, Spliericce Elementa, p. 98. 
 
140 HISTORY OF ASTRONOMY. PARTI. 
 
 unwisdom of the Government, both limited its production, and, 
 by rendering experiments too costly for repetition, barred its 
 improvement. 
 
 Up to this time, Great Britain had left foreign competitors 
 far behind in the instrumental department of astronomy. The 
 quadrants and circles of Bird, Gary and Kamsden were un- 
 approached abroad. The reflecting telescope came into existence 
 and reached maturity on British soil. The refracting telescope 
 was cured of its inherent vices by British ingenuity. But with 
 the opening of the nineteenth century, the almost unbroken 
 monopoly of skill and contrivance which our countrymen had 
 succeeded in establishing was invaded, and British workmen 
 had to be content to exchange a position of supremacy for one 
 of at least partial and temporary inferiority. 
 
 Somewhere about the time that Herschel set about polishing 
 his first speculum, Pierre Louis Guinand, a Swiss artisan, living 
 near Chaux-de-Fonds, in the canton of Neuchatel, began to grind 
 spectacles for his own use, and was thence led on to the rude 
 construction of telescopes by fixing lenses in pasteboard tubes. 
 The sight of an English achromatic, however, stirred a higher 
 ambition, and he took the first opportunity of procuring some 
 flint-glass from England (then the only source of supply), with 
 the design of imitating an instrument the full capabilities of 
 which he was destined to be the humble means of developing. 
 The English glass proving of inferior quality, he conceived the 
 possibility, unaided and ignorant of the art as he was, of himself 
 making better, and spent seven years (1784-90) in fruitless 
 experiments directed to that end. Failure only stimulated him 
 to enlarge their scale. He bought some land near Les Brenets, 
 constructed upon it a furnace capable of melting two quintals of 
 glass, and reducing himself and his family to the barest necessaries 
 of life, he poured his earnings (he at this time made bells for 
 repeaters) unstintingly into his crucibles. 1 His undaunted 
 resolution triumphed. In 1799 he carried to Paris and there 
 showed to Lalande several discs of flawless crystal four to six 
 inches in diameter. Lalande advised him to keep his secret, 
 
 1 Wolf, Biograpliien , Bd. ii., p. 301. 
 
CHAP. vi. INSTRUMENTAL ADVANCES. 141 
 
 but in 1805 he was induced to remove to Munich, where he 
 became the instructor of the immortal Fraunhofer. His return 
 to Les Brenets in 1814 was signalised by the discovery of an 
 ingenious mode of removing striated portions of glass by break- 
 ing and re-soldering the product of each melting, and he 
 eventually attained to the manufacture of perfect discs up to 1 8 
 inches in diameter. An object-glass for which he had furnished 
 the material to Cauchoix, procured him, in 1823, a royal invita- 
 tion to settle in Paris ; but he was no longer equal to the 
 change, and died at the scene of his labours, February 13 
 following. 
 
 This same lens (12 inches across) was afterwards purchased 
 by Sir James South, and the first observation made with it, 
 February 13, 1830, disclosed to Sir John Herschel the sixth 
 minute star in the central group of the Orion nebula, known as 
 the "trapezium." 1 Bequeathed by South to Trinity College, 
 Dublin, it has been employed at the Dunsink Observatory by 
 Briinnow and Ball in their investigations of stellar parallax. A 
 still larger objective (of 13^2 inches) made of Guinand's glass 
 was secured about the same time in Paris, by Mr. Edward 
 Cooper of Markree Castle, Ireland. The peculiarity of the 
 method discovered at Les Brenets resided in the manipulation, 
 not in the quality of the ingredients ; the secret, that is to say, 
 was not chemical, but mechanical. 2 It was communicated by 
 Henry Guinand (a son of the inventor) to Bontemps, one of the 
 directors of the glassworks at Choisy-le-Koi, and by him trans- 
 mitted to Messrs. Chance of Birmingham, with whom he entered 
 into partnership when the revolutionary troubles of 1 848 obliged 
 him to quit his native country. ' The celebrated American 
 opticians, Alvan Clark & Sons, derived from the Birmingham 
 firm the materials for some of their earlier telescopes, notably 
 the iQ-inch Chicago and 26-inch Washington equatoreals; but 
 the discs for the great Lick refractor, and others shaped by them 
 in recent years, have been supplied by Feil of Paris. 
 
 Two distinguished amateurs, meanwhile, were preparing to 
 
 1 Month. Not., vol. i., p. 153, note. ' 2 Henrivaux, Encyclopedic Chimique, 
 t. v., fasc. 5, p. 363. 
 
142 HISTORY OF ASTRONOMY. PARTI. 
 
 reassert on behalf of reflecting instruments their claim to the 
 place of honour in the van of astronomical discovery. Of 
 Mr. Lassell's specula something has already been said. 1 They 
 were composed of an alloy of copper and tin, with a minute 
 proportion of arsenic (after the example of Newton 2 ), and were 
 remarkable for perfection of figure and brilliancy of surface. 
 
 The resources of the Newtonian system were developed still 
 more fully it might almost be said to the uttermost by the 
 enterprise of an Irish nobleman. William Parsons, known as 
 Lord Oxmantown until 1841, when, on his father's death, he 
 succeeded to the title of Earl of Kosse, was born at York, 
 June 17, 1800. His public duties began before his education 
 was completed. He was returned to Parliament as member for 
 King's County while still an undergraduate at Oxford, and 
 continued to represent the same constituency for thirteen years 
 (1821-34). From 1845 until his death, which took place at 
 Birr Castle, Parsonstown, October 31, 1867, he sat, silent but 
 assiduous, in the House of Lords as an Irish representative 
 peer ; he held the not unlaborious post of President of the 
 Royal Society from 1849 to 1854 ; presided over the meeting of 
 the British Association at Cork in 1843, an d was elected Vice- 
 Chancellor of Dublin University in 1862. In addition to these 
 extensive demands upon his time and thoughts, were those 
 derived from his position as (practically) the feudal chief of a 
 large body of tenantry in times of great and anxious responsi- 
 bility, to say nothing of the more genial claims of an unstinted 
 hospitality. Yet, while neglecting no public or private duty, 
 this model nobleman found leisure to render to science services 
 so conspicuous as to entitle his name to a lasting place in its 
 annals. 
 
 He early formed the design of reaching the limits of the 
 attainable in enlarging the powers of the telescope, and the- 
 qualities of his mind conspired with the circumstances of his 
 fortune to render the design a feasible one. From refractors it 
 was obvious that no such vast and rapid advance could be 
 
 1 See ante, p. 103. 2 Phil. Trans., vol. vii., p. 4007. 
 
CHAP. vi. INSTRUMENTAL ADVANCES. 143 
 
 expected. English glass-manufacture was still in a backward 
 state. So late as 1839, Simms (successor to the distinguished 
 instrumentalist Edward Troughton) reported a specimen of 
 crystal scarcely J\ inches in diameter, and perfect only over six, 
 to be unique in the history of English glass-making. 1 Yet at 
 that time the fifteen-inch achromatic of Pulkowa had already left 
 the workshop of Fraunhofer's successors at Munich. It was not 
 indeed until 1845, when the impost which had so long hampered 
 their efforts was removed, that the optical artists of these islands 
 were able to compete on equal terms with their rivals on the 
 Continent. In the case of reflectors, however, there seemed no 
 insurmountable obstacle to an almost unlimited increase of light- 
 gathering capacity ; and it was here, after some unproductive 
 experiments with fluid lenses, that Lord Oxmantown concentrated 
 his energies. 
 
 He had to rely entirely on his own invention, and to earn his 
 own experience. James Short had solved the problem of giving 
 to metallic surfaces a perfect parabolic figure (the only one by 
 which parallel incident rays can be brought to an exact focus ) ; 
 but so jealous was he of his secret, that he caused all his tools to 
 be burnt before his death ; 2 nor was anything known of the pro- 
 cesses by which Herschel had achieved his astonishing results. 
 Moreover, Lord Oxmantown had no skilled workmen to assist him. 
 His implements, both animate and inanimate, had to be formed 
 by himself. Peasants taken from the plough were educated by 
 him into efficient mechanics and engineers. The delicate and 
 complex machinery needed in operations of such hairbreadth 
 nicety as his enterprise involved, the steam-engine which was to 
 set it in motion, at times the very crucibles in which his specula 
 were cast, issued from his own workshops. 
 
 In 1827 experiments on the composition of speculum-metal 
 were set on foot, and the first polishing-machine ever driven 
 by steam-power was contrived. But twelve arduous years of 
 struggle with recurring difficulties passed before success began 
 to dawn. A material less tractable than the alloy selected of 
 
 1 J. Herschel, The Telescope, p. 39. 3 Month. Not,, vol. xxix., p. 125. 
 
144 HISTORY OF ASTRONOMY. PART i. 
 
 four chemical equivalents of copper to one of tin l can scarcely 
 be conceived. It is harder than steel, yet brittle as glass, 
 crumbling into fragments with the slightest inadvertence of 
 handling or treatment ; 2 and the precision of figure requisite 
 to secure good definition is almost beyond the power of lan- 
 guage to convey. The quantities involved are so small as 
 not alone to elude sight, but to confound imagination. Sir 
 John Herschel tells us that "the total thickness to be abraded 
 from the edge of a spherical speculum 48 inches in diameter 
 and 40 feet focus, to convert it into a paraboloid, is only 
 a ! a a a of an inch ; " 3 yet upon this minute difference of form 
 depends the clearness of the image, and, as a consequence, 
 the entire efficiency of the instrument. "Almost infinite," 
 indeed (in the phrase of the late Dr. Robinson), must be the 
 exactitude of the operation adapted to bring about so delicate 
 a result. 
 
 At length, in 1840, two specula, each three feet in diameter, 
 were turned out in such perfection as to prompt a still bolder 
 experiment. The various processes needed to ensure success 
 were now ascertained and under control ; all that was neces- 
 sary was to repeat them on a larger scale. A gigantic mirror, 
 six feet across and fifty-four in focal length, was accordingly 
 cast on the I3th of April 1842 ; in two months it was ground 
 down to figure by abrasion with emery and water, and daintily 
 polished with rouge; and by the month of February 1845 tne 
 "leviathan of Parsonstown " was available for the examination 
 of the heavens. 
 
 The suitable mounting of this vast machine was a problem 
 scarcely less difficult than its construction. The shape of a 
 speculum needs to be maintained with an elaborate care equal 
 to that used in imparting it. In fact, one of the most for- 
 midable obstacles to increasing the size of such reflecting 
 surfaces consists in their liability to bend under their own 
 
 1 A slight excess of copper renders the metal easier to work, but liable to 
 tarnish. Kobinson, Proc. Roy. Irish. Ac., vol. ii., p. 4. 2 Brit. Ass., 1843, 
 Dr. Rob'inson's closing Address. Athenceunv, Sept. 23, p. 866. 3 The Tele- 
 scope, p. 82. 
 
CHAP. vi. INSTRUMENTAL ADVANCES, 145 
 
 weight. That of the great Eosse speculum was no less than 
 four tons. Yet, although six inches in thickness, and composed 
 of a material only a degree inferior in rigidity to wrought iron, 
 the strong pressure of a man's hand at its back produced sufficient 
 flexure to distort perceptibly the image of a star reflected in it. 1 
 Thus the delicacy of its form was perishable equally by the stress 
 of its own gravity, and by the slightest irregularity in the means 
 taken to counteract that stress. The problem of affording a 
 perfectly equable support in all possible positions was solved by 
 resting the speculum upon twenty-seven platforms of cast iron, 
 felt-covered, and carefully fitted to the shape of the areas they 
 were to carry, which platforms were themselves borne by a com- 
 plex system of triangles and levers, ingeniously adapted to 
 distribute the weight with complete uniformity. 2 
 
 A tube which resembled, when erect, one of the ancient round 
 towers of Ireland, 3 served as the habitation of the great mirror. 
 It was constructed of deal staves bound together with iron hoops, 
 was fifty-eight feet long (including the speculum-box), and seven 
 in diameter. A reasonably tall man could walk through it (as 
 Dean Peacock once did) with umbrella uplifted. Two piers of 
 solid masonry, about fifty feet high, seventy long, and twenty- 
 three apart, flanked the huge engine on either side. Its lower 
 extremity rested on an universal joint of cast iron ; above, it was 
 slung in chains, and even in a gale of wind remained perfectly 
 steady. The weight of the entire, although amounting to fifteen 
 tons, was so skilfully counterpoised, that the tube could with 
 ease be raised or depressed by two men working a windlass. Its 
 horizontal range was limited by the lofty walls erected for its 
 support to about ten degrees on each side of the meridian ; but 
 it moved vertically from near the horizon through the zenith as 
 far as the pole. Its construction was of the Newtonian kind, the 
 observer looking into the side of the tube near its upper end, 
 which a series of galleries and sliding stages enabled him to reach 
 
 1 Lord Rosse in Phil Trans., vol. cxl., p. 302. a This method is the same 
 in principle with that applied by Grubb in 1834 to a 1 5-inch speculum for the 
 observatory of Armagh. Phil. Trans., vol. clix., p. 145. 3 Robinson, Proc* 
 Roy. Ir. Ac., vol. iii., p. 120. 
 
 IO 
 
146 HISTORY OF ASTRONOMY. PART i. 
 
 in any position. It has also, though rarely, been used without a 
 second mirror, as a " Herschelian " reflector. 
 
 The splendour of the celestial objects as viewed with this vast 
 " light-grasper" surpassed all expectation. " Never in my life," 
 exclaimed Sir James South, " 'did I see such glorious sidereal 
 pictures ! " l The orb of Jupiter produced an effect compared 
 to that of the introduction of a coach-lamp into the telescope ; 2 
 and certain star-clusters exhibited an appearance (we again 
 quote Sir James South) " such as man before had never seen, 
 and which for its magnificence baffles all description." But it 
 was in the examination of the nebulae that the superiority of the 
 new instrument was most strikingly displayed. A large number 
 of these misty objects, which the utmost powers of Herschel's 
 specula had failed to resolve into stars, yielded at once to the 
 Parsonstown reflector; while many others were, so to speak, 
 metamorphosed through the disclosure of previously unseen 
 details of structure. 
 
 One extremely curious result of the increase of light was the 
 abolition of the distinction between the two classes of " annular " 
 and "planetary" nebulae. Up to that time, only four ring- 
 shaped systems two in the northern and two in the southern 
 hemisphere were known to astronomers ; they were now 
 reinforced by five of the planetary kind, the discs of which were 
 observed to be centrally perforated ; while the sharp marginal 
 definition visible in weaker instruments was replaced by ragged 
 edges or filamentous fringes. 
 
 Still more striking was the discovery of an entirely new and 
 highly remarkable species of nebulae. These were termed 
 " spiral," from the more or less regular convolutions, resembling 
 the whorls of a shell, in which the matter composing them 
 appeared to be distributed. The first and most conspicuous 
 specimen of this class was met with in April 1845 ; it is situated 
 in Canes Venatici, close to the tail of the Great Bear, and wore, 
 in Sir J. Herschel's instruments, the aspect of a split ring en- 
 compassing a bright nucleus, thus presenting, as he supposed, a 
 complete analogue to the system of the Milky Way. In the 
 
 1 Astr. Nacli.. No. 536. - Airy, Month. Not., vol. ix., p. '120.' 
 
CHAP. vi. INSTRUMENTAL ADVANCES. 147 
 
 Bosse mirror it shone out as a vast whirlpool of light a 
 stupendous witness to the presence of cosmical activities on the 
 grandest scale,* yet regulated by laws as to the nature of which 
 we are profoundly ignorant. Professor Stephen Alexander 
 of New Jersey, however, concluded, from an investigation 
 (necessarily founded on highly precarious data) of the mechani- 
 cal condition of these extraordinary agglomerations, that we see 
 in them " the partially scattered fragments of enormous masses 
 once rotating in a state of dynamical equilibrium." He further 
 suggested "that the separation of these fragments may still be 
 in progress," 1 and traced back their origin to the disruption, 
 through its own continually accelerated rotation, of a " primitive 
 spheroid " of inconceivably vast dimensions. Such also, it was 
 added (the curvilinear form of certain outliers of the Milky 
 Way, giving evidence of a spiral structure), is probably the 
 history of our own cluster ; the stars composing which, no 
 longer held together in a delicately adjusted system like that of 
 the sun and planets, are advancing through a period of seeming 
 confusion towards an appointed goal of higher order and more 
 perfect and harmonious adaptation. 2 
 
 The class of spiral nebulae included, in 1850, fourteen mem- 
 bers, besides several in which the characteristic arrangement 
 seemed partial or dubious. 3 A tendency in the exterior stars of 
 many clusters to gather (as in our Galaxy) into curved branches 
 was likewise noted ; and the existence of unsuspected analogies 
 was proclaimed by the significant combination in the "Owl" 
 nebulas (a large planetary in Ursa Major) 4 of the twisted forms 
 of a spiral with the perforation distinctive of an annular nebula. 
 
 Once more, by the achievements of the Parsonstown reflector, 
 the supposition of a " shining fluid" filling vast regions of space 
 was brought into (as it has since proved) undeserved discredit. 
 Although Lord Rosse himself rejected the inference, that because 
 many nebulas had been resolved, all were resolvable, very few 
 
 1 Astronomical Journal (Gould's), vol. ii., p. 97. ' 2 JbicL, vol. ii., p. 160. 
 3 Lord Kosse in Phil. Trans., vol. cxl., p. 505. 4 No. 2343 of Herschel's 
 (1864) Catalogue. Before 1850 a star marked each of the two larger openings 
 by which it is pierced ; since then one only has been seen. Webb, Celestial 
 Objects (4th ed.), p. 409. 
 
148 HISTORY OF ASTRONOMY. PARTI. 
 
 imitated Ms truly scientific caution ; and the results of Bond's 
 investigations 1 with the Harvard College refractor quickened 
 and strengthened the current of prevalent opinion. It is now 
 certain that the evidence furnished on both sides of the Atlantic 
 as to the stellar composition of some conspicuous objects of this 
 class, notably the Orion and " Dumb-bell " nebulae, was delusive ; 
 but the spectroscope alone was capable of peeting it with a 
 categorical denial. Meanwhile there seemed good ground for 
 the persuasion, which now, for the last time, gained the upper 
 hand, that nebulae are, without exception, true " island-universes," 
 or assemblages of distant suns. 
 
 Lord Eosse's telescope possesses a nominal power of 6000 
 that is, it shows the moon as if viewed with the naked eye at a 
 distance of forty miles. But this seeming advantage is neutral- 
 ised by the weakening of the available light through excessive 
 diffusion, as well as by the troubles of the surging sea of air 
 through which the observation must necessarily be made. 
 Professor Newcomb, in fact, doubts whether with any telescope 
 our satellite has ever been seen to such advantage as it would be 
 if brought within 500 miles of the unarmed eye. 2 
 
 The French opticians' rule of doubling the number of milli- 
 metres contained in the aperture of an instrument to find the 
 highest magnifying power usefully applicable to it, would give 
 3600 as the maximum for the leviathan of Birr Castle ; but in a 
 climate like that of Ireland the occasions must be rare when even 
 that limit can be reached. Indeed, the experience acquired by 
 its use plainly shows that atmospheric rather than mechanical 
 difficulties impede a still further increase of telescopic power. 
 Its construction may accordingly be said to mark the ne plus 
 ultra of effort in one direction, and the beginning of its conver- 
 sion towards another. It became thenceforward more and more 
 obvious that the conditions of observation must be ameliorated 
 before any added efficacy could be given to it. The full effect of 
 an uncertain climate in nullifying optical improvements was 
 recognised, and the attention of astronomers began to be turned 
 
 1 Mem. Am. Ac., vol. iii., p. 87 : and Astr. Nad., No. 611. 2 Pop. Astr., 
 p. 145- 
 
CHAP. vi. INSTRUMENTAL ADVANCES. 149 
 
 towards the advantages offered by more tranquil and more 
 translucent skies. 
 
 Scarcely less important for the practical uses of astronomy 
 than the optical qualities of the telescope is the manner of its 
 mounting. The most admirable performance of the optician can 
 render but unsatisfactory service if its mechanical accessories are 
 ill-arranged or inconvenient. Thus the astronomer is ultimately 
 dependent upon the mechanician ; and so excellently have his 
 needs been served, that the history of the ingenious contrivances 
 by which discoveries have been prepared would supply a subject 
 (here barely glanced at) not far inferior in extent and instruction 
 to the history of those discoveries themselves. 
 
 There are two chief modes of using the telescope, to which all 
 others may be considered subordinate. 1 Either it may be in- 
 variably directed towards the south, with no motion save in the 
 plane of the meridian, so as to intercept the heavenly bodies at 
 the moment of transit across that plane ; or it may be arranged 
 so as to follow the daily revolution of the sky, thus keeping the 
 object viewed permanently in sight, instead of simply noting the 
 instant of its flitting across the telescopic field. The first plan 
 is that of the "transit instrument," the second that of the 
 " equatoreal." Both were, by a remarkable coincidence, intro- 
 duced about i69O 2 by Olaus Komer, the brilliant Danish astro- 
 nomer who first measured the velocity of light. 
 
 The uses of each are entirely different. With the transit, 
 the really fundamental task of astronomy the determination 
 of the movements of the heavenly bodies is mainly accom- 
 plished; while the investigation of their nature and peculi- 
 arities is best conducted with the equatoreal. One is the 
 
 1 This statement must be taken in the most general sense. Supplement- 
 ary observations of great value are now made at Greenwich with the alti- 
 tude and azimuth instrument, which likewise served Piazzi to determine the 
 places of his stars ; while a " prime vertical instrument " is prominent at 
 Pulkowa. 2 As early as 1620, according to R. Wolf (Gasch. der Astr., p. 587)* 
 Father Scheiner made the experiment of connecting a telescope with an axis 
 directed to the pole, while Chinese " equatoreal armillas," dating from the 
 thirteenth century, still exist at Pekin. J. L. E. Dreyer, Copernicus, vol. i., 
 P. 134- 
 
150 HISTORY OF ASTRONOMY. PARTI- 
 
 instrument of mathematical, the other of descriptive astronomy. 
 One furnishes the materials with which theories are constructed, 
 and the tests by which they are corrected ; the other registers 
 new facts, takes note of new appearances, sounds the depths and 
 pries into every nook of the heavens. 
 
 The great improvement of giving to a telescope equatoreally 
 mounted an automatic movement by connecting it with clock- 
 work, was proposed in 1674 by Eobert Hooket- Bradley in 1721 
 actually observed Mars with a telescope "moved by a machine 
 that made it keep pace with the stars ; >?1 and Von Zach relates 2 
 that he had once followed Sirius for twelve hours with a 
 " heliostat " of Kamsden's construction. But these eighteenth- 
 century attempts were of no practical effect. Movement by 
 clockwork was virtually a complete novelty when it was adapted 
 by Fraunhofer in 1824 to the Dorpat refractor. By simply 
 giving to an axis unvaryingly directed towards the celestial pole 
 an equable rotation with a period of twenty-four hours, a tele- 
 scope attached to it, and pointed in any direction, will trace out 
 on the sky a parallel of declination, thus necessarily accom- 
 panying the movement of any star upon which it may be fixed. 
 It thus forms part of the large sum of Fraunhofer's merits to 
 have secured this inestimable advantage to observers. 
 
 It was considered by Sir John Herschel that Lassell's applica- 
 tion of equatoreal mounting to a nine-inch Newtonian in 1 840 
 made an epoch in the history of "that eminently British instru- 
 ment, the reflecting telescope." 3 Nearly a century earlier, 4 it is 
 true, Short had fitted one of his Gregorians to a complicated 
 system of circles in such a manner that, by moving a handle, it 
 could be made to follow the revolution of the sky; but the 
 arrangement did not obtain, nor did it deserve, general adoption. 
 Lassell's plan was a totally different one ; he employed the 
 crossed axes of the true equatoreal, and his success removed, to 
 a great extent, the fatal objection of inconvenience in use, until 
 then unanswerably urged against reflectors. The very largest 
 of these can now be mounted equatoreally ; even the Rosse 
 
 1 Miscellaneous Works, p. 350. - Astr. Jolirluclt, 1799 (published 1796), 
 p. 115. 3 Month. Not., vol. xli., p. 189. 4 Phil. Trans., vol. xlvi., p. 242. 
 
CHAP. vi. INSTRUMENTAL ADVANCES. 151 
 
 was eventually provided with a movement by clockwork within 
 its limited range along declination-parallels. 
 
 The art of accurately dividing circular arcs into the minute 
 equal parts which serve as the units of astronomical measure- 
 ment, remained, during the whole of the eighteenth century, 
 almost exclusively in English hands. It was brought to a high 
 degree of perfection by Graham, Bird and Eamsden, all of whom, 
 however, gave the preference to the old-fashioned mural quad- 
 rant and zenith sector over the entire circle, which Komer had 
 already found the advantage of employing. The five-foot 
 vertical circle, which Piazzi with some difficulty induced Kams- 
 den to complete for him in 1789, was the first divided instru- 
 ment constructed in what may be called the modern style. It 
 was provided with magnifiers for reading off the divisions (one 
 of the neglected improvements of Komer), and was set up above 
 a smaller horizontal circle, forming an " altitude and azimuth " 
 combination (again Komer's invention), by which both the ele- 
 vation of a celestial object above the horizon, and its position as 
 referred to the horizon could be measured. In the same year 
 Borda invented the " repeating circle " (the principle of which 
 had been suggested by Tobias Mayer in 1756 *), a device for ex- 
 terminating, so far as possible, errors of graduation by repeating 
 an observation with different parts of the limb. This was 
 perhaps the earliest systematic effort to correct the imperfections 
 of instruments by the manner of their use. 
 
 The manufacture of astronomical circles was brought to a 
 very refined state of excellence early in the present century by 
 Eeichenbach at Munich, and after 1 8 1 8 by Eepsold at Hamburg. 
 Bessel states 2 that the " reading-off " on an instrument of the 
 kind by the latter artist was accurate to about -^th of a human 
 hair. Meanwhile the traditional reputation of the English 
 school was fully sustained ; and Sir George Airy did not hesitate 
 to express his opinion that the new method of graduating 
 circles, published by Troughton in iSop, 3 was tne "greatest 
 improvement ever made in the art of instrument-making." 4 
 
 1 Grant, Hist, of Astr., p. 487. - Pop. VorL, p. 546. 3 Phil. Trans., vol. 
 xcix., p. 105. 4 fieport Brit. Ass., 1832, p. 132. 
 
152 HISTORY OF ASTRONOMY. PARTI. 
 
 But a more secure road to improvement than that of mere 
 mechanical exactness was pointed out by Bessel. His introduc- 
 tion of a regular theory of instrumental errors might almost be 
 said to have created a new art of observation. Every instru- 
 ment, he declared in memorable words, 1 must be twice made 
 once by the artist, and again by the observer. Knowledge is 
 power. Defects that are ascertained and can be allowed for are 
 as good as non-existent. Thus the truism that the best 
 instrument is worthless in the hands of a careless or clumsy 
 observer, became supplemented by the converse maxim, that 
 defective appliances may, by skilful use, be made to yield 
 valuable results. The Konigsberg observations of which the 
 first instalment was published in 1815 set the example of 
 regular " reduction " for instrumental errors. Since then, it has 
 become an elementary part of an astronomer's duty to study the 
 idiosyncrasy of each one of the mechanical contrivances at his 
 disposal, in order that its inevitable, but now certified deviations 
 from ideal accuracy may be included amongst the numerous 
 corrections by which the pure essence of even approximate truth 
 is distilled from the rude impressions of sense. 
 
 Nor is this enough ; for the casual circumstances attending 
 each observation have to be taken into account with no less 
 care than the inherent or constitutional peculiarities of the 
 instrument with which it is made. There is no " once for all " 
 in astronomy. Vigilance can never sleep ; patience can never 
 tire. Variable as well as constant sources of error must 
 be anxiously heeded; one infinitesimal inaccuracy must be 
 weighed against another; all the forces and vicissitudes of 
 nature frosts, dews, winds, the interchanges of heat, the dis- 
 turbing effects of gravity, the shiverings of the air, the tremors 
 of the earth, the weight and vital warmth of the observer's 
 own body, nay, the rate at which his brain receives and trans- 
 mits its impressions, must all enter into his calculations, and 
 be sifted out from his results. 
 
 It was in 1823 that Bessel drew attention to discrepancies 
 
 1 Pop. Vorl., p. 432. 
 
CHAP. vi. INSTRUMENTAL ADVANCES. 153 
 
 in the times of transits given by different astronomers. 1 The 
 quantities involved were far from insignificant. He was him- 
 self nearly a second in advance of all his contemporaries, 
 Argelander lagging behind him as much as a second and a 
 quarter. Each individual, in fact, was found to have a certain 
 definite rale of perception, which, under the name of "personal 
 equation," now forms so important an element in the correction 
 of observations that a special instrument for accurately deter- 
 mining its amount in each case is in actual use at Greenwich. 
 
 Such are the refinements upon which modern astronomy 
 depends for its progress. It -is a science of hairbreadths and 
 fractions of a second. It exists only by the rigid enforcement 
 of arduous accuracy and unwearying diligence. Whatever 
 secrets the universe still has in store for man will only be 
 communicated on these terms. They are, it must be acknow- 
 ledged, difficult to comply with. They involve an unceasing 
 struggle against the infirmities of his nature and the insta- 
 bilities of his position. But the end is not unworthy the 
 sacrifices demanded. One additional ray of light thrown on 
 the marvels of creation a single, minutest encroachment upon 
 the strongholds of ignorance is recompense enough for a lifetime 
 of toil. Or rather, the toil is its own reward, if pursued in the 
 lofty spirit which alone becomes it. For it leads through the 
 abysses of space and the unending vistas of time to the very 
 threshold of that infinity and eternity of which the disclosure is 
 reserved for a life to come. 
 
 1 C. T. Anger, Grundzuge der neueren astronomischen Iteobachtungs-Kunst, 
 P-3- 
 
PART II. 
 
 i 
 
 RECENT PROGRESS OF ASTRONOMY. 
 
 CHAPTER I. 
 FOUNDATION OF ASTRONOMICAL PHYSICS. 
 
 IN the year 1826, Heinrich Scliwabe of Dessau, elated with the 
 hope of speedily delivering himself from the hereditary incubus 
 of an apothecary's shop, 1 obtained from Munich a small telescope 
 and began to observe the sun. His choice of an object for his 
 researches was instigated by his friend Harding of Gottingeii. 
 It was a peculiarly happy one. The changes visible in the solar 
 surface were then generally regarded as no less capricious than 
 the changes in the skies of our temperate regions. Consequently, 
 the reckoning and registering of sun-spots was a task hardly 
 more inviting to an astronomer than the reckoning and regis- 
 tering of summer clouds. Cassini, Keill, Lemonnier, Lalande, 
 were unanimous in declaring that no trace of regularity could 
 be detected in their appearances or eifacements. 2 Delambre 
 pronounced them "more curious than really useful." 3 Even 
 Herschel, profoundly as he studied them, and intimately as he 
 was convinced of their importance as symptoms of solar activity, 
 saw no reason to suspect that .their abundance and scarcity 
 were subject to orderly alternation. One man alone in the 
 eighteenth century, Christian Horrebow of Copenhagen, divined 
 their periodical character, and foresaw the time when the effects 
 of the sun's vicissitudes upon the globes revolving round him 
 
 - Wolf, Gesch. tier Astr., p. 655. - Manuel Johnson, Mem. B. A. P., vol. 
 xxvi., p. 197. 3 Astronomic Theoriqne it Pratique, t. iii., p. 20. 
 
156 HISTORY OF ASTRONOMY. PART n. 
 
 might be investigated with success ; but this prophetic utterance 
 was of the nature of a soliloquy rather than of a communica- 
 tion, and remained hidden away in an unpublished journal until 
 1859, when it was .brought to light in a general ransacking of 
 archives. 1 
 
 Indeed, Schwabe himself was far from anticipating the dis- 
 covery which fell to his share. He compared his fortune to that 
 of Saul, who, seeking his father's asses, found a kingdom. 2 For 
 the hope which inspired his early resolution lay in quite another 
 direction. His patient ambush was laid for a possible intra- 
 mercurial planet, which, he thought, must sooner or later betray 
 its existence in crossing the face of the sun. He took, however, 
 the most effectual measures to secure whatever new knowledge 
 might be accessible. During forty-three years his " impertur- 
 bable telescope " 3 never failed (weather and health permitting) 
 to bring in its daily report as to how many, or if any, spots 
 were visible on the sun's disc, the information obtained being 
 day by day recorded on a simple and unvarying system. In 
 1843 ne made his first announcement of a probable decennial 
 period, 4 but it met with no general attention ; although Julius 
 Schmidt of Bonn (afterwards director of the Athens Observatory), 
 and Gautier of Geneva were impressed with his figures, and 
 Littrow had himself, in i836, 5 hinted at the likelihood of some 
 kind of regular recurrence. Schwabe, however, worked on, 
 gathering each year fresh evidence of a law such as he had in- 
 dicated ; and when Humboldt published in 1851, in the third 
 volume of his Kosmos* a table of the sun-spot statistics collected 
 by him from 1826 downwards, the strength of his case was per- 
 ceived with, so to speak, a start of surprise; the reality and 
 importance of the discovery were simultaneously recognised, 
 and the persevering Hofrath of Dessau found himself famous 
 among astronomers. His merit recognised by the bestowal of 
 the Astronomical Society's Gold Medal in 1857 consisted in 
 his choice of an original and appropriate line of work, and in the 
 
 1 Wolf, Gesch. der Astr., p. 654. ~ Month. Xot., vol. xvii., p. 241. 3 Mem. 
 JR. A. /&, vol. xxvi., p. 200. 4 Astr. Nach., No. 495. 5 Gehler's Physika- 
 lisches Worterbuch, art. Sonnwflecken, p. 851. ti Ziceite Abth., p. 401. 
 
CHAP. i. ASTRONOMICAL PHYSICS. 157 
 
 admirable tenacity of purpose with which he pursued it. His 
 resources and acquirements were those of an ordinary amateur ; 
 lie was distinguished solely by the unfortunately rare power of 
 turning both to the best account. He died where he was born 
 and had lived, April n, 1875, at the ripe age of eighty-six. 
 
 Meanwhile, an investigation of a totally different character, 
 and conducted by totally different means, had been prosecuted 
 to a very similar conclusion. Two years after Schwabe began 
 his solitary observations, Humboldt gave the first impulse, at 
 the Scientific Congress of Berlin in 1828, to a great inter- 
 national movement for attacking simultaneously, in various 
 parts of the globe, the complex problem of terrestrial magnetism. 
 Through the genius and energy of Gauss. Gottingen became its 
 centre. Thence new apparatus, and a new system for its employ- 
 ment, issued; there, in 1833, the first regular magnetic observa- 
 tory was founded, whilst at Gottingen was fixed the universal 
 time-standard for magnetic observations. The letter addressed 
 by Humboldt in April 1836 to the Duke of Sussex as President 
 of the Royal Society, enlisted the co-operation of England. 
 A network of magnetic stations was spread all over the 
 British dominions, from Canada to Van Diemen's Land; 
 measures were concerted with foreign authorities, and an 
 expedition was fitted out, under the able command of Captain 
 (afterwards Sir James) Clark Boss, for the special purpose of 
 bringing intelligence on the subject from the dismal neigh- 
 bourhood of the South Pole. In 1841, the elaborate organisa- 
 tion created by the disinterested efforts of scientific "agitators " 
 was complete; Gauss's "magnetometers" were vibrating tinder 
 the view of attentive observers in five continents, and simul- 
 taneous results began to be recorded. 
 
 Ten years later, in September 1851, Dr. John Lamont, the 
 Scotch director of the Munich Observatory, in reviewing the 
 magnetic observations made at Gottingen and Munich from 
 1835 to 1850, perceived with some surprise that they gave 
 unmistakable indications of a period which he estimated at lOg 
 years. 1 The manner in which this periodicity manifested itself 
 
 1 Annalen der Physik (Poggendorff's), Bd. Ixxxiv., p. 580. 
 
158 HISTORY OF ASTRONOMY. PART n. 
 
 requires a word of explanation. The observations in question 
 referred to what is called the " declination " of the magnetic 
 needle that is, to the position assumed by it with reference to 
 the points of the compass when moving freely in a horizontal 
 plane. Now this position as was discovered by Graham in 
 1722 is subject to a small daily fluctuation, attaining its 
 maximum towards the east about 8 A;M., and its maximum 
 towards the west shortly before 2 P.M. In other words, the 
 direction of the needle approaches (in these countries at the 
 present time) nearest to the true north some four hours before 
 noon, and departs furthest from it between one and two hours 
 after noon. It was the range of this daily variation that Lament 
 found to increase and diminish once in every ioj years. 
 
 In the following winter, Sir Edward Sabine, ignorant as yet 
 of Lament's conclusion, undertook to examine a totally different 
 set of observations. The materials in his hands had been 
 collected at the British colonial stations of Toronto and Hobar- 
 ton from 1843 to 1848, and had reference, not to the regular 
 diurnal swing of the needle, but to those curious spasmodic 
 vibrations, the inquiry into the laws of which was the primary 
 object of the vast organisation set on foot by Humboldt and 
 Gauss. Yet the upshot was practically the same. Once in 
 about ten years magnetic disturbances (termed by Humboldt 
 "storms") were perceived to reach a maximum of violence and 
 frequency. Sabine was the first to note the coincidence be- 
 tween this unlooked-for result and Schwabe's sun-spot period. 
 He showed that, so far as observation had yet gone, the two 
 cycles of change agreed perfectly both in duration and phase, 
 maximum corresponding to maximum, minimum to minimum. 
 What the nature of the connection could be that bound together 
 by a common law effects so dissimilar as the rents in the lumin- 
 ous garment of the sun, and the swayings to and fro of the 
 magnetic needle, was, and still remains, beyond the reach of well- 
 founded conjecture ; but the fact was from the first undeniable. 
 
 The memoir containing this remarkable disclosure was pre- 
 santed to the Koyal Society, March 18, and read May 6, 1852. 1 
 1 Phil. Trans., vol. cxlii., p. 103. 
 
CHAP. i. ASTRONOMICAL PHYSICS. 159 
 
 On the 3 1st of July following, Rudolf Wolf at Berre, 1 and on 
 the 1 8th of August, Alfred Gautier at Sion, 2 announced, separately 
 and independently, perfectly similar conclusions. This triple 
 event is perhaps the most striking instance of the successful 
 employment of the Baconian method of co-operation in dis- 
 covery, by which "particulars" are amassed by one set of 
 investigators corresponding to the " Depredators " and "Inocu- 
 lators" of Solomon's House while inductions are drawn from 
 them by another and a higher class the "Interpreters of 
 Nature." Yet even here the convergence of two distinct lines of 
 research was wholly fortuitous, and skilful combination owed the 
 most brilliant part of its success to the unsought bounty of what 
 we call Fortune. 
 
 The exactness of the coincidence thus brought to light was 
 fully confirmed by further inquiries. A diligent search through 
 the scattered records of sun-spot observations, from the time of 
 Galileo and Scheiner onwards, put Wolf 3 in possession of 
 materials by which he was enabled to correct Schwabe's loosely- 
 indicated decennial period to one of slightly over eleven (ii.n) 
 years ; and he further showed that this fell in with the ebb and 
 flow of magnetic change even better than Lament's loj-year 
 cycle. The analogy was also pointed out between the " light- 
 curve," or zig-zagged line representing on paper the varying 
 intensity in the lustre of certain stars, and the similar delinea- 
 tion of spot-frequency ; the ascent from minimum to maximum 
 being, in both cases, usually steeper than the descent from 
 maximum to minimum; while an additional point of resem- 
 blance was furnished by the irregularities in height of the 
 various maxima. In other words, both the number of spots on 
 the sun and the brightness of variable stars increase, as a rule, 
 more rapidly than they decrease ; nor does the amount of that 
 increase, in either instance, show any approach to uniformity. 
 
 The endeavour, suggested by the very nature of the pheno- 
 menon, to connect sun-spots with weather was less successful. 
 
 1 Mitthettunyen der Naturforschenden Gesellscliaft, 1852, p. 183. 2 Archives 
 des Sconces, t. xxi., p. 194. 3 Neue Untersuchungen, Mitth. Naturf. Ges. 
 1852, p. 249. 
 
160 HISTORY OF ASTRONOMY. PART n. 
 
 The earliest attempt of the kind was made by Sir William Herschel 
 in the first year of the present century, and a very notable one 
 it was. Meteorological statistics, save of the scantiest and most 
 casual kind, did not then exist ; but the price of corn from year 
 to year was on record, and tnis, with full recognition of its 
 inadequacy, he adopted as his criterion. Nor was he much better 
 off for information respecting the solar condition. "What little 
 he could obtain, however, served, as he believed, to confirm his 
 surmise that a copious emission of light and heat accompanies 
 an abundant formation of "openings" in the dazzling substance 
 whence our supply of those indispensable commodities is derived. 1 
 He gathered, in short, from his inquiries very much what he had 
 expected to gather, namely, that the price of wheat was high 
 when the sun showed an unsullied surface, and that food and 
 spots became plentiful together. 2 
 
 This plausible inference, however, was scarcely borne out by a 
 more exact collocation of facts. Schwabe failed to detect any 
 reflection of the sun-spot period in his meteorological register. 
 Gautier 3 reached a provisional conclusion the reverse though 
 not markedly the reverse of Herschel's. Wolf, in 1852, derived 
 from an examination of Vogel's collection of Zurich Chronicles 
 (1000-1800 A.D.) evidence showing (as he thought) that minimum 
 years were usually wet and stormy, maximum years dry and 
 genial; 4 but a subsequent review of the subject in 1859 con- 
 vinced him that no relation of any kind between the two classes of 
 phenomena was traceable. 5 With the singular affection of our at- 
 mosphere known as the Aurora Borealis (more properly Aurora 
 Polaris) the case was different. Here the Zurich Chronicles set 
 Wolf on the right track in leading him to associate such lumi- 
 nous manifestations with a disturbed condition of the sun ; since 
 
 1 Phil. Trans., vol. xci., p. 316. - Evidence of an eleven-yearly fluctuation 
 in the price of food-grains in India was in 1886 alleged by Mr. Frederick 
 Chambers in Nature, vol. xxxiv., p. 100. 3 Bibl. Un. de Geneve, t. li., p. 336. 
 4 Neue Untvrsuchungen, p. 269. 5 Die Sonne und ihre Flecken, p. 30. Arago 
 was the first who attempted to decide the question by keeping, through a 
 series of years, a parallel register of sun-spots and weather ; but the data 
 regarding the solar condition collected at the Paris Observatory from 1822 to 
 1830 were not sufficiently precise to found any inference upon. 
 
CHAP. i. ASTRONOMICAL PHYSICS. 161 
 
 subsequent detailed observation has exhibited the curve of 
 auroral frequency as following with such fidelity the jagged lines 
 figuring to the eye the fluctuations of solar and magnetic 
 activity, as to leave no reasonable doubt that all three rise and 
 sink together under the influence of a common cause. As long- 
 ago as I/I6, 1 Halley had conjectured that the Northern Lights 
 were due to magnetic " effluvia," but there was no evidence on 
 the subject forthcoming until Hiorter observed at Upsalain 1741 
 their agitating influence upon the magnetic needle. That the 
 effect was no casual one was made superabundantly clear by 
 Arago's researches in 1819 and subsequent years. Now both 
 were perceived to be swayed by the same obscure power of 
 cosmical disturbance. 
 
 The sun is not the only one of the heavenly bodies by which 
 the magnetism of the earth is affected. Proofs of a similar kind 
 of lunar action were laid by Kreil in 1841 before the Bohemian 
 Society of Sciences, and were fully substantiated, though with 
 minor corrections, by Sabine's more extended researches. It 
 has thus been ascertained that each lunar day, or the interval of 
 twenty-four hours and about fifty-four minutes between two 
 successive meridian passages of our satellite, is marked by a 
 perceptible, though very small double oscillation of the needle 
 two progressive movements from east to west, and two returns 
 from west to east. 2 Moreover, the lunar, like the solar 
 influence (as was proved in each case by Sabine's analysis of 
 the Hobarton and Toronto observations), extends to all three 
 " magnetic elements," affecting not only the position of the 
 horizontal or declination needle, but also the dip and intensity. 
 It seems not unreasonable to attribute some portion of the 
 same subtle power to the planets, and even to the stars, though 
 with effects rendered imperceptible by distance. 
 
 We have now to speak of the discovery and application to 
 the heavenly bodies of a totally new method of investigation. 
 Spectrum analysis may be shortly described as a mode of distin- 
 guishing the various species of matter by the kind of light 
 
 1 Phil. Trans., vol. xxix., p. 421. " Phil. Trans., vol. cxliii., p. 558, and 
 vol. cxlvi., p. 505. 
 
 II 
 
162 HISTORY OF ASTRONOMY. PART n. 
 
 proceeding from each. This definition at once explains how it 
 is that, unlike every other system of chemical analysis, it has 
 proved available in astronomy. Light, so far as quality is con- 
 cerned, ignores distance. No intrinsic change, that we yet know 
 of, is produced in it by a journey from the farthest bounds of 
 the visible universe ; so that, provided only that in quantity it 
 remain sufficient for the purpose, its peculiarities can be equally 
 well studied whether the source of its vibrations be one foot or 
 a hundred billion miles distant. Now the most obvious distinc- 
 tion between one kind of light and another resides in colour. But 
 of this distinction the eye takes cognisance in an aesthetic, not 
 in a scientific sense. It finds gladness in the "thousand tints" 
 of nature, but can neither analyse nor define them. Here the 
 refracting prism or the combination of prisms known as the 
 " spectroscope " comes to its aid, teaching it to measure as well as 
 to perceive. It furnishes, in a word, an accurate scale of colour 
 The various fays which, entering the eye together in a confused 
 crowd, produce a compound impression made up of undistinguish- 
 able elements, are, by the mere passage through a triangular 
 piece of glass, separated one from the other, and ranged side by 
 side in orderly succession, so that it becomes possible to tell at a 
 glance what kinds of light are present, and what absent. Thus, 
 if we could only be assured that the various chemical substances 
 when made to glow by heat, emit characteristic rays rays, that 
 is, occupying a place in the spectrum reserved for them, and 
 for them only we should at once be in possession of a mode of 
 identifying such substances with the utmost readiness and cer- 
 tainty. This assurance, which forms the solid basis of spectrum 
 analysis, was obtained slowly and with difficulty. 
 
 The first to employ the prism in the examination of various 
 flames (for it is only in a state of vapour that matter emits dis- 
 tinctive light) was a young Scotchman named Thomas Melvill, 
 who died in 1753, at the age of twenty-seven. He studied the 
 spectrum of burning spirits, into which were introduced succes- 
 sively sal ammoniac, potash, alum, nitre, and sea-salt, and observed 
 the singular predominance, under almost all circumstances, of a 
 particular shade of yellow light, perfectly definite in its degree of 
 
CHAP. i. ASTRONOMICAL PHYSICS. 163 
 
 refraiigibility l in other words, taking up a perfectly definite 
 position in the spectrum. His experiments were repeated by 
 Morgan, 2 Wollaston, and with far superior precision and dili- 
 gence by Fraunhofer. 3 The great Munich optician, whose work 
 was completely original, rediscovered Melvill's deep yellow ray 
 and measured its place in the colour-scale. It has since become 
 well known as the " sodium line," and has played a very im- 
 portant part in the history of spectrum analysis. Nevertheless, 
 its ubiquity and conspicuousness long impeded progress. It was 
 elicited by the combustion of a surprising variety of substances 
 sulphur, alcohol, ivory, wood, paper; its persistent visibility 
 suggesting the accomplishment of some universal process 
 of nature rather than the presence of one individual kind of 
 matter. But if spectrum analysis were to exist as a science at 
 all, it could only be by attaining certainty as to the unvarying 
 association of one special substance with each special quality of 
 light. 
 
 Thus perplexed, Fox Talbot 4 hesitated in 1826 to enounce this 
 fundamental principle. He was inclined to believe that the 
 presence in the spectrum of any individual ray told unerringly 
 of the volatilisation in the flame under scrutiny of some body as 
 whose badge or distinctive symbol that ray might be regarded ; 
 but the continual prominence of the yellow beam staggered him. 
 It appeared, indeed, without fail where sodium was; but it also 
 appeared where it might be thought only reasonable to conclude 
 that sodium was not. Nor was it until thirty years later that 
 William Swan, 5 by pointing out the extreme delicacy of the 
 spectral test, and the singularly wide dispersion of sodium, made 
 it appear probable (but even then only probable) that the 
 questionable yellow line was really due invariably to that 
 substance. Common salt (chloride of sodium) is, in fact, the 
 most diffusive of solids. It floats in the air ; it flows with water ; 
 every grain of dust has its attendant particle ; its absolute 
 
 1 Observations on Light and Colours, p. 35. - Phil. Trans., vol. Ixxv., p. 190. 
 3 Denkschriften (Munich Ac. of So.) 1814-15, Bd. v., p. 197. 4 Edinburgh 
 Journal rf Science, vol. v., p. 77. See also Phil. Mag., Feb. 1834, vol. iv., 
 p. 112. 5 Ed. Phil. Trans., vol. xxi., p. 411. 
 
1 64 HISTORY OF ASTRONOMY. PART n. 
 
 exclusion approaches the impossible. And withal, the light that 
 it gives in burning is so intense and concentrated, that if a 
 single grain be divided into 1 80 million parts, and one alone of 
 such inconceivably minute fragments be present in a source of 
 light, the spectroscope will show unmistakably its characteristic 
 beam. 
 
 Among the pioneers of knowledge in this, direction were Sir 
 John Herschel 1 who, however, applied himself to the subject in 
 the interests of optics, not of chemistry W. A. Miller, 2 and 
 Wheatstone. The last especially made a notable advance when, 
 in the course of his studies on the "prismatic decomposition" of 
 the electric light, he reached the significant conclusion that the 
 rays visible in its spectrum were different for each kind of metal 
 employed as " electrodes." 3 Thus indications of a wider principle 
 were to be found in several quarters, but no positive certainty 011 
 any single point was obtained, until, in 1859, Gustav Kirchhoff, 
 professor of physics in the University of Heidelberg, and his 
 colleague, the eminent chemist Robert Bunsen, took the matter 
 in hand. By them the general question as to the necessary and 
 invariable connection of certain rays in the spectrum with 
 certain kinds of matter, was first resolutely confronted, and 
 first definitely answered. It was answered affirmatively else 
 there could have been no science of spectrum analysis as the 
 result of experiments more numerous, more stringent, and more 
 precise than had previously been undertaken. 4 And the 
 assurance of their conclusion was rendered doubly sure by the 
 discovery, through the peculiarities of their light alone, of two 
 new metals, named from the blue and red rays by which they 
 were respectively distinguished, "Caesium," and "Rubidium." 5 
 Both were immediately afterwards actually obtained in small 
 quantities by evaporation of the Durckheim mineral waters. 
 
 1 On the Absorption of Light by Coloured Media, Ed. Phil. Trans., vol. ix., 
 p. 445 (1823). 2 Phil. Mag., vol. xxvii. (ser. iii.), p. 81. 3 Report Brit. Ass. 
 1835, p. ii (pt. ii.). Electrodes are the terminals from one to the other of 
 which the electric spark passes, volatilising and rendering incandescent in its 
 transit some particles of their substance, the characteristic light of which 
 accordingly flashes out in the spectrum. 4 Phil. Mag., vol. xx., p. 93. 
 5 Annalen der Physik, Bd. cxiii.. p. 357. 
 
CHAP. i. ASTRONOMICAL PHYSICS. 165 
 
 The link connecting this important result with astronomy 
 may now be indicated. In the year 1802 William Hyde 
 Wollaston hit upon the idea of substituting for the round hole 
 used by Newton and his successors to admit light about 
 to be examined with the prism, an elongated "crevice" ~^th 
 of an inch in width. He thereupon perceived that the 
 spectrum, thus formed of light purified by the abolition of 
 overlapping images, was traversed by seven dark lines. These 
 he took to be natural boundaries of the various colours, 1 and 
 satisfied with this quasi-explanation, allowed the subject to 
 drop. It was independently taken up after twelve years by 
 a man of higher genius. In the course of experiments 
 on light, directed towards the perfecting of his achromatic 
 lenses, Fraunhofer, by means of a slit and a telescope, 
 made the surprising discovery that the solar spectrum is 
 crossed, not by seven, but by thousands of obscure trans- 
 verse streaks. 2 Of these he counted some 600, and carefully 
 mapped 324, while a few of the most conspicuous he set up 
 (if we may be permitted the expression) as landmarks, mea- 
 suring their distances apart with a theodolite, and affixing 
 to them the letters of the alphabet by which they are still 
 universally known. Nor did he stop here. The same system 
 of examination applied to the rest of the heavenly bodies 
 showed the mild effulgence of the moon and planets to be 
 deficient in precisely the same rays as sunlight; while in the 
 stars it disclosed the differences in likeness which are always 
 an earnest of increased knowledge. The spectra of Sirius and 
 Castor, instead of being delicately ruled crosswise throughout, 
 like that of the sun, were seen to be interrupted by three 
 massive bars of darkness two in the blue and one in the 
 green; 3 the light of Pollux, on the other hand, seemed pre- 
 cisely similar to sunlight attenuated by distance or reflection, 
 and that of Capella, Betelgeux, and Procyon to share some 
 of its peculiarities. One solar line especially that marked 
 in his map with the letter D proved common to all the four 
 
 1 Phil. Trans., vol. xcii., p. 378. ' 2 DenJcschriften, Bd. v., p. 202. 3 Ibid., 
 p. 220 ; Edin. Jour, of Science, vol. viii., p. . 
 
1 66 HISTORY OF ASTRONOMY. PART lu 
 
 last-mentioned stars ; and it was remarkable that it exactly 
 coincided in position with the conspicuous yellow beam (after- 
 wards, as we have said, identified with the light of glowing 
 sodium) which he had already found to accompany most kinds 
 of combustion. Moreover, bo*n the dark solar and the bright 
 terrestrial "D-lines" were displayed by the refined Munich 
 appliances as double. 
 
 In this striking correspondence, discovered by Fraunhofer 
 in 1815, was contained the very essence of solar chemistry: 
 but its true significance did not become apparent until long 
 afterwards. Fraunhofer was by profession, not a physicist, 
 but a practical optician. Time pressed; he could not and 
 would not deviate from his appointed track ; all that was 
 possible to him was to indicate the road to discovery, and exhort 
 others to follow it. 1 
 
 Partially and inconclusively at first this was done. The 
 "fixed lines" (as they were called) of the solar spectrum took 
 up the position of a standing problem, to the solution of 
 which no approach seemed possible. Conjectures as to their 
 origin were indeed rife. An explanation put forward by 
 Zantedeschi 2 and others, and dubiously favoured by Sir David 
 Brewster and Dr. J. H. Gladstone, 3 was that they resulted from 
 " interference " that is, a destruction of the motion producing 
 in our eyes the sensation of light, by the superposition of two 
 light-waves in such a manner that the crests of one exactly 
 fill up the hollows of the other. This effect was supposed to 
 be brought about by imperfections in the optical apparatus 
 employed. 
 
 A more plausible view was that the atmosphere of the earth 
 was the agent by which sunlight was deprived of its missing* 
 beams. For some of them, this is actually the case. Brewster 
 found in 1832 that certain dark lines, which were invisible 
 when the sun stood high in the heavens, became increasingly 
 conspicuous as he approached the horizon. 4 These are the 
 well-known " atmospheric lines ; " but the immense majority of 
 
 1 Denkschriften, Bd. v., p. 222. - Arch, cles Sciences, 1849, p. 43. :>> Phil. 
 Trans., vol. cl., p. 159, note. 4 Ed. Phil. Trans., vol. xii., p. 528. 
 
CHAP. i. ASTRONOMICAL PHYSICS. 167 
 
 their companions in the spectrum remain quite unaffected by 
 the thickness of the stratum of air traversed by the sunlight 
 containing them. They are then obviously due to another cause. 
 
 There remained the true interpretation absorption in the 
 sun's atmosphere ; and this, too, was extensively canvassed. 
 But a remarkable observation made by Professor Forbes of 
 Edinburgh 1 on the occasion of the annular eclipse of May 15, 
 1836, appeared to throw discredit upon it. If the problematical 
 dark lines were really occasioned by the stoppage of certain 
 rays through the action of a vaporous envelope surrounding 
 the sun, they ought, it seemed, to be strongest in light pro- 
 ceeding from his edges, which, cutting that envelope obliquely, 
 passed through a much greater depth of it. But the circle 
 of light left by the interposing moon, and of course derived 
 entirely from the rim of the solar disc, yielded to Forbes's 
 examination precisely the same spectrum as light coming from 
 its central parts. This circumstance helped to baffle inquirers, 
 already sufficiently perplexed. It still remains an anomaly, 
 of which no completely satisfactory explanation has been 
 offered. 
 
 Convincing evidence as to the true nature of the solar lines 
 was however at length, in the autumn of 1859, brought for- 
 ward at Heidelberg. Kirchhoff's cxperimentum crucis in the 
 matter was a very simple one. He threw bright sunshine 
 across a space occupied by vapour of sodium, and perceived 
 with astonishment that the dark Fraunhofer line D, instead 
 of being effaced by flame giving a luminous ray of the same 
 refrangibility, was deepened and thickened by the superposition. 
 
 He tried the same experiment, substituting for sunbeams 
 light from a Drummond lamp, and with similar result. A 
 dark furrow, corresponding in every respect to the solar D- 
 line, was instantly seen to interrupt the otherwise unbroken 
 radiance of its spectrum. The inference was irresistible, that 
 
 1 Phil. Trans., vol. cxxvi., p. 453. "I conceive," he says, "that this result 
 proves decisively that the sun's atmosphere has nothing to do with the pro- 
 duction of this singular phenomenon " (p. 455). And Brewster's well-founded 
 opinion that it had much to do with it was thereby, in fact, overthrown. 
 
168 HISTORY OF ASTRONOMY. PART n. 
 
 the effect thus produced artificially was brought about naturally 
 in the same way, and that sodium formed an ingredient in the 
 glowing atmosphere of the sun. 1 This first discovery was 
 quickly followed up by the identification of numerous bright 
 rays in the spectra of other metallic bodies with others of the 
 hitherto mysterious Fraunhofer lines. Kirchhoff was thus led 
 to the conclusion that (besides sodium) iron, magnesium, 
 calcium, and chromium, are certainly solar constituents, 
 and that copper, zinc, barium, and nickel are also present, 
 though in smaller quantities. 2 As to cobalt, he hesitated 
 to pronounce, but its existence in the sun has since been 
 established. 
 
 These memorable results were founded upon a general prin- 
 ciple first enunciated by Kirchhoff in a communication to the 
 Berlin Academy, December 15, 1859, and afterwards more 
 fully developed by him. 3 It may be expressed as follows : 
 Substances of every kind are opaque to the precise rays which 
 they emit at the same temperature ; that is to say, they stop the 
 kinds of light or heat which they are then actually in a condi- 
 tion to radiate. But it does not follow that cool bodies absorb 
 the rays which they would give out if sufficiently heated. 
 Hydrogen at ordinary temperatures, for instance, is almost 
 perfectly transparent, but if raised to the glowing point 
 as by the passage of electricity it then becomes capable of 
 arresting, and at the same time of displaying in its own spectrum, 
 light of four distinct colours. 
 
 This principle is fundamental to solar chemistry. It gives 
 the key to the hieroglyphics of the Fraunhofer lines. The 
 identical characters which are written bright in terrestrial 
 spectra are written dark in the unrolled sheaf of sun-rays ; the 
 meaning remains unchanged. It must, however, be remembered 
 that they are only relatively dark. The substances stopping 
 
 1 Monatsberichte, Berlin, 1859, p. 664. - Abhandlungen, Berlin, 1861, 
 pp. 80, 81. 3 Ibid., 1861. p. 77 ; Annalen der Physik, Bd. cxix., p. 275. A 
 similar conclusion, reached by Balfour Stewart in 1858, for heat-rajs (Ed. 
 Phil. Trans., vol. xxii., p. 13), was, in 1860, without previous knowledge of 
 Kirchhoff 's work, extended to light (Phil. Mag., vol. xx., p. 534) ; but his 
 experiments wanted the precision of those executed at Heidelberg. 
 
CHAP. i. ASTRONOMICAL PHYSICS. 169 
 
 those particular tints in the neighbourhood of the sun are at the 
 same time vividly glowing with the very same. Remove the 
 dazzling solar background, by contrast with which they show as 
 obscure, and they will be seen, and have, under certain circum- 
 stances, actually been seen, in all their native splendour. It is 
 because the atmosphere of the sun is cooler than the globe it 
 envelops that the different kinds of vapour constituting that 
 atmosphere take more than they give, absorb more light than 
 they are capable of emitting ; raise them to the same tem- 
 perature as the sun itself, and their powers of emission and 
 absorption being brought exactly to the same level, the thousands 
 of dusky rays in the solar spectrum will be at once obliterated. 
 
 The establishment of the terrestrial science of spectrum analy- 
 sis was due, as we have seen, equally to Kirchhoff and Bunsen, 
 but its celestial application to Kirchhoff alone. He effected this 
 object of the aspirations, more or less dim, of many other 
 thinkers and workers, by the union of two separate though 
 closely related lines of research the study of the different kinds 
 of light emitted by various bodies, and the study of the different 
 kinds of light absorbed by them. The latter branch appears to 
 have been first entered upon by Dr. Thomas Young in 1 803 ; x 
 it was pursued by the younger Herschel, 2 by William Allen 
 Miller, Brewster, and Gladstone. Brewster indeed made, in 
 1 83 3, 3 a formal attempt to found what might be called an inverse 
 system of analysis with the prism based upon absorption ; and 
 his efforts were repeated, just a quarter of a century later, by 
 Gladstone. 4 But no general point of view was attained ; nor, it 
 may be added, was it by this path attainable. 
 
 Kirchhoff's map of the solar spectrum, drawn to scale with 
 exquisite accuracy, and printed in three shades of ink to convey 
 the graduated obscurity of the lines, was published in the 
 Transactions of the Berlin Academy for 1861 and i862. 5 
 Representations of the principal lines belonging to various 
 
 1 Miscellaneous Works, vol. i., p. 189. " Ed. Phil. Trans., vol. ix., p. 458. 
 3 Ibid., vol. xii., p. 519. 4 (Juart. Jour. Chain. /Soc., vol. x., p. 79. 5 A fac- 
 simile accompanied Sir H. Roscoe's translation of Kirchhoff's " Researches on 
 the Solar Spectrum " (London, 1862-63). 
 
i;o HISTORY OF ASTRONOMY. PART n. 
 
 elementary bodies formed, as it were, a series of marginal notes 
 accompanying the great solar scroll, and enabling the veriest 
 tyro in the new science to decipher its meaning at a glance. 
 Where the dark solar and bright metallic rays agreed in position, 
 it might safely be inferred that the metal emitting them was a 
 solar constituent; and such coincidences were numerous. In 
 the case of iron alone, no less than sixty occurred in one-half of 
 the spectral area, rendering the chances l absolutely overwhelm- 
 ing against mere casual conjunction. The preparation of this 
 elaborate picture proved so trying to the eyes that Kirchhoff 
 was compelled by failing vision to resign the latter: half of the 
 task to his pupil Hofniann. The complete map measured nearly 
 eight feet in length. 
 
 The conclusions reached by Kirchhoff were no sooner an- 
 nounced than they took their place, with scarcely a dissenting* 
 voice, among the established truths of science. The broad result, 
 that the dark lines in the spectrum of the sun afford an index to 
 its chemical composition no less reliable than any of the tests 
 used in the laboratory, was equally captivating to the imagina- 
 tion of the vulgar, and authentic in the judgment of the learned ; 
 and, like all genuine advances in the knowledge of Nature, it 
 stimulated curiosity far more than it gratified it. Now the 
 history of how discoveries were missed is often quite as instruc- 
 tive as the history of how they were made ; it may then be worth 
 while to expend a few words on the thoughts and trials by which, 
 in the present case, the actual event was heralded. 
 
 Three times it seemed on the verge of being anticipated. The 
 experiment, which in Kirchhoff's hands proved decisive, of 
 passing sunlight through glowing vapours and examining the 
 superposed spectra, was performed by Professor W. A. Miller of 
 King's College in i845- 2 Nay, more, it was performed with 
 express reference to the question, then already (as has been 
 noted) in debate, of the possible production of Fraunhofers lines 
 by absorption in a solar atmosphere. Yet it led to nothing. 
 
 Again, at Paris in 1849, with a view to testing the asserted 
 
 1 Estimated by Kirchhoff at & trillion to one. AbkandL. i86i,p. 79. - Phil. 
 Mag., vol. xxvii. (3rd series), p. 90. 
 
CHAP. i. ASTRONOMICAL PHYSICS. 171 
 
 coincidence between the solar D-line and the bright yellow beam 
 in the spectrum of the electric arc (really due to the unsuspected 
 presence of sodium), Leon Foucault threw a ray of sunshine 
 across the arc and observed its spectrum. 1 He was surprised to 
 see that the D-line was rendered more intensely dark by the 
 combination of lights. To assure himself still further, he sub- 
 stituted a reflected image of one of the white-hot carbon-points 
 for the sunbeam, with an identical result. The same ray was 
 missing. It needed but another step to have generalised this 
 result, and thus laid hold of a natural truth of the highest 
 importance ; but that step was not taken. Foucault, keen and 
 brilliant though he was, rested satisfied with the information 
 that the voltaic arc had the power of stopping the kind of 
 light emitted by it; he asked no further question, and was 
 consequently the bearer of no further intelligence on the 
 subject. 
 
 The truth conveyed by this remarkable experiment was, how- 
 ever, divined by one eminent man. Professor (now Sir Gabriel) 
 Stokes of Cambridge stated to Sir William Thomson, shortly after 
 it had been made, his conviction that an absorbing atmosphere 
 of sodium surrounded the sun. And so forcibly was his hearer 
 impressed with the weight of the arguments based upon the 
 absolute agreement of the D-line in the solar spectrum with the 
 yellow ray of burning sodium (then freshly certified by W. A. 
 Miller), combined with Foucault's "reversal" of that ray, that he 
 regularly inculcated, in his public lectures on natural philosophy 
 at Glasgow, five or six years before KirchhofFs discovery, not 
 only the fact of the presence of sodium in the solar neighbour- 
 hood, but also the principle of the study of solar and stellar 
 chemistry in the spectra of flames. 2 Yet it does not appear to 
 have occurred to either of these two distinguished professors 
 themselves among the foremost of their time in the successful 
 search for new truths to verify practically a sagacious con- 
 jecture in which was contained the possibility of a scientific 
 revolution. It is just to add, that Kirchhoff was unacquainted, 
 
 1 L'Institut, Feb. 7, 1849. p. 45; Phil. Mag., vol. xix. (4th series), p. 193. 
 2 Ann. (1. Phys., vol. cxviii., p. no. 
 
i?2 HISTORY OF ASTRONOMY. PART 11. 
 
 when he undertook his investigation, either with the experiment 
 of Foucault or the speculation of Stokes. 
 
 For C. J. Angstrom, on the other hand, perhaps somewhat too 
 much has been claimed in the way of anticipation. His Optical 
 Researches appeared at Upsala* in 1853, and in their English 
 garb two years later. 1 They were undoubtedly pregnant with 
 suggestion, yet made no epoch in discovery. The old perplexities 
 continued to prevail after, as before then' publication. To 
 Angstrom, indeed, belongs the great merit of having revived 
 Euler's principle of the equivalence of emission and absorption ; 
 but he revived it in its original crude form, and without the 
 qualifying proviso which alone gave it value as a clue to new 
 truths. According to his statement, a body absorbs all the 
 series of vibrations it is, under any circumstances, capable of 
 emitting, as well as those connected with them by simple 
 harmonic relations. This is far too wide. To render it either 
 true or useful, it had to be reduced to the cautious terms 
 employed by Kirchhoff. Radiation strictly and necessarily 
 corresponds with absorption only when the temperature is the 
 same. In point of fact, Angstrom, though convinced that their 
 explanation embraced that of the luminous lines in the spectrum 
 of the electric arc, was still, in 1853, divided between absorption 
 and interference as the mode of origin of the Fraunhofer dark 
 rays. Very important, however, was his demonstration of the 
 compound nature of the spark-spectrum, which he showed to be 
 made up of the spectrum of the metallic electrodes super- 
 posed upon that of the gas or gases across which the discharge 
 passed. 
 
 It may here be useful since without some clear ideas on the 
 subject no proper understanding of recent astronomical progress 
 is possible to take a cursory view of the elementary principles 
 of spectrum analysis. To many of our readers they are doubtless 
 already familiar ; but it is better to be trite in repetition than 
 obscure from lack of explanation. 
 
 The spectrum, then, of a body is simply the light proceeding 
 
 1 Phil. May., vol. ix. (4th series*), p. 327. 
 
CHAP. i. ASTRONOMICAL PHYSICS. 173 
 
 from it spread out by refraction x into a brilliant variegated 
 band, passing from brownish-red through crimson, orange, 
 yellow, green, and azure into dusky violet. The reason of this 
 spreading-out or " dispersion " is that the various colours have 
 different wave-lengths, and consequently meet with different 
 degrees of retardation in traversing the denser medium of the 
 prism. The shortest and quickest vibrations (producing the 
 sensation we call " violet ") are thrown farthest away from their 
 original path in other words, suffer the widest deviation; 
 the longest and slowest (the red) travel much nearer to it. Thus 
 the sheaf of rays which would otherwise combine into a patch of 
 white light are separated through the divergence of their tracks 
 after refraction by a prism, so as to form a tinted riband. This 
 visible spectrum is prolonged invisibly at both ends by a long* 
 range of vibrations, either too rapid or too tardy to affect the 
 eye as light, but recognisable through their chemical and heating 
 effects. 
 
 Now all incandescent solid or liquid substances, and even 
 gases ignited under great pressure, give what is called a " con- 
 tinuous spectrum ; " that is to say, the light derived from them 
 is of every conceivable hue. Sorted out with the prism, its tints 
 merge imperceptibly one into the other, uninterrupted by any 
 dark spaces. No colours, in short, are missing. But gases and 
 vapours rendered luminous by heat emit rays of only a few 
 tints sometimes of one only which accordingly form an in- 
 terrupted spectrum, usually designated as one of lines or bands. 
 And since these rays are perfectly definite and characteristic 
 not being the same for any two substances it is easy to tell 
 what kind of matter is concerned in producing them. We may 
 suppose that the inconceivably minute particles which by their 
 rapid thrilling agitate the ethereal medium so as to produce 
 light, are free to give out their peculiar tone of vibration onlv 
 when floating apart from each other in gaseous form ; but when 
 crowded together into a condensed mass, the clear ring of the 
 distinctive note is drowned, so to speak, in an universal mole- 
 
 1 Spectra may be produced by diffraction as well as by refraction; but we 
 are here only concerned with the matter in its simplest aspect. 
 
174 HISTORY OF ASTRONOMY. PART n. 
 
 cular clang. Thus prismatic analysis has no power to identify 
 individual kinds of matter, except when they present themselves 
 as glowing vapours. 
 
 A spectrum is said to be "reversed" when lines previously 
 seen bright on a dark background appear dark on a bright 
 background. In this form it is equally representative of che- 
 mical composition with the "direct" spectrum, being due to 
 absorption, as the latter is to emission. And absorption and 
 emission are, by Kirchhoff's law, strictly correlative. This is 
 easily understood by the analogy of sound. For just as a 
 tuning-fork responds to sound-waves of its own pitch, but 
 remains indifferent to those of any other, so those particles of 
 matter whose nature it is, when set swinging by heat, to vibrate 
 a certain number of times in a second, thus giving rise to light of 
 a particular shade of colour, appropriate those same vibrations, 
 and those only, when transmitted past them, or, phrasing it 
 otherwise, are opaque to them, and transparent to all others. 
 
 It should further be explained that the shape of the bright or 
 dark spaces in the spectrum has nothing whatever to do with 
 the nature of the phenomena. The " lines " and " bands " so 
 frequently spoken of are seen as such for no other reason than 
 because the light forming them is admitted through a narrow, 
 straight opening. Change that opening into a fine crescent or 
 a sinuous curve, and the " lines " will at once appear as crescents 
 or curves. 
 
 Kesuming in a sentence what has been already explained, we 
 find that the prismatic analysis of the heavenly bodies was 
 founded upon three classes of facts : First, the unmistakable 
 character of the light given by each different kind of glowing 
 vapour; secondly, the identity of the light absorbed with the 
 light emitted by each ; thirdly, the coincidences observed be- 
 tween rays missing from the solar spectrum and rays absorbed 
 by various terrestrial substances. Thus, a realm of knowledge, 
 pronounced by Morinus ! in the seventeenth century, and no 
 less dogmatically by Auguste Comte 2 in the nineteenth, hope- 
 
 1 Astrologia Gallica (1661), p. 189. - Pos. Phil., vol. i., pp. 114-115 (Mar- 
 tineau's trans.). 
 
CHAP. i. ASTRONOMICAL PHYSICS. 175 
 
 lessly out of reach of the human intellect, was thrown freely 
 open, and the chemistry of the sun and stars took its place 
 among the foremost of the experimental sciences. 
 
 The immediate increase of knowledge was not the chief 
 result of Kirchhoff's labours ; still more important was the 
 change in the scope and methods of astronomy, which, set on 
 foot in 1852 by the detection of a common period affecting at 
 once the spots on the sun and the magnetism of the earth, was 
 extended and accelerated by the discovery of spectrum analysis. 
 The nature of that change is concisely indicated by the heading 
 of the present chapter ; we would now ask our readers to en- 
 deavour to realise somewhat distinctly what is implied by the 
 " foundation of astronomical physics." 
 
 Nearly three centuries ago, Kepler drew a forecast of what 
 he called a "physical astronomy" a science treating of the 
 efficient causes of planetary motion, and holding the "key 
 to the inner astronomy." 1 What Kepler dreamed of and 
 groped after, Newton realised. He showed the beautiful and 
 symmetrical revolutions of the solar system to be governed 
 by a uniformly acting cause, and that cause no other than 
 the familiar force of gravity, which gives stability to all our 
 terrestrial surroundings. The world under our feet was thus 
 for the first time brought into physical connection with the 
 worlds peopling space, and a very tangible relationship was 
 demonstrated as existing between what used to be called the 
 "corruptible" matter of the earth and the "incorruptible" 
 matter of the heavens. 
 
 This process of unification of the cosmos this levelling of 
 the celestial with the sublunary was carried no farther until 
 the fact unexpectedly emerged from a vast and complicated mass 
 of observations, that the magnetism of the earth is subject to 
 subtle influences, emanating, certainly from some, and pre- 
 sumably (were their amount sufficient to be perceptible) from 
 all of the heavenly bodies ; the inference being thus rendered 
 at least plausible, that a force not less universal than gravity 
 itself, but with whose modes of action we are as yet un- 
 
 1 Proem. Astronomies Pars Optica (1604), Op., t. ii. 
 
176 HISTORY OF ASTRONOMY. PART n. 
 
 acquainted, pervades the universe, and forms, it might be said, 
 an intangible bond of sympathy between its parts. Now for 
 the investigation of this influence two roads are open. It may 
 be pursued by observation either of the bodies from which it 
 emanates, or of the effects which it produces that is to say. 
 either by the astronomer or by the physicist, or, better still, by 
 both concurrently. Their acquisitions are mutually profitable ; 
 nor can either be considered as independent of- the other. Any 
 important accession to knowledge respecting the sun, for 
 example, may be expected to cast a reflected light on the still 
 obscure subject of terrestrial magnetism; while discoveries in 
 magnetism or its alter ego electricity must profoundly affect 
 solar inquiries. 
 
 The establishment .of the new method of spectrum analysis 
 drew far closer this alliance between celestial and terrestrial 
 science. Indeed, they have come to merge so intimately one 
 into the other, that it is no easier to trace their respective 
 boundaries than it is to draw a clear dividing-line between the 
 animal and vegetable kingdoms. Yet up to the middle of the 
 present century, astronomy, while maintaining her strict union 
 with mathematics, looked with indifference on the rest of the 
 sciences ; it was enough that she possessed the telescope and 
 the calculus. Now the materials for her inductions are supplied 
 by the chemist, the electrician, the inquirer into the most re- 
 condite mysteries of light and the molecular constitution of 
 matter. She is concerned with what the geologist, the meteor- 
 ologist, even the biologist, has to say ; she can afford to close 
 her ears to no new truth of the physical order. Her position 
 of lofty isolation has been exchanged for one of community and 
 mutual aid. The astronomer has become, in the highest sense 
 of the term, a physicist; while the physicist is bound to be 
 something of an astronomer. 
 
 This, then, is what is designed to be conveyed by the " founda- 
 tion of astronomical or cosmical physics." It means the estab- 
 lishment of a science of Nature whose conclusions are not only 
 presumed by analogy, but are ascertained by observation, to be 
 valid wherever light can travel and gravity is obeyed a science 
 
CHAP. i. ASTRONOMICAL PHYSICS. 177 
 
 by which, the nature of the stars can be studied upon the earth, 
 and the nature of the earth can be made better known by study 
 of the stars a science, in a word, which is, or aims at being, 
 one and universal, even as Nature the visible reflection of the 
 invisible highest Unity is one and universal. 
 
 It is not too much to say that a new birth of knowledge has 
 ensued. The astronomy so signally promoted by Bessel 1 the 
 astronomy placed by Comte 2 at the head of the hierarchy of the 
 physical sciences was the science of the movements of the 
 heavenly bodies. And there were those who began to regard it 
 as a science which, from its very perfection, had ceased to be in- 
 teresting whose tale of discoveries was told, and whose further 
 advance must be in the line of minute technical improvements, 
 not of novel and stirring disclosures. But the science of the 
 nature of the heavenly bodies is one only in the beginning of its 
 career. It is full of the audacities, the inconsistencies, the im- 
 perfections, the possibilities of youth. It promises everything ; 
 it has already performed much ; it will doubtless perform much 
 more. The means at its disposal are vast and are being daily 
 augmented. What has so far been secured by them it must now 
 be our task to extricate from more doubtful surroundings and 
 place in due order before our readers. 
 
 1 Pop. For?., pp. 14, 19 408. 2 Pos. Phil, p. 115. 
 
 12 
 

 CHAPTER II. 
 
 SOLAR OBSERVATIONS AND THEORIES. 
 
 THE zeal with which solar studies have been pursued during 
 the last quarter of a century has already gone far to redeem 
 the neglect of the two preceding ones. Since Schwabe's 
 discovery was published in 1851, observers have multiplied, 
 new facts have been rapidly accumulated, and the previous 
 comparative quiescence of thought on the great subject of the 
 constitution of the sun, has been replaced by a bewildering 
 variety of speculations, conjectures, and more or less justifiable 
 inferences. It is satisfactory to find this novel impulse not 
 only shared, but to a large extent guided, by our country- 
 men. 
 
 William Butter Dawes, one of many clergymen eminent in 
 astronomy, observed, in 1852, with the help of a solar eye- 
 piece of his own devising, some curious details of spot- 
 structure. 1 The umbra heretofore taken for the darkest part 
 of the spot was seen to be suffused with a mottled, nebulous 
 illumination, in marked contrast with the striated appearance 
 of the penumbra; while through this "cloudy stratum" a 
 "black opening" permitted the eye to divine farther unfathom- 
 able depths beyond. The hole thus disclosed evidently the 
 true nucleus was found to be present in all considerable, as 
 well as in many small maculae. 
 
 Again, the whirling motions of some of these objects were 
 noticed by him. The remarkable form of one sketched at 
 Wateringbury, in Kent. January 17, 1852, gave him the means 
 
 1 Mem. R. A. S. t vol. xxi., p. 157. 
 
CHAP. ii. SOLAR THEORIES. 179 
 
 of detecting and measuring a rotatory movement of the whole 
 spot round the black nucleus at the rate of 100 degrees in 
 six days. "It appeared/' he said, "as if some prodigious 
 ascending force of a whirlwind character, in bursting through 
 the cloudy stratum and the two higher and luminous strata, 
 had given to the whole a movement resembling its own." l An 
 interpretation founded, as is easily seen, on the Herschelian 
 theory, then still in full credit. 
 
 An instance of the same kind was observed by the late 
 Mr. W. R. Birt in i86o, 2 and cyclonic movements are now a 
 recognised feature of sun-spots. They are, however, as Father 
 Secchi 3 concluded from his long experience, but temporary 
 and casual. Scarcely three per cent, of all spots visible exhibit 
 the spiral structure which should invariably result if a conflict 
 of opposing, or the friction of unequal, currents were essential, 
 and not merely incidental to their origin. A whirlpool phase 
 not unfrequently accompanies their formation, and may be 
 renewed at periods of recrudescence or dissolution ; but it is 
 both partial and inconstant, sometimes affecting only one side 
 of a spot, sometimes slackening gradually its movement in one 
 direction, to resume it, after a brief pause, in the opposite. 
 Persistent and uniform motions, such as the analogy of terres- 
 trial storms would absolutely require, are not to be found. 
 So that the "cyclonic theory" of sun-spots, suggested by 
 Herschel in 1847,* and urged, from a different point of view, 
 by Faye in 1872, may be said to have completely broken 
 down. 
 
 The drift of spots over the sun's surface was first systematic- 
 ally investigated by Carrington, a self-constituted astronomer, 
 gifted with the courage and the instinct of thoughtful labour. 
 Born at Chelsea in May 1826, Richard Christopher Carrington 
 entered Trinity College, Cambridge, in 1844. He. was intended 
 for the Church, but Professor Challis's lectures diverted him to 
 astronomy, and he resolved, as soon as he had taken his degree, 
 to prepare, with all possible diligence, to follow his new vocation. 
 
 1 Mem. It. A. /, vol. xxi., p. 160. ' 2 Month. Not., vol. xxi., p. 144. 
 
 3 Le Soleil, t. i., pp. 87-90 (2d ed. 1875). 4 See ante, p. 70. 
 
i8o HISTORY OF ASTRONOMY. PART n. 
 
 His father, who was a brewer on a large scale at Brentford, 
 offered no opposition ; ample means were at his disposal ; never- 
 theless, he chose to serve an apprenticeship of three years as 
 observer in the University of Durham, as though his sole object 
 had been to earn a livelihood. '" He quitted the post only when 
 he found that its restricted opportunities offered no further 
 prospect of self -improvement. 
 
 He now built an observatory of his own at Redhill in Surrey, 
 with the design of completing Bessel's and Argelander's survey 
 of the northern heavens by adding to it the circumpolar stars 
 omitted from their view. This project, successfully carried out 
 between 1854 and 1857, had another and still larger one super- 
 posed upon it before it had even begun to be executed. In 1852, 
 while the Redhill Observatory was in course of erection, the 
 discovery of the coincidence between the sun-spot and magnetic 
 periods was announced. Carrington was profoundly interested, 
 and devoted his enforced leisure to the examination of records, 
 both written and depicted, of past solar observations. Struck 
 with their fragmentary and inconsistent character, he resolved 
 to " appropriate," as he said, by " close and methodical research," 
 the eleven-year period next ensuing. 1 He calculated rightly 
 that he should have the field pretty nearly to himself ; for many 
 reasons conspire to make public observatories slow in taking 
 up new subjects, and amateurs with freedom to choose, and 
 means to treat them effectually, were even scarcer then than 
 they are now. 
 
 The execution of this laborious task was commenced Nov- 
 ember 9, 1853. It was intended to be merely a parergon a 
 " second subject," upon which daylight energies might be spent, 
 while the hours of night were reserved for cataloguing those 
 stars that " are bereft of the baths of ocean." Its results, how- 
 ever, proved of the highest interest, although the vicissitudes of 
 life barred the completion, in its full integrity, of the original 
 design. By the death, in 1858, of the elder Carrington, the 
 charge of the brewery devolved upon his son ; and eventually 
 absorbed so much of his care that it was found advisable to 
 
 1 Observations at Redldll (1863), Introduction. 
 
CHAP. ii. SOLAR THEORIES. 181 
 
 bring the solar observations to a premature close, March 24, 
 1861. 
 
 His scientific life may be said to have closed with them. 
 Attacked four years later with severe, and, in its results per- 
 manent illness, he disposed of the Brentford business, and with- 
 drew to Churt, near Farnham, in Surrey. There, in a lonely 
 spot, on the top of a detached conical hill known as the 
 " Devil's Jump," he built a second observatory, and erected an 
 instrument which he was no longer able to use with pristine 
 effectiveness; and there, November 27, 1875, ne died of the 
 rupture of a blood-vessel on the brain, before he had completed 
 his fiftieth year. 1 
 
 His observations of sun-spots were of a geometrical character. 
 They concerned positions and movements, leaving out of sight 
 physical peculiarities. Indeed, the prudence with which he 
 limited his task to what came strictly within the range of his 
 powers to accomplish, was one of Carrington's most valuable 
 qualities. The method of his observations, moreover, was chosen 
 with the same practical sagacity as their objects. As early as 1 847, 
 Sir John Herschel had recommended the daily self-registration 
 of sun-spots, 2 and he enforced the suggestion, with more imme- 
 diate prospect of success, in i854. 3 The art of celestial photo- 
 graphy, however, was even then in a purely tentative stage, and 
 Carrington wisely resolved to waste no time on dubious experi- 
 ments, but employ the means of record and measurement 
 actually at his command. These were very simple, yet very 
 effective. To the " helioscope " employed by Father Scheiner 4 
 two centuries and a quarter earlier a species of micrometer was 
 added. The image of the sun was projected upon a screen by 
 means of a firmly clamped telescope, in the focus of which were 
 placed two cross wires forming angles of 45 with the meridian. 
 The six instants were then carefully noted at which these were 
 met by the edges of the disc as it traversed the screen and by the 
 nucleus of the spot to be measured. 5 A short process of cal- 
 
 1 Month. Not., vol. xxxvi., p. 142. 2 Cape Observations, p. 435, note. 3 Month. 
 Not., vol. x., p. 158. 4 liosa Ursina, lib. iii., p. 348. 5 Observations at 
 Redldll, p. 8. 
 
1 82 HISTORY OF ASTRONOMY. PART n. 
 
 dilation then gave the exact position of the spot as referred to 
 the sun's centre. 
 
 From a series of 5 290 observations made in this way, together 
 with a great number .of accurate drawings, Carrington derived 
 conclusions of great importance on each of the three points 
 which he had proposed to himself to investigate. These were : 
 the law of the sun's rotation, the existence^ and direction of 
 systematic currents, and the distribution of spots on the solar 
 surface. 
 
 Grave discrepancies were early perceived to exist between the 
 determination of the sun's rotation by different observers. 
 Galileo, with " comfortable generality," estimated the period 
 at "about a lunar month"; 1 Schemer, at twenty-seven days. 2 
 Cassini, in 1678, made it 25.58; Delambre, in 1775, reduced 
 it to twenty-five days. Later inquiries brought these diver- 
 gences within no more tolerable limits. Laugier's result of 
 25.34 days obtained in 1841 enjoyed the highest credit, yet 
 it differed widely in one direction from that of Bohm (1852), 
 giving 25.52 days, and in the other from that of Kysseus (1846), 
 giving 25.09 days. Now the cause of these variations was really 
 obvious from the first, although for a long time strangely over- 
 looked. Father Scheiner pointed out in 1630 that different 
 spots gave different periods, adding the significant remark that 
 one at a distance from the solar equator revolved more slowly 
 than those nearer to it. 3 But the hint was wasted. For up- 
 wards of two centuries ideas on the subject were either retro- 
 grade or stationary. What were called the "proper motions" 
 of spots were, however, recognised by Schroter, 4 and utterly 
 baffled Laugier, 5 who despaired of obtaining any concordant 
 result as to the sun's rotation except by taking the mean of a 
 number of discordant ones. At last, in 1855, a valuable course 
 of observations made at Capo di Monte, Naples, in 1845-6, 
 
 1 Op. t. iii., p. 402. 2 Rosa Ursina, lib. iv., p. 601. Both Galileo and 
 Scheiner spoke of the apparent or " sy nodical " period, which is about one 
 and a third days longer than the true or " sidereal " one. The difference is 
 caused by the revolution of the earth in its orbit in the same direction with 
 the sun's rotation on its axis. 3 Rosa Ursina, lib. iii., p. 260. 4 Faye, Comptes. 
 Rendus, t. lx., p. 818. 5 Comptes Rendus, t. xii., p. 648. 
 
CHAP. ii. SOLAR THEORIES. 183 
 
 enabled C. H. F. Peters l (later of Hamilton College, Clinton, 
 N.Y.) to set in the clearest light the insecurity of determinations 
 based on the assumption of fixity in objects plainly affected by 
 movements uncertain both in amount and direction. 
 
 Such was the state of affairs when Carrington entered upon 
 his task. Everything was in confusion ; the most that could be 
 said was that the confusion had come to be distinctly admitted 
 and referred to its true source. What he discovered was this : 
 that the sun, or at least the outer shell of the sun visible to us, 
 has no single period of rotation, but drifts round, carrying the 
 spots with it, at a rate continually accelerated from the poles to 
 the equator. In other words, the time of axial revolution is 
 shortest at the equator and lengthens with increase of latitude. 
 Carrington devised a mathematical formula by which the rate or 
 " law " of this lengthening was conveniently expressed ; but it 
 was a purely empirical one. It was a concise statement, but 
 implied no physical interpretation. It summarised, but did not 
 explain the facts. An assumed " mean period " for the solar 
 rotation of 25.38 days (twenty-five days nine hours, very nearly), 
 was thus found to be actually conformed to only in two parallels 
 of solar latitude (14 north and south), while the equatoreal 
 period was slightly less than twenty-five, and that of latitudes 
 50 rose to twenty-seven days and a half. 2 These curious results 
 gave quite a new direction to ideas on solar physics. 
 
 The other two "elements" of the sun's rotation were also 
 ascertained by Carrington with hitherto unattained precision. 
 He fixed the inclination of its axis to the ecliptic at 82 45' ; 
 the longitude of the ascending node at 73 40' (both for the 
 epoch I85OA.D.) These data which have scarcely yet been 
 improved upon suffice to determine the position in space of the 
 sun's equator. Its north pole is directed towards a star in the 
 coils of the Dragon, midway between Vega and the Pole-star ; 
 its plane intersects that of the earth's orbit in such a way that 
 our planet finds itself in the same level on or about the 3rd of 
 June and the 5th of December, when any spots visible on the 
 
 1 Proc. Am. Ass. Adv. of Science, 1855, p. 85. 2 Observations at Redhill, 
 p. 221. 
 
184 HISTORY OF ASTRONOMY. PART n. 
 
 disc cross it in apparently straight lines. At other times, the 
 paths pursued by them seem curved downward (to an observer 
 in the northern hemisphere) between June and December, up- 
 ward between December and June. 
 
 7 . -f 
 
 A singular peculiarity in the distribution of sun-spots emerged 
 from Carrington's studies at the time of the minimum of 1856. 
 Two broad belts of the solar surface, as we Ijiave seen, are fre- 
 quented by them, of which the limits may be put at 6 and 35 
 of north and south latitude. Individual equatoreal spots are not 
 uncommon, but nearer to the poles than 35 they are a rare 
 exception. Carrington observed as an extreme instance in 
 July 1858, one in south latitude 44 ; and Peters, in June 1846, 
 watched, during several days, a spot in 50 24' north latitude. 
 But beyond this no true macula has ever been seen ; for Lahire's 
 reported observation of one in latitude 70 is now believed to 
 have had its place on the solar globe erroneously assigned ; and 
 the " veiled spots" described by Trouvelot in 1875 1 as occurring 
 within 10 of the pole can only be regarded as, at the most, the 
 same kind of disturbance in an undeveloped form. 
 
 But the novelty of Carrington's observations consisted in the 
 detection of certain changes in distribution concurrent with the 
 progress of the eleven-year period. As the minimum approached, 
 the spot-zones contracted towards the equator, and there finally 
 vanished; then, as if by a fresh impulse, spots suddenly re- 
 appeared in high latitudes, and spread downwards with the 
 development of the new phase of activity. Scarcely had this 
 remark been made public, 2 when Wolf 3 found a confirmation of 
 its general truth in Bohm's observations during the years 1833- 
 36 ; and a perfectly similar behaviour was noted both by Sporer 
 and Secchi at the minimum epoch of 1 867. The ensuing period 
 gave corresponding indications ; and it may now be looked upon 
 as established that the spot-zones close in towards the equator 
 with the advance of each cycle, their activity culminating, as a 
 rule, in a mean latitude of about 16, and expiring when it is 
 reduced to 6. Before this happens, however, a completely new 
 
 1 Am. Jour, of Science, vol. xi., p. 169. 2 Month. Not., vol. xix., p. i. 
 
 3 Vierteljahrsschrift der Naturfors. Gesellsckaft (Zurich), 1859, p. 252. 
 
CHAP. ii. SOLAR THEORIES. 185 
 
 disturbance will have manifested itself some 35 north and south 
 of the equator, and will have begun to travel over the same 
 course as its predecessor. Each series of sun-spots is thus, to 
 some extent, overlapped by the ensuing one ; so that while the 
 average interval from one maximum to the next is eleven years, 
 the period of each distinct wave of agitation is twelve or four- 
 teen. 1 Curious evidence of the retarded character of the maxi- 
 mum of 1883-4 was to be found in the unusually low latitude of 
 the spot-zones when it occurred. Their movement inwards 
 having gone on regularly while the crisis was postponed, its 
 final symptoms were displaced locally as well as in time. The 
 "law of zones" (as it is called) was duly obeyed at the recent 
 minimum of iSQO, 2 and Sporer has found evidence of conformity 
 to it so far back as i6i9. 3 His researches, however, also show 
 that it was in abeyance for some seventy years previously to 
 1716, during which period sun-spots remained persistently scarce, 
 and auroral displays were feeble and infrequent even in high 
 northern latitudes. An unaccountable suspension at that time 
 of solar activity is thus unmistakably indicated. 
 
 Gustav Sporer, born at Berlin in 1822, began to observe sun- 
 spots with the view of assigning the law of solar rotation in 
 December 1 860. His means were at first very limited, but his 
 assiduity and success attracted attention, and a Government 
 endowment was procured for the little solar observatory organised 
 by him at Anclam, in Pomerania. Unaware of Carrington's 
 discovery (first made known in January 1859), he arrived at and 
 published, in June i86i, 4 a similar conclusion as to the equatoreal 
 quickening of the sun's movement on its axis. His sun-spot 
 observations were continued at Anclam until the end of 1873, 
 and have since been pursued at the astro-physical observatory of 
 Potsdam. 
 
 The time had now evidently come for a fundamental revision 
 of current notions respecting the nature of the sun. Herschel's 
 theory of a cool, dark, habitable globe, surrounded by, and 
 protected against the radiations of a luminous and heat-giving 
 
 1 Lockyer, Chemistry of the Sun, p. 428. 2 Maunder, Knowledge, vol. xv. , 
 
 p. 130. 3 Month. Not., vol. 1., p. 251. 4 Astr. Nach., No. 1315. 
 
186 HISTORY OF ASTRONOMY. PART n. 
 
 envelope, was shattered by the first dicta of spectrum analysis. 
 Traces of it may be found for a few years subsequent to I859, 1 
 but they are obviously survivals from an earlier order of ideas, 
 doomed to speedy extinction. It needs only a moment's con- 
 sideration of what was implied in the discovery of the origin 
 of the Fraunhofer lines to see the incompatibility of the new 
 facts with the old conceptions. It implied, not only the presence 
 near the sun, as glowing vapours, of bodies highly refractory to 
 heat, but that these glowing vapours formed the relatively cool 
 envelope of a still hotter internal mass. Kirchhoff, accordingly, 
 included in his great memoir " On the Solar Spectrum," read 
 before the Berlin Academy of Sciences, July 1 1 , 1 86 1 , an ex- 
 position of the views on the subject to which his memorable 
 investigations had led him. They may be briefly summarised as 
 follows. 
 
 Since the body of the sun gives a continuous spectrum, it 
 must be either solid or liquid, 2 while the interruptions in its 
 light prove it to be surrounded by a complex atmosphere of 
 metallic vapours, somewhat cooler than itself. Spots are simply 
 clouds due to local depressions of temperature, differing in no 
 respect from terrestrial clouds except as regards the kinds of 
 matter composing them. These sun-clouds take their origin in 
 the zones of encounter between polar and equatoreal currents in 
 the solar atmosphere. 
 
 This explanation was liable to all the objections urged against 
 the ''cumulus theory " on the one hand, and the "trade-wind 
 theory " on the other. Setting aside its propounder, it was con- 
 sistently upheld perhaps by no man eminent in science except 
 Sporer ; and his advocacy of it tended rather to delay the recogni- 
 tion of his own merits than to promote its general adoption. 
 
 M. Faye, of the Paris Academy of Sciences, was the first to 
 propose a coherent scheme of the solar constitution covering 
 
 1 As late as 1866 an elaborate treatise in its support was written by M. F. 
 Coyteux, entitled Quest ce que k Soleil? Peut-il etre halite? and answering the 
 question in the affirmative. - The subsequent researches of Pliicker, 
 
 Frankland, Wullner, and others showed that gases strongly compressed give 
 an absolutely unbroken spectrum. 
 
CHAP. ii. SOLAR THEORIES. 187 
 
 the whole range of new discovery. The fundamental ideas on 
 the subject now in vogue here made their first connected 
 appearance. Much, indeed, remained to be modified and cor- 
 rected ; but the transition was finally made from the old to the 
 new order of conceptions, The essence of the change of view 
 thus effected may be conveyed in a single sentence. The sun 
 was thenceforth regarded, not as a mere heated body, or still 
 more remotely from the truth as a cool body unaccountably 
 spun round with a cocoon of fire, but as a vast heat-radiating 
 machine. The terrestrial analogy was abandoned in one more 
 particular besides that of temperature. The solar system of 
 circulation, instead of being adapted, like that of the earth, to 
 the distribution of heat received from without, was seen to be 
 directed towards the transportation towards the surface of the 
 heat contained within. Polar and equatoreal currents, tending 
 to a purely superficial equalisation of temperature, were replaced 
 by vertical currents bringing up successive portions of the 
 intensely heated interior mass, to contribute their share in turn 
 to the radiation into space which might be called the proper 
 function of a sun. 
 
 Faye's views, which were communicated to the Academy of 
 Sciences, January 16, I865, 1 were avowedly based on the 
 anomalous mode of solar rotation discovered by Carrington, 
 This may be regarded either as an acceleration increasing from 
 the poles to the equator, or as a retardation increasing from 
 the equator to the poles, according to the rate of revolution 
 we choose to assume for the unseen nucleus. Faye preferred 
 to consider it as a retardation produced by ascending currents 
 continually left behind as the sphere widened in which the 
 matter composing them was forced to travel. He further 
 supposed that the depth from which these vertical currents rose, 
 and consequently the amount of retardation effected by their 
 ascent to the surface, became progressively greater as the poles 
 were approached, owing to the considerable flattening of the 
 spheroidal surface from which they started ; 2 but the adoption 
 1 Comptes jRendus, t. lx., pp. 89, 138. 2 Ibid., t. c., p. 595. 
 
1 88 HISTORY OF ASTRONOMY. PART n. 
 
 of this expedient has been shown to involve inadmissible con- 
 sequences. 
 
 The extreme internal mobility betrayed by Carrington's and 
 Sporer's observations led to the inference that the matter 
 composing the sun was mainly or wholly gaseous. This had 
 already been suggested by Father Secchi 1 in January, and 
 by Sir John Herschel in April i864; 2 but it first obtained 
 general currency through Faye's more elaborate presentation. 
 A physical basis was afforded for the view by Cagniard de la 
 Tour's experiments in i822, 3 proving that, under conditions 
 of great heat and pressure, the vaporous state was compatible 
 with a very considerable density. The position was strength- 
 ened when Andrews showed, in i869, 4 that above a fixed limit 
 of temperature, varying for different bodies, true liquefaction 
 is impossible, even though the pressure be so tremendous 
 as to retain the gas within the same space that enclosed the 
 liquid. The opinion that the mass of the sun is gaseous now 
 commands a very general assent; although the gaseity ad- 
 mitted is of such a nature as to afford the consistence rather of 
 honey or pitch than of the aeriform fluids with which we are 
 familiar. 
 
 On another important point the course of subsequent thought 
 was powerfully influenced by Faye's conclusions in 1865. 
 Arago somewhat hastily inferred from experiments with the 
 polariscope the wholly gaseous nature of the visible disc of the 
 sun. Kirchhoff, on the contrary, believed (erroneously, as we 
 now know) that the brilliant continuous spectrum derived from 
 it proved it to be a white-hot solid or liquid. Herschel and 
 Secchi 5 indicated a cloud-like structure as that which would 
 best harmonise the whole of the evidence at command. The 
 novelty introduced by Faye consisted in regarding the photo- 
 sphere no longer "as a defined surface, in the mathematical 
 sense, but as a limit to which, in the general fluid mass, 
 ascending currents carry the physical or chemical phenomena of 
 
 1 Bull. Meteor, dell Osservatorio dell Coll Bom., Jan. i, 1864, p. 4. 2 Quart. 
 Jour, of Science, vol. i., p. 222. 3 Ann. de Ckim. et de Phys., t. xxii., p. 127. 
 4 Phil. Trans., vol. clix., p. 575. 5 Les Jllondes, Dec. 22, 1864, p. 707. 
 
CHAP. ii. SOLAR THEORIES. 189 
 
 incandescence." x Uprushing floods of mixed vapours with strong 
 mutual affinities say of calcium or sodium and oxygen at last 
 attain a region cool enough to permit their combination ; a fine 
 dust of solid or liquid compound particles (of lime or soda, for 
 example) there collects into the photospheric clouds, and 
 descending by its own weight in torrents of incandescent rain, 
 is dissociated by the fierce heat below, and replaced by ascending 
 and combining currents of similar constitution. 
 
 This first attempt to assign the part played in cosmical physics 
 by chemical affinities, was marked by the importation into the 
 theory of the sun of the now familiar phrase dissociation. It is 
 indeed tolerably certain that no such combinations as those 
 contemplated by Faye occur at the photospheric level, since the 
 temperature there must be enormously higher than would be 
 needed to reduce all metallic earths and oxides ; but molecular 
 changes of some kind, dependent perhaps in part upon electrical 
 conditions, in part upon the effects of radiation into space, most 
 likely replace them. The conjecture (originally thrown out, it 
 would seem, by Faye himself) was countenanced by Angstrom, 2 
 and subsequently advocated by Professor Hastings of Baltimore, 5 
 that the photospheric clouds are composed of particles of some 
 member of the carbon-triad 4 precipitated from its mounting 
 vapour just where the temperature is lowered by expansion and 
 radiation to the boiling-point of that substance. This view is, 
 however, open to grave objections. 
 
 In Faye's theory, sun-spots were regarded as simply breaks 
 in the photospheric clouds, where the rising currents had 
 strength to tear them asunder. It followed that they were 
 regions of increased heat regions, in fact, where the tempera- 
 ture was too high to permit the occurrence of the precipitations 
 to which the photosphere is due. Their obscurity was attributed, 
 as in Dr. Brester's recently published Thdorie du Soleilf to defi- 
 ciency of emissive power. Yet here the verdict of the spectro- 
 scope is adverse and irreversible. 
 
 1 Comptes Rendus, t. lx., p. 147. " Becliercltes sur le Spectre Solaire, p. 38. 
 3 Am. Jour, of Science, 1881, vol. xxi., p. 41. 4 Carbon, silicon, and boron. 
 5 Amsterdam, 1892. 
 
190 HISTORY OF ASTRONOMY. PART n. 
 
 After every deduction, however, has been made, we still find 
 that several ideas of permanent value were embodied in this 
 comprehensive sketch of the solar constitution. The principal 
 of these were : first, that the sun is a mainly gaseous body ; 
 secondly, that its stores of hetit are rendered available at the 
 surface by means of vertical convection-currents by the bodily 
 transport, that is to say, of intensely hot matter upwards, and 
 of comparatively cool matter downwards ;'" thirdly, that the 
 photosphere is a surface of condensation, forming the limit set 
 by the cold of space to this circulating process, and that a 
 similar formation may be supposed to attend, at a certain stage, 
 the cooling of every cosmical body. 
 
 To Mr. Warren De la Rue belongs the honour of having 
 obtained the earliest results of substantial value in celestial 
 photography. What had been done previously was interesting 
 in the way of promise, but much could not be claimed for it 
 as actual performance. Some pioneering experiments were 
 made by Dr. J. W. Draper of New York in 1840, resulting in 
 the production of a few " moon-pictures " one-inch in diameter ; 1 
 but slight encouragement was derived from them, either to him- 
 self or others. Bond of Cambridge (U.S.), however, secured in 
 1850 with the Harvard 15 -inch refractor that daguerreotype of 
 the moon with which the career of extra-terrestrial photography 
 may be said to have formally opened. It was shown in London 
 at the Great Exhibition of 1851, and determined the direction 
 of De la Rue's efforts. Yet it did little more than prove the art 
 to be a possible one. 
 
 Warren De la Rue was born in Guernsey in 1815, was 
 educated at the Ecole Sainte-Barbe in Paris, and made a large 
 fortune as a paper manufacturer in England. 2 The material 
 supplies for his scientific campaign were thus amply and early 
 provided. Towards the end of 1853 he took some successful 
 lunar photographs. They were remarkable as the first examples 
 of the application to astronomical light-painting of the collodion 
 process, invented by Archer in 1851 ; and also of the use of 
 
 1 H. Draper, Quart. Jou^n. of 8c., vol. i., p. 381 ; also Phil. Mag., vol. xvii., 
 1840. p. 222. - He died in London, April 19, 1889, aged 74. 
 
CHAP. ii. SOLAR THEORIES. 191 
 
 reflectors (Mr. De la Eue's was one of thirteen inches, constructed 
 by himself) for that kind of work. The absence of a driving 
 apparatus was, however, very sensibly felt ; the difficulty of 
 moving the instrument by hand so as accurately to follow the 
 moon's apparent motion being such as to cause the discontinu- 
 ance of the experiments until 1857, when the want was supplied. 
 Mr. De la Rue's new observatory, built in that year at Cranford, 
 twelve miles west of Hyde Park, was expressly dedicated to celes- 
 tial photography; and there he immediately applied to the heavenly 
 bodies the stereoscopic method of obtaining relief, and turned 
 his attention to the delicate business of photographing the sun. 
 A solar daguerreotype 1 was taken at Paris, April 2, 1845, by 
 MM. Foucault and Fizeau, acting on a suggestion from Arago. 
 But the attempt, though far from being unsuccessful, does not, 
 at that time, seem to have been repeated. Its great difficulty 
 consisted in the enormous light-power of the object to be 
 represented, rendering an inconceivably short period of expo- 
 sure indispensable, under pain of getting completely " burnt- 
 up " plates. In 1857 Mr. De la Rue was commissioned by 
 the Royal Society to construct an instrument specially adapted 
 to the purpose for the Kew Observatory. The resulting " photo- 
 heliograph" may be described as a small telescope (of 3^ 
 inches aperture and 50 focus), with a plate-holder at the eye-end, 
 guarded in front by a spring-slide, the rapid movement of which 
 across the field of view secured for the sensitive plate a virtually 
 instantaneous exposure. By its means the first solar light-pic- 
 tures of real value were taken, and the autographic record of 
 the solar condition recommended by Sir John Herschel was com- 
 menced and continued at Kew during fourteen years 1858-72. 
 The work of photographing the sun is now carried on in every 
 quarter of the globe, from the Mauritius to Massachusetts, and 
 the days are few indeed on which the self-betrayal of the 
 camera can be evaded by our chief luminary. In the year 1883 
 the incorporation of Indian with Greenwich pictures afforded a 
 record of the state of the solar surface on 340 days ; and 360 
 were similarly provided for in 1885. 
 
 1 Reproduced in Arago's Popular Astronomy, plate xii., vol. i. 
 
192 HISTORY OF ASTRONOMY. PART n. 
 
 The conclusions reached by photographic means at Kew 
 were communicated to the Royal Society in a series of papers 
 drawn up jointly by Messrs. De la Rue, Balfour Stewart, and 
 Benjamin Loewy, in.. 1865 and subsequent years. They influ- 
 enced materially the progress ; bf thought on the subject they 
 were concerned with. 
 
 By its rotation the sun itself offers opportunities for bringing 
 the stereoscope to bear upon it. Two pictures, taken at an 
 interval of twenty-six minutes, show just the amount of difference 
 needed to give, by their combination, the maximum effect of 
 solidity. 1 Mr. De la Rue thus obtained, in 1861, a stereoscopic 
 view of a sun-spot and surrounding faculae, representing the 
 various parts in their true mutual relations. " I have ascertained 
 in this way," he wrote, 2 " that the faculse occupy the highest 
 portions of the sun's photosphere, the spots appearing like holes 
 in the penumbrae, which appeared lower than the regions sur- 
 rounding them ; in one case, parts of the faculae were discovered 
 to be sailing over a spot apparently at some considerable height 
 above it." Thus Wilson's inference as to the depressed nature 
 of spots received, after the lapse of not far from a century, 
 proof of the most simple, direct, and convincing kind. A careful 
 application of Wilson's own geometrical test gave results only a 
 trifle less decisive. Of 694 spots observed, 78 per cent, showed, 
 as they traversed the disc, the expected effects of perspective ; 3 
 and their absence in the remaining 22 per cent, might be easily 
 explained by internal commotions producing irregularities of 
 structure. The absolute depth of spot-cavities at least of their 
 sloping sides was determined by Father Seech i through mea- 
 sures of the " parallax of profundity" 4 that is, of apparent 
 displacements attendant on the sun's rotation, due to depression 
 below the sun's surface. He found that in every case it fell 
 short of 4000 miles, and averaged not more than 1-321, corre- 
 
 1 Report Brit. Ass., 1859, p. 148. " Phil Trans., vol. clii., p. 407. 
 8 Researches in Solar Physics, part i., p. 20. 4 Both the phrase and the 
 method were suggested by Faye, who estimates the average depth of the 
 luminous sheath of spots at 2160 miles. Comptes Rendus, t. Ixi., p. 1082 ; 
 t. xcvi., p. 356. 
 
CHAP. ii. SOLAR THEORIES. 193 
 
 spending, on the terrestrial scale, to an excavation in the earth's 
 crust of I J miles. There may be, however, and probably are, 
 depths below this depth, of which the eye takes not even indirect 
 cognisance ; so that it would be hasty to pronounce spots to be 
 a merely superficial phenomenon. 
 
 The opinion of the Kew observers as to the nature of such 
 disturbances was strongly swayed by another curious result of 
 the statistical method of inquiry. They found that of 1137 
 instances of spots accompanied by faculae, 584 had those faculae 
 chiefly or entirely on the left, 508 showed a nearly equal distri- 
 bution, while 45 only had faculous appendages mainly on the 
 right side. 1 Now, the rotation of the sun, as we see it, is per- 
 formed from left to right ; so that the marked tendency of the 
 faculse was a lagging one. This was easily accounted for by 
 supposing the matter composing them to have been flung up- 
 wards from a considerable depth, whence it would reach the 
 surface with the lesser absolute velocity belonging to a smaller 
 circle of revolution, and would consequently fall behind the 
 cavities or " spots " formed by its abstraction. A result incon- 
 sistent with this deduction was, it is true, reached by M. Wilsing 
 at Potsdam in 1 888, 2 through an attempt to determine the sun's 
 rotation from faculae photographically recorded during six months 
 of the year 1884. They unexpectedly gave a uniform period. No 
 trace of the retardation shown by the spots, poleward from the 
 equator, could be detected in their movements. All seemed to 
 rotate together in a period of nearly 25 J days, which is just that 
 of spots in 10 of heliographic latitude. Beyond these limits, 
 accordingly, faculse ought, if the case really stood thus, to gain 
 steadily upon spots. Observation, however, tells nothing of 
 any such effect. Faculae have never been seen to press onward 
 in front of the spots to which they originally appertained. M. 
 Wilsing's experiment must then be set aside as inconclusive. 3 
 And, indeed, the materials available for it were not only insuf- 
 ficient in amount, but dubious in quality, the identity and 
 
 1 Proc. Hoy. Soc., vol. xiv., p. 39. 2 Potsdam PuUicationen, No. 18 ; Astr. 
 Nach., Nos. 3000, 3153. 3 Faye, Oomptes Rendus, t. cxi., p. 77 ; Belopolsky, 
 Astr. Nacli., No. 2991. 
 
 13 
 
194 HISTORY OF ASTRONOMY. PART n. 
 
 permanence of faculse from one week to another being exceed- 
 ingly ill-assured. 
 
 The ideas of M. Faye were, on two fundamental points, con- 
 tradicted by the Kew investigators. He held spots to be regions 
 of uprush and of heightened temperature ; they believed their 
 obscurity to be due to a downrush of comparatively cool vapours. 
 Now M. Chacornac, observing at Ville-Urbanne, March, 6, 1865, 
 saw floods of photospheric matter visibly precipitating themselves 
 into the abyss opened by a great spot, and carrying with them 
 small neighbouring maculae. 1 Similar instances were repeatedly 
 noted by Father Secchi, who considered the existence of a kind 
 of suction in spots to be quite beyond question. 2 The tendency 
 in their vicinity, to put it otherwise, is centripetal, not centrifugal; 
 and this alone seems to negative the supposition of a central 
 uprush. 
 
 A fresh witness was by this time at hand. The application of 
 the spectroscope to the direct examination of the sun's surface 
 dates from March 4, 1866, when Mr. Norman Lockyer began his 
 inquiry into the cause of the darkening in spots. 3 It was made 
 possible by the simple device of throwing upon the slit of the 
 spectroscope an image of the sun, any part of which could be 
 subjected to special scrutiny, instead of (as had hitherto been 
 done) admitting rays from every portion of his surface indis- 
 criminately. The answer to the inquiry was prompt and 
 unmistakable, and was again, in this case, adverse to the French 
 theorist's view. The obscurations in question were found to be 
 produced by no deficiency of emissive power, but by an increase 
 of absorptive action. The background of variegated light 
 remains unchanged, but more of it is stopped by the interposition 
 of a dense mass of relatively cool vapours. The spectrum of a 
 sun-spot is crossed by the same set of multitudinous dark lines, 
 with some minor differences, visible in the ordinary solar 
 spectrum. We must then conclude that the same vapours 
 (speaking generally) which are dispersed over the unbroken solar 
 surface, are accumulated in the umbral cavity, the compression 
 
 1 Lockyer, Contributions to Solar Physics, p. 70. -' Le Soleil, p. 87. 
 
 3 Proc. Boy. Soc., vol. xv., p. 256. 
 
CHAP. ii. SOLAR THEORIES. 195 
 
 incidental to such accumulation being betrayed by the thickening 
 of certain lines of absorption. But there is also a general 
 absorption, extending almost continuously from one end of the 
 spot-spectrum to the other. Using, however, a spectroscope of 
 exceptionally high dispersive power, Professor Young of 
 Princeton, New Jersey, succeeded in 1883 in resolving, for 
 the most part, the supposed continuous obscurity of spot-spectra 
 into a countless multitude of fine dark lines set very close 
 together. 1 This was done, still more perfectly, about five years 
 later, by M. Duner, 2 Director of the Upsala observatory, who was 
 besides able to trace some bare shadows, as it were, of the 
 crowded doublets and triplets forming the array, from the spots 
 into the general solar spectrum. He hence drew the inference 
 that, between the photospheric and the spot-spectra, there is no 
 fundamental difference, but that the latter is merely an intensifi- 
 cation of the former. 
 
 As to the movements of the constipated vapours forming 
 spots, the spectroscope is also competent to supply information. 
 The principle of the method by which it is procured will be 
 explained farther on. Suffice it here to say that the transport* 
 at any considerable velocity, to or from the eye of the gaseous 
 material giving bright or dark lines, can be measured by the 
 displacement of such lines from their previously known normal 
 positions. In this way movements have been detected in or 
 above spots of enormous rapidity, ranging up to 320 miles per 
 second. But the result, so far, has been to negative the ascription 
 to them of any systematic direction. Uprushes and downrushes 
 are doubtless, as Father Cortie remarks, 3 " correlated phenomena 
 in the production of a sun-spot " ; but neither seem to pre- 
 dominate as part of its regular internal economy. 
 
 The same kind of spectroscopic evidence tells heavily against 
 a theory of sun-spots, started by Paye in 1872, and still advocated 
 by *him. He had been foremost in pointing out that the 
 observations of Carrington and Sporer absolutely forbade the 
 supposition that any phenomenon at all resembling our trade- 
 
 1 Phil. Mag., vol. xvi., p. 460. - Becker dies sur la Rotation du Soleil, p. 12, 
 Upsala, 1891. 3 Jour. Brit. Astr. Ass., vol. i., p. 177. 
 
196 HISTORY OF ASTRONOMY. PART n. 
 
 winds exists in the sun. They showed, indeed, that beyond the 
 parallels of 20 there is a general tendency in spots to a slow 
 poleward displacement, while within that zone they incline to 
 approach the equator; but their "proper movements" gave 
 no evidence of uniformly flowing currents in latitude. The 
 systematic drift of the photosphere is strictly a drift in longi- 
 tude ; its direction is everywhere parallel to thue equator. This 
 fact being once clearly recognised, the " solar tornado " hypo- 
 thesis at once fell to pieces ; but M. Faye 1 perceived another 
 source of vorticose motion in the unequal rotating velocities of 
 contiguous portions of the photosphere. The "pores" with 
 which the whole surface of the sun is studded he took to be the 
 smaller eddies resulting from these inequalities ; the spots to be 
 such eddies developed into whirlpools. It only needs to thrust 
 a stick into a stream to produce the kind of effect designated. 
 And it happens that the differences of angular movement 
 adverted to attain a maximum just about the latitudes where 
 spots are most frequent and conspicuous. 
 
 There are, however, grave difficulties in identifying the two 
 kinds of phenomena. One (already mentioned) is the total 
 absence of the regular swirling motion in a direction contrary 
 to that of the hands of a watch north of the solar equator, in the 
 opposite sense south of it which should impress itself upon 
 every lineament of a sun-spot if the cause assigned were a 
 primary producing, and not merely, as it possibly may be, a 
 secondary determining one. The other, pointed out by Professor 
 Young, 2 is that the cause is inadequate to the effect. The differ- 
 ence of movement, or relative drift, supposed to occasion such 
 prodigious disturbances, amounts, at the utmost, for two portions 
 of the photosphere 123 miles apart, to about five yards a minute. 
 Thus the friction of contiguous sections must be quite insig- 
 nificant. 
 
 A view better justified by observation was urged by Father 
 
 1 Comptes Rendus, t. Ixxv. , p. 1664 ; Revue Scientifique, t. v., p. 359 (1883). 
 Mr. Herbert Spencer had already (in The Reader, Feb. 25, 1865) put forward 
 an opinion that spots were of the nature of " cyclonic clouds." 2 Tlie Sun, 
 p. 174. For Faye's answer to the objection, see Comptes Rendus, t. xcv., p. 
 1310. 
 
CHAP. ii. SOLAR THEORIES. 197 
 
 Secchi in and after the year 1872, and was presented in an 
 improved form by Professor Young in his excellent little book 
 on The Sun, published in 1882. Spots are manifestly associated 
 with violent eruptive action, giving rise to the faculge and 
 prominences which usually garnish their borders. It is accord- 
 ingly contended that upon the withdrawal of matter from below 
 by the flinging up of a prominence must ensue a sinking-in of 
 the surface, into which the partially cooled erupted vapours rush 
 and settle, producing just the kind of darkening by increased 
 absorption told of by the spectroscope. Bound the edges of the 
 cavity the rupture of the photospheric shell will form lines of 
 weakness provocative of further eruptions, which will, in their 
 turn, deepen and enlarge the cavity. The phenomenon will thus 
 tend to perpetuate itself, until equilibrium is at last restored by 
 internal processes. A sun-spot might then be described as an 
 inverted terrestrial volcano, in which the outbursts of heated 
 matter take place on the borders instead of at the centre of the 
 crater, while the cooled products gather in the centre instead of 
 at the borders. 
 
 But on the earth, the solid crust forcibly represses the steam 
 gathering beneath until it has accumulated strength for an 
 explosion, while there is no such restraining power that we know 
 of in the sun. Zollner, indeed, adapted his theory of the solar 
 constitution to the special purpose of procuring it ; yet with very 
 partial success, since almost every new fact ascertained has 
 proved adverse to his assumptions. Volcanic action is essentially 
 spasmodic. It implies habitual constraint varied by temporary 
 outbreaks, inconceivable in a gaseous globe, such as we believe 
 the sun to be. 
 
 If the "volcanic hypothesis" represented the truth, no spot 
 could possibly appear without a precedent eruption. The real 
 order of the phenomena is exceedingly difficult to ascertain ; 
 nor is it perhaps invariable. Yet, some instances to the 
 contrary notwithstanding, spots seem as a rule to begin the 
 visible disturbance, while f aculge show subsequently to the opening 
 of a rent in the photosphere. 
 
 This sequence forms an integral part of Professor Lockyer's 
 
198 HISTORY OF ASTRONOMY. PART 11. 
 
 theory of sun-spots, communicated to the Royal Society. May 6, 
 1 886, 1 and further developed some months later in his work on The- 
 Chemistry of the Sun. Spots are represented in it as incidental 
 to a vast system of solar .atmospheric circulation, starting with 
 the polar out- and up-flows indicated by observations during 
 some total eclipses, and eventuating in the plunge downward 
 from great heights upon the photosphere of prodigious masses 
 of condensed materials. From these falls result, primarily, 
 spots ; secondarily, through the answering uprushes in which 
 chemical and mechanical forces co-operate, their girdles of flame- 
 prominences. The limitation of spots to middle latitudes is 
 accounted for as follows. Above the equator, the height of fall 
 is so great that the products of condensation are volatilised 
 before reaching the photosphere. Near the poles, on the contrary, 
 the cooled substances have attained too small an elevation to give 
 the velocity needed to bring them to the surface in a solid form. 
 Hence their fall can produce nothing more conspicuous than 
 pores and "veiled spots." This augmented height of descent 
 with diminishing latitude offers besides an explanation of Car- 
 rington's law of solar rotation. For the nearer a spot lies to the 
 equator, the faster it must travel, because its materials, falling 
 from a greater elevation, bring down with them the increased 
 velocity belonging to a wider circumference. Local changes of 
 temperature and extent in the sun's atmosphere, due to the 
 violent disturbances affecting it, are supposed, on this hypothesis,, 
 to occasion periodical fluctuations in spot-frequency, as well as 
 the observed oscillations of the spot-zones. Nevertheless, the 
 evidence is far from clear that a circulatory system such as it 
 postulates is really in operation in the sun's atmosphere. A 
 similar objection applies to M. E. von Oppolzer's recently 
 published rationale of solar disturbances. 2 
 
 A singular circumstance has now to be recounted. On the 
 1st of September 1859, while Carrington was engaged in his 
 daily work of measuring the positions of sun-spots, he was 
 startled by the sudden appearance of two patches of peculiarly 
 intense light within the area of the largest group visible. His 
 
 1 Proc. Roy. Sac., No. 244. 2 Astr. Nach., No. 3146. 
 
CHAP. ii. SOLAR THEORIES. 199 
 
 first idea was that a ray of unmitigated sunshine had penetrated 
 the screen employed to reduce the brilliancy of the image ; but, 
 having quickly convinced himself to the contrary, he ran to 
 summon an additional witness of an obviously remarkable 
 occurrence. On his return he was disappointed to find the 
 strange luminous outburst already on the wane ; shortly after- 
 wards the last trace of it vanished. Its entire duration was five 
 minutes from 11.18 to II. 23 A.M., Greenwich time ; and during 
 those five minutes it had traversed a space estimated at 35,000 
 miles ! No perceptible change took place in the details of the 
 group of spots visited by this transitory conflagration, which, it was 
 accordingly inferred, took place at a considerable height above it. 1 
 
 Carrington's account was precisely confirmed by an observation 
 made at Highgate. Mr. R. Hodgson described the appearance 
 seen by him as that "of a very brilliant star of light, much 
 brighter than the sun's surface, most dazzling to the protected eye, 
 illuminating the upper edges of the adjacent spots and streaks, 
 not unlike in effect the edging of the clouds at sunset." 2 
 
 This unique phenomenon seemed as if specially designed to 
 accentuate the inference of a sympathetic relation between the 
 earth and the sun. From the 28th of August to the 4th of 
 September 1859, a magnetic storm of unparalleled intensity, 
 extent, and duration, was in progress over the entire globe. 
 Telegraphic communication was everywhere interrupted except, 
 indeed, that it was, in some cases, found practicable to work the 
 lines without batteries, by the agency of the earth-currents alone ; 3 
 sparks issued from the wires ; gorgeous auroras draped the skies 
 in solemn crimson over both hemispheres, and even within the 
 tropics ; the magnetic needle lost all trace of continuity in its 
 movements, and darted to and fro as if stricken with inexplicable 
 panic. The coincidence was drawn even closer. At the very 
 instant* of the solar outburst witnessed by Carrington and 
 Hodgson, the photographic apparatus at Kew registered a 
 marked disturbance of all the three magnetic elements ; while, 
 
 1 Month. Not., vol. xx., p. 13. ' 2 Jbid., p. 15. 3 Am. Jour., vol. xxix. 
 
 (and series), pp. 94-95. 4 The magnetic disturbance took place at 11.15 A.M. 
 three minutes before the solar blaze compelled the attention of Carrington. 
 
200 HISTORY OF ASTRONOMY. PART n. 
 
 shortly after the ensuing midnight, the electric agitation 
 culminated, thrilling the earth with subtle vibrations, and light- 
 ing up the atmosphere from pole to pole with the coruscating 
 splendours which, perhaps, dimly recall the times when our 
 ancient planet itself shone as a star. 
 
 Here then, at least, the sun was in Professor Balfour 
 Stewart's phrase "taken in the act" 1 of stirring up terrestrial 
 commotions. Nor have instances since been wanting of an 
 indubitable connection between outbreaks of individual spots 
 and magnetic disturbances. Four such were registered in 1882 ; 
 and symptoms of the same kind markedly attended the progress 
 across the sun of the enormous spot-group of February 1892 
 the largest ever recorded at Greenwich. This extraordinary 
 formation, which covered about ^J^ of the sun's disc, preserved 
 its individuality, though not its gigantic proportions, through 
 five complete rotations. 2 It was remarkable for a persistent 
 drift in latitude, its place altering progressively from 17 to 30 
 south of the solar equator. 
 
 The nearest approach to the event of September i, 1859, was 
 photographically observed by Professor George E. Hale at 
 Chicago, July 15, i892. 3 An active spot in the southern 
 hemisphere was the scene of this curiously sudden manifestation. 
 During an interval of 1 2 minutes between two successive expo- 
 sures, a bridge of dazzling light was found to have spanned the 
 boundary-line dividing the twin-nuclei of the spot ; and these, 
 after another 27 minutes, were themselves almost obliterated by 
 an overflow of far-spreading brilliancy. Yet two hours later, no 
 trace of the outburst remained, the spot and its attendant f aculge 
 remaining just as they had been previously to its occurrence. 
 Unlike that seen by Carrington, it was accompanied by no 
 exceptional magnetic phenomena, although a " storm " set in 
 next day. 4 Possibly a terrestrial analogue to the former might 
 be discovered in the " auroral beam " which traversed the heavens 
 
 1 Phil. Trans., vol. cli. , p. 428. * Maunder, Journal Brit. Astr. Ass. , vol. 
 ii., p. 386; Miss E. Brown, Ibid., p. 210; Month. Not., vol. lii., p. 354. 
 3 Astr. and Astro-Physics, Aug. 1892, p. 611. 4 Ibid., Nov. 1892, p. 819 
 
 (Sidgreaves). 
 
CHAP. ii. SOLAR THEORIES. 201 
 
 during a vivid display of polar lights, November 17, 1882, and 
 shared, there is every reason to believe, their electrical origin 
 and character. 1 
 
 Meantime M. Kudolf Wolf, transferred to the direction of 
 the Zurich Observatory, had relaxed none of his zeal in the 
 investigation of sun-spot periodicity. A laborious revision of 
 the entire subject with the aid of fresh materials led him, in 
 I&59, 2 to the conclusion that while the mean period differed 
 little from that arrived at in 1852 of ii.n years, very consider- 
 able fluctuations on either side of that mean were rather the rule 
 than the exception. Indeed, the phrase " sun-spot period " 
 must be understood as fitting very loosely the great fact it is 
 taken to represent ; so loosely, that the interval between two 
 maxima may rise to sixteen and a half, or sink below seven and 
 a half years. 3 In 1861 4 Wolf showed, and the remark was 
 fully confirmed by the Kew observations, that the shortest 
 periods brought the most acute crises, and -vice versd ; as if for 
 each wave of disturbance a strictly equal amount of energy 
 were available, which might spend itself lavishly and rapidly, or 
 slowly and parsimoniously, but could in no case be exceeded. 
 The further inclusion of recurring solar commotions within a 
 cycle of fifty-five and a half years was simultaneously pointed 
 out; and Hermann Fritz showed soon after that the aurora 
 borealis is subject to an identical double periodicity. 5 The 
 same inquirer detected besides both for auroras and sun- 
 spots a "secular period" of 222 years, 6 and the Kew 
 observations indicate for the latter, oscillations accomplished 
 within twenty-six and twenty-four days, 7 depending most likely 
 upon the rotation of the sun. This is certainly reflected in 
 
 1 See J. Rand Capron, Phil. Mag., vol. xv.,p. 318. ' 2 Mitiheilungen uber 
 die Sonnenflecken, No ix., Vierteljahrssclirift der Naturforschenden Gesellschaft 
 in Zurich, Jahrgang 4. 8 Mitth., No. lii., p. 58 (1881). 4 Ibid., No. xii., 
 p. 192. Mr. Joseph Baxendell, of Manchester, reached independently a similar 
 conclusion. See Month. Not., vol. xxi., p. 141. 5 Wolf, Mitth., No. xv. 
 p. 107, &c. Olmsted, following Hansteen, had already, in 1856, sought to 
 establish an auroral period of sixty-five years. /Smithsonian Contributions, 
 vol. viii., p. 37. 6 Hahn, Ueber die Beziehungen der Sonnenflecltenperiode 
 zu meteorologiscJien JErscheinungen, p. 99 (1877). 7 Report Brit. Ass., 1881, 
 
 p. 518 ; 1883, p. 418. 
 
202 HISTORY OF ASTRONOMY. PART n. 
 
 magnetic, and, according to the statistics collected by Mr. M. A. 
 Veeder, 1 of Lyons, N.Y., also in auroral periodicity. The more 
 closely spot-fluctuations are looked into, indeed, the more complex 
 they prove. Maxima -of one orjier are superposed upon, or 
 in part neutralised by, maxima of another order; originating 
 causes are masked by modifying causes ; the larger waves of 
 the commotion are indented with minor undulations, and these 
 again crisped with tiny ripples, while the whole rises and falls 
 with the swell of the great secular wave, scarcely perceptible 
 in its progress because so vast in scale. 
 
 The idea that solar maculation depends in some way upon 
 the position of the planets occurred to Galileo in i6i2. 2 It 
 has been industriously sifted by a whole bevy of modern solar 
 physicists. Wolf in i859 3 found reason to believe that the 
 eleven-year curve is determined by the action of Jupiter, modified 
 by that of Saturn, and diversified by influences proceeding from 
 the earth and Venus. Its tempting approach to agreement 
 with Jupiter's period of revolution round the sun, indeed, 
 irresistibly suggested a causal connection ; yet it does not seem 
 that the most skilful "coaxing" of figures can bring about a 
 fundamental harmony. Carrington pointed out in 1863, that 
 while, during eight successive periods, from 1770 downwards, 
 there were approximate coincidences between Jupiter's aphelion 
 passages and sun-spot maxima, the relation had been almost 
 exactly reversed in the two periods preceding that date ; 4 and 
 the latest conclusion of M. Wolf himself is that the Jovian 
 origin must be abandoned. 5 M. Duponchel 6 of Paris was 
 nevertheless not wholly unsuccessful in accommodating dis- 
 crepancies with the help of perturbations by the large exterior 
 planets ; since his prediction of an abnormal lengthening of the 
 maximum of 18834, through certain peculiarities in the positions 
 of Uranus and Neptune about the time it fell due, was partially 
 verified by the event. The previous maximum having occurred 
 in June 1870, the next phase of agitation should, if punctual, 
 
 1 Astr. and Astro- Phyaics, March 1893, p. 264. - Opere, t. iii., p. 412. 
 3 Mitth., Nos. vii. and xviii. 4 Observations at Eedldll, p. 248. 5 Comptes 
 Eendus, t. xcv.,p. 1249. 6 Ibid., t. xciii., p. 827 ; t. xcvi., p. 1418. 
 
CHAP. ii. SOLAR THEORIES. 203 
 
 have culminated about August 1881 ; whereas, after an abortive 
 effort at completion in April 1882, the final outburst was post- 
 poned to November 1883. The interval was thus 13.3 instead of 
 I I.I years ; and it is noticeable that the delay affected chiefly the 
 southern hemisphere. Alternations of activity in the solar 
 hemispheres were indeed a marked feature of the maximum 
 of 1883-4, which, in M. Faye's view, 1 derived thence its inde- 
 cisive character, while sharp, strong crises arise with the simul- 
 taneous advance of agitation north and south of the solar 
 equator. The curve of magnetic disturbance, it should be 
 added, followed with its usual strict fidelity the anomalous 
 fluctuations of the sun-spot curve. The ensuing minimum 
 occurred in February 1890. 
 
 It cannot be said that much progress has yet been made 
 towards the disclosure of the cause, or causes, of the sun- 
 spot cycle. No external influence adequate to the effect has, at 
 any rate, been pointed out. The Kew observers adduced evi- 
 dence to show that solar outbreaks are modified by planetary 
 configurations; 2 but even this secondary kind of relationship 
 has of late evaded notice. Other thinkers on this difficult 
 subject have provided a quasi-explanation of the periodicity in 
 question through certain assumed vicissitudes affecting internal 
 processes ; 3 while Professor Lockyer reaches the same end by 
 establishing self-compensatory fluctuations in the solar atmo- 
 spheric circulation. In all these theories, however, the flow 
 of change is arbitrarily arranged to suit a period, which 
 imposes itself as a fact necessarily admitted, although not 
 easily accounted for. 
 
 The question so much discussed, as to the influence of sun- 
 spots on weather, does not admit of a satisfactory answer. The 
 facts of meteorology are too complex for easy or certain classi- 
 fication. Effects owning dependence on one cause often wear 
 the livery of another ; the meaning of observed particulars may 
 be inverted by situation ; and yet it is only by the collection and 
 
 1 Comptes Ren>ius,t. c.,p. 593. ' 2 Ed. Phil. Trans., vol. xxiii., p. 499 ; Proc- 
 Roy. Soc., vols. xiv., p. 59 ; xx., p. 210. a Schulz, Astr. Nach., Nos. 2817-18, 
 2847-8; Wilsing, Ibid., Nos. 3000, 3039; Belopolsky, Ibid., Nos. 2722, 2954. 
 
204 HISTORY OF ASTRONOMY. PART n. 
 
 collocation of particulars that we can hope to reach any general 
 law. There is, however, a good deal of evidence to support the 
 opinion the grounds for which were primarily derived from 
 the labours of Mr. Meldrum a the Mauritius that increased 
 rainfall and atmospheric agitation attend spot-maxima; while 
 Herschel's conjecture of a more copious emission of light and 
 heat about the same epochs has received little support from 
 direct investigations. The inequality seems indeed to be the 
 other way. Radiation, according to the best recent authorities, 
 gains strength at times of minimum. 1 
 
 The examination of what we may call the texture of the 
 sun's surface derived new interest from a remarkable announce- 
 ment made by Mr. James Nasmyth in i862. 2 He had (as he 
 supposed) discovered the entire luminous stratum of the sun 
 to be composed of a multitude of elongated shining objects on 
 a darker background, shaped much like willow-leaves, of vast 
 size, crossing each other in all possible directions, and possessed 
 of unceasing relative motions. A lively controversy ensued. 
 In England and abroad, the most powerful telescopes were 
 directed to a scrutiny encompassed with varied difficulties. The 
 results, on the whole, were such as to invalidate the precision of 
 the disclosures made by the Hammerfield reflector. Mr. Dawes 
 was especially emphatic in declaring that Nasmyth's " willow- 
 leaves" were nothing more than the " nodules" of Sir William 
 Herschel seen under a misleading aspect of uniformity; and 
 there is little doubt that he was right. It is, however, admitted 
 that something of the kind may be seen in the penumbrse and 
 "bridges" of spots, presenting an appearance compared by 
 Dawes himself in 1852 to that of a piece of coarse straw-thatching 
 left untrimmed at the edges. 3 
 
 The term "granulated," suggested by Dawes in 1 864,* best 
 describes the mottled aspect of the solar disc as shown by 
 modern telescopes and cameras. The grains, or rather the 
 
 1 Blanford, Nature, vol. xliii., p. 583. ' 2 Report Brit. Ass., 1862, p. 16 
 (pt. ii.). 3 Mem. JR. A. /Soc., vol. xxi., p. 161. 4 Month. Not., vol. xxiv., 
 p. 162. 
 
CHAP. ii. SOLAR THEORIES. 205 
 
 " floccules," with which it is thickly strewn, have been resolved 
 by Langley, under exceptionally favourable conditions, into 
 " granules " not above 100 miles in diameter; and from these 
 relatively minute elements, composing, jointly, about one-fifth of 
 the visible photosphere, 1 he estimates that three quarters of the 
 entire light of the sun are derived, 2 Janssen goes so far as to 
 say that if the whole surface were as bright as its brightest 
 parts, its luminous emission would be ten to twenty times greater 
 than it actually is. 3 
 
 The rapid changes in the forms of these solar cloud-summits 
 are beautifully shown in the marvellous photographs taken by 
 Janssen at Meudon, with exposures reduced at times to TO^O-ITO" 
 of a second ! By their means, also, the curious phenomenon 
 known as the rdseau photospkerique has been made evident. 4 
 This consists in the diffusion over the entire disc of fleeting 
 blurred patches, separated by a reticulation of sharply outlined, 
 and regularly arranged granules. The appearance of imperfect 
 definition in the smudged areas is doubtless due to agitations in 
 the intervening solar atmosphere. M. Janssen considers that 
 the photospheric cloudlets change their shape and character with 
 the progress of the sun-spot period ; 5 but nothing is as yet quite 
 certain upon this point. 
 
 The " grains," or more brilliant parts of the photosphere, are 
 now generally held to represent the upper termination of ascend- 
 ing and condensing currents, while the darker interstices 
 (Herschel's "pores") mark the positions of descending cooler 
 ones. In the penumbrae of spots, the glowing streams rushing 
 up from the tremendous sub-solar furnace are bent sideways by 
 the powerful indraught, so as to change their vertical for a 
 nearly horizontal motion, and are thus taken, as it were, in flank 
 by the eye, instead of being seen end-on in mamelon form. Thus 
 a plausible explanation is afforded of the channelled penumbral 
 
 1 Am. Jour, of Science, vol. vii., 1874, p. 92. ' 2 Young, The Sun, 
 
 p. 103. 3 Ann. Bur. Lone/., 1879, p. 679. 4 Ibid., 1878, p. 689. 
 
 5 Eanyard, Knowledge, vol. xiv., p. 14, where compare the accompanying 
 photographs. 
 
206 HISTORY OF ASTRONOMY. PART n. 
 
 structure which suggested the comparison to a rude thatch. 
 Accepting this theory as in the main correct, we perceive that 
 the very same circulatory process which, in its spasms of activity, 
 gives rise to spots, produces in^its regular course the singular 
 " marbled " appearance, for the recording of which we are no 
 longer at the mercy of the fugitive or delusive impressions of the 
 human retina. And precisely this circulatory .process it is which 
 gives to our great luminary its permanence as a sun, or warming 
 and illuminating body. 
 
CHAPTER III. 
 RECENT SOLAR ECLIPSES. 
 
 BY observations made during a series of five remarkable eclipses, 
 comprised within a period of eleven years, knowledge of the solar 
 surroundings was advanced nearly to its present stage. Each of 
 these events brought with it a fresh disclosure of a definite and 
 unmistakable character. We will now briefly review this orderly 
 sequence of discovery. 
 
 Photography was first systematically applied to solve the 
 problems presented by the eclipsed sun, July 18, 1860. It is 
 true that a very creditable daguerreotype, 1 taken by Barkowski 
 with the Konigsberg heliometer during the eclipse of 1851, is 
 still valuable as a record of the corona of that year ; and some 
 subsequent attempts were made to register partial phases of 
 solar occupation, notably by Professor Bartlett at West Point in 
 i854; 2 but the ground remained practically unbroken until 
 i860. 
 
 In that year the track of totality crossed Spain, and thither, 
 accordingly, Mr. Warren De la Rue transported his photo- 
 heliograph, and Father Secchi his six-inch Cauchoix refractor. 
 The question then primarily at issue was that relating to the 
 nature of the red protuberances. Although, as already stated, 
 the evidence collected in 1851 gave a reasonable certainty of 
 their connection with the sun, objectors were not silenced ; and 
 when the side of incredulity was supported by so considerable an 
 authority as M. Faye, it was impossible to treat it with contempt. 
 Two crucial tests were available. If it could be shown that the 
 
 1 Vierteljahrsschrift As r. Ges., Jahrg. xxvi., p. 274. ' 2 Astr. Jour., vol. iv., 
 P- 33- 
 
2oS HISTORY OF ASTRONOMY. PART n. 
 
 fantastic shapes suspended above the edge of the dark moon 
 were seen under an identical aspect from two distant stations, 
 that fact alone would annihilate the theory of optical illusion or 
 "mirage"; while the -certainty .that they were progressively 
 concealed by the advancing moon on one side, and uncovered on 
 the other, would effectually detach them from dependence on 
 our satellite, and establish them as solar appendages. 
 
 Now both these tests were eminently capable of being applied 
 by photography. But the difficulty arose that nothing was 
 known as to the chemical power of the rosy prominence-light, 
 while everything depended on its right estimation. A shot had 
 to be fired, as it were, in the dark. It was a matter of some 
 surprise, and of no small congratulation, that, in both cases, the 
 shot took effect. 
 
 Mr. De la Rue occupied a station at Eivabellosa, in the Upper 
 Ebro valley ; Father Secchi set up his instrument at Desierto de 
 las Palmas, about 250 miles to the south-east, overlooking the 
 Mediterranean. From the totally eclipsed sun, with its strange 
 garland of flames, each observer derived several perfectly success- 
 ful impressions, which were found, on comparison, to agree in 
 the most minute details. This at once settled the fundamental 
 question as to the substantial reality of these objects ; while 
 their solar character was demonstrated by the passage of the 
 moon in front of them, indisputably attested by pictures taken 
 at successive stages of the eclipse. That forms seeming to defy 
 all laws of equilibrium were, nevertheless, not wholly evanescent, 
 appeared from their identity at an interval of seven minutes, 
 during which the lunar shadow was in transit from one station 
 to the other ; and the singular energy of their actinic rays was 
 shown by the record on the sensitive plates of some prominences 
 invisible in the telescope. Moreover, photographic evidence 
 strongly confirmed the inference previously drawn by Grant 
 and others, and now repeated with fuller assurance by F. Secchi 
 that an uninterrupted stratum of prominence-matter encom- 
 passes the sun on all sides, forming a reservoir from which 
 gigantic jets issue, and into which they subside. 
 
 Thus a first-fruits of accurate knowledge regarding the solar 
 
CHAP. in. RECENT ECLIPSES. 209 
 
 surroundings was gathered, and the value of the brief moments 
 of eclipse indefinitely increased, by supplementing transient 
 visual impressions with the faithful and lasting records of the 
 camera. 
 
 In the year 1 868 the history of eclipse spectroscopy virtually 
 began, as that of eclipse photography in 1 860 ; that is to say, 
 the respective methods then first gave definite results. On the 
 1 8th of August 1868, the Indian and Malayan peninsulas were 
 traversed by a lunar shadow producing total obscuration during 
 five minutes and thirty-eight seconds. Two English and two 
 French expeditions were despatched to the distant regions 
 favoured by an event so propitious to the advance of knowledge, 
 chiefly to obtain the verdict of the prism as to the composition 
 of prominences. Nor were they despatched in vain. An identical 
 discovery was made by nearly all the observers. At Jamkandi, 
 in the Western Ghauts, where Lieutenant (now Colonel) Herschel 
 was posted, unremitting bad weather threatened to bafile his eager 
 expectations ; but during the lapse of the critical five and a half 
 minutes the clouds broke, and across the driving wrack a " long, 
 finger-like projection " jutted out over the margin of the dark 
 lunar globe. In another moment the spectroscope was pointed 
 towards it ; three bright lines red, orange, and blue flashed 
 out, and the problem was solved. 1 The problem was solved in 
 this general sense, that the composition out of glowing vapours 
 of the objects infelicitously termed " protuberances " or " pro- 
 minences " was no longer doubtful ; although further inquiry 
 was needed for the determination of the particular species to 
 which those vapours belonged. 
 
 Similar, but more complete observations were made, with less 
 atmospheric hindrance, by Tennant and Janssen at Guntoor, by 
 Pogson at Masulipatam, and by Rayet at Wha-Tonne, on the 
 coast of the Malay peninsula, the last observer counting as 
 many as nine bright lines. 2 Among them it was not difficult 
 to recognise the characteristic light of hydrogen ; and it was 
 generally, though over-hastily, assumed that the orange ray 
 
 1 Proc. Boy. Soc., vol. xvii., p. 116. 2 Comptes Bendus, t. Ixvii., p. 757. 
 
210 HISTORY OF ASTRONOMY. PART n. 
 
 matched the luminous emissions of sodium. But fuller oppor- 
 tunities were at hand. 
 
 The eclipse of 1 868 is chiefly memorable for having taught 
 astronomers to do without eclipses, so far, at least, as one 
 particular branch of solar inquiry is concerned. Inspired by the 
 beauty and brilliancy of the variously tinted prominence-lines 
 revealed to him by the spectroscope, Janssen qxclaimed to those 
 about him, " Je verrai ces lignes-la en dehors des eclipses ! " On 
 the following morning he carried into execution the plan which 
 formed itself in his brain while the phenomenon which suggested 
 it was still before his eyes. It rests upon an easily intelligible 
 principle. 
 
 The glare of our own atmosphere alone hides the appendages 
 of the sun from our daily view. To a spectator on an airless 
 planet, the central globe would appear attended by all its 
 splendid retinue of crimson prominences, silvery corona, and far- 
 spreading zodiacal light, projected on the star-spangled black 
 background of an absolutely unilluminated sky. Now the 
 spectroscope offers the means of indefinitely weakening atmo- 
 spheric glare by diffusing a constant amount of it over an 
 indefinitely widened area. But monochromatic or "bright-line" 
 light is, by its nature, incapable of being so diffused. It can, of 
 course, be deviated by refraction to any extent desired ; but it 
 always remains equally concentrated, in whatever direction it 
 may be thrown. Hence, when it is mixed up with continuous 
 light as in the case of the solar flames shining through our 
 atmosphere it derives a relative gain in intensity from every 
 addition to the dispersive power of the spectroscope with which 
 the heterogeneous mass of beams is analysed. Employ prisms 
 enough, and eventually the undiminished rays of persistent 
 colour will stand out from the continually fading rainbow-tinted 
 band, by which they were at first effectually veiled. 
 
 This Janssen saw by a flash of intuition while the eclipse was 
 in progress ; and this he realised at 10 A.M. next morning, 
 August 19, 1868 the date of the beginning of spectroscopic 
 work at the margin of the unobscured sun. During the whole 
 of that day and many subsequent ones, he enjoyed, as he said, 
 
CHAP. in. RECENT ECLIPSES. 211 
 
 the advantage of a prolonged eclipse. The intense interest 
 with which he surveyed the region suddenly laid bare to his 
 scrutiny was heightened by evidences of rapid and violent 
 change. On the i8th of August, during the eclipse, a huge 
 spiral structure, at least 89,000 miles high, was perceived, 
 planted in surprising splendour on the rim of the interposed 
 moon. It was formed, as Major Tennant judged from its 
 appearance in his photographs, by the encounter of two mount- 
 ing torrents of flame, and was distinguished as the " Great 
 Horn." Next day it was in ruins ; hardly a trace remained to 
 show where it had been. 1 Janssen's spectroscope furnished him 
 besides with the strongest confirmation of what had already 
 been reported by the telescope and the camera as to the 
 continuous nature of the scarlet " sierra " lying at the base of 
 the prominences. Everywhere at the sun's edge the same bright 
 lines appeared. 
 
 It was not until the ipth of September that Janssen thought 
 fit to send news of his discovery to Europe. He little dreamed 
 of being anticipated; nor did he indeed grudge that science 
 should advance at the expense of his own undivided fame. A 
 few minutes before his despatch was handed to the Secretary of 
 the Paris Academy of Sciences, a communication similar in 
 purport had been received from Mr. Norman Lockyer. There 
 is no need to discuss the narrow and wearisome question of 
 priority ; each of the competitors deserves, and has obtained, 
 full credit for his invention. With noteworthy and confident 
 prescience, Mr, Lockyer, in 1866, before anything was yet 
 known regarding the constitution of the " red flames," ordered a 
 strongly dispersive spectroscope for the express purpose of 
 viewing, apart from eclipses, the bright-line spectrum which he 
 expected them to give. Various delays, however, supervened, 
 and the instrument was not in his hands until October 16, 1868. 
 On the 2Oth he picked up the vivid rays, of which the presence 
 and (approximately) the positions had in the interim become 
 known. But there is little doubt that, even without that pre- 
 vious knowledge, they would have been found ; and that the 
 1 Comptes Rendus, t. Ixvii., p. 839. 
 
212 HISTORY OF ASTRONOMY. PART n. 
 
 eclipse of August 18 only accelerated a discovery already 
 assured. 
 
 Dr. Huggins, meanwhile, had been tending towards the same 
 goal during two and a half years in his observatory at Tulse Hill. 
 The principle of the spectroscopic visibility of prominence-lines 
 at the edge of an uneclipsed sun was quite explicitly stated by 
 him in February I868, 1 and he devised various apparatus for 
 bringing them into actual view ; but not until he knew where 
 to look did he succeed in seeing them. 
 
 Astronomers, thus liberated, by the acquisition of power to 
 view them at any time, from the necessity of studying pro- 
 minences during eclipses, were able to concentrate the whole of 
 their attention on the corona. The first thing to be done was 
 to ascertain the character of its spectrum. This was seen in 
 1 868 only as a faintly continuous one ; for Rayet, who seems to 
 have perceived its distinctive bright line far above the summits 
 of the flames, connected it, nevertheless, with those objects. 
 On the other hand, Lieutenant Campbell ascertained on the 
 same occasion the polarisation of the coronal light in planes 
 passing through the sun's centre, 2 thereby showing that light to 
 be, in whole or in part, reflected sunshine. But if reflected sun- 
 shine, it was objected, the chief at least of the dark Fraunhofer 
 lines should be visible in it, as they are visible in moonbeams, 
 sky illumination, and all other sun-derived light. The objec- 
 tion was well founded, but was prematurely urged, as we 
 shall see. 
 
 On the /th of August 1869, a track of total eclipse crossed 
 the continent of North America diagonally, entering at Behring's 
 Straits, and issuing on the coast of North Carolina. It was 
 beset with observers ; but the most effective work was done in 
 Iowa. At Des Moines, Professor Harkness of the Naval 
 Observatory, Washington, obtained from the corona an " abso- 
 lutely continuous spectrum," slightly less bright than that of 
 the full moon, but traversed by a single green ray. 3 The same 
 green ray was seen at Burlington, and its position measured by 
 
 1 Month. Not., vol. xxvii., p. 88. 2 Proc. Hoy. Soc., vol. xvii., p. 123, 
 3 Washington Observations, 1867, App. ii., Harkness's Keport, p. 60. 
 
CHAP. in. RECENT ECLIPSES 213 
 
 Professor Young. 1 It was found to coincide with that of a dark 
 line of iron in the solar spectrum, numbered 1474 on Kirch- 
 hoff's scale. This was perplexing, since it seemed, at first 
 sight, to compel the inference that the corona was actually 
 composed of vapour of iron, 2 so attenuated as to give only one 
 line of secondary importance out of the many hundreds be- 
 longing to it. But in 1876 Young was able, by the use of 
 greatly increased dispersion, to resolve the Fraunhofer line 
 '1474" into a pair, of which one component is due to iron, 
 the other (the more refrangible) to the coronal gas. 3 This 
 substance, now distinguished as " coronium," of which nothing 
 is known to terrestrial chemistry, appears luminous at least 
 half a million of miles above the sun's surface, and is hence 
 presumably much lighter even than hydrogen. 
 
 A further trophy was carried off by American skill 4 sixteen 
 months after the determination due to it of the distinctive 
 spectrum of the corona. The eclipse of December 22, 1870, 
 though lasting only two minutes and ten seconds, drew observers 
 from the New, as well as from the Old World to the shores of 
 the Mediterranean. Janssen issued from beleagured Paris in a 
 balloon, carrying with him the vital parts of a reflector specially 
 constructed to collect evidence about the corona. But he 
 reached Oran only to find himself shut behind a cloud-curtain 
 more impervious than the Prussian lines. Everywhere the sky 
 was more or less overcast. Mr. Lockyer's journey from England 
 to Sicily, and shipwreck in the Psyche, were recompensed with 
 a glimpse of the solar aureola during one second and a half ! 
 Three parties stationed at various heights on Mount Etna, saw 
 absolutely nothing. Nevertheless important information was 
 snatched in despite of the elements. 
 
 The prominent event was Young's discovery of the " reversing 
 layer." As the surviving solar crescent narrowed before the 
 encroaching moon, "the dark lines of the spectrum," he tells 
 us, "and the spectrum itself, gradually faded away, until all at 
 
 1 Am. Jour., vol. xlviii. (2nd series), p. 377. ' 2 This view was never 
 assented to by either Young or Lockyer. 3 Am. Jour., vol. xi. (3rd series), 
 p. 429. 4 Everything in such observations depends upon the proper manipu- 
 lation of the slit of the spectroscope. 
 
214 HISTORY OF ASTRONOMY. PART n. 
 
 once, as suddenly as a bursting rocket shoots out its stars, the 
 whole field of view was filled with bright lines more numerous 
 than one could count. The phenomenon was so sudden, so un- 
 expected, and so wonderfully beautiful, as to force an involuntary 
 exclamation." 1 Its duration was about two seconds, and the 
 impression produced was that of a complete reversal of the 
 Fraunhofer spectrum that is, the substitution of a bright for 
 every dark line. 
 
 Now something of the kind was theoretically necessary to 
 account for the dusky rays in sunlight which have taught us so 
 much, and have yet much more to teach us ; so that, although 
 surprising from its transitory splendour, the appearance could 
 not strictly be called "unexpected." Moreover, its premonitory 
 symptom in the fading out of those rays had been actually 
 described by Father Secchi in i868, 2 and looked for by Young 
 as the moon covered the sun in August 1869. But with the 
 slit of his spectroscope placed normally to the sun's limb, the 
 bright lines gave a flash too thin to catch the eye. In 1870 the 
 position of the slit was tangential it ran along the shallow bed 
 of incandescent vapours, instead of cutting across it : hence his 
 success. 
 
 The same observation was made at Xerez de la Frontera by 
 Mr. Pye, a member of Young's party ; and, although an exceed- 
 ingly delicate one, has since frequently been repeated. The 
 whole Fraunhofer series appeared bright (omitting other in- 
 stances) to Maclear, Herschel, and Fyers in 1871, at the 
 beginning or end of totality ; to Pogson during a period (perhaps 
 erroneously estimated) of from five to seven seconds, at the 
 break up of an annular eclipse, June 6, 1872 ; to Stone at Klip- 
 fontein, April 16, 1874, when he saw "the field full of bright 
 lines." 3 But between the picture presented by the " veritable 
 pluie de lignes brillantes," 4 which descended into M. Trepied's 
 spectroscope for three seconds after the disappearance of the 
 sun, May 17, 1882, and the familiar one of the dark-line solar 
 spectrum, certain differences were perceived, showing their 
 
 1 Mem. B. A. Soc., vol. xli., p. 435. 2 Comptes Rendus, t. Ixvii., p. 1019. 
 3 Mem. H. A. Soc., vol. xli., p. 43. 4 Comptes Rendus, t. xciv., p. 1640. 
 
CHAP. in. RECENT ECLIPSES. 215 
 
 relation to be not simply that of a positive to a negative 
 impression. 
 
 A " reversing layer," or stratum of mixed vapours, glowing, 
 but at a lower temperature than that of the actual solar surface, 
 was an integral part of Kirchhoff 's theory of the production of 
 the Fraunhofer lines. Here it was assumed that the missing rays 
 were stopped, and here also it was assumed that the missing rays 
 would be seen bright, could they be isolated from the over- 
 powering splendour of their background. This isolation is 
 effected by eclipses, with the result beautifully confirmatory of 
 theory of reversing, or turning from dark to bright, the Fraun- 
 hofer spectrum. But there is a difficulty. If absorption be in 
 truth thus localised, it should appear greatly strengthened near 
 the edges of the solar disc. This, however, is not the case. 
 Kirchhoff met the objection by giving a great depth to the 
 reversing stratum, whereby the difference in length of the paths 
 across that stratum traversed by rays from the sun's limb and 
 centre became relatively insignificant. In other words, he 
 supposed that the chief part of the light absent from the 
 spectrum was arrested in the region of the corona. 
 
 Faye, on the other hand, abolished the reversing layer alto- 
 gether (there was at that time no ocular demonstration of its 
 existence) ; or rather, sunk it out of sight below the visible level 
 of the photosphere, and got the necessary absorption done in the 
 interstices of the photospheric clouds by the vapours in which 
 they float, and from which they condense. It was, however, at 
 once seen that the lines thus produced would be bright, not 
 dark, since the brilliant cumuli would be cooled, by their greater 
 power of radiation, below the temperature of the surrounding 
 medium. Less obviously out of accord with facts was an 
 explanation offered by Professor Hastings of Baltimore in iSSi. 1 
 Young's stratum, with its estimated thickness of 600 miles, 
 represented in his view only the upper margin of a reversing 
 ocean, in which the granules of the photosphere float at various 
 depths. The necessary difference of temperature is derived from 
 
 1 Am. Jour, of Science, vol. xxi., p. 33 ; cf. Scheiner, Die /Spectralanalyse der 
 Gestirne, p. 196. 
 
216 HISTORY OF ASTRONOMY. PART n. 
 
 the coolness of the descending vapours, which bathe the radiat- 
 ing particles and rob them of certain characteristic beams. Some 
 observations, however, made by Professor Lockyer during the 
 total eclipse of 1882, and by the late Father Perry of Stony- 
 hurst 1 and Mr. Turner of the Greenwich Observatory during 
 that of 1886, threw a new light upon the matter. They seem 
 to prove the brief prismatic display seen at ^he beginning and 
 end of totality to be only a part of a varied and extensive 
 phenomenon. By careful watching, it was found to be preceded 
 (in the advancing phase) by the stealing out, first of short, vivid 
 lines close to the limb, corresponding to the highest known 
 temperature, then of long, faint lines telling of absorption high 
 up in the coronal regions. Just such effects had been predicted 
 by Lockyer, 2 and are required by his theory of the origin of 
 the Fraunhofer spectrum through the combined and varying 
 absorption of all the successive layers of the sun's atmosphere, 
 each at a lower temperature, and with a higher molecular 
 complexity than those beneath. Thus a strict correspondence 
 between the bright rays of the so-called " reversing layer " and the 
 solar dusky rays is not to be expected, and would, in fact, prove 
 somewhat embarrassing. The question may eventually be de- 
 cided by an instantaneous photograph of the complete " rainbow- 
 flash " accompanying totality. 
 
 The last of the five eclipses which we have grouped together 
 for separate consideration was visible in Southern India and 
 Australia, December 12, 1871. Some splendid photographs 
 were secured by the English parties on the Malabar coast, 
 showing, for the first time, the remarkable branching forms of 
 the coronal emanations ; but the most conspicuous result was 
 Janssen's detection of some of the dark Fraunhofer lines, long 
 vainly sought in the continuous spectrum of the corona. Chief 
 amongst these was the D-line of sodium, the original index, it 
 might be said, to solar chemistry. No proof could be afforded 
 more decisive than this faint echoing back of the distinctive 
 notes of the Fraunhofer spectrum, that the polariscope had 
 spoken the truth in asserting a large part of the coronal radiance 
 
 1 Phil. Trans., vol. clxxx., pp. 387, 359. 2 Chemist, oftlie Sun, p. 359. 
 
CHAP. in. RECENT ECLIPSES. 217 
 
 to be reflected sunlight. But it is (especially at certain epochs) 
 so drenched in original luminous emissions, that its characteristic 
 features are almost obliterated. Janssen's success in seizing 
 them was due in part to the extreme purity of the air at Sholoor, 
 in the Neilgherries, where he was stationed ; in part to the use 
 of an instrument adapted by its large aperture and short focus 
 to give an image of the utmost possible luminosity. 
 
 His observations further " peremptorily demonstrated " the 
 presence of hydrogen far outside the region of prominences, 
 and forming an integral constituent of the corona. This im- 
 portant fact was simultaneously attested by Lockyer at Baikul, 
 and by Eespighi at Poodacottah, each making separate trial 
 of a " slitless spectroscope " devised for the occasion. This 
 consists simply of a prism placed outside the object-glass of a 
 telescope or the lens of a camera, whereby the radiance encom- 
 passing the eclipsed sun is separated into as many differently 
 tinted rings as it contains different kinds of light. These 
 tinted rings were viewed by Respighi through a telescope, and 
 were photographed by Lockyer, with the same result of showing 
 hydrogen to ascend uniformly from the sun's surface to a height 
 of fully 200,000 miles. Another notable observation made by 
 Herschel and Tennant at Dodabetta showed the green ray of 
 coronium to be just as bright in a rift as in the adjacent 
 streamer. The visible structure of the corona was thus seen to 
 be independent of the distribution of the gases which enter into 
 its composition. 
 
 By means, then, of the five great eclipses of 1 860-7 1 it was 
 ascertained : first, that the prominences, and at least the lower 
 part of the corona, are genuine solar appurtenances ; secondly, 
 that the prominences are composed of hydrogen and other 
 gases in a state of incandescence, and rise, as irregular outliers, 
 from a continuous envelope of the same materials, some 
 thousands of miles in thickness ; thirdly, that the corona is of a 
 highly complex constitution, being made up in part of glowing 
 vapours, in part of matter capable of reflecting sunlight. 
 We may now proceed to consider the results of subsequent 
 eclipses. 
 
218 HISTORY OF ASTRONOMY. PART n. 
 
 These have raised, and have helped to solve, some very curious 
 questions. Indeed, every carefully watched total eclipse of the 
 sun stimulates as well as appeases curiosity, and leaves a legacy 
 of outstanding doubt,, continually, as time and inquiry go on, 
 removed, but continually replaced. It cannot be denied that the 
 corona is a perplexing phenomenon, and that it does not become 
 less perplexing as we know more about it. It presented itself 
 under quite a new and strange aspect on the occasion of the 
 eclipse which visited the Western States of North America^ 
 July 29, 1878. The conditions of observation were peculiarly 
 favourable. The weather was superb ; above the Rocky Moun- 
 tains the sky was of such purity as to permit the detection, with 
 the naked eye, of Jupiter's satellites on several successive nights. 
 The opportunity of advancing knowledge was made the most of. 
 Nearly a hundred astronomers (including several Englishmen) 
 occupied twelve separate posts, and prepared for an attack in 
 force. 
 
 The question had often suggested itself, and was a natural one 
 to ask, whether the corona sympathises with the general condi- 
 tion of the sun ? whether, either in shape or brilliancy, it varies 
 with the progress of the sun-spot period ? A more propitious 
 moment for getting this question answered could hardly have 
 been chosen than that at which the eclipse occurred. Solar dis- 
 turbance was just then at its lowest ebb. The development of 
 spots for the month of July 1878 was represented on Wolfs 
 system of "relative numbers" by the f raction o. I , as against 
 135.4 for December 1870, an epoch of maximum activity. The 
 " chromosphere" 1 was, for the most part, shallow and quiescent ; 
 its depth, above the spot-zones, had sunk from about 6000 to 
 2OOO miles ; prominences were few and faint. Obviously, if 
 a type of corona corresponding to a minimum of sun-spots 
 existed, it should be seen then or never. It was seen ; but 
 while, in some respects, it agreed with anticipation, in others it 
 completely set it at naught. 
 
 The corona of 1878, as compared with those of 1869, 1870, 
 
 1 The rosy envelope of prominence- matter was so named by Lockyer in 
 1868 (Phil. Trans., vol. clix., p. 430). 
 
CHAP. in. RECENT ECLIPSES. 219 
 
 and 1871, was generally admitted to be shrunken in its main 
 outlines, and much reduced in brilliancy. Mr. Lockyer pro- 
 nounced it ten times fainter than in 1871; Professor Harkness 
 estimated its light at less than one-seventh that derived from 
 the mist-blotted aureola of I87O. 1 In shape, too, it was mark- 
 edly different. When sun-spots are numerous, the corona 
 appears to be most fully developed above the spot-zones, thus 
 offering to our eyes a rudely quadrilateral contour. The four 
 great luminous sheaves forming the corners of the square are 
 made up of rays curving together from each side into " synclinal ' r 
 or ogival groups, each of which may be compared to the petal of 
 a flower. To Janssen, in 1871, the eclipsing moon seemed like 
 the dark heart of a gigantic dahlia, painted in light on the sky ; 
 and the similitude to the ornament on a compass-card, used by 
 Sir George Airy in 1851, well conveys the decorative effect of 
 the beamy, radiated kind of aureola, never, it would appear, 
 absent when solar activity is at a tolerably high pitch. In his 
 splendid volume on eclipses, 2 with which the systematic study of 
 coronal structure may be said to have begun, Mr. Ranyard first 
 generalised the synclinal peculiarity by a comparison of records ; 
 but the symmetry of the arrangement, though frequently 
 striking, is liable to be confused by secondary formations. He 
 further pointed out, with the help of careful drawings from 
 the photographs of 1871 made by Mr. Wesley, the curved and 
 branching shapes assumed by the component filaments of massive 
 bundles of rays. Nothing of all this, however, was visible in 
 1878. Instead, there was seen, as the groundwork of the corona, 
 a ring of pearly light, nebulous to the eye, but shown by tele- 
 scopes and in photographs to have a fibrous texture, as if made 
 up of tufts of fine hairs. North and south, a series of short, 
 vivid, electrical-looking flame-brushes diverged with conspicuous 
 regularity from each of tiie solar poles. Their direction was not 
 towards the centre of the sun, but towards each summit of his 
 axis, so that the farther rays on either side started almost tan- 
 gentially to the surface. It is difficult not to connect this 
 
 1 Bull. Phil. Soc. Washington, vol. iii., p. 118. 2 Mem. R. A. Soc., vol. xli., 
 1879. 
 
220 HISTORY OF ASTRONOMY. PART n. 
 
 unusual display of polar activity l with the great relative depth of 
 the chromosphere in those regions, noticed by Trouvelot previous 
 to the eclipse. 2 
 
 But the leading, and a truly, amazing, characteristic of the 
 phenomenon was formed by two vast, faintly luminous wings 
 of light, expanded on either side of the sun in the direction of 
 the ecliptic. These were missed by very few careful onlookers ; 
 but the extent assigned to them varied with skill in, and 
 facilities for seeing. By far the most striking observations 
 were made by Newcomb at Separation (Wyoming), by Cleve- 
 land Abbe from the shoulder of Pike's Peak, and by Langley 
 at its summit, an elevation of 14,100 feet above the sea. 
 Never before had an eclipse been viewed from anything 
 approaching that altitude, or under so translucent a sky. A 
 proof of the great reduction in atmospheric glare was afforded 
 by the perceptibility of the corona for above four minutes after 
 totality was over. During the 165 seconds of its duration, more- 
 over, the remarkable streamers above alluded to continued "per- 
 sistently visible," stretching away right and left of the sun to 
 a distance of at least ten million miles ! One branch was traced 
 over an apparent extent of fully twelve lunar diameters, without 
 sign of a definite termination having been reached ; and there 
 were no grounds for supposing the other more restricted. The 
 axis of the longest ray was found to coincide exactly, so far as 
 could be judged, with the ecliptic. 3 Pale cross-beams were seen 
 by Young and Abbe. 
 
 The resemblance to the zodiacal light was striking; and a 
 community of origin between that enigmatical member of our 
 system and the corona was irresistibly suggested. We should, 
 indeed, expect to see, under such exceptionally favourable 
 atmospheric conditions as Professor Langley enjoyed on Pike's 
 Peak, the roots of the zodiacal light presenting near the sun 
 
 1 Professor W. A. Norton observed a similar phenomenon in 1869, accom- 
 panied by some symptoms of equatorial emission. This is the more remark- 
 able as 1869 was a year of many sun-spots. His evidence, though unsup- 
 ported, and adverse to the theory of varying types, should not be overlooked. 
 See Am. Jour, of Sc., vol. i. (3rd ser.), p. I. ' 2 Wash. Obs., 1876, App. iii., 
 p. 80. 3 Ibid., p. 209. 
 
CHAP. in. RECENT ECLIPSES. 221 
 
 just such an appearance as lie witnessed ; but we can imagine 
 110 reason why their visibility should be associated with a low 
 state of solar activity. Nevertheless this seems to be the case 
 with the streamers which astonished astronomers in 1878. 
 For in August 1867. when similar equatoreal emanations, 
 accompanied by similar symptoms of polar excitement, were 
 described and depicted by Grosch 1 of the Santiago Observa- 
 tory, sun-spots were at a minimum; while the corona of 1715, 
 which appears from the record of it by Roger Cotes 2 to have 
 been of the same type, preceded by three years the ensuing 
 maximum. The eclipsed sun was seen by him at Cambridge, 
 May 2, 1715, encompassed with a ring of light about one-sixth 
 of the moon's diameter in breadth, upon which was superposed a 
 luminous cross formed of long bright branches lying very nearly 
 in the plane of the ecliptic, combined with shorter polar arms so 
 faint as to be only intermittently visible. The resemblance 
 between his sketch and Cleveland Abbe's drawing of the corona 
 of 1878 is extremely striking. It should, nevertheless, be noted 
 that some conspicuous spots were visible on the sun's disc at the 
 time of Cotes's eclipse, and that the preceding minimum 
 (according to Wolf) occurred in 1712. Thus, the coincidence of 
 epochs is imperfect. 
 
 Professor Cleveland Abbe was fully persuaded that the long 
 rays carefully observed by him from Pike's Peak were nothing 
 else than streams of meteorites rushing towards or from peri- 
 helion ; and it is quite certain that the solar neighbourhood 
 must be crowded with such bodies. But there are no grounds 
 for supposing that they affect the ecliptic more than any other 
 of the infinite number of planes passing through the sun's 
 centre. On the contrary, everything we know leads us to 
 believe that meteorites, like their cometary allies, yield no 
 obedience to the rules of the road which bind the planets, 
 but travel in either direction indifferently, and in paths in- 
 clined at any angle to the fundamental plane of our system. 
 Besides, the peculiar structure at the base of the streamers 
 
 1 Astr. Nach., No. 1737. 2 Correspondence with Newton, pp. 181-184 ; 
 Ranyard, Mem. JR. Astr. 8oc., vol. xli., p. 501. 
 
222 HISTORY OF ASTRONOMY. PART n. 
 
 displayed in the photographs, the curved rays meeting in pointed 
 arches like Gothic windows, the visible upspringing tendency, 
 the filamentous texture, speak unmistakably of the action of 
 forces proceeding from the sup, not of extraneous matter 
 circling round him. 
 
 Again, it may be asked what possible relation can exist 
 between the zodiacal plane and the sun's internal activity? 
 For it is a remarkable fact that to this approximately, and not 
 to the level of the solar equator, the streamers conformed. We 
 are acquainted with no such relation ; but it may be remarked 
 that the coronal axis of symmetry has frequently been observed 
 during eclipses to be inclined at an appreciable angle to the 
 solar axis of rotation, and the corresponding "magnetic 
 equator" might quite conceivably be the scene of emanations 
 induced by some form of electrical repulsion. 
 
 The surest, though not the most striking, proof of sympa- 
 thetic change in the corona is afforded by the analysis of its 
 light. In 1878 the bright lines so conspicuous in the coronal 
 spectrum in 1870 and 1871 were discovered to have faded to the 
 very limits of visibility, Several skilled observers failed to see 
 them at all ; but Young and Eastman succeeded in tracing both 
 the hydrogen and the green " 1474 " rays all round the sun, to 
 a height estimated at 340,000 miles. The substances emitting 
 them were thus present, though in a low state of incandescence. 
 The continuous spectrum was relatively strong ; a faint reflec- 
 tion of the Fraunhofer lines was traced in it ; and polarisation 
 was undoubted, increasing towards the limb, whereas in 1870 
 it reached a maximum at a considerable distance from it. 
 Experiments with Edison's tasimeter showed that the corona 
 radiates a sensible amount of heat. 
 
 The next promising eclipse occurred May 17, 1882. The 
 concourse of astronomers which has become usual on such 
 occasions assembled this time at Sohag, in Upper Egypt. 
 Karely have seventy-four seconds been turned to such account. 
 To each observer a special task was assigned, and the advan- 
 tages of a strict division of labour were visible in the variety and 
 amount of the information gained. 
 
CHAP. in. RECENT ECLIPSES. 223 
 
 The year 1882 was one of numerous sun-spots. On the eve 
 of the eclipse twenty-three separate maculae were counted. If 
 there were any truth in the theory which connected coronal 
 forms with fluctuations in solar activity, it might be anticipated 
 that the vast ecliptical expansions and polar "brushes " of 1878 
 would be found replaced by the star-like structure of 1871. 
 This expectation was literally fulfilled. No zodiacal streamers 
 were to be seen. The universal failure to perceive them, after 
 express search in a sky of the most transparent purity, justifies 
 the emphatic assertion that they were not there. Instead, the 
 type of corona observed in India eleven years earlier, was repro- 
 duced, with its shining aigrettes, complex texture, and brilliant 
 radiated aspect. 
 
 Concordant testimony was given by the spectroscope. The 
 reflected light derived from the corona was weaker than in 1878, 
 while its original emissions were proportionately intensified. A 
 complex spectrum testified to their heterogeneous nature. Many 
 new bright lines were discovered. Tacchini determined four in 
 the red end of the spectrum ; Thollon perceived several in the 
 violet ; and Dr. Schuster measured and photographed about 
 thirty. 1 The Fraunhofer lines autographically recorded in the 
 continuous spectrum were not less numerous. This was the 
 first successful attempt to photograph the spectrum of the 
 corona as seen with an ordinary slit-spectroscope. The slitless 
 spectroscope, or "prismatic camera," although its statements 
 are necessarily of a far looser character, was, however, also 
 employed. And with profit, since it served to bring out at least 
 one important fact that of the uncommon strength in the 
 chromospheric regions of the violet light concentrated in the 
 two lines H and K, attributed to calcium. Professor Lockyer 
 observed the continuous part of the coronal spectrum to be 
 curiously ribbed and fluted ; and in the spectrum 'of one pro- 
 minence twenty-nine rays were photographed, including the 
 hydrogen ultra-violet series, discovered by Dr. Huggins in the 
 emissions of white stars. 2 
 
 1 Proc. Roy. /Sbc., vol. xxxv. , p. 154. ' 2 Abney, Phil. Trans., vol. clxxv., 
 p. 267. 
 
224 HISTORY OF ASTRONOMY. PART 11. 
 
 Dr. Schuster's photographs of the corona itself were the most 
 extensive, as well as the most detailed, of any yet secured. 
 One rift imprinted itself on the plates to a distance of nearly a 
 diameter and a half from the limb ; and the transparency of the 
 streamers was shown by the delineation through them of the 
 delicate tracery beyond. The singular and picturesque feature 
 was added of a bright comet, self-depicted ir*. all the exquisite 
 grace of swift movement betrayed by the fine curve of its tail, 
 hurrying away from, possibly, its only visit to our sun, and 
 rendered momentarily visible by the withdrawal of the splendour 
 in which it had been, and was again quickly veiled. 
 
 From a careful study of these valuable records Dr. Huggins 
 derived the idea of a possible mode of photographing the 
 corona without an eclipse. 1 As already stated, its ordinary 
 invisibility is entirely due to the "glare" or reflected light 
 diffused through our atmosphere. But Dr. Huggins found, on 
 examining Schuster's negatives, that a large proportion of the 
 light in the coronal spectrum, both continuous and interrupted, 
 is collected in the violet region between the Fraunhofer lines 
 G and H. There, then, he hoped that, all other rays being ex- 
 cluded, it might prove strong enough to vanquish inimical glare, 
 and stamp on prepared plates, through local superiority in illumin- 
 ative power, the forms of the appendage by which it is emitted. 
 
 His experiments were begun towards the end of May 1882, 
 and by September 28 he had obtained a fair earnest of success. 
 The exclusion of all other qualities of light save that with 
 which he desired to operate, was accomplished (screens of tinted 
 glass having been tried and discarded) by using chloride 
 of silver as his sensitive material, that substance being chemically 
 inert to all other but those precise rays in which the corona has 
 the advantage. 2 The genuineness of the impressions left upon 
 his plates was strongly attested. "Not only the general 
 
 1 Proc. Roy. >Sbc., vol. xxxiv., p. 409 ; Report Brit. Ass., 1883, p. 346. Experi- 
 ments directed to the same end had been made by Dr. O. Lohse at Potsdam, 
 1878-80 ; not without some faint promise of ultimate success. Astr. Nach., 
 No. 2486. - The sensitiveness of chloride of silver extends from h to H ; 
 that is, over the upper or more refrangible half of the space in which the 
 main part of the coronal light is concentrated. 
 
CHAP. in. RECENT ECLIPSES. 225 
 
 features," Captain Abney affirmed, 1 " are the same, but details, 
 such as rifts and streamers, have the same position and form." 
 It was found, moreover, that the corona photographed during 
 the total eclipse of May 6, 1883, was intermediate in shape 
 between the coronas photographed by Dr. Huggins before and 
 after that event, each picture taking its proper place in a 
 series of progressive modifications highly interesting in them- 
 selves, and full of promise for the value of the method 
 employed to record them. 2 But experiments on the subject 
 were singularly interrupted. The volcanic explosion in the 
 Straits of Sunda in August 1883 brought to astronomers a 
 peculiarly unwelcome addition to their difficulties. The mag- 
 nificent sunglows due to the diffractive effects on light of the 
 vapours and fine dust flung in vast volumes into the air, 
 and rapidly diffused all round the globe, betokened an atmo- 
 spheric condition of all others the most prejudicial to delicate 
 researches in the solar vicinity. The filmy coronal forms, 
 accordingly, which had been hopefully traced on Dr. Huggins's 
 plates ceased to appear there ; nor were any substantially better 
 results obtained by Mr. 0. Ray Woods, in the purer air either of 
 the Eiffel or the Cape of Good Hope, during the three ensuing 
 years. Nay, doubts were expressed as to the genuineness of 
 what had at first seemed to be accomplished ; and the eclipse of 
 the sun on August 29, 1886, was anticipated as an opportunity 
 for resolving them in one sense or the other. For. evidently, in 
 a true coronal photograph taken during the partial phases, the 
 contour of the moon off the sun must stand out against the faint 
 radiance beyond ; while deceptive appearances caused by air- 
 glare would take no notice of the moon, for the simple reason 
 that they would originate in front of her. No trace of the lunar 
 globe, however, was visible on any of the plates exposed on 
 August 29, at Grenada ; and what vestiges of " structure " there 
 were, came out almost better upon the moon than beside her, thus 
 stamping themselves at once as spurious. It is hence quite certain 
 that they had not been appreciably affected by coronal light. 
 This was discouraging ; but when all the circumstances are 
 
 1 Proc. Roy. Soc., vol. xxxiv., p. 414. 2 Report Brit. Assoc., 1883, p. 351. 
 
 15 
 
226 HISTORY OF ASTRONOMY. PART n. 
 
 taken into account, scarcely surprising. 1 The corona was, on 
 that occasion, shorn of much of its splendour through the action 
 of moisture-laden air ; the sun, when the observations were 
 made, had less than nineteen degrees of altitude, so that a large 
 proportion of the highly refrangible rays selected by silver 
 chloride must have been cut off by atmospheric absorption ; 
 finally, the eruptive products from Krakatao were still far from 
 having wholly subsided. That the effect sought is a possible 
 one is proved by the distinct appearance of the moon projected 
 on the corona, in photographs of the partially eclipsed sun in 
 1858, 1889, and 1890.2 
 
 For the unequivocal success of Dr. Huggins's method rarely 
 favourable conditions are undoubtedly requisite. Eesults of 
 substantial value from it can hardly be hoped for in this climate, 
 and at the sea-level. Every yard of ascent, however, tells in 
 its favour ; and its effectiveness may not improbably be en- 
 hanced, through changes in the coronal spectrum, at epochs of 
 sun-spot maximum. Thus, we may hopefully look forward to 
 the results of experiments shortly to be made in Colorado or 
 Arizona with an apparatus devised by Professor Hale for the 
 purpose of carrying out its essential principle of chromatic 
 isolation ; 3 and M. Deslandres has already at Paris, by different 
 means, obtained some apparently genuine coronal impressions. 4 
 
 The prosperous result of the Sohag observations stimulated 
 the desire to repeat them on the first favourable opportunity. 
 This offered itself one year later, May 6, 1883, yet not without 
 the drawbacks incident to terrestrial conditions. The eclipse 
 promised was of rare length, giving no less than five minutes 
 and twenty-three seconds of total obscurity, but its path was 
 almost exclusively a " water-track." It touched land only on 
 the outskirts of the Marquesas group in the Southern Pacific, 
 
 1 Captain L. Darwin, Phil. Trans., vol. clxxx., p. 311. 2 Liais, Corrptes 
 
 Rendus. t. xlvii., p. 789 ; Reports on Observations of the Total Eclipse of Jan. I, 
 1889, published by the Lick Observatory, p. 164; Trepied, C. Rendus, t. ex., 
 p. 1321. For instances of the same visual appearance, see Trouvelot, Observa- 
 tory, vol. ix> p. 395 ; G. Dollond's observation of Nov. 29, 1826, Month. Notice*, 
 vol. i., p. 26; Hill, Jan. i, 1889, Lick Reports, p. 75, Stanoiewitch, 19 Aug- 
 1887, C. Rendus, t. cvi., p. 43. 3 Astr. and Astro- Physics, March 1893, P- 26 - 
 4 Comptes Rendus, Jan. 23, 1893. 
 
CHAP. in. RECENT ECLIPSES. 227 
 
 and presented, as the one available foothold for observers, a 
 coral reef named Caroline Island, seven and a half miles long by 
 one and a half wide, unknown previously to 1874, and visited only 
 for the sake of its stores of guano. Seldom has a more striking 
 proof been given of the vividness of human curiosity as to the 
 condition of the worlds outside our own, than in the assemblage 
 of a group of distinguished men from the chief centres of civilis- 
 ation, on a barren ridge, isolated in a vast and tempestuous 
 ocean, at a distance, in many cases, of 11,000 miles and upwards 
 from the ordinary scene of their labours. And all these sacri- 
 fices the cost and care of preparation, the transport and 
 readjustment of delicate instruments, the contrivance of new 
 and more subtle means of investigating phenomena on the 
 precarious chance of a clear sky during one particular five 
 minutes ! The event, though fortunate, emphasised the hazard of 
 the venture. The observation of the eclipse was made possible only 
 by the happy accident of a serene interval between two storms. 
 The American expedition was led by Professor Edward S. 
 Holden, and to it were courteously permitted to be attached 
 Messrs. Lawrance and Woods, photographers, sent out by the 
 Royal Society of London. M. Janssen was chief of the French 
 Academy mission; he was accompanied from Meudon by 
 Trouvelot, and joined from Vienna by Palisa, and from Rome 
 by Tacchini. A large share of the work done was directed to 
 assuring or negativing previous results. The circumstances 
 of an eclipse favour illusion. A single observation by a single 
 observer, made under unfamiliar conditions, and at a moment of 
 peculiar excitement, can scarcely be regarded as offering more 
 than a suggestion for future inquiry. But incredulity may be 
 carried too far. Janssen, for instance, felt compelled by the 
 survival of unwise doubts, to devote some of the precious 
 minutes of obscurity at Caroline Island to confirming what, in 
 his own persuasion, needed no confirmation that is, the 
 presence of reflected Fraunhofer lines in the spectrum of the 
 corona. Trouvelot and Palisa, on the other hand, instituted 
 an exhaustive, but fruitless search for the spurious " intra- 
 mercurian" planet announced by Swift and Watson in 1878. 
 
228 HISTORY OF ASTRONOMY. PART ir. 
 
 New information, however, was not deficient. The corona 
 proved identical in type with that of I882, 1 agreeably to what 
 was expected at an epoch of protracted solar activity. Its light 
 was, however, less violet in tyige, owing to the absence of 
 diffused "JET and K" illumination from the prominences. The 
 characteristic aigrettes (of which five appeared in Mr. Dixon's 
 sketch) were of even greater brilliancy than ^in the preceding 
 year, and the chemical effects of the coronal light proved un- 
 usually intense. Janssen's photographs, owing to the con- 
 siderable apertures (six and eight inches) of his object-glasses, 
 and the long exposures permitted by the duration of totality, 
 were singularly perfect ; they gave a greater extension to the 
 corona than could be traced with the telescope, 2 and showed its 
 forms as absolutely fixed and of remarkable complexity. 
 
 The English pictures, taken with exposures up to sixty 
 seconds, were likewise of great value. They exhibited details 
 of structure from the limb to the tips of the streamers, which ter- 
 minated definitely, and as it seemed actually, where the impres- 
 sions on the plates ceased. The coronal spectrum was also suc- 
 cessfully photographed, with a number of bright and dark lines ; 
 and a print was caught of some of the more prominent rays of 
 the reversing layer just before and after totality. The use of 
 the prismatic camera was baffled by the anomalous scarcity of 
 prominences. 
 
 Another of the observations made at Caroline Island, although 
 probably through some unexplained cause delusive, merits some 
 brief notice. Using an ingenious apparatus for viewing simul- 
 taneously the spectrum from both sides of the sun, Professor 
 Hastings noticed alternations, with the advance of the moon, in 
 the respective heights above the right and left solar limbs of the 
 coronal line "1474," which were thought to imply that the 
 old exploded idea was after all a true one, and that the corona, 
 with its rifts and sheaves and " tangled hanks " of rays, is an 
 illusive appearance produced by the diffraction of sunlight at the 
 moon's edge. 3 But the whole course of recent research is against 
 such a supposition, even were the validity of Professor Hastings's 
 1 Abney, Phil. Trans., vol. clxxx., p. 119. * Comptes Bendus, t. xcvii. 
 
CHAP. III. 
 
 RECENT ECLIPSES. 229 
 
 arguments in favour of its optical possibility admitted. Atmo- 
 spheric diffusion may indeed, under favouring circumstances, be 
 effective in deceptively enlarging solar appendages ; but always 
 to a very limited extent. 
 
 The controversy is an old one as to the part played by our air 
 in producing the radiance visible round the eclipsed sun. In its 
 original form, it is true, it came to an end when Professor 
 Harkness, in I 869, l pointed out that the shadow of the moon 
 falls equally over the air and on the earth, and that if the sun 
 had no luminous appendages, a circular space of almost absolute 
 darkness would consequently surround the apparent places of 
 the superposed sun and moon. Mr. Proctor, 2 with his usual 
 ability, impressed this mathematically certain truth (the precise 
 opposite of the popular notion) upon public attention ; and Sir 
 John Herschel calculated that the diameter of the " negative 
 halo " thus produced would be, in general, no less than 23. 
 
 But about the same time a noteworthy circumstance relating 
 to the state of things in the solar vicinity was brought into 
 view. On February 1 1 , I 869, Messrs. Frankland and Lockyer 
 communicated to the Eoyal Society a series of experiments on 
 gaseous spectra under varying conditions of heat and density, 
 leading them to the conclusion that the higher solar prominences 
 exist in a medium of excessive tenuity, and that even at the 
 base of the chromosphere the pressure is far below that at the 
 earth's surface. 3 This inference was fully borne out by the 
 researches of Wiillner ; and Janssen expressed the opinion that 
 the chromospheric gases are rarefied almost to the degree of an 
 air-pump vacuum. 4 Hence was derived a general and fully 
 justified conviction that there could be outside, and incumbent 
 upon the chromosphere, no such vast atmosphere as the corona 
 appeared to represent. Upon the strength of which conviction 
 the " glare " theory entered, chiefly under the auspices of Mr. 
 Lockyer, upon the second stage of its existence. 
 
 The genuineness of the "inner corona" to the height of 5' or 
 6' from the limb was admitted ; but it was supposed that by the 
 detailed reflection of its light in our air the far more extensive 
 
 1 Wash. 06*., 1867, App. ii., p. 64. 2 The Sun, p. 357. 3 Proc. Boy. JSoc., 
 vol. xvii., p. 289. 4 Canutes Rendus, t. Ixxiii., p. 434. 
 
230 HISTORY OF ASTRONOMY. PART IL 
 
 "outer corona" was optically created, the irregularities of the 
 moon's edge being called in to account for the rays and rifts by 
 which its structure was varied. This view received some coun- 
 tenance from Captain (now Admiral) Maclear's observation, 
 during the eclipse of 1870, of bright lines " everywhere" even 
 at the centre of the lunar disc. Here, indeed, was an undoubted 
 case of atmospheric diffusion ; but here, also> was a safe index 
 to the extent of its occurrence. Light scatters equally in all 
 directions ; so that when the moon's face at the time of an 
 eclipse shows (as is the common case) a blank in the spectro- 
 scope, it is quite certain that the corona is not noticeably 
 enlarged by atmospheric causes. A sky drifted over with thin 
 cirrous clouds, and air charged with aqueous vapour amply 
 accounted for the abnormal amount of scattering in 1 87 o. 
 
 But even in 1870 positive evidence was obtained of the sub- 
 stantial reality of the radiated outer corona, in the appearance 
 on the photographic plates exposed by Willard in Spain and by 
 Brothers in Sicily, of identical dark rifts. The truth is, that 
 far from being developed by misty air, it is peculiarly liable to be 
 effaced by it. The purer the sky, the more extensive, brilliant, and 
 intricate in the details of its structure the corona appears. Take 
 as an example General Myer's description of the eclipse of 1 869. 
 as seen from the summit of White Top Mountain, Virginia, at 
 an elevation above the sea of 5530 feet, in an atmosphere of 
 peculiar clearness. 
 
 "To the unaided eye," he wrote, 1 "the eclipse presented, 
 during the total obscuration, a vision magnificent beyond de- 
 scription. As a centre stood the full and intensely black disc of 
 the moon, surrounded by the aureola of a soft bright light, 
 through which shot out, as if from the circumference of the 
 moon, straight, massive, silvery rays, seeming distinct and 
 separate from each other, to a distance of two or three diameters 
 of the solar disc; the whole spectacle showing as on a back- 
 ground of diffused rose-coloured light." 
 
 On the same day, at Des Moines, Newcomb could perceive, 
 through somewhat hazy air, no long rays, and the four-pointed 
 
 1 Wash. Obs., 1867, App. ii., p. 195. 
 
CHAP. in. RECENT ECLIPSES. 231 
 
 outline of the corona reached at its farthest only a single semi- 
 diameter of the moon from the limb. The plain fact, that our 
 atmosphere acts rather as a veil to hide the coronal radiance 
 than as the medium through which it is visually formed, emerges 
 from the records of innumerable other observations. 
 
 None of importance were made during the eclipse of Sep- 
 tember 9, 1885. The path of total obscuration touched land 
 only on the shores of New Zealand, and two minutes was 
 the outside limit of available time. Hence local observers had 
 the phenomenon to themselves ; nor were they even favoured by 
 tlie weather in their efforts to make the most of it. One striking 
 appearance was, however, disclosed. It was that of two " white " 
 prominences of unusual brilliancy, shining like a pair of electric 
 lamps hung one at each end of a solar diameter, right above the 
 places of two large spots. 1 This coincidence of diametrically 
 opposite disturbances is of too frequent occurrence to be acci- 
 dental. M. Trouvelot observed at Meudon, June 26, 1885, two 
 active and evanescent prominences thus situated, each rising to 
 the enormous height of 300,000 miles, and on August 16, one 
 scarcely less remarkable, balanced by an antipodal spot-group. 2 
 It towered upward, as if by a process of unrolling, to a quarter 
 of a million of miles ; after which, in two minutes, the light died 
 out of it ; it had become completely extinct. 
 
 The eclipse of August 29, 1886, was total during about four 
 minutes over tropical Atlantic regions ; and an English expe- 
 dition, led by Professor Lockyer, was accordingly despatched 
 to Grenada in the West Indies, for the purpose of using the 
 opportunity it offered. But the rainy season was just then at its 
 height ; clouds and squalls were the order of the day ; and the 
 elaborately planned programme of observation could only in part 
 be carried through. Some good work, none the less, was done. 
 Observations of great theoretical importance were made, tending 
 (as already stated) to widen the interpretation given to the 
 fugitive shower of bright lines marking the beginning and end 
 of totality. Professor Tacchini, who had been invited to accom- 
 
 1 Stokes, Anniversary Address, Nature, vol. xxxv., p. 114. 2 Comptes 
 
 Rendus, t. ci., p. 50. 
 
232 HISTORY OF ASTRONOMY. PART n. 
 
 pany the party, ascertained besides some significant facts about 
 prominences. From a comparison of their forms and sizes 
 during and after the eclipse, it appeared that only the glowing 
 vaporous cores of these objects ,are shown by the spectroscope 
 under ordinary circumstances; their upper sections, giving a 
 faint continuous spectrum, and composed of presumably cooler 
 materials, can only be seen when the veil pf scattered light 
 usually drawn over them is removed by an eclipse. Thus all 
 moderately tall prominences have silvery summits ; but all do 
 not appear to possess the "red heart of flame," by which alone 
 they can be rendered perceptible to daylight observation. Some 
 are now found to be ordinarily invisible, because silvery through- 
 out " sheeted ghosts," as it were, met only in the dark. 
 
 Specimens of the class had been noted as far back as 1842, 
 but Tacchini first drew particular attention to them. The one 
 observed by him in 1886 rose in a branching form to a height of 
 1 50,000 miles, and gave a brilliantly continuous spectrum, with 
 bright lines at H and K, but no hydrogen-lines. 1 Hence 
 the total invisibility of the object before and after the eclipse. 
 During the eclipse, it was seen framed, as it were, in a pointed 
 arch of coronal light, the symmetrical arrangement of which 
 with regard to it could scarcely have been accidental. Both 
 its upspringing shape, and the violet rays of calcium 
 strongly emitted by it, contradicted the supposition that 
 " white prominences " represent a downrush of refrigerated 
 materials. 
 
 The corona of 1886, as photographed by Dr. Schuster and 
 Mr. Maunder, showed neither the petals and plumes of 1871, 
 nor the streamers of 1878. It might be called of an inter- 
 mediate type. 2 Wide polar rifts were filled in with tufted 
 radiations, and bounded on either side by irregularly disposed, 
 compound luminous masses. In the south-western quadrant, a 
 triangular ray was conspicuous, and seemed to Mr. W. H. 
 Pickering the visual presentation of a huge, hollow cone. 3 
 Branched, and recurving jets were curiously associated with it. 
 
 1 Harvard Annals, vol. xviii., p. 99. 2 Wesley, Phil. Trans., vol. clxxx., 
 p. 350. 3 Harvard Annals, vol. xviii., p. 108. 
 
CHAP. in. RECENT ECLIPSES. 233 
 
 The intrinsic photographic brightness of the corona proved, from 
 Pickering's measures, to be about -^ that of the average surface 
 of the full moon. 1 
 
 The Eussian eclipse of August 19, 1887, can only be remem- 
 bered as a disastrous failure. Much was expected of it. The 
 shadow-path ran overland from Leipsic to the Japanese sea, 
 so that the solar appurtenances would, it was hoped, be dis- 
 closed to observers echeloned along a line of 6000 miles. But 
 the incalculable element of weather rendered all forecasts 
 nugatory. The clouds never parted, during the critical three 
 minutes, over Central Eussia, where Mr. Turner's, Professor 
 Young's, and many other parties were stationed ; and a supple- 
 mentary American expedition, led by Professor D. P. Todd, was 
 equally unfortunate in Japan. Some good photographs were, 
 nevertheless, secured by Professor Arai, Director of the Tokio 
 Observatory, as well as by MM. Belopolsky and Glasenapp at 
 Petrovsk and Jurjevitch respectively. They showed a corona of 
 simpler form than that of the year before, but not yet of the 
 pronounced type first associated by Mr. Eanyard with the lowest 
 stage of solar activity. 
 
 The genuineness of the association was ratified by the duplicate 
 spectacle of the next-ensuing minimum year. Two total eclipses 
 of the sun distinguished 1 889. The first took place on New Year's 
 Day, when a narrow shadow-path crossed California, allowing 
 less than two minutes for the numerous experiments prompted 
 by the varied nature of modern methods of research. American 
 astronomers availed themselves of the occasion to the full. The 
 heavens were propitious. Photographic records were obtained 
 in unprecedented abundance, and of unusual excellence. Their 
 comparison and study placed it beyond reasonable doubt that 
 the radiated corona belonging to periods of maximum sun-spots 
 gives place, at periods of minimum, to the "winged" type of 
 1878. Professor Holden perceived further that the equatoreal 
 extensions characterising the latter are, to a certain extent, 
 "trumpet-shaped." 2 Their extremities diverge, as if mutually 
 repellent, instead of flowing together along a medial plane. The 
 
 1 Harvard Annals, vol. xviii., p. 108. 2 Lick Report, p. 20. 
 
234 HISTORY OF ASTRONOMY. PART n. 
 
 maximum actinic brilliancy of the corona of January i, 1889, 
 was determined at Lick to be twenty-one times less than that of 
 the full moon. 1 Its colour was described as " of an intense lumi- 
 nous silver, with a bluish tinge^ similar to the light of an electric 
 arc." 2 Its spectrum was comparatively simple. Very few bright 
 lines besides those of hydrogen and coronium, and apparently no 
 dark ones, stood out from the prismatic background. 
 
 " The marked structural features of the corona, as presented by 
 the negatives " taken by Professors Nipher and Charroppin, were, 
 according to the statement of Professor Pritchett of St. Louis, 3 
 "the so-called filaments, and the streamers extending approxi- 
 mately in the direction of the ecliptic. The filaments extend," 
 he continues, "over a region of twenty degrees or more on each 
 side of the poles. Comparing our negatives with a copy of a 
 negative taken by the Lick Observatory party, I find the number 
 and arrangement of these filaments to coincide accurately. 
 There are slight differences in the lengths of some of the fila- 
 ments, but no greater than might be accounted for by differences 
 of exposure and atmospheric conditions. 
 
 "The broad and strongly marked equatorial belt stretches 
 directly across this mass of filaments, apparently cutting off the 
 filaments, at the somewhat irregular line of separation. The 
 impression conveyed to the eye is that the equatorial stream of 
 denser coronal matter extends across and through the filaments, 
 simply obscuring them by its greater brightness. The effect is 
 just as if the equatorial belt were superposed upon, or passed 
 through, the filamentary structure. There is nothing in the 
 photographs to prove that the filaments do not exist all round 
 the sun. 4 The testimony from negatives of different lengths of 
 exposure goes to show that the equatorial streamers are made 
 up of numerous interlacing parts inclined at varying angles to 
 the sun's equator, but all trending, in a general way, along it, or 
 roughly speaking, along the ecliptic. The direction and character 
 of these component streamers can be best studied at the edges 
 
 1 Lick fieport, p. 14. 2 Ibid., p. 155. 3 Pull. Astr. Soc. of the Pacific, 
 
 vol. iii., p. 158. 4 Professor Holden concluded, with less qualification, " that 
 so-called 'polar' rays exist at all latitudes on the sun's surface." Lick 
 Report, p. 19. 
 
CHAP. in. RECENT ECLIPSES. 23$ 
 
 of the photographs, where, on account of the smaller number 
 shown, their direction and force can be made out. It seems 
 probable that, could we have a faithful reproduction of the 
 extreme outer corona, where individual streamers could be traced 
 out for a considerable distance, our knowledge of the coronal 
 structure would be materially increased." 
 
 So far, the coronal extensions have not been photographed, 
 Senor Valle, who came, for the purposes of the eclipse, from 
 the Tacubaya Observatory, in Mexico, to Norman, a Californian 
 hamlet, was able to trace the " fish-tail" structures, with the 
 naked eye, to a distance of more than three degrees from the 
 sun; while on Father Charroppin's best negative, exposed at 
 the same place during twenty-eight seconds, they appeared 
 barely one-third of that length. Nor could much more of them 
 be seen either in Mr. Barnard's exquisite miniature pictures^ or 
 in the photographs obtained by Mr. W. H. Pickering with a 
 thirteen-inch refractor the largest instrument ever used in 
 eclipse-photography. 
 
 The self-portraiture of the corona thus remains incomplete, 
 and opinions are divided as to how its deficiencies can be supplied. 
 Professor Holden points out l that the problem under attack is 
 totally different from that of photographing very faint nebulae ; 
 for these are relieved against almost absolute darkness, while 
 coronal streamers fade out into a partially illuminated sky. 
 What has to be caught is hence a differential effect, which long 
 exposures, or excessive light-power in the instrument employed 
 would tend to obliterate. Father Charroppin, 2 and Mr. A. 
 Taylor, 3 on the other hand, advocate long exposures for the wing- 
 tips, but short exposures for the polar filaments, and the mass 
 of inner equatoreal details. No single photograph, indeed, can 
 properly depict the different sections of this enigmatical appen- 
 dage to the sun. 
 
 The total eclipse of December 22, 1889, held out the hope, 
 which unfortunately proved delusive, of a practical decision of 
 these questions. Messrs. Burnham and Schaeberle secured at 
 
 1 Iteport on Eclipse of Dec. 1889, p. 18. 2 Publ Astr. Pacific Society, 
 vol. iii., p. 26. a Observatory, vol. xvi., p. 95. 
 
236 HISTORY OF ASTRONOMY. PART n. 
 
 Cayenne some excellent photographs showing enough of the 
 corona to prove its identical character with that exhibited in the 
 beginning of the year, but not enough to convey additional 
 information about its terminal forms or innermost structure. 
 Any better result was indeed impossible, the moisture-laden air 
 having cut down the actinic power of the coronal light to one- 
 fourth its previous value. ^ 
 
 Two English expeditions organised by the Royal Astronomical 
 Society fared still worse. Mr. Taylor was stationed on the 
 West Coast of Africa, one hundred miles south of Loanda ; 
 Father Perry chose as the scene of his operations the Salut 
 Islands, off French Guiana. Each was supplied with a reflector 
 constructed by Dr. Common, endowed, by its extremely short 
 focal length of forty-five, combined with an aperture of twenty 
 inches, with a light-concentrating force capable, it was hoped, 
 of compelling the very filmiest coronal branches to self -registra- 
 tion. Had things gone well, two sets of coronal pictures, 
 absolutely comparable in every respect, and taken at an interval 
 of two hours and a half, would have been at the disposal of 
 astronomers. But things went very far from. well. Clouds 
 altogether obscured the sun in Africa ; they only separated to 
 allow of his shining through a saturated atmosphere in South 
 America. Father Perry's observations were the last heroic 
 effort of a dying man. Stricken with malaria, he crawled to 
 the hospital as soon as the eclipse was over, and expired five 
 days later, at sea, on board the Comiis. He was buried at 
 Barbados. And the sacrifice of his life had, after all, purchased 
 no decisive success. Most of the plates exposed by him suffered 
 deterioration from the climate, or from an inevitably delayed 
 development. A drawing from the best of them by Miss Violet 
 Common 1 represented a corona differing from its predecessor 
 of January I, chiefly through the oppositely unsymmetrical 
 relations of its parts. Then, the western wing had been broader 
 at its base than the eastern ; now, the inequality was conspi- 
 cuously the other way. 2 
 
 1 Published as the Frontispiece to the Observatory, No. 160. ' 2 Wesley, 
 
 ibid., p. 107. 
 
CHAP. in. RECENT ECLIPSES. 237 
 
 An opportunity for retrieving the mischances of the past was 
 offered April 16, 1893. The line of totality charted for that 
 day ran from Chili to Cape Verde. English expeditions were 
 sent both to Brazil and Senegambia ; MM. Bigourdan and 
 Deslandres of Paris stationed themselves near the Senegal ; and 
 at least three American parties occupied posts in various pro- 
 vinces of South America. But the upshot cannot become known 
 in time for inclusion in the present record. 
 
 The problem of deducing the real shape of the corona, as it 
 exists in space, from its projection upon a flat surface hung up 
 in the sky for our inspection during total eclipses, was first grap- 
 pled with in its entirety by Professor J. M. Schaeberle of the 
 Lick Observatory. 1 His resulting " mechanical theory " makes 
 the corona to consist, in its physical reality, of streams of matter 
 shot out with great velocity from the spot-zones by forces acting 
 perpendicularly to the sun's surface. The rays thus formed are 
 not however straight. The particles composing them, since 
 they bring the rotational speed of a smaller into a larger sphere, 
 lag behind as they ascend, and eventually return to the sun,, 
 after describing sections of extremely elongated ellipses. That 
 is, unless their initial velocity equals or exceeds the critical rate 
 of 383 miles a second, by which they would be finally driven off 
 into space. This state of things is perennial; but the visual 
 effect changes with the point of view. "According as the 
 observer is above, below, or in the plane of the sun's equator, >r 
 our author remarks, " the perspective overlapping and inter- 
 lacing of the two sets of streamers cause the observed apparent 
 variations in the type of the corona." The corona, then, is 
 always essentially the same, and only certain limited portions of 
 the solar surface are concerned in its production. What seem 
 like polar emanations are really, on this hypothesis, the tips of 
 zonal streamers issuing from behind the moon. These speculative 
 conclusions were strikingly illustrated by Professor Schaeberle's 
 device of photographing, in various positions, a model globe 
 bristling with properly arranged needles. But the objection 
 
 1 Lick Eeport on Eclipse of Dec. 22, 1889, p. 47 ; Month. Notices, vol. 1., 
 P- 372. 
 
238 HISTORY OF ASTRONOMY. PART n. 
 
 immediately occurs that, if coronal shapes depended only upon 
 the situation of the earth with regard to the sun's equator, 1 they 
 should be subject to an annual periodicity. The same type 
 should recur at the. same time of year. This, however, is far 
 from being the case. "Winged" coronas have been recorded 
 in May (at Cotes's eclipse of 1715), August, July, December, 
 and January ; radiated coronas in July, December, May, and 
 August. Nothing could well be more unlike the corona of 
 December 21, 1870, than the corona of December 22, 1889. 
 Yet according to the "mechanical theory," they ought to have 
 been perfectly similar., The agreement with the spot-cycle, on 
 the contrary, of the physical changes in this beautiful, if com- 
 monly hidden, solar appendage, appear so far unfailing ; although 
 optical changes, such as those pointed out by Professor Schaeberle, 
 are doubtless added. Coronal types, then, however fundamental 
 their relations to the state of the sun's activity, are necessarily 
 modified in accordance with geometrical laws, through variations 
 in the standpoint from which they are regarded. The effects 
 of perspective should also be constantly kept in view in study- 
 ing internal coronal structure. Their intricate nature has 
 been sufficiently shown by Professor Schaeberle's painstaking- 
 research. 
 
 Professor Bigelow's theory of the corona 2 goes deep into the 
 nature of things. He indeed claims for it no more than an 
 abstract character ; but it is a case in which correct mathema- 
 tical construction is scarcely separable from physical truth. By 
 an able discussion of the beautiful American photographs of 
 January I, 1889, he showed a marked coincidence between the 
 distinctive coronal forms and the calculated effects of a repulsive 
 force acting according to the laws of electric potential. Finely 
 subdivided matter is, on this view, expelled from the sun along 
 lines of force emanating from points near his poles, and tends 
 to accumulate at " equipotential surfaces." " The straight polar 
 
 1 Prof. Schaeberle evidently admits some genuine change in coronal forms 
 through the shifting of the spot-zones, but he does not include the effect in 
 his formal investigation. - The Solar Corona discussed by Splierical Har- 
 monics, Smithsonian Institution, Washington, 1889. 
 
CHAP. in. RECENT ECLIPSES. 239 
 
 rays of high tension carry the lightest substances, as hydrogen, 
 meteoric matter, debris of comets, and other coronal material, 
 away from the sun, and they become soon invisible by disper- 
 sion." The strong quadrilateral rays, conspicuous at periods of 
 great solar activity, come next in strength of potential, which, 
 however, falls off rapidly outward. "The explanation of the 
 long equatorial wings, with absence of well-marked quadri- 
 laterals, is that they are due to the closing of the lines of force 
 about the equator. The re-entrance of these lines forms along 
 the equator, the place of zero potential, a sort of pocket or 
 receptacle wherein the coronal matter is gradually carried by the 
 forces, accumulated and retained as a solar accompaniment." 
 The appurtenance thus formed and fed may, Professor Bigelow 
 thinks, be no other than the mysterious Zodiacal light. The 
 entire theory is, however, expressly left floating in the analytical 
 empyrean ; it is too soon to attempt its establishment upon a 
 secure physical basis. "In deference," its deviser says, "to 
 the doubt that free electricity can exist at such temperatures as 
 prevail on the sun's surface," he has avoided "speaking. of the 
 apparent coronal structure as a phenomenon of electricity." Its 
 details, if it were, he limits himself to pointing out, would be 
 just what they are. Hence the great interest and value of their 
 precise photographic registration. 
 
 Summing up what we have learned about the corona during 
 some fifty minutes of scrutiny in as many years, we may state, 
 to begin with, that it is not a solar atmosphere. It does not 
 gravitate upon the sun's surface and share his rotation, as our 
 air gravitates upon and shares the rotation of the earth ; and 
 this for the simple reason that there is no visible growth of 
 pressure downwards (such as the spectroscope would infallibly 
 give notice of) in its gaseous constituents ; whereas under the 
 sole influence of the sun's attractive power, their density should 
 be multiplied many million times in the descent through a mere 
 fraction of their actual depth. 1 
 
 The corona is properly described as a solar appendage ; and 
 
 1 See Huggins, Proc. Roy. Soc., vol. xxxix., p. 108 ; and Young, North Am. 
 Iteview, Feb. 1885, p. 179. 
 
240 HISTORY OF ASTRONOMY. PART n. 
 
 may be conjecturally defined as matter in a perpetual state of 
 efflux from, and influx to our great luminary, under the stress 
 of electrical repulsion in one direction and of gravity in the 
 other. 1 Its constitution is of a composite character. It is partly 
 made up of self-luminous gases, chiefly hydrogen, and the 
 unknown substance named coronium giving the green ray 
 "1474"; partly of white-hot solid or liquid particles, shining 
 with continuous light, both reflected and original. There is a 
 probability amounting almost to certainty that it is affected both 
 in shape and constitution by the periodic ebb and flow of solar 
 activity, its low-tide form being winged, its high-tide form 
 stellate ; while the rays emitted by the gases contained in it fade, 
 and the continuous spectrum brightens, at times of minimum sun- 
 spots, as if by a fall of temperature producing, on the one hand, 
 a decline in luminosity of the incandescent materials existing near 
 the sun, and, on the other, a condensation of vapours previously 
 invisible into compact particles of some reflective capacity. 
 The coronal materials must be of inconceivable tenuity, since 
 comets cut their way through them without experiencing sensible 
 retardation. Not even Mr. Crookes's vacua can give an idea of 
 the rarefaction which this fact implies. Yet the observed lumi- 
 nous effects may not in reality bear witness contradictory of it. 
 One solitary molecule in each cubic inch of space, might, in 
 Professor Young's opinion, produce them ; while in the same 
 volume of ordinary air at the sea-level, the molecules number 
 (according to Mr. Johnstone Stoney) 20,000 trillions ! 
 
 The most important lesson, however, derived from eclipses is 
 that of independence of them. Some of its fruits in the daily 
 study of prominences the next chapter will collect ; and the 
 harvest has been rendered more abundant, as well as more 
 valuable, since it has been found possible to enlist, in this 
 department too, the versatile usefulness of the camera. 
 
 1 Professor W. A. Norton, of Yale College, appears to have been the earliest 
 formal advocate of the Expulsion Theory of the solar surroundings, in the 
 second (1845) an< ^ later editions of his Treatise on Astronomy. 
 
CHAPTER IV. 
 
 SOLAR SPECTROSCOPY. 
 
 THE new way struck out by Janssen and Lockyer was at once 
 and eagerly followed. In every part of Europe, as well as in 
 North America, observers devoted themselves to the daily study 
 of the chromosphere and prominences. Foremost among these 
 were Lockyer in England, Zollner at Leipzig, Sporer at Anclam, 
 Young at Hanover, New Hampshire, Secchi and Respighi at 
 Rome. There were many others, but these names are conspicuous 
 from the outset. 
 
 The first point to be cleared up was that of chemical composi- 
 tion. Leisurely measurements verified the presence above the 
 sun's surface of hydrogen in prodigious masses, but showed that 
 sodium had nothing to do with the orange-yellow ray identified 
 with it in the haste of the eclipse. From its vicinity to the D 
 pair (than which it is slightly more refrangible), the prominence- 
 line was, however, designated D3, and the unknown substance 
 emitting it was named by Frankland " helium." Young is in- 
 clined to associate with it two other faint but persistent lines in 
 the spectrum of the chromosphere ; l and Messrs. Liveing and 
 Dewar pointed out, in i879, 2 that the wave-lengths of all three 
 are bound together with that of the coronal ray " I474" 3 by 
 numerical ratios virtually the same with those underlying the 
 vibrations of hydrogen, and also conformed to by certain lines of 
 lithium and magnesium. Professor Lockyer, however, considers 
 both helium and coronium to be " modifications " of hydrogen ; 
 
 1 Phil. Mag., vol. xlii., 1871, p. 380. 2 Proc. Boy. Hoc., vol. xxviii., p. 475. 
 3 Wave-length on Angstrom's scale = 5315.9 ten millionths of a millimetre. 
 
 16 
 
242 HISTORY OF ASTRONOMY. PART n. 
 
 and Professor Griinwald l of Prague classes them more definitely, 
 on the grounds of a plausible, though perhaps misleading theory, 
 as its constituents. But the actual relation would seem to be 
 one of analogy rather than of identity. 
 
 The importance of the part played in the prominence- spectrum 
 by the violet lines of calcium, did not escape Professor Young's 
 notice; but since H and K lie near the limit of the visible 
 spectrum, photography was needed for a thorough investigation 
 of their appearances. Aided by its resources, Professor George 
 E. Hale, a young American spectroscopist already of high distinc- 
 tion, made in 1891 the curious discovery of their unfailing and 
 conspicuous presence. 2 The substance emitting them, accordingly, 
 not only constitutes a fundamental ingredient of the chromo- 
 sphere, but rises in the fantastic jets thence issuing, to greater 
 heights than hydrogen itself. Whether this substance be 
 calcium, such as we know it, or some unknown element sepa- 
 rated from calcium, is a question not yet decided ; what is certain 
 is that the H and K derived from solar prominences have no 
 permanent companionship with any other of the lines terres- 
 trially characteristic of the metal. 
 
 Hydrogen and helium form the chief and unvarying materials 
 of the solar sierra and its peaks ; but a number of metallic 
 elements make their appearance spasmodically under the in- 
 fluence of disturbances in the layers beneath. In September 
 1871, Young 3 drew up at Dartmouth College a list of 103 lines 
 significant of injections into the chromosphere of iron, titanium, 
 calcium, magnesium, and many other substances. During two 
 months' observation in the pure air of Mount Sherman (8335 
 feet high) in the summer of 1872, these tell-tale lines mounted 
 up to 273 ; and he believes their number might still be doubled 
 by steady watching. Indeed, both Young and Lockyer have 
 more than once seen the whole field of the spectroscope moment- 
 arily inundated with bright rays, as if the " reversing layer " 
 seen at the beginning and end of eclipses had been suddenly 
 
 1 Astr. Nach., No. 2797. * Ibid., No. 3053; Amer. Jour., vol. xlii., 
 
 p. 160 ; see also Deslandres, Comptes Hendus, t. cxiii., p. 307. 3 Phil Mag., 
 vol. xlii. , p. 377. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 243 
 
 thrust upwards in the chromosphere, and as quickly allowed to 
 drop back again. It would thus appear that the two form one 
 continuous region, of which the lower parts are habitually occu- 
 pied by the heaviest vapours, but where orderly arrangement is 
 continually overturned by violent eruptive disturbances. 
 
 The study of the forms of prominences practically began 
 with Dr. Huggins's observation of one through an " open slit," 
 February 13. I869. 1 At first it had been thought possible to 
 study them only in sections that is, by admitting mere narrow 
 strips or " lines " of their various kinds of light ; while the 
 actual shape of the objects emitting those lines had been 
 arrived at by such imperfect devices as that of giving to the 
 slit of the spectroscope a vibratory movement rapid enough 
 to enable the eye to retain the impression of one part while 
 others were successively presented to it. It was an immense 
 gain to find their rays strong enough to bear so much of dilu- 
 tion with ordinary light as was involved in opening the spectro- 
 scopic shutter wide enough to exhibit the tree-like, or horn- 
 like, or flame-shaped bodies rising over the sun's rim in their 
 undivided proportions. Several diversely-coloured images of 
 them are formed in the spectroscope ; each prominence may be 
 seen under a crimson, a yellow, a green, and a deep blue aspect. 
 The crimson, however (built up out of the C-line of hydrogen), 
 is the most intense, and is commonly used for purposes of ob- 
 servation and illustration. 
 
 Friedrich Zollner was, by a few days, beforehand with 
 Huggins in describing the open-slit method, but was somewhat 
 less prompt in applying it. His first survey of a complete pro- 
 minence, pictured in, and not simply intersected by, the slit of 
 his spectroscope, was obtained July i, iS6g. 2 Shortly after- 
 wards the plan was successfully adopted by the whole band of 
 investigators. 
 
 A difference in kind was very soon perceived to separate 
 these objects into two well-marked classes. Its natural and 
 obvious character was shown by its having struck several 
 observers independently. The distinction of " cloud-promi- 
 
 1 Proc. Roy. Soc., vol. xvii., p. 302. 2 Astr. Nach., No. 1769. 
 
244 HISTORY OF ASTRONOMY. PART n. 
 
 nences " from " flame-prominences " was announced by Lockyer, 
 April 27 ; by Zollner, June 2 ; and by Kespighi, Dec. 4, 1 870. 
 
 The first description is tranquil and relatively permanent, 
 sometimes enduring -without striking change for many days. 
 The species it includes mimic terrestrial cloud-scenery now 
 appearing like fleecy cirrus transpenetrated with the red glow of 
 sunset now like prodigious masses of cumulo-stratus hanging 
 heavily above the horizon. The solar clouds, however, have the 
 peculiarity of possessing stems. Slender columns can ordinarily 
 be seen to connect the surface of the chromosphere with its 
 outlying portions. Hence the fantastic likeness to forest scenery 
 presented by the long ranges of fiery trunks and foliage at 
 times seeming to fringe the sun's limb. But while this form- 
 ation suggests an actual outpouring of incandescent material, 
 certain facts require a different interpretation. At a distance, 
 and quite apart from the chromosphere, prominences have been 
 perceived, both by Secchi and Young, to form, just as clouds 
 form in a clear sky, condensation being replaced by ignition. 
 Filaments were then thrown out downward towards the chromo- 
 sphere, and finally the usual appearance of a " stemmed promi- 
 nence " was assumed. Still more remarkable was an observation 
 made by Trouvelot at Harvard College Observatory, June 26, 
 1 874- 1 A gigantic comma-shaped prominence, 82,000 miles 
 high, vanished from before his eyes by a withdrawal of light as 
 sudden as the passage of a flash of lightning. The same 
 observer has frequently witnessed a gradual illumination or 
 gradual extinction of such objects, testifying to changes in the 
 thermal or electrical condition of matter already in situ. 
 
 The first photograph of a prominence, as shown by the 
 spectroscope in daylight, was taken by Professor Young in 
 i87O. 2 But neither his method, nor that described by Dr. 
 Braun in i872, 3 had any practical success. This was reserved to 
 crown the efforts towards the same end of Professor Hale. 
 They were begun at Harvard College in 1889, and continued 
 
 1 Am. Jour, of Science, vol. xv., p. 85. 2 Jour. Franklin Institute, vol. Ix^ 
 p. 2320;. 3 Pogg. Annalen, Bd. cxlviii., p. 475; Astr. Nacli., No. 3014. For 
 a complete history of the subject, see Hale, Astr. and Astro-Physics, March 
 1893, p. 241. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 245 
 
 at Chicago in I89O. 1 The great difficulty was to extricate 
 the coloured image of the gaseous structure spectroscopically 
 visible at the sun's limb, from the encompassing glare, a 
 very little of which goes a long way in fogging sensitive 
 plates. To counteract its mischievous effects, a second slit, 
 besides the usual narrow one in front of the collimator, 
 was placed on guard, as it were, behind the dispersing ap- 
 paratus, so as to shut out from the sensitised surface all light 
 save that of the required quality. The sun's image being then 
 allowed to drift across the outer slit, while the plate-holder was 
 kept moving at the same rate, the successive sectional impres- 
 sions thus rapidly obtained finally "built up" a complete picture 
 of the prominence. A still better expedient was, however, soon 
 afterwards contrived. 2 The H and K rays of calcium are always, 
 as we have seen, bright in the spectrum of prominences. They 
 are besides fine and sharp, while the corresponding absorption- 
 lines in the ordinary solar spectrum are wide and diffuse. Hence, 
 prominences formed by the spectroscope out of these particular 
 qualities of violet light, can be photographed entire and at once, 
 for the simple reason that they are projected upon a naturally 
 darkened background. Atmospheric glare is abolished by local 
 absorption. This beautiful method was first realised by Professor 
 Hale in June 1891 at the Kenwood Physical Observatory, 
 Chicago ; and he designs its extension to those evasive white 
 prominences, of which, but for the calcium-element in their light, 
 no tidings could possibly be procured outside of total eclipses. 
 
 A " spectroheliograph," consisting of a spectroscopic and a 
 photographic apparatus of special type, attached to the eye-end 
 of an equatoreal twelve inches in aperture, was erected at Ken- 
 wood in March 1891 ; and with its aid, Professor Hale has 
 entered upon original researches of high promise for the advance- 
 ment of solar physics. Noteworthy above all is his achievement 
 of photographing both prominences and faculee on the very face 
 
 1 Astr. Nach., Nos. 3006, 3037 ; Ainer. Jour, of Science, vol. xlii., p. 160. 
 Janssen (Report Brit. Ass., 1869, ii. p. 23) invented, but did not apply the 
 double slit method of isolating light of a definite refrangibility, independently 
 arrived at by Hale twenty years later. a Astr. Nach., No. 3053. 
 
 * Astronomy and Astro-Physics, May 1892, p. 407. 
 
246 HISTORY OF ASTRONOMY. PART n. 
 
 of the sun. Tlie latter had, until then, been very imperfectly 
 observed. They were only visible, in fact, when relieved by 
 their brilliancy against the dusky edge of the solar disc. Their 
 convenient emission of calcium light, however, makes it possible 
 to photograph them in all positions, and emphasises their close 
 relationship to prominences, whether white or red. The simul- 
 taneous picturing, moreover, of the entire chromospheric ring, 
 with whatever trees or fountains of fire chance to be at the moment 
 issuing from it, has been accomplished by a very simple device. 
 The disc of the sun itself having been screened with a circular 
 metallic diaphragm, it is only necessary to cause the slit to 
 traverse the virtually eclipsed luminary, in order to get an 
 impression of the whole round of its fringing appendages, such 
 as is exhibited in the lower part of Plate I. (the upper part re- 
 produces six photographs of a prominence, taken in ih. 4Oni., 
 November 15, 1892). And the record can be still further com- 
 pleted by removing the screen, and carrying the slit back at 
 a quicker rate, when "an image of the sun's surface, with the 
 faculge and spots, is formed on the plate exactly within the 
 image of the chromosphere formed during the first exposure. 
 The whole operation," Professor Hale continues, "is completed 
 in less than a minute, and the resulting photographs give the 
 first true pictures of the sun, showing all of the various pheno- 
 mena at its surface." l 
 
 The ultra-violet prominence spectrum was, for the first time, 
 photographed from an uneclipsed sun, in June 1891, at Chicago. 
 Besides H and K, three of a remarkable series of hydrogen- 
 lines, discovered by Dr. Huggins in the spectra of stars like 
 Sirius, imprinted themselves on the plate. 2 Meanwhile M. 
 Deslandres, 3 who had been carrying on at the Paris Observatory 
 the novel work initiated by Professor Hale, was enabled, by 
 fitting quartz lenses to his spectroscope, and substituting a 
 reflecting for a refracting telescope, to get rid of the obstructive 
 action of glass upon the shorter light-waves, and thus to extend 
 greatly the scope of his inquiry into the peculiarities of those 
 
 1 Astr. and Astro- Physics, Aug. 1892, p. 604. ~ Amer. Jour, of Science, 
 
 vol. xlii., p. 163. 3 Comptes Rendas,t. cxiii., p. 307. 
 
PLATE I. 
 
 PHOTOGRAPHS OF THE SOLAR CHROMOSPHERE AND 
 PROMINENCES. 
 
 Taken with the Spectroheliograph of the Kenwood Observatory, Chicago. 
 
 T>_. T) I, 
 
CHAP. iv. SOLAR SPECTROSCOPY. 247 
 
 derived from prominences. 1 As the result, not only all the nine 
 white-star lines were photographed from a brilliant sun-flame, 
 but five additional ones were found to continue the series 
 upward. The wave-lengths of these last had, moreover, been 
 calculated beforehand with singular exactness, from a simple 
 formula known as "Balmer's Law," 2 depending upon a principle 
 analogous to that determining the harmonics of a vibrating 
 string. The new quintette of lines, accordingly, filled places in 
 a manner already prepared for them, and were thus unmistak- 
 ably associated with the hydrogen-spectrum. This is now found 
 to be represented in prominences by nineteen lines, forming a 
 kind of harmonic progression, only four of which are visibly 
 darkened in the Fraunhofer spectrum of the sun. 
 
 The chemistry of " cloud-prominences " is simple. Hydrogen, 
 helium, and calcium are their only recognised constituents. 
 " Flame-prominences," on the other hand, show, in addition, the 
 characteristic rays of a number of metals, among which iron, 
 titanium, barium, sodium, and magnesium are conspicuous. 
 They are intensely brilliant ; sharply defined in their varying 
 forms of jets, spikes, fountains, waterspouts ; of rapid formation 
 and speedy dissolution, seldom attaining to the vast dimensions 
 of the more tranquil kind. They are of eruptive or explosive 
 origin, and are closely connected with spots ; whether causally, 
 the materials ejected as " flames " cooling and settling down as 
 dark, depressed patches of increased absorption ; 3 or consequen- 
 tially, as a reactive effect of falls of solidified substances from 
 great heights in the solar atmosphere. 4 The two classes of 
 phenomena, at any rate, stand in a most intimate relation ; they 
 obey the same law of periodicity, and are confined to the same 
 portions of the sun's surface, while quiescent prominences may 
 be found right up to the poles and close to the equator. 
 
 The general distribution of prominences, including both 
 species, follows that of faculae much more closely than that 
 of spots. From Father Secchi's and Professor Respighi's ob- 
 servations, 186971, were derived the first clear ideas on the 
 
 1 Astr. and Astro- Physics, April 1892, p. 314. 2 Wiedemann's Annalen der 
 Physik, Bd. xxv., p. 80. 3 Secchi, Le Soleil, t. ii., p. 294. 4 Lockyer, 
 Chemistry of the Sun, p. 418. 
 
248 HISTORY OF ASTRONOMY. PART n. 
 
 subject, which have been supplemented and modified by the later 
 researches of Professors Tacchini and Ricco at Rome and 
 Palermo. The results are somewhat complicated, but may be 
 stated broadly as follows. The district of greatest prominence- 
 frequency covers and overlaps by several degrees that of greatest 
 spot-frequency. That is to say, it extends to about 40 north 
 and south of the equator. 1 There is a visible tendency to a 
 second pair of maxima nearer the poles, which are themselves, 
 like the equator, regions of minimum occurrence. Distribution 
 in time is governed by the spot-cycle, but the maximum lasts 
 longer for prominences than for spots. 
 
 The structure of the chromosphere was investigated in 1869 
 and subsequent years by the late Professor Respighi, director of 
 the Capitoline Observatory, as well as by Sporer, and Bredichin, 
 then of Moscow, now director of the Pulkowa Observatory. 
 They found this supposed solar envelope to be of the same 
 eruptive nature as the vast protrusions from it, and to be 
 made up of a congeries of minute flames 2 set close together 
 like blades of grass. " The appearance," Professor Young 
 writes. 3 "which probably indicates a fact, is as if countless jets 
 of heated gas were issuing through vents and spiracles over 
 the whole surface, thus clothing it with flame which heaves and 
 tosses like the blaze of a conflagration." 
 
 The summits of these filaments of fire are commonly inclined, 
 as if by a wind sweeping over them, when the sun's activity is 
 near its height, but erect during his phase of tranquillity. 
 Sporer, in 1871, inferred the influence of permanent polar cur- 
 rents, 4 but Tacchini showed in 1 876 that the deflections upon 
 which this inference was based ceased to be visible as the spot- 
 minimum drew near. 5 
 
 Another peculiarity of the chromosphere, denoting the re- 
 moteness of its character from that of a true atmosphere 6 is 
 the irregularity of its distribution over the sun's surface. There 
 
 1 V Astronomic, August 1884, p. 292 (Kicco) ; see also Evershed, Jour. 
 British Astr. Ass., vol. ii., p. 174. ~ Averaging about 100 miles across and 
 300 high. L", Soleil, t. ii., p. 35. 3 The Sun, p. 180. 4 Astr. Nach., 
 
 No. 1854. 5 Mem. degli Spettroscopisti Italiani, t. v., p. 4. Kestated by 
 Secchi, ibid., t. vi., p. 56. 6 Its non-atmospheric character was early 
 defined by Proctor, Month. Not., vol. xxxi., p. 196. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 249 
 
 are no signs of its bulging out at the equator, as the laws of 
 fluid equilibrium in a rotating mass would require ; but there 
 are some that the fluctuations in its depth are connected with 
 the phases of solar agitation. At times of minimum it seems 
 to accumulate and concentrate its activity at the poles ; while 
 maxima probably bring a more equable general distribution, with 
 local depressions at the base of great prominences and above spots. 
 
 The reality of the appearance of violent disturbance pre- 
 sented by the " flaming " kind of prominence can be tested in 
 a very remarkable manner. Christian Doppler, 1 professor of 
 mathematics at Prague, enounced in 1842 the theorem that 
 the colour of a luminous body, like the pitch of a sonorous 
 body, must be changed by movements of approach or recession. 
 The reason is this. Both colour and pitch are physiological 
 effects, depending, not upon absolute wave-length, but upon the 
 number of waves entering the eye or ear in a given interval of 
 time. And this number, it is easy to see, must be increased 
 if the source of light or sound is diminishing its distance, and 
 diminished if it is decreasing it. In the one case, the vibrating 
 body pursues and crowds together the waves emanating from 
 it ; in the other, it retreats from them, and so lengthens out 
 the space covered by an identical number. The principle may 
 be thus illustrated. Suppose shots to be fired at a target at 
 fixed intervals of time. If the marksman advances, say 
 twenty paces between each discharge of his rifle, it is evident 
 that the shots will fall faster on the target than if he stood 
 still ; if, on the contrary, he retires by the same amount, they 
 will strike at correspondingly longer intervals. The result will 
 of course be the same whether the target or the marksman be 
 in movement. 
 
 So far Doppler was altogether right. As regards sound, any 
 one can convince himself that the effect he predicted is a real 
 one, by listening to the alternate shrilling and sinking of the 
 steam-whistle when an express train rushes through a station. 
 But in applying this principle to the colours of stars he went 
 widely astray; for he omitted from consideration the double 
 range of invisible vibrations which partake of, and to the eye 
 
 1 Abh. d. Ron. ohm. Ges. d. Wiss., Bd. ii., 1841-42, p. 467. 
 
250 HISTORY OF ASTRONOMY. PART n. 
 
 exactly compensate, changes of refrangibility in the visible rays. 
 There is, then, no possibility of finding a criterion of velocity in 
 the hue of bodies shining, like the sun and stars, with continuous 
 light. There is a slight shift ofjbhe entire spectrum up or down 
 in the scale of refrangibility; certain rays normally visible 
 become exalted or degraded (as the case may be) into invi- 
 sibility, and certain other rays at the opposite end undergo the 
 converse process ; but the sum-total of impressions on the retina 
 continues the same. 
 
 We are not, however, without the means of measuring this 
 sub-sensible transportation of the light-gamut. Once more the 
 wonderful Fraunhofer lines came to the rescue. They were 
 called by the earlier physicists "fixed lines;" but it is just 
 because they are not fixed that, in this instance, we find them 
 useful. They share, and in sharing betray, the general shift of 
 the spectrum. This aspect of Doppler's principle was adverted 
 to by Fizeau in 1848,* and the first tangible results in the 
 estimation of movements of approach and recession between the 
 earth and the stars, were communicated by Dr. Huggins to the 
 Koyal Society, April 23, 1868. Eighteen months later, Zollner 
 devised his " reversion-spectroscope " 2 for doubling the measure- 
 able effects of line-displacements ; aided by which ingenious 
 instrument, and following a suggestion of its inventor, Professor 
 H. C. Vogel succeeded at Bothkamp, June 9, 1871 , 3 in detecting 
 effects of that nature due to the solar rotation. This application 
 constitutes at once the test and the triumph of the method. 4 
 
 The eastern edge of the sun is continually moving towards 
 us with an equatoreal speed of about a mile and a quarter per 
 second, the western edge retreating at the same rate. The dis- 
 placements towards the violet on the east, towards the red on 
 the west corresponding to this velocity are very small ; so 
 small that it seems hardly credible that they should have been 
 laid bare to perception. They amount to but T |^th part of the 
 interval between the two constituents of the D-line of sodium ; 
 
 1 In a paper read before the Societe Philomathique de Paris, December 23, 
 1848, and first published in extenso in Ann. de Chim. et de Phys., t. xix., p. 211 
 (1870). 2 Astr. Nach., No. 1772. 3 Ibid., No. 1864. 4 See Cornu, tiur 
 la Methode Doppler-Fizeau, p. D. 23. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 251 
 
 and the D-line of sodium itself can be separated into a pair only 
 by a powerful spectroscope. Nevertheless, Professor Young l 
 was able to show quite satisfactorily, in 1 876, not only deviations 
 in the solar lines from their proper places indicating a velocity 
 of rotation (1.42 miles per second) slightly in excess of that 
 given by observations of spots, but the exemption of terrestrial 
 lines (those produced by absorption in the earth's atmosphere) 
 from the general push upwards or downwards. Shortly after- 
 wards, Professor Langley, director of the Allegheny Observatory, 
 having devised a means of comparing with great accuracy light 
 from different portions of the sun's disc, found that while the 
 obscure rays in two juxtaposed spectra derived from the solar 
 poles were absolutely continuous, no sooner was the instrument 
 rotated through ninety degrees, so as to bring its luminous 
 supplies from opposite extremities of the equator, than the 
 same rays became perceptibly " notched." The telluric lines, 
 meanwhile, remained unaffected, so as to be " virtually mapped" 
 by the process. 2 This rapid and unfailing mode of distinction 
 was used by Cornu with perfect ease during his investigation of 
 atmospheric absorption near Loiret in Aug. and Sept. i883. 3 
 
 A beautiful experiment of the same kind was .performed by 
 M. Thollon, of M. Bischoffsheim's observatory at Nice, in the 
 summer of i88o. 4 He confined his attention to one delicately 
 denned group of four lines in the orange, of which the inner 
 pair are solar (iron) and the outer terrestrial. At the centre of 
 the sun the intervals separating them were sensibly equal ; but 
 when the light was taken alternately from the right and left 
 limbs, a relative shift in alternate directions of the solar, towards 
 and from the stationary telluric rays became apparent. A 
 parallel observation was made at Dunecht, December 14, 1883, 
 when it was noticed that a strong iron-line in the yellow part of 
 the solar spectrum is permanently double on the sun's eastern, 
 but single on his western limb ; 5 opposite motion-displacements 
 bringing about this curious effect of coincidence with, and 
 separation from, an adjacent stationary line of our own atmo- 
 
 1 Am. Jour, of Sc., vol. xii., p. 321. 2 Ibid., vol. xiv., p. 140. 3 Bull. 
 Astronom., Feb. 1884, p. 77. 4 Comptes Rendus, t. xci., p. 368. 5 Month. 
 Not., vol. xliv., p. 170. 
 
252 HISTORY OF ASTRONOMY. PART n. 
 
 sphere's production, according as the spectrum is derived from 
 the retreating or advancing margin of the solar globe. State- 
 ments of fact so precise and authoritative amount to a demon- 
 stration that results of this kind are worthy of confidence ; and 
 they already occupy an important place among astronomical data. 
 
 The subtle method of which they served to assure the validity 
 was employed in 1887-9 by M. Duner to. test and extend 
 Carrington's and Sporer's conclusions as to the anomalous 
 nature of the sun's axial movement. 1 His observations on the 
 shiftings of Fraunhofer lines at various heliographic latitudes, 
 made with a fine diffraction-spectroscope just then mounted at 
 the observatory of Upsala, were published in iSgi. 2 Their 
 upshot was to confirm and widen the law of retardation with 
 increasing latitude derived from the progressive motions of 
 spots. Determinations made within fifteen degrees of the pole, 
 consequently far beyond the region of spots, gave a rotation- 
 period of 38 J, that of the equatoreal belt being of 25 J days. 
 Spots near the equator indeed complete their rounds in a period 
 shorter than this last by at least half a day ; and proportionate 
 differences were found to exist in each corresponding latitude ; 
 but Duner's observations, it must be remembered, apply to a dis- 
 tinct part of the complex solar machine from the disturbed photo- 
 spheric surface. It is amply possible that the absorptive strata 
 producing the Fraunhofer lines, significant by their varying 
 displacements at either limb, of the inferred varying rates of 
 rotation, may gyrate more slowly than the spot-generating level. 
 "This peculiar law of the sun's rotation," Professor Holden 
 remarks, 3 " shows conclusively that it is not a solid body, in 
 which case, every one of its layers in every latitude must neces- 
 sarily rotate in the same time. It is more like a vast whirlpool 
 where the velocities of rotation depend not only on the situation 
 of the rotating masses as to latitude, but also as to depth 
 beneath the exterior surface." 
 
 Professor Lockyer 4 was the first to perceive the applicability of 
 
 1 See ante, p. 183. - Reclierclies sur la Rotation du Soleil, Upsal, 1891. 
 3 Putt. Astr. tioc. of the Pacific, vol. ii., p. 193. 4 Proc. Roy. 8oc., vols. xvii., 
 p. 415; xviii., p. 120. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 253. 
 
 the surprising discovery of line-shiftings through motion in line 
 of sight to the study of prominences, the discontinuous light of 
 which affords precisely the same means of detecting movement 
 without seeming change of place, as do lines of absorption in a 
 continuous spectrum. Indeed, his observations at the sun's 
 edge almost compelled him to have recourse to an explanation 
 made available just when the need of it began to be felt. He 
 saw bright lines, not merely pushed aside from their normal 
 places by a barely perceptible amount, but bent, torn, broken,, 
 as if by the stress of some tremendous violence. These remark- 
 able appearances were quite simply interpreted as the effects of 
 movements varying in amount and direction in the different 
 parts of the extensive mass of incandescent vapours falling 
 within a single field of view. Very commonly they are of a 
 cyclonic character. The opposite distortions of the same coloured 
 rays betray the fury of " counter-gales " rushing along at the- 
 rate of 1 20 miles a second ; while their undisturbed sections 
 prove the persistence of a " heart of peace " in the midst of that 
 unimaginable fiery whirlwind. Velocities up to 250 miles a 
 second, or 15,000 times that of an express train at the top of its 
 speed, were thus observed by Young during his trip to Mount 
 Sherman, August 2, 1872 ; and these were actually doubled in 
 an extraordinary outburst observed by Father Jules Fenyi, on 
 June 17, 1891, at the Haynald Observatory in Hungary, as well 
 as by M. Trouvelot at Meudon. 1 
 
 Motions ascertainable in this way near the limb are, of course,, 
 horizontal as regards the sun's surface ; the analogies they 
 present might, accordingly, be styled meteorological rather than 
 volcanic. But vertical displacements on a scale no less stupen- 
 dous can also be shown to exist. Observations of the spectra 
 of spots centrally situated (where motions in the line of sight 
 are vertical) disclose the progress of violent uprushes and down- 
 rushes of ignited gases, for the most part in the penumbral or 
 outlying districts. They appear to be occasioned by fitful and 
 irregular disturbances, and have none of the systematic quality 
 which would be required for the elucidation of sun-spot theories. 
 
 3 Comptes JRendus, t. cxii,, p. 1421 ; t. cxiii., p. 310. 
 
.254 HISTORY OF ASTRONOMY. PART n. 
 
 Indeed, they almost certainly take place at a great height above 
 the actual opening in the photosphere. 
 
 As to vertical motions above the limb, on the other hand, 
 we have direct visual -evidence pf a truly amazing kind. The 
 projected glowing matter has, by the aid of the spectroscope, 
 been watched in its ascent. On September 7, 1871, Young 
 examined at noon a vast hydrogen cloud, ioo,qoo miles long, as 
 it showed to the eye, and 54,000 high. It floated tranquilly 
 above the chromosphere at an elevation of some 15,000 miles, 
 and was connected with it by three or four upright columns, 
 presenting the not uncommon aspect compared by Lockyer to 
 that of a grove of banyans. Called away for a few minutes at 
 12.30, on returning at 12.55 the observer found 
 
 " That in the meantime the whole thing had been literally 
 blown to shreds by some inconceivable uprush from beneath. 
 In place of the quiet cloud I had left, the air, if I may use the 
 expression, was filled with flying debris a mass of detached, 
 vertical, fusiform filaments, each from 10" to 30" long by 2" 
 >or 3" wide, 1 brighter and closer together where the pillars had 
 formerly stood, and rapidly ascending. They rose, with a 
 velocity estimated at 166 miles a second, to fully 200,000 miles 
 above the sun's surface, then gradually faded away like a 
 dissolving cloud, and at 1.15 only a few filmy wisps, with some 
 brighter streamers low down near the photosphere, remained to 
 mark the place." 2 
 
 A velocity of projection of at least 500 miles per second was, 
 according to Proctor's calculation, 3 betrayed by this extra- 
 ordinary display. It was marked by the simultaneous record 
 at Greenwich of a magnetic disturbance, and was succeeded, 
 the same evening, by a fine aurora. It has proved by no 
 means an isolated occurrence. Young saw its main features 
 repeated, October 7, i88i, 4 on a still vaster scale; for the 
 exploded prpminence attained, this time, an altitude of 350,000 
 miles the highest yet chronicled. Professor Lockyer, more- 
 
 1 At the sun's distance, one second of arc represents about 450 miles. 
 2 Am. Jour, of 8c., vol. ii., 1871, p. 468. 3 Month. Not., vol. xxxii., p. 51. 
 4 Nature, vol. xxiii., p. 281. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 255 
 
 over, saw a prominence 40,000 miles high blown to pieces 
 in' ten minutes; while uprushes have been witnessed by 
 Respighi, of which the initial velocities were judged by him to 
 be 400 or 500 miles a second. When it is remembered that a 
 body starting from the sun's surface at the rate of 379 miles a 
 second would, if it encountered no resistance, escape for ever 
 from his control, it is obvious that we have, in the enormous 
 forces of eruption or repulsion manifested in the outbursts just 
 described, the means of accounting for the vast diffusion of 
 matter in the solar neighbourhood. Nor is it possible to ex- 
 plain them away, as Cornu, 1 Faye, 2 and others have sought to 
 do, by substituting for the rush of matter in motion, progressive 
 illumination through electric discharges, chemical processes, 3 or 
 even through the mere reheating of gases cooled by expansion. 4 
 All the appearances are against such evasions of the difficulty 
 presented by velocities stigmatised as "fabulous" and "im- 
 probable," but which, there is the strongest reason to believe, 
 really exist. 
 
 On the 1 2th of December, 1878, Professor Lockyer formally 
 expounded before the Royal Society his famous hypothesis of 
 the compound nature of the " chemical elements." 5 An hypo- 
 thesis, it is true, over and over again propounded from the 
 simply terrestrial point of view. What was novel was the 
 supra-terrestrial evidence now adduced in its support ; and even 
 this had been, in a general and speculative way, anticipated by 
 Professor F. W. Clarke of Washington. 6 Professor Lockyer 
 had been led to his conclusion along several converging lines of 
 research. In a letter to M. Dumas, dated December 3, 1873, 
 he had sketched out the successive stages of " celestial dissocia- 
 tion " which he conceived to be represented in the sun and stars. 
 The absence from the solar spectrum of metalloidal absorption 
 he explained by the separation, in the fierce solar furnace, of 
 such substances as oxygen, nitrogen, sulphur, chlorine, &c., 
 
 1 Comptes Rendus, t. Ixxxvi., p. 532. 2 Ibid., t. xcvi., p. 359. 3 A. Brester, 
 Theorie du Soleil, p. 66. 4 Such prominences as have been seen to grow by 
 the spread of incandescence are of the quiescent kind, and present no decep- 
 tive appearance of violent motion. 5 Proc. Boy. Soc., vol. xxviii., p. 157. 
 " Evolution and the Spectroscope," Pop. Science Monthly, Jan. 1873. 
 
256 HISTORY OF ASTRONOMY. PART n. 
 
 into simpler constituents possessing unknown spectra ; while 
 metals were at that time still admitted to be capable of existing 
 there in a state of integrity. Three years later he shifted his 
 position onward. He .announced, as the result of a comparative 
 study of the Fraunhofer and ele'ctric-arc spectra of calcium, that 
 the "molecular grouping" of that metal, which at low tempera- 
 tures gives a spectrum with its chief line in the blue, is nearly 
 broken up in the sun into another or others with lines in the 
 violet. 1 The further progress of his work showed him this 
 discrepancy between solar and terrestrial spectra as no excep- 
 tion, but " a truly typical case." 2 
 
 During four years (1875-78 inclusive) this diligent observer 
 was engaged in mapping a section of the more refrangible part 
 of the solar spectrum (wave-lengths 3800-4000) on a scale of 
 magnitude such that, if completed down to the infra-red, its 
 length would have been about half a furlong. The attendant 
 laborious investigation, by the aid of photography, of metallic 
 spectra, seemed to indicate the existence of what he called 
 " basic lines." These held their ground persistently in the 
 spectra of two or more metals after all possible " impurities " 
 had been eliminated, and were therefore held to attest the 
 presence of a common substratum of matter in a simpler state 
 of aggregation than any with which we are ordinarily ac- 
 quainted. 
 
 Later inquiries have shown, however, that between the 
 spectral lines of different substances there are probably no 
 absolute coincidences. " Basic " lines are really formed of 
 doublets or triplets merged together by insufficient dispersion. 
 Of Thalen's original list of seventy rays common to several 
 spectra, 3 very few have so far resisted Thollon's and Young's 
 powerful spectroscopes ; the process of resolution may in fact 
 be regarded as practically complete. Thus the argument from 
 
 1 Proc, Roy. Soc., vol. xxiv., p. 353. These are the H and K of prominences. 
 Their chemical meaning is still obscure. H. W. Vogel discovered in 1879 a 
 hydrogen line nearly coincident with H (Monatsb. Preuss. Ak., Feb. 1879, 
 p. 118). " Proc. Roy. 8oc., vol. xxviii., p. 444. 3 Many of these were 
 shown by Lockyer, who was the first to sift the matter, to be due to very slight- 
 admixtures of the several metals concerned. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 257 
 
 community of lines to community of substance has virtually 
 collapsed. Moreover, the singularity noted by Professor Lockyer 
 as a strong argument for solar dissociation of these twin-lines 
 these spots of rendezvous, it might be said, for different sets 
 of vibrations being specially selected for display in solar disturb- 
 ances, has not been re-attested by the Stonyhurst observations. 1 
 They have nevertheless proved confirmatory of results ob- 
 tained at South Kensington under Professor Lockyer's direction 
 since November 1879. Down to February 1888, the spectra 
 of 850 spots had been studied on a fixed plan, 2 and the pro- 
 cured data tabulated with rigid impartiality. The principle 
 of the method employed is this. The whole range of Fraun- 
 hofer lines is visible when the light from a spot is examined 
 with the spectroscope ; but relatively few are widened. Now 
 these widened lines presumably constitute the true spot- 
 spectrum ; they, and they alone, tell what kinds of vapour are 
 thrust down into the strange dusky pit of the nucleus, the 
 unaffected lines taking their accustomed origin from the over- 
 lying strata of the normal solar atmosphere. Here then we 
 have the criterion that was wanted the means of distinguish- 
 ing, spectroscopically and chemically, between the cavity and 
 the absorbing layers piled up above it. By its persistent 
 employment some marked peculiarities have been brought out 
 qualified, however, where negative conclusions are in question, by 
 the necessary limitations of the method of research. Positive 
 results, meanwhile, of an indubitable kind, are not wanting. 
 Such are the unfamiliar character of numerous lines in spot- 
 spectra, especially at epochs of disturbance ; above all, the 
 strange individuality in the behaviour of every one of these 
 darkened and distended rays. 3 Each seems to act on its own 
 account ; it comports itself as if it were the sole representative 
 of the substance emitting it ; its appearance is unconditional 
 upon that of any of its terrestrial companions in the same 
 spectrum. 
 
 For some metals, as cobalt, chromium, and calcium, the lines 
 
 : 1 Kev. A. L. Cortie, Month. Not., vol. li., p. 77. 2 Chemistry of the Sun, 
 
 p. 312. 3 Ibid., p. 314. 
 
 17 
 
258 HISTORY OF ASTRONOMY. PART n. 
 
 widened in spots are the same with those brightened in the 
 uprushing flames at the sun's edge ; for a good many others, 
 they are, as a rule, totally dissimilar, the spot-spectrum of iron, 
 for example, having only a remote relationship to its " storm " 
 spectrum. It is, moreover, pretty clear, from the character of 
 the lines severally affected, that the change is connected with a 
 difference of temperature, and that the prominences are much 
 hotter than the spots. Hence two well-defined heat-levels are 
 placed, as it were, at the disposal of the solar chemist, while a 
 third, assumed to be lower than either, characterised by the 
 ordinary Fraunhofer spectrum, is found at the photospheric 
 surface. 
 
 By far the most curious fact, however, elicited by these 
 inquiries, was that of the attendance of chemical vicissitudes upon 
 the advance of the sun-spot period. As the maximum approached, 
 unknown replaced known components of the spot-spectra in a 
 most pronounced and unmistakable way. 1 It would really seem 
 as if the vapours emitting lines of iron, titanium, nickel, &c., had 
 ceased to exist as such, and their room been taken by others, 
 total strangers in terrestrial laboratories. These, in Professor 
 Lockyer's view, are simply the finer constituents of their prede- 
 cessors, dissociation having been effected by the higher tempera- 
 ture ensuing upon increased solar activity. And for the present 
 there is at hand no other explanation of the striking facts 
 brought forward by him, and supported by others collected on 
 his method by Father Cortie. 2 
 
 But the strongest point of the "dissociation theory " has yet 
 to be mentioned. It is that the contortions or displacements due 
 to motion are frequently seen to affect a single line belonging to 
 a particular substance, while the other lines of that same substance 
 remain imperturbable. Now, how is this most singular fact, 
 which seems at first sight to imply that a body may be at rest 
 and in motion at one and the same instant, to be accounted for ? 
 It is accounted for, on the present hypothesis, easily enough, by 
 supposing that the rays thus discrepant in their testimony, do 
 not belong to one kind of matter, but to several, combined at ordi- 
 
 1 Chemistry of the Sun, p. 324. 3 Month. Not., vol. li., p. 76. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 259 
 
 nary temperatures, to form a body in appearance elementary. 
 Of these different vapours, one or more may of course be 
 rushing rapidly towards or from the observer, while the others 
 remain still ; and since the line of sight across the average 
 prominence region penetrates, at the sun's edge, a depth of 
 about 300,000 miles, 1 all the incandescent materials separately 
 occurring along which line are projected into a single " flame " 
 or " cloud," it will be perceived that there is ample room for 
 diversities of behaviour. 
 
 The alternative mode of escape from the perplexity consists 
 in assuming that the vapour in motion is rendered luminous 
 under conditions which reduce its spectrum to a few rays, the 
 unaffected lines being derived from a totally distinct mass of the 
 same substance shining with its ordinary emissions. 2 But these 
 conditions are assuredly not those prevailing in solar cyclones ; 
 while the seemingly capricious choice of lines associated to indi- 
 cate rest or motion, negative a supposition implying orderly and 
 invariable sequence. It is thus difficult to resist the conclusion 
 that distinct kinds of matter are really aligned before the eye, 
 embarrassing us with the contradictory testimony of their 
 integrated light. 
 
 Professor Lockyer's view has the argument from continuity in 
 its favour. It only asks us to believe that processes which we 
 know to take place on the earth under certain conditions, are 
 carried further in the sun, where the same conditions are, it may 
 be presumed, vastly exalted. We find that the bodies we call 
 " compound " split asunder at fixed degrees of heat within the 
 range of our resources. Why should we hesitate to admit that 
 the bodies we call " simple " do likewise at degrees of heat without 
 the range of our resources ? The term " element " simply ex- 
 presses terrestrial incapability of reduction. That, in celestial 
 laboratories, the means and their effect here absent should be 
 
 1 Thollon's estimate (Comptes Eendus, t. xcvii., p. 902) of 300,000 Idlometres 
 seems considerably too low. Limiting the " average prominence region " to a 
 shell 54,000 miles deep (2' of arc as seen from the earth), the visual line will, 
 at mid-height (27,000 miles from the sun's surface), travel through (in round 
 numbers) 320,000 miles of that region. ' 2 Liveing and Dewar, Phil. Mag., 
 vol. xvi. (5th ser.), p. 407. 
 
260 HISTORY OF ASTRONOMY. PART n. 
 
 present, would be an inference challenging, in itself, no expres- 
 sion of incredulity. 
 
 Yet it is, in point of fact, a revolutionary one. and its 
 acceptance will involve the reconstruction of more than one 
 fair edifice of scientific thought. It appears, none the less, 
 likely to become inevitable. There are indeed theoretical 
 objections, which, though probably not insuperable, are un- 
 questionably grave. Our seventy chemical "elements," for 
 instance, are placed by the law of specific heats on a separate 
 footing from their known compounds. We are not, it is true, 
 compelled by it to believe their atoms to be really and abso- 
 lutely such to contain, that is, the "irreducible minimum" 
 of material substance ; but we do certainly gather from it 
 that they are composed on a different principle from the salts 
 and oxides made and unmade at pleasure by chemists. Then 
 the multiplication of the species of matter with which Professor 
 Lockyer's results menace us, is at first sight startling. They 
 may lead, we are told, to eventual unification, but the prospect 
 seems remote. For the present each terrestrial '' element " 
 is asserted by them to be broken up in the sun into several, 
 and the existence of even a single common constituent is 
 uncertain. The components of iron alone, for instance, should 
 be counted by the dozen. And there are other metals, such 
 as cerium, which, giving a still more complex spectrum, would 
 doubtless be still more numerously resolved. It is true Pro- 
 fessor Lockyer interprets the observed phenomena as indicating 
 the successive combinations, in varying proportions, of a very 
 few original ingredients ; 1 but if the emission, at exalted 
 temperatures, of a single quality of light be admitted as the 
 criterion of a truly elementary body, then the independent 
 behaviour of a considerable number of lines in the spectrum 
 of each metal seems to assert that its formative units are 
 numerous. 
 
 Thus, added complexity is substituted for that fundamental 
 unity of matter which has long formed the dream of speculators. 
 And it is extremely remarkable that Mr. Crookes, working along 
 
 1 Chemistry of the /Sun, p. 260. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 261 
 
 totally different lines, has been led to analogous conclusions. To 
 take only one example. As the outcome- of processes of sifting 
 and testing of extreme delicacy carried on for years, he finds 
 that the metal yttrium splits up into five, if not eight con- 
 stituents. 1 Evidently, old notions are doomed, nor are any 
 preconceived ones likely to take their place. But, whatever 
 comes of it, we have no choice but to admit facts. 
 
 There can be no doubt that the spectra of bodies are an index 
 to changes in their molecular constitution of every kind and 
 degree, from a complete disruption of the molecule into atoms, 
 homogeneous or heterogeneous, to some unspeakably minute, yet 
 orderly and harmonious rearrangement of parts in the complex 
 little system of which the movements are the source of light. 
 Professor Lockyer's "working hypothesis" thus raises questions 
 which science is not yet prepared to answer. It brings us face 
 to face with the mysteries of the ultimate constitution of matter, 
 and of its relations to the vibrating medium filling space. It 
 makes our ignorance on these subjects seem at once more dense 
 and more definite. Nevertheless, this in itself (though the saying 
 appears paradoxical) constitutes an advance. Unfelt ignorance 
 persists. Ignorance that is stricken with uneasy self -conscious- 
 ness is already on the way to be turned into knowledge. 
 
 Professor A. J. Angstrom of Upsala takes rank after Kirchhoff 
 as a subordinate founder, so to speak, of solar spectroscopy. His 
 great map of the " normal " solar spectrum 2 was published in 
 1868, two years before he died. Robert Thalen was his coadjutor 
 in its execution, and the immense labour which it cost was amply 
 repaid by its eminent and lasting usefulness. For more than a 
 score of years it held its ground as the universal standard of 
 reference in all spectroscopic inquiries within the range of the 
 visible emanations. Those that are invisible by reason of the 
 quickness of their vibrations, were mapped by the late Dr. Henry 
 
 1 Nature, Oct. 14, 1886. 2 The normal spectrum is that depending exclu- 
 sively upon wave-length the fundamental constant given by nature as regards 
 light. It is obtained by the interference of rays, in the manner first exem- 
 plified by Fraunhofer, and affords the only unvarying standard for measure- 
 ment. In the refraction-spectrum (upon which Kirchhoff 's map was founded), 
 the relative positions of the lines vary with the material of the prisms. 
 
262 HISTORY OF ASTRONOMY. PART n. 
 
 Draper of New York in 1873, anc ^ wi-tli superior accuracy by M. 
 Cornu in 1881. The infra-red part of the spectrum, investigated 
 by Langley, Abney, and Knut Angstrom, reaches perhaps no- 
 definite end. The radiations oscillating too slowly to affect the 
 eye as light, may pass by insensible gradations into the long- 
 Hertzian waves of electricity. 1 
 
 Professor Rowland's photographic map of the solar spectrum, 
 published in 1886, and in a second enlarged' edition in 1889, 
 opened fresh possibilities for its study, from far down in the red 
 to high up in the ultra-violet. The accompanying scale of 
 absolute wave-lengths, 2 destined perhaps to supersede that of 
 Angstrom, varies so little from the new Potsdam scale, based on 
 determinations by MM. Miiller and Kempf , that the two systems 
 may be said to guarantee each the other's accuracy. Knowledge 
 of the solar spectrum has in fact of late so far outstripped know- 
 ledge of terrestrial spectra, that the recognition of their common 
 lines is hampered by intolerable uncertainties. Thousands of 
 the solar lines charted with minute precision thus remain 
 unidentified for want of a corresponding precision in the regis- 
 tration of metallic lines. Their much-needed revision has, 
 however, been undertaken by Professor Hasselberg at Stock- 
 holm ; 3 and a special study of the complex and fundamentally 
 important iron-spectrum was published by Thalen in i885- 4 
 Rowland, too, has for some years been engaged, with great suc- 
 cess, on photographic comparisons of the solar with elemental 
 spectra ; and Mr. Frank McClean has done excellent work of the 
 same kind. So the prospect is improving. 
 
 Another condition sine qua non of progress in this department 
 is the separation of true solar lines from those produced by 
 absorption in our own atmosphere. And here little remains to 
 be done. Thollon's great Atlas 5 was designed for this purpose 
 of discrimination. Each of its thirty-three maps (covering 
 the region A to &) exhibits in quadruplicate a subdivision of 
 
 1 Scheiner, Die Spectralanalyse der Gestirne, p. 168. 2 Phil. Mag., vol. 
 xxvii., p. 479. 3 Astr. and Astro-Physics, Nov. 1892, p. 793. 4 Scheiner r 
 Spectralanalyse, p. 401, reprints a list of 1172 of his wave-lengths. 5 Annalcz 
 de r Observatoire de Nice, t. iii. ; 1890. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 263 
 
 the solar spectrum under varied conditions of weather and 
 zenith-distance. Telluric effects are thus made easily legible, 
 and they account wholly for 866, partly for 246, out of a 
 total of 3200 lines. The lamented death of the artist, April 8, 
 1887, interrupted, however, the half-finished task of the last 
 seven years of his life, the completion of which has been 
 undertaken by the Abbe Spee, of Brussels. A most satis- 
 factory record, meanwhile, of selective atmospheric action has 
 been supplied by Dr. Becker's valuable drawings, 1 and by Mr. 
 McClean's photographs of the analysed light of the sun at high, 
 low, and medium altitudes ; and the autographic pictures ob- 
 tained by Mr. George Higgs of Liverpool, of certain rhythmical 
 groups in the red, emerging with surprising strength near 
 sunset, excite general and well-deserved admiration. 2 The main 
 interest, however, of all these documents resides in the informa- 
 tion afforded by them regarding the chemistry of the sun. 
 
 The discovery that hydrogen exists in the atmosphere of the 
 sun was made by Angstrom in 1862. His list of solar elements 
 published in that year, 3 the result of an investigation separate 
 from though conducted on the same principle as Kirchhoff's, 
 included the substance which we now know to be predominant 
 among them. Dr. Pliicker of Bonn had identified in 1859 the 
 Fraunhofer line F with the green ray of hydrogen, but drew no 
 inference from his observation. The agreement was verified by 
 Angstrom; two further coincidences were established; and in 
 1 866 a fourth hydrogen line in the extreme violet (named Ji) was 
 detected in the solar spectrum. With Thalen, he besides added 
 manganese, titanium, and cobalt to the constituents of the sun 
 enumerated by Kirchhoff, and raised the number of identical rays 
 in the solar and terrestrial spectra of iron to no less than 460.* 
 
 Thus, when Professor Lockyer entered on that branch of 
 inquiry in 1 872, fourteen substances were recognised as common 
 to the earth and sun. Early in 1878 he was able to increase 
 the list provisionally to thirty-three. 5 All these are metals ; 
 
 1 Trans. Royal Society of Edinburgh, vol. xxxvi., p. 99. 2 Kev. A. L. 
 Cortie, Astr. and Astro-Physics, May 1892, p. 401. 3 Ann. d. Phys., Bd. cxvii., 
 p. 296. 4 Comptes Rendus, t. Ixiii., p. 647. 5 Ibid., t. Ixxxvi., p. 317. Some 
 half dozen of these identifications have proved fallacious. 
 
264 HISTORY OF ASTRONOMY PART n. 
 
 for there is strong reason to believe that hydrogen presents a 
 solitary instance of an ordinarily gaseous metal, just as mercury 
 does of an ordinarily liquid one. This rapid success was due to 
 his adoption of the test of length in lieu of that of strength in 
 the comparison of lines. He measured their relative significance, 
 in other words, rather by their persistence through a wide range 
 of temperature than by their brilliancy at any one temperature. 
 The distinction was easily drawn. Photographs of the electric 
 arc, in which any given metal had been volatilised, showed some 
 of the rays emitted by it stretching across the axis of the light 
 to a considerable distance on either side, while many others 
 clung more or less closely to its central hottest core. The 
 former " long lines," regarded as certainly representative, were 
 those primarily sought in the solar spectrum ; while the 
 attendant " short lines," often, in point of fact, due to foreign 
 admixtures, were set aside as likely to be misleading. 1 The 
 criterion is a valuable one, and its employment has greatly helped 
 to quicken the progress of solar chemistry. 
 
 Carbon was the first non-metallic element discovered in the 
 sun. Messrs. Trowbridge and Hutchiris of Harvard College 
 concluded in I 887,2 on the ground of certain spectral coincidences, 
 that this protean substance is vaporised in the solar atmosphere 
 at a temperature approximately that of the voltaic arc. Partial 
 evidence to the same effect had earlier been alleged by Lockyer, 
 as well as by Liveing and Dewar ; and the case was rendered 
 tolerably complete by the photographs taken by MM. Kayser 
 and Kunge in iSSp. 3 The work of photographically comparing 
 the spectrum of the sun with the spectra of the various 
 chemical elements, lately carried on by Professor Kowland at 
 the Johns Hopkins University, Baltimore, has shown it to be 
 irresistible. Two hundred carbon-lines have actually been thus 
 sifted out from sunlight, and it contains others significant of 
 the presence of silicon a related substance, and one as 
 important to rock-building on the earth as carbon is to the 
 maintenance of life. The general result, so far, of Professor 
 
 1 Chemistry of the Sun, p. 143. - Amer. Jour, of Science, vol. xxxiv., p. 348. 
 3 Berlin Abhandlungen, 1889. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 265 
 
 Rowland's labours lias been the establishment among solar 
 materials, not only of these two out of the fourteen metalloids, 
 or non-metallic substances, but of thirty-three metals, including 
 silver and tin. Gold, mercury, bismuth, antimony, and arsenic 
 are discarded from the catalogue ; platinum and uranium, with 
 six other metals, are recorded as doubtful ; while iron crowds 
 the spectrum with over two thousand obscure rays. 1 
 
 The question whether the sun contains oxygen must still be 
 answered negatively. Dr. Henry Draper 2 announced, in 1877, 
 his imagined discovery, in the solar spectrum, of eighteen 
 especially brilliant spaces corresponding to its direct emissions. 
 But the discrimination of bright lines from ' a very slightly less 
 lucid background must, it is plain, be always a matter of much 
 delicacy and some uncertainty, especially when the lines to be 
 discriminated are not sharp, but more or less blurred and 
 widened. And in fact, the correspondences which had appeared 
 so striking in Dr. Draper's photographs, proved, when put to the 
 test of very high dispersion, to be wholly illusory. 3 
 
 The solar spectrum, it is now admitted, includes no significant 
 bright rays. 4 The book of solar chemistry must be read in 
 characters exclusively of absorption. Nevertheless, the whole 
 truth is unlikely to be written there. That a substance displays 
 none of its distinctive beams in the spectrum of the sun or of a 
 star, affords scarcely a presumption against its presence. For 
 it may be situated below the level where absorption occurs, 
 or under a pressure such as to efface lines by continuous lustre ; 
 it may be at a temperature so high that it gives out more 
 light than it takes up, and yet its incandescence may be masked 
 by the absorption of other bodies ; finally, it may just balance 
 absorption by emission, with the result of complete spectral 
 neutrality. An instructive example is that of helium, the 
 enigmatical chromospheric element. Father Secchi remarked 
 in 1 868 5 that there is no dark line in the solar spectrum 
 
 1 Amer. Jour, of Science, vol. xli., p. 243. See Appendix, Table II. 
 2 Ibid., vol. xiv., p. 89; Nature, vol. xvi., p. 364; Month. Not., vol. xxxix., 
 p. 440. 3 Month. Not., vol. xxxviii. , p. 473; Trowbridge and Hutchins, 
 Amer. Jour, of Science, vol. xxxiv., p. 263. 4 Scheiner, Die Spectralanalyse , 
 p. 180. 5 Comptes Rendus, t. Ixvii., p. 1123. 
 
266 HISTORY OF ASTRONOMY. PART n. 
 
 matching its light ; and his observation has been fully confirmed. 1 
 The substance, for some unknown reason, seems incapable of 
 exercising the least absorption. 
 
 Our terrestrial vital element might then easily wear an 
 impenetrable disguise in the sun ; more especially since the 
 inner organisation of the oxygen molecule is a considerably 
 plastic one. It is readily modified by heat. . and these modi- 
 fications are reflected in its varying modes of radiating 
 light. Dr. Schuster enumerated in 1 879 - four distinct oxygen 
 spectra, corresponding to various stages of temperature, or 
 phases of electrical excitement; and a fifth has been added 
 by M. Egoroff's discovery in i883 3 that certain well-known 
 groups of dark lines in the red end of the solar spectrum 
 (Fraunhof er's A and B) are due to absorption by the cool oxygen 
 of our air. These persist down to the lowest temperatures, and 
 even survive a change of state. They are produced essentially 
 the same by liquid as by aerial oxygen. 4 
 
 It seemed, however, possible to M. Janssen that these bands 
 owned a joint solar and terrestrial origin. Oxygen in a fit 
 condition to produce them might, he considered, exist in the 
 outer atmosphere of the sun; and he resolved to decide the 
 point. No one could bring more skill and experience to bear 
 upon it than he. 5 By observations on the summit of the Faul- 
 horn, as well as by direct experiment, he demonstrated, nearly 
 thirty years ago, the leading part played by water-vapour in 
 generating the atmospheric spectrum ; and he had recourse to 
 similar means for appraising the share in it assignable to oxygen. 
 An electric beam, transmitted from the Eiffel Tower to Meudon 
 in the summer of 1888, having passed through a weight of 
 oxygen about equal to that piled above the surface of the earth, 
 showed the groups A and B just as they appear in the high-sun 
 spectrum. 6 Atmospheric action is then adequate to produce 
 them. But M. Janssen desired to prove, in addition, that they 
 
 1 Rev. A. L. Cortie, Month. Not., vol. li., p. 18. - Phil. Trans., vol. 
 
 clxx., p. 46. s Comptes Hendus, t. xcvii., p. 555 ; t. ci., p. 1145. 4 Liveing 
 and Dewar, Astr. and Astro- Physics, Oct. 1892, p. 705. 5 Comptes Rendusi 
 t. lx., p. 213 ; t. Ixiii., p. 289. 6 Ibid., t. cviii., p. 1035. 
 
CHAP. iv. SOLAR SPECTROSCOPY. 267 
 
 diminish proportionately to its amount. His ascent of Mont 
 Blanc 1 in 1890 was undertaken with this object. It was 
 perfectly successful. In the solar spectrum, examined from 
 that eminence, oxygen-absorption was so much enfeebled as to 
 leave no possible doubt of its purely telluric origin. Our 
 pabulum vitce is not then at present recognisable as a solar 
 element. 
 
 The rays of the sun, besides being stopped selectively in our 
 atmosphere, suffer also a marked general absorption. This tells 
 chiefly upon the shortest wave-lengths ; the ultra-violet spectrum 
 is in fact closed, as if by the interposition of an opaque screen. 
 Nor does the screen appear very sensibly less opaque from an 
 elevation of 10,000 feet. Dr. Simony's spectral photographs, 
 taken on the Peak of Teneriffe, 2 extended but slightly further up 
 than M. Cornu's, taken in the valley of the Loire. Could the veil 
 be withdrawn, some indications as to the originating temperature 
 of the solar spectrum might be gathered from its range, since 
 the proportion of quick vibrations given out by a glowing body 
 grows with the intensity of its incandescence. And this brings 
 us to the subject of our next Chapter. 
 
 1 Comptes Rendus, t. cxi., p. 431. 2 Ibid., p. 941 ; Huggins, Proc. Soy. 
 Soc., vol. xlvi., p. 1 68. 
 
CHAPTER V. 
 TEMPERATURE OF THE SUN. 
 
 NEWTON was the first who attempted to measure the quantity of 
 heat received by the earth from the sun. His object in making 
 the experiment was to ascertain the temperature encountered by 
 the comet of 1680 at its passage through perihelion. He found 
 it, by multiplying the observed heating effects of direct sunshine 
 according to the familiar rule of the " inverse squares of the 
 distances," to be about 2000 times that of red-hot iron." 1 
 
 Determinations of the sun's thermal power made with some 
 scientific exactness, date, however, from 1837. A few days 
 previous to the beginning of that year, Herschel began observ- 
 ing at the Cape of Good Hope with an " actinometer," and 
 obtained results agreeing quite satisfactorily with those derived 
 by Pouillet from experiments made in France some months later 
 with a " pyrheliometer." 2 Pouillet found that the vertical rays 
 of the sun falling on each square centimetre of the earth's sur- 
 face are competent (apart from atmospheric absorption) to raise 
 the temperature of 1.7633 grammes of water one degree Centi- 
 grade per minute. This number (1.7633) he called the " solar 
 constant " ; and the unit of heat chosen is known as the 
 " calorie." Hence it was computed that the total amount of 
 solar heat received during a year would suffice to melt a layer of 
 ice covering the entire earth to a depth of 30.89 metres, or 
 100 feet ; while the heat emitted would melt, at the sun's sur- 
 face, a stratum 11.80 metres thick each minute. A careful 
 series of observations showed that nearly half the heat incident 
 upon our atmosphere is stopped in its passage through it. 
 
 1 Principia, p. 498 (ist ed.). 2 Comptes Rendus, t. vii., p. 24. 
 
CHAP. v. TEMPERATURE OF THE SUN. 269 
 
 Herschel got somewhat larger figures, though he assigned 
 only a third as the spoil of the air. Taking a mean between his 
 own and Pouillet's, he calculated that the ordinary expenditure 
 of the sun per minute would have power to melt a cylinder of 
 ice 184 feet in diameter, reaching from his surface to that of 
 a Centauri ; or, putting it otherwise, that an ice-rod 45.3 miles 
 across, continually darted into the sun with the velocity of light, 
 would scarcely consume, in dissolving, the thermal supplies now 
 poured abroad into space. 1 It is nearly certain that this esti- 
 mate should be increased by about two-thirds in order to bring 
 it up to the truth. 
 
 Nothing would, at first sight, appear simpler than to pas& 
 from a knowledge of solar emission a strictly measurable 
 quantity to a knowledge of the solar temperature ; this being 
 defined as the temperature to which a surface thickly coated 
 with lamp-black (that is, of standard radiating power) should be 
 raised to enable it to send us, from the sun's distance, the* 
 amount of heat actually received from the sun. Sir John 
 Herschel showed that heat-rays at the sun's surface must be- 
 92,000 times as dense as when they reach the earth ; but it by 
 no means follows that either the surface emitting or a body 
 absorbing those heat-rays must be 92,000 times hotter than a 
 body exposed here to the full power of the sun. The reason is, 
 that the rate of emission consequently the rate of absorption, 
 which is its correlative increases very much faster than the 
 temperature. In other words, a body radiates or cools at a con- 
 tinually accelerated pace as it becomes more and more intensely 
 heated above its surroundings. 
 
 Newton, however, took it for granted that radiation and 
 temperature advance pari passu that you have only to ascertain 
 the quantity of heat received from and the distance of a remote 
 body in order to know how hot it is. 2 And the validity of this 
 principle, known as "Newton's Law" of cooling, was never 
 questioned until De la Eoche pointed out, in i8i2, 3 that it was 
 
 1 Results o/Astr. Observations, ^. 446. - " Est enim calor soils ut radiorum 
 densitas, hoc est, reciproce ut quadratum distantise locorum a sole." 
 Principia, p. 508 (3d ed. 1726). 3 Jour, de Physique, t. Ixxv., p. 215. 
 
270 HISTORY OF ASTRONOMY. PART n. 
 
 .approximately true only over a low range of temperature ; while 
 five years later, Dulong and Petit generalised experimental 
 results into the rule, that while temperature grows by arithmet- 
 ical, radiation increases by geometrical progression. 1 Adopting 
 this formula, Pouillet derived from his observations on solar 
 heat a solar temperature of somewhere between 1461 and 
 1761 Cent. Now, the higher of these pointfe which is nearly 
 that of melting platinum is undoubtedly surpassed at the focus 
 of certain burning-glasses which have been constructed of such 
 power as virtually to bring objects placed there within a quarter 
 of a million of miles of the photosphere. In the rays thus con- 
 centrated, platinum and diamond become rapidly vaporised, 
 notwithstanding the great loss of heat by absorption, first in 
 passing through the air, and again in traversing the lens. 
 Pouillet's maximum is then manifestly too low, since it involves 
 the absurdity of supposing a radiating mass capable of heating 
 a distant body more than it is itself heated. 
 
 Less demonstrably, but scarcely less surely, Mr. J. J. Water- 
 ston, who attacked the problem in 1860, erred in the opposite 
 direction. Working up, on Newton's principle, data collected 
 by himself in India and at Edinburgh, he got for the " potential 
 temperature" of the sun 12,880,000 Fahr., 2 equivalent to 
 7,156,000 Cent. The phrase potential temperature (for which 
 Violle substituted, in 1876, effective temperature) was designed 
 to express the accumulation in a single surface, postulated for 
 the sake of simplicity, of the radiations not improbably received 
 from a multitude of separate solar layers reinforcing each other ; 
 and might thus (it was explained) be considerably higher than 
 the actual temperature of any one stratum. 
 
 At Eome, in 1861, Father Secchi repeated Waterston's ex- 
 periments, and reaffirmed his conclusion ; 3 while Soret's observ- 
 ations, made on the summit of Mont Blanc in 1 867, 4 furnished 
 him with materials for a fresh and even higher estimate of ten 
 million degrees Centigrade. 5 Yet from the very same data, 
 
 1 Ann. de Chimie, t. vii., 1817, p. 365. - Phil. Mag., vol. xxiii. (4th ser.), 
 p. 505. 3 Nuovo Cimento, t. xvi., p. 294. 4 Comptes Itendus, t. Ixv., p. 526. 
 .5 The direct result of 5^ million degrees was doubled in allowance for absorp- 
 tion in the sun's own atmosphere. Comptes Itendus, t. Ixxiv., p. 26. 
 
CHAP. v. TEMPERATURE OF THE SUN. 271 
 
 substituting Dulong and Petit's for Newton's law, Vicaire 
 deduced in 1872 a provisional solar temperature of I398 . 1 
 This is below that at which iron melts, and we know that iron- 
 vapour exists high up in the sun's atmosphere. The matter was 
 taken into consideration on the other side of the Atlantic by 
 Ericsson in 1871. He attempted to re-establish the shaken 
 credit of Newton's principle, and arrived, by its means, at a 
 temperature of four million degrees of Fahrenheit. 2 More 
 recently, what he considered an " underrated computation," based 
 upon observations of the quantity of heat received by his " sun 
 motor," gave him three million degrees. This, he rightly 
 asserted, must be accepted, if only the temperature produced by 
 radiant heat be proportional to its density, or inversely as its 
 diffusion. 3 Could this be granted, the question would be much 
 simplified ; but there is no doubt that the case is far otherwise 
 when heat becomes intensified. 
 
 In 1 876 the sun's temperature was proposed as the subject of 
 a prize by the Paris Academy of Sciences ; but although the 
 essay of M. Jules Violle was crowned, the problem was declared 
 to remain unsolved. Violle (who adhered to Dulong and Petit's 
 formula) arrived at an effective temperature of 1500 C., but con- 
 sidered that it might actually reach 2500 C., if the emissive 
 power of the photospheric clouds fell far short (as seemed 
 probable) of the lamp-black standard. 4 Experiments made in 
 April and May 1881 giving a somewhat higher result, he raised 
 this figure to 3000 C. 5 
 
 Appraisements so outrageously discordant as those of Water- 
 ston, Secchi, and Ericsson on the one hand, and those of the 
 French savans on the other, served only to show that all were 
 based upon a vicious principle. The late Professor F. Rosetti, 6 
 accordingly, of the Paduan University, at last perceived the 
 necessity for getting out of the groove of " laws " plainly in 
 contradiction with facts. The temperature, for instance, of 
 the oxy-hydrogen flame was fixed by Bunsen at 2800 C. an 
 
 1 Comptes JRendus, t. Ixxiv., p. 31. 2 Nature, vols. iv., p. 204 ; v., p. 505. 
 3 Ibiff., vol. xxx., p. 467. 4 Ann. de Chim., t. x. (5th ser.), p. 361. 5 Comptes 
 Rendus, t, xcvi., p. 254. (! Phil. Mag., vol. viii., p. 324, 1879. 
 
272 HISTORY OF ASTRONOMY. PART n. 
 
 estimate certainly not very far from the truth. But if the two 
 systems of measurement applied to the sun be used to determine 
 the heat of a solid body rendered incandescent in this flame, 
 it comes out, by Newton's -mo^de of calculation, 45,000 C. ; by 
 Dulong and Petit's, 870 C. 1 Both, then, are justly discarded, 
 the first as convicted of exaggeration, the second of undervalua- 
 tion. The formula substituted by Rosetti wa^s tested successfully 
 up to 2000 C. ; but since it is, like its predecessors, a purely 
 empirical rule, is guaranteed by no principle, and can, in con- 
 sequence, not be trusted out of sight, it may, like them, break 
 down at still higher elevations. All that can be said is that it 
 gives the most plausible results. Radiation, so far as it obeys 
 this new prescription, increases as the square of the absolute 
 temperature that is. of the number of degrees counted from 
 the "absolute zero" of 273 C. Its employment gives for the 
 sun's radiating surface an effective temperature of 20,380 C. 
 (including a supposed loss of one-half in the solar atmosphere) ; 
 and setting a probable deficiency in emission (as compared with 
 lamp-black) against a probable mutual reinforcement of super- 
 posed strata, Professor Rosetti considered " effective" as nearly 
 equivalent to "actual" temperature. Yet another "law of 
 cooling " was proposed by M. Stefan at Vienna in i879- 2 It is 
 that emission grows as the fourth power of absolute temperature. 
 Hence the temperature of the photosphere would be proportional 
 to the square root of the square root of its heating effects at a 
 distance, and appeared, by Stefan's calculations from Violle's 
 measures of solar radiative intensity, to be just 6000 C. M. H. 
 Le Chatelier 3 makes it 7600, and he is the latest investigator on 
 the subject. His formula, based on a series of laborious experi- 
 ments, conveys an intricate and unaccountable relation between 
 the temperature of an incandescent body and the intensity of its 
 red radiations. It proved, however, trustworthy up to 1800 C. ; 
 and he hence believes that his result for the sun cannot well be 
 more than one thousand degrees in error, one way or the other. 
 
 1 Phil. Mag., vol. viii., p. 325, 1879. 2 jSitzungsberichte, Wien, Bd. Ixxix., ii., 
 p. 391. 3 Comptes Rendus, March 28, 1892 ; Astr. and Astro-Physics, June 
 1892, p. 517- 
 
CHAP. v. TEMPERATURE .OF THE SUN. 273 
 
 A new line of inquiry was struck out by Zollner in 1870. 
 Instead of tracking the solar radiations backward with the 
 dubious guide of empirical formulas, he investigated their in- 
 tensity at their source. He showed 1 that, taking prominences 
 to be simple effects of the escape of powerfully compressed 
 gases, it was possible, from the known mechanical laws of heat 
 and gaseous constitution, to deduce minimum values for the 
 temperatures prevailing in the area of their development. These 
 came out 27,700 C. for the strata lying immediately above, and 
 68,400 C. for the strata lying immediately below the photo- 
 sphere, the former being regarded as the region into which, and 
 the latter as the region from which, the eruptions took place. In 
 this calculation, no prominences exceeding 40,000 miles (1.5') in 
 height were included. But in 1884, G. A. Hirn of Colmar, 
 taking into account the enormous velocities of projection observed 
 in the interim, fixed two million degrees Centigrade as the lowest 
 internal temperature by which they could be accounted for; 
 although of opinion that the condensations, presumed to give 
 origin to the photospheric clouds, were incompatible with a 
 higher external temperature than 50,000 to 100,000 C. 2 
 
 This method of going straight to the sun itself, observing what 
 goes on there, and inferring conditions, has much to recommend 
 it ; but its profitable use demands knowledge we are still very 
 far from possessing. We are quite ignorant, for instance, of the 
 actual circumstances attending the birth of the solar flames. The 
 assumption that they are nothing but phenomena of elasticity is 
 a purely gratuitous one. Spectroscopic indications, again, give 
 hope of eventually affording a fixed point of comparison with 
 terrestrial heat-sources ; but their interpretation is still beset with 
 uncertainties ; nor can, indeed, the expression of transcendental 
 temperatures in degrees of impossible thermometers be, at the 
 best, other than a futile attempt to convey notions respecting a 
 state of things altogether outside the range of our experience. 
 
 A more tangible, as well as a less disputable proof of solar 
 radiative intensity than any mere estimates of temperature, was 
 provided in some experiments made by Professor Langley in 
 
 1 Astr. Nach., Nos. 1815-16. 3 L' Astronomic, Sept. 1884, p. 334. 
 
 18 
 
274 HISTORY OF ASTRONOMY. PART n. 
 
 1878.* Using means of unquestioned validity, he found the 
 sun's disc to radiate 87 times as much heat, and 5300 times as 
 much light, as an equal area of metal in a Bessemer converter 
 after the air-blast had continued about twenty minutes. The 
 brilliancy of the incandescent steel, nevertheless, was so blinding, 
 that melted iron, flowing in a dazzling white-hot stream into the 
 crucible, showed " deep brown by comparison, presenting a 
 contrast like that of dark coffee poured into a white cup." Its 
 temperature was estimated (not quite securely, as Young has 
 pointed out) 2 at 1 800 to 2000 C. ; and no allowances were 
 made, in computing relative intensities, for atmospheric ravages 
 on sunlight, for the extra impediments to its passage presented 
 by the smoke-laden air of Pittsburg, or for the obliquity of its 
 incidence. Thus a very large balance of advantage lay on the 
 side of the metal. 
 
 A further element of uncertainty in estimating the intrinsic 
 strength of the sun's rays has still to be considered. From the 
 time that his disc first began to be studied with the telescope, it 
 was perceived to be less brilliant near the edges. Lucas Valerius 
 of the Lyncean Academy seems to have been the first to note 
 this fact, which, strangely enough, was denied by Galileo in a 
 letter to Prince Cesi of January 25, i6i3. 3 Father Schemer, 
 however, fully admitted it, and devoted some columns of his 
 bulky tome to the attempt to find an appropriate explanation. 4 
 In 1729, Bouguer measured, with much accuracy, the amount of 
 this darkening ; and from his data, Laplace, adopting a principle 
 of emission now known to be erroneous, concluded that the sun 
 loses eleven-twelfths of his light through absorption in his own 
 atmosphere. 5 The real existence of this atmosphere, which is 
 totally distinct from the beds of ignited vapours producing the 
 Fraunhofer lines, is not open to doubt, although its nature is still 
 a matter of conjecture. The separate effects of its action on 
 luminous, thermal, and chemical rays were carefully studied by 
 Father Secchi, who in 1 870 6 inferred the total absorption to be 
 
 1 Jour, of Science, vol. i. (3d ser.), p. 653. " The /Sun, p. 269. 3 Op., 
 t. vi., p. 198. 4 Rosa Ursina, lib. iv., p. 618. 5 Mec. Cef., liv. x., p. 323. 
 6 Le Soleil (ist ed.), p. 136. 
 
CHAP. v. TEMPERATURE OF THE SUN. 275 
 
 jy*j. of all radiations taken together, and added the important 
 observation that the light from the limb is no longer white, but 
 reddish-brown. Absorptive effects were thus seen to be unequally 
 distributed ; and they could evidently be studied to advantage 
 only by taking the various rays of the spectrum separately, and 
 finding out how much each had suffered in transmission. 
 
 This was done by H. C. Vogel in I877- 1 Using a polarising 
 photometer, he found that only 13 per cent, of the violet rays 
 escape at the edge of the solar disc, 16 of the blue and green, 
 25 of the yellow, and 30 per cent, of the red. Midway between 
 centre and limb, 88.7 of violet light and 96.7 of red penetrate 
 the absorbing envelope, the removal of which would leave the 
 sun's visible spectrum of above two and a half times its present 
 intensity in the most, and one and a half times in the least 
 refrangible parts. The nucleus of a small spot was ascertained 
 to be of the same luminous intensity as a portion of the 
 unbroken surface about two and a half minutes from the limb. 
 These experiments having been made during a spot-minimum 
 when there is reason to think that absorption is below its 
 average strength, Vogel suggested their repetition at a time of 
 greater activity. They were extended to the heat-rays by 
 Edwin B. Frost at Potsdam in iSpi. 2 His careful experiment? 
 went to show that, were the sun's atmosphere removed, his 
 thermal power, as regards ourselves, would be increased 1.7 
 times. They established, too, the intrinsic uniformity in radia- 
 tion of all parts of his disc. 
 
 Professor Langley, now of Washington, gave to measures of 
 the kind a refinement previously undreamt of. Reliable deter- 
 minations of the energy of the individual spectral rays were, 
 for the first time, rendered possible by his invention of the 
 "bolometer" in i88o. 3 This exquisitely sensitive instrument 
 affords the means of measuring heat, not directly, like the 
 thermopile, but in its effects upon the conduction of electricity. 
 It represents, in the phrase of the inventor, the finger laid upon 
 the throttle-valve of a steam-engine. A minute force becomes 
 
 1 Monatsber., Berlin, 1877, p. 104. 2 Astr. Nach., Nos. 3105-3106 ; Astr. 
 and Astro-Physics, Oct. 1892, p. 720. Cf. Wilson and Kambaut, Proc. R. Irish 
 Acad., vol. ii., p. 299, 1892. 3 Am. Jour, of Sc., vol. xxi., p. 187. 
 
276 HISTORY OF ASTRONOMY. PART n. 
 
 the modulator of a much greater force, and thus from imper- 
 ceptible becomes conspicuous. By locally raising the tempera- 
 ture of an inconceivably fine strip of platinum serving as the 
 conducting-wire in a circuit, ; *he flow of electricity is impeded 
 at that point, and the included galvanometer records a disturb- 
 ance of the electrical flow. Amounts of heat have, in this way, 
 been detected in less than ten seconds, whiteh, expended during 
 a thousand years on the melting of a kilogramme of ice, would 
 leave a part of the work still undone. 
 
 The heat contained in the diffraction spectrum is, with 
 equal dispersions, barely one-tenth of that in the prismatic 
 spectrum. It had, accordingly, never previously been found 
 possible to measure it in detail that is, ray by ray. But it 
 is only from the diffraction, or normal spectrum that any 
 true idea can be gained as to the real distribution of energy 
 among the various constituents, visible and invisible, of a 
 sunbeam. The effect of passage through a prism is to crowd 
 together the red rays very much more than the blue. To this 
 prismatic distortion was owing the establishment of a pseudo- 
 maximum of heat in the infra-red, which disappeared when 
 the natural arrangement by wave-length was allowed free play. 
 Professor Langley's bolometer has shown that the hottest part 
 of the normal spectrum virtually coincides with its most 
 luminous part, both lying in the orange, close to the D line. 1 
 Thus the last shred of evidence in favour of the threefold 
 division of solar radiations vanished, and it became obvious that 
 the varying effects thermal, luminous, or chemical produced 
 by them are due, not to any distinction of quality in themselves, 
 but to the different properties of the substances they impinge 
 upon. They are simply bearers of energy, conveyed in shorter 
 or longer vibrations ; the result in each separate case depending 
 upon the capacity of the material particles meeting them for 
 taking up those shorter or longer vibrations, and turning them 
 variously to account in their inner economy. 
 
 A long series of experiments at Allegheny was completed 
 
 1 For J. W. Draper's partial anticipation of this result, see Am. Jour, of tic., 
 vol. iv., 1872, p. 174. 
 
CHAP. v. TEMPERATURE OF THE SUN. 277 
 
 in the summer of 1881 011 the crest of Mount Whitney in the 
 Sierra Nevada. Here, at an elevation of 14,887 feet, in the 
 driest and purest air, perhaps, in the world, atmospheric 
 absorptive inroads become less sensible, and the indications of 
 the bolometer, consequently, surer and stronger. An enor- 
 mous expansion was at once given to the invisible region in 
 the solar spectrum below the red. Captain Abney had got 
 chemical effects from undulations twelve ten-thousandths of a 
 millimetre in length. These were the longest recognised as, 
 or indeed believed, on theoretical grounds, to be capable of 
 existing. Professor Langley now got heating effects from 
 rays of above twice that wave-length, his delicate thread of 
 platinum groping its way down nearly to thirty ten-thousandths 
 of a millimetre, or three "microns." The known extent of the 
 solar spectrum was thus at ouce more than doubled. Its visible 
 portion covers a range of about one octave; bolometric indi- 
 cations already in 1884 comprised between three and four. The 
 great importance of the newly explored region appears from 
 the fact that three-fourths of the entire energy* of sunlight 
 reside in the infra-red, while scarcely more than one-hundredth 
 part of that amount is found in the better known ultra-violet 
 space. 1 These curious facts were reinforced, in i886, 2 by further 
 particulars learned with the help of rock-salt lenses and prisms, 
 glass being impervious to very slow as to very rapid vibrations. 
 Traces were thus detected of solar heat distributed into bands of 
 transmission alternating with bands of atmospheric absorption, 
 down to a wave-length of eighteen microns. The meteorological 
 importance of this circumstance, as Professor Langley has not 
 failed to note, is very great. It means that radiations corre- 
 sponding to the temperature of freezing water, can reach space 
 from the earth's surface, as well as the earth's surface from 
 space. 
 
 Atmospheric absorption had never before been studied with 
 such precision as it was by Professor Langley on Mount Whitney. 
 
 1 Phil. Mag., vol. xiv., p. 179 (March 1883). 2 " The Solar and the Lunar 
 Spectrum," Memoirs National Acad. of Sciences, vol. iv. ; " On hitherto 
 Unrecognised Wave-lengths," Atner. Jour, of Science, vol. xxxii., Aug. 1886. 
 
278 HISTORY OF ASTRONOMY. PART n. 
 
 Aided by simultaneous observations from Lone Pine, at the foot 
 of the Sierra, he was able to calculate the intensity belonging to 
 each ray before entering the earth's gaseous envelope in other 
 words, to construct -an extra-atmospheric curve of energy in the 
 spectrum. The result showed that the blue end suffered far 
 more than the red, absorption varying inversely as wave-length. 
 This property of stopping predominantly tfye quicker vibrations 
 is shared, as both Vogel and Langley l have conclusively shown, 
 by the solar atmosphere. The effect of this double absorption 
 is as if two plates of reddish glass were interposed between us 
 and the sun, the withdrawal of which would leave his orb, not 
 only three or four times more brilliant, but in colour of a distinct 
 greenish-blue, not very different from the tint of the second (F) 
 line of hydrogen. 2 
 
 The fact of the unveiled sun being blue has an important 
 bearing upon the question of his temperature, to afford a some- 
 what more secure answer to which was the ultimate object of 
 Professor Langley's persevering researches ; for it is well known 
 that, as bodies grow hotter, the proportionate representation in 
 their spectra of the more refrangible rays becomes greater. The 
 lowest stage of incandescence is the familiar one of red heat. 
 As it gains intensity, the quicker vibrations come in, and an 
 optical balance of sensation is established at white heat. The 
 final term of bhw heat, as we now know, is attained by the 
 photosphere. On this ground alone, then, of the large original 
 preponderance of blue light, we must raise our estimate of solar 
 heat ; and actual measurements show the same upward tendency. 
 Until quite lately, Pouillet's figure of 1.7 calories per minute per 
 square centimetre of terrestrial surface, was the received value 
 for the "solar constant." Forbes had, it is true, got 2.85 from 
 observations on the Faulhorn in 1 842 ; 3 but they failed to 
 obtain the confidence they merited. Pouillet's result was not 
 definitely superseded until Violle, from actinometrical measures 
 at the summit and base of Mont Blanc in 1875, computed the 
 intensity of solar radiation at 2.54- 4 and Crova, about the same 
 
 1 Comptes Bendus, t. xcii., p. 701. 2 Nature, vol. xxvi., p. 589. 3 Phil. 
 Trans., vol. cxxxii., p. 273. 4 Ann. de CJiim., t. x., p. 321. 
 
CHAP. v. TEMPERATURE OF THE SUN. 279 
 
 time, at Montpellier, showed it to be above two calories. 1 
 Langley went higher still. Working out the results of the 
 Mount Whitney expedition, he was led to conclude atmospheric 
 absorption to be fully twice as effective as had hitherto been sup- 
 posed. Scarcely sixty per cent., in fact, of those solar radiations 
 which strike perpendicularly through a seemingly translucent 
 sky, attain the sea-level. The rest are reflected, dispersed, or 
 absorbed. This discovery involved a large addition to the 
 original supply so mercilessly cut down in transmission, and the 
 solar constant rose at once to three calories its actual standard 
 value. 2 Phrased otherwise, this means that the sun's heat 
 reaching the outskirts of our atmosphere is capable of doing 
 without cessation the work of an engine of three-horse power 
 for each square yard of the earth's surface. Thus, modern 
 inquiries, though they give no signs of agreement, within any 
 tolerable limits of error, as to the probable temperature of the 
 sun, tend, with growing certainty, to render more and more 
 evident the vastness of the thermal stores contained in the 
 great central reservoir of our system. 
 
 1 Ann. de Chim., t. xi., p. 505. - From actinometrical observations made 
 at Kieff in 1890, M. Savelieff deduces a solar constant of 3.47 (Comptes Itendus, 
 t. cxii., p. 1200), but there seems no sufficient reason at present for advancing 
 beyond Limgley's value of 3.0. 
 
f 
 
 CHAPTEE VI. ^ 
 
 77f SUN'S DISTANCE. 
 
 THE question of the sun's distance arises naturally from the 
 consideration of his temperature, since the intensity of the 
 radiations emitted as compared with those received and mea- 
 sured, depends upon it. But the knowledge of that distance 
 has a value quite apart from its connection with solar physics. 
 The semi-diameter of the earth's orbit is our standard measure 
 for the universe. It is the great fundamental datum of 
 astronomy the unit of space, any error in the estimation of 
 which is multiplied and repeated in a thousand different ways, 
 both in the planetary and sidereal systems. Hence its deter- 
 mination has been called by Airy "the noblest problem in 
 astronomy." It is also one of the most difficult. The quantities 
 dealt with are so minute that their sure grasp tasks all the 
 resources of modern science. An observational inaccuracy 
 which would set the moon nearer to or farther from us than 
 she really is by one hundred miles, would vitiate an estimate of 
 the sun's distance to the extent of sixteen million ! * What is 
 needed in order to attain knowledge of the desired exactness is 
 no less than this : to measure an angle about equal to that 
 subtended by a halfpenny 2000 feet from the eye, within a 
 little more than a thousandth part of its value. 
 
 The angle thus represented is what is called the " horizontal 
 parallax " of the sun. By this amount the breadth of a half- 
 penny at 2OOO feet he is, to a spectator on the rotating earth, 
 removed at rising and setting from his meridian place in the 
 heavens. Such, in other terms, would be the magnitude of the 
 
 1 Airy, Month. JVot., vol. xvii., p. 210. 
 
CHAP. vi. THE SUN'S DISTANCE. 281 
 
 terrestrial radius as viewed from the sun. If we knew this 
 magnitude with certainty and precision, we should also know 
 with certainty and precision the dimensions of the earth 
 being, as they are, well ascertained the distance of the sun. 
 In fact, the one quantity commonly stands for the other in 
 works treating professedly of astronomy. But this angle of 
 parallax or apparent displacement cannot be directly measured 
 cannot even be perceived with the finest instruments. Not 
 from its smallness. The parallactic shift of the nearest of the 
 stars as seen from opposite sides of the earth's orbit, is many 
 times smaller. But at the sun's limb, and close to the horizon r 
 where the visual angle in question opens out to its full extent, 
 atmospheric troubles become overwhelming, and altogether 
 swamp the far more minute effects of parallax. 
 
 There remain indirect methods. Astronomers are well ac- 
 quainted with the proportions which the various planetary 
 orbits bear to each other. They are so connected, in the 
 manner expressed by Kepler's Third Law, that the periods 
 being known, it only needs to find the interval between any 
 two of them in order to infer at once the distances separating 
 them all from one another and from the sun. The plan is given ; 
 what we want to discover is the scale upon which it is drawn ; 
 so that, if we can get a reliable measure of the distance of a 
 single planet from the earth, our problem is solved. 
 
 Now some of our fellow-travellers in our unending journey 
 round the sun, come at times well within the scope of celestial 
 trigonometry. The orbit of Mars lies at one point not more 
 than thirty-five million miles outside that of the earth, and 
 when the two bodies happen to arrive together in or near the 
 favourable spot a conjuncture which recurs every fifteen years 
 the desired opportunity is granted. Mars is then " in oppo- 
 sition," or on the opposite side of us from the sun, crossing the 
 meridian consequently at midnight. 1 It was from an opposition 
 of Mars, observed in 1672 by Richer at Cayenne in concert with 
 
 1 Mars comes into opposition once in about 780 days ; but owing to the 
 eccentricity of both orbits, his distance from the earth at those epochs varies 
 from thirty-five to sixty-two million miles. 
 
282 HISTORY OF ASTRONOMY. PART n. 
 
 Cassini in Paris, that the first scientific estimate of the sun's 
 distance was derived. It appeared to be nearly eighty-seven 
 millions of miles (parallax 9.5"); while Flamsteed deduced 
 81,700,000 (parallax 10") from his independent observations of 
 the same occurrence a difference quite insignificant at that 
 stage of the inquiry. But Picard's result was just half Flam- 
 steed's (parallax 20" distance forty-one million miles) ; and 
 Lahire considered that we must be separated from the hearth 
 of our system by an interval of at least 136 million miles. 1 So 
 that uncertainty continued to be on a gigantic scale. 
 
 Venus', on the other hand, comes closest to the earth when 
 she passes between it and the sun. At such times of " inferior 
 conjunction " she is, however, still twenty-six million miles, or 
 (in round numbers) 109 times more distant than the moon. More- 
 over, she is so immersed in the sun's rays that it is only when 
 her path lies across his disc that the requisite facilities for 
 measurement are afforded. These " partial eclipses of the sun 
 by Venus " (as Encke termed them) are coupled together in 
 pairs, 2 of which the components are separated by eight years, 
 recurring at intervals alternately of 105^ and I2ij years. 
 Thus, the first calculated transit took place in December 1631, 
 and its companion (observed by Horrocks) in the same month 
 (N.S.) 1639. Then, after the lapse of 12 ij years, came the 
 June couple of 1761 and 1769; and again, after 105^, the two 
 recently observed, December 8, 1874, and December 6, 1882. 
 Throughout the twentieth century there will be no transit of 
 Venus ; but the astronomers of the twenty-first will only have 
 to wait four years for the first of a June pair. The rarity of 
 
 1 J. D. Cassini, Hist. Abregee tie la Parallaxe du Soleil, p. 122, 1772. - The 
 present period of coupled eccentric transits will, in the course of ages, be 
 succeeded by a period of single, nearly central transits. The alignments by 
 which transits are produced, of the earth, Venus, and the sun, close to the 
 place of intersection of the two planetary orbits, now occur, the first a little 
 in front of, the second, after eight years less two and a half days, a little 
 behind the node. But when the first of these two meetings takes place very 
 near the node, giving a nearly central transit, the second falls too far from it, 
 and the planet escapes projection on the sun. The reason of the liability to 
 an eight-yearly recurrence is that eight revolutions of the earth are accom- 
 plished in only a very little more time than thirteen revolutions of Venus. 
 
CHAP. vi. THE SUN'S DISTANCE. 283 
 
 these events is due to the fact that the orbits of the earth and 
 Venus do not lie in the same plane. If they did, there would 
 be a transit each time that our twin-planet overtakes us in her 
 more rapid circling that is, on an average, every 584 days. 
 As things are actually arranged, she passes above or below the 
 sun, except when she happens to be very near the line of inter- 
 section of the two tracks. 
 
 Such an occurrence as a transit of Venus seems, at first sight, 
 full of promise for solving the problem of the sun's distance. 
 For nothing would appear easier than to determine exactly either 
 the duration of the passage of a small, dark orb across a large 
 brilliant disc, or the instant of its entry upon or exit from it. 
 And the differences in these times (which, owing to the compa- 
 rative nearness of Venus, are quite considerable), as observed from 
 remote parts of the earth, can be translated into differences of 
 space that is, into apparent or parallactic displacements, 
 whereby the distance of Venus becomes known, and thence, by 
 a simple sum in proportion, the distance of the sun. But in that 
 word "exactly" what snares and pitfalls lie hid! It is so easy 
 to think and to say; so indefinitely hard to realize. The 
 astronomers of the eighteenth century were full of hope and 
 zeal. They confidently expected to attain, through the double 
 opportunity offered them, to something like a permanent settle- 
 ment of the statistics of our system. They were grievously 
 disappointed. The uncertainty as to the sun's distance, which 
 they had counted upon reducing to a few hundred thousand 
 miles, remained at many millions. 
 
 In 1822, however, Encke, then director of the Seeberg Obser- 
 vatory near Gotha, undertook to bring order out of the confusion 
 of discordant and discordantly interpreted observations. His 
 combined result for both transits (1761 and 1769) was published 
 in I824, 1 and met universal acquiescence. The parallax of the 
 sun thereby established was 8. 5 776", corresponding to a mean 
 distance 2 of 95^ million miles. Yet this abolition of doubt was 
 
 1 Die Entfernung der Sonne : Fortsetzung, p. 108. Encke slightly corrected 
 his result of 1824 in Berlin Abh., 1835, p. 295. ~ Owing to the ellipticity of 
 its orbit, the earth is nearer to the sun in January than in June by 3,100,000 
 
284 HISTORY OF ASTRONOMY. PART n. 
 
 far from being so satisfactory as it seemed. Serenity on the 
 point lasted exactly thirty years. It was disturbed in 1854 by 
 Hansen's announcement l that the observed motions of the moon 
 could be drawn into accord ^ith theory only on the terms of 
 bringing the sun considerably nearer to us than he was supposed 
 to be. 
 
 Dr. Matthew Stewart, professor of mathematics in the 
 University of Edinburgh, bad made a futile attempt in 1763 to 
 deduce the sun's distance from his disturbing power over our 
 satellite. 2 Tobias Mayer of Gottingen, however, whose short 
 career was so fruitful of suggestions, struck out the right way to 
 the same end ; and Laplace, in the seventh book of the Mtcanigue 
 Celeste, 8 gave a solar parallax derived from the lunar "parallactic 
 inequality " substantially identical with that issuing from Encke's 
 subsequent discussion of the eighteenth century transits. Thus 
 two wholly independent methods the trigonometrical, or method 
 by survey, and the gravitational, or method by perturbation 
 seemed to corroborate each the upshot of the use of the other 
 until the nineteenth century was well past its meridian. It is 
 singular how often errors conspire to lead conviction astray. 
 
 Hansen's note of alarm in 1854 was echoed by Leverrier in 
 1858.* He found that an apparent monthly oscillation of the 
 sun which reflects a real monthly movement of the earth round 
 its common centre of gravity with the moon, and which depends 
 for its amount solely on the mass of the moon and the distance 
 of the sun, required a diminution in the admitted value of that 
 distance by fully four million miles. Three years later he pointed 
 out that certain perplexing discrepancies between the observed 
 and computed places both of Venus and Mars, would vanish on 
 the adoption of a similar measure. 5 Moreover, a favourable 
 
 miles. The quantity to be determined, or " mean distance," is that lying 
 midway between these extremes is, in other words, half the major axis of 
 the ellipse in which the earth travels. 1 Month. Not., vol. xv., p. 9. - The 
 Distance of the Sun from the Earth determined by the Theory of Gravity, Edin- 
 burgh, 1763. 3 Opera, t. iii., p. 326. 4 Comptes Rendus, t. xlvi., p. 882. 
 The parallax 8.95" derived by Leverrier from the above-described inequality 
 in the earth's motion, was corrected by Stone to 8.91. Month. Not., vol. 
 xxviii., p. 25. 5 Month. Not., vol. xxxv., p. 156. 
 
CHAP. vi. THE SUN'S DISTANCE. 285 
 
 opposition of Mars gave the opportunity in 1862 for fresh 
 observations, which, separately worked out by Stone and 
 Winnecke, agreed with all the newer investigations in fixing 
 the great unit at slightly over 91 million miles. In Newcomb's 
 hands they gave 92 J million. 1 The accumulating evidence in 
 favour of a large reduction in the sun's distance was just then 
 reinforced by an auxiliary result of a totally different and 
 unexpected kind. 
 
 The discovery that light does not travel instantaneously from 
 point to point, but takes some short time in transmission, was 
 made by Olaus Komer in 1675, through observing that the 
 eclipses of Jupiter's satellites invariably occurred later, by a 
 considerable interval, when the earth was on the far side, than 
 when it was on the near side of its orbit. Half this interval, or 
 the time spent by a luminous vibration in crossing the " mean 
 radius " of the earth's orbit, is called the " light-equation " ; 
 and the determination of its precise value has claimed the 
 minute care distinctive of modern astronomy. Delambre in 
 1792 made it 493 seconds. Glasenapp, a Eussian astronomer, 
 raised the estimate in 1874 to 501, Prof essor Harkness adopting a 
 safe medium value of 498 seconds. Hence, if we had any in- 
 dependent means of ascertaining how fast light travels, we could 
 tell at once how far off the sun is. 
 
 There is yet another way by which knowledge of the swift- 
 ness of light would lead us straight to the goal. The' heavenly 
 bodies are perceived, when carefully watched and measured, to 
 be pushed forward out of their true places, in the direction of 
 the earth's motion, by a very minute quantity. This effect 
 (already adverted to) has been known since Bradley's time as 
 " aberration." It arises from a combination of the two move- 
 ments of the earth round the sun and of the light-waves through 
 the ether. If the earth stood still, or if light spent no time on 
 the road from the stars, such an effect would not exist. Its 
 amount represents the proportion between the velocities with 
 which the earth and the light-rays pursue their respective 
 journeys. This proportion is, roughly, one to ten thousand. 
 1 Wash. Ob., 1865, App. ii., p. 28. 
 
286 HISTORY OF ASTRONOMY. PART n. 
 
 So that here again, if we knew the rate per second of luminous 
 transmission, we should also know the rate per second of the 
 earth's movement, consequently the size of its orbit and the 
 distance of the sun. - ^ 
 
 But, until lately, instead of finding the distance of the sun 
 from the velocity of light, there has been no means of ascer- 
 taining the velocity of light except through the imperfect 
 knowledge possessed as to the distance of the sun. The first 
 successful terrestrial experiments on the point date from 
 1 849 ; and it is certainly no slight triumph of human ingenuity 
 to have taken rigorous account of the delay of a sunbeam in 
 flashing from one mirror to another. Fizeau led the way, 1 
 and he was succeeded, after a few months, by Leon Foucault,2 
 who, in 1862, had so far perfected Wheatstone's method of 
 revolving mirrors, as to be able to announce with authority that 
 light travelled slower, and that the sun was in consequence nearer, 
 than had been supposed. 3 Thus a third line of separate research 
 was found to converge to the same point with the two others. 
 
 Such a conspiracy of proof was not to be resisted, and at 
 the anniversary meeting of the Royal Astronomical Society in 
 February 1864, the correction of the solar distance took the 
 foremost place in the annals of the year. Lest, however, a 
 sudden bound of four million miles nearer to the centre of our 
 system should shake public faith in astronomical accuracy, it 
 was explained that the change in the solar parallax correspond- 
 ing to that huge leap, amounted to no more than the breadth 
 of a human hair 125 feet from the eye ! 4 The Nautical Almanac 
 gave from 1870 the altered value of 8.95", for which Newcomb's 
 result of 8.85", admitted since 1869 into the Berlin Ephemeris, 
 was substituted some ten years later. In astronomical literature 
 the change was initiated by Sir Edmund Beckett in the first 
 edition (1865) of his Astronomy without Mathematics. 
 
 1 Comptes Kendus, t. xxix., p. 90. 2 Ibid., t. xxx., p. 551. 3 Ibid., t. lv., 
 p. 501. The previously admitted velocity was 308 million metres per second ; 
 Foucault reduced it to 298 million. Combined with Struve's " constant of 
 aberration " this gave 8. 86" for the solar parallax, which exactly agreed with 
 Cornu's result from a repetition of Fizeau's experiments in 1872. Comptes 
 Rendus, t. Ixxvi., p. 338. 4 Month. Not., vol. xxiv., p. 103. 
 
CHAP. vi. THE SUN'S DISTANCE. 287 
 
 If any doubt remained as to the misleading character of 
 Encke's deduction, so long implicitly trusted in, it was removed 
 by Powalky's and Stone's re-discussions, in 1864 an ^ 1868 
 respectively, of the transit observations of 1769. Using im- 
 proved determinations of the longitude of the various stations 
 (very imperfectly known to their able predecessor), and a selec- 
 tive judgment in dealing with their materials, which, however 
 indispensable, did not escape adverse criticism, they brought out 
 results confirmatory of the no longer disputed necessity for 
 largely increasing the solar parallax, and proportionately dimin- 
 ishing the solar distance. Once more in 1890, and this time 
 with better success, the eighteenth century transits were investi- 
 gated by Professor Newcomb. 1 Turning to account the experi- 
 ence gained in the interim regarding the optical phenomena 
 accompanying such events, he elicited from the mass of some- 
 what discordant observations at his command, a parallax (8.79") 
 remarkably close to agreement with the value towards which 
 several lines of recent research converge. 
 
 Conclusions on the subject, however, were still regarded as* 
 purely provisional. A transit of Venus was fast approaching, 
 and to its arbitrament, as to that of a court of final appeal, the 
 pending question was to be referred. It is true that the verdict 
 in the same case by the same tribunal a century earlier had 
 proved of so indistinct a character as to form only a starting- 
 point for fresh litigation ; but that century had not passed in 
 vain, and it was confidently anticipated that observational diffi- 
 culties, then equally unexpected and insuperable, would yield to 
 the elaborate care and skill of forewarned modern preparation. 
 
 The conditions of the transit of December 8, 1874, were 
 sketched out by the then Astronomer-Royal (Sir George Airy) 
 in i857, 2 and formed the subject of eager discussion in this and 
 other countries down to the very eve of the occurrence. In 
 these Mr. Proctor took a leading part, supplying official omis- 
 sions, and working out, with geometrical accuracy, the details 
 of the relations between the different parts of the earth and 
 
 1 Astr. Papers of the American Epltemeris, vol. ii., p. 263. - Month. Not. r 
 vol. xvii., p. 208. 
 
288 HISTORY OF ASTRONOMY. PART n. 
 
 Venus's shadow-cone ; and it was due to his urgent representa- 
 tions that provision was made for the employment of the method 
 identified with the name, of Halley, 1 which had been too hastily 
 assumed inapplicable, to the first of each transit-pair. It depends 
 upon the difference in the length of time taken by the planet to 
 cross the sun's disc, as seen from various points of the terrestrial 
 surface, and requires, accordingly, the visibility of both entrance 
 .and exit at the same station. Since these were, in 1874, sepa- 
 rated by about three and a half hours, and the interval is often 
 much longer, the choice of posts for the successful use of the 
 'method of durations " is a matter of some difficulty. 
 
 The system described by Delisle in 1760, on the other hand, 
 involves merely noting the instant of ingress or egress (accord- 
 ing to situation) from opposite extremities of a terrestrial dia- 
 meter ; the disparity in time giving a measure of the planet's 
 apparent displacement, hence of its actual rate of travel in miles 
 per minute, from which its distances severally from earth and 
 sun are immediately deducible. Its chief attendant difficulty is 
 the necessity for accurately fixing the longitudes of the points 
 of observation. This, however, was much more sensibly felt a 
 century ago than it is now, and the improved facility and cer- 
 tainty of modern determinations have tended to ive the Delislean 
 plan a decided superiority over its rival. 
 
 These two traditional methods were supplemented in 1874 by 
 the camera and the heliometer. From photography, above all, 
 much was expected. Observations made by its means would 
 have the advantages of impartiality, multitude, and permanence. 
 Peculiarities of vision and bias of judgment would be eliminated ; 
 the slow progress of the phenomenon would permit an indefinite 
 number of pictures to be taken, their epochs fixed to a fraction 
 of a second ; while subsequent leisurely comparison and measure- 
 ment could hardly fail, it was thought, to educe approximate 
 truth from the mass of accumulated evidence. The use of the 
 heliometer (much relied on by German observers) was so far 
 similar to that of the camera that the object aimed at by both 
 was the determination of the relative positions of the centres of 
 
 1 Because closely similar to that proposed by him in Phil. Trans, for 1716. 
 
CHAP. vi. THE SUN'S DISTANCE. 289 
 
 the sun and Venus viewed, at the same absolute instant, from 
 opposite sides of the globe. So that the two older methods seek 
 to ascertain the exact times of meeting between the solar and 
 planetary limbs ; while the two modern methods work by 
 measurement of the position of the dark body already thrown 
 into complete relief by its shining background. The former 
 are '-methods by contact," the latter "methods by projection." 
 
 Every country which had a reputation to keep or to gain for 
 scientific zeal was forward to co-operate in the great cosmo- 
 politan enterprise of the transit. France and Germany each 
 sent out six expeditions ; twenty-six stations were in Russian, 
 twelve in English, eight in American, three in Italian, one 
 (equipped with especial care) in Dutch occupation. In all, at a 
 cost of nearly a quarter of a million, some fourscore distinct 
 posts of observation were provided ; among them such inhos- 
 pitable and all but inaccessible rocks in the bleak Southern 
 Ocean, as St. Paul's and Campbell Islands, swept by hurricanes, 
 and fitted only for the habitation of seabirds, where the daring 
 votaries of science, in the wise prevision of a long leaguer by 
 the elements, were supplied with stores for many months, or even 
 a whole year. Siberia and the Sandwich Islands were thickly 
 beset with obser ers ; parties of three nationalities encamped 
 within the mists of Kerguelen Island, expressively termed the 
 " Land of Desolation," in the sanguine, though not wholly 
 frustrated hope of a glimpse of the sun at the right moment. 
 M. Janssen narrowly escaped destruction from a typhoon in the 
 China seas on his way to Nagasaki ; Lord Lindsay (now Earl of 
 Crawford and Balcarres) equipped, at his private expense, an 
 expedition to the Mauritius, which was in itself an epitome of 
 modern resource and ingenuity. 
 
 During several years the practical methods best suited to 
 ensure success for the impending enterprise, formed a subject of 
 European debate. Official commissions were appointed to receive 
 and decide upon evidence; and experiments were in progress 
 for the purpose of defining the actual circumstances of the 
 contacts, the accurate determination of which constituted the 
 only tried, though by no means an assuredly safe road to the 
 
 19 
 
290 HISTORY OF ASTRONOMY. PART n. 
 
 end in view. In England, America, France, and Germany, 
 artificial transits were mounted, and the members of the various 
 expeditions were carefully trained to unanimity in estimating 
 the phases of junction and separation between a moving dark 
 circular body and a broad illuminated disc. In the last century, 
 a formidable and prevalent phenomenon had swamped all pre- 
 tensions to rigid accuracy. This was an ejfect analogous to 
 " Baily's Beads," which acquired notoriety as the " Black Drop '' 
 or "Black Ligament." It may be described as substituting 
 adhesion for contact, the limbs of the sun and planet, instead of 
 meeting and parting with the desirable clean definiteness. 
 clinging together as if made of some glutinous material, and 
 prolonging their connection by means of a dark band or dark 
 threads stretched between them. Some astronomers ascribe 
 this baffling appearance entirely to instrumental imperfections ; 
 others to atmospheric agitation : others again to the optical 
 encroachment of light upon darkness known as " irradiation." 
 It is probable that all these causes conspire, in various measure, 
 to produce it ; but it is certain that by suitable precautions, 
 combined with skill in the observer, and a reasonably tranquil 
 air, its conspicuous appearance may, in most cases, be obviated. 
 
 The organisation of the British forces reflected the utmost 
 credit on the energy and ability of Lieutenant-Colonel Tupman, 
 of the Royal Marine Artillery, who was responsible for the 
 whole. No useful measure was neglected. Each observer went 
 out ticketed with his " personal equation," his senses drilled into 
 a species of martial discipline, his powers absorbed, so far as 
 possible, in the action of a cosmopolitan observing machine. 
 Instrumental uniformity and uniformity of method were attain- 
 able, and were attained ; but diversity of judgment unhappily 
 survived the best-directed efforts for its extirpation. 
 
 The eventful day had no sooner passed than telegrams began 
 to pour in, announcing an outcome of considerable, though not 
 unqualified success. The weather had proved generally favour- 
 able ; all the manifold arrangements had (save for some casual 
 mishaps) worked well ; contacts had been plentifully observed ; 
 photographs in lavish abundance had been secured ; a store of 
 
CHAP. vi. THE SUN'S DISTANCE. 291 
 
 materials, in short, had been laid up, of which it would take 
 years to work out the full results by calculation. Gradually, 
 however, it came to be known that the hope of a definitive issue 
 must be abandoned. Unanimity was found to be as remote as 
 ever. The dreaded "black ligament" gave, indeed, less trouble 
 than was expected ; but another appearance supervened which 
 took most observers by surprise. This was the illumination due 
 to the atmosphere of Venus. Astronomers, it is true, were not 
 ignorant that the planet had, on previous occasions, been seen 
 girdled with a lucid ring ; but its power to mar observations by 
 the distorting effect of refraction had scarcely been reckoned 
 with. It proved, however, to be very great. Such was the 
 difficulty of determining the critical instant of internal contact, 
 that (in Colonel Tupman's words) " observers side by side, with 
 adequate optical means, differed as much as twenty or thirty 
 seconds in the times they recorded for phenomena which they 
 have described in almost identical language." l 
 
 Such uncertainties in the data admitted of a corresponding 
 variety in the results. From the British observations of ingress 
 and egress Sir George Airy 2 derived, in 1 877, a solar parallax 
 of 8. 76" (corrected to 8.754"), indicating a mean distance of 
 93,375,000 miles. Mr. Stone obtained a value of ninety-two 
 millions (parallax 8.88"), and held any parallax less than 8.84" 
 or more than 8.93" to be " absolutely negatived " by the docu- 
 ments available. 3 Yet, from the same, Colonel Tupman deduced 
 8.8i", 4 implying a distance 700,000 miles greater than Stone had 
 obtained. The French observations of contacts gave (the best 
 being selected) a parallax of about 8.88"; French micrometric 
 measures the obviously exaggerated one of 9-O5". 5 
 
 Photography, as practised by most of the European parties, was 
 a total failure. Utterly discrepant values of the microscopic 
 displacements designed to give the scale of the solar system, 
 issued from attempts to measure even the most promising 
 pictures. " You might as well try to measure the zodiacal 
 light," it was remarked to Sir George Airy. Those taken on 
 
 montn. Not., vol. xxxviii., p. 447. 2 Ibid., p. n. 3 Ibid., p. 294. 
 4 Ibid., p. 334. 5 Comptes Bendus, t. xcii., p. 812. 
 
2Q2 HISTORY OF ASTRONOMY. PART n. 
 
 the American plan (adopted by Lord Lindsay), of using tele- 
 scopes of so great focal length as to afford, without further 
 enlargement, an image of the requisite size, gave notably better 
 results. From an elaborate comparison of these (some dating 
 from Vladivostock, Nagasaki, and Pekin, others from Kerguelen 
 and Chatham Islands), Professor D. P. Todd, director of the 
 Amherst College Observatory, deduced a solar ^distance of about 
 ninety-two million miles (parallax 8.883 // O.O34 // ), 1 a value, as 
 Mr. Stone has pointed out, favoured by a considerable accumula- 
 tion of independent evidence. 
 
 On the whole, estimates of the great spatial unit cannot be 
 said to have gained any security from the combined effort of 
 1874. A few months before the transit, Mr. Proctor considered 
 that the uncertainty then amounted to 1,448,000 miles; 2 five 
 years after the transit, Professor Harkness judged it to be still 
 1,575,950 miles ; 3 yet it had been hoped that it would have been 
 brought down to 100,000. As regards the end for which it had 
 been undertaken, the grand campaign had come to nothing. 
 Nevertheless, no sign of discouragement was apparent. There 
 was a change of view, but no relaxation of purpose. The 
 problem, it was seen, could be solved by no single heroic effort, 
 but by the patient approximation of gradual improvements. 
 Astronomers, accordingly, looked round for fresh means, or more 
 refined expedients for applying those already known. A new 
 phase of exertion was entered upon. 
 
 On September 5, 1877, Mars came into opposition close to the 
 part of his orbit which lies nearest to that of the earth, and Dr. Gill 
 (since 1 879 Her Majesty's astronomer at the Cape of Good Hope) 
 took advantage of the circumstance to appeal once more to him 
 for a decision on the qucestio vexata of the sun's distance. He 
 chose, as the scene of his labours, the Island of Ascension, and 
 for their plan a method recommended by Airy in 1857,* but 
 never before fairly tried. This is known as the "diurnal method 
 of parallaxes." Its principle consists in substituting successive 
 morning and evening observations from the same spot, for 
 simultaneous observations from remote spots, the rotation of 
 
 1 Observatory, No. 51, p. 205. - Transits of Venus, p. 89 (ist ed.): 3 Am. 
 Jour, of Sc., vol. xx., p. 393. 4 Month. Not., vol. xvii., p. 219. 
 
CHAP. vi. THE SUN'S DISTANCE. 293 
 
 the earth supplying the necessary difference in the points of 
 view. . Its great advantage is that of unity in performance. 
 A single mind, looking through the same pair of eyes, reinforced 
 with the same optical appliances, is employed throughout, and 
 the errors inseparable from the combination of data collected 
 under different conditions are avoided. There are many cases 
 in which one man can do the work of two better than two men 
 can do the work of one. The result of Dr. Gill's skilful deter- 
 minations (made with Lord Lindsay's heliometer) was a solar 
 parallax of 8.78", corresponding to a distance of 93,080,000 
 miles. 1 The bestowal of the Koyal Astronomical Society's gold 
 medal stamped the merit of this distinguished service. 
 
 But there are other subjects for this kind of inquiry besides 
 Mars and Venus. Professor Galle of Breslau suggested in 1872 2 
 that some of the minor planets might be got to repay astronomers 
 for much disinterested toil spent in unravelling their motions, by 
 lending aid to their efforts towards a correct celestial survey. 
 Ten or twelve come near enough, and are bright enough for the 
 purpose ; and, in fact, the absence of sensible magnitude is one 
 of their chief recommendations, since a point of light offers far 
 greater facilities for exact measurement than a disc. The first 
 attempt to work this new vein was made at the opposition of 
 Phocasa in 1872 ; and from observations of Flora in the following 
 year at twelve observatories in the northern and southern hemi- 
 spheres, Galle deduced a solar parallax of 8.87". 3 At the 
 Mauritius in 1874, Lord Lindsay and Dr. Gill applied the 
 "diurnal method" to Juno, then conveniently situated for the 
 purpose ; and the continued use of similar occasions affords an 
 unexceptionable means for improving knowledge of the sun's 
 distance. . They frequently recur; they need no elaborate 
 preparation ; a single astronomer armed with a heliometer can 
 do all the requisite work. Dr. Gill, however, organised a more 
 complex plan of operations upon Iris in 1888, and upon Victoria 
 .and Sappho in 1889. A novel method was adopted. Its object 
 was to secure simultaneous observations made from opposite sides 
 
 1 Mem. Boy. Astr. Soc., vol. xlvi., p. 163. ' 2 Astr. Nach., No. 1897. 
 3 Hilfiker, Bern Mittkeilunyen, 1878, p. 109. 
 
294 HISTORY OF ASTRONOMY. PAK*T n. 
 
 of the globe just when the planet lay in the plane passing through 
 the centre of the earth and the two observers, the same pair of 
 reference-stars being used on each occasion. The displacements 
 caused by parallax were thus in.^a sense doubled, since the star 
 to which the planet seemed approximated in the northern hemi- 
 sphere, showed as if slightly removed from .it in the southern, 
 and vice versd. And as the planet pursued its Bourse, during the 
 weeks that the favourable period lasted, fresh star-couples came 
 into play. In these determinations, only heliometers were 
 employed. Dr. Elkin, of Yale College, co-operated throughout, 
 and the heliometers of Dresden, Gottingen, Bamberg, and 
 Leipzig, shared in the work, while Dr. Auwers of Berlin was Dr. 
 Gill's personal coadjutor at the Cape for the opposition of 
 Victoria. The photographic registration of planet and star-places 
 was not neglected ; and thus the question of the sun's distance 
 is at last likely to be definitively settled. Already a result has 
 been deduced by the Cape Astronomer from the opposition of 
 Victoria which amply repays the time and pains expended in 
 procuring it. They were of formidable amount. Meridian 
 observations of the comparison-stars occupied twenty-one ob- 
 servatories during four months ; the direct work of triangulation 
 kept four heliometers in almost exclusive use for the best part 
 of a year ; and the data thus collected were rendered communi- 
 cative only by the computative toil of three years, 011 the part 
 of Dr. Gill himself and a too scanty staff of assistants. The 
 upshot has been to assign a parallax to the sun of 8.809', corre- 
 sponding (in round numbers) to a distance of 92,700,000 miles ; 
 and the extremely small appended probable error of 0.007" 
 shows that this dictum can be trusted within narrow limits. 
 Moreover, a nearly identical result has been derived from the 
 opposition of Iris. 
 
 The second of the nineteenth-century pair of Venus-transits 
 was looked forward to with much-abated enthusiasm. Russia 
 refused her active co-operation in observing it, on the ground 
 that oppositions of the minor planets were trigonometrically more 
 useful, and financially far less costly; and her example was 
 followed by Austria, while Italian astronomers limited their 
 
CHAP. vi. THE SUN'S DISTANCE. 295 
 
 sphere of action to their own peninsula. Nevertheless, it was 
 generally held that a phenomenon which the world could not 
 again witness until it was four generations older should, at the 
 price of any effort, not be allowed to pass in neglect. 
 
 An International Conference, accordingly, met at Paris in 
 1 88 1 with a view to concerting a plan of operations. America, 
 however, preferring independent action, sent no representative ; 
 and the European breakdown of photography in recording 
 transit-phases was admitted by its official abandonment. It was 
 decided to give Delisle's method another trial ; and the ambi- 
 guities attending and marring its use were sought to be obviated 
 by careful regulations for ensuring agreement in the estimation 
 of the critical moments of ingress and egress. 1 But, in fact (as 
 M. Puiseux had shown 2 ), contacts between the limbs of the sun and 
 planet, so far from possessing the geometrical simplicity long 
 attributed to them, are really made up of a prolonged succession 
 of various and varying phases, impossible either to predict or 
 identify with anything like rigid exactitude. Sir Eobert Ball 
 compared the task of determining the precise instant of their 
 meeting or parting, to that of telling the hour with accuracy on 
 a watch without a minute-hand ; and the comparison is admittedly 
 inadequate. For not only is the apparent movement of Venus 
 across the sun extremely slow, being but the excess of her real 
 motion over that of the earth ; but three distinct atmospheres 
 the solar, terrestrial, and cytherean combine to deform outlines^ 
 and mask the geometrical relations which it is desired to connect 
 with a strict count of time. 
 
 The result was very much what had been expected. The 
 arrangements were excellent, and were only in a few cases 
 disconcerted by bad weather. The British parties, under the 
 experienced guidance of Mr. Stone, the Badcliffe observer, 
 took up positions scattered (not at random) over the globe 
 from Queensland to Bermuda, and accumulated an ample 
 supply of skilful observations ; the Americans gathered in a 
 whole library of photographs, among them a fine series taken 
 at the new Lick Observatory on Mount Hamilton ; the Germans 
 
 1 Comptes Bendus, t. xciii., p. 569. - Ibid., t. xcii., p. 841. 
 
296 HISTORY OF ASTRONOMY. PART n. 
 
 and Belgians trusted to the heliometer ; the French used the 
 camera as an adjunct to the method of contacts. Yet little or 
 no approach was made to solving the problem. The range 
 of doubt as to the sun's distance remained as wide as before. 
 The value published in I884, 1 by M. Houzeau, late director 
 of the Brussels Observatory, forcibly illustrates this un- 
 welcome conclusion. ' From 606 measures of Venus on the 
 sun, taken with a new kind of heliometer at Santiago in 
 Chili, he derived a solar parallax of 8.907", and a distance 
 of 91,727,000 miles. But the "probable error" of this deter- 
 mination amounted to 0.084" either way ; that is, it was subject 
 to a "more or less" of 900,000 miles, or to a total uncertainty 
 of 1,800,000. The probable error of the English result, 
 published in 1887, was less formidable, 2 yet the details of the 
 discussion showed that no great confidence could be placed in 
 it. The sun's distance came out 92,560,000 miles; while 
 92,360,000 was given by Professor Harkness's investigation of 
 1475 American photographs. 3 Finally, Dr. Auwers derived from 
 the German heliometric measures the unsatisfactorily small 
 value of 92,000,000 miles. 4 The transit of 1882 had not then 
 brought about the desired unanimity. 
 
 The state and progress of knowledge on this important 
 subject were summed up by Faye and Harkness in i88i. 5 
 The methods employed in its investigation fall (as we have 
 seen) into three separate classes the trigonometrical, the 
 gravitational, and the " phototachymetrical " an ungainly ad- 
 jective used to describe the method by the velocity of light. 
 Each has its special difficulties and sources of error; each has 
 counter-balancing advantages. The only distinct and trust- 
 worthy results, so far, from celestial surveys, have been furnished 
 by Dr. Gill's observations of Mars in 1877, and of Iris and 
 Victoria in 1888-9. But the method by lunar and planetary 
 disturbances is unlike all the others in having time on its side. 
 It is this which Leverrier declared with emphasis must in- 
 
 1 Bull, de I'Acad., t. vi., p. 842. ~ Month. Not., vol. xlviii., p. 201. 
 
 s Astr. Jour., No. 182. * Astr. Nach., No. 3066. 5 Contptes Bendus, 
 
 t. xcii., p. 375 ; Am. Jour, of Sc., vol. xxii., p. 375. 
 
CHAP. vi. THE SUN'S DISTANCE. 297 
 
 evitably prevail, because its accuracy is continually growing. 1 
 The scarcely perceptible errors which still impede its application 
 are of such a nature as to accumulate year by year ; eventually, 
 then, they will challenge, and must receive, a more and more 
 perfect correction. 
 
 The light-velocity method has, however, some immediate 
 advantages. By a beautiful series of experiments on Foucault's 
 principle, Master A. A. Michelson, of the United States Navy, 
 fixed in 1879 the rate of luminous transmission at 299,930 
 (corrected later to 299,910) kilometres a second. 2 This de- 
 termination was held by Professor Todd to be entitled to four 
 times as much confidence as any previous one; and if the 
 solar parallax of 8. 7 5 8" deduced from it by Professor Harkness 
 errs somewhat by defect, it is doubtless because Glasenapp's 
 "light-equation," with which it was combined, errs slightly 
 by excess. But all earlier efforts of the kind were thrown into 
 the shade by Professor Newcomb's arduous operations at 
 Washington in i88o-i882. 3 The scale upon which they were 
 conducted was in itself impressive. Foucault's entire apparatus 
 in 1862 had been enclosed in a single room; Newcomb's 
 revolving and fixed mirrors, between which the rays of light 
 were to run their timed course, were set up on opposite shores 
 of the Potomac, at a distance of nearly four kilometres. This 
 advantage was turned to the utmost account by ingenuity and 
 skill in contrivance and execution ; and the deduced velocity of 
 299,860 kilometres = 186,328 miles a second, had an estimated 
 error (30 kilometres) only one-tenth that ascribed by Cornu to 
 his own result in 1874. 
 
 Just as these experiments were concluded in 1882, M. Magnus 
 Nyren, of St. Petersburg, published an elaborate investigation 
 of the small annular displacements of the stars due to the 
 successive transmission of light, involving an increase of Struve's 
 "constant of aberration " from 20.44$" t 20.492". And from 
 the new value combined with Newcomb's light-velocity, was 
 
 1 Month. Xot., vol. xxxv., p. 401. - Am. Jour, of Sc., vol. xviii., p. 393. 
 s Mature, vol. xxxiv., p. 170 ; Astron. Papers of the American JZpJiemeris, vol. ii., 
 p. 113. 
 
298 HISTORY OF ASTRONOMY. PART n. 
 
 derived a valuable approximation to the sun's distance, concluded 
 at 92,905,021 miles (parallax = 8. 794")- Yet it is not quite 
 certain that Nyren's correction was an improvement. A 
 differential method o determining the amount of aberration, 
 struck out by M. Loewy of Paris, l avoids most of the snares and 
 pitfalls of the absolute method previously in vogue. There are, 
 indeed, drawbacks to its prerogatives ; nevertheless, the upshot 
 of its application in 1891 deserves notice. It went to show 
 that Struve's constant might better be retained than altered, 
 Loewy's of 20.447" varying from it only to an insignificant 
 extent. Professor Hall had, moreover, deduced nearly the same 
 value (20.4 5 4") from the Washington observations since 1862, of 
 a Lyrse (Vega) ; whence, in conjunction with Newcomb's rate 
 of light transmission, he arrived at a solar parallax of 8.8i". 2 
 
 Now it is noteworthy that this precise value was derived by 
 Professor Harkness in 1891 3 through the combination, accord- 
 ing to the method of least squares that is, by the mathematical 
 rules of probability of all the various quantities upon which the 
 sun's parallax depends. His result thus sums up and harmonises 
 the whole of the multifarious evidence bearing upon the point ; 
 and confidence in it is strengthened by its perfect agreement 
 no less with the most authentic verdict yet pronounced by the 
 swift traveller, light, than with the issue of Dr. Gill's recent 
 trigonometrical operations. We may then, at least provisionally, 
 accept 92, 700,000 miles as the length of the earth's mean orbital 
 radius. Nor do we hazard much in fixing 1 50,000 miles as the 
 outside limit of its future correction. 
 
 1 Comptes Rendus, i. cxii., p. 549. - Astr. Jour., Nos. 169, 170. 3 Th> 
 Solar Parallax and its Related Constants, Washington, 1891. 
 
CHAPTER VII. 
 
 PLANETS AND SATELLITES. 
 
 JOHANN HIERONYMUS ScHROTER was the Herschel of Germany, 
 He did not, it is true, possess the more brilliant gifts of his 
 rival. HerscheFs piercing discernment, comprehensive intelli- 
 gence, and inventive splendour were wanting to him. He was, 
 nevertheless, the founder of descriptive astronomy in Germany, 
 as Herschel was in England. 
 
 Born at Erfurt in 1745, he prosecuted legal studies at Got- 
 tingen, and there imbibed from Kastner a life-long devotion to 
 science. From the law, however, he got the means of living, 
 and, what was to the full as precious to him, the means of 
 observing. Entering the sphere of Hanoverian officialism in 
 1788, he settled a few years later at Lilienthal, near Bremen, as 
 <; 0beramtmann," or chief magistrate. Here he built a small 
 observatory, enriched in 1785 with a seven-foot reflector by 
 Herschel, then one of the most powerful instruments to be 
 found anywhere out of England. It was soon surpassed, 
 through his exertions, by the first-fruits of native industry in 
 that branch. Schrader of Kiel transferred his workshops to 
 Lilienthal in 1792, and constructed there, under the super- 
 intendence and at the cost of the astronomical Oberamtmann, 
 a thirteen-foot reflector, declared by Lalande to be the finest 
 telescope in existence, and one twenty-seven feet in focal length, 
 probably as inferior to its predecessor in real efficiency as it was 
 superior in size. 
 
 Thus, with instruments of gradually increasing power, 
 Schroter studied during thirty-four years the topography of the 
 
300 HISTORY OF ASTRONOMY. PART n. 
 
 moon and planets. The field was then almost untrodden ; he 
 had but few and casual predecessors, and has since had no 
 equal in the sustained and concentrated patience of his hourly 
 watchings. Both their prolixity* and their enthusiasm are faith- 
 fully reflected in his various treatises. Yet the one may be 
 pardoned for the sake of the other, especially when it is re- 
 membered that he struck out a substantially new line, and that 
 one of the main lines of future advance. Moreover, his infec- 
 tious zeal communicated itself ; he set the example of observing 
 when there was scarcely an observer in Germany ; and under 
 his roof Harding and Bessel received their training as practical 
 astronomers. 
 
 But he was reserved to see evil days. Early in 1813 the 
 French under Vandamme occupied Bremen. On the night of 
 April 20, the Vale of Lilies was, by their wanton destructiveness, 
 laid waste with fire ; the Government offices were destroyed, and 
 with them the chief part of Schroter's property, including the 
 whole stock of his books and writings. There was worse behind. 
 A few days later, his observatory, which had escaped the con- 
 flagration, was broken into, pillaged, and ruined. His life was 
 wrecked with it. He survived the catastrophe three years with- 
 out the means to repair, or the power to forget it, and gradually 
 sank from disappointment into decay, terminated by death, 
 August 29, 1816. He had, indeed, done all the work he was 
 capable of ; and though not of the first quality, it was far from 
 contemptible. He laid the foundation of the comparative study 
 of the moon's surface, and the descriptive particulars of the 
 planets laboriously collected by him constituted a store of more 
 or less reliable information hardly added to during the ensuing 
 half century. They rested, it is true, under some shadow of 
 doubt ; but the most recent observations have tended on several 
 points to rehabilitate the discredited authority of the Lilienthal 
 astronomer. We may now briefly resume, and pursue in its 
 further progress the course of his studies, taking the planets in 
 the order of their distances from the sun. 
 
 In April 1792 Schroter first saw reason to conclude, from the 
 gradual degradation of light on its partially illuminated disc, 
 
CHAP. vii. PLANETS AND SATELLITES. 301 
 
 that Mercury possesses a tolerably dense atmosphere. 1 During 
 the transit of May 7, 1/99, he was, moreover, struck with the 
 appearance of a ring of softened luminosity encircling the 
 planet to an apparent height of three seconds, or about a quarter 
 of its own diameter. 2 Although a " mere thought " in texture, 
 yet it remained persistently visible both with the seven-foot and 
 the thirteen-foot reflectors, armed with powers up to 288. It 
 had a well-marked greyish boundary, and reminded him, though 
 indefinitely fainter, of the penumbra of a sun-spot. A similar 
 appendage, but more distinctly bright, had been noticed by De 
 Plantade at Montpellier, November n, 1736, and again in 1786 
 and 1789 by Prosperin and Flaugergues. Mercury projected on 
 the sun, November 9, 1802. appeared to Ljunberg at Copen- 
 hagen surrounded with a dark zone ; but Herschel, on the same 
 day, saw its "preceding limb cut the luminous solar clouds with 
 the most perfect sharpness." 3 The presence, however, of a 
 " halo," appearing to some observers a little darker, to others a 
 little brighter than the solar surface, was unmistakable in 1832. 
 Professor Moll of Utrecht described it as " a nebulous ring of a 
 darker tinge, approaching to the violet colour." 4 To Huggins 
 and Stone, November 5, 1868, it showed as lucid and most 
 distinct. No change in the colour of the glasses used, or the 
 powers applied, could get rid of it, and it lasted throughout the 
 transit. 5 It was again well seen by Christie and Dunkin at 
 Greenwich, May 6, i878, 6 and with much precision of detail by 
 Trouvelot at Cambridge (U.S.). 7 Professor Holden, on the 
 other hand, noted at Hastings-on-Hudson the total absence of 
 all anomalous appearances. 8 No observations of much interest 
 were made during the transit of November 8, 1881. Dr. Little, 
 at Shanghai, perceived an unvarying " darkish halo," of which, 
 however, neither Mr. Ellery at Melbourne, nor Mr. Tebbutt at 
 Windsor, New South Wales, saw any trace. 9 They, however^ 
 
 1 Neueste Beytrdye zur Erweiteruny der /Sternkunde, Bd. iii., p. 14 (1800}. 
 '2 Ibid., p. 24. 3 Phil. Trans., vol. xciii., p. 215. 4 Mem. Roy. Astr. Soc.> 
 vol. vi., p. 116. 5 Month. Not., vol. xxix., pp. n, 25. 6 Ibid., vol. xxxviii.,. 
 p. 398. 7 Am. Jour, of Sc., vol. xvi., p. 124. 8 Wash. Obss. for 1876, Ft. ii., 
 p. 34. 9 Month. Not., vol. xlii., pp. 101-104. 
 
302 HISTORY OF ASTRONOMY. PART n 
 
 took note of a certain whitish spot on the planet's disc, which, 
 ever since 1697, when it was detected by Wurzelbauer at Erfurt, 
 has been one of the most frequent attendant phenomena of a 
 transit of Mercury. It is not ajbvays centrally situated, and is 
 sometimes seen in duplicate, so that Powell's explanation by 
 diffraction is obviously insufficient. Nevertheless there can 
 scarcely be a doubt that it is an optical effect qf some kind. 
 
 As to the " halo," it is less easy to decide. That Mercury 
 possesses a considerable refractive atmosphere is certified by 
 the observation of De Plantade in 17 36,* and the still more 
 definite observation of Simms in i832, 2 of a luminous edge to 
 the part of the disc outside the sun at ingress or egress. The 
 natural complement to this appearance would be a dusky 
 annulus round the planet on the sun precisely such as was 
 seen by Moll and Little due to the imperfect transparency of 
 its gaseous envelope. But the brilliant ring vouched for by 
 others is not so readily explicable. Airy has shown that it 
 cannot possibly be caused by refraction, and must accordingly 
 be set down as " strictly an ocular nervous phenomenon." 3 It 
 is the less easy to escape from this conclusion that we find the 
 virtually airless 'moon capable of exhibiting a like appendage. 
 Professor Stephen Alexander of the United States Survey, with 
 two other observers, perceived, during the eclipse of the sun of 
 July 1 8, 1 860, the advancing lunar limb to be bordered with a 
 bright band ; 4 and photographic effects of the same kind appear 
 in pictures of transits of Venus and partial solar eclipses. Little 
 fresh information on the point was gleaned during the transit of 
 May 9, 1891. No uncommon effects of any kind were perceived 
 at the Lick Observatory, either with the great telescope, or 
 any of the other instruments employed. 5 The daylight defini- 
 tion there is, however, always exceptionally bad. The late 
 Professor Naegamvala of Poona, on the other hand, saw steadily 
 the well-known aureola as "brighter than the sun." 6 There 
 cannot be much doubt of its mainly illusory nature. 
 
 1 Mem. (Je I' Ac., 1736, p. 440. - Month. Not., vol. ii., p. 103. a Ibid., vol. 
 xxiv., p. 18. 4 Ibid., vol. xxiii., p. 234 (Challis). 5 Publ. Astr. Pacific Society 
 -vol. iii., p. 225. (i Month. Not., vol. li., p. 501. 
 
CHAP. vii. PLANETS AND SATELLITES. 303 
 
 As to the constitution of this planet, the spectroscope has 
 little to tell. Its light is of course that of the sun reflected, and 
 its spectrum is consequently a faint echo of the Fraunhofer 
 spectrum. Dr. H. C. Vogel, who first examined it in April 
 1871, suspected traces of the action of an atmosphere like ours, 1 
 but, it would seem, on slight grounds. It is, however, certainly 
 very poor in blue rays. More definite conclusions were, in 
 i874, 2 derived by Zollner from photometric observations of 
 Mercurian phases. A similar study of the waxing and waning 
 moon had afforded him the curious discovery that light-changes 
 dependent upon phase vary with the nature of the reflecting 
 surface, following a totally different law on a smooth homo- 
 geneous globe and on a rugged and mountainous one. Now 
 the phases of Mercury so far as could be determined from 
 only two sets of observations correspond with the latter kind 
 of structure. Strictly analogous to those of the moon, they 
 seem to indicate an analogous superficial conformation. It is at 
 any rate certain that the reflective capacity of Mercury does not 
 differ much from the lunar standard. The measurements of 
 Zollner and Winnecke 3 concur in assigning to it an "albedo " 
 represented by the fraction 0.13 (that of the moon being = 0.17), 
 signifying that it absorbs all but thirteen per cent, of the light 
 with which the fierce near sun inundates it. The inferred ab- 
 sence of an atmosphere is indeed scarcely reconcilable with some 
 of the transit-phenomena just adverted to ; but heights and 
 hollows in abundance seem to exist. 
 
 On March 26, 1800, Schroter, observing with his 1 3-foot 
 reflector in a peculiarly clear sky, perceived the southern horn of 
 Mercury's crescent to be quite distinctly blunted. 4 Interception 
 of sunlight by a Mercurian mountain rather more than eleven 
 English miles high, explained the effect to his satisfaction. By 
 carefully timing its recurrence, he concluded rotation on an axis 
 in a period of 24. hours 4 minutes. This was the first determina- 
 tion of the kind, and was the reward of twenty years' unceasing 
 vigilance. It was confirmed by watching the successive appear- 
 
 1 Untersuchungen iiber die Spectra der Planeten, p. 9. 2 Sirius, vol. vii., 
 
 4 Neueste Beytraye, Bd. iii., p. 50. 
 
304 HISTORY OF ASTRONOMY. PART n. 
 
 ances of a dusky streak and blotch in May and June iSoi. 1 
 These, however, were inferred to be no permanent markings on 
 the body of the planet, but atmospheric formations, the streak 
 at times drifting forwards (it was thought) under the fluctuating 
 influence of Mercurian breezes. From a rediscussion of these 
 somewhat doubtful observations Bessel inferred that Mercury 
 rotates on an axis inclined 70 to the plane of ^ its orbit in 24 
 hours 53 seconds. 
 
 The rounded appearance of the southern horn seen by 
 Schroter was more or less doubtfully caught by Noble (1864). 
 Burton, and Franks (i877); 2 but was obvious to Mr. W. F. 
 Denning at Bristol on the morning of November 5, 1882.^ 
 That the southern polar regions are usually less bright than the 
 northern, is well ascertained ; but the cause of the deficiency 
 remains dubious. If inequalities of surface are in question, they 
 must be on a considerable scale ; and a similar explanation 
 might be given of the deformations of the "terminator" or 
 dividing-line between darkness and light in the planet's phases 
 first remarked by Schroter, and again clearly seen by Trouvelot 
 in 1878 and 1 88 1. 4 The displacement, during four days, of 
 certain brilliant and dusky spaces on the disk indicated to Mr. 
 Demiiiig in 1882 rotation in about twenty-five hours; while the 
 general aspect of the planet reminded him of that of Mars. 5 
 But the difficulties in the way of its observation are enormously 
 enhanced by its constant close attendance on the sun. 
 
 In his sustained study of the aspect of Mercury, Schroter had 
 no imitator until Schiaparelli took up the task at Milan in 1882. 
 His observations were made in daylight. It was found that 
 much more could be seen, and higher magnifying powers used, 
 high up iri the sky near the sun, than at low altitudes, through 
 the agitated air of morning or evening twilight. A notable 
 discovery ensued. 6 Following the planet hour by hour, instead 
 of making necessarily brief inspections at intervals of about a 
 
 1 Astr. Jahrbuch, 1804, pp. 97-102. " Webb, Celestial Objects, p. 46 
 
 (4th ed. ), 3 L'Astronomie, t. ii., p. 141. 4 Observations sur les Planetes Venus 
 et Mercure, p. 87. 5 Observatory No. 82, p. 40. 6 Atti deW Accad. del Lincei, 
 t. v. ii., p. 283, 1889 ; Astr. Nach., No. 2944. 
 
CHAP. vii. PLANETS AND SATELLITES. 305 
 
 day, as previous observers had done, it was found that the 
 markings faintly visible remained sensibly fixed, hence, that 
 there was no rotation in a period at all comparable with that of 
 the earth. And after long and patient watching, the conclusion 
 was at last reached that Mercury turns on his axis in the same 
 time needed to complete a revolution in his orbit. One of his 
 hemispheres, then, is always averted from the sun, as one of the 
 moon's hemispheres from the earth, while the other never shifts 
 from beneath his torrid rays. The " librations," however, of 
 Mercury are on a larger scale than those of the moon, because 
 he travels in a more eccentric path. The temporary inequalities 
 arising between his " even pacing " on an axis and his alternately 
 accelerated and retarded elliptical movement occasion, in fact, an 
 oscillation to and fro of the boundaries of light and darkness on 
 his globe over an arc of 47 22', in the course of his year of 88 
 days. Thus the regions of perpetual day and perpetual night 
 are separated by two segments, amounting to one-fourth of the 
 entire surface, where the sun rises and sets once in 88 days. No 
 variation from the fierce glare on one side of the globe, and the 
 nocturnal blackness on the other, can, indeed, take place. Yet 
 these apparently intolerable climatic conditions may be somewhat 
 mitigated by the vigorous atmospheric circulation to which they 
 would naturally give rise. 
 
 To Schiaparelli's attentive scrutiny, Mercury appeared as a 
 " spotty globe," enveloped in a tolerably dense atmosphere. The 
 brownish stripes and streaks, discerned on his rose-tinged disc, 
 were judged to be permanent, and formed the basis of a chart 
 constructed by him. But they were not always equally well 
 seen. They disappeared regularly near the limb, owing doubtless 
 to the increased depth of the air-strata in that direction, and they 
 were at times veiled even when centrally situated. Some of them 
 had been clearly perceived by De Ball at Bothkamp in I882. 1 
 
 The theory of Mercury's movements has always given trouble. 
 In Lalande's, 2 as in Mastlin's time, the planet seemed to exist for 
 no other purpose than to throw discredit on astronomers ; and 
 even to Leverrier's powerful analysis it long proved recalcitrant. 
 
 1 Astr. Nach., No. 2479. - Hist, de VAstr., p. 682. 
 20 
 
306 HISTORY OF ASTRONOMY. PART n. 
 
 On the 1 2th of September 1859, however, he was able to 
 announce before the Academy of Sciences 1 the terms of a com- 
 promise between observation and calculation. They involved 
 the addition of a new member ta.the solar system. The hitherto 
 unrecognised presence of a body about the size of Mercury itself, 
 revolving at somewhat less than half its mean distance from the 
 sun (or, if farther, then of less mass, and vice versd), would, it 
 was pointed out, produce exactly the effect required, of displac- 
 ing the perihelion of the former planet 38 seconds a century 
 more than could otherwise be accounted for. The planes of the 
 two orbits, however, should not He far apart, as otherwise a 
 nodal disturbance would arise not perceived to exist. It was 
 added that a ring of asteroids similarly placed would answer 
 the purpose equally well, and was more likely to have escaped 
 notice. 
 
 Upon the heels of this forecast followed promptly a seeming 
 verification. Dr. Lescarbault, a physician residing at Orgeres, 
 whose slender opportunities had not blunted his hopes of 
 achievement, had, ever since 1845, when he witnessed a transit 
 of Mercury, cherished the idea that an unknown planet might 
 be caught thus projected on the solar background. Unable to 
 observe continuously until 1858, he, on March 26. 1859, saw 
 what he had expected a small, perfectly round object slowly 
 traversing the sun's disc. The fruitless expectation of re- 
 observing the phenomenon, however, kept him silent, and it 
 was not until December 22, after the news of Leverrier's pre- 
 diction had reached him, that he wrote to acquaint him with 
 his supposed discovery. 2 The Imperial Astronomer thereupon 
 hurried down to Orgeres, and by personal inspection of the 
 simple apparatus used, by searching cross-examination and 
 local inquiry, convinced himself of the genuine character and 
 substantial accuracy of the reported observation. He named 
 the new planet "Vulcan," and computed elements giving it a 
 period of revolution slightly under twenty days. 3 But it has 
 never since been seen. M. Liais, director of the Brazilian 
 Coast Survey, thought himself justified in asserting that it 
 
 1 Comptes Bendus, t. xlix., p. 379. 2 Ibid., t. 1., p. 40. 3 Ibid., p. 46. 
 
CHAP. vii. PLANETS AND SATELLITES. 307 
 
 never had been seen. Observing the sun for twelve minutes 
 after the supposed ingress recorded at Orgeres, he noted those 
 particular regions of its surface as " tres uniformes d'in- 
 tensite." 1 He subsequently, however, admitted Lescarbault's 
 good faith, at first rashly questioned. The planet-seeking 
 doctor was, in truth, only one among many victims of similar 
 illusions. 
 
 Waning interest in the subject was revived by a fresh 
 announcement of a transit witnessed, it was asserted, by Weber 
 at Peckeloh, April 4, i876. 2 The pseudo-planet, indeed, was 
 -detected shortly afterwards on the Greenwich photographs, 
 and was found to have been seen by M. Ventosa at Madrid 
 in its true character of a sun-spot without penumbra; but 
 Leverrier had meantime undertaken the investigation of a list 
 -of twenty similar dubious appearances, collected by Haase, 
 and republished by Wolf in i872. 3 From these five were picked 
 out as referring in all likelihood to the same body, the reality 
 of whose existence was now confidently asserted, and of which 
 more or less probable transits were fixed for March 22, 1877, 
 and October 15, i882. 4 But, widespread watchfulness not- 
 withstanding, no suspicious object came into view at either 
 epoch. 
 
 The next announcement of the discovery of " Vulcan " was 
 on the occasion of the total solar eclipse of July 29, i878. 5 
 This time it was stated to have been seen at some distance 
 south-west of the obscured sun, as a ruddy star with a~ minute 
 planetary disc ; and its simultaneous detection by two observers 
 the late Professor James C. Watson, stationed at Eawlins 
 (Wyoming Territory), and Professor Lewis Swift at Denver 
 (Colorado) was at first readily admitted. But their separate 
 observations could, on a closer examination, by no possibility 
 be brought into harmony, and, if valid, certainly referred to 
 two distinct objects, if not to four ; each astronomer eventually 
 claiming a pair of planets. Nor could any one of the four be 
 
 1 Astr. Nach., Nos. 1248 and 1281. 2 Comptes Rendus, t. Ixxxiii., pp. 510, 
 561. 3 Handbuch der Mathematik, Bd. ii., p. 327. 4 Comptes Bendus, 
 .t. Ixxxiii. p. 721. 5 Nature, vol. xviii., pp. 461, 495, 539. 
 
3o8 HISTORY OF ASTRONOMY. PART n. 
 
 identified with Lescarbault's and Leverrier's Vulcan, which, 
 if a substantial body revolving round the sun, must then (as 
 Oppolzer showed) l have been found on the east side of that 
 luminary. The most feasible explanation of the puzzle seems to 
 be that Watson and Swift merely saw each the same two stars 
 in Cancer : haste and excitement doing the rest. 2 Nevertheless 
 they strenuously maintained their opposite conviction. 3 
 
 Intra-Mercurian planets have since been diligently searched 
 for when the opportunity of a total eclipse offered, especially 
 during the long obscuration at Caroline Island. Not only did 
 Professor Holden " sweep " in the solar vicinity, but Palisa and 
 Trouvelot agreed to divide the field of exploration, and thus 
 make sure of whatever planetary prey there might be within 
 reach ; yet with only negative results. Belief in the presence of 
 any considerable body or bodies within the orbit of Mercury 
 is, accordingly, now at a low ebb. Yet the existence of the 
 anomaly in the Mercurian movements indicated by Leverrier 
 has been made only surer by further research. 4 Its elucidation 
 constitutes one of the " pending problems " of astronomy. 
 
 From the observation at Bologna in 1666-67 of some very 
 faint spots, Domenico Cassini concluded a rotation or libration 
 of Venus he was not sure which in about twenty-three hours. 6 
 By Bianchini in 1726 the period was augmented to twenty-four 
 days eight hours. J. J. Cassini, however, in 1740, showed that 
 the data collected by both observers were consistent with rota- 
 tion in twenty-three hours twenty minutes. 6 So the matter 
 rested until Schroter's time. He, after watching nine years in 
 vain, perceived at last, February 28, 1788, the ordinarily uniform 
 brightness of the planet's disc to be marbled with a filmy streak, 
 
 1 Astr. Nach., No. 2239. 2 Ibid., Nos. 2253-2254 (C. H. F. Peters). 
 3 Ibid., Nos. 2263 and 2277. See also Tisserand in Ann. Bur. des Long., 1882, 
 p. 729. 4 See J. Bauschinger's Untersucliungen (1884), summarised in Bull. 
 Astr., t. i., p. 506, and Astr. Nach., No. 2594. Newcomb finds the anomalous 
 motion of the perihelion to be even larger (43" instead of 38") than Leverrier 
 made it. Month. Not., Feb. 1884, p. 187. Harzer's attempt to account for it 
 in Astr. Nach., No. 3030, is more ingenious than successful. 5 Jour, des 
 Sgavans, Dec. 1667, p. 122. 6 Elemens dAstr., p. 525. 
 
CHAP. vii. PLANETS AND SATELLITES. 309 
 
 which returned periodically to the same position in about twenty- 
 three hours twenty-eight minutes. This approximate estimate 
 was corrected by the application of a more definite criterion. 
 On December 28, 1789, the southern horn of the crescent Venus 
 was seen truncated, an outlying lucid point interrupting the 
 darkness beyond. Precisely the same appearance recurred two 
 years later, giving for the planet's rotation a period of twenty- 
 three hours twenty-one minutes. 1 To this only twenty-two 
 seconds were added by De Vico, as the result of over 10,000 
 observations made with the Cauchoix refractor of the Collegio 
 Romano, 1839-4 1. 2 The axis of rotation was found to be much 
 more bowed towards the orbital plane than that of the earth, 
 the equator making with it an angle of 53 1 1'. 
 
 These conclusions inspired, it is true, much distrust, conse- 
 quently there were no received ideas on the subject to be sub- 
 verted. Nevertheless, vivid surprise was excited by Schiaparelli's 
 announcement, early in 1890,* that Venus rotates after the 
 fashion just previously ascribed to Mercury. His assertion, 
 however, that the day and year of Venus agree in length was 
 not positive. He claimed only to have demonstrated that her 
 period of rotation must lie between six and nine months, a 
 strong probability of exact coincidence with the 225 days of her 
 orbital revolution being urged by reasons of the physical order. 
 A continuous series of observations, from November 1877 to 
 February 1878, with their records in above a hundred drawings, 
 supplied the chief part of the data upon which he rested his 
 conclusions. They certainly appeared exceptionally well- 
 grounded. Most observers have depended, in their attempts 
 to ascertain the rotation-period of Venus, upon evanescent 
 shadings, most likely of atmospheric origin, and scarcely recog- 
 nisable from day to day. Schiaparelli fixed his attention 
 upon round, defined, lustrously white spots, the presence of 
 which near the cusps of the illuminated crescent has been 
 
 1 Beobachtunyen uber die sehr betrdchtlichen Gebirye und Rotation der Venus, 
 !792, p. 35- Schroter's final result in 1811 was 230. 2im. 7.9773. Monat. Corr. t 
 Bd. xxv., p. 367. a Astr., Nuch., No. 404. 3 Jtendiconti del It. Istituto 
 JLombardo, t. xxiii. , serie ii. 
 
310 HISTORY OF ASTRONOMY. PART n. 
 
 attested for close upon two centuries. His steady watch over 
 them showed the invariability of their position with regard to- 
 the terminator ; and this is as much as to say that the regions 
 of day and night do riot shift QH. the surface of the planet. In 
 other words, she keeps the same face always turned towards the 
 sun. Moreover, since her orbit is nearly circular, libratory 
 effects are very small. They amount in fact^-to only just one- 
 thirtieth of those serving to modify the severe contrasts of 
 climate in Mercury. 
 
 Confirmatory evidence of Schiaparelli's result for Venus is 
 not wanting. Thus, observations irreconcilable with a swift 
 rate of rotation were made at Bothkamp in 1871 by Vogel and 
 Lohse ; x and a drawing executed by Professor Holden with the 
 great Washington refractor, December 15, 1877, showed the 
 same markings in the positions recorded at Milan to have been 
 occupied by them eight hours previously. Above all, a series of 
 observations, carried out by M. Perrotin at Nice, May 15 to 
 October 4, 1 890, with the special aim of testing the inference of 
 synchronous rotation and revolution, proved strongly corrobora- 
 tive of it. 2 Yet it has been vigorously controverted. M. Niesten 
 of the Brussels Observatory, finds the numerous drawings of 
 Venus made by him and M. Stuyvaert during the years 1881 to 
 1 890, to fit in with De Vico's elements of rotation ; 3 and 
 M. Trouvelot 4 so far agrees with him as to adopt a period of 
 about twenty-four hours, while preferring Schiaparelli's nearly 
 upright axis to one deviating from the vertical by an angle of 
 no less than 53. The testimony of these two able observers is 
 thus so very far from being concordant that it can scarcely be 
 said to invalidate the daring hypothesis of the Milanese astro- 
 nomer. 
 
 Effects attributed to great differences of level in the surface 
 of Venus have struck many observers. Francesco Fontana at 
 Naples in 1643 noticed irregularities along the inner edge of the 
 crescent. 5 Lahire in 1700 considered them regard being had 
 
 1 Bothkamp Beobacldunyen, Heft ii., p. 120. 2 Comptes Rendus, t. cxi., 
 p. 587. 3 Bull de TAcad. rotj. de Belyique, t. xxi., p. 452, 1891. 4 Observa- 
 tions sur les Planetes Venus et Mercure, 1892. 5 Novce Observations, p. 92. 
 
CHAP. vii. PLANETS AND SATELLITES. 311 
 
 to difference of distance to be much, more strongly marked 
 than those visible in the moon. 1 Schroter's assertions to the 
 same effect, though scouted with some unnecessary vehemence 
 by Herschel, 2 have since been repeatedly confirmed ; amongst 
 others by Madler, De Vico, Langdon, who in 1873 saw the 
 broken line of the terminator with peculiar distinctness through 
 a veil of auroral cloud; 3 by Denning, 4 March 30, 1 88 1, despite 
 preliminary impressions to the contrary, as well as by C. V. 
 Zenger at Prague, January 8, 1883. The great mountain mass, 
 presumed to occasion the periodical blunting of the southern 
 horn, was precariously estimated by the Lilienthal observer to 
 rise to the prodigious height of nearly twenty-seven miles, or 
 just five times the elevation of Mount Everest ! Yet the phe- 
 nomenon persists, whatever may be thought of the explanation. 
 Moreover, the speck of light beyond, .interpreted as the visible 
 sign of a detached peak rising high enough above the encircling 
 shadow to catch the first and last rays of the sun, was frequently 
 discerned by Baron Van Ertborn in I8/6; 5 while an object 
 near the northern horn of the crescent, strongly resembling a 
 lunar ring-mountain, was delineated both by De Vico in 1841 
 and by Denning forty years later. 
 
 We are almost equally sure that Venus as that the earth 
 is encompassed with an atmosphere. Yet, notwithstanding 
 luminous appearances plainly due to refraction during the 
 transits both of 1761 and 1769, Schroter, in 1792, took the 
 initiative in coming to a definite conclusion on the subject. 6 It 
 was founded, first, on the rapid diminution of brilliancy towards 
 the terminator, attributed to atmospheric absorption ; next, on 
 the extension beyond a semicircle of the horns of the crescent ; 
 lastly, on the presence of a bluish gleam illuminating the early 
 hours of the Cytherean night with what was taken to be 
 genuine twilight. Even Herschel admitted that sunlight, by 
 the same effect through which the heavenly bodies show visibly 
 
 1 Mem de I' Ac., 1700, p. 296. 2 Phil. Trans., vol. Ixxxiii., p. 201. 3 Webb, 
 Cel Objects, p. 58. 4 Month. Not., vol. xlii., p. in. 5 Bull. Ac. de Bruxelles, 
 t. xliii., p. 22. 6 Phil. Trans., vol. Ixxxii., p. 309; Aphroditogr aphis che 
 Fragmente, p. 85 (1796). 
 
312 HISTORY OF ASTRONOMY. PART n. 
 
 above our horizons while still geometrically below them, appeared 
 to be bent round the shoulder of the globe of Venus. Ample 
 confirmation of the fact has since been afforded. At Dorpat 
 in May 1849, the planet being;-* within 3 26' of inferior con- 
 junction, Madler found the arms of waning light upon the disc 
 to embrace no less than 240 of its extent ; 1 and in December 
 1842, Mr. Guthrie, of Bervie, N.B., actually observed, under 
 similar conditions, the whole circumference to be lit up with a 
 faint nebulous glow. 2 Here the solar rays evidently pierced 
 the planet's atmosphere from behind, pursuing a curved path, 
 as if through a lens. The same curious phenomenon was 
 intermittently seen by Mr. Leeson Prince at Uckfield in 
 September, 1 863 ; 3 but with more satisfactory distinctness by 
 Mr. C. S. Lyman of Yale College, 4 before and after the con- 
 junction of December n, 1866, and during nearly five hours 
 previous to the transit of 1874, when the yellowish ring of 
 refracted light showed at one point an approach to interruption, 
 possibly through the intervention of a bank of clouds. These 
 effects can be accounted for, as Mr. Neison pointed out, 5 only 
 by supposing the atmosphere of Venus to be nearly twice as 
 dense at the surface of its globe, and to possess nearly twice as 
 much refractive power as that of the earth. Mr. Proctor gives, 
 however, a considerably lower estimate. 6 
 
 Similar appearances are conspicuous during transits. But 
 while the Mercurian halo is characteristically seen on the sun, 
 the "silver thread " round the limb of Venus commonly shows 
 on the part q^the sun. There are, however, instances of each 
 description in both cases. Mr. Grant, in collecting the records 
 of physical phenomena accompanying the transits of 1761 and 
 1769, remarks that no one person saw both kinds of amiulus, 
 and argues thus a dissimilarity in their respective modes of 
 production. 7 Such a dissimilarity probably exists, in the sense 
 that the inner section of the ring is due to absorption, the outer 
 
 1 Astr. Nach., No. 679. 2 Month. Not., vol. xiv., p. 169. 3 Ibid., vol. xxiv., 
 p. 25. 4 Am. Jour, of Sc., vol. xliii., p. 129 (26. ser.) ; vol. ix., p. 47 (36! ser.). 
 5 Month. Not., vol. xxxvi., p. 347. 6 Old and New Astronomy, p. 448. 7 Hist. 
 Phys. Astr., p. 431. 
 
CHAP. vii. PLANETS AND SATELLITES. 313 
 
 to refraction by the same planetary atmosphere ; but the dis- 
 tinction of separate visibility has not been borne out by recent 
 experience. Several of the Australian observers during the 
 transit of 1874 witnessed the complete phenomenon. Mr. J. 
 Macdonnell, at Eden, saw a " shadowy nebulous ring " surround 
 the whole disc when ingress was two-thirds accomplished ; Mr. 
 Tornaghi, at Goulburn, perceived a halo, entire and un- 
 mistakable, at half egress. 1 Similar observations were made at 
 Sydney, 2 and were renewed in 1882 by Lescarbault at Orgeres, 
 by Metzger in Java, and by Barnard atVanderbilt University. 3 
 
 Spectroscopic indications of aqueous vapour as present in 
 the atmosphere of Venus, were obtained in 1874 and 1882, by 
 Tacchini and Kiccd in Italy, and by Young in New Jersey. 4 
 Janssen, however, who made a special study of the point 
 subsequently to the transit of 1882, found them much less 
 certain than he had anticipated ; 5 and Vogel, by repeated 
 examinations, 1871-73, could detect only the very slightest 
 variations from the pattern of the solar spectrum. Some 
 additions there indeed seem to be in the thickening of certain 
 water-lines, and also of a group (B) since shown by Egoroff 
 to be developed through the absorptive action of cool oxygen ; 
 but so nearly evanescent as to induce the persuasion that 
 the light we receive from Venus is reflected from a heavy 
 cloud-stratum, and has traversed, consequently, only the rarer 
 upper portion of its atmosphere. 6 This would also account for 
 the extreme brilliancy of the planet. On the 26th and 27th of 
 September 1878, a close conjunction gave Mr. James Nasmyth 
 the rare opportunity of watching Venus and Mercury for several 
 hours side by side in the field of his reflector ; when the former 
 appeared to him like clean silver, the latter as dull as lead or 
 zinc. 7 Yet the light incident upon Mercury is, on an average, 
 three and a half times stronger than the light reaching Venus. 
 Thus the reflective power of Venus must be singularly strong. 
 
 1 Mem. Roy. Astr. Soc., vol. xlvii., pp. 77, 84. 2 Astr. Reg., vol. xiii., 
 p. 132. 3 L 'Astronomic, t. ii., p. 27; Astr. Nach., No. 2021; Am. Jour, of 
 /Sc., vol. xxv., p. 430. 4 Mem. Spectr. ItaL, Dicembre 1882 ; Am. Jour, of Sc., 
 vol. xxv., p. 328. 5 Oomptes Rendus, t. cxvi., p. 288. 6 Vogel, Spectra der 
 Planeten, p. 15. 7 Nature, vol. xix., p. 23. 
 
3H HISTORY OF ASTRONOMY. PART n. 
 
 And we find accordingly, from a combination of Zollner's with 
 Pickering's results, that its albedo is but little inferior to that of 
 new-fallen snow ; in other words, it gives back 72 per cent, of the 
 luminous rays impinging upon Jfc. 
 
 This view, that we see only a Cytherean cloud-canopy, has been 
 confirmed by M. Landerer's observation of the non-polarised char- 
 acter of the light reflected by the crescent Vefeius ; 1 yet it is not 
 easily reconcilable with the supposed permanence of many of its 
 spots, or with the perception of shadow effects on a rugged crust. 
 It is, however, with some reservation, shared by M. Trouvelot, 
 who in 1875 and some subsequent years pursued at Cambridge 
 (U.S.) a diligent telescopic study of the planet, continued later at 
 Meudon. Not the least surprising fact about this sister-globe is 
 that the axis on which it rotates is hooded at each end with some 
 shining substance. These polar appendages were discovered in 
 1813 by Gruithuisen, 2 who set them down as polar snow-caps like 
 those of Mars. Nor is it altogether certain that he was wrong. 
 Trouvelot, indeed, in January 1878, perceived (or thought that 
 he perceived) the southern one to be composed of isolated peaks 
 thrown into relief against the sky, and hence concluded each to 
 represent a lofty group of mountains penetrating the vapour- 
 stratum supposed to form the greater part of the visible disc. 
 He pointed out, moreover, that the place of the southern spot 
 might be called identical with that of a projection above the limb 
 detected by MM. Bouquet de la Grye and Arago in measuring 
 photographs of Venus in transit taken at Puebla and Port-au- 
 Prince in i882. 3 This projection corresponded to a real elevation 
 of about sixty-five miles. But it was more probably due to 
 "photographic irradiation" from a local excess of brilliancy, the 
 result according to the French investigator's conjecture of 
 accumulations of ice and snow, or the continuous formation of 
 vast cloud-masses. 
 
 The same photographs seem to show that in figure Venus very 
 closely resembles our earth, the estimated equatorial bulging 
 produced by rotation being -J^ of the radius. 
 
 1 Comptes Bendus, t. cxiv., p. 1524. 2 Nova Ada Acad. Nature Curiosorum, 
 Bd.x., 239. 3 Observatory, vols.iii., p. 416, vii., p. 239 ; L 1 Astronomie, t. x., p. 261. 
 
CHAP. vii. PLANETS AND SATELLITES. 315 
 
 The '-'secondary," or "ashen light," of Venus was first noticed 
 by Biccioli in 1643 j ^ was seen by Derham about 171 5, by Kirch 
 in 1721, by Schroter and Harding in 1806; l and the reality of 
 the appearance has since been authenticated by numerous and 
 trustworthy observations. It is precisely similar to that of the 
 " old moon in the new moon's arms ; " and Zenger, who witnessed 
 it with unusual distinctness, January 8, i883, 2 supposes it due to 
 the same cause namely, to the faint gleam of reflected earth- 
 light from the night-side of the planet. When we remember, 
 however, that "full earth-light" on Venus, at its nearest, has 
 little more than y^-J^nr its intensity on the moon, we see at once 
 that the explanation is inadequate. Nor can Professor Safarik's, 3 
 by phosphorescence of the warm and teeming oceans with which 
 Zollner 4 regarded the globe of Venus as mainly covered, be 
 seriously entertained. Vogel's suggestion is more plausible. He 
 and 0. Lohse, at Bothkamp, November 311, 1871, saw th& 
 dark hemisphere partially illuminated by secondary light, 
 extending 30 from the terminator, and thought the effect 
 might be produced by a very extensive twilight. 5 Others have 
 had recourse to the analogy of our auroras, and J. Lamp 
 suggested that the greyish gleam, visible to him at Bothkamp, 
 October 21 and 26, i887, 6 might be an accompaniment of 
 electrical processes connected with the planet's meteorology. 
 Whatever the origin of the phenomenon, it may serve, on a 
 night-enwrapt hemisphere, to dissipate some of the thick 
 darkness otherwise encroached upon only by " the pale light of 
 stars." 
 
 Venus was once supposed to possess a satellite. But belief in 
 its existence has died out. No one, indeed, has caught even a 
 deceptive glimpse of such an object during the last 125 years. 
 Yet it was repeatedly, and one might have thought, well 
 observed in the seventeenth and eighteenth centuries. Fontana 
 "discovered" it in 1645; Cassini an adept in the art of 
 
 1 Astr.Jahrbuck, 1809, p. 164. 2 Month. Not., vol. xliii., p. 331. 3 Report 
 Brit. Ass., 1873, P- 47- Tne paper contains a valuable record of observations 
 of the phenomenon. 4 Photom. Untersuckungen, p. 301. 5 Botlikamp 
 Beobaclitungen, Heft ii., p. 126. G Astr. Nach., No. 2818. 
 
316 HISTORY OF ASTRONOMY. PART n. 
 
 seeing recognised it in 1672, and again in 1686; Short 
 watched it for a full hour in 1740 with varied instrumental 
 means,- Tobias Mayer in 1759, Montaigne in 1761, several 
 astronomers at Copenhagen in* March 1764, noted what they 
 considered its unmistakable presence; as did Horrebow in 1768. 
 But M. Paul Stroobaiit, 1 who in 1887 submitted all the available 
 data on the subject to a searching examination, identified 
 Horrebow's satellite with Librae, a fifth-magnitude star ; and 
 a few other apparitions were, by his industry, similarly explained 
 away. Nevertheless, several withstood all efforts to account for 
 them, and together form a most curious case of illusion. For it 
 is quite certain that Venus has no such conspicuous attendant. 
 
 The third planet encountered in travelling outward from the 
 sun is the abode of man. He has in consequence opportunities 
 for studying its physical habitudes altogether different from the 
 baffling glimpses afforded to him of the other members of the 
 solar family. Kegarding the earth, then, a mass of knowledge 
 so varied and comprehensive has been accumulated as to form 
 a science or rather several sciences apart. But underneath 
 all lie astronomical relations, the recognition and investigation 
 of which constitute one of the most significant intellectual 
 events of the present century. 
 
 It is indeed far from easy to draw a line of logical distinction 
 between items of knowledge which have their proper place 
 here, and those which should be left to the historian of geology. 
 There are some, however, of which the cosmical connections 
 are so close that it is impossible to overlook them. Amongst 
 these is the ascertainment of the solidity of the globe. At first 
 sight it seems difficult to conceive what the apparent positions 
 of the stars can have to do with subterranean conditions ; yet 
 it was from star measurements alone that Hopkins, in 1839, 
 concluded the earth to be solid to a depth of at least 800 or 
 IOOO miles. 2 His argument was, that if it were a mere shell 
 filled with liquid, precession and nutation would be much 
 
 1 Memoir es del' A cad. de Bruxelies, t. xlix., No. 5, 4to ; Astr. Nach., No. 2809 ; 
 Cf. Schorr, Der Venusmond, 1875. 3 Phil Trans., 1839, 1841, 1842. 
 
CHAP. vii. PLANETS AND SATELLITES. 317 
 
 larger than they are observed to be. For the shell alone would 
 follow the pull of the sun and moon on its equatorial girdle,, 
 leaving the liquid behind ; and being thus so much the lighter, 
 would move the more readily. There is, it is true, grave reason 
 to doubt whether this reasoning corresponds with the actual 
 facts of the case ; x but the conclusion to which it led has been 
 otherwise substantiated. 
 
 Indications to an identical effect have been derived from 
 another kind of external disturbance, affecting our globe 
 through the same agencies. Lord Kelvin (then Sir William 
 Thomson) pointed out in 1 862 2 that tidal influences are brought 
 to bear on land as well as 011 water, although obedience to them 
 is perceptible only in the mobile element. Some bodily 
 distortion of the earth's figure must however take place, unless 
 we suppose it of absolute or " preternatural " rigidity, and the 
 amount of such distortion can be determined from its effect in 
 diminishing oceanic tides below their calculated value. For if 
 the earth were perfectly plastic to the stresses of solar and 
 lunar gravity, tides in the ordinary sense would not exist- 
 Continents and oceans would swell and subside together. It is 
 to the difference in the behaviour of solid and liquid terrestrial 
 constituents that the ebb and flow of the waters are due. 
 
 Six years later, the distinguished Glasgow professor suggested 
 that this criterion might, by the aid of a prolonged series of 
 exact tidal observations, be practically applied to test the 
 interior condition of our planet. 3 In 1882, accordingly, suitable 
 data extending over thirty-three years having at length become 
 available, Professor Gr. H. Darwin performed the laborious task of 
 their analysis, with the general result that the " effective rigidity " 
 of the earth's mass must be at least as great as that of steel. 4 
 
 1 Delaunay objected (Comptes Rendus, t. Ixvii., p. 65) that the viscosity of 
 the contained liquid (of which Hopkins took no account) would, where the 
 movements were so excessively slow as those of the earth's axis, almost 
 certainly cause it to behave like a solid. Sir W. Thomson, however (Report 
 Brit. Ass., 1876, ii., p. i), considers Hopkins's argument valid as regards the 
 comparatively quick solar semi-annual and lunar fortnightly nutations. 
 2 Phil. Trans., vol. cliii., p. 573. 3 Report Brit. Ass., 1868, p. 494. 4 Ibid.* 
 1882, p. 474. 
 
3i8 HISTORY OF ASTRONOMY. PART n. 
 
 Ratification of this conclusion has lately been derived from 
 an unexpected quarter. The question of a possible mobility in 
 the earth's axis of rotation has often been mooted. Now at 
 last it has received an affirmative^, reply. Dr. Kiistner detected, 
 in his observations of 1884-5, effects apparently springing from 
 a minute variation in the latitude of Berlin. The matter having 
 been brought before the International Geodetic Association in 
 1888, special observations were set on foot at Berlin, Potsdam, 
 Prague, and Strasbourg, the upshot of which was to bring 
 plainly to view synchronous, and seemingly periodic fluctuations 
 of latitude to the extent of half a second of arc. The reality of 
 these was verified by an expedition to Honolulu in 1891-2, the 
 variations there corresponding inversely to those simultaneously 
 determined in Europe. 1 Their character was completely defined 
 by Dr. S. C. Chandler's discussion in October iSgi. 2 He 
 showed that they could be explained by supposing the pole of 
 the earth to describe a circle with a radius of thirty feet in a 
 period of about fourteen months, or 427 days. Confirmation of 
 this hypothesis was found by Dr. B. A. Gould in the Cordoba 
 observations, 3 and it was provided with a physical basis through 
 the able co-operation of Professor Newcomb. 4 The earth, owing 
 to its ellipsoidal shape, should, apart from disturbance, rotate 
 upon its " axis of figure," or shortest diameter ; since thus alone 
 can the centrifugal forces generated by its spinning balance 
 each other. Temporary causes, however, such as heavy falls of 
 snow or rain limited to one continental area, the shifting of 
 ice-masses, even the movements of winds, may render the globe 
 slightly lop-sided, and thus oblige it to forsake its normal axis, 
 and rotate on one somewhat divergent from it. This " instan- 
 taneous axis " (for it is incessantly changing) should, by mathe- 
 matical theory, revolve round the axis of figure in a period of 
 306 days. Provided, that is to say, the earth were a perfectly 
 rigid body. But it is far from being so ; it yields sensibly to 
 every strain put upon it ; and this yielding tends to protract 
 the time of circulation of the displaced pole. The length of its 
 period, then, serves as a kind of measure of the plasticity of the 
 
 1 Albrecht, Astr. Nach., No. 3131 ; Marcuse, ibid., No. 3139. * Astr. Jour., 
 Nos. 248, 249, 277. 3 Ibid., No. 258. 4 Month. Not., vol. lii., p. 336. 
 
CHAP. vii. PLANETS AND SATELLITES. 319 
 
 globe ; which, according to Newcomb's calculation, 1 seems to be 
 a little less than that of steel. In an earth compacted of steel, 
 the instantaneous axis would revolve in 441 days ; in the actual 
 earth, hence concluded to have an advantage, in point of stiff- 
 ness, over the fictitious metallic one, the process is accomplished 
 in, at the most, 430 days. By this new path, accordingly, 
 astronomers have been led to a virtually identical estimate of 
 the consistence of our globe with that derived from tidal investi- 
 gations. 
 
 Variations of latitude are by their nature ephemeral and 
 irregular. They alternately subside, and are renewed. For the 
 shifting axis strives of itself to get back to its natural place, 
 although it can never rest long absolutely fixed there. That its 
 deviations to the extent of no more than thirty feet should have 
 been betrayed and determined by means of stellar observations, 
 bears emphatic witness to the extraordinary accuracy of modern 
 celestial measurements. 
 
 In a paper read before the Geological Society, December 15, 
 i83O, 2 Sir John Herschel threw out the idea that the perplexing 
 changes of climate revealed by the geological record might be 
 explained through certain slow fluctuations in the eccentricity 
 of the earth's orbit, produced by the disturbing action of the 
 other planets. Shortly afterwards, however, he abandoned the 
 position as untenable ; 3 and it was left to the late Dr. James 
 Croll, in 1 864 4 and subsequent years, to reoccupy and fortify it. 
 Within restricted limits (as Lagrange and, more certainly and 
 definitely, Leverrier proved), the path pursued by our planet 
 round the sun alternately contracts, in the course of ages, into 
 a moderate ellipse, and expands almost to a circle, the major 
 axis, and consequently the mean distance, remaining invariable. 
 Even at present, when the eccentricity approaches a minimum, 
 the sun is nearer to us in January than in July by above three 
 million miles, and some 850,000 years ago this difference was 
 more than four times as great. Dr. Croll brought together 5 a 
 
 1 A sir. Nach., No. 3097. 2 Trans. Geol. Soc., vol. iii. (2nd ser. ), p. 293. 
 
 3 See his Treatise on Astronomy, p. 199 (1833). 4 ^"^ Mag., vol. xxviii., 
 
 (4th ser.), p. 121. 5 Climate and Time, 1875 ; Discussions on Climate and 
 
 Cosmology, 1885. 
 
320 HISTORY OF ASTRONOMY. PART n. 
 
 mass of evidence to support the view that, at epochs of con- 
 siderable eccentricity, the hemisphere of which the winter, 
 occurring at aphelion, was both intensified and prolonged, 
 must have undergone extensive glaciation ; while the opposite 
 hemisphere, with a short, mild 'winter, and long, cool summer, 
 enjoyed an approach to perennial spring. These conditions, 
 through the shifting of the earth's perihelion combined with the 
 precession of the equinoxes, were exactly reversed at the end of 
 10,500 years, the frozen hemisphere blooming into a luxuriant 
 garden as its seasons came round to occur at the opposite sides 
 of the terrestrial orbit, and the vernal hemisphere subsiding 
 simultaneously into ice-bound rigour. 1 Thus a plausible ex- 
 planation was offered of fche anomalous alternations of glacial 
 and semi-tropical periods, attested, on incontrovertible geo- 
 logical evidence, as having succeeded each other in times past 
 over what are now temperate regions. They succeeded each 
 other, it is true, with much less frequency and regularity than 
 the theory demanded ; but the discrepancy was overlooked or 
 smoothed away. The most recent glacial epoch was placed by 
 Dr. Groll about 200,000 years ago, when the eccentricity of the 
 earth's orbit was 3.4 times as great as it now is. At present, a 
 faint representation of such a state of things is afforded by the 
 southern hemisphere. One condition of glaciation in the coin- 
 cidence of winter with the maximum of remoteness from the sun, 
 is present ; the other a high eccentricity is deficient. Yet 
 the ring of ice-bound territory hemming in the southern pole is 
 well known to be far more extensive than the corresponding 
 region in the north. 
 
 The verification of this ingenious hypothesis depends upon a 
 variety of intricate meteorological conditions, some of which have 
 been adversely interpreted by competent authorities. 2 What is 
 still more serious, its acceptance seems precluded by time- 
 relations of a simple kind. Dr. Wright 3 has established with 
 some approach to certainty that glacial conditions ceased in 
 
 1 See for a popular account of the theory, Sir R. Ball's The Cause of an Ice 
 Age, 1892. 2 See A. Woeikof, Phil. Mag., vol. xxi., p. 223. 3 The Ice Age 
 in North America, London, 1890. 
 
CHAP. vii. PLANETS AND SATELLITES. 321 
 
 Canada and the United States about ten or twelve thousand 
 years ago. The erosive action of the Falls of Niagara qualifies 
 them to serve as a clepsydra, or water-clock on a grand scale ; 
 and their chronological indications have been amply corroborated 
 elsewhere and otherwise on the same continent. The astro- 
 nomical Ice Age, however, should have been enormously more 
 antique. No reconciliation of the facts with the theory appears 
 possible. 
 
 The first attempt at an experimental estimate of the " mean 
 density " of the earth was Maskelyne's observation in 1 774 of 
 the deflection of a plumb-line through the attraction of Schehal- 
 lien. The conclusion thence derived, that our globe weighs 4^ 
 times as much as an equal bulk of water, 1 was not very exact. 
 It was considerably improved upon by Cavendish, who, in 1798, 
 brought into use the " torsion-balance " constructed for the 
 same purpose by John Michell. The resulting estimate of 5.48 
 was raised to 5.66 by Francis Baily's elaborate repetition of the 
 process in 183842. From experiments on the subject made in 
 187273 by Cornu and Bailie the slightly inferior value of 5.56 
 was obtained ; and it was further shown that the data collected 
 by Baily, when corrected for a systematic error, gave practically 
 the same result (5-S5). 2 The research, which was begun in 1867, 
 is being continued at Paris with extraordinary precautions 
 against recondite sources of error, 3 and will presumably yield 
 before long a definitive result. Meantime, M. Wilsing obtained, 
 in 1889, by means of a pendulum-apparatus, a mean density for 
 our planet of 5.58 ; 4 Professor Poynting's result with a common 
 balance, in 1890, coming out S-49- 5 Newton's guess at the 
 average weight of the earth as five or six times that of water has 
 thus been curiously verified. 
 
 Operations for determining the figure of the earth have been 
 carried out during the present century on an unprecedented 
 scale. The Russo-Scandinavian arc, of which the measurement 
 was completed under the direction of the elder Struve in 1855, 
 
 1 Phil Trans., vol. Ixviii., p. 783. - Comptes fiendus, t. Ixxvi., p. 954, 
 
 3 Observatory, vol. xiv., p. 249. 4 Potsdam Pull., Nos. 22, 23. 5 Phil 
 
 Trans., vol. clxxxii., p. 565. 
 
 21 
 
322 HISTORY OF ASTRONOMY. PART n. 
 
 reached from Hammerfest to Ismailia on the Danube, a length 
 of 25 20'. But little inferior to it was the Indian arc, begun 
 by Lambton in the first years of the century, continued by 
 Everest, revised and extended b^ Walker. The general upshot 
 is to show that the polar compression of the earth is somewhat 
 greater than had been supposed. The admitted fraction until 
 lately was -^^ that is to say, the thickness qf the protuberant 
 equatorial ring was taken to be -yj^ of the equatorial radius. 
 But Sabine's pendulum experiments, discussed by Airy in 1826. 
 gave $ ; l and arc measurements tend more and more towards 
 agreement with this figure. A fresh investigation led the late 
 J. B. Listing in 1878 2 to state the dimensions of the terrestrial 
 spheroid as follows : equatorial radius = 6,377,377 metres ; 
 polar radius = 6,355,270 metres ; ellipticity = SFS-.TF- The 
 fraction, however, at present adopted is ^4^. 
 
 But it is far from certain that the figure of the earth is 
 one of strict geometrical regularity. Nay, it is by no means 
 clear that even its main outlines are best represented by what 
 is called an " ellipsoid of revolution " in other words, by a 
 globe flattened at top and bottom, but symmetrical on every 
 side. From a survey of geodetical results all over the world, 
 Colonel Clarke concluded that different meridians possess 
 different amounts of curvature ; 3 so that the equator, instead of 
 being a circle, as it should be apart from perturbing causes 
 in a rotating body, must, on this view, be itself an ellipse, and 
 our planet be correctly described as in shape " an ellipsoid of 
 three unequal axes." But the point is still siib judice. Opera- 
 tions towards its decision are in active progress both in Europe 
 and India. 
 
 The moon possesses for us an unique interest. She in all 
 probability shared the origin of the earth ; she perhaps pre- 
 figures its decay. She is at present its minister and companion. 
 Her existence, so far as we can see, serves no other purpose 
 than to illuminate the darkness of terrestrial nights, and to 
 
 1 Phil. Trans., vol. cxvi., p. 548. ' 2 Astr. Nach., No. 2228. 3 Phil. Mag., 
 vol. vi. (5th ser.), p. 92. 
 
CHAP. vii. PLANETS AND SATELLITES, 323 
 
 measure, by swiftly-recurring and conspicuous changes of aspect, 
 the long span of terrestrial time. Inquiries stimulated by 
 visible dependence, and aided by relatively close vicinity, have 
 resulted in a wonderfully minute acquaintance with the features 
 of the single lunar hemisphere open to our inspection. 
 
 Selenography, in the modern sense, is not yet a hundred years 
 old. It originated with the publication in 1791 of Schroter's 
 Sdenotopographisclie Fragmented Not but that the lunar surface 
 had already been diligently studied, chiefly by Hevelius, Cassini, 
 and Tobias Mayer ; the idea, however, of investigating the 
 moon's physical condition, and detecting symptoms of the 
 activity there of natural forces through minute topographical 
 inquiry, first obtained effect at Lilienthal. Schroter's delinea- 
 tions, accordingly, imperfect though they were, afforded a 
 starting-point for a comparative study of the superficial features 
 of our satellite. 
 
 The first of the curious objects which he named " rills " was 
 noted by him in 1787. Before 1801 he had found eleven ; 
 Lohrmarm added 75 ; Ma'dler 55 ; Schmidt published in 1866 a 
 catalogue of 425, of which 278 had been detected by himself ; 2 
 and he eventually brought the number up to nearly 1000. 
 They are, then, a very persistent lunar feature, though wholly 
 without terrestrial analogue. There is no difference of opinion 
 as to their nature. They are quite obviously clefts in a rocky 
 surface, 100 to 500 yards deep (the depression of the great rill 
 near Aristarchus was estimated by Schmidt at 554y ar ds), usually 
 a couple of miles across, and pursuing straight, curved, or 
 branching tracks up to 1 50 miles in length. As regards their 
 origin, the most probable view is that they are fissures produced 
 in cooling ; but Neison inclines to consider them rather as dried 
 watercourses. 3 
 
 On February 24, 1792, Schroter perceived what he took to be 
 distinct traces of a lunar twilight, and continued to observe them 
 during nine ensuing years. 4 They indicated, he thought, the 
 presence of a shallow atmosphere (not reaching a height of more 
 
 1 The second volume was published at Gottingen in 1802. - Ueber Bitten 
 aufdem Monde, p. 13. 3 The Moon, p. 73. 4 Selen. fragm., Th. ii , p. 399. 
 
324 HISTORY OF ASTRONOMY. PART n. 
 
 than 8400 feet), about ^th as dense as our own. Bessel, on the 
 other hand, considered that the only way of " saving " a lunar 
 atmosphere was to deny it any refractive power, the sharpness 
 and suddenness of star-occultatfoiis negativing the possibility of 
 gaseous surroundings of greater density (admitting an extreme 
 supposition) than ^^ that of terrestrial air. 1 Newcomb places 
 the maximum at T ^. Sir John Herschel cdncluded "the non- 
 existence of any atmosphere at the moon's edge having one- 
 ipSoth part of the density of the earth's atmosphere." 2 
 
 This decision was fully borne out by Dr. Huggins's spectro- 
 scopic observation of the disappearance behind the moon's limb 
 of the small star c Piscium, January 4, i865. 3 Not the slightest 
 sign of selective absorption or unequal refraction was discernible. 
 The entire spectrum went out at once, as if a slide had suddenly 
 dropped over it. The spectroscope has uniformly told the same 
 tale ; for M. Thollon's observation during the total solar eclipse 
 at Sohag of a supposed thickening at the moon's rim, of certain 
 dark lines in the solar spectrum, is now acknowledged to have 
 been illusory. Moonlight, analysed with the prism, is found to 
 be pure reflected sunlight, diminished in quantity, owing to the 
 low reflective capability of the lunar surface, to less than one- 
 fifth its incident intensity, but wholly unmodified in quality. 
 
 Yet there is little or no doubt that the diameter of the moon, 
 as determined from occupations, is 4" smaller than it appears by 
 direct measurement. This fact, which emerged from Sir George 
 Airy's discussion, in 1865,* of an extensive series of Greenwich 
 and Cambridge observations, would naturally result from lunar 
 atmospheric refraction. He showed, however, that even if the 
 entire effect were thus produced (a certain share is claimed by 
 irradiation) the atmosphere involved would be 2OOO times thinner 
 than our own air at the sea-level. A gaseous stratum of such 
 extreme tenuity could scarcely produce any spectroscopic effect. 
 It is certain (as Mr. Neisoii has pointed out 5 ) that a lunar 
 atmosphere of very great extent and of no inconsiderable mass, 
 
 i Astr. Nach., No. 263 (1834) ; Pop. Vorl, pp. 615-620 (1838). - Outlines of 
 Astr., par. 431. 3 Month. Not., vol. xxv., p. 61. 4 Ibid., p. 264. 5 The 
 Moon. p. 25. 
 
CHAP. vii. PLANETS AND SATELLITES. 325 
 
 would possess, owing to the low power of lunar gravity, a very 
 small surface density, and might thus escape direct observation 
 while playing a very important part in the economy of our 
 satellite. Some renewed evidence of actual crepuscular gleams 
 on the moon has been gathered by MM. Paul and Prosper Henry 
 of the Paris Observatory, 1 as well as by Mr. W. H. Pickering, in 
 the pure air of Arequipa, at an altitude of 8000 feet above the 
 sea. An occupation of Jupiter, too, observed by him August I2 
 1892,2 was attended with a slight flattening of the planet's disc 
 through effects of refraction, it was supposed, in a lunar atmo- 
 sphere but in a lunar atmosphere possessing, at the most, ^Vo- 
 the density at the sea-level, or terrestrial air, and capable of 
 holding, in equilibrium, no more than ^^ of an inch of mercury. 
 Yet this small barometric value corresponds, Mr. Pickering 
 remarks, " to a pressure of hundreds of tons per square mile of 
 the lunar surface." 
 
 The first to emulate Schroter's selenographical zeal was 
 Wilhelm Gotthelf Lohrmann, a land-surveyor of Dresden, who, in 
 1824, published four out of twenty-five sections of the first 
 scientifically executed lunar chart, on a scale of 37 J inches to a 
 lunar diameter. His sight, however, began to fail three years 
 later, and he died in 1 840, leaving materials from which the work 
 was completed and published in 1878 by Dr. Julius Schmidt, late 
 director of the Athens Observatory. Much had been done in the 
 interim. Beer and Madler began at Berlin in 1830 their great 
 trigonometrical survey of the lunar surface, as yet neither revised 
 nor superseded. A map, issued in four parts, 1834-36, on nearly 
 the same scale as Lohrmann's, but more detailed and authoritative, 
 embodied the results. It was succeeded, in 1837, by a descriptive 
 volume bearing the imposing title, Der Mond; oder allgemeine 
 vergkichende Selenographie. This summation of knowledge in 
 that branch, though in truth leaving many questions open, had 
 an air of finality which tended to discourage further inquiry. 3 It 
 gave form to a reaction against the sanguine views entertained 
 by Hevelius, Schroter, Herschel and Gruithuisen as to the possi- 
 
 1 Webb, Cel Objects, p. 79. 3 Astr. and Astro-Physics, Nov. 1892, p. 778. 
 
 * Nelson, The Moon, p. 104. 
 
326 HISTORY OF ASTRONOMY. PART n. 
 
 bilities of agreeable residence on the moon, and relegated the 
 " Selenites," one of whose cities Schroter thought he had dis- 
 covered, and of whose festal processions Gruithuisen had not 
 despaired of becoming a spectator, to the shadowy land entered 
 through the Ivory Gate. All examples of change in lunar forma- 
 tions were, moreover, dismissed as illusory. The light contained 
 in the work was, in short, a " dry light," not stimulating to the 
 imagination. " A mixture of a lie," Bacon shrewdly remarks, 
 " doth ever add pleasure." For many years, accordingly, Schmidt 
 had the field of selenography almost to himself. 
 
 Reviving interest in the subject was at once excited and dis- 
 played by the appointment, in 1864, of a Lunar Committee of 
 the British Association. The indirect were of greater value 
 than the direct fruits of its labours. An English school of 
 selenography rose into importance. Popularity was gained for 
 the subject by the diffusion of works conspicuous for ingenuity 
 and research. Messrs. Nasmyth's and Carpenter's beautifully 
 illustrated volume (1874) was succeeded, after two years, by a 
 still more weighty contribution to lunar science in Mr. Nelson's 
 well-known book, accompanied by a map, based on the survey of 
 Beer and Madler, but adding some 500 measures of positions, . 
 besides the representation of several thousand new objects. 
 With Schmidt's Charte der G-ebirge des Mondes, Germany once 
 more took the lead. This splendid delineation the result of 
 thirty-four years' labour was built upon Lohrmann's founda- 
 tion, but embraced the detail contained in upwards of 3000 
 original drawings. No less than 32,856 craters are represented 
 in it, on a scale of seventy-five inches to a diameter. An 
 additional help to lunar inquiries was provided at the same time 
 in this country by the establishment, through the initiative of 
 the late Mr. W. B. Birt, of the Selenographical Society. 
 
 But the strongest incentive to diligence in studying the 
 rugged features of our celestial helpmate has been the idea of 
 probable or actual variation in them. A change always seems 
 to the inquisitive intellect of man like a breach in the defences 
 of Nature's secrets, through which it may hope to make its 
 way to the citadel. What is desirable easily becomes credible ; 
 
CHAP. vii. PLANETS AND SATELLITES. 327 
 
 and thus statements and rumours of lunar convulsions have 
 successively, during the last hundred years, obtained credence, 
 and successively, on closer investigation, been rejected. The 
 subject is one as to which illusion is peculiarly easy. Our 
 view of the moon's surface is a bird's-eye view. Its conforma- 
 tion reveals itself indirectly through irregularities in the dis- 
 tribution of light and darkness. The forms of its elevations 
 and depressions can be inferred only from the shapes of the 
 black, unmitigated shadows cast by them. But these shapes 
 are in a state of perpetual and bewildering fluctuation, partly 
 through changes in the angle of illumination, partly through 
 changes in our point of view, caused by what are called the 
 moon's " librations." x The result is, that no single observation 
 can be exactly repeated by the same observer, since identical 
 conditions recur only after the lapse of a great number of 
 years. 
 
 Local peculiarities of surface, besides, are liable to produce 
 perplexing effects. The reflection of earth-light at a particular 
 angle from certain bright summits completely, though tem- 
 porarily, deceived Herschel into the belief that he had wit- 
 nessed, in 1783 and 1787, volcanic outbursts on the dark side 
 of the moon. The persistent recurrence, indeed, of similar 
 appearances under circumstances less amenable to explanation 
 inclined Webb to the view that effusions of native light actually 
 occur. 2 More cogent proofs must, however, be adduced before 
 a fact so intrinsically improbable can be admitted as true. 
 
 But from the publication of Beer and Madler's work until 
 1866, the received opinion was that no genuine sign of activity 
 had ever been seen, or was likely to be seen, on our satellite ; 
 that her face was a stereotyped page, a fixed and irrevisable 
 
 * The combination of a uniform rotational, with an unequal orbital 
 movement causes a slight swaying of the moon's globe, now east, now west, by 
 which we are enabled to see round the edges of the averted hemisphere. 
 There is also a " parallactic " libration, depending on the earth's rotation ; and 
 a species of nodding movement the " libration in latitude " is produced by 
 the inclination of the moon's axis to her orbit, and by her changes of position 
 with regard to the terrestrial equator. Altogether, about y T of the invisible 
 side come into view. - CeL Objects, p. 58 (4th ed.). 
 
328 HISTORY OF ASTRONOMY. PART n. 
 
 record of the past. A profound sensation, accordingly, was 
 produced by Schmidt's announcement, in October 1866, that 
 the crater " Linne," in the Mare Serenitatis, had disappeared, 1 
 effaced, as it was supposed, by ,an igneous outflow. The case 
 seemed undeniable, and is still Dubious. Linne had been known 
 to Lohrmann and Madler, 1822-32, as a deep crater, five or six 
 miles in diameter, the third largest in the dusky plain known 
 as the " Mare Serenitatis " ; and Schmidt had observed and 
 drawn it, 1840-43, under a practically identical aspect. Now 
 it appears under high light as a whitish spot, in the centre of 
 which, as the rays begin to fall obliquely, a pit, probably under 
 two miles across, emerges into view. The crateral character of 
 this comparatively minute depression was detected by Father 
 Secchi, February n, 1867. 
 
 This, however, is not all. Schroter's description of Linne, 
 as seen by him November 5, 1788, tallies quite closely with 
 modern observation; 2 while its inconspicuousness in 1797 is 
 shown by its omission from Russell's lunar globe and maps. 3 
 We are thus driven to adopt one of two suppositions : either 
 Lohrmann, Madler, and Schmidt were entirely mistaken in the 
 size and importance of Linne, or a real change in its outward 
 semblance supervened during the first half of the century, and 
 has since passed away, perhaps again to recur. The latter 
 hypothesis seems the more probable ; and its probability is 
 strengthened by much evidence of actual obscuration or varia- 
 tion of tint in other parts of the lunar surface, more especially 
 on the floor of the great " walled plain " named " Plato." 4 From 
 a re-examination of this region, and of the Mare Serenitatis, 
 with a thirteen-iiich Clark refractor at Arequipa in 1891-2, Mr. 
 W. H. Pickering inclines to the belief that lunar volcanic action, 
 once apparently so potent, is not yet wholly extinct. 5 
 
 An instance of an opposite kind of change was alleged by 
 Dr. Hermann J. Klein of Cologne in March i878. 6 In Linne, 
 
 1 Astr. Nach., No. 1631. 2 Eespighi, Les blondes, t. xiv., p. 294 ; Huggins, 
 Month. Not , vol. xxvii., p. 298. 3 Birt, Ibid., p. 95. 4 JReport Brit. Ass., 
 
 1872, p. 245. 5 Observatory, vol. xv., p. 250. 6 Astr. Reg., vol. xvi., p. 265 ; 
 Astr. Nach., No. 2275. 
 
CHAP. vii. PLANETS AND SATELLITES. 329 
 
 the obliteration of an old crater had been assumed ; in 
 " Hyginus N.," the formation of a new crater was asserted. 
 Yet, quite possibly, the same cause may have produced the 
 effects thought to be apparent in both. It is, however, far 
 from certain that any real change has affected the neighbour- 
 hood of Hyginus. The novelty of Klein's observation of May 
 19, 1877, may have consisted simply in the detection of a 
 hitherto unrecognised feature. The region is one of complex 
 formation, consequently of more than ordinary liability to 
 deceptive variations in aspect under rapid and entangled fluctu- 
 ations of light and shade. 1 Moreover, it seems to be certain, from 
 Messrs. Pratt and Capron's attentive study, that " Hyginus N." is 
 no true crater, but a shallow, saucer-like depression, difficult 
 of clear discernment. 2 Under suitable illumination, neverthe- 
 less, it contains, and is marked by, an ample shadow. 3 
 
 In both these controverted instances of change, lunar photo- 
 graphy was invoked as a witness ; but, notwithstanding the 
 great advances made in the art by Mr. De la Rue in this 
 country, by Dr. Henry Draper, and above all by Mr. Lewis M. 
 Rutherfurd, in America, without decisive results. The auto- 
 graphic moon-pictures, however, lately obtained at Paris and on 
 Mount Hamilton, are on a larger scale, and show a vastly 
 greater amount of detail than those available when Linne 
 became obliterated ; and Dr. Weinek's discovery of a previously 
 unknown crater on one of the Lick plates 4 forecasts perhaps 
 the eventual importance to selenography of the method. But no 
 lunar photograph yet taken can compete with a really fine 
 telescopic view. 
 
 Melloni was the first to get undeniable heating effects from 
 moonlight. His experiments were made on Mount Vesuvius 
 early in i846, 5 and were repeated with like result by Zantedeschi 
 at Venice four years later. A rough measure of the intensity of 
 those effects was arrived at by Piazzi Smyth at Guajara, on the 
 
 1 See Lord Lindsay and Dr. Copeland in Month. Not., vol. xxxix., p. 195. 
 2 Observatory, vols. ii., p. 296 ; iv., p. 373. Mr. N. E. Green (Astr. fieg., 
 vol. xvii., p. 144) concludes the object a mere " spot of colour," dark under 
 oblique light. 3 Webb, Gel Objects, p. 101. * Astr. Nac/i. t No. 3130. 
 
 5 Comptes Rendus, t. xxii., p. 541. 
 
330 HISTORY OF ASTRONOMY. PART n. 
 
 Peak of Teneriffe, in 1856. At a distance of fifteen feet from 
 the thermomultiplier, a Price's candle was found to radiate just 
 twice as much heat as the full moon. 1 But by far the most 
 exact and extensive series of observations on the subject were 
 those made by the present Earl of Kosse, 1869-72. The lunar 
 radiations, from the first to the last quarter, displayed, when 
 concentrated with the Parsonstown three-foot mirror, appreciable 
 thermal energy, increasing with the phase, and largely due to 
 *' dark heat." distinguished from the quicker-vibrating sort by 
 inability to traverse a plate of glass. This was supposed to 
 indicate an actual heating of the surface, during the long lunar 
 day of 300 hours, to about 500 F. 2 (corrected later to I97), 3 
 the moon thus acting as a direct radiator no less than as a 
 reflector of heat. But the conclusion was very imperfectly borne 
 out by Dr. Boeddicker's observations with the same instrument 
 and apparatus during the total lunar eclipse of October 4, i884. 4 
 This first opportunity of measuring the heat-phases of an 
 eclipsed moon was used with the remarkable result of showing 
 that the heat disappeared almost completely, though not quite 
 simultaneously, with the light. Confirmatory evidence of the 
 extraordinary promptitude with which our satellite parts with 
 heat already to some extent appropriated, was afforded by 
 Professor Langley's bolometric observations at Allegheny of the 
 partial eclipse of September 23, i885. 5 Yet it is certain that 
 the moon sends us a perceptible quantity of heat on its own 
 account, besides simply throwing back solar radiations. For 
 in February 1885, Professor Langley succeeded, after many 
 fruitless attempts, in getting measures of a " lunar heat- 
 spectrum." The incredible delicacy of the operation may be 
 judged of from the statement that the sum-total of the thermal 
 energy dispersed by his rock-salt prisms was insufficient to raise 
 a thermometer fully exposed to it one-thousandth of a degree 
 Centigrade ! The singular fact was, however, elicited that this 
 almost evanescent spectrum is made up of two superposed 
 
 1 Phil. Trans., vol. cxlviii., p. 502. - Proc. Roy. 8oc., vol. xvii., p. 443. 
 * Phil. Trans., vol. clxiii., p. 623. 4 Trans. R. Dublin 8oc., vol. iii., p. 321. 
 /Science, vol. vii., p. 9. 
 
CHAP. viz. PLANETS AND SATELLITES. 331 
 
 spectra, one due to reflection, the other, with a maximum far 
 down in the infra-red, to radiation. 1 The corresponding tempe- 
 rature of the moon's sunlit surface Professor Langley considers 
 to be about that of freezing water. 2 Repeated experiments 
 having failed to get any thermal effects from the dark part of 
 the moon, it was inferred that our satellite " has no internal heat 
 sensible at the surface " ; so that the radiations from the lunar 
 soil giving the low maximum in the heat-spectrum, " must be due 
 purely to solar heat which has been absorbed and almost im- 
 mediately re-radiated." Professor Langley's explorations of the 
 terra incognita of immensely long wave-lengths where lie the 
 unseen heat-emissions from, the earth into space, led him to the 
 discovery that these, contrary to the received opinion, are in good 
 part transmissible by our atmosphere, although they are com- 
 pletely intercepted by glass. Another important result of the 
 Allegheny work was the abolition of the anomalous notion of 
 the "temperature of space," fixed by Pouillet at 142 C. For 
 space in itself can have no temperature, and stellar radiation is 
 a negligible quantity. Thus, it is safe to assume "that a 
 perfect thermometer suspended in space at the distance of the 
 earth or moon from the sun, but shielded from its rays, would 
 sensibly indicate the absolute zero ; 3 which absolute zero is 
 ordinarily placed at - 273 C. 
 
 A " Prize Essay on the Distribution of the Moon's Heat " (The 
 Hague, 1891), by Mr. Frank W. Very, who had shared actively 
 in Professor Langley's long-sustained inquiry, embodies the 
 fruits of its continuation. They show the lunar disc to be 
 tolerably uniform in thermal power. The brighter parts are also 
 indeed hotter, but not much. The traces perceived of a slight 
 retention of heat by the substances forming the lunar surface, 
 agreed well with Dr. Boeddicker's observations of the total eclipse 
 of the moon, January 28, i888. 4 For they showed as before an 
 unmistakable divergence between the heat and light-phases. A 
 
 1 Amer. Jour, of /Science, vol. xxxviii., p. 428. 2 " The Temperature of the 
 Moon," Memoirs National Acad. of /Sciences, vol. iv., p. 193, 1889. 3 Ibid., 
 p. in ; see also App. ii., p. 206. 4 Trans. B. Dublin Society, vol. iv., 
 
 p. 481, 1891. 
 
332 HISTORY OF ASTRONOMY. PART n. 
 
 curious decrease of heat previous to the first touch of the earth's 
 shadow upon the lunar globe remains unexplained, unless it be 
 admissible to suppose the terrestrial atmosphere capable of 
 absorbing heat at an elevation of 190 miles. 
 
 Although that fundamental part of astronomy known as 
 " celestial mechanics " lies outside the scope of this work, and 
 we must therefore pass over in silence the immense labours of 
 Plana, Damoiseau, Hansen, Delaunay, G. W. Hill, and Airy in 
 reconciling the observed and calculated motions of the moon, 
 there is one slight but significant discrepancy which is of such 
 importance to the physical history of the solar system, that 
 some brief mention must be made of it. 
 
 Halley discovered in 1693, by examining the records of 
 ancient eclipses, that the moon was going faster then than 
 2000 years previously so much faster, as to have got ahead 
 of the place in the sky she would otherwise have occupied, by 
 about two of her own diameters. It was one of Laplace's 
 highest triumphs to have found an explanation of this puzzling 
 fact. He showed, in 1787, that it was due to a very slow 
 change in the ovalness of the earth's orbit, tending, during the 
 present age of the world, to render it more nearly circular. 
 The pull of the sun upon the moon is thereby lessened; the 
 counter-pull of the earth gets the upper hand ; and our satellite, 
 drawn nearer to us by something less than an inch each year, 1 
 proportionately quickens her pace. Many thousands of years 
 hence the process will be reversed; the terrestrial orbit will 
 close in at the sides, the lunar orbit will open out under the 
 growing stress of solar gravity, and our celestial chronometer 
 will lose instead of gaining time. 
 
 This is all quite true as Laplace put it ; but it is not enough. 
 Adams, the virtual discoverer of Neptune, found with surprise 
 in 1853 that the received account of the matter was " essentially 
 incomplete," and explained, when the requisite correction was 
 introduced, only half the observed acceleration. 2 What was to 
 be done with the remaining half ? Here Delaunay, the eminent 
 
 1 Airy, Observatory, vol. iii., p. 420. ' 2 Phil. Trans., vol. cxliii., p. 397 ; 
 
 Proc. Hoy. Soc., vol. vi., p. 321. 
 
CHAP. vii. PLANETS AND SATELLITES. 333 
 
 French mathematical astronomer, unhappily drowned at Cher- 
 bourg in 1872 by the capsizing of a pleasure-boat, came to the 
 rescue. 1 
 
 It is obvious to any one who considers the subject a little 
 attentively, that the tides must act to some extent as a friction- 
 brake upon the rotating earth. In other words, they must 
 bring about an almost infinitely slow lengthening of the day. 
 For the two masses of water piled up by lunar influence on 
 the hither and farther sides of our globe, strive, as it were, to 
 detach themselves from the unity of the terrestrial spheroid, and 
 to follow the movements of the moon. The moon, accordingly y 
 holds them against the whirling earth, which revolves like a 
 shaft in a fixed collar, slowly losing motion and gaining heat, 
 eventually dissipated through space. 2 This must go on (so far 
 as we can see) until the periods of the earth's rotation and of 
 the moon's revolution coincide. Nay, the process will be con- 
 tinued should our oceans survive so long by the feebler tide- 
 raising power of the sun, ceasing only when day and night cease 
 to alternate, when one side of our planet is plunged in perpetual 
 darkness and the other seared by unchanging light. 
 
 Here, then, we have the secret of the moon's turning always 
 the same face towards the earth. It is that in primeval times, 
 when the moon was liquid or plastic, an earth-raised tidal wave 
 rapidly and forcibly reduced her rotation to its present exact 
 agreement with her period of revolution. This was divined by 
 Kant 3 nearly a century before the necessity for such a mode of 
 action presented itself to any other thinker. In a weekly paper 
 published at Konigsberg in 1754, the modern doctrine of " tidal 
 friction " was clearly outlined by him, both as regards its effects 
 actually in progress on the rotation of the earth, and as regards 
 its effects already consummated on the rotation of the moon the 
 whole forming a preliminary attempt at what he called a " natural 
 
 1 Comptes fiendus, t. Ixi., p. 1023. 2 Professor Darwin calculates that the 
 heat generated by tidal friction in the course of lengthening the earth's period 
 of rotation from 23 to 24 hours, equalled 23 million times the amount of its 
 present annual loss by cooling. Nature, vol. xxxiv., p. 422. 3 Sammtl. 
 
 Werke (ed. 1839), Th. vi., pp. 5-12. See also Mr. C. J. Monro's useful 
 indications in Nature, vol. vii., p. 241. 
 
334 HISTORY OF ASTRONOMY. PART n. 
 
 history " of the heavens. His sagacious suggestion, however, 
 remained entirely unnoticed until revived it would seem inde- 
 pendently by Julius Robert Mayer in 1 848 ; l while similar, 
 and probably original conclusjions were reached by William 
 Ferrel of Allensville, Kentucky, in 1 858.2 
 
 Delaunay was not then the inventor or discoverer of tidal 
 friction ; he merely displayed it as an effective cause of change. 
 He showed reason for believing that its action in checking the 
 earth's rotation, far from being, as Ferrel had supposed, com- 
 pletely neutralised by the contraction of the globe through 
 cooling, was a fact to be reckoned with in computing the move- 
 ments, as well as in speculating on the history of the heavenly 
 bodies. The outstanding acceleration of the moon was thus at 
 once explained. It was explained as apparent only the reflec- 
 tion of a real lengthening, by one second in 100,000 years, of the 
 day. But on this point the last word has not yet been spoken. 
 
 Professor Newcomb undertook in 1870 the onerous task of 
 investigating the errors of Hansen's Lunar Tables as compared 
 with observations prior to 1750. The results, published in i878, 3 
 have proved somewhat perplexing. They tend, in general, to 
 reduce the amount of acceleration left unaccounted for by 
 Laplace's gravitational theory, and proportionately to diminish 
 the importance of the part played by tidal friction. But, in 
 order to bring about this diminution, and at the same time con- 
 ciliate Alexandrian and Arabian observations, it is necessary to 
 reject as total the ancient solar eclipses known as those of Thales 
 and Larissa. This may be a necessary, but it must be admitted 
 to be a hazardous expedient. Its upshot was to indicate a 
 possibility that the observed and calculated values of the moon's 
 acceleration might after all prove to be identical ; and the small 
 outstanding discrepancy was still further diminished by Tis- 
 serand's investigation, differently conducted, of the same Arab 
 eclipses discussed by Newcomb. 4 The necessity of having re- 
 course to a lengthening day is then less pressing than it seemed 
 
 1 Dynamik des ffimmels, p. 40. 2 Gould's Astr. Jour., vol. iii., p. 138. 
 
 3 Wash. Obs. for 1875, vol. xxii., App. ii. 4 Comptes Rendus, t. cxiii., p. 669 ; 
 Annuaire, Paris, 1892. 
 
CHAP. vii. PLANETS AND SATELLITES. 335 
 
 some time ago ; and the effect, if perceptible in the moon's 
 motion, should, as M. Tisseraiid remarks, be proportionately so 
 in the motions of all the other heavenly bodies. The presence 
 of the apparent general acceleration that should ensue can be 
 tested with most promise of success, according to the same 
 authority, by delicate comparisons of past and future transits of 
 Mercury. 
 
 Newcomb further showed that small residual irregularities are 
 still found in the movements of our satellite, inexplicable either 
 by any known gravitational influence, or by any uniform value 
 that could be assigned to secular acceleration. 1 If set down to 
 the account of imperfections in the " time-keeping " of the earth, 
 it could only be on the arbitrary supposition of fluctuations in 
 its rate of going themselves needing explanation. This, it is 
 true, might be found, as Sir W. Thomson pointed out in 1876,2 
 in very slight changes of figure, not altogether unlikely to occur. 
 But into this cloudy and speculative region astronomers for 
 the present decline to penetrate. They prefer, if possible, 
 to deal only with calculable causes, and thus to preserve for 
 their " most perfect of sciences " its special prerogative of 
 assured prediction. 
 
 1 Newcomb, Pop. Astr. (4th ed.). p. 101. ' 2 Jteport Brit. Ass. 1876, p. 12. 
 
CHAPTER VIII. 
 
 PLANETS AND SATELLITES (continued). 
 
 " THE analogy between Mars and the earth is perhaps by far the 
 greatest in the whole solar system." SoHerschel wrote in I783, 1 
 and so it may safely be repeated to-day, after an additional 
 hundred years of scrutiny. The circumstance lends a particular 
 interest to inquiries into the physical habitudes of our exterior 
 planetary neighbour. 
 
 Pontana was the first to catch glimpses, at Naples in 1636 
 and 1 63 8, 2 of dusky stains on the ruddy disc of Mars. They 
 were next seen by Hooke and Cassini in 1666, and this time with 
 sufficient distinctness to serve as indexes to the planet's rotation, 
 determined by the latter as taking place in a period of twenty- 
 four hours forty minutes. 8 Increased confidence was given to 
 this result through Maraldi's precise verification of it in 17 19.* 
 Among the spots observed by him, he distinguished two as 
 stable in position, though variable in size. They were of a 
 peculiar character, showing as bright patches round the poles, 
 and had already been noticed during sixty years back. A current 
 conjecture of their snowy nature obtained validity when Herschel 
 connected their fluctuations in extent with the progress of the 
 Martian seasons. It was hard to resist the inference of frozen 
 precipitations when once it was clearly perceived that the shining 
 polar zones did actually diminish alternately and grow with the 
 alternations of summer and winter in the corresponding hemi- 
 sphere. 
 
 This, it may be said, was the opening of our acquaintance 
 
 1 Phil. Trans., vol. Ixxiv., p. 260. 2 Novce Observations, p. 105. 3 Phil. 
 Trans., vol. i., p. 243. 4 Mem. de VAc., 1720, p. 146. 
 
CHAP. viir. PLANETS AND SATELLITES. 337 
 
 with the state of things prevailing on the surface of Mars. It 
 was accompanied by a steady assertion, on Herschel's part, of 
 permanence in the dark markings, notwithstanding partial 
 obscurations by clouds and vapours floating in a " considerable 
 but moderate atmosphere." Hence the presumed inhabitants of 
 the planet were inferred to " probably enjoy a situation in many 
 respects similar to ours." l 
 
 Schroter, on the other hand, went altogether wide of the 
 truth as regards Mars. He held that the surface visible to us 
 is a mere shell of drifting cloud, deriving a certain amount of 
 apparent stability from the influence on evaporation and con- 
 densation of subjacent but unseen areographical features ; 2 
 and his opinion prevailed with his contemporaries. It was, 
 however, rejected by Kunowsky in 1822, and finally overthrown 
 by Beer and Madler's careful studies during five consecutive 
 oppositions, 1830-39. They identified at each the same dark 
 spots, frequently blurred with mists, especially when the local 
 winter prevailed, but fundamentally unchanged. 3 In 1862 
 Lockyer established a " marvellous agreement " with Beer and 
 Madler's results of 1830, leaving no doubt as to the com- 
 plete fixity of the main features, amid "daily, nay, hourly," 
 variations of detail through transits of clouds. 4 On seventeen 
 nights of the same opposition, F. Kaiser of Leyden obtained 
 drawings in which nearly all the markings noted at Berlin 
 in 1830 reappeared, besides spots frequently seen respectively 
 by Arago in 1813, by Herschel in 1783, and one sketched by 
 Huygens in 1672 with a writing-pen in his diary. 5 From these 
 data the Leyden observer arrived at a period of rotation of 
 24h. 37m. 22.623., being just one second shorter than that 
 deduced, exclusively from their own observations, by Beer 
 and Madler. The exactness of this result has been practically 
 confirmed by the inquiries of Professor Bakhuyzen of Leyden. 6 
 Using for a middle term of comparison the disinterred 
 
 1 Phil. Trans., vol. Ixxiv., p. 273. " A large work, entitled Areographisclte 
 Iraymente, in which Schroter embodied the results of his labours on Mars, 
 1785-1803, narrowly escaped the conflagration of 1813, and was published at 
 Leyden in 1881. 3 Beitrdge, p. 124. 4 Mem. E. A. Soc., vol. xxxii., p. 183. 
 
 Astr. Nach., No. 1468. 6 Observatory, vol. viii., p. 437. 
 
338 HISTORY OF ASTRONOMY. PART n. 
 
 observations of Schroter, with those of Huygens at one, 
 and of Schiaparelli at the other end of an interval of 220 
 years, he was enabled to show, with something like certainty, 
 that the time of rotation (^h. 37m. 22.7355.) ascribed to 
 Mars by Mr. Proctor x in reliance on a drawing executed by 
 Hooke in 1666, was too long by nearly one-tenth of a second. 
 The minuteness of the correction indicates*- the nicety of care 
 employed. Nor employed vainly ; for, owing to the com- 
 parative antiquity of the records available in this case, an 
 almost infinitesimal error becomes so multiplied by frequent 
 repetition as to produce palpable discrepancies in the positions 
 of the markings at distant dates. Hence Bakhuyzen's period 
 of 24h. 37m. 22.66s. is undoubtedly of a precision unap- 
 proached as regards any other heavenly body save the earth 
 itself. 
 
 Two facts bearing on the state of things at the surface of 
 Mars were, then, fully acquired to science in or before the year 
 1862. The first was that of the seasonal fluctuations of the 
 polar spots; the second, that of the permanence of certain 
 dark grey or greenish patches, perceived with the telescope as 
 standing out from the deep yellow ground of the disc. The 
 opinion has steadily gained consistency during the last half- 
 century that these varieties of tint correspond to the real 
 diversities of a terraqueous globe, the " ripe cornfield " 2 sections 
 representing land, the dusky spots and streaks, oceans and 
 straits. Sir J. Herschel in 1830 led the way in ascribing the 
 redness of the planet's light to an inherent peculiarity of soil. 3 
 Previously it had been assimilated to our sunset glows rather 
 than to our red sandstone formations set down, that is, to an 
 atmospheric stoppage of blue rays. But the extensive Martian 
 atmosphere, implicitly believed in on the strength of some 
 erroneous observations by Cassini and Komer in the seven- 
 teenth century, vanished before the sharp occupation of a 
 small star in Leo, witnessed by Sir James South in 1822;* 
 
 1 Month. Not., vols. xxviii., p. 37; xxix., p. 232; xxxiii., p. 552. 2 Flam- 
 marion, L' Astronomic, t. i., p. 266. 3 Smyth, Cel Cycle, vol. i., p. 148 (ist ed.). 
 4 Phil. Trans., vol. cxxi., p. 417. 
 
CHAP. viii. PLANETS AND SATELLITES. 339 
 
 and Dawes's observation in I865, 1 that the ruddy tinge is 
 deepest near the central parts of the disc, certified its non- 
 atmospheric origin. The absolute whiteness of the polar snow- 
 caps was alleged in support of the same inference by Dr. 
 Muggins in 1 867.2 
 
 All recent observations tend to show that the atmosphere of 
 Mars is much thinner than our own. This was to have been 
 expected d priori, since the same proportionate mass of air 
 would, owing to the small size and inferior specific gravity of 
 Mars, as compared with the earth, form a very much sparser 
 covering over each square mile of his surface. 3 Besides, gravity 
 there possesses less than four-tenths its force here, so that this 
 sparser covering would weigh less, and be less condensed, than if 
 it enveloped the earth. Atmospheric pressure would accordingly 
 be of about two and a quarter, instead of fifteen terrestrial 
 pounds per square inch. This corresponds with what the tele- 
 scope shows us. It is extremely doubtful whether any features 
 of the earth's actual surface could be distinguished by a planet- 
 ary spectator, however well provided with optical assistance. 
 Professor Langley's inquiries 4 have led him to conclude that 
 fully twice as much light is absorbed by our air as had pre- 
 viously been supposed say forty per cent, of vertical rays in 
 a clear sky. Of the sixty reaching the earth, less than a quarter 
 would be reflected even from white sandstone ; and this quarter 
 would again pay heavy toll in escaping back to space. Thus not 
 more than perhaps ten or twelve out of the original hundred 
 sent by the sun would, under the most favourable circumstances, 
 and from the very centre of the earth's disc, reach the eye of a 
 Martian or lunar observer. The light by which he views our 
 world is, there is little doubt, light reflected from the various 
 strata of our atmosphere, cloud- or mist-laden or serene, as the 
 case may be, with an occasional snow-mountain figuring as a 
 permanent white spot. 
 
 This consideration at once shows us how much more tenuous 
 
 1 Month. Xot., vol. xxv., p. 227. 2 Phil. Mag., vol. xxxiv., p. 75. 3 Proctor, 
 'Quart. Jour, of Science, vol. x., p. 185; Maunder, Sunday Mag., Jan., Feb. 
 JMarch, 1882. 4 Am. Jour, of Sc., vol. xxviii., p. 163. 
 
340 HISTORY OF ASTRONOMY. PART n. 
 
 the Martian air must be, since it admits of topographical delinea- 
 tions of the Martian globe. The clouds, too, that form in it 
 seem in general to be rather of the nature of ground-mists than 
 of heavy cumulus. 1 Aqueous'' vapour is indeed present. A 
 characteristic group of dark rays, due to its absorptive action r 
 was detected by Dr. Huggins in the analysed light of the 
 planet in 1 867, 2 and serves to raise the conjecture of " snowy 
 poles " to a verisimilitude scarcely to be distinguished from 
 certainty. 
 
 The climate of Mars seems to be unexpectedly mild. Its 
 theoretical mean temperature, taking into account both distance 
 from the sun and albedo, is thirty-four Centigrade degrees below 
 freezing. 3 Yet its polar snows are both less extensive and less 
 permanent than those on the earth. The southern white hood, 
 always eccentrically situated, was noticed by Schiaparelli in 1 877 
 to have survived the summer only as a small lateral patch, the 
 pole itself being quite free from snow. Moreover, Mr. W. BL 
 Pickering observed with astonishment the disappearance, in the 
 course of thirty-three days of June and July 1892, of 1,600,000 
 square miles of southern snow. Curiously enough, the initial 
 stage of shrinkage in the white calotte was marked by its division 
 into two unequal parts, as if in obedience to the mysterious 
 principle of duplication governing so many Martian phenomena. 4 
 Changes of the hues associated respectively with land and 
 water, simultaneously noted in lower latitudes, were thought to be 
 occasioned by floods ensuing upon this rapid antarctic thaw. 5 It 
 is true that scarcity of moisture would account for the scantiness 
 and transitoriness of snowy deposits easily liquefied because 
 thinly spread. But we might expect to see the whole wintry 
 hemisphere, at any rate, frostbound, since the sun radiates less 
 than half as much heat on Mars as on the earth. Water seems, 
 nevertheless, to remain, as a rule, uncongealed everywhere out- 
 side the polar regions. We are at a loss to imagine by what 
 beneficent arrangement the rigorous conditions naturally to be 
 
 1 Burton, Trans. Boy. Dublin 8oc., vol. i., 1880, p. 169. 2 Month. Not., vol. 
 xxvii., p. 179. 3 C. Christiansen, Beiblatter, 1886, p. 532. 4 Flamtnarion, La 
 Planete Mars, p. 574. 5 Astr. and Astro- Physics, Oct. 1892, p. 671. 
 
CHAP. vni. PLANETS AND SATELLITES. 341 
 
 looked for, can be modified into a climate which might be found 
 tolerable by creatures constituted like ourselves. 
 
 Martian topography may be said to form nowadays a separate 
 sub-department of descriptive astronomy. The amount of detail 
 become legible by close scrutiny of a little disc which, once in 
 fifteen years, attains a maximum of about ^oW the area ^ the 
 full moon, must excite surprise and might provoke incredulity. 
 Spurious discoveries, however, have little chance of holding their 
 own where there are so many competitors quite as ready to dispute 
 as to confirm. 
 
 The first really good map of Mars was constructed in 1869 
 by Proctor from drawings by Dawes. Kaiser of Leyden fol- 
 lowed in 1872 with a representation founded upon data of his 
 own providing in 1 862-64 ; and M. Terby, in his valuable 
 Arfagraphie, presented to the Brussels Academy in 1 874* a 
 careful discussion of all important observations from the time of 
 Fontana downwards, thus virtually adding to knowledge by 
 summarising and digesting it. The memorable opposition of 
 September 5, 1877, marked a fresh epoch in the study of Mars. 
 While executing a trigonometrical survey (the first attempted) 
 of the disc, then of the unusual size of 25" across, Signer G. V. 
 Schiaparelli, director of the Milan Observatory, detected a novel 
 and curious feature. What had been taken for Martian conti- 
 nents were found to be, in point of fact, agglomerations of 
 islands, separated from each other by a network of so-called 
 "canals" (more properly channels). 2 These are obviously ex- 
 tensions of the " seas," originating and terminating in them, 
 and sharing their grey-green hue, but running sometimes to a 
 length of three or four thousand miles in a straight line, and 
 preserving throughout a nearly uniform breadth of about sixty 
 miles. Further inquiries have fully substantiated the discovery 
 made at the Brera Observatory. The " canals " of Mars are an 
 actually existent and permanent phenomenon. An examination 
 of the drawings in his possession showed M. Terby that they 
 had been seen, though not distinctively recognised, by Dawes, 
 
 1 Mtmoires Couronnes, t. xxxix. 2 Lockyer, Nature vol. xlvi., p. 447. 
 
342 HISTORY OF ASTRONOMY. PART n. 
 
 Secchi, and Holden ; several were independently traced out by 
 Burton at the opposition of 1879; and all were recovered by 
 Schiaparelli himself in 1879 and 1881-82. 
 
 When the planet culminated at midnight, and was therefore in 
 opposition, December 26, i88r,'its distance was greater, and its 
 apparent diameter less than in 1877, in the proportion of sixteen 
 to twenty-five. Its atmosphere was, however, more transparent, 
 and ours of less impediment to northern observers, the object of 
 scrutiny standing considerably higher in northern skies. Never 
 before, at any rate, had the true aspect of Mars come out so 
 clearly as at Milan, with the 8f-inch Merz refractor of the 
 observatory, between December 1881 and February 1882. The 
 canals were all again there, but this time they were in as many 
 as twenty cases seen in duplicate. That is to say, a twin-canal 
 ran parallel to the original one at an interval of 200 to 400 miles. 1 
 
 We are here brought face to face with an apparently insoluble 
 enigma. Schiaparelli regards the " gemination " of his canals as 
 a periodical phenomenon depending on the Martian seasons. It 
 is, at any rate, not an illusory one, since it was plainly apparent, 
 during the opposition of 1886, to MM. Perrotin and Thollon at 
 Nice, 2 and to the former, using the new thirty-inch refractor of 
 that observatory, in 1888 ; Mr. A. Stanley Williams, with the help 
 of only a 6^-inch reflector, distinctly perceived in 1 890 seven of 
 the duplicate objects noted at Milan, 8 and the Lick observations, 
 both of 1890 and of 1892, brought unequivocal confirmation to the 
 accuracy of Schiaparelli's impressions. 4 Various conjectures have 
 been hazarded in explanation of this bizarre appearance. The 
 difficulty of conceiving a physical reality corresponding to it has 
 suggested recourse to an optical rationale. Proctor regarded it as 
 an effect of diffraction, 5 Stanislas Meunier, of oblique reflection 
 from overlying mist-banks ; 6 Flammarion considers it possible that 
 companion-canals might, under special circumstances, be evoked as 
 a kind of mirage, 7 by refraction. But none of these speculations 
 
 1 Mem. Spettr. Italiani, t. xi., p. 28. ~ Bull. Astr., t. iii., p. 324. 3 Jour. 
 Brit. Astr. Ass., vol. i., p. 88. 4 Publ. Pac. Astr. Soc., vol. ii., p. 299. 5 Old 
 and New Astr., p. 545. ti L'Astronomie, t. xi., p. 445. 7 La Plan&te Mars, 
 p. 588. 
 
CHAP. vin. PLANETS AND SATELLITES. 343 
 
 are really admissible, when all the facts are taken into account. 
 The view that the canals of Mars are vast rifts due to the cooling 
 of the globe, is recommended by the circumstance that they 
 always follow great circles ; nevertheless, it would break down if, 
 as Schiaparelli holds, the fluctuations in their visibility depend 
 upon actual obliterations and re-emergences. 
 
 The closeness of the terrestrial analogy has thus of late been 
 much impaired. The distribution of land and water on Mars, at 
 any rate, appears to be of a completely original type. The inter- 
 lacing everywhere of continents with arms of the sea (if that be 
 the correct interpretation of the visual effects) implies that their 
 levels scarcely differ ; l and Schiaparelli carries most observers with 
 him in holding that their outlines are not absolutely constant, 
 encroachments of dusky upon bright tints suggesting extensive 
 inundations. Mr. N. E. Green's observations at Madeira in 1877 
 seem to indicate, on the other hand, a rugged south polar region. 
 The contour of the snow-cap not only appeared to him indented, as 
 if by valleys and promontories, but brilliant points were discerned 
 outside the white area, attributed to isolated snow-peaks. 2 Still 
 more elevated, if similarly explained, must be the " ice island " first 
 seen in a comparatively low latitude by Dawes in January 1865. 
 On August 4, 1892, Mars stood opposite to the sun at a 
 distance of only 34,865,000 miles from the earth. In point of 
 vicinity, then, its situation was scarcely less favourable than 
 in 1877. Yet the low altitude of the planet practically 
 neutralised this advantage for northern observers, and public 
 expectation, which had been raised to the highest pitch by th e 
 announcements of sensation-mongers, was somewhat disappointed 
 at the "meagreness" of the news authentically received from 
 Mars. Valuable series of observations were, nevertheless, made 
 at Lick and Arequipa; and they unite in testifying to the 
 genuine prevalence of surface-variability, especially in certain 
 regions of intermediate tint, and perhaps of the "crude consist- 
 ence" of " boggy Syrtis, neither sea, nor good dry land." 
 Professor Holden insists on the " enormous difficulties in the way 
 
 1 L'Astronomie, t. viii. 2 Month. Not., vol. xxxviii., p. 41; Memoirs E. Astr. 
 Soc., vol. xliv., p. 123. 
 
344 HISTORY OF ASTRONOMY. PART n. 
 
 of completely explaining the recorded phenomena by terrestrial 
 analogies"; 1 Mr. W. H. Pickering speaks of " conspicuous and 
 startling changes." They however merely overlay, and partially 
 disguise a general stability. " Among the novelties detected by 
 Mr. Pickering, were a number of " lakes," under the aspect of 
 black dots at the junctions of two or more canals ; 2 and he, no 
 less than the Lick astronomers and M. Perrbtin at Nice, 3 observed 
 brilliant clouds projecting beyond the limb, while carried round 
 by the planet's rotation. They seemed to float at an altitude of 
 at least twenty miles, or about four times the height of terrestrial 
 cirrus ; but this was not wonderful, considering the low power of 
 gravity acting upon them. Great capital was made in the 
 journalistic interest out of these imaginary signals from intelli- 
 gent Martians, desirous of opening communications with (to 
 them) problematical terrestrial beings. Their exemplary per- 
 severance was certified by the previous occurrence of similar 
 effects, witnessed by M. Terby at Louvain in 1888, and at the 
 Lick Observatory in 1890. 
 
 The first photograph of Mars was taken by Gould at Cordoba 
 in 1879. Little real service in planetary delineation has, it is 
 true, been so far rendered by the art, yet one achievement must 
 be recorded to its credit. A set of photographs obtained by Mr. 
 W. H. Pickering on Wilson's Peak, California, April 9, 1890, 
 showed the southern polar cap of Mars as of moderate dimensions, 
 but with a large dim adjacent area. Twenty-four hours later, on 
 a corresponding set, the dim area was brilliantly white. The 
 polar cap had become enlarged in the interim, apparently through 
 a wide-spreading snow-fall, by the annexation of a territory equal 
 to that of the United States. The season was towards the close 
 of winter in Mars. This was the first time that the process of 
 glacial extension had been actually (it might be said) super- 
 intended in that distant globe. 
 
 Mars was gratuitously supplied with a pair of satellites long 
 before he was found actually to possess them. Kepler interpreted 
 Galileo's anagram of the "triple" Saturn in this sense; they 
 
 1 Astr. and Astro-Physics, Oct. 1892, p. 668. 2 Hid., Dec. 1892, p. 850. 
 
 3 Comptes fiendus, t. cxv., p. 379. 
 
CHAP. vin. PLANETS AND SATELLITES. 345 
 
 were perceived by Micromegas on his long voyage through space ; 
 and the Laputan astronomers had even arrived at a knowledge, 
 curiously accurate under the circumstances, of their distances 
 and periods. But terrestrial observers could see nothing of them 
 until the night of August II, 1877. The planet was then 
 within one month of its second nearest approach to the earth 
 during this century ; and in 1 845 the Washington 26-inch 
 refractor was not in existence. 1 Professor Asaph Hall, accord- 
 ingly, determined to turn the conjuncture to account for an 
 exhaustive inquiry into the surroundings of Mars. Keeping his 
 glaring disc just outside the field of view, a minute attendant 
 speck of light was "glimpsed "August II. Bad weather, how- 
 ever, intervened, and it was not until the 1 6th that it was ascer- 
 tained to be what it appeared a satellite. On the following 
 evening a second, still nearer to the primary, was discovered, which, 
 by the bewildering rapidity of its passages hither and thither, 
 produced at first the effect of quite a crowd of little moons. 2 
 
 Both these delicate objects have since been repeatedly 
 observed, both in Europe and America, even with comparatively 
 small instruments. At the opposition of 1884, indeed, the 
 distance of the planet was too great to permit of the detection 
 of both elsewhere than at Washington. But the Lick equatoreal 
 showed them, July 18, 1 888, when their brightness was only o. 12 
 its amount at the time of their discovery ; so that they can now 
 be followed for a considerable time before and after the least 
 favourable oppositions. 
 
 The names chosen for them were taken from the Iliad, where 
 "Deimos" and "Phobos" (Fear and Panic) are represented 
 as the companions in battle of Ares. In several respects, they 
 are interesting and remarkable bodies. As to size, they may 
 be said to stand midway between meteorites and satellites. 
 From careful photometric measures executed at Harvard in 
 1877 and 1879, Professor Pickering concluded their diameters 
 to be respectively six and seven miles. 3 This is on the assump- 
 
 1 See Mr. Wentworth Erck's remarks in Trans. Roy. Dublin Soc., vol. i., 
 p. 29. 2 Month. Not., vol. xxxviii., p. 206. 3 Annals Harvard Coll. Obs., 
 
 vol. xi., pt. ii., p. 217. 
 
346 HISTORY OF ASTRONOMY. PART 11. 
 
 tion that they reflect the same proportion of the light incident 
 upon them that their primary does. But it may very well be 
 that they are less reflective, in which case they would be more 
 extensive. The albedo of Mars according to Zollner, is 0.28; 
 his surface, in other words, returns 28 per cent, of the rays 
 striking it. If we put the albedo of his satellites equal to that 
 of our moon, 0.17, their diameters will be increased from 6 and 
 7 to 8 and 9 miles, Phobos, the inner one, being the larger. 
 Their actual dimensions are unlikely to exceed this estimate. 
 It is interesting to note that Deimos, according to Professor 
 Pickering's very distinct perception, does not share the reddish 
 tint of Mars. 
 
 Both satellites move quickly in small orbits. Deimos completes 
 a revolution in thirty hours eighteen minutes, at a distance from 
 the surface of its ruling body of 12,500 miles; Phobos in seven 
 hours thirty-nine minutes twenty-two seconds, at a distance of 
 only 3760 miles. This is the only known instance of a satellite 
 circulating faster than its primary rotates, and is a circumstance 
 of some importance as regards theories of planetary development. 
 To a Martian spectator the curious effect would ensue of a 
 celestial object, seemingly exempt from the general motion of 
 the sphere, rising in the west, setting in the east, and culmi- 
 nating twice, or even thrice a day ; which, moreover, in latitudes 
 above 69 north or south, would be permanently and altogether 
 hidden by the intervening curvature of the globe. 
 
 The detection of new members of the solar system has come 
 to be one of the most ordinary of astronomical events. Since 
 1 846 no single year has passed without bringing its tribute of 
 asteroidal discovery. In the last of the seventies alone, a full 
 score of miniature planets were distinguished from the thronging 
 stars amid which they seem to move ; 1875 brought seventeen 
 such recognitions ; their number touched a minimum of one in 
 1 88 1 ; it rose in 1882, and again in 1886, to eleven; dropped to 
 six in 1889, remounted to fifteen in 1890, to twenty-one in 1891, 
 and attained a maximum of twenty-nine in 1892. On January 
 J > I ^93, 352 asteroids were recognised as revolving between 
 the orbits of Mars and Jupiter. Of these, no less than 83 are 
 
CHAP. vin. PLANETS AND SATELLITES. 347 
 
 claimed by a single observer Professor J. Palisa of Vienna; 
 Dr. C. H. F. Peters of Clinton (N.Y.), whose varied and useful 
 career terminated July 19, 1890, comes second with 48; 
 Charlois claims 40 ; Watson, Borrelly, Luther, Hind, Gold- 
 schmidt, Paul and Prosper Henry, and many others have 
 each contributed numerously to swell the sum-total. The 
 construction by Chacornac and his successors at Paris, and 
 more recently by Peters at Clinton, of ecliptical charts showing 
 all stars down to the thirteenth and fourteenth magnitudes 
 respectively, rendered the picking out of moving objects above 
 that brightness a mere question of time and diligence. Both, 
 however, are vastly economised by the photographic method. 
 Tedious comparisons of the sky with charts are no longer 
 needed for the identification of unrecorded, because simulated 
 stars. Planetary bodies declare themselves by appearing upon 
 properly exposed sensitive plates, not in circular, but in linear 
 form. Their motion converts their images into trails, long or 
 short according to the time of exposure. The first asteroid 
 (No. 323) thus detected was by M. Max Wolf, at Heidelberg, 
 December 22, 1891. Fifteen others were similarly discovered 
 in 1892, by the same skilful operator; and ten more through 
 Charlois's adoption at Nice of the novel plan for picking up 
 errant light specks. Its continued use which promises soon 
 to become exclusive led, during the first three months of 1893, 
 to the reinforcement, at the same two observatories, of the 
 asteroidal band by no less than twenty-three new members. Far 
 more onerous than the task of their discovery is that of keeping 
 them in view once disco vered of tracking out their paths, fixing 
 their places, and calculating the disturbing effects upon them of 
 the mighty Jovian mass. These complex operations have come 
 to be centralised at Berlin under the superintendence of Professor 
 Tietjen, and their results are given to the public through the 
 medium of the Berliner Astronomisches Jdhrlmcli. 
 
 The crowd of orbits thus disclosed invites attentive study. 
 
 D' Arrest remarked in I85I, 1 when only thirteen minor planets 
 
 were known, that supposing their paths to be represented by 
 
 solid hoops, not one of the thirteen could be lifted from its 
 
 1 Astr. Nach., No. 752. 
 
348 HISTORY OF ASTRONOMY. PART n. 
 
 place without bringing the others with it. The complexity of 
 interwoven tracks thus illustrated has grown almost in the 
 numerical proportion of discovery. Yet no two actually intersect, 
 because no two lie exactly in the same plane, so that the chances 
 of collision are at present nil. There is only one case, indeed, 
 in which it seems to be eventually possible. M. Lespiault has 
 pointed out that the curves traversed by " Fides " and " Maia " 
 approach so closely that a time may arrive when the bodies in 
 question will either coalesce or unite to form a binary system. 1 
 
 The maze threaded by the 375 asteroids contrasts singularly 
 with the harmoniously ordered and rhythmically separated orbits 
 of the larger planets. Yet the seeming confusion is not without 
 a plan. The established rules of our system are far from being 
 totally disregarded by its minor members. The orbit of Pallas, 
 with its inclination of 34 42', touches the limit of departure 
 from the ecliptic level ; the average plane of the asteroidal paths 
 differs by little more than one degree from that of the sun's 
 equator ; 2 their mean eccentricity is below that of the curve 
 traced out by Mercury, and- all without exception are pursued in 
 the planetary direction from west to east. 
 
 The zone in which these small bodies travel is about three 
 times as wide as the interval separating the earth from the sun. 
 It extends perilously near to Jupiter, and actually encroaches 
 upon the sphere of Mars. In one of his lectures at Gresham 
 College in 1 879, 3 Mr. Ledger remarked that the minor planet 
 Aethra, when in perihelion, gets inside Mars in aphelion by as 
 much as five millions of miles, though at so different a level in 
 space that there is no close approach. 
 
 The distribution of the asteroids over the zone frequented by 
 them is very unequal. They are most densely congregated 
 about the place where a single planet ought, by Bode's Law, to 
 revolve; it may indeed be said that only stragglers from the 
 main body are found more than fifty million miles within or 
 without a mean distance from the sun 2.8 times that of the 
 
 1 L, Niesten, Annuaire, Bruxelles, 1881, p. 269. ~ According to Svedstrup 
 (Astr. Nack., Nos. 2240-41), the inclination to the ecliptic of the "mean 
 asteroid's" orbit is = 6. 3 /Sun and Planets, p. 267. 
 
CHAP. viii. PLANETS AND SATELLITES. 349 
 
 earth. Significant gaps, too, occur where some force prohibitive 
 of their presence would seem to be at work. What the nature of 
 that force may be, Professor Daniel Kirkwood, then of the Indiana 
 University, indicated, first in 1866 when the number of known 
 asteroids was only eighty-eight, and again with more confidence 
 in 1876 from the study of a list then run up to I/2. 1 It appears 
 that these bare spaces are found just where a revolving body 
 would have a period connected by a simple relation with that of 
 Jupiter. It would perform two or three circuits to his one, five 
 to his two, nine to his five, and so on. Kirk wood's inference is 
 that the gaps in question were cleared of asteroids by the 
 attractive influence of Jupiter. For disturbances recurring time 
 after time owing to commensurability of periods nearly at 
 the same part of the orbit, would have accumulated until th& 
 shape of that orbit was notably changed. The body thus 
 displaced would have come in contact with other cosmical particles, 
 of the same family with itself in former ages, it may be assumed, 
 more evenly distributed than now would have coalesced with 
 them, and permanently left its original track. In this way the 
 regions of maximum perturbation would gradually have become 
 denuded of their occupants. 
 
 We can scarcely doubt that this law of commensurability has- 
 largely influenced the present distribution of the asteroids. But 
 its effects must have been produced while they were still in an 
 unformed, perhaps a nebular condition. In a system giving* 
 room for a considerable modification through disturbance, the 
 recurrence of conjunctions with a dominating mass at the same 
 orbital point, need not involve instability. 2 Thus, if Menippe 
 really performs five while Jupiter performs two revolutions, she 
 may, serious perturbations notwithstanding, continue to do so. 
 The elements, however, computed for this small body are as yet 
 unverified by a recapture since its discovery in 1878. And in 
 general, the correspondence of facts with Professor Kirkwood's 
 hypothesis has become closer the more numerously they have 
 been ascertained. 3 
 
 1 Smiths. Iteport, 1876, p. 358 ; The Asteroids (Kirkwood), p. 42, 1888. 
 - Tisserand, Annuaire, Paris, 1891, p. B. 15. 3 Berberich, Astr. jYac/i.,No. 3088. 
 
350 HISTORY OF ASTRONOMY. PART n. 
 
 The existence, too, of numerous pairs and groups of asteroids 
 travelling in strikingly similar tracks, must date from a remote 
 antiquity. They result, in Professor Kirkwood's 1 opinion, from 
 the divellent action of Jupiter pon embryo pygmy planets, just 
 as comets moving in pursuit of one another are a consequence 
 of the sundering influence of the sun. 
 
 Leverrier fixed, in i853, 2 one-fourth of the earth's mass as 
 the outside limit for the combined masses of all the bodies 
 circulating between Mars and Jupiter; but it is far from 
 probable that this maximum is at all nearly approached. M. 
 Berberich 3 thinks that the moon would more than outweigh the 
 whole of them, a million of the lesser bodies shining like stars of 
 the twelfth magnitude being needed, according to his judgment, 
 to constitute her mass. And M. Niesten estimated that the 
 whole of the 216 asteroids discovered up to August 1880 
 amounted in volume to only 4^^- of our globe, 4 and we may safely 
 add since they are tolerably certain to be lighter, bulk for bulk, 
 than the earth that their proportionate mass is smaller still. 
 Professor Pickering, from determinations of light-intensity, 
 assigns to Vesta a diameter of 319 miles, to Pallas 167, to Juno 
 94, down to twelve and fourteen for the smaller members of the 
 group. 5 An albedo equal to that of Mars is assumed as the basis 
 of the calculation. Still more recently, Professor G. Miiller 6 of 
 Potsdam has examined photometrically the phases of seven 
 minor planets, of which four namely, Vesta, Iris, Massalia, and 
 Amphitrite were found to conform precisely to the behaviour 
 of Mars as regards light-change from position, while Ceres, 
 Pallas, and Irene varied after the manner of the moon and 
 Mercury. The first group were hence inferred to resemble Mars 
 in physical constitution, nature of atmosphere, and reflective 
 capacity ; the second to be moon-like bodies. The low albedo 
 of these last rendering them less conspicuous proportionately 
 to their extent of surface than the others, the value of their 
 
 1 The Asteroids, p. 48; Publ. Astr. Pac. Soc., vols. ii., p. 48, iii., p. 95; 
 Astr. and Astro-Physics, Nov. 1892, p. 785. 2 Comptes Eendus, t. xxxvii., 
 p. 797. 3 Bull. Astr.,t. v.,p. 180. 4 Annuaire, Bruxelles, 1881, p. 243. 
 5 Harvard Annals, vol. xi., part ii., p. 294. 6 Astr. Nach., Nos. 2724-5. 
 
CHAP. vin. PLANETS AND SATELLITES. 351 
 
 diameters arrived at by Pickering probably require some increase. 
 Vesta, however (of which the Harvard estimate of size has 
 been still further assured at Potsdam), still holds its place 
 as the largest minor planet. The rapid and regular changes of 
 its brightness, observed by Professor M. W. Harrington x at Ann 
 Arbor, and supposed by him to be those of a swiftly rotating 
 and unequally reflective globe, are unconfirmed and disbelieved 
 in by Miiller. 
 
 There is no direct evidence that any of the minor planets 
 possess atmospheres. The aureolae seen by Schroter to surround 
 Ceres and Pallas have been dissipated by optical improvements. 
 Vogel in 1872 thought he had detected an air-line in the 
 spectrum of Vesta ; 2 but admitted that its presence required 
 confirmation, which has not been forthcoming. 
 
 Crossing the zone of asteroids on our journey outward from 
 the sun, we meet with a group of bodies widely different from 
 the " inferior " or terrestrial planets. Their gigantic size, low 
 specific gravity, and rapid rotation, obviously from the first 
 threw the " superior " planets into a class apart ; and modern 
 research has added qualities still more significant of a dissimilar 
 physical constitution. Jupiter, a huge globe 86,000 miles in 
 diameter, stands pre-eminent among them. He is, however, 
 only primus inter pares ; all the wider inferences regarding his 
 condition may be extended, with little risk of error, to his 
 fellows ; and inferences in his case rest on surer grounds than in 
 the case of the others, from the advantages offered for telescopic 
 scrutiny by his comparative nearness. 
 
 Now the characteristic modern discovery concerning Jupiter 
 is that he is a body midway between the solar and terrestrial 
 stages of cosniical existence a decaying sun or a developing 
 earth, as we choose to put it whose vast unexpended stores of 
 internal heat are mainly, if not solely, efficient in producing the 
 interior agitations betrayed by the changing features of his 
 visible disc. This view was anticipated in the last century. 
 
 1 Am. Jour, of So., vol. xxvi. (3rd ser.), p. 464. 2 Spectra der Planeten, p. 24. 
 
352 HISTORY OF ASTRONOMY. PART n. 
 
 Buffon wrote in his Epoques de la Nature (1778) : 1 " La surface 
 de Jupiter est, comme Ton sait, sujette a des changemens sen- 
 sibles, qui semblent indiquer que cette grosse planete est encore 
 dans un etat d'inconBtance et de bouillonnement." 
 
 Primitive incandescence, attendant, in his fantastic view, on 
 planetary origin by cometary impacts with the sun, combined, 
 he concluded, with vast bulk to bring about this result. Jupiter 
 has not yet had time to cool. Kant thought similarly in 1785 ; 2 
 but the idea did not commend itself to the astronomers of the 
 time, and dropped out of sight until Mr. Nasmyth arrived at it 
 afresh in i853- 3 Even still, however, terrestrial analogies held 
 their ground. The dark belts running parallel to the equator, 
 first seen at Naples in 1630, continued to be associated as 
 Herschel had associated them in 1781 with Jovian trade-winds, 
 in raising which the deficient power of the sun was supposed to 
 be compensated by added swiftness of rotation. But opinion 
 was not permitted to halt here. 
 
 In 1860 G. P. Bond of Cambridge (U.S.) derived some 
 remarkable indications from experiments on the light of Jupiter. 4 
 They showed that fourteen times more of the photographic rays 
 striking it are reflected by the planet than by our moon, and" 
 that, unlike the moon, which sends its densest rays from the 
 margin, Jupiter is brightest near the centre. But the most 
 perplexing part of his results was that Jupiter actually seemed 
 to give out more light than he received. Bond, however, con- 
 sidered his data too uncertain for the support of so bold an 
 assumption as that of original luminosity, and, even if the 
 presence of native light were proved, thought that it might 
 emanate from auroral clouds of the terrestrial kind. The con- 
 ception of a sun-like planet was still a remote, and seemed an 
 extravagant one. 
 
 Only since it was adopted and enforced by Zollner in 1865,^ 
 can it be regarded as permanently acquired to science. The 
 rapid changes in the cloud-belts both of Jupiter and Saturn, he 
 remarked, attest a high internal temperature. For we know 
 
 1 Tome i., p. 93. 2 Berlinische Monatssclmft, 1785, p. 211. 3 Month. Not., 
 vol. xiii., p. 40. 4 Mem. Am. Ac., vol. viii., p. 221. 5 Photom. Unters., p. 303. 
 
CHAP. vin. PLANETS AND SATELLITES. 353 
 
 that all atmospheric movements on the earth are sun-heat trans- 
 formed into motion. But sun-heat at the distance of Jupiter 
 possesses but ^ T , at that of Saturn y^ of its force here. The 
 large amount of energy, then, obviously exerted in those remote 
 firmaments must have some other source, to be found nowhere 
 else than in their own active and all-pervading fires, not yet 
 banked in with a thick solid crust. 
 
 The same acute investigator dwelt, in I87I, 1 on the similarity 
 between the modes of rotation of the great planets and of the 
 sun, applying the same principles of explanation to each case. 
 The fact of this similarity is undoubted. Cassini 2 and Schroter 
 both noticed that markings on Jupiter travelled quicker the 
 nearer they were to his equator; and Cassini even hinted at 
 their possible assimilation to sun-spots. 3 It is now well ascer- 
 tained that, as a rule (not without exceptions), equatorial spots 
 give a period some 5j minutes shorter than those in latitudes of 
 about 30. But, as Mr. Denning has pointed out, 4 no single 
 period will satisfy the observations either of different markings 
 at the same epoch, or of the same markings at different epochs. 
 Accelerations and retardations, depending upon processes of 
 growth or change, take place in very much the same kind of 
 way as in solar maculae, inevitably suggesting similarity of 
 origin. 
 
 Among popular writers, Mr. Proctor was foremost in real- 
 ising the highly primitive condition of these giant orbs, and 
 in impressing the facts and their logical consequences upon the 
 public mind. The inertia of ideas on the subject was overcome 
 largely through the arguments reiterated in the various and 
 well-known works published by him subsequently to 1870. It 
 should be added that Mr. Mattieu Williams in his Fuel of the 
 Sun adopted, equally early, similar views. 
 
 The interesting query as to Jupiter's surface incandescence 
 has been studied since Bond's time with the aid of all the 
 appliances furnished to physical inquirers by modern inventive- 
 ness, yet without bringing to it a categorical reply. Zollner 
 
 1 Astr. Nach. t No. 1851. 2 Mtm. del Ac., t. x., p. 514. 3 Ibid., 1692, 
 p. 7. 4 Month. Not., vol. xliv., p. 63. 
 
 23 
 
354 HISTORY OF ASTRONOMY. PART n. 
 
 in 1865 estimated his albedo at 0.62, that of fresh-fallen snow 
 being 0.78, and of white paper o./O. 1 But the disc of Jupiter 
 is by no means purely white. The general ground is tinged 
 with ochre, the polar zones are leaden or fawn-coloured, large 
 spaces are at times stained or diffused with chocolate-browns 
 and rosy hues. It is occasionally seen ruled from pole to pole 
 with dusky bars, and is never wholly free from obscure mark- 
 ings. The reflection then by it, as a whole, of 62 per cent, of 
 the rays impinging upon it, might well suggest some original 
 reinforcement. 
 
 Nevertheless, the spectroscope gives little countenance to the 
 supposition of any considerable permanent light-emission. The 
 spectrum of Jupiter, as examined by Huggins, 1862-64, and by 
 Vogel, 187173, shows the familiar Fraunhofer rays belonging to- 
 reflected sunlight. But it also shows lines of native absorption. 
 Some of these are identical with those produced by the action 
 of our own atmosphere, especially one or more groups due to 
 aqueous vapour ; others are of unknown origin, and it is remark- 
 able that one amongst the latter a strong band in the red 
 agrees in position with a dark line in the spectra of some ruddy 
 stars. 2 There is, besides, a general absorption of blue rays y 
 intensified as Le Sueur observed at Melbourne in 1 869 3 in the 
 dusky markings, evidently through an increase of depth in the 
 atmospheric strata traversed by the light proceeding from them. 
 
 All these observations, however (setting aside the stellar line 
 as of doubtful significance), point to a cool planetary atmo- 
 sphere. There is, we believe, only one on record evincing un- 
 mistakably the presence of intrinsic light. On September 2/ r 
 1879, Dr. Henry Draper obtained a photograph of Jupiter's 
 spectrum, in which a strengthening of the impression was visible 
 in the parts corresponding to the planet's equatoreal regions.* 
 This is just the right sort of evidence, but it is altogether 
 exceptional. We are driven then to conclude that native emis- 
 sions from Jupiter's visible surface are local and fitful, not 
 
 1 Photom, Unters., pp. 165, 273. Zollner's photometric methods are, however,, 
 seriously called in question by Seeliger, F. /. 8. Astr. Ges., Jahrg. xxi. , p. 225. 
 2 Vogel, Sp. d. Planeten, p. 33, note. 3 Proc. Roy. Soc., vol. xviii., p. 250. 
 4 Month. Not., vol. xl., p. 433. 
 
CHAP. viii. PLANETS AND SATELLITES. 355 
 
 permanent and general. Indeed, the blackness of the shadows 
 cast by his satellites on his disc sufficiently proves that he sends 
 out virtually none but reflected light. 1 This conclusion, however, 
 by no means invalidates that of his high internal temperature. 
 
 The curious phenomena attending Jovian satellite-transits 
 may be explained partly as effects of contrast, partly as due to 
 temporary obscurations of the small discs projected on the large 
 disc of Jupiter. At their first entry upon its marginal parts, 
 which are several times less luminous than those near the centre, 
 they invariably show as bright spots, then usually vanish as the 
 background gains lustre, to reappear, after crossing the disc, 
 thrown into relief, as before, against the dusky limb. But 
 instances are not rare, more especially of the third and fourth 
 satellites standing out, during the entire middle part of their 
 course, in such inky darkness as to be mistaken for their own 
 shadows. The earliest witness of a " black transit " was Cassini, 
 September 2, 1665 ; Komer in 1677, and Maraldi in 1707 and 
 1713, made similar observations, which have been multiplied 
 during the present century. In some cases the process of 
 darkening has been visibly attended by the formation, or emer- 
 gence into view, of spots on the transiting body, as noted by 
 the two Bonds at Harvard, March 18, i848. 2 The third satellite 
 was seen by Dawes, half dark, half bright, when crossing Jupiter's 
 disc, August 21, 1 867 ; 3 one-third dark by Davidson of Cali- 
 fornia, January 15, 1884, under the same circumstances; 4 and 
 unmistakably spotted, both on and off the planet, by Schroter, 
 Secchi, Dawes, and Lassell. 
 
 The first satellite sometimes looks dusky, but never abso- 
 lutely black, in travelling over the disc of Jupiter. The 
 second appears uniformly white a circumstance attributed 
 by Dr. Spitta 5 to its high albedo. The singularly different 
 aspects, even during successive transits of the third and fourth 
 satellites, are connected by Professor Holden 6 with the varied 
 
 1 The anomalous shadow-effects recorded by Webb (Cel. Objects, p. 170, 
 4th ed.) are obviously of atmospheric and optical origin. 2 Engelmann, 
 Ueber die HeUigkeitsverhdltnisse der Jvpiterstrabanten, p. 59. 3 Month. Not., 
 vol. xxviii., p. ii. 4 Observatory, vol. vii., p. 175. 5 Month. Not., vol. 
 xlviii., p. 43. 6 Puhl. Astr. Pac. Soc., vol. ii., p. 296. 
 
356 HISTORY OF ASTRONOMY. PART n. 
 
 luminosity of the segments of the planetary surface they are 
 projected upon ; but fluctuations in their own brightness 1 must 
 be at least a concurrent cause ; and these seem to depend, in 
 some degree, upon their orbital positions. This amounts to 
 saying that, as Herschel concluded in 1797, they always, like 
 our moon, turn the same face towards their primary, thus always 
 presenting to us, when in the same relative situations, the same 
 obscure or brilliant sections of their globes. As regards the 
 outer satellite, Engelmann's researches in 1871, and the late 
 C. E. Burton's in 1873, make this almost certain; and there 
 is a strong probability that it also applies to the other three. 
 The phenomena, however, are quite too irregular to be com- 
 pletely rationalised on so simple and obvious a principle. We 
 must also admit changes in the power of reflecting light of the 
 satellites themselves, which Vogel's detection of lines in their 
 spectra or traces of such indicative of gaseous envelopes 
 similar to that of Jupiter, entitle us to regard as possibly of 
 atmospheric production. 
 
 This is rendered almost certain, in the case of the third 
 satellite, by a duplicate series of observations made upon it by 
 Messrs. Schaerberle and Campbell at Lick, in the autumn of 
 1 89 1. 2 They perceived, with the great telescope, its small 
 disc (less than 2" in diameter) to be usually crossed by three 
 dusky and broken bands, fading off towards each limb, as if 
 through atmospheric effacement, a constitution analogous to that 
 of Jupiter himself being thus unexpectedly suggested. Rotation 
 on an axis perpendicular to the shadings was evident, and is most 
 likely performed in seven days four hours, the period of the 
 satellite's revolution. 
 
 The rule of isochronous rotation and circulation is also un- 
 doubtedly obeyed by the first satellite. On September 8, 1890, 
 Professor Barnard saw it elongated, and as if double, during one 
 of its dark transits; 3 and his observation, repeated August 3, 
 
 1 There is a consensus among observers as to the variability of all Jupiter's 
 satellites, though Pickering strangely finds no trace of it in his exact 
 measures of their light. See Harvard Annals, vol. xi., pt. ii., p. 245. 2 Publ. 
 Astr. Pac. Soc., vol. iii., p. 359. 3 Astr. Nach., No. 2995 5 -Month. Not.,vol. li., 
 P- 556. 
 
CHAP. vin. PLANETS AND SATELLITES. 357 
 
 1891, was completely verified by Messrs. Schaeberle and Camp- 
 bell, who ascertained, moreover, that the longer axis of the egg- 
 shaped moon was directed towards Jupiter's centre. 1 The 
 elongation represents, in fact, a fixed tidal wave, by the arresting 
 action of which the satellite's rotation relative to its primary was, 
 long ages ago, brought to a standstill. 
 
 The system of Jupiter, as it was discovered by Galileo and 
 investigated by Laplace, appeared in its outward aspect so 
 symmetrical, and displayed in its inner mechanism such har- 
 monious dynamical relations, that it might well have been 
 deemed complete. Nevertheless, a new member has been added 
 to it. Near midnight on September 9, 1892, Professor Barnard 
 discerned with the Lick 36-inch, " a tiny speck of light " 
 closely following the planet. 2 He instantly divined its nature, 
 watched its hurried disappearance in the adjacent glare, and 
 made sure of the reality of his discovery on the ensuing night. 
 It was a delicate business throughout, the liliputian luminary 
 subsiding into invisibility before the slightest glint of Jovian 
 light, and tarrying, only for brief intervals, far enough from the 
 disc to admit of its exclusion by means of an occulting plate. 
 The new satellite is estimated to be of the thirteenth stellar 
 magnitude, and, if equally reflective of light with its next 
 neighbour, lo (satellite No. i), its diameter must be of about one 
 hundred miles. It revolves at a distance of 112,000 miles from 
 Jupiter's centre, and of 67,000 from his bulging equatorial sur- 
 face. 3 Its period of nh. 57m. 233. is just two hours longer than 
 Jupiter's period of rotation, so that Phobos still remains a unique 
 example of a secondary body revolving faster than its primary 
 rotates. Jupiter's innermost moon conforms in its motions 
 strictly, and indeed inevitably, to the plane of his equatorial 
 protuberance, following, however, a sensibly elliptical path. Its 
 very insignificance raises the suspicion that it may not prove 
 solitary. Possibly it belongs to a zone peopled by asteroidal 
 satellites. More than fifteen thousand such small bodies could 
 be furnished out of the materials of a single full-sized satellite 
 
 1 PuU. A fir, Pac. Soc., voL iii., p. 355. - Astr. Jour., No. 275 ; Observa- 
 tory, vol. xv. p. 425. 3 Astr. Jour., Nos. 285-6. 
 
358 HISTORY OF ASTRONOMY. PART n. 
 
 spoiled in the making. But we must be content for the present 
 to register the fact without seeking to penetrate the meaning of 
 its existence. Very high and very fine telescopic power is 
 needed for its perception. In America it has been observed 
 by Professor Stone with the 26-inch of the University of 
 Virginia; by Professor Young's assistant, Mr. Keed, with the 
 Halstead 2 3 -inch ; and by Professor Hougji with the i8J-inch 
 of the Dearborn Observatory, Chicago ; all three refractors by 
 Alvan Clark. Outside of the United States, it has been 
 seen, so far, only with Dr. Common's five-foot reflector. But 
 several of the great Continental refractors are doubtless fully 
 capable of showing it. 
 
 In the course of his observations on Jupiter at Brussels in 
 1878, M. Niesten was struck with a rosy cloud attached to a 
 whitish zone beneath the dark southern equatorial band. 1 Its 
 size was enormous. At the distance of Jupiter, its measured 
 dimensions of 13" by 3" implied a real extension in longitude 
 of 30,000, in latitude of something short of 7000 miles. The 
 earliest record of its appearance seems to be by Professor 
 Pritchett, director of the Morrison Observatory (U.S.), who 
 figured and described it July 9, i878. 2 It was again delineated 
 August 9, by Tempel at Florence. 3 In the following year it 
 attracted the wonder and attention of almost every possessor of 
 a telescope. Its colour had by that time deepened into a full 
 brick-red, and was set off by contrast with a white equatorial 
 spot of unusual brilliancy. During three ensuing years these 
 remarkable objects continued to offer a visible and striking 
 illustration of the compound nature of the planet's rotation. 
 The red spot completed a circuit in nine hours fifty-five minutes 
 thirty-six seconds; the white spot in about five and a half 
 minutes less. Their relative motion was thus no less than 260 
 miles an hour, bringing them together in the same meridian at 
 intervals of forty-four days ten hours forty-two minutes. 
 Neither, however, preserved continuously the same uniform rate 
 of travel. The period of each had lengthened by some seconds 
 
 1 Butt. Ac. B. Bruxelles, t. xlviii., p. 607. z Astr. Nach., No. 2294. 
 3 Ibid., No. 2284. 
 
CHAP. vin. PLANETS AND SATELLITES. 359 
 
 in 1883, while sudden displacements, associated with the 
 recovery of lustre after recurrent fadings, were observed in 
 the position of the white spot, 1 recalling the leap forward 
 of a reviving sun-spot. Just the opposite effect attended the 
 rekindling of the companion object. While semi-extinct in 
 1882-4, it lost little motion; but a fresh access of retardation 
 was observed by Professor Young 2 in connection with its 
 brightening in 1 8 86. This suggests very strongly that the red 
 spot is fed from below. A shining aureola of " faculae," described 
 by Bredichin at Moscow, and by Lohse at Potsdam, as encir- 
 cling it in September i879, 3 was held to strengthen the solar 
 analogy. 
 
 The conspicuous visibility of this astonishing object lasted 
 three years. When the planet returned to opposition in 
 1882-83, it had faded so considerably that Ricco's uncertain 
 glimpse of it at Palermo, May 31, 1883, was expected to be the 
 last. It had, nevertheless, begun to recover in December, and 
 in the beginning of 1886 presented to Mr. Denning much the 
 same aspect as in October i882. 4 Observed by him in an 
 intermediate stage, February 25, 1885, when "a mere skeleton 
 of its former self," it bore a striking likeness to an " elliptical 
 ring " observed in the same latitude by Mr. Gledhill at Halifax 
 in 186970. This, indeed, might be called the preliminary 
 sketch for the famous object brought to perfection ten years 
 later, but which Mr. H. C. Kussell of Sydney saw and drew in 
 June i8/6, 5 in what might be called an unfinished condition, 
 before it had separated from its matrix, the dusky south tropical 
 belt. In earlier times, too, a marking " at once fixed and 
 transient" had been repeatedly perceived attached to the 
 southernmost of the central belts. It gave Cassini in 1665 a 
 rotation-period of nine hours fifty-six minutes, 6 reappeared 
 and vanished eight times during the next forty-three years, 
 and was last seen by Maraldi in 1713. It was, however, very 
 
 1 Denning, Month. Not., vol. xliv., pp. 64, 66; Nature, vol. xxv., p. 226. 
 2 Sidereal Mess., Dec. 1886, p. 289. 3 Astr. Nach., Nos. 2280, 2282. 
 4 Month. Not., vol. xlvi., p. 117. 5 Proc. Boy. Soc. N. S. Wales, vol. xiv., p. 68. 
 6 Phil. Trans., vol. i., p. 143. 
 
3<5o HISTORY OF ASTRONOMY. PART n. 
 
 much smaller than the recent object, and showed no unusual 
 colour. 1 
 
 The assiduous observations made on the " Great Ked Spot " 
 by Mr. Denning at Bristol and by Professor Hough at Chicago 
 afforded grounds only for negative conclusions as to its nature. 
 It certainly did not represent the outpourings of a Jovian 
 volcano ; it was in no sense attached to the * Jovian soil if the 
 phrase have any application to that planet ; it was not a mere 
 disclosure of a glowing mass elsewhere seethed over by rolling 
 vapours. It was, indeed, almost certainly not self-luminous, a 
 satellite projected upon it in transit having been seen to show 
 as bright as upon the dusky equatorial bands. A fundamental 
 objection to all three hypotheses is that the rotation of the spot 
 was variable. It did not then ride at anchor, but floated free. 
 Some held that its surface was depressed below the average 
 cloud-level, and that the cavity was filled with vapours. Pro- 
 fessor H. C. Wilson, on the other hand, observing with the six- 
 teen-inch equatoreal of the Goodsell Observatory, in Minnesota, 
 received the uniform impression of the object " being at a higher 
 level than the other markings." 2 A crucial experiment on this 
 point was proposed by Mr. Stanley Williams in 1 890. 3 A dark 
 spot moving faster along the same parallel was timed to overtake 
 the red spot towards the end of July. A unique opportunity 
 hence appeared to be at hand of determining the relative vertical 
 depths of the two formations, one of which must inevitably, it 
 was thought, pass above the other. No forecast included a 
 third alternative, which was nevertheless adopted by the dark 
 spot. It evaded the obstacle in its path by skirting round its 
 southern edge. 4 Nothing, then, was gained by the conjunction, 
 beyond an additional proof of the singular repellent influence 
 exerted by the red spot over the markings in its vicinity. It 
 has, for example, gradually carved out a deep bay for its accom- 
 
 1 For indications relative to the early history of the red spot, see Holden, 
 Pull. Astr. Pac. Soc., vol. ii., p. 77; Noble, Month. Not., vol. xlvii., p. 515; 
 A. S. Williams, Observatory, vol. xiii., p. 338. 2 Astr. and Astro-Physics, 
 March 1892. p. 192. 3 Month. Not., vol. 1., p. 520. 4 Observatory, vol. xiii., 
 pp. 297, 326. 
 
CHAP. viii. PLANETS AND SATELLITES. 36* 
 
 modation in the grey belt just north of it. The effect was not at 
 first steadily present. Trouvelot, Barnard, and Elvins of Toronto, 
 delineated the excavation in 1879; yet there was no sign of it 
 in the following year. Its development can be traced in Dr. 
 Boeddicker's beautiful drawings of Jupiter, made with the Par- 
 soiistown three-foot reflector from 1881 to I886. 1 They record 
 the belt as straight in 1881, but as strongly indented from 
 January 1883 ; and the cavity now promises to outlast the spot. 
 So long as it survives, however, the forces at work in the spot 
 can have lost little of their activity. For it must be remembered 
 that the belt has a rotation-period five and a half minutes shorter 
 than the red spot, which, accordingly (as Mr. Elvins has re- 
 marked), breasts and diverts, by its interior energy, a current 
 of flowing matter, ever ready to fill up its natural bed, and 
 override the barrier of obstruction. 
 
 The famous spot was described by Keeler in 1889 as " of a 
 pale pink colour, slightly lighter in the middle. Its outline was 
 a fairly true ellipse, framed in by bright white clouds." 2 The 
 fading continuously in progress from 1887 was temporarily 
 interrupted in 1891. The revival, however, was brief. Pro- 
 fessor Barnard wrote in August 1892 : " The great red spot is 
 still visible, but it has just passed through a crisis that seemingly 
 threatened its very existence. For the past month it has been 
 all but impossible to catch the feeblest trace of the spot, though 
 the ever-persistent bay in the equatorial belt close north of it, 
 and which has been so intimately connected with the history of 
 the red spot, has been as conspicuous as ever. For a while there 
 was only the feeblest glow of warmth where the spot ought to 
 be. This has been the case under the very best seeing. It is 
 now, however, possible to detect a feeble outline of the following 
 end, and the feeblest traces of the entire spot. An obscuring 
 medium seems to have been passing over it, and has now drifted 
 somewhat preceding the spot." 3 
 
 Jupiter was systematically photographed with the Lick 
 thirty-six inch telescope during the oppositions of 1 890, 1 89 1 , 
 
 1 Trans. E. Dublin Soc., vol. iv., p. 271, 1889. 2 Pull. A*tr. Poc. /S'oc., 
 vol. ii.. p. 289. 3 Astr. and Astro-Physics, Oct. 1892, p. 686. 
 
362 HISTORY OF ASTRONOMY. PART n. 
 
 and 1892, the image thrown on the plates (after eightfold 
 direct enlargement) being one inch in diameter. Mr. Stanley 
 Williams's measurements and discussion of the set for 1891 
 showed the high value of the materials thus collected, although 
 much more minute details can be seen than can at present be 
 photographed. The red spot shows as " very distinctly annular " 
 in several of these pictures. 1 ^ 
 
 In 1 870, Mr. Ranyard, 2 acting upon an earlier suggestion of 
 Dr. Huggins, collected records of unusual appearances on the 
 disc of Jupiter, with a view to investigate the question of their 
 recurrence at regular intervals. He concluded that the develop- 
 ment of the deeper tinges of colour, and of the equatorial " port- 
 hole " markings girdling the globe in regular alternations of 
 bright and dusky, agreed, so far as could be ascertained, with 
 epochs of sun-spot maximum. The further inquiries of Dr. 
 Lohse at Bothkamp in 1873 3 went to strengthen the coincidence, 
 which had been anticipated a priori by Zollner in i87i. 4 Yet 
 subsequent experience has rather added to than removed doubts 
 as to the validity of that first conclusion. It may, indeed, be 
 taken for granted that what Hahn terms the universal pulse of 
 the solar system 5 affects the vicissitudes of Jupiter ; but the law 
 of those vicissitudes is far from being so obviously subordinate 
 to the rhythmical flow of central disturbance as are certain 
 terrestrial phenomena. The great planet being in fact himself 
 a "semi-sun," may be regarded as an originator, no less than a 
 recipient of agitating influences, the combined effects of which 
 may well appear insubordinate to any obvious law. 
 
 It is likely that Saturn is in a still earlier stage of planetary 
 development than Jupiter. He is the lightest for his size of all 
 the planets. In fact, he would float in water. And since his 
 density is shown, by the amount of his equatorial bulging to in- 
 crease centrally, 6 it follows that his superficial materials must be 
 
 1 Publ. Astr. Pac. Soc., vol. iv., p. 176. 2 Month. Not., vol. xxxi., p. 34. 
 
 3 Beobachtungen, Heft ii., p. 99. 4 Ber. Sac.h*. Get. der. Wiss., 1871, 
 
 p. 553. 5 Iteziehungen der Sonntnflecken periode, p. 175. 6 A. Hall, Astr. 
 Naeh., No. 2269. 
 
CHAP. vin. PLANETS AND SATELLITES. 363 
 
 of a specific gravity so low as to be inconsistent, on any probable 
 supposition, with the solid or liquid states. Moreover, the chief 
 arguments in favour of the high temperature of Jupiter apply, 
 with increased force, to Saturn ; so that it may be concluded, 
 without much risk of error, that a large proportion of his 
 bulky globe, 71,000 miles in diameter, is composed of heated 
 vapours, kept in active and agitated circulation by the process of 
 cooling. 
 
 His unique set of appendages has, since the middle of the 
 century, formed the subject of searching and fruitful inquiries, 
 both theoretical and telescopic. The mechanical problem of the 
 stability of Saturn's rings was left by Laplace in a very unsatis- 
 factory condition. Considering them as rotating solid bodies, 
 he pointed out that they could not maintain their position unless 
 their weight were in some way unsymmetrically distributed; 
 but made no attempt to determine the kind or amount of 
 irregularity needed to secure this end. Some observations by 
 Herschel gave astronomers an excuse for taking for granted the 
 fulfilment of the condition thus vaguely postulated ; and the 
 question remained in abeyance until once more brought 
 prominently forward by the discovery of the dusky ring in 
 1850. 
 
 The younger Bond led the way, among modern observers, in 
 denying the solidity of the structure. The fluctuations in its 
 aspect were, he asserted in I85I, 1 inconsistent with such an 
 hypothesis. The fine dark lines of division, frequently detected 
 in both bright rings, and as frequently relapsing into imper- 
 ceptibility, were due, in his opinion, to the real mobility of their 
 particles, and indicated a fluid formation. Professor Benjamin 
 Peirce of Harvard University immediately followed with a demon- 
 stration, on abstract grounds, of their non-solidity. 2 Streams of 
 some fluid denser than water were, he maintained, the physical 
 reality giving rise to the anomalous appearance first disclosed 
 by Galileo's telescope. 
 
 The mechanism of Saturn's rings, proposed as the subject 
 of the Adams Prize, was dealt with by the late James Clerk 
 1 Astr. Jour. (Gould's), vol. ii., p. 17. 2 Ibid., p. 5. 
 
364 HISTORY OF ASTRONOMY. PART n. 
 
 Maxwell in 1857. His investigation forms the groundwork of all 
 that is at present known in the matter. Its upshot was to show 
 that neither solid nor fluid rings could continue to exist, and 
 that the only possible composition of the system was by an 
 aggregated multitude of unconnected particles, each revolving 
 independently in a period corresponding to its distance from the 
 planet. 1 This idea of a satellite-formation had been, remarkably 
 enough, several times entertained and lost sight of. It was first 
 put forward by Roberval in the seventeenth century, again by 
 Jaques Cassini in 1715, and with perfect definiteness by Wright 
 of Durham in I75<D. 2 Little heed, however, was taken of these 
 casual anticipations of a truth which reappeared, a virtual 
 novelty, as the legitimate outcome of the most refined modern 
 methods. 
 
 The details of telescopic observation accord, on the whole, 
 admirably with this hypothesis. The displacements or dis- 
 appearance of secondary dividing-lines the singular striated 
 appearance, first remarked by Short in the eighteenth century, 
 last by Perrotin and Lockyer at Nice, March 18, i884 3 show 
 the effects of waves of disturbance traversing a moving mass of 
 gravitating particles ; 4 the broken and changing line of the 
 planet's shadow on the ring gives evidence of variety in the 
 planes of the orbits described by those particles. The whole 
 ring-system, too, appears to be somewhat elliptical. 5 
 
 The satellite-theory has derived unlooked-for support from 
 photometric inquiries. Professor Seeliger pointed out in 1888 6 
 that the unvarying brilliancy of the outer rings under all angles 
 of illumination, from o to 30, can be explained from no other 
 point of view. Nor does the constitution of the obscure inner 
 ring offer any difficulty. For it is doubtless formed of similar 
 small bodies to those aggregated in the lucid members of 
 the system, only much more thinly strewn, and reflecting, 
 
 1 On the Stability of the Motion of Saturn's Rings, p. 67. 2 Mem. de I' Ac., 
 1715, p. 47 ; Montucla, Hist. des. Math., t. iv., p. 19 ; An Original Theory of 
 the Universe, p. 115. 3 Comptes Rendus, t. xcviii., p. 718. 4 Proctor, 
 Saturn and its System (1865), p. 125. 5 Perrotin, Comptes Rendus, t. cvi. , 
 p. 1716. 6 Abhandl. Akad. der Wiss., Munich Bd. xvi., p. 407. 
 
CHAP. vni. PLANETS AND SATELLITES. 365 
 
 consequently, much less light. It is not, indeed, at first easy 
 to see why these sparser flights should show as a dense dark 
 shading on the body of Saturn. Yet this is invariably the case. 
 The objection has been urged by Professor Hastings of Baltimore. 
 The brightest parts of these appendages, he remarks, 1 are more 
 lustrous than the globe they encircle ; but if the inner ring 
 consists of identical materials, possessing presumably an equal 
 reflective capacity, the mere fact of their scanty distribution 
 would not cause them to show as dark against the same globe. 
 Professor Seeliger, however, replies 2 that the darkening is due 
 to the never-ending swarms of their separate shadows transiting 
 the planet's disc. Sunlight is not, indeed, wholly excluded. 
 Many rays come and go between the open ranks of the meteorites. 
 For the dusky ring is transparent. The planet it encloses shows 
 through it, as if veiled with a strip of crape. A beautiful illus- 
 tration of its quality in this respect was derived by Professor 
 Barnard from an eclipse of Japetus, November i, iSSp. 3 The 
 eighth moon remained steadily visible during its passage through 
 the shadow of the inner ring, but with a progressive loss of 
 lustre in approaching its bright neighbour. There was no 
 breach of continuity. The satellite met no gap, corresponding 
 to that between the dusky ring and the body of Saturn, through 
 which it could shine with undiminished light, but was slowly 
 lost sight of as it plunged into deeper and deeper gloom. The 
 important facts were thus established, that the brilliant and 
 obscure rings merge into each other, and that the latter thins 
 out towards the Saturnian globe. 
 
 A question of singular interest, and one which we cannot 
 refrain from putting to ourselves, is whether we see in the 
 rings of Saturn a finished structure, destined to play a perma- 
 nent part in the economy of the system ; or whether they 
 represent merely a stage in the process of development out of the 
 chaotic state in which it is impossible to doubt that the mate- 
 rials of all planets were originally merged. M. Otto Struve 
 
 1 Smiths. Heport, 1880 (Holden). 2 Quoted by Dr. E. Anding, Astr. NacJi., 
 No. 2881. 3 Astr. and Astro-Physics, Feb. 1892, p. 119; Month. Not., vol. 1., 
 p. 108. 
 
366 HISTORY OF ASTRONOMY. PART ir. 
 
 has attempted to give a definite answer to this important 
 query. 
 
 A study of early and later records of observations disclosed to 
 him, in 1851, an apparent progressive approach of the inner 
 edge of the bright ring to the planet. The rate of approach he 
 estimated at about sixty English miles a year, or 11,000 
 miles during the 194 years elapsed since the fcime of Huygens. 1 
 Were it to continue, a collapse of the system must be far 
 advanced within three centuries. But was the change real or 
 illusory a plausible, but deceptive inference from insecure data ? 
 M. Struve resolved to put it to the test. A set of elaborately 
 careful micrometrical measures of the dimensions of Saturn's 
 rings, executed by himself at Pulkowa in the autumn of 1851, 
 was provided as a standard of future comparison ; and he was 
 enabled to renew them, under closely similar circumstances, in 
 i882. 2 But the expected diminution of the space between 
 Saturn's globe and his rings had not taken place. There was, 
 indeed, a slight extension in the width of the system, both out- 
 ward and inward ; but so slight that it could hardly be considered 
 to lie outside the limits of probable error. Still it is worth 
 notice that just such a separation of the rings was indicated by 
 Clerk Maxwell's theory, so that there is an a priori likelihood of 
 its being in progress. Moreover, since 1657, when Huygens 
 described the interval between the ring and the planet as rather 
 exceeding the width of the ring, it is all but certain that a 
 growth inward has actually occurred. For the two bright rings 
 together, instead of being narrower than the interval, are now 
 more than once and a half times as broad. Hence the expres- 
 sions used by Huygens, no less than most of the old drawings, 
 are glaringly inconsistent with the planet's present appearance. 
 
 The rings of Saturn replace, in Professor G. H. Darwin's 
 view, 3 an abortive satellite, scattered by tidal action into annular 
 form. For they lie closer to the planet than is consistent with 
 
 1 Mem. de I Ac. Imp. (St. Petersb.), t. vii., 1853, p. 464. 2 A sir. Nach., No. 
 2498. Professor A. Hall's measures in 1886, with the great Washington 
 refractor, were absolutely negative as regards evidence of change : Washington 
 Observations, App. ii., p. 22. 3 Harper's Magazine, June 1889. 
 
CHAP. vin. PLANETS AND SATELLITES. 367 
 
 the integrity of a revolving body of reasonable bulk. The limit 
 of possible existence for such a mass was fixed by Roche of 
 Montpellier, in I848, 1 at 2.44 mean radii of its primary; while 
 the outer edge of the ring-system is distant 2.38 radii of Saturn 
 from his centre. The virtual discovery of its pulverulent con- 
 dition dates, then, according to Professor Darwin, from 1848. 
 He conjectures that the appendage will eventually disappear, 
 partly through the dispersal of its constituent particles inwards, 
 and their gradual collapse upon the planet's surface, partly by 
 their dispersal outwards, to a region beyond " Roche's limit," 
 where coalescence might proceed unhindered by the strain of 
 unequal attractions. One modest satellite, revolving inside 
 Mimas, would then be all that was left of the singular appur- 
 tenances we now contemplate with admiration. 
 
 There seems reason to admit that Kirkwood's law of com- 
 mensurability has had some effect in bringing about the present 
 distribution of the matter composing these appendages. Hera 
 the disturbing bodies are Saturn's moons, while the divisions 
 and boundaries of the rings represent the spaces where their 
 disturbing action conspires to eliminate revolving particles. 
 Kirkwood, in fact, showed, in 1 86/, 2 that a body circulating in 
 the chasm between the bright rings known as " Cassini's division," 
 would have a period nearly commensurable with those of four out 
 of the eight moons ; and Dr. Meyer of Geneva has since calcu- 
 lated all such combinations with the result of bringing out 
 coincidences between regions of maximum perturbation and the 
 limiting and dividing lines of the system. 3 This is in itself a 
 strong confirmation of the view that the rings are made up of 
 independently revolving small bodies. 
 
 On December 7, 1876, Professor Asaph Hall discovered at 
 Washington a bright equatorial spot on Saturn, which he 
 followed and measured through above sixty rotations, each 
 performed in ten hours fourteen minutes twenty-four seconds. 4 
 
 1 See also, Mem*, de I'Acad. de Montpellier, t. viii., p. 296, 1873. ~ Meteoric 
 Astronomy, chap. xii. He carried the subject somewhat farther in 1871. See 
 Observatory, vol. vi., p. 335. s Asfr. Nach., No. 2527. 4 Am. Jour, of Sc. y 
 vol. xiv., p. 325. 
 
368 HISTORY OF ASTRONOMY. PART n. 
 
 He was careful to add that this represents the period, not 
 necessarily of the planet, but only of the individual spot. The 
 only previous determination of Saturn's axial movement (setting 
 aside some insecure -estimates .^by Schroter) was Herschel's in 
 1794, giving a period of ten hours sixteen minutes. The 
 substantial accuracy of Hall's result was verified by Mr. Den- 
 ning in 1 89 1. 1 From ten white equatorial -spots observed in 
 May and June of that year by Mr. Stanley Williams, he derived 
 a rotation-period only two seconds longer than that deter- 
 mined at Washington, to which a dark spot conformed exactly. 
 Individual discrepancies, however, to the extent of twenty 
 seconds, showed that the spots were not devoid of motions of 
 their own. 
 
 Saturn's outermost satellite, Japetus, is markedly variable 
 so variable that it sends us, when brightest, just 4^ times as 
 much light as when faintest. Moreover, its fluctuations depend 
 upon its orbital position in such a way as to make it a con- 
 spicuous telescopic object when west, a scarcely discernible one 
 when east of the planet. Herschel's inference 2 of a partially 
 obscured globe turning always the same face towards its 
 primary, seems the only admissible one, and is confirmed by 
 Pickering's measurements of the varying intensity of its light. 
 He remarks further that the dusky and brilliant hemispheres 
 must be so disposed as to divide the disc, viewed from Saturn, 
 into nearly equal parts ; so that this Saturnian moon, even when 
 *' full," appears very imperfectly illuminated over one-half of its 
 surface. 3 
 
 The spectrum of Saturn is closely similar to that of Jupiter. 
 It shows the distinctive dark line in the red (wave-length 
 6 1 8), which we may call the "red-star line"; and Janssen, 
 examining it from the summit of Etna in 1867,* found 
 unmistakable traces of aqueous absorption. The light from 
 the ring appears to be entirely unmodified by original atmo- 
 spheric action. 5 
 
 1 Observatory, vol. xiv., p. 369. 2 Phil. Trans., vol. Ixxxii., p. 14. 
 3 SmitJis. Report, 1880. 4 Comptes Rendus, t. Ixiv. p. 1304. 5 Huggins, 
 Proc. fi. Soc., vol. xlvi., p. 231 ; Keeler, Astr. Nach., No. 2927. 
 
CHAP. vni. PLANETS AND SATELLITES. 369 
 
 Uranus, when favourably situated, can easily be seen with the 
 naked eye as a star between the fifth and sixth magnitudes. 
 There is, indeed, some reason to suppose that he had been 
 detected as a wandering orb by savage " watchers of the skies " 
 in the Pacific long before he swam into Herschel's ken. Never- 
 theless, inquiries into his physical habitudes are still in an early 
 stage. They are exceedingly difficult of execution, even with 
 the best and largest modem telescopes ; and their results remain 
 so far more or less clouded with uncertainty. 
 
 It will be remembered that Uranus presents the unusual 
 spectacle of a system of satellites travelling nearly at right 
 angles to the plane of the ecliptic. The existence of this 
 anomaly gives a special interest to investigations of his axial 
 movement, which might be presumed, from the analogy of the 
 other planets, to be executed in the same tilted plane. Yet this 
 is far from being certainly the case. 
 
 Mr. Buffham in 187072 caught traces of bright markings on 
 the Uranian disc, doubtfully suggesting a rotation in about 
 twelve hours in a plane not coincident with that in which his 
 satellites circulate. 1 Dusky bands resembling those of Jupiter, 
 but very faint, were barely perceptible to Professor Young at 
 Princeton in 1883. Yet, though almost necessarily inferred to 
 be equatorial, they made a considerable angle with the trend of 
 the satellites' orbits. 2 More distinctly by the brothers Henry, 
 with the aid of their fine refractor, two grey parallel rulings, 
 separated by a brilliant zone, were discerned every clear night 
 at Paris from January to June i884. 3 What were taken to be 
 the polar regions appeared comparatively dusky. The direction 
 of the equatorial rulings (for so we may safely call them) made 
 an angle of 40 with he satellites' line of travel. Similar 
 observations were made at Nice by MM. Perrotiii and Thollon, 
 March to June 1884, a lucid spot near the equator, in addi- 
 tion, indicating rotation in a period of about ten hours. 4 
 The discrepancy was, however, considerably reduced by Per- 
 rotin's study of the planet in 1889 with the new thirty-inch 
 
 1 Month. Not., vol. xxxiii., p. 164. 2 Astr. Rack., No. 2545. 3 Comptes 
 Rendus, t. xcviii., p. 1419. 4 Ibid., pp. 718, 967. 
 
 24 
 
370 HISTORY OF ASTRONOMY. PART n. 
 
 equatoreal. 1 The dark bands, thus viewed to better advantage 
 than in 1884, appeared to deviate no more than 10 from the 
 satellites' orbit-plane. No definitive results, on the other hand, 
 were derived by Professors Holden, Schaeberle, and Keeler, 
 from their observations of Uranus in 188990 with the potent 
 instrument on Mount Hamilton. Shadings, it is true, were 
 almost always, though faintly, seen ; but. they showed an 
 anomalous character. They consisted, not of parallel, but of 
 forked bands. 2 The possibility thus suggests itself that Uranus 
 may display two systems of markings, one indicating a vertical 
 circulation on the model of that of Jupiter, the other resulting 
 from horizontal convection-currents, such as prevail in the ter- 
 restrial atmosphere, necessarily rendered oblique by the effects of 
 rotation. Inconsistent observations could, if this were the case, be 
 explained by differences in the relative conspicuousness of the 
 straight or oblique shadings. 
 
 Measurements of the little sea-green disc which represents 
 to us the massive bulk of Uranus, by Young, Schiaparelli, 3 
 Safarik, H. C. Wilson of Cincinnati, 4 and Perrotin, prove it to 
 be quite distinctly bulged ; and all agree that the bulging lies 
 just in the plane of the satellites' orbits. If this be so, there 
 can be no question but that the same plane is that of the 
 planet's rotation, the spheroidal shape of a rotating globe being 
 the necessary consequence of the greater equatorial velocity of 
 its particles. The tendency to conformity, too, of late visible 
 in the dusky streaks, renders the peculiarities of the Uranian 
 aspect less puzzling. 
 
 The spectrum of this planet was first examined by Father 
 Secchi in 1869, and later, though with more advantages for 
 accuracy, by Huggins and Vogel. It is a very remarkable 
 one. In lieu of the reflected Fraunhofer lines, imperceptible 
 perhaps through feebleness of light, six broad bands of original 
 absorption appear, 5 one corresponding to the blue-green ray 
 
 1 V. J. S. Astr. Ges., Jahrg. xxiv., p. 267. - Publ. Astr. Pac. Soc., vol. iii., 
 p. 287. 3 Astr. Nach., No. 2526. 4 Jlid., No. 2730. Seeliger found no 
 perceptible compression in 1884 ; Sitzungsb., Munich, p. 278. 5 Vogel, 
 Annalen der Pliys., vol. clviii., p. 470. 
 
CHAP. vin. PLANETS AND SATELLITES. 371 
 
 of hydrogen (F), another to the " red-star line " of Jupiter and 
 Saturn, the rest as yet unidentified. The hydrogen band seems 
 much too strong and diffuse to be the mere echo of a solar line, 
 and implies accordingly the presence of free hydrogen in the 
 "[Iranian atmosphere, where a temperature must thus prevail 
 sufficiently high to reduce water to its constituent elements. 
 Notwithstanding its high albedo 0.62, according to Zollner 
 there is at present no evidence that any of the light of Uranus 
 is inherent. Mr. Albert Taylor announced indeed, in 1889, his 
 detection, with Dr. Common's giant reflector, of bright flutings 
 in its spectrum ; l but Professor Keeler's examination proved 
 them to be merely contrast effects. 2 Dr. and Mrs. Huggins, 
 moreover, obtained about the same time a photograph purely 
 solar in character. The spectrum it represented was crossed by 
 numerous Fraunhofer lines, and by no others. It was then 
 composed, presumably, entirely of reflected light. 
 
 Judging from the indications of an almost evanescent spectrum, 
 Neptune, as regards physical condition, is the twin of Uranus, 
 as Saturn of Jupiter. Of the circumstances of his rotation we 
 are as good as completely ignorant. Mr. Maxwell Hall, indeed, 
 noticed at Jamaica, in November and December 1883, certain 
 rhythmical fluctuations of brightness, suggesting revolution on 
 an axis in slightly less than eight hours; 3 but Professor 
 Pickering reduces the supposed variability to an amount alto- 
 gether too small for certain perception, and Dr. G. Miiller denies 
 its existence in toto. It is true their observations were not. 
 precisely contemporaneous with those of Mr. Hall, 4 who believes 
 the partial obscurations recorded by himself to have been of a 
 passing kind, and to have suddenly ceased after a fortnight of 
 prevalence. Their less conspicuous renewal was visible to him 
 in November 1884, confirming a rotation-period of 7.92 hours. 
 
 It has been ascertained by indirect means that the orbit of 
 Neptune's satellite is inclined at least 20 to his equator. Mr. 
 
 1 Month. Not., vol. xlix., p. 405. " Astr. Nach., No. 2927; Schemer's 
 jSpectralanalyse, p. 221. 3 Month. Not., vol. xliv., p. 257. 4 Observatory, 
 voL vii., pp. 134, 221, 264. 
 
372 HISTORY OF ASTRONOMY. PART n. 
 
 Marth 1 having drawn attention to the rapid shifting of its plane 
 of motion, M. Tisserand and Professor Newcomb 2 independ- 
 ently published the conclusion that such shifting is a necessary 
 result of Neptune's ellipsoidal shape. The movement is of 
 the kind exemplified although with inverted relations in 
 the precession of the equinoxes. The pole of the satellite's 
 orbit, owing to the pull of Neptune's equatorial protuberance, 
 describes a circle round the pole of his equator, in a retrograde 
 direction, and in a period of over five hundred years. The 
 amount of compression indicated for the primary body is, at the 
 outside, gV ; whence it can be inferred that Neptune possesses 
 a lower rotatory velocity than the other giant planets. 
 
 The possibility that Neptune may not be the most remote 
 body circling round the sun has been contemplated ever since 
 he has been known to exist. Within the last few years the 
 position at a, given epoch of a planet far beyond him has been 
 approximately fixed by two separate investigators. 
 
 Professor George Forbes of Edinburgh adopted in 1880 a 
 novel plan of search for unknown members of the solar system, 
 the first idea of which was thrown out by M. Flammarion in 
 November i879. 3 It depends upon the movements of comets. 
 It is well known that those of moderately short periods are, 
 for a reason already explained, connected with the larger planets 
 in such a way that the cometary aphelia fall near some planetary 
 orbit. Jupiter claims above a score of such partial dependants, 
 Neptune owns five, and there are two considerable groups, the 
 farthest distances of which from the sun lie respectively near 
 100 and 300 times that of the earth. At each of these vast 
 intervals, one involving a period of IOOO, the other of .5000 
 years, Professor Forbes maintains that an unseen planet circu- 
 lates. He has even computed elements for the nearer of the 
 two, and fixed its place on the celestial sphere. 4 
 
 1 Month. Not, vol. xlvi., p. 507. 2 Comptes Rendus, t. cvii., p. 804; Astr. 
 Jour., No. 1 86. 3 Astr. Pop., p. 66 1 ; La Nature, Jan. 3, 1880. 4 Proc. 
 Hoy. Soc. Ediiib. , vols. x.,p. 429; xi.,p. 89. M. Deichmiiller (VierteljaJirsschrift 
 Astr. Ges., Jahrg. xxi., p. 206) pointed out in 1886 that cometary orbits ex- 
 tending beyond the distance of Neptune are too imperfectly known to serve 
 as the basis of anv trustworthv conclusion. 
 
CHAP. viii. PLANETS AND SATELLITES. 373 
 
 In the meantime, Professor Todd had been groping for the 
 same object by the help of a totally different set of indications. 
 The old approved method of perturbations was that adopted by 
 him ; but those of Neptune have scarcely yet had time to 
 develop, so that he was thrown back upon the "residual errors " 
 of Uranus. They gave him a virtually identical situation for 
 the new planet with that arrived at by Professsor Forbes. 1 
 Yet its assigned distance was little more than half that of the 
 nearer of Professor Forbes's remote pair, and it completed a 
 revolution in 375, instead of 1000 years. The agreement in 
 position, then, of the two hypothetical bodies was merely an 
 odd coincidence; and the telescopic search for the one has 
 proved as fruitless as the photographic search for the other. 2 
 
 1 Amer. Jour, of Science, vol. xx., p. 225. 2 Koberts, Month. Not., vol. lii., 
 p. 501. 
 
CHAPTER IX. 
 THEORIES OF PLANETARY EVOLUTION. 
 
 WE cannot doubt that the solar system, as we see it, is the 
 result of some process of growth that, during innumerable 
 ages, the forces of Nature were at work upon its materials, 
 blindly modelling them into the shape appointed for them from 
 the beginning by Omnipotent Wisdom. To set ourselves to 
 inquire what that process was, may be an audacity, but it is a 
 legitimate, nay, an inevitable one. For man's implanted in- 
 stinct to " look before and after " does not apply to his own little 
 life alone, but regards the whole history of creation, from 
 the highest to the lowest from the microscopic germ of an 
 alga or a fungus to the visible frame and furniture of the 
 heavens. 
 
 Kant considered that the inquiry into the mode of origin of 
 the world was one of the easiest problems set by Nature ; but it 
 cannot be said that his own solution of it was a satisfactory one. 
 He, however, struck out in 1755 a track which thought still 
 pursues. In his Allgemeine Naturgescliiclite the growth of sun 
 and planets was traced from the cradle of a vast and formless 
 mass of evenly diffused particles, and the uniformity of their 
 movements was sought to be accounted for by the uniform action 
 of attractive and repulsive forces, under the dominion of which 
 their development was carried forward. 
 
 In its modern form, the " Nebular Hypothesis " made its 
 appearance in I796. 1 It was presented by Laplace with 
 diffidence, as a speculation unfortified by numerical buttresses 
 
 1 Exposition du Systeme du Monde, t. ii., p. 295. 
 
CHAP. ix. PLANETARY EVOLUTION. 375 
 
 of any kind, yet with visible exultation in having, as he thought, 
 penetrated the birth-secret of our system. He demanded, 
 indeed, more in the way of postulates than Kant had done. He 
 started with a sun ready made, 1 and surrounded with a vast 
 glowing atmosphere, extending into space out beyond the orbit 
 of the farthest planet, and endowed with a slow rotatory motion. 
 As this atmosphere or nebula cooled, it contracted ; and as it 
 contracted, its rotation, by a well-known mechanical law, became 
 accelerated. At last, a point arrived when tangential velocity at 
 the equator increased beyond the power of gravity to control, 
 and equilibrium was restored by the separation of a nebulous 
 ring revolving in the same period as the generating mass. After 
 a time, the ring broke up into fragments, all eventually reunited 
 in a single revolving and rotating body. This was the first and 
 farthest planet. 
 
 Meanwhile the parent nebula continued to shrink and whirl 
 quicker and quicker, passing, as it did so, through successive 
 crises of instability, each resulting in, and terminated by, the 
 formation of a planet, at a smaller distance from the centre, and 
 with a shorter period of revolution than its predecessor. In 
 these secondary bodies the same process was repeated on a 
 reduced scale, the birth of satellites ensuing upon their contrac- 
 tion, or not, according to circumstances. Saturn's ring, it was 
 added, afforded a striking confirmation of the theory of annular 
 separation, 2 and appeared to have survived in its original form 
 in order to throw light on the genesis of the whole solar system ; 
 while the four first discovered asteroids offered an example in 
 which the ddbris of a shattered ring had failed to coalesce into a 
 single globe. 
 
 This scheme of cosmical evolution was a characteristic 
 bequest from the eighteenth century to the nineteenth. It 
 possessed the self-sufficing symmetry and entireness appropriate 
 to the ideas of a time of renovation, when the complexity of 
 -nature was little accounted of in comparison with the imperious 
 orderliness of the thoughts of man. Since it was propounded, 
 
 1 In later editions a retrospective clause was added admitting a prior condi- 
 tion of all but evanescent nebulosity. 2 Mec. Cd., lib. xiv., ch. iii. 
 
376 HISTORY OF ASTRONOMY. PART n. 
 
 however, knowledge has transgressed many boundaries, and set 
 at naught much ingenious theorising. How has it fared with 
 Laplace's sketch of the origin of the world ? It has at least not 
 been discarded as effete. The .groundwork of speculation on the 
 subject is still furnished by it. It is, nevertheless, admittedly 
 inadequate. Of much that exists it gives no account, or an 
 erroneous one. The march of events certainly did not every- 
 where even if it did anywhere follow the exact path prescribed 
 for it. Yet modern science attempts to supplement, but scarcely 
 ventures to supersede it. 
 
 Thought has, in many directions, been profoundly modified 
 by Mayer's and Joule's discovery, in 1842, of the equivalence 
 between heat and motion. Its corollary was the grand idea of 
 the " conservation of energy," now one of the cardinal principles 
 of science. This means that, under the ordinary circumstances 
 of observation, the old maxim ex nihilo nihil fit applies to force 
 as well as to matter. The supplies of heat, light, electricity, 
 must be kept up, or the stream will cease to flow. The question 
 of the maintenance of the sun's heat was thus inevitably raised ; 
 and with the question of maintenance that of origin is indis- 
 solubly connected. 
 
 Dr. Julius Robert Mayer, a physician residing at Heilbronn, 
 was the first to apply the new light to the investigation of what 
 Sir John Herschel had termed the " great secret." He showed 
 that if the sun were a body either simply cooling or in a state 
 of combustion, it must long since have " gone out." Had an 
 equal mass of coal been set alight, four or five centuries after 
 the building of the Pyramid of Cheops, and kept burning at 
 such a rate as to supply solar light and heat during the interim, 
 only a few cinders would now remain in lieu of our undiminished 
 glorious orb. Mayer looked round for an alternative. He found 
 it in the " meteoric hypothesis " of solar conservation. 1 The 
 importance in the economy of our system of the bodies known 
 as falling stars was then (in 1848) beginning to be recognised. 
 It was known that they revolved in countless swarms round the 
 sun ; that the earth daily encountered millions of them ; and it 
 1 Beitrage zur Dyn.am.ik des Himmels, p. 12. 
 
CHAP. ix. PLANETARY EVOLUTION. 377 
 
 was surmised that the cone of the zodiacal light represented 
 their visible condensation towards the attractive centre. From 
 the zodiacal light, then, Mayer derived the store needed for 
 maintaining the sun's radiations. He proved that, by the stop- 
 page of their motion through falling into the sun, bodies would 
 evolve from 4600 to 9200 times as much heat (according to their 
 ultimate velocity) as would result from the burning of equal 
 masses of coal, their precipitation upon the sun's surface being 
 brought about by the resisting medium observed to affect the 
 revolutions of Encke's comet. . There was, however, a difficulty. 
 The quantity of matter needed to keep, by the sacrifice of its 
 movement, the hearth of our system warm and bright, would 
 be very considerable. Mayer's lowest estimate put it at 94,000 
 billion kilogrammes per second, or a mass equal to that of our 
 moon bi-annually. But so large an addition to the gravitating 
 power of the sun would quickly become sensible in the move- 
 ments of the bodies dependent upon him. Their revolutions 
 would be notably accelerated. Mayer admitted that each year 
 would be shorter than the previous one by a not insignificant 
 fraction of a second, and postulated an unceasing waste of sub- 
 stance, such as Newton had supposed must accompany emission 
 of the material corpuscles of light, to neutralise continual re- 
 inforcement. 
 
 Mayer's views obtained a very small share of publicity, and 
 owned Mr. Waterston as their independent author in this 
 country. The meteoric, or "dynamical" theory of solar sus- 
 tentation was expounded by him before the British Association 
 in 1853. It was developed with his usual ability by Sir William 
 Thomson in the following year. The inflow of meteorites, he 
 remarked, " is the only one of all conceivable causes of solar 
 heat which we know to exist from independent evidence." 1 We 
 know it to exist, but we now also know it to be entirely in- 
 sufficient. The supplies presumed to be contained in the 
 zodiacal light would be quickly exhausted; a constant inflow 
 from space would be needed to meet the demand. But if moving 
 bodies were drawn into the sun at anything like the required 
 1 Trans. Roy. Soc. of Edinburgh, vol. xxi., p. 66. 
 
378 HISTORY OF ASTRONOMY. PART 11. 
 
 rate, the air, even out here at ninety-three millions of miles 
 distance, would be thick with them ; the earth would be red-hot 
 from their impacts ; * geological deposits would be largely 
 meteoric; 2 to say nothing of.^the effects on the mechanism of 
 the heavens. Sir William Thomson himself urged the in- 
 admissibility of the "extra-planetary" theory of meteoric 
 supply on the very tangible ground that, if it were true, the 
 year would be shorter now, actually by six weeks, than at the 
 opening of the Christian era. The " intra-planetary " supply, 
 however, is too scanty to be anything more than a temporary 
 makeshift. 
 
 The meteoric hypothesis was naturally extended from the 
 maintenance of the sun's heat to the formation of the bodies 
 circling round him. The earth no less doubtless than the 
 other planets is still growing. Cosmical matter in the shape 
 of falling stars and aerolites, to the amount, adopting Professor 
 Newton's estimate, of 100 tons daily, is swept up by it as it 
 pursues its orbital round. Inevitably the idea suggested itself 
 that this process of appropriation gives the key to the life-history 
 of our globe, and that the momentary streak of fire in the 
 summer sky represents a feeble survival of the glowing hail- 
 storm by which, in old times, it was fashioned and warmed. 
 Mr. E. W. Brayley supported this view of planetary production 
 in i864, 3 and it has recommended itself to Haidinger, Helmholtz, 
 Proctor, and Faye. But the negative evidence of geological 
 deposits appears fatal to it. 
 
 The theory of solar energy now generally regarded as the true 
 one, was enounced by Helmholtz in a popular lecture in 1854. 
 It depends upon the same principle of the equivalence of heat 
 and motion which had suggested the meteoric hypothesis. But 
 here the movement surrendered and transformed belongs to the 
 particles, not of any foreign bodies, but of the sun itself. Drawn 
 together from a wide ambit by the force of their own gravity, 
 their fall towards the sun's centre must have engendered a vast 
 thermal store, of which |f are computed to be already spent. 
 
 1 Newcomb, Pop. Aslr., p. 521 (2nd ed.). 2 M. Williams, Nature, vol. iii., 
 p. 26. 3 Comp. Brit. Almanac, p. 94. 
 
CHAP. ix. PLANETARY EVOLUTION. 379 
 
 Presumably, however, this stream of reinforcement is still 
 flowing. In the very act of parting with heat, the sun develops 
 a fresh stock. His radiations, in short, are the direct result of 
 shrinkage through cooling. A diminution of the solar diameter 
 by 380 feet yearly would just suffice to cover the present rate of 
 emission, and would for ages remain imperceptible with our 
 means of observation, since, after the lapse of 6000 years, the 
 lessening of angular size would scarcely amount to one second. 1 
 But the process, though not terminated, is strictly a terminable 
 one. In less than five million years the sun will have contracted 
 to half its present bulk. In seven million more, it will be as 
 dense as the earth. It is difficult to believe that it will then be 
 a luminous body. 2 Nor can an unlimited past duration be 
 admitted. Helmholtz considered that radiation might have 
 gone on with its actual intensity for twenty-two, Langley allows 
 only eighteen million years. The period can scarcely be stretched, 
 by the most generous allowances, to double the latter figure. 
 But this is far from meeting the demands of geologists and 
 biologists. 
 
 An attempt was made in 1881 to supply the sun with 
 machinery analogous to that of a regenerative furnace, enabling 
 it to consume the same fuel over and over again, and so to pro- 
 long indefinitely its beneficent existence. The inordinate 
 " waste " of energy, which shocks our thrifty ideas, was simul- 
 taneously abolished. The earth stops and turns variously to 
 account one 225o-millionth part of the solar radiations ; each of 
 the other planets and satellites takes a proportionate share ; the 
 rest, being all but an infinitesimal fraction of the whole, is 
 dissipated through endless space, to serve what purpose we 
 know not. Now, on the late Sir William Siemens's plan, this 
 reckless expenditure would cease ; the solar incomings and out- 
 goings would be regulated on approved economic principles, and 
 the inevitable final bankruptcy would be staved off to remote 
 ages. Let us see how it is to be done. 
 
 We must first imagine space to be filled with combustible 
 substances hydrogen, hydro-carbons, and oxygen in an ex- 
 1 Eadau, Bull. Astr., t. ii., p. 316. 2 Newcomb, Pop. Astr., pp. 521-525. 
 
380 HISTORY OF ASTRONOMY. PART n. 
 
 cessively rarefied state. Next, that the sun keeps up, by its 
 rotation, a fan-like action on this floating matter, drawing it 
 inward at the polar surfaces, and projecting it outward at the 
 equator "in a continuous dislike stream." 1 But it will not 
 travel from the sun unchanged. Combustion will have inter- 
 vened. In other words, the particles sucked in will have 
 surrendered their stored up energy in the 'shape of heat and 
 light, and they will depart, no longer combustible, but the mere 
 inert products of combustion. By the very power of the radia- 
 tions they had contributed to supply, however, they may be 
 restored to activity. Sir W. Siemens obtained, as he thought, 
 some experimental evidence that carbonic acid and water may 
 possibly be dissociated in space, as they undoubtedly are in the 
 leaves of plants by the power of direct sunshine. Their par- 
 ticles, thus compulsorily separated, and by the act restocked 
 with energy, are ready to rush together again with fresh 
 evolution of heat and light. A mechanical circulation is, 
 in this way, combined with a pendulum-swing of chemical 
 change, and the round might go on for ever, if only one 
 condition were granted. That one condition is an unlimited 
 supply of motive power. It is, however, an inexorable law 
 of nature that there is no work without waste. Ex nihilo 
 nihil Jit. 
 
 In this case, the heart-throb of the circulating system resides 
 in the rotation of the sun. Therein is contained a certain 
 definite amount of mechanical power enough, according to Sir 
 W. Thomson, if directly converted into heat, to keep up the 
 sun's emission during 116 years and six days a mere moment 
 in cosmical time. More economically applied, it would no doubt 
 go farther. Its exhaustion would nevertheless, under the most 
 favourable circumstances, ensue in a comparatively short period. 2 
 Many other objections equally unanswerable have been urged to 
 the "regenerative" hypothesis, but this one suffices. 
 
 1 Proc. Boy. Soc., vol. xxxiii. , p. 393. 2 To this hostile argument, as urged 
 by Mr. E. Douglas Archibald, Sir W. Siemens opposed the increase of rotative 
 velocity through contraction (Nature, vol. xxv. p. 505). But contraction 
 cannot restore lost momentum. 
 
CHAP. ix. PLANETARY EVOLUTION. 381 
 
 Dr. Croll's collision-hypothesis is 1 less demonstrably unsound, 
 but scarcely less unsatisfactory. By the mutual impact of two 
 dark masses rushing together with tremendous speed, he sought 
 to provide the solar nebula with an immense original stock of 
 heat for the reinforcement of that subsequently evolved in the 
 course of its progressive contraction. The sun, while still living 
 on its capital, would thus have a larger capital to live on, and 
 the time-demands of the less exacting geologists and biologists 
 might be successfully met. But the primitive event, assumed 
 for the purpose of dispensing them from the inconvenience of 
 " hurrying up their phenomena," is not one that a sane judg- 
 ment can readily admit to have ever, in point of actual fact, 
 happened. 
 
 There remains, then, as the only intelligible rationale of solar 
 sustentation, Helmholtz's shrinkage theory. And this has a 
 very important bearing upon the nebular view of planetary 
 formation ; it may, in fact, be termed its complement. For it 
 involves the postulate that the sun's materials, once enormously 
 diffused, gradually condensed to their present volume with 
 development of heat and light, and, it may plausibly be added, 
 with the separation of dependent globes. The data furnished by 
 spectrum analysis, too, favour the supposition of a common origin 
 for sun and planets by showing their community of substance ; 
 while gaseous nebulae present examples of vast masses of tenuous 
 vapour, such as our system may plausibly be conjectured to have 
 primitively sprung from. 
 
 But recent science raises many objections to the details, if 
 it supplies some degree of confirmation to the fundamental 
 idea of Laplace's cosmogony. The detection of the retrograde 
 movement of Neptune's satellite made it plain that the anoma- 
 lous conditions of the Uranian world were due to no extra- 
 ordinary disturbance, but to a systematic variety of arrangement 
 at the outskirts of the solar domain. So that, were a trans- 
 Neptunian planet discovered, we should be fully prepared to find 
 it rotating, and surrounded by satellites circulating from east 
 to west. The uniformity of movement, upon the probabilities 
 1 Stellar Evolution, and its Relations to Geological Time, 1889. 
 
382 HISTORY OF ASTRONOMY. PART n. 
 
 connected with which the French geometer mainly based his 
 scheme, thus at once vanishes. 
 
 The excessively rapid revolution of the inner Martian moon 
 is a further stumbling-block. <* On Laplace's view, no satellite 
 can revolve in a shorter time than its primary rotates ; for in 
 its period of circulation survives the period of rotation of the 
 parent mass which filled the sphere of its o*bit at the time of 
 giving it birth. And rotation quickens as contraction goes on ; 
 therefore, the older time of axial rotation should invariably be 
 the longer. This obstacle can, however, it seems, be turned. 
 
 More serious is one connected with the planetary periods, 
 pointed out by Babinet in I86I. 1 In order to make them fit in 
 with the hypothesis of successive separation from a rotating and 
 contracting body, certain arbitrary assumptions have to be made 
 of fluctuations in the distribution of the matter forming that 
 body at the various epochs of separation. 2 Such expedients 
 usually merit the distrust which they inspire. 
 
 Again, it was objected by Professor Kirkwood in i869 3 that 
 there could be no sufficient cohesion in such an enormously 
 diffused mass as the planets are supposed to have sprung from, 
 to account for the wide intervals between them. The matter 
 separated through the growing excess of centrifugal speed, would 
 have been cast off, not by rarely recurring efforts, but continually, 
 fragmentarily, pari passu with condensation and acceleration. 
 Each wisp of nebula, as it found itself unduly hurried, would 
 have declared its independence, and set about revolving and 
 condensing on its own account. The result would have been a 
 meteoric, not a planetary system. 
 
 Moreover, it is a question whether the relative ages of the 
 planets do not follow an order just the reverse of that concluded 
 by Laplace. Professor Newcomb holds the opinion that the 
 rings which eventually constituted the planets divided from the 
 main body of the nebula almost simultaneously, priority, if there 
 were any, being on the side of the inner and smaller ones ; * 
 
 1 Comptes Bendus, t. Hi., p. 481. See also Kirkwood, Observatory, vol. iii., 
 p. 409. 2 Fouche, Comptes Rendus, t. xcix., p. 903. 3 Month. Not., vol. 
 xxix., p 96. 4 Pop. Astr., p. 257. 
 
CHAP. ix. PLANETARY EVOLUTION. 383 
 
 while in M. Faye's cosmogony, 1 the retrograde motion of the 
 systems formed by the two outer planets is ascribed on 
 grounds, it is true, of dubious validity to their comparatively 
 late origin. 
 
 This ingenious scheme is designed, not merely to complete, 
 but to supersede that of Laplace, which, undoubtedly, through 
 the inclusion by our system of oppositely directed rotations, 
 forfeits its claim simply and singly to account for the funda- 
 mental peculiarities of its structure. 
 
 M. Faye's leading contention is that, under the circumstances 
 assumed by Laplace, not the two outer planets alone, but the 
 whole company, must have been possessed of retrograde rotation. 
 For they were formed ex hypothesi after the sun ; central con- 
 densation had reached an advanced stage when the rings they 
 were derived from separated ; the principle of inverse squares 
 consequently held good, and Kepler's Laws were in full operation. 
 Now particles circulating in obedience to these laws can only 
 since their velocity decreases outward from the centre of 
 attraction coalesce into a globe with a backward axial move- 
 ment. Nor was Laplace blind to this flaw in his theory ; but 
 his effort to remove it, though it passed muster for the best part 
 of a century, 2 was scarcely successful. His planet-forming 
 rings were made to rotate all in one piece, their outer parts 
 thus necessarily travelling at a swifter linear rate than their 
 inner parts, and eventually uniting, equally of necessity, into a 
 fonoard-spuming body. The strength of cohesion involved may, 
 however, safely be called impossible, especially when it is con- 
 sidered that nebulous materials were in question. 
 
 The reform proposed by M. Faye consists in admitting that 
 all the planets inside Uranus are of pre-solar origin that they 
 took globular form in the bosom of a nearly homogeneous nebula, 
 revolving in a single period, with motion accelerated from centre 
 to circumference, and hence agglomerating into masses with a 
 direct rotation. Uranus and Neptune owe their exceptional 
 characteristics to their later birth. When they came into exist- 
 
 1 Sur VOriginp, du Monde, 1884. 2 Kirkwood adverted to it in 1864, Am. 
 Jour., vol. xxxviii., p. i. 
 
384 HISTORY OF ASTRONOMY. PART n. 
 
 ence, the development of the sun was already far advanced, 
 central force had acquired virtually its present strength, unity of 
 period had been abolished by its predominance, and motion was 
 retarded outward. 
 
 Thus, what we may call the relative chronology of the solar 
 system is thrown once more into confusion. The order of 
 seniority of the planets is now no easier to'-determine than the 
 " Who first, who last ? " among the victims of Hector's spear. 
 For M. Faye's arrangements, notwithstanding the skill with 
 which he has presented them, cannot be unreservedly accepted. 
 The objections to them, thoughtfully urged by M. C. Wolf 1 
 and Professor Darwin, 2 are grave. Not the least so is his 
 omission to take account of an agency of change presently to be 
 noticed. 
 
 We now come to a most remarkable investigation one, 
 indeed, unique in its profession to lead us back with mathe- 
 matical certainty towards the origin of a heavenly body. We 
 refer to Professor Darwin's inquiries into the former relations of 
 the earth and moon. 3 
 
 They deal exclusively with the effects of tidal friction, and 
 primarily with those resulting, not from oceanic, but from 
 " bodily" tides, such as the sun and moon must have raised in 
 past ages on a liquid or viscous earth. The immediate effect 
 of either is, as already explained, to destroy the rotation of the 
 body on which the tide is raised, as regards the tide-raising 
 body, bringing it to turn always the same face towards its dis- 
 turber. This, we can see, has been completely brought about in 
 the case of the moon. There is, however, a secondary or reactive 
 effect. Action is always mutual. Precisely as much as the 
 moon pulls the terrestrial tidal wave backward, the tidal wave 
 pulls the moon forward. But pulling a body forward in its 
 orbit implies the enlargement of that orbit ; in other words, the 
 moon is, as a consequence of tidal friction, very slowly receding 
 from the earth. This will go on (other circumstances remaining 
 unchanged) until the lengthening day overtakes the more tardily 
 
 1 Bull. Astr., t. ii. 2 Nature, vol. xxxi., p. 506. 3 Phil. Trans., vol. 
 clxxi., p. 713. 
 
CHAP. ix. PLANETARY EVOLUTION. 385 
 
 lengthening month, when each will be of about 1400 hours. 1 A 
 position of what we may call tidal equilibrium between earth 
 and moon will (apart from disturbance by other bodies) then be 
 attained. 
 
 If, however, it be true that, in the time to come, the moon 
 will be much farther from us, it follows that in the time past 
 she was much nearer to us than she now is. Tracing back her 
 history by the aid of Professor Darwin's clue, we at length find 
 her revolving in a period of somewhere between two and four 
 hours, almost in contact with an earth rotating just at the same 
 rate. This was before tidal friction had begun its work of 
 grinding down axial velocity and expanding orbital range. But 
 the position was not one of stable equilibrium. The slightest 
 inequality must have set on foot a series of uncompensated 
 changes. If the moon had whirled the least iota faster than the 
 earth spun, she must have been precipitated upon it. Her actual 
 existence shows that the trembling balance inclined the other 
 way. By a second or two to begin with, the month exceeded 
 the day ; the tidal wave crept ahead of the moon ; tidal friction 
 came into play, and our satellite started on its long spiral 
 journey outward from the parent globe. This must have 
 occurred, it is computed, at least fifty-four million years ago. 
 
 That this kind of tidal reactive effect played its part in 
 bringing the moon into its present position, and is still, to some 
 slight extent, at work in changing it, there can be no doubt 
 whatever. An irresistible conjecture carried the explorer of its 
 rigidly deducible consequences one step beyond them. The 
 moon's time of revolution, when so near the earth as barely to 
 escape contact with it, must have been, by Kepler's Law, more 
 than two and less than two and a half hours. Now, it happens 
 that the most rapid rate of rotation of a fluid mass of the earth's 
 average density, consistent with spheroidal equilibrium, is two 
 hours and twenty minutes. Quicken the movement but by one 
 second and the globe must fly asunder. Hence the inference 
 
 1 Mr. J. Nolan has pointed out (Xature, vol. xxxiv., p. 287) that the length 
 of the equal day and month will be reduced to about 1240 hours by the 
 effects of solar tidal friction. 
 
 25 
 
386 HISTORY OF ASTRONOMY. PART n. 
 
 seems justified that the earth actually did fly asunder through 
 over-fast spinning, the ensuing disruption representing the birth- 
 throes of the moon. It is likely that the event was hastened or 
 helped by solar tidal disturbance. 
 
 To recapitulate. Analysis tracks backward the two bodies 
 until it leaves them in very close contiguity, one rotating and 
 the other revolving in approximately the same time, and that 
 time certainly not far different from, and quite possibly identical 
 with, the critical period of instability for the terrestrial spheroid. 
 " Is this," Professor Darwin asks, " a mere coincidence, or does 
 it not rather point to the break-up of the primeval planet into 
 two masses in consequence of a too rapid rotation ? " * 
 
 We are tempted, but are not allowed to give an unqualified 
 assent. Mr. James Nolan of Victoria has made it clear that the 
 moon could not have subsisted as a continuous mass under the 
 powerful disruptive strain which would have acted upon it when 
 revolving almost in contact with the present surface of the 
 earth ; and Professor Darwin, admitting the objection, concedes 
 to our satellite, in its initial stage, the alternative form of a 
 flock of meteorites. 2 But such a congregation must have been 
 quickly dispersed, by tidal action, into a meteoric ring. The same 
 investigator subsequently fixed 650x3 miles from centre to centre- 
 as the minimum distance at which the moon could have revolved 
 in its entirety ; and he concluded it " necessary to suppose that, 
 after the birth of a satellite, if it takes place at all in this way, 
 a series of changes occur which are quite unknown." 3 The 
 evidence, however, for the efficiency of tidal friction in bringing 
 about the actual configuration of the lunar-terrestrial system, is 
 not invalidated by this failure to penetrate its natal mystery. 
 Under its influence the principal elements of that system fall 
 into interdependent mutual relations. It connects, causally and 
 quantitatively, the periods of the moon's revolution and of the 
 earth's rotation, the obliquity of the ecliptic, the inclination 
 and eccentricity of the lunar orbit. All this can scarcely be 
 accidental. 
 
 1 Phil Trans , vol. clxxi., p. 835. 2 Nature, vol. xxxiii., p. 368; see also 
 Nolan, Ibid., vol. xxxiv., p. 286. 3 Phil. Trans., vol. clxxviii,, p. 422. 
 
CHAP. ix. PLANETARY EVOLUTION. 387 
 
 Professor Darwin's first researches on this subject were com- 
 municated to the Koyal Society, December 18, 1879. They 
 were followed January 20, 1 88 1, 1 by an inquiry on the same 
 principles into the earlier condition of the entire solar system. 
 The results were a warning against hasty generalisation. They 
 showed that the lunar-terrestrial system, far from being a 
 pattern for their development, was a singular exception among 
 the bodies swayed by the sun. Its peculiarity resides in the 
 fact that the moon is proportionately by far the most massive 
 attendant upon any known planet. Its disturbing power over 
 its primary is thus abnormally great, and tidal friction has, in 
 consequence, played a predominant part in bringing their mutual 
 relations into their present state. 
 
 The comparatively late birth of the moon tends to ratify this 
 inference. The dimensions of the earth did not differ (according 
 to our present authority) very greatly from what they now are 
 when her solitary offspring came, somehow, into existence. 
 'This is found not to have been the case with any other of the 
 planets. It is unlikely that the satellites of Jupiter, Saturn, or 
 Mars (we may safely add, of Uranus or Neptune) ever revolved 
 in much narrower orbits than those they now traverse ; it is 
 practically certain that they did not, like our moon, originate 
 very near the present surfaces of their primaries. 2 What fol- 
 lows ? The tide-raising power of a body grows with vicinity in 
 a rapidly accelerated ratio. Lunar tides must then have been 
 on an enormous scale when the moon swung round at a fraction 
 of its actual distance from the earth. But no other satellite 
 with which we are acquainted occupied at any time a corre- 
 sponding position. Hence no other satellite ever possessed tide- 
 raising capabilities in the least comparable to those of the moon. 
 We conclude once more that tidal friction had an influence here 
 ^quite different from its influence elsewhere. 
 
 There is, however, another branch of the same subject. We 
 know that the sun as well as the moon causes tides in our 
 oceans. There must then be solar, no less than lunar, tidal 
 friction. The question at once arises : What part has it played 
 
 1 Phil. Trans,, vol. clxxii., p. 491. 2 Ibid., p. 530. 
 
388 HISTORY OF ASTRONOMY. PART n. 
 
 in the development of the solar system ? Has it ever been one 
 of leading importance, or has its influence always been, as it 
 now is, subordinate, almost negligeable ? To this, too, Professor 
 Darwin supplies an answer. 
 
 It can be stated without hesitation that the sun did not give 
 birth to the planets, as the earth has been supposed to have 
 given birth to the moon, by the disruption f 'of its already con- 
 densed, though plastic and glowing mass, pushing them then 
 gradually backward from its surface into their present places. 
 For the utmost possible increase in the length of the year 
 through tidal friction is one hour ; and five minutes is a more 
 probable estimate. 1 So far as the pull of tide-waves raised on 
 the sun by the planets is concerned, then, the distances of the 
 latter have never been notably different from what they now 
 are ; though that cause may have converted the paths traversed 
 by them from circles into ellipses. 
 
 Over their physical history, however, it was probably in a large 
 measure influential. The first vital issue for each of them was 
 satellites or no satellites ? Were they to be governors as well 
 as governed, or should they revolve in sterile isolation throughout 
 the aeons of their future existence ? Here there is strong 
 reason to believe that solar tidal friction was the overruling 
 power. It is remarkable that planetary fecundity increases at 
 least so far outward as Saturn with distance from the sun. 
 Can these two facts be in any way related ? In other words, is- 
 there any conceivable way by which tidal influence could prevent 
 or impede the thro wing-off of secondary bodies ? We have only 
 to think for a moment in order to see that this is precisely on& 
 of its direct results. 2 
 
 Tidal friction, whether solar or lunar, tends to reduce the 
 axial movement of the body it acts upon. But the separation 
 of satellites depends according to the received view upon the 
 attainment of a disruptive rate of rotation. Hence, if solar 
 tidal friction were strong enough to keep down the pace below 
 this critical point, the contracting mass would remain intact 
 there would be no satellite-production. This, in all probability, 
 
 1 Phil Trans., vol. clxxii., p. 533. 2 This was perceived by M. d. Roche 
 in 1872. Mem. de TAcad. iles Sciences de Montpellier, t. viii., p. 247. 
 
CHAP. ix. PLANETARY EVOLUTION. 389 
 
 actually occurred in the case both of Mercury and Venus. They 
 cooled without dividing, because the solar friction-brake applied 
 to them was too strong to permit acceleration to pass the limit 
 of equilibrium. The complete destruction of their relative axial 
 movement has been rendered probable by Schiaparelli's obser- 
 vations ; and that the process went on rapidly, is a reasonable 
 further inference. The earth barely escaped the fate of loneli- 
 ness incurred by her neighbours. Her first and only epoch of 
 instability was retarded until she had nearly reached maturity. 
 The late appearance of the moon accounts for its large relative 
 size through the increased cohesion of an already strongly 
 condensed parent mass and for the distinctive peculiarities of 
 its history and influence on the producing globe. 
 
 Solar tidal friction, although it did not hinder the formation of 
 two minute dependants of Mars, has been invoked to explain 
 the anomalously rapid revolution of one of them. Phobos, we 
 have seen, completes more than three revolutions while Mars 
 rotates once. But this was probably not always so. The two 
 periods were originally nearly equal. The difference, it is alleged, 
 was brought about by tidal waves raised by the sun on the vis- 
 cous spheroid of Mars. Rotatory velocity was thereby destroyed, 
 the Martian day slowly lengthened, and, as a secondary conse- 
 quence, the period of the inner satellite, become shorter than the 
 augmented day, began progressively to diminish. So that 
 Phobos, unlike our moon, was in the beginning farther from its 
 primary than now. 
 
 But here again Mr. Nolan enters a caveat. Applying the 
 simple test of numerical evaluation, he shows that before solar 
 tidal friction can lengthen the rotation-period of Mars by so 
 much as one minute, Phobos must already have been precipitated 
 upon its surface. 1 For the enormous disparity of mass between 
 it and the sun is so far neutralised by the enormous disparity 
 in their respective distances from Mars, that solar tidal force 
 there is only fifty times that of the little satellite. But the 
 tidal effects of a satellite circulating quicker than its primary 
 rotates, exactly reverse those of one moving, like our moon, 
 comparatively slowly, so that the tides raised by Phobos tend 
 
 1 Nature, vol. xxxiv., p. 287. 
 
390 HISTORY OF ASTRONOMY. PART n. 
 
 to shorten both, periods. Its orbital momentum, however, is 
 so extremely small in proportion to the rotational momentum of 
 Mars, that any perceptible inroad upon the latter is attended by 
 a lavish and ruinous -expenditure of the former. It is as if a 
 man owning a single five-pound note were to play for equal 
 stakes with a man possessing a million. The bankruptcy sure 
 to ensue is typified by the coming fate of ;the Martian inner 
 satellite. The catastrophe of its fall needs to bring it about 
 only a very feeble reactive pull compared with the friction which 
 should be applied in order to protract the Martian day by one 
 minute. And from the proportionate strength of the forces at 
 work, it is quite certain that one result cannot take place with- 
 out the other. Nor can things have been materially different in 
 the past ; hence the idea must be abandoned that the primitive 
 time of rotation of Mars survives in the period of its inner 
 satellite. 
 
 The anomalous shortness of the latter may, however, in 
 M. Wolfs opinion, 1 be explained by the "trainees elliptiques " 
 with which Koche supplemented nebular annulation. 2 These 
 are traced back to the descent of separating strata from the 
 shoulders of the great nebulous spheroid towards its equatorial 
 plane. Their rotational velocity being thus relatively small, 
 they formed " inner rings," very much nearer to the centre of 
 condensation than would have been possible on the unmodified 
 theory of Laplace. Phobos might, in this view, be called a polar 
 offset of Mars; and the rings of Saturn are thought to own a 
 similar origin. 
 
 Outside the orbit of Mars, solar tidal friction can scarcely 
 be said at present to possess any sensible power. But it is 
 far from certain that this was always so. It seems not un- 
 likely that its influence was the overruling one in determining 
 the direction of planetary rotation. M. Faye, as we have seen, 
 objected to Laplace's scheme that only retrograde secondary 
 systems could be produced by it. In this he was anticipated 
 by Kirkwood, who, however, supplied an answer to his own 
 objection. 3 
 
 1 Bull. Astr., t. ii., p. 223. 2 Montpellier Mems., t. viii., p. 242. 3 Amer. 
 Jour., vol. xxxviii. (1864), p. I. 
 
CHAP. ix. PLANETARY EVOLUTION. 391 
 
 Sun-raised tides must have acted with great power on the 
 diffused masses of the embryo planets. By their means they 
 doubtless very soon came to turn (in lunar fashion) the same 
 hemisphere always towards their centre of motion. This 
 amounts to saying that even if they started with retrograde 
 rotation, it was, by solar tidal friction, quickly rendered direct. 
 For it is scarcely necessary to point out that a planet turning 
 an invariable face to the sun, rotates in the same direction in 
 which it revolves, and in the same period. As, with the pro- 
 gress of condensation, tides became feebler, and rotation more 
 rapid, the accelerated spinning necessarily proceeded in the 
 sense thus prescribed for it. Hence the backward axial move- 
 ments of Uranus and Neptune may very well be a survival, due 
 to the inefficiency of solar tides at their great distance, of a 
 state of things originally prevailing universally throughout the 
 system. 
 
 The general outcome of Professor Darwin's researches has been 
 to leave Laplace's cosmogony untouched. He concludes nothing 
 against it, and, what perhaps tells with more weight in the 
 long run, has nothing to substitute for it. In one form or 
 another, if we speculate at all on the development of the planetary 
 system, our speculations are driven into conformity with the 
 broad lines of the Nebular Hypothesis so far, at least, as 
 admitting an original material unity and motive uniformity. 
 But we can see now, better than formerly, that these supply a 
 bare and imperfect sketch of the truth. We should err gravely 
 were we to suppose it possible to reconstruct, with the help of 
 any knowledge our race is ever likely to possess, the real and 
 complete history of our admirable system. "The subtlety of 
 nature," Bacon says, "transcends in many ways the subtlety 
 of the intellect and senses of man." By no mere barren formula 
 of evolution, indiscriminately applied all round, the results we 
 marvel at, and by a fragment of which our life is conditioned, 
 were brought forth; but by the manifold play of interacting 
 forces, variously modified and variously prevailing, according 
 to the local requirements of the design they were appointed to 
 execute. 
 
CHAPTEK X. 
 
 RECENT COMETS. 
 
 ON the 2nd of June 1858, Giambattista Donati discovered at 
 Florence, in the constellation Leo, a feeble round nebulosity 
 about one-tenth the diameter of the full moon. It proved to be 
 a comet approaching the sun. But it changed little in apparent 
 place or brightness for some weeks. The gradual development 
 of a central condensation of light was the first symptom of 
 coming splendour. At Harvard, in the middle of July, a strong 
 stellar nucleus was seen ; on August 14 a tail began to be thrown 
 out. As the comet wanted still over six weeks of the time of 
 its perihelion-passage, it was obvious that great things might be 
 expected of it. They did not fail of realisation. 
 
 Not before the early days of September was it generally 
 recognised with the naked eye, though it had been detected 
 without a glass at Pulkowa, August 19. But its growth was 
 thenceforward surprisingly rapid, as it swept with acceler- 
 ated motion under the hindmost foot of the Great Bear, and 
 past the starry locks of Berenice. A sudden leap upward in 
 lustre was noticed on September 12, when the nucleus shone 
 with about the brightness of the pole-star, and the tail, not- 
 withstanding large foreshortening, could be traced with the 
 lowest telescopic power over six degrees of the sphere. The 
 appendage, however, attained its full development only after 
 perihelion, September 30, by which time, too, it lay nearly square 
 to the line of sight from the earth. On October 10 it stretched 
 in a magnificent scimitar-like curve over a third and upwards of 
 the visible hemisphere, representing a real extension in space 
 of fifty-four million miles. But the most striking view was 
 
CHAP. x. RECENT COMETS. 393 
 
 presented on October 5, when the brilliant star Arcturus became 
 involved in the brightest part of the tail, and during many hours 
 contributed, its lustre undiminished by the interposed nebulous 
 screen, to heighten the grandeur of the most majestic celestial 
 object of which living memories retain the impress. Donati's 
 comet was, according to Admiral Smyth's testimony, 1 outdone 
 " as a mere sight-object " by the great comet of 181 1 ; but what 
 it lacked in splendour, it surely made up in grace, and variety of 
 what we may call " scenic" effects. 
 
 Some of these were no less interesting to the student than 
 they were impressive to the spectator. At Pulkowa, on the 
 1 6th September, Winnecke 2 observed a faint outer envelope 
 resembling a veil of almost evanescent texture flung somewhat 
 widely over the head. Next evening, the first of the " secondary " 
 tails appeared, possibly as parts of the same phenomenon. This 
 was a narrow straight ray, forming a tangent to the strong 
 curve of the primary tail, and reaching to a still greater distance 
 from the nucleus. It continued faintly visible for about three 
 weeks, during part of which time it was seen in duplicate. For 
 from the chief train itself, at a point where its curvature 
 abruptly changed, issued, as if through the rejection of part of 
 its materials, a second beam nearly parallel to the first, the 
 rigid line of which contrasted singularly with the softly diffused 
 and waving aspect of the plume of light from which it sprang. 
 Olbers's theory of unequal repulsive forces was never more 
 beautifully illustrated. The triple tail seemed a visible solar 
 analysis of cometary matter. 
 
 The processes of luminous emanation going on in this body 
 forcibly recalled the observations made on the comets of 1 7/1/1 
 and 1835. From the middle of September, the nucleus, estimated 
 by Bond to be under five hundred miles in diameter, was the 
 centre of action of the most energetic kind. Seven distinct 
 "envelopes" were detached in succession from the nebulosity 
 surrounding the head, and after rising towards the sun during 
 periods of from four to seven days, finally cast their material 
 backward to form the right and left branches of the great train. 
 
 1 Month. Not., vol. xix., p. 27. - Mem. de I" Ac. Imp., t. ii., 1859, p. 46. 
 
394 HISTORY OF ASTRONOMY. PART n. 
 
 The separation of these by an obscure axis apparently as black, 
 quite close up to the nucleus, as the sky indicated for the tail 
 a hollow, cone-like structure; 1 while the repetition of certain 
 spots and rays in the same* corresponding situation on one 
 envelope after another, served to show that the nucleus to 
 some local peculiarity of which they were doubtless due had 
 no proper rotation, but merely shifted sufficiently on an axis to 
 preserve the same aspect towards the sun as it moved round 
 it. 2 This observation of Bond's was strongly confirmatory of 
 Bessel's hypothesis of opposite polarities in such bodies' opposite 
 sides. 
 
 The protrusion towards the sun, on September 25, of a 
 brilliant luminous fan-shaped sector completed the resemblance 
 to Halley's comet. The appearance of the head was now some- 
 what that of a "bat's-wing" gaslight. There were, however, no 
 oscillations to and fro, such as Bessel had seen and speculated 
 upon in 1835. As the size of the nucleus contracted with 
 approach to perihelion, its intensity augmented. On October 2, 
 it outshone Arcturus, and for a week or ten days was a con- 
 spicuous object half an hour after sunset. Its lustre setting 
 aside the light emitted from the tail was, at that date, 6300 
 times what it had been on June 15, though theoretically taking 
 into account, that is, only the differences of distance from sun 
 and earth it should have been only $ of that amount. Here, 
 it might be thought, was convincing evidence of the comet itself 
 becoming ignited under the growing intensity of the solar radi- 
 ations. Yet experiments wit 1 ^. the polariscope were interpreted 
 in an adverse sense, and Bond's conclusion that the comet sent 
 us virtually unmixed reflected sunshine was generally acquiesced 
 in. It did not, however, survive the first application of the 
 spectroscope to these bodies. 
 
 Very few comets have been so well or so long observed as 
 Donati's. It was visible to the naked eye during 1 1 2 days ; it 
 was telescopically discernible for 275, the last observation having 
 been made by Mr. William Mann at the Cape of Good Hope, 
 March 4, 1859. Its course through the heavens combined 
 
 1 Harvard Annals, vol. iii., p. 368. - Ibid., p. 371. 
 
CHAP. x. RECENT COMETS. 395 
 
 singularly with the orbital place of the earth to favour curious 
 inspection. The tail, when near its greatest development, lost 
 next to nothing by the effects of perspective, and at the same 
 time lay in a plane sufficiently inclined to the line of sight to 
 enable it to display its exquisite curves to the greatest advantage. 
 Even the weather was, on both sides of the Atlantic, propitious 
 during the period of greatest interest, and the moon as little 
 troublesome as possible. The splendid volume compiled by the 
 younger Bond is a monument to the care and skill with which 
 these advantages were turned to account. Yet this stately 
 apparition marked no turning-point in the history of cometary 
 science. By its study knowledge was indeed materially advanced, 
 but along the old lines. No quick and vivid illumination broke 
 upon its path. Quite insignificant objects as we have already 
 partly seen have often proved more vitally instructive. 
 
 Donati's comet has been identified with no other. Its path is 
 an immensely elongated ellipse, lying in a plane far apart from 
 that of the planetary movements, carrying it at perihelion 
 considerably within the orbit of Venus, and at aphelion out into 
 space to 5j times the distance from the sun of Neptune. The 
 entire circuit occupies over 2000 years, and is performed in a 
 retrograde direction, or against the order of the Signs. Before 
 its next return, about the year 4000 A.D., the enigma of its 
 presence and its purpose may have been to some extent though 
 we may be sure not completely penetrated. 
 
 On June 30, 1 86 1, the earth passed, for the second time in 
 this century, through the tail of a great comet. Many of our 
 readers must remember the unexpected disclosure, on the with- 
 drawal of the sun below the horizon on that evening, of an object 
 so remarkable as to challenge universal attention. A golden- 
 yellow planetary disc, wrapt in dense nebulosity, shone out 
 while the June twilight of these latitudes was still in its first 
 strength. The number and complexity of the envelopes sur- 
 rounding the head produced, according to the late Mr. Webb, 1 a 
 magnificent effect. Portions of six distinct emanations were 
 traceable. " It was as though a number of light, hazy clouds 
 
 1 Month. Not., vol. xxii., p. 306. 
 
396 HISTORY OF ASTRONOMY. PART 11. 
 
 were floating round a miniature full moon." As the sky 
 darkened the tail emerged to view. 1 Although in brightness 
 and sharpness of definition it could not compete with the display 
 of 1858, its dimensions proved to be extraordinary. It reached 
 upwards beyond the zenith when the head had already set. By 
 some authorities its extreme length was stated at 118, and it 
 showed no trace of curvature. Most remarkable, however, was 
 the appearance of two widely divergent rays, each pointing 
 towards the head, though cut off from it by sky-illumination, of 
 which one was seen by Mr. Webb, and both by Mr. Williams at 
 Liverpool, a quarter of an hour before midnight. There seems 
 no doubt that Mr. Webb's interpretation was the true one, and 
 that these beams were, in fact, "the perspective representation 
 of a conical or cylindrical tail, hanging closely above our heads, 
 and probably just being lifted up out of our atmosphere." 2 The 
 cometary train was then rapidly receding from the earth, so that 
 the sides of the " outspread fan " of light, as which it showed 
 when we were right in the line of its axis, must have appeared 
 (as they did) to close up in departure. The swiftness with which 
 the visually opened fan shut, proved its vicinity; and indeed 
 Mr. Hind's calculations showed that we were not so much near, 
 as actually within its folds at that very time. 
 
 Already M. Liais, from his observations at Eio de Janeiro, 
 June II to 14, and Mr. Tebbutt in New South W T ales on June 
 15, had anticipated such an encounter, while the former subse- 
 quently proved that it must have occurred in such a way as to 
 cause an immersion of the earth in cometary matter to a depth 
 of 300,000 miles. 3 The comet then lay between the earth and 
 the sun at a distance of about fourteen million miles from the 
 former ; its tail stretched outward just along the line of inter- 
 section of its own with the terrestrial orbit to an extent of 
 fifteen million miles ; so that our globe, happening to pass at 
 the time, found itself during some hours involved in the flimsy 
 appendage. 
 
 No perceptible effects were produced by the meeting ; it was 
 
 1 Stothard, Month. Xot., vol. xxi., p. 243. " Intell. Observer, vol. i., p. 65. 
 3 Comptes Hendus, t. Ixi., p. 953. 
 
CHAP. x. RECENT COMETS. 397 
 
 known to have occurred by theory alone. A peculiar glare in 
 the sky, thought by some to have distinguished the evening of 
 June 30, was, at best, inconspicuous. Nor were there any symp- 
 toms of unusual electrical excitement. The Greenwich instru- 
 ments were, indeed, disturbed on the following night ; but it 
 would be rash to infer that the comet had art or part in their 
 agitation. 
 
 The perihelion-passage of this body occurred June 1 1, 1861 ; 
 and its orbit has been shown by M. Kreutz of Bonn, from a 
 very complete investigation founded on observations extending 
 over nearly a year, to be an ellipse traversed in a period of 
 409 years. 1 
 
 Towards the end of August 1862, a comet became visible to 
 the naked eye high up in the northern hemisphere, with a 
 nucleus equalling in brightness the lesser stars of the Plough 
 and a feeble tail 20 in length. It thus occupied quite a 
 secondary position among the members of its class. It was, 
 nevertheless, a splendid object in comparison with a telescopic 
 nebulosity discovered by Tempel at Marseilles, December 19, 
 1865. This, the sole comet of 1866, slipped past perihelion, 
 January 1 1 , without pomp of train or other appendages, and 
 might have seemed hardly worth the trouble of pursuing. 
 Fortunately, however, this was not the view entertained by 
 observers and computers ; since upon the knowledge acquired of 
 the movements of these two bodies has been founded one of the 
 most significant discoveries of modern times. The first of them 
 is now styled the comet (1862 iii.) of the August meteors, the 
 second (1866 i.) that of the November meteors. The steps by 
 which this curious connection came to be ascertained were 
 many, and were taken in succession by a number of individuals. 
 But the final result was reached by Schiaparelli of Milan, and 
 remains deservedly associated with his name. 
 
 The idea prevalent in the last century as to the nature of 
 shooting stars was that they were mere aerial ignes fatui 
 inflammable vapours accidentally kindled in our atmosphere. 
 But Halley had already entertained the opinion of their cosmical 
 
 1 Smiths. Xeport, 1881 (Holden). 
 
398 HISTORY OF ASTRONOMY. PART n. 
 
 origin ; and Chladni in 1 794 formally broached the theory that 
 space is filled with minute circulating atoms, which, drawn by 
 the earth's attraction, and ignited by friction in its gaseous 
 envelope, produce the luminou^ effects so frequently witnessed. 1 
 Acting on his suggestion, Brandes and Benzenberg, two students 
 at the University of Gottingen, began in 1 798 to determine the 
 heights of falling stars by simultaneous enervations at a dis- 
 tance. They soon found that they move with planetary veloci- 
 ties in the most elevated regions of our atmosphere, and by the 
 ascertainment of this fact laid a foundation of distinct know- 
 ledge regarding them. Some of the data collected, however, 
 served only to perplex opinion, and even caused Chladni tempo- 
 rarily to renounce his. Many high authorities, headed by 
 Laplace in 1802, declared for the lunar-volcanic origin of 
 meteorites ; but thought on the subject was turbid, and inquiry 
 seemed only to stir up the mud of ignorance. It needed one of 
 those amazing spectacles, at which man assists, not now as 
 formerly in abject terror for his own frail fortunes, but with 
 keen curiosity, and the vivid expectation of new knowledge, to 
 bring about a clarification. 
 
 On the night of November 12-13, J 833, a tempest of falling 
 stars broke over the earth. North America bore the brunt of 
 its pelting. From the Gulf of Mexico to Halifax, until daylight 
 with some difficulty put an end to the display, the sky was 
 scored in every direction with shining tracks and illuminated 
 with majestic fireballs. At Boston, the frequency of meteors 
 was estimated to be about half that of flakes of snow in an 
 average snowstorm. Their numbers, while the first fury of 
 their coming lasted, were quite beyond counting; but as it 
 waned, a reckoning was attempted, from which it was computed, 
 on the basis of that much diminished rate, that 240,000 must 
 have been visible during the nine hours they continued to 
 fall. 2 
 
 Now there was one very remarkable feature common to the 
 innumerable small bodies which traversed, or were consumed 
 
 1 Ueber den tirsprung der von Pallas gcfandenen Eisenmassen, p. 24. 
 - Arago, Annuaire, 1836, p. 294. 
 
CHAP. x. RECENT COMETS. 399 
 
 in our atmosphere that night. They all seemed to come from 
 the same part of the sky. Traced backward, their paths were 
 invariably found to converge to a point in the constellation Leo. 
 Moreover, that point travelled with the stars in their nightly 
 round. In other words, it was entirely independent of the earth 
 and its rotation. It was a point in inter-planetary space. 
 
 The effective perception of this fact l amounted to a discovery, 
 as Olmsted and Twining, who had " simultaneous ideas " on the 
 subject, were the first to realize. Denison Olmsted was then 
 professor of mathematics in Yale College. He showed early in 
 i834 2 that the emanation of the showering meteors from a fixed 
 '-radiant" proved their approach to the earth along nearly 
 parallel lines, appearing to diverge by an effect of perspective ; 
 and that those parallel lines must be sections of orbits described 
 by them round the sun and intersecting that of the earth. For 
 the November phenomenon was now seen to be a periodical one. 
 On the same night of the year 1832, although with less dazzling 
 and universal splendour than in America in 1833, it had been 
 witnessed over great part of Europe and in Arabia. Olmsted 
 accordingly assigned to the cloud of cosmical particles (or 
 "comet," as he chose to call it), by terrestrial encounters with 
 which he supposed the appearances in question to be produced, 
 a period of about 182 days ; its path a narrow ellipse, meeting, 
 near its farthest end from the sun, the place occupied by the 
 earth on November 12. 
 
 Once for all, then, as the result of the star-fall of 1833, the 
 study of luminous meteors became an integral part of astro- 
 nomy. Their membership of the solar system was no longer 
 a theory or a conjecture it was an established fact. The dis- 
 covery might be compared to, if it did not transcend in impor- 
 tance, that of the asteroidal group. " C'est un nouveau monde 
 planetaire," Arago wrote, 3 " qui commence a se reveler a nous." 
 
 Evidences of periodicity continued to accumulate. It was 
 
 1 Humboldt had noticed the emanation of the shooting stars of 1799 from 
 a single point, or "radiant," as Greg long afterwards termed it; hut no 
 reasoning was founded on the observation. 2 Am. Jour, of Sc., vol. xxvi., 
 p. 132. 3 Annuaire, 1836, p. 297. 
 
400 HISTORY OF ASTRONOMY. PART n. 
 
 remembered that Humboldt and Bonpland had been the spec- 
 tators at Cumana, after midnight on November 12, 1799, of a 
 fiery shower little inferior to that of 1833, and reported to have 
 been visible from the equator to Greenland. Moreover, in 1834 
 and some subsequent years, there were waning repetitions of the 
 display, as if through the gradual thinning-out of the meteoric 
 supply. The extreme irregularity of its distribution was noted 
 by Olbers in 1837, who conjectured that we might have to wait 
 until 1 867 to see the phenomenon renewed on its former scale of 
 magnificence. 1 This was the first hint of a thirty-three or thirty- 
 four year period. 
 
 The falling stars of November did not alone attract the 
 attention of the learned. Similar appearances were traditionally 
 associated with August 10 by the popular phrase in which they 
 figured as " the tears of St. Lawrence." But the association 
 could not be taken on trust from mediaeval authority. It had to 
 be proved scientifically, and this Quetelet of Brussels succeeded 
 in doing in December 1 836.2 
 
 A second meteoric revolving system was thus shown to exist. 
 But its establishment was at once perceived to be fatal to the 
 " cosmical cloud " hypothesis of Olmsted. For if it be a viola- 
 tion of probability to attribute to one such agglomeration a 
 period of an exact year, or sub-multiple of a year, it would be 
 plainly absurd to suppose the movements of two or more 
 regulated by such highly artificial conditions. An alternative 
 was proposed by Adolf Erman of Berlin in i839- 3 No longer 
 in clouds, but in closed rings, he supposed meteoric matter to 
 revolve round the sun. Thus the mere circumstance of inter- 
 section by a meteoric, of the terrestrial orbit, without any coin- 
 cidence of period, would account for the earth meeting some 
 members of the system at each annual passage through the 
 " node " or point of intersection. This was an important step 
 in advance, yet it decided nothing as to the forms of the orbits 
 of such annular assemblages ; nor was it followed up in any 
 direction for a quarter of a century. 
 
 1 Ann. de i'Observ., Bruxelles, 1839, p. 248. - Ibid., 1837, p. 272. 3 Astr. 
 Nacli., Nos. 385, 390. 
 
CHAP. x. RECENT COMETS. 401 
 
 Professor Hubert A. Newton, of Yale College, took up 
 in I864 1 the dropped thread of inquiry. A search through 
 old records carried the November phenomenon back to the year 
 902 A.D., long distinguished as " the year of the stars." For in the 
 same night in which Taormina was captured by the Saracens, 
 and the cruel Aghlabite tyrant Ibrahim ibn Ahmed died " by 
 the judgment of God " before Cosenza, stars fell from heaven in 
 such abundance as to amaze and terrify beholders far and near. 
 This was on October 13, and recurrences were traced down 
 through the subsequent centuries, always with a day's delay in 
 about seventy years. It was easy, too, to derive from the dates 
 a cycle of 33 J years, so that Professor Newton did not hesitate 
 to predict the exhibition of an unusually striking meteoric 
 spectacle on November 13-14, i866. 2 
 
 For the astronomical explanation of the phenomena, recourse 
 was had to a method introduced by Erman of computing 
 meteoric orbits. It was found, however, that conspicuous 
 recurrences every thirty-three or thirty-four years could be 
 explained on the supposition of five widely different periods, 
 combined with varying degrees of extension in the revolving 
 group. Professor Newton himself gave the preference to the 
 shortest of the five of 354^ days, but indicated the means of 
 deciding with certainty upon the true one. It was furnished by 
 the advancing motion of the node, or that day's delay of the 
 November shower every seventy years, which the old chronicles 
 had supplied data for detecting. For this is a strictly measurable 
 effect of gravitational disturbance by the various planets, the 
 amount of which naturally depends upon the course pursued by 
 the disturbed bodies. Here the great mathematical resources of 
 Professor Adams were brought to bear. By laborious processes 
 of calculation, he ascertained that four out of Newton's five 
 possible periods were entirely incompatible with the observed 
 nodal displacement, while for the fifth that of 33 J years 
 a perfectly harmonious result was obtained. 3 This was the 
 last link in the chain of evidence proving that the November 
 
 1 Am. Jour, of Sc., vol. xxxvii. (2nd. ser.), p. 377. 2 Ibid., vol. xxxviii., 
 p. 61. 3 Month. Not., vol. xxvii., p. 247. 
 
 26 
 
402 HISTORY OF ASTRONOMY. PART IK 
 
 meteors or " Leonids," as they had by that time come to be 
 called revolve round the sun in a period of 33.27 years, in an 
 ellipse spanning the vast gulf between the orbits of the earth 
 and Uranus, the group being,*so extended as to occupy six or 
 eight years in defiling past the scene of terrestrial encounters. 
 But before it was completed in March 1867, the subject had 
 assumed a new aspect and importance. $ 
 
 Professor Newton's prediction of a remarkable star-shower 
 in November 1866 was punctually fulfilled. This time, Europe 
 served as the main target of the celestial projectiles, and ob- 
 servers were numerous and forewarned. The display, although, 
 according to Mr. Baxendell's memory, 1 inferior to that of 1833, 
 was of extraordinary impressiveness. Dense crowds of meteors, 
 equal in lustre to the brightest stars, and some rivalling Venus 
 at her best, 2 darted from east to west across the sky with 
 enormous apparent velocities, and with a certain determinate- 
 aess of aim, as if let fly with a purpose, and at some definite 
 object. 3 Nearly all left behind them trains of emerald green or 
 clear blue light, which occasionally lasted many minutes, before 
 they shrivelled and curled up out of sight. The maximum rush 
 occurred a little after one o'clock on the morning of November 
 14, when attempts to count were overpowered by frequency. 
 But during a previous interval of seven minutes five seconds r 
 four observers at Mr. Bishop's observatory at Twickenham 
 reckoned 514, and during an hour H2O. 4 Before daylight the 
 earth had fairly cut her way through the star-bearing stratum ^ 
 the "ethereal rockets" had ceased to fly. 
 
 This event brought the subject of shooting stars once more 
 vividly to the notice of astronomers. Schiaparelli had, indeed, 
 been already attracted by it. The results of his studies were 
 made known in four remarkable letters, addressed, before the 
 close of the year 1 866, to Father Secchi, and published in the. 
 Bulletino of the Roman Observatory. 5 Their upshot was to 
 show, in the first place, that meteors possess a real velocity 
 
 1 Am. Jour, of Sc., vol. xliii. (2nd ser.), p. 87. ' 2 Grant, Month. Not., 
 vol. xxvii., p. 29. 3 P. Smyth, Month. Not., vol. xxvii., p. 256. 4 Hind r 
 Ibid., p. 49. r> Reproduced in Les Mondes, t. xiii. 
 
CHAP. x. RECENT COMETS. 403 
 
 considerably greater than that of the earth, and travel, accord- 
 ingly, to enormously greater distances from the sun along 
 tracks resembling those of comets in being very eccentric, in 
 lying at all levels indifferently, and in being pursued in either 
 direction. It was next inferred that comets and meteors equally 
 have an origin foreign to the solar system, but are drawn into 
 it temporarily by the sun's attraction, and occasionally fixed in 
 it by the backward pull of some planet. But the crowning fact 
 was reserved for the last. It was the astonishing one that the 
 August meteors move in the same orbit with the bright comet 
 of 1 862 that the comet, in fact, is but a larger member of the 
 family of Perseids (so named because their radiant point is 
 situated in the constellation Perseus). 
 
 This discovery was quickly capped by others of the same 
 kind. Leverrier published, January 21, I86/, 1 elements for the 
 November swarm, at once identified by Dr. C. F. W. Peters of 
 Altona, with Oppolzer's elements for Tempel's comet of i866. 2 
 A few days later, Schiaparelli, having re-calculated the orbit of 
 the meteors from improved data, arrived at the same con- 
 clusion ; while Professor Weiss of Vienna pointed to the agree- 
 ment between the orbits of a comet which had appeared in. 1861 
 and of a star shower found to recur on April 20 (Lyraids), as 
 well as between those of Biela's comet and certain conspicuous 
 meteors of November 28. 3 
 
 These instances do not seem to be exceptional. The number 
 of known or suspected accordances of cometary tracks with 
 meteor streams contained in a list drawn up in 1878* by 
 Professor Alexander S. Herschel (who has made the subject 
 peculiarly his own), amounts to seventy-six ; although the four 
 first detected still remain the most conspicuous, and perhaps the 
 only absolutely sure examples of a relation as significant as it 
 was, to most astronomers, unexpected. 
 
 There had, indeed, been anticipatory ideas. Not that Kepler's 
 comparison of shooting stars to " minute comets," or Maskelyne's 
 "forse risultera che essi sono comete," in a letter to the Abate 
 
 1 Comptes Bendus, t. Ixiv., p. 96. 2 Astr. Nach., No. 1626. 3 Ibid., 
 No. 1632. 4 Month. Not., vol. xxxviii., p. 369. 
 
404 HISTORY OF ASTRONOMY. PART n. 
 
 Cesaris, December 12, 1 78 3, 1 need count for much. But Chladni, 
 in iSip, 2 considered both to be fragments or particles of the 
 same primitive matter, irregularly dispersed through space as 
 nebulas; and Morstadt of Prague suggested about i83/ 3 that 
 the November meteors might be dispersed atoms from the tail 
 of Biela's comet, the path of which is cut across by the earth 
 near that epoch. Professor Kirkwood, however, by a luminous 
 intuition, penetrated the whole secret, so far as it has yet been 
 made known. In an article published, or rather buried, in the 
 Danville Quarterly Review for December 1861, he argued, from 
 the observed division of Biela, and other less noted instances of 
 the same kind, that the sun exercises a "divellent influence" 
 on the nuclei of comets, which may be presumed to continue 
 its action until their corporate existence (so to speak) ends 
 in complete pulverisation. " May not," he continued, " our 
 periodic meteors be the debris of ancient, but now disintegrated 
 comets, whose matter has become distributed round their 
 orbits?" 4 
 
 The gist of Schiaparelli's discovery could not be more clearly 
 conveyed. For it must be borne in mind that with the ultimate 
 destiny of comets' tails this had nothing to do. The tenuous 
 matter composing them is, no doubt, permanently lost to the 
 body from which it emanated ; but science does not pretend to 
 track its further wanderings through space. It can, however, 
 state categorically that these will no longer be conducted along 
 the path forsaken under solar compulsion. From the central, 
 and probably solid parts of comets, on the other hand, are 
 derived the granules by the swift passage of which our skies are 
 seamed with periodic fires. It is certain that a loosely agglom- 
 erated mass (such as there is every reason to believe cometary 
 nuclei to be) must gradually separate through the unequal 
 action of gravity on its various parts through, in short, solar 
 tidal influence. Thenceforward its fragments will revolve 
 independently in parallel orbits, at first as a swarm, finally 
 
 1 Schiaparelli, Le Stelle Cadenti, p. 54. 2 Uebcr Feuer- Meteor e, p. 406. 
 3 A str. Nach., No. 347 (Madler) ; see also Boguslawski, Die Kometen, p. 98, 
 1857. 4 Nature, vol. vi., p. 148. 
 
CHAP. x. RECENT COMETS. 405 
 
 when time has been given for the full effects of the lagging of 
 the slower moving particles to develop as a closed ring. The 
 first condition is still, more or less, that of the November 
 meteors ; those of August have already arrived at the second. 
 For this reason, Leverrier pronounced, in 1867, the Perseid to 
 be of older formation than the Leonid system. He even assigned 
 a date at which the introduction of the last-named bodies into 
 their present orbit was probably effected through the influence 
 of Uranus. In 126 A.D. a close approach must have taken place 
 between the planet and the parent comet of the November stars, 
 after which its regular returns to perihelion, and the consequent 
 process of its disintegration, set in. Though not complete, it 
 is already far advanced. 
 
 The view that meteorites are the dust of decaying comets 
 was now to be put to a definite test of prediction. Biela's 
 comet had not been seen since its duplicate return in 1852. 
 Yet it had been carefully watched for with the best telescopes ; 
 its path was accurately known ; every perturbation it could 
 suffer was scrupulously taken into account. Under these cir- 
 cumstances, its repeated failure to come up to time might fairly 
 be thought to imply a cessation from visible existence. Might 
 it not, however, be possible that it would appear under another 
 form that a star-shower might have sprung from and would 
 commemorate its dissolution ? 
 
 An unusually large number of falling stars were seen by 
 Brandes, December 6, 1798. Nearly anniversary displays 
 attracted notice in the years 1830, 1838, and 1847, and the point 
 from which they emanated was shown by Heis at Aix-la-Chapelle 
 to be situated near the bright star y Andromedae. 1 Now this is 
 precisely the direction in which the orbit of Biela's comet would 
 seem to lie, as it runs down to cut the terrestrial track very close 
 to the earth's place at the above dates. The inference was then an 
 easy one, that the meteors were pursuing the same path with the 
 comet ; and it was separately arrived at, early in 1 867, by 
 Weiss, D' Arrest, and Galle. 2 But Biela travels in the opposite 
 
 1 A. S. Herschel, Month. Not., vol. xxxii., p. 355. Astr. Nach., Nos. 
 
 1632, 1633, 1635. 
 
406 HISTORY OF ASTRONOMY. PART n. 
 
 direction to Tempel's comet, and its attendant " Leonids " ; its 
 motion is direct, or from west to east, while theirs is retrograde. 
 Consequently, the motion of its node is in the opposite direction 
 too. In other words,. .the meeting-place of its orbit with that of 
 the earth retreats (and very rapidly) along the ecliptic instead 
 of advancing. So that if the " Andromedes " stood in the 
 supposed intimate relation to Biela's comet, they might be 
 expected to anticipate the times of their recurrence by us much 
 as a week (or thereabouts) in half a century. All doubt as to 
 the fact may be said to have been removed by Signor Zezioli's 
 observation of the annual shower in more than usual abundance 
 at Bergamo, November 30, 1867. 
 
 The missing comet was next due at perihelion in the year 
 1872, and the probability was contemplated by both Weiss and 
 Galle of its being replaced by a copious discharge of falling 
 stars. The precise date of the occurrence was not easily deter- 
 minable, but Galle thought the chances in favour of November 
 28. The event anticipated the prediction by twenty-four hours. 
 Scarcely had the sun set in Western Europe on November 27 
 when it became evident that Biela's comet was shedding over 
 us the pulverised products of its disintegration. The meteors 
 came in volleys from the foot of the Chained Lady, their 
 numbers at times baffling the attempt to keep a reckoning. At 
 Moncalieri, about 8 P.M., they constituted (as Father Denza 
 said *) a " real rain of fire." Four observers counted, on an 
 average, four hundred each minute and a half ; and not a few 
 fireballs, equalling the moon in diameter, traversed the sky. 
 On the whole, however, the stars of 1872, though about equally 
 numerous, were less brilliant than those of 1 866 ; the phosphor- 
 escent tracks marking their passage were comparatively 
 evanescent and their movements sluggish. This is easily under- 
 stood when we remember that the Andromedes overtake the earth, 
 while the Leonids rush to meet it ; the velocity of encounter for 
 the first class of bodies being under twelve, for the second above 
 forty-four miles a second. The spectacle was, nevertheless, 
 magnificent. It presented itself successively to various parts of 
 
 1 Nature, vol. vii., p. 122. 
 
CHAP. x. RECENT COMETS. 407 
 
 the earth, from Bombay and the Mauritius to New Brunswick 
 and Venezuela, and was most diligently and extensively 
 observed. Here it had well-nigh terminated by midnight. 1 
 
 It was attended by a slight aurora, and although Tacchini had 
 telegraphed that the state of the sun rendered some show of 
 polar lights probable, it has too often figured as an accompani- 
 ment of star-showers to permit the coincidence to rank as 
 fortuitous. Admiral Wrangel was accustomed to describe how, 
 during the prevalence of an aurora on the Siberian coast, the 
 passage of a meteor never failed to extend the luminosity to 
 parts of the sky previously dark ; 2 and the power of exciting 
 electrical disturbance seems to belong to all such flitting 
 cosmical atoms. 
 
 A singular incident connected with the meteors of 1872 has 
 now to be recounted. The late Professor Klinkerfues, who had 
 observed them very completely at Gottingen, was led to believe 
 that not merely the debris strewn along its path, but the comet 
 itself must have been in the closest proximity to the earth 
 during their appearance. 3 If so, it might be possible, he 
 thought, to descry it as it retreated in the diametrically opposite 
 direction from that in which it had approached. On November 
 30, accordingly, he telegraphed to Mr. Pogson, the Madras 
 astronomer, " Biela touched earth November 27 ; search near 
 Theta Centauri" the "anti-radiant," as it is called, being 
 situated close to that star. Bad weather prohibited observation 
 during thirty-six hours, but when the rain-clouds broke on the 
 morning of December 2, there a comet was, just in the indicated 
 position. In appearance it might have passed well enough for 
 one of the Biela twins. It had no tail, but a decided nucleus, 
 and was about 45 seconds across, being thus altogether below 
 the range of naked-eye discernment. It was again observed 
 December 3, when a short tail was perceptible; but overcast 
 skies supervened, and it has never since been seen. Its identity 
 accordingly remains in doubt. It seems tolerably certain, how- 
 ever, that it was not the lost comet, which ought to have passed 
 
 1 A. S. Herschel, Report Brit. Ass., 1873, p. 390. 2 Humboldt, Cosmos, 
 vol. i., p. 114 (Otto's trans.). 3 Month. Not., vol. xxxiii., p. 128. 
 
408 HISTORY OF ASTRONOMY. PART 11. 
 
 that spot twelve weeks earlier, and was subject to no conceivable 
 disturbance capable of delaying to that extent its revolution. 
 On the other hand, there is the strongest likelihood that it 
 belonged to the same, system 1 -^-that it was a third fragment, 
 torn from the parent-body of the Andromedes at a period 
 anterior to our first observations of it. 
 
 In thirteen years, Biela's comet (or its relics) travels nearly 
 twice round its orbit, so that a renewal of the meteoric shower of 
 1872 was looked for on the same day of the year 1885, the 
 probability being emphasised by an admonitory circular from 
 Dunecht. Astronomers were accordingly on the alert, and were 
 not disappointed. In England, observation was partially 
 impeded by clouds ; but at Malta, Palermo, Beyrout, and other 
 southern stations, the scene was most striking. The meteors 
 were both larger and more numerous than in 1872. Their 
 numbers in the densest part of the drift were estimated by 
 Professor Newton at 75,000 per hour, visible from one spot to 
 so large a group of spectators that practically none could be 
 missed. Yet each of these multitudinous little bodies was 
 found by him to travel in a clear cubical space of which the 
 edge measured twenty miles ! 2 Thus the dazzling effect of a 
 luminous throng was produced without jostling or overcrowding, 
 by particles, it might almost be said, isolated in the void. 
 
 Their aspect was strongly characteristic of the Andromede 
 family of meteors. " They invariably," Mr. Denning wrote, 3 
 " traversed short paths with very slow motions, and became 
 extinct in evolved streams of yellowish sparks." The conclu- 
 sion seemed obvious " that these meteors are formed of very 
 soft materials, which expand while incalescent, and are imme- 
 diately crumbled and dissipated into exiguous dust." 
 
 The Biela meteors of 1885 did not merely gratify astronomers 
 with a fulfilled prediction, but were the means of communicating 
 to them some valuable information. Although their main body 
 was cut through by the moving earth in six hours, and was not 
 more than 100,000 miles across, skirmishers were thrown out to 
 
 1 Even this was denied by Bruhns, Astr. Nach., No. 2054. - Am. Jour., 
 vol. xxxi., p. 425. 3 Month. Not., vol. xlvi., p. 69. 
 
CHAP. x. RECENT COMETS. 409 
 
 nearly a million miles on either side of the compact central 
 battalions. Members of the system were, on the 26th of 
 November, recorded by Mr. Denning at the hourly rate of about 
 130; and they did not wholly cease to be visible until December 
 I. They afforded besides a particularly well-marked example 
 of that diffuseness of radiation, previously observed in some less 
 conspicuous displays. Their paths seemed to diverge from an 
 area, rather than from a point in the sky. They came so ill to 
 focus, that divergences of several degrees were found between 
 the most authentically determined radiants. These incon- 
 gruities are attributed by Professor Newton to the irregular 
 shape of the meteoroids, evoking unsymmetrical resistance 
 from the air, and hence causing them to glance from their 
 original direction on entering it. Thus their luminous tracks 
 did not always represent (even apart from the effects of the 
 earth's attraction) the true prolongation of their course through 
 space. 
 
 The Andromedes of 1872 were laggards behind the comet 
 from which they sprang : those of 1885 were its avant-couriers. 
 That wasted and disrupted body was not due at the node until 
 January 26, 1886, sixty days, that is, after the earth's encounter 
 with its meteoric fragments. These are now probably scattered 
 over more than five hundred million miles of its orbit ; 1 yet 
 Professor Newton considers that all must have formed one com- 
 pact group with Biela at the time of its close approach to Jupiter 
 about the middle of 1841. For otherwise, both comet and 
 meteorites could not have experienced, as they seem to have 
 done, the same kind and amount of disturbance. The rapidity 
 of cometary disintegration is thus curiously illustrated. 
 
 A short-lived per suasion thatthe missing heavenly bodyitself had 
 been recovered, was created by Mr. Edwin Holmes's discovery, 
 at London, November 6, 1892, of a tolerably bright, tailless 
 comet, just in a spot which Biela's comet must have traversed in 
 approaching the intersection of its orbit with that of the earth. 
 A hasty calculation by Berberich assigned elements to the new- 
 
 1 In Schiaparelli's opinion, centuries must have elapsed while the observed 
 amount of scattering was being produced. Le titelle Cadenti, 1886, p. 112. 
 
410 HISTORY OF ASTRONOMY. PART n. 
 
 comer seeming, not only to ratify the identity, but to promise a 
 quasi-encounter with the earth on November 21. The only 
 effect of the prediction, however, was to raise a panic among the 
 negroes of the Southern States of America. The comet quietly 
 ignored it, and moved away^from, instead of towards the 
 appointed meeting-place. Its projection then, on the night of 
 its discovery, upon a point of the Biela-orbit, was by a mere 
 caprice of chance. North America, nevertheless, was visited on 
 November 23 by a genuine Andromede shower. Although the 
 meteors were less numerous than in 1885, Professor Young 
 estimated that 30,000, at the least, of their orange fire-streaks 
 came, during five hours, within the range of view at Princeton. 1 
 The anticipation of their due time by four days implied if they 
 were a prolongation of the main Biela group, the nucleus of 
 which passed the spot of encounter five months previously a 
 recession of the node since 1885 by nearly four degrees. 
 Unless, indeed, Mr. Denning were right in supposing the display 
 to have proceeded from "an associated branch of the main 
 swarm through which we passed in 1872 and i885." 2 The 
 existence of separated detachments of Biela meteors, attributed 
 to disturbing planetary action, was contemplated as highly 
 probable by Schiaparelli. 3 Such may have been the belated 
 flights met with in 1830, 1838, 1841, and 1847, and such the 
 advance-flight plunged through in 1892. 
 
 Biela does not offer the only example of cometary disruption. 
 Setting aside the unauthentic reports of early chroniclers, we 
 meet the " double comet" discovered by Liais at Olinda (Brazil), 
 February 27, 1860, of which the division appeared recent, and 
 about to be carried farther. 4 But a division once established, 
 separation must continually progress. The periodic times of 
 the fragments can never be identical ; one must drop a little 
 behind the other at each revolution, until at length they come 
 to travel in remote parts of nearly the same orbit. Thus the 
 comet predicted by Klinkerfues and discovered by Pogson had 
 
 1 Astr. and Astro-Physics, Dec. 1892, p. 943. 2 Observatory, vol. xvi., p. 55. 
 3 Le Stelle Cadenti, p. 133 ; Rendiconti dell Istituto Lombardo, t. iii., ser. ii., p. 
 23. Cf. Bredichin, Astr. Nacli., Nos. 3154, 3156. 4 Month. Not., vol. xx., p. 336. 
 
CHAP. x. RECENT COMETS. 411 
 
 already lagged to the extent of twelve weeks, and we shall meet 
 instances farther on where the retardation is counted, not by 
 weeks, but by years. Here, original identity emerges only from 
 calculation and comparison of orbits. 
 
 Comets then die, as Kepler wrote long ago, sicut 'bombyccs filo 
 fundendo. This certainty, anticipated by Kirkwood in 1861, we 
 have at least acquired from the discovery of their generative 
 connection with meteors. Nay, their actual materials become, 
 in smaller or larger proportions, incorporated with our globe. 
 It is not indeed universally admitted that the ponderous masses 
 of which, according to Daubree's estimate, 1 at least 600 fall 
 annually from space upon the earth, ever formed part of the 
 bodies known to us as comets. Some follow Tschermak in 
 attributing to aerolites a totally different origin from that of 
 periodical shooting-stars. That no clear line of demarcation can 
 be drawn is no valid reason for asserting that no real distinction 
 exists ; and it is certainly remarkable that a meteoric fusilade 
 may be kept up for hours without a single solid projectile 
 reaching its destination. It would seem as if the celestial army 
 had been supplied with blank cartridges. Yet, since a few 
 detonating meteors have been found to proceed from ascertained 
 radiants of shooting-stars, it is difficult to suppose that any 
 generic difference separates them. 
 
 Their assimilation is further urged though not with any 
 demonstrative force by two instances, the only two on record, 
 of the tangible descent of an aerolite during the progress of a 
 star-shower. On April 4, 1095, the Saxon Chronicle informs us 
 that stars fell " so thickly that no man could count them," and 
 adds that one of them having struck the ground in France, a 
 bystander " cast water upon it, which was raised in steam with 
 a great noise of boiling." 2 And again, on November 27, 1885, 
 while the skirts of the Andromede-tempest were trailing over 
 Mexico, a '-'ball of fire" was precipitated from the sky at 
 Mazapil, within view of a ranchman. 3 Scientific examination 
 proved it to be a " siderite," or mass of " nickel-iron ; " its weight 
 
 1 Revue d. d. Mondes, Dec. 15, 1885, p. 889. * Palgrave, Phil Trans., vol, 
 cxxv., p. 175. 3 W. E. Hidden, Century Mag., vol. xxxiv., p. 534. 
 
412 HISTORY OF ASTRONOMY. PART n. 
 
 exceeded eight pounds, and it contained many nodules of 
 graphite. We are not, however, authorised by the circumstances 
 of its arrival in regarding the Mazapil fragment of cosmic metal 
 as a specimen torn, from Biela's comet. In this, as in the 
 preceding case, the coincidence of the fall with the shower may 
 have been purely casual. No hint is given of any sort of 
 agreement between the sample provided for, curious study, and 
 the swarming meteors consumed in the upper air. 
 
 Professor Newton's inquiries into the tracks pursued by 
 meteorites previous to their collisions with the earth, tend to 
 distinguish them, at least specifically, from shooting-stars. He 
 found that nearly all had been travelling with a direct movement 
 in orbits the perihelia of which lay in the outer half of the space 
 separating the earth from the sun. 1 Shooting-stars, on the 
 other hand, are entirely exempt from such limitations. The Yale 
 College Professor concludes " that the larger meteorites moving 
 in our solar system are allied much more closely with the group 
 of comets of short period than with the comets whose orbits are 
 nearly parabolic." They would thus seem to be more at home 
 than might have been expected amid the planetary family. 
 Father Carbonelle has moreover shown 2 that meteorites, if 
 explosion-products of the earth or moon, should, with rare 
 exceptions, follow just the kind of paths assigned to them, from 
 data of observation, by Professor Newton. Yet it is altogether 
 improbable that projectiles from terrestrial volcanoes should, at 
 any geological epoch, have received impulses powerful enough 
 to enable them, not only to surmount the earth's gravity, but to 
 penetrate its atmosphere. 
 
 A striking indeed, an almost startling peculiarity, on the 
 other hand, divides from their congeners a class of meteors 
 identified by Mr. Denning during ten years' patient watching of 
 such phenomena at Bristol. 3 These are described as " meteors 
 with stationary radiants," since for months together they seem 
 to come from the same fixed points in the sky. Now this 
 
 1 Amer. Jour, of /Science, vol. xxxvi., p. i., 1888. 2 Revue des Questions 
 Scientifiques, Jan. 1889, p. 194; Tisserand, Bull. Astr., t. viii., p. 460. 
 3 Month. Not., vol. xlv., p. 93. 
 
CHAP. x. RECENT COMETS. 413 
 
 implies quite a portentous velocity. The direction of meteor- 
 radiants is affected by a kind of aberration, analogous to the 
 aberration of light. It results from a composition of terrestrial 
 with meteoric motion. Hence, unless that of the earth in its 
 orbit be by comparison insignificant, the visual line of encounter 
 must shift, if not perceptibly from day to day, at any rate 
 conspicuously from month to month. The fixity, then, of half- 
 a-dozen or more systems observed by Mr. Denning seems to 
 demand the admission that their members travel so fast as to 
 throw the earth's movement completely out of the account. The 
 required velocity would be, by Mr. Eanyard's calculation, at 
 least 880 miles a second. 1 But the aspect of the meteors justi- 
 fies no such extravagant assumption. Their seeming swiftness 
 is very various, and what is highly significant it is notably 
 less when they pursue than when they meet the earth. Yet the 
 "incredible and unaccountable " 2 fact of the existence of these 
 " long radiants," although doubted by Tisserand 3 on the ground 
 of its theoretical refractoriness, must apparently be admitted. 
 
 The Perseids afford, on the contrary, a remarkable instance of 
 a " shifting radiant." Mr. Denning's observations of these 
 yellowish, leisurely meteors extended over nearly six weeks, from 
 July 8 to August 16 ;' the point of radiation meantime pro- 
 gressing no less than 57 in right ascension. Doubts as to 
 their common origin were hence freely expressed, especially by 
 Mr. Monck of Dublin. 4 But the late Dr. Kleiber 5 proved by 
 strict geometrical reasoning, that the forty-nine radiants succes- 
 sively determined for the shower were all in reality comprised 
 within one narrowly limited region of space. In other words, 
 the application of the proper corrections for the terrestrial move- 
 ment, and for the effects of attraction by which each individual 
 shooting-star is compelled to describe a hyperbola round the 
 earth's centre, reduces the extended line of radiants to a compact 
 group with the cometary radiant for its central point; the 
 cometary radiant being the spot in the sky met by a tangent to 
 
 1 Observatory, vol. viii., p. 4. 2 Denning, Month. Not., vol. xxxviii., p. 114. 
 3 Comptes Rendus, t. cix., p. 344. 4 Publ Astr. Pac. toe., vol. iii., p. 114. 
 3 Month. Not., vol. Hi., p. 341. 
 
414 HISTORY OF ASTRONOMY. PART n. 
 
 the orbit of the Perseid comet of 1862 at its intersection with 
 the orbit of the earth. The reality of the connection between 
 the comet and the meteors could scarcely be more cogently de- 
 monstrated ; while the vast dimensions of the stream into which 
 the latter are found to be diffused, cannot but excite astonish- 
 ment not unmixed with perplexity. 
 
 The first successful application of the spectroscope to comets 
 was by Donati in I864. 1 A comet discovered by Tempel, July 4, 
 brightened until it appeared like a star somewhat below the 
 second magnitude, with a feeble tail 30 in length. It was 
 remarkable as having, on August 7, almost totally eclipsed a 
 small star a very rare occurrence. 2 On August 5 Donati 
 admitted its light through his train of prisms, and found it, 
 thus analysed, to consist of three bright bands yellow, green, 
 and blue separated by wider dark intervals. This implied a 
 good deal. Comets had previously been considered, as we have 
 seen, to shine mainly, if not wholly, by reflected sunlight. They 
 were now perceived to be self-luminous, and to be formed, to a 
 large extent, of glowing gas. The next step was to determine 
 what kind of gas it was that was thus glowing in them ; and 
 this was taken by Dr. Huggins in i868. 3 
 
 A comet of subordinate brilliancy, known as comet 1868 ii., 
 or sometimes as Winnecke's, was the subject of his experiment. 
 On comparing its spectrum with that of an olefiant-gas ' vacuum 
 tube ' rendered luminous by electricity, he found the agreement 
 exact. It has since been abundantly confirmed. All the 
 eighteen comets tested by light-analysis, between 1868 and 
 1880, showed the typical hydro-carbon spectrum 4 common to 
 the whole group of those compounds, but probably due immedi- 
 ately to the presence of acetylene. Some minor deviations from 
 the laboratory pattern, in the shifting of the maxima of light 
 from the edge towards the middle of the yellow and blue bands, 
 have been experimentally reproduced by Vogel and Hasselberg 
 in tubes containing a mixture of carbonic oxide with olefiant 
 
 1 Astr. Nacli., No. 1488. 2 Annuaire, Paris, 1883, p. 185. 3 Phil. Trans., 
 vol. clviii., p. 556. 4 Hasselberg, Mim. de I 1 Ac. Imp. de /St. Peter sloury, 
 t. xxviii. (7th ser.), No. 2 p. 66. 
 
CHAP. x. RECENT COMETS. 415 
 
 gas. 1 Their illumination by disruptive electric discharges was, 
 however, a condition sine qua non for the exhibition of the 
 cometary type of spectrum. When a continuous current was 
 employed, the carbonic oxide bands asserted themselves to the 
 exclusion of the hydro-carbons. The distinction has great 
 significance as regards the nature of comets. For it indicates, 
 in the first place, that combinations of carbon with oxygen, as 
 well as with hydrogen, are present in them ; while it affords, in 
 the second, a criterion as to the quality of the electrical processes 
 by which their constituent vapours are rendered luminous. Of 
 particular interest in this connection is the circumstance that 
 carbonic oxide is one of the gases evolved by meteoric stones 
 and irons under stress of heat. 2 It must then apparently have 
 formed part of an aeriform mass in which they were immersed at 
 an earlier stage of their history. These facts afford the only 
 trustworthy evidence of the extra-terrestrial existence of oxygen. 
 
 In a few exceptional comets with large perihelion-distances, 
 the usual carbon-bands have been missed. Two such were 
 observed by Dr. Huggins in 1866 and 1867 respectively. 3 In 
 each, a green ray, approximating in position to the fundamental 
 nebular line, crossed an otherwise unbroken spectrum. And 
 Holmes's comet of 1 892 4 displayed only a faint prismatic band 
 devoid of any characteristic feature. 5 Now the path of this 
 body is inscribed, as it were, between the orbits of Jupiter and 
 Mars ; and of Dr. Huggins's pair, the first (comet 1 866 i. of 
 the November meteors) approaches very little nearer than the 
 earth does to the sun; while the second (comet 1867 ii.) main- 
 tains a still greater distance from it. Hence possibly the 
 unusual nature of the spectra of all three. 
 
 The earliest comet of first-class lustre to present itself for 
 spectroscopic examination was that discovered by Coggia at 
 Marseilles, April 17, 1874. Invisible to the naked eye till 
 June, it blazed out in July a splendid ornament of our northern 
 
 1 Scheiner, Die Spectralanalyse der Gestirne, p. 234. 2 Dewar, Proc. Hoy. 
 Inst., vol. xi., p. 541. 3 Proc. JK. Soc., vol. xv.,p. 5 ; Month. Not., vol. xxvii., 
 p. 288. 4 Keeler, Astr. and Astro- Physics, Dec. 1892, p. 929. 5 Vogel, 
 Astr. Nacli., No. 3142. 
 
416 HISTORY OF ASTRONOMY. PART n. 
 
 skies, with a just perceptibly curved tail, reaching more than 
 half-way from the horizon to the zenith, anil a nucleus surpassing 
 in brilliancy the brightest stars in the Swan. Bredichin, Vogel, 
 and Huggins x were unanimous- in pronouncing its spectrum to 
 be that of marsh or olefiant gas. Father Secchi, in the clear 
 sky of Rome, was able to push the identification even closer 
 than had heretofore been done. The com^/^ejiydro-carbon spec- 
 trum consists of five zones of variously coloured light. Three of 
 these only the three central ones had till then been obtained 
 from comets ; owing, it was supposed, to their temperature not 
 being high enough to develop the others. The light of Coggia's 
 comet, however, was found to contain all five, traces of the 
 violet band emerging June 4, of the red, July 2. 2 Presumably, 
 all five would show universally in cometary spectra, were the 
 dispersed rays strong enough to enable them to be seen. 
 
 The gaseous surroundings of comets are then largely made up 
 of a compound of hydrogen with carbon. Other materials are 
 also present ; but the hydro-carbon element is probably unfailing 
 and predominant. Its luminosity is, there is little doubt, an effect 
 of electrical excitement. Zollner showed in 1 872 3 that owing 
 to evaporation and other changes produced by rapid approach to 
 the sun, electrical processes of considerable intensity must take 
 place in comets ; and that their original light is immediately 
 connected with these, and depends upon solar radiation, rather 
 through its direct or indirect electrifying effects, than through 
 its more obvious thermal power, may be considered a truth 
 permanently acquired to science. 4 They are not, it thus seems, 
 bodies incandescent through heat, but glowing by electricity; 
 and this is compatible, under certain circumstances, with a 
 relatively low temperature. 
 
 The gaseous spectrum of comets is accompanied, in varying 
 degrees, by a continuous spectrum. This is usually derived most 
 strongly from the nucleus, but extends, more or less, to the nebu- 
 lous appendages. In part, it is certainly due to reflected sunlight ; 
 in part, most likely, to the ignition of minute solid particles. 
 
 1 Proc. Roy. Soc., vol. xxiii.,p. 154. 2 Hasselberg, loc. cit.,p. 58. 3 Ueber 
 die Natur der Cometen, p. 112. 4 Hasselberg, loc. cit., p. 38. 
 
CHAPTER XI. 
 RECENT COMETS (continued). 
 
 THE mystery of comets' tails has been to some extent penetrated ; 
 so far, at least, that, by making certain assumptions strongly 
 recommended by the facts of the case, their forms can be, with 
 very approximate precision, calculated beforehand. We have, 
 then, the assurance that these extraordinary appendages are 
 composed of no ethereal or super-sensual stuff, but of matter 
 such as we know it, and subject to the ordinary laws of motion, 
 though in a state of extreme tenuity. This is unquestionably 
 one of the most remarkable discoveries of our time. 
 
 Olbers, as already stated, originated in 1812 the view that the 
 tails of comets are made up of particles subject to a force of 
 electrical repulsion proceeding from the sun. It was developed 
 and enforced by Bessel's discussion of the appearances presented 
 by Halley's comet in 1835. He, moreover, provided a formula 
 for computing the movement of a particle under the influence of 
 a repulsive force of any given intensity, and thus laid firmly the 
 foundation of a mathematical theory of cometary emanations. 
 Professor W. A. Norton, of Yale College, considerably improved 
 this by inquiries begun in 1844, and resumed on the apparition 
 of Donati's comet ; and Dr. C. F. Pape at Altona l gave numeri- 
 cal values for the impulses outward from the sun, which must 
 have actuated the materials respectively of the curved and 
 straight tails adorning the same beautiful and surprising 
 object. 
 
 The physical theory of repulsion, however, was, it might be 
 said, still in the air. Nor did it assume an aspect of even 
 
 1 Astr. NacJi., Nos. 1172-74. 
 27 
 
4i8 HISTORY OF ASTRONOMY. PART n. 
 
 moderate plausibility until Zollner took it in hand in I87I. 1 
 It is perfectly well ascertained that the energy of the push or 
 pull produced by electricity depends (other things being the 
 same) upon the surface of the* body acted on ; that of gravity, 
 upon its mass. The efficacy of solar electrical repulsion relatively 
 to solar gravitational attraction grows, consequently, as the size 
 of the particle diminishes. Make this small , enough, and it will 
 virtually cease to gravitate, and will unconditionally obey the 
 impulse to recession. 
 
 This principle Zollner was the first to realise in its application 
 to comets. It gives the key to their constitution. Admitting 
 (as we seem bound to do) that the sun and they are similarly 
 electrified, their more substantially aggregated parts will still 
 follow the solicitations of his gravity, while the finely-divided 
 particles escaping from them will, simply by reason of their 
 minuteness, fall under the sway of his repellent electric power. 
 They will, in other words, form "tails." Nor is any extravagant 
 assumption called for as to the intensity of the electrical charge 
 concerned in producing these effects. Zollner, in fact, showed 2 
 that it need not be higher than that attributed by the best 
 authorities to the terrestrial surface. 
 
 It is now more than thirty years since Professor Bredichin 
 directed his attention to these curious phenomena. His 
 persistent inquiries on the subject, however, date from the 
 appearance of Coggia's comet in 1874. On computing the 
 value of the repulsive force exerted in the formation of its tail, 
 and comparing it with values of the same force arrived at 
 by him in 1862 for some other conspicuous comets, it struck 
 him that the numbers representing them fell into three well- 
 defined classes. "I suspect," he wrote in 1877, "that comets 
 are divisible into groups, for each of which the repulsive force 
 is perhaps the same." 3 This idea was confirmed on fuller inves- 
 tigation. In 1882 the appendages of thirty-six well-observed 
 comets had been reconstructed theoretically, without a single 
 exception being met with to the rule of the three types. 
 
 1 Berichte Sachs. Ges., 1871, p. 174. 2 Natur der Cometen, p. 124; Astr. 
 Nach., No. 2086. 3 Annales de lObs. de Moscou, t. iii., pt. i., p. 37. 
 
CHAP. xi. RECENT COMETS. 419 
 
 A further study of forty comets led, however, in 1885, to a 
 modification of the numerical results previously arrived at. 
 
 In the first of these, the repellent energy of the sun is four- 
 teen times as strong as his attractive energy; 1 the particles 
 forming the enormously long straight rays projected outward 
 from this kind of comet, leave the nucleus with a mean velocity 
 of just seven kilometres per second, which, becoming constantly 
 accelerated, carries them in a few days to the limit of visibility. 
 The great comets of 1811, 1843, and 1861, that of 1744 (so far 
 as its principal tail was concerned), and Halley's comet at its 
 various apparitions, belonged to this class. In the second 
 type, the value of the repulsive force is less narrowly limited. 
 For the axis of the tail, it exceeds by one-tenth (=i'i) the 
 power of solar gravity ; for the anterior edge, it is more than 
 twice (2*2), for the posterior only half as strong. The corre- 
 sponding initial velocity (for the axis) is 1 500 metres a second, 
 and the resulting appendage a scimitar-like or plumy tail, such 
 as Donati's and Coggia's comet furnished splendid examples 
 of. Tails of the third type are constructed with forces of 
 repulsion from the sun ranging from one-tenth to three-tenths 
 that of his gravity, producing an accelerated movement of 
 attenuated matter from the nucleus, beginning at the leisurely 
 rate of 300 to 600 metres a second. They are short, strongly 
 bent, brush-like emanations, and in bright comets seem to 
 be only found in combination with tails of the higher classes. 
 Multiple tails, indeed that is, tails of different types emitted 
 simultaneously by one comet are perceived, as experience 
 advances and observation becomes closer, to be rather the rule 
 than the exception. 2 
 
 Now, what is the meaning of these three types ? Is any 
 translation of them into physical fact possible? To this 
 question Bredichin supplied in 1879 a plausible answer. 3 It 
 was already a current surmise that multiple tails are composed 
 
 1 Butt. Aktr., t. iii., p. 598. The value of the repellent force for the comet 
 of 1811 (which offered peculiar facilities for its determination) was found = 
 17.5. 2 Faye, Comptes fiendus, t. xciii., p. 13. 3 Annales, t. v., pt. ii., 
 P- 137- 
 
4^0 HISTORY OF ASTRONOMY. PART IK 
 
 of different kinds of matter, differently acted on by the sun. 
 Both Olbers and Bessel had suggested this explanation of the 
 straight and curved emanations from the comet of 1 807 ; Norton 
 had applied it to the faint light-tracks proceeding from that of 
 Donati ; l Winnecke, to the varying deviations of its more brilliant 
 plumage. Bredichin defined and ratified the conjecture. He 
 undertook to determine (provisionally as yet) Hhe several kinds 
 of matter appropriated severally to the three classes of tails. 
 These he found to be hydrogen for the first, hydro-carbons for 
 the second, and iron for the third. The ground of this appor- 
 tionment is that the atomic weights of these substances bear to- 
 each other the same inverse proportion as the repulsive forces 
 employed in producing the appendages they are supposed to 
 form ; and Zollner had pointed out in 1875 that the " heliofugal " 
 power by which comets' tails are developed, would, in fact, be 
 effective just in that ratio. 2 Hydrogen, as the lightest known 
 element that is, the least under the influence of gravity was 
 naturally selected as that which yielded most readily to the 
 counter-persuasions of electricity. Hydro-carbons had been 
 shown by the spectroscope to be present in comets, and were 
 fitted by their specific weight, as compared with that of hydrogen r 
 to form tails of the second type ; while the atoms of iron were 
 just heavy enough to. compose those of the third, and, from the 
 plentifulness of their presence in meteorites, might be presumed 
 to enter, in no inconsiderable proportion, into the mass of comets. 
 These three substances, however, were by no means supposed to 
 be the sole constituents of the appendages in question. On the 
 contrary, the great breadth of what, for the present, were taken 
 to be characteristically "iron" tails, was attributed to the 
 presence of many kinds of matter of high and slightly different 
 specific weights ; 3 while the expanded plume of Donati was 
 shown to be, in reality, a whole system of tails, made up of many 
 substances, each spreading into a separate hollow cone, more or 
 less deviating from, and partially superposed upon the others. 
 These felicities of explanation must not, however, make us 
 
 1 Am. Jour, of Sc., vol. xxxii. (2nd ser.), p. 57. 2 Astr. Nacli., No. 2082. 
 3 Annales, t. vi., pt. i., p. 60. 
 
CHAP. xi. RECENT COMETS. 421 
 
 forget that the chemical composition attributed to the first type 
 of cometary train has, so far, received no countenance from the 
 spectroscope. The emission lines of free, incandescent hydrogen 
 have never been derived from any part of these bodies. Opinions 
 are, accordingly, far from being unanimous as to the cause of 
 their structural peculiarities. Ranyard, 1 Zenker, and Faye 
 advocate the agency of heat-repulsion in producing them ; 
 Kiaer somewhat obscurely explains them through the evolution 
 of gases by colliding Articles ; 2 Herz of Vienna concludes tails 
 to be mere illusory appendages produced by electrical discharges 
 through the rare medium assumed to fill space. 3 But Hirn 4 has 
 conclusively shown that no such medium could possibly exist 
 without promptly bringing ruin upon our " daedal earth " and 
 its revolving companions. 
 
 Never was a theory more promptly or profusely illustrated 
 than this of Bredichin. Within three years of its promulgation, 
 five bright comets made their appearance, each presenting some 
 distinctive peculiarity by which knowledge of these curious 
 objects was materially helped forward. The first of these is 
 remembered as the " Great Southern Comet." It was never 
 visible in these latitudes, but made a short though stately pro- 
 gress through southern skies. Its earliest detection was at 
 Cordoba on the last evening of January, 1880 ; and it was seen 
 on February I, as a luminous streak, reaching just after sunset 
 from the south-west horizon towards the pole, in New South 
 Wales, at Monte Video, and the Cape of Good Hope. The head 
 was lost in the solar rays until February 4, when Dr. Gould, 
 director of the National Observatory of the Argentine Eepublic 
 at Cordoba, caught a glimpse of it very low in the west ; and on 
 the following evening, Mr. Eddie, at Graham's Town, discovered 
 a faint nucleus, of a straw-coloured tinge, about the size of the 
 annular nebula in Lyra. Its condensation, however, was very 
 imperfect, and the whole apparition was of an exceedingly filmy 
 texture. The tail was enormously long. On February 5 it 
 extended large perspective retrenchment notwithstanding 
 
 1 Astr. Register, March, 1883. 2 Aatr. Nach., No. 3018. 3 Ibid., No. 3093. 
 4 Constitution de I'Espace Celeste, p. 224. 
 
422 HISTORY OF ASTRONOMY. PART n. 
 
 over an arc of 50 ; but its brightness nowhere exceeded that of 
 the Milky Way in Taurus. There was little curvature percep- 
 tible ; the edges of the appendage ran parallel, forming a nebulous 
 causeway from star to star ; and the comparison to an auroral 
 beam was appropriately used. r $ie aspect of the famous comet 
 of 1843 was forcibly recalled to the memory of Mr. Janisch, 
 governor of St. Helena ; and the resemblance proved not merely 
 superficial. But the comet of 1880 was less brilliant, and even 
 more evanescent. After only eight days of visibility, it had 
 faded so much as no longer to strike, though still discoverable 
 by the unaided eye ; and on February 20 it was invisible with 
 the great Cordoba equatoreal pointed to its known place. 
 
 But the most astonishing circumstance connected with this 
 body is the identity of its path with that of its predecessor in 
 1843. This is undeniable. Dr. Gould, 1 Mr. Hind, and Dr. 
 Copeland, 2 each computed a separate set of elements from the 
 first rough observations, and each was struck with an agreement 
 between the two orbits so close as to render them virtually in- 
 distinguishable. " Can it be possible," Mr. Hind wrote to Sir 
 George Airy, " that there is such a comet in the system, almost 
 grazing the sun's surface in perihelion, and revolving in less 
 than thirty-seven years ? I confess I feel a difficulty in admit- 
 ting it, notwithstanding the above extraordinary resemblance of 
 orbits." 3 
 
 Mr. Hind's difficulty was shared by other astronomers. It 
 would, indeed, be a violation of common sense to suppose that a 
 celestial visitant so striking in appearance had been for centuries 
 back an unnoticed frequenter of our skies. Various expedients 
 accordingly were resorted to for getting rid of the anomaly. The 
 most promising at first sight was that of the resisting medium. 
 It was hard to believe that a body, largely vaporous, shooting 
 past the sun at a distance of less than a hundred thousand miles 
 from his surface, should have escaped powerful retardation. It 
 must have passed through the very midst of the corona. It 
 might easily have had an actual encounter with a prominence. 
 Escape from such proximity might, indeed, very well have been 
 
 1 Astr. Nach., No. 2307. 2 Ibid., No. 2304. 3 Observatory, vol. iii., p. 390. 
 
CHAP. xr. RECENT COMETS. 423 
 
 judged beforehand to be impossible. Even admitting no other 
 kind of opposition than that dubiously supposed to have affected 
 Encke's comet, the result in shortening the period ought to be of 
 the most marked kind. It was proved by Oppolzer 1 that if the 
 comet of 1843 na( ^ entered our system from stellar space with 
 parabolic velocity, it would, by the action of a medium such as 
 Encke postulated (varying in density inversely as the square of 
 the distance from the sun), have been brought down, by its first 
 perihelion passage, to elliptic movement in a period of twenty- 
 four years, with such rapid diminution that its next return would 
 be in about ten. But such restricted observations as were 
 available on either occasion of its visibility, gave no sign of such 
 a rapid progress towards engulfment. 
 
 Another form of the theory was advocated by Klinkerf ues. He 
 supposed that four returns of the same body had been witnessed 
 within historical memory the first in 371 B.C., the next in 1668, 
 besides those of 1843 an d 1880 ; an original period of 2039 years 
 being successively reduced by the withdrawal at each perihelion 
 passage of T^T of the velocity acquired by falling from the far 
 extremity of its orbit towards the sun, to 175 and 37 years. A 
 continuance of the process would bring the comet of 1880 back 
 in 1897. 
 
 Unfortunately, the earliest of these apparitions cannot be 
 identified with the recent ones unless by doing violence to the 
 plain meaning of Aristotle's words in describing it. He states 
 that the comet was first seen "during the frosts and in the 
 clear skies of winter," setting due west nearly at the same time 
 as the sun. 2 This implies some considerable north latitude. 
 But the objects lately observed had practically no north latitude. 
 They accomplished their entire course above the ecliptic in two 
 hours and a quarter, during which space they were barely 
 separated a hand's-breadth (one might say) from the sun's sur- 
 face. For the purposes of the desired assimilation, Aristotle's 
 comet should have appeared in March. It is not credible, 
 however, that even a native of Thrace should have termed 
 March "winter." 
 
 1 Astr. Nach., No. 2319." 2 Meteor., lib. i., cap. 6. 
 
424 HISTORY OF ASTRONOMY. PART n. 
 
 With the comet of 1668 the case is more dubious. The 
 circumstances of its appearance are barely reconcilable with the 
 identity attributed to it, although too vaguely known to render 
 certainty one way or the other ^attainable. It might, however, 
 be expected that recent observations would at least decide the 
 questions whether the comet of 1 843 could have returned in less 
 than thirty-seven years, and whether the cornet of 1880 was to 
 be looked for at the end of i/J. But the truth is that both 
 these objects were observed over so small an arc 8 and 3 
 respectively that their periods remained virtually undetermined. 
 For while the shape and position of their orbits could be and 
 were fixed with a very close approach to accuracy, the length of 
 those orbits might vary enormously without any very sensible 
 difference being produced in the small part of the curves traced 
 out near the sun. It is, however, remarkable that Dr. Wilhelm 
 Meyer arrived, by an elaborate discussion, at a period of thirty- 
 seven years for the comet of I88O, 1 while the observations of 
 1843 are admittedly best fitted by Hubbard's ellipse of 533 
 years ; but these Dr. Meyer supposes to be affected by some 
 constant source of error, such as would be produced by a mis- 
 taken estimate of the position of the comet's centre of gravity. 
 He infers finally that, in spite of previous non-appearances, we 
 really have to do with a regular denizen of our system, returning 
 once in thirty-seven years along an orbit of such extreme 
 eccentricity that its movement might be described as one of 
 precipitation towards and rapid escape from the sun, rather 
 than of sedate circulation round it. 
 
 The geometrical test of identity has hitherto been the only one 
 which it was possible to apply to comets, and in the case before 
 us it may fairly be said to have broken down. We may, then, 
 tentatively, and with much hesitation, try a physical test, though 
 scarcely yet, properly speaking, available. We have seen that 
 the comets of 1843 an ^ J 88o were strikingly alike in general 
 appearance, though the absence of a formed nucleus in the 
 latter, and its inferior brilliancy, detracted from the convincing 
 effect of the resemblance. Nor was it maintained when tried 
 
 1 Mem. 8oc. Phys. de Geneve, t. xxviii., p. 23. 
 
CHAP. xi. RECENT COMETS. 425 
 
 by exact methods of inquiry. Professor Bredichin found that the 
 gigantic ray emitted in 1843 belonged to his type No. I ; that 
 of 1 880 to type No. 2. 1 The particles forming the one were 
 actuated by a repulsive force ten times as powerful as those 
 forming the other. It is true that a second noticeably curved 
 tail was seen in Chili, March I, and at Madras, March 1 1, 1843 j 
 and M. Bredichin, accordingly, thinks the conjecture justified 
 that the materials composing on that occasion the principal 
 appendage having become exhausted, those of the secondary one 
 remained predominant, and reappeared alone in the " hydro- 
 carbon " train of 1880. But the one known instance in point is 
 against such a supposition. Halley's comet, the only great 
 comet of which the returns have been securely authenticated 
 and carefully observed, has preserved its " type " unchanged 
 through many successive revolutions. The dilemma presented 
 to astronomers by the Great Southern Comet of 1880 was unex- 
 pectedly renewed in the following year. 
 
 On the 22nd of May 1881, Mr. John Tebbutt of Windsor, 
 New South Wales, scanning the western sky, discerned a hazy- 
 looking object which he felt sure was a strange one. A marine 
 telescope at once resolved it into two small stars and a comet, 
 the latter of which quickly attracted the keen attention of 
 astronomers ; for Dr. Gould, computing its orbit from his first 
 observations at Cordoba, found it to agree so closely with that 
 arrived at by Bessel for the comet of 1 807, that he telegraphed 
 to Europe, June i, announcing the unexpected return of that 
 body. So unexpected, that theoretically it was not possible 
 before the year 3346 ; and Bessel's investigation was one which 
 inspired and eminently deserved confidence. Here then once 
 more the perplexing choice had to be made between a premature 
 and unaccountable re-appearance, and the admission of a plura- 
 lity of comets moving nearly in the same path. But in this 
 case facts proved decisive. 
 
 Tebbutt's comet passed the sun June 1 6, at a distance of 
 sixty-eight millions of miles, and became visible in Europe six 
 days later. It was, in the opinion of some, the finest object of 
 1 Annales de V0bs. de Moscou, t. vii., pt. i., p. 60. 
 
426 HISTORY OF ASTRONOMY. PART n. 
 
 the kind since 1861. In traversing the constellation Auriga, 
 on its ddbut in these latitudes, it outshone Capella. On June 
 24 and some subsequent nights, it was unmatched in brilliancy 
 by any star in the heavens. Jji the telescope, the "two inter- 
 lacing arcs of light " which had adorned the head of Coggia's 
 comet were reproduced ; while a curious dorsal spine of strong 
 illumination formed the axis of the tail, which extended in clear 
 skies over an arc of 20. It belonged to the same "type" as 
 Donati's great plume ; the particles composing it being driven 
 from the sun by a force twice as powerful as that urging them 
 towards it. 1 But the appendage was, for a few nights, and by 
 two observers, perceived to be double. Tempel, on June 27, 
 and Lewis Boss, at Albany (N.Y.), June 26 and 28, saw a long 
 straight ray corresponding to a far higher rate of emission than 
 the curved train, and shown by Bredichin to be a member of the 
 (so-called) hydrogen class. It had vanished by July I, but 
 made a temporary reappearance July 22. 2 
 
 The appendages of this comet were of remarkable trans- 
 parency. Small stars shone wholly undimmed across the tail, 
 and a very nearly central transit of the head over one of the 
 seventh magnitude on the night of June 29, produced if any 
 change an increase of brilliancy in the object of this spon- 
 taneous experiment. 3 Yet Dr. Meyer, at the Geneva Observa- 
 tory, found distinct evidence of refraction suffered by stellar 
 rays under these circumstances. Three times he pursued with 
 micrometric measurements the course of a star across the 
 cometary surroundings ; and on each occasion the uniformity of 
 its progress was disturbed in a manner corresponding to the 
 optical action of a gaseous mass increasing in density and 
 refractive power as the square of the distance from the nucleus 
 diminished. Supposing olefiant gas to be in question, its 
 density, 102,000 kilometres from the nucleus, was estimated to 
 be -nnro- that of our atmosphere at the sea-level. 4 This was the 
 
 1 Bredichin, Annales, t. viii., p. 68. 2 Am. Jour, of Sc., vol. xxii., p. 305. 
 3 Messrs. Burton and Green observed a dilatation of the stellar image into a 
 nebulous patch by the transmission of its rays through a nuclear jet of the 
 comet. Am. Jour, of Sc., vol. xxii., p. 163. 4 Archives des Sciences, t. viii., 
 p. 535. Meyer founded his conclusions on the theory of M. Gustave Cellerier. 
 
CHAP. xi. RECENT COMETS. 427 
 
 first successful attempt to measure the effects of cometary 
 refraction, and will doubtless be renewed on a favourable oppor- 
 tunity. 
 
 The track pursued by this comet gave peculiar advantages for 
 its observation. Ascending from Auriga through Camelopardus, 
 it stood, July 19, on a line between the Pointers and the Pole, 
 within 8 of the latter, thus remaining for a considerable period 
 constantly above the horizon of northern observers. Its bright- 
 ness, too, was no transient blaze, but had a lasting quality 
 which enabled it to be kept steadily in view during nearly nine 
 months. Visible to the naked eye until the end of August, the 
 last telescopic observation of it was made February 14, 1882, 
 when its distance from the earth considerably exceeded 300 
 million miles. Under these circumstances, the knowledge 
 acquired of its orbit was of more than usual accuracy, and 
 showed conclusively that the comet was not a simple return of 
 Bessel's ; for this would involve a period of seventy-four years, 
 whereas Tebbutt's comet cannot revisit the sun until after the 
 lapse of close upon three millenniums. Nevertheless, the two 
 bodies move so nearly in the same path that an original con- 
 nection of some kind is obvious; and the recent example of 
 Biela readily suggested a conjecture as to what the nature of 
 that connection might have been. The comets of 1807 and 1881 
 are then regarded with much probability as fragments of a 
 primitive disrupted body, one following in the wake of the other 
 at an interval of seventy-four years. 
 
 Tebbutt's comet was the first of which a satisfactory photo- 
 graph was obtained. The difficulties to be overcome were very 
 great. The chemical intensity of cometary light is, to begin 
 with, extraordinarily small. Janssen estimated it at soAznj- f 
 moonlight. 1 So that, if the ordinary process by which lunar 
 photographs are taken had been applied to the comet of 1881, 
 an exposure of at least three days would have been required in 
 order to get an impression of the head with about a tenth part 
 of the tail. But by that time a new method of vastly increased 
 sensitiveness had been rendered available, by which dry gelatine- 
 
 1 Annuaire, Paris, 1882, p. 781. 
 
428 HISTORY OF ASTRONOMY. PART n. 
 
 plates were substituted for the wet collodion-plates previously in 
 use ; and this improvement alone reduced the necessary time of 
 exposure to two hours. It was brought down to half an hour 
 by Janssen's employment of a reflector specially adapted to give 
 an image illuminated eight or ten times as strongly as that pro- 
 duced in the focus of an ordinary telescope. 1 
 
 The photographic feebleness of cometary. rays was not the 
 only obstacle in the way of success. The proper motion of these 
 bodies is so rapid as to render the usual devices for keeping a 
 heavenly body steadily in view quite inapplicable. The 
 machinery by which the diurnal movement of the sphere is 
 followed, must be specially modified to suit each eccentric 
 career. This too was done, and on June 30, 1881, Janssen secured 
 a perfect photograph of the brilliant object then visible, showing 
 the structure of the tail with beautiful distinctness to a distance 
 of 2j from the head. An impression to nearly 10 was obtained 
 about the same time by Dr. Henry Draper at New York, with 
 an exposure of 162 minutes. 2 
 
 Tebbutt's (or comet 1881 iii.) was also the first comet of 
 which the spectrum was so much as attempted to be chemically 
 recorded. Both Dr. Huggins and Dr. Draper were successful 
 in this respect, but Dr. Huggins was more completely so. 3 The 
 importance of the feat consisted in its throwing open to investi- 
 gation a part of the spectrum invisible to the eye, and so afford- 
 ing an additional test of cometary constitution. The result was 
 fully to confirm the origin from carbon-compounds assigned to 
 the visible rays, by disclosing additional bands belonging to the 
 same series in the ultra-violet ; as well as to establish unmistakably 
 the presence of a not inconsiderable proportion of reflected solar 
 light by the clear impression of some of the principal Fraunhof er 
 lines. Thus the polariscope was found to have told the truth, 
 though not the whole truth. 
 
 The photograph so satisfactorily communicative was taken by 
 Dr. Huggins on the night of June 24 ; and on the 29th, at 
 Greenwich, the tell-tale Fraunhof er lines were perceived to in- 
 
 1 Annuaire, p. 776. 2 Am. Jour, of Sc., vol. xxii., p. 134. 3 Report Brit. 
 Ass., p. 520. 
 
CHAP. xi. RECENT COMETS. 429 
 
 terrupt the visible range of the spectrum. This was at first so 
 vividly continuous, that the characteristic cometary bands could 
 scarcely be detached from their bright background. But as the 
 nucleus faded, towards the end of June, they came out strongly, 
 and were more and more clearly seen, both at Greenwich and at 
 Princeton, to agree, not with the spectrum of hydro-carbons lit 
 up in a vacuum tube by an electric discharge, but with that of 
 the same substances burning in a Bunsen flame. 1 Here we have 
 an additional clue to the molecular condition of cometary mate- 
 rials. It need not, however, be inferred that they are really in 
 a state of combustion. This, from all that we know, may be 
 called an impossibility. The truth pointed to seems rather to be 
 that the electricity by which comets are rendered luminous is of 
 very low intensity. 2 
 
 The spectrum of the tail was, in this comet, found to be not 
 essentially different from that of the head. Professor Wright 
 of Yale College ascertained a large, but probably variable per- 
 centage of its light to be polarised in a plane passing through 
 the sun, and hence to be reflected sunlight. 3 A faint continuous 
 spectrum corresponded to this portion of its radiance ; but 
 gaseous emissions were also present. At Potsdam, on June 30, 
 the hydro-carbon bands were traced by Vogel to the very end of 
 the tail ; 4 and they were kept in sight by Young at a greater 
 distance from the nucleus than the more equably dispersed light. 
 There seems little doubt that, as in the solar corona, the relative 
 strength of the two orders of spectrum is subject to fluctua- 
 tions. 
 
 The comet of 1881 iii. was thus of signal service to science. 
 It afforded, when compared with the comet of 1807, the first 
 undeniable example of two such bodies travelling so nearly in 
 the same orbit as to leave absolutely no doubt of the existence of 
 a genetic tie between them. Cometary photography came to its 
 earliest fruition with it ; and cometary spectroscopy made a 
 notable advance by means of it. Before it was yet out of sight, 
 it was provided with a successor. 
 
 1 Month. Not., vol. xlii., p. 14 ; Am. Jour, of Sc., vol. xxii., p. 136. 2 Piazzi 
 Smyth, Nature, vol. xxiv., p. 430. 3 Astr. Nach., No. 2395. 4 Ibid. 
 
430 HISTORY OF ASTRONOMY. PART n. 
 
 At Ann Arbor Observatory, Michigan, on July 14, a comet 
 was discovered by Dr. Schaeberle, which, as his claim to priority 
 is undisputed, is often allowed to bear his name. In strict 
 scientific parlance, however, it ie designated comet 1881 iv. It 
 was observed in Europe after three days, became just discernible 
 with the naked eye at the end of July, and brightened consistently 
 up to its perihelion passage, August 22, wheti it was still about 
 fifty million miles from the sun. During many days of that 
 month, the uncommon spectacle was presented of two bright 
 comets circling together, though at widely different distances, 
 round the north pole of the heavens. The newcomer, however, 
 never approached the pristine lustre of its predecessor. Its 
 nucleus, when brightest, was comparable to the star Cor Caroli, 
 a narrow, perfectly straight ray proceeding from it to a distance 
 of 10. This was easily shown by Bredichin to belong to the 
 hydrogen type of tails ; x while a " strange, faint second tail, or 
 bifurcation of the first one," observed by Captain Noble, August 
 24, 2 fell into the hydro-carbon class of emanations. It was 
 seen, August 22 and 24, by Dr. F. Terby of Louvain, 3 as a short 
 nebulous brush, like the abortive beginnings of a congeries of 
 curving trains; but appeared no more. Its well-attested presence 
 was, however, significant of the complex constitution of such 
 bodies, and the manifold kinds of action progressing in them. 
 
 The only peculiarity in the spectrum of Schaeberle's comet 
 consisted in the almost total absence of continuous light. The 
 carbon-bands were nearly isolated and very bright. Barely from 
 the nucleus proceeded a rainbow-tinted streak, indicative of solid 
 or liquid matter, which, in this comet, must have been of very 
 scanty amount. Its visit to the sun in 1881 was, so far as is 
 known, the first. The elements of its orbit showed no resem- 
 blance to those of any previous comet, nor any marked signs of 
 periodicity. So that, although it may be considered probable, 
 we do not knoiv that it is moving in a closed curve, or will ever 
 again penetrate the precincts of the solar system. It was last 
 seen from the southern hemisphere, October 19, 1881. 
 
 1 Astr. Nock., No. 2411. - Month. Not, vol. xlii., p. 49. 3 Astr. Nach., 
 No. 2414. 
 
CHAP. xi. RECENT COMETS. 431 
 
 The third of a quartette of lucid comets visible within sixteen 
 months, was discovered by Mr. C. S. Wells at the Dudley 
 Observatory, Albany, March 17, 1882. Two days later it was 
 described by Mr. Lewis Boss as " a great comet in miniature," 
 so well defined and regularly developed were its various parts 
 and appendages. It was discernible without optical aid early in 
 May ; and on June 5 it was observed on the meridian at Albany 
 just before noon an astronomical event of extreme rarity. 
 Comet Wells, however, never became an object so conspicuous 
 as to attract general attention, owing to its immersion in the 
 evening twilight of our northern June. 
 
 But the study of its spectrum revealed new facts of the 
 utmost interest. All the comets till then examined had been 
 found (with the two transiently observed exceptions already 
 mentioned) to conform to one invariable type of luminous 
 emission. Individual distinctions there had been, but no specific 
 differences. Now all these bodies had kept at a respectful 
 distance from the sun; for of the great comet of 1880 no 
 spectroscopic inquiries had been made. Comet Wells, on the 
 other hand, approached its surface within little more than five 
 million miles on June 10, 1882; and it is not doubtful that 
 to this circumstance the novel feature in its incandescence 
 was due. 
 
 During the first half of April its spectrum was of the normal 
 type, though the carbon-bands were unusually weak ; but with 
 increasing vicinity to the sun they died out, and the entire 
 light seemed to become concentrated into a narrow, unbroken, 
 brilliant streak, hardly to be distinguished from the spectrum 
 of a star. This unusual behaviour excited attention, and a 
 strict watch was kept. It was rewarded at the Dunecht 
 Observatory, May 27, by the discernment of what had never 
 before been seen in a comet the yellow ray of sodium. 1 By 
 June I, this had kindled into a blaze overpowering all other 
 emissions. The light of the comet was practically mono- 
 chromatic ; and the image of the entire head, with the root of 
 the tail, could be observed, like a solar prominence, depicted, in 
 1 Copernicus, vol. ii., p. 229. 
 
432 HISTORY OF ASTRONOMY. PART n. 
 
 its new saffron vesture of vivid illumination, within the jaws of 
 an open slit. 
 
 At Potsdam, the bright yellow line was perceived with 
 astonishment by Vogel on May 31, and was next evening 
 identified with Fraunhofer's " D." Its character led him to 
 infer a very considerable density in the glowing vapour emit- 
 ting it. 1 Hasselberg founded an additiorial argument in 
 favour of the electrical origin of cometary light on the changes 
 in the spectrum of comet Wells. 2 For they were closely 
 paralleled by some earlier experiments of Wiedemann, in 
 which the gaseous spectra of vacuum tubes were at once 
 effaced on the introduction of metallic vapours. It seemed 
 as if the metal had no sooner been rendered volatile by heat, 
 than it usurped the entire office of carrying the discharge, the 
 resulting light being thus exclusively of its production. Had 
 simple incandescence by heat been in question, the effect 
 would have been different; the two spectra would have been 
 superposed without prejudice to either. Similarly, the replace- 
 ment of the hydro-carbon bands in the spectrum of the comet 
 by the sodium line, proved electricity to be the exciting agent. 
 For the increasing thermal power of the sun might, indeed, 
 have ignited the sodium, but it could not have extinguished the 
 hydro-carbons. 
 
 Dr. Huggins succeeded in photographing the spectrum of 
 comet Wells by an exposure of one hour and a quarter. 3 The 
 result was to confirm the novelty of its character. None of the 
 ultra-violet carbon groups were apparent ; but certain bright 
 rays, as yet unidentified, had imprinted themselves. Other- 
 wise the spectrum was strongly continuous, uninterrupted even 
 by the Fraunhofer lines detected in the spectrum of Tebbutt's 
 comet. Hence it was concluded that a smaller proportion of 
 reflected light was mingled with the native emissions of the 
 later arival. 
 
 All that is certainly known about the extent of the orbit 
 traversed by the first comet of 1882 is that it came from, and 
 
 1 Astr. Nach., Nos. 2434, 2437. 2 Ibid., No. 2441. 3 Report Brit. Ass., 
 1882, p. 442. 
 
CHAP. xi. RECENT COMETS. 433 
 
 is now retreating towards, vastly remote depths of space. An 
 American computer l found a period indicated for it of no less 
 than 400,000 years ; A. Thraen of Dingelstadt arrived at one of 
 36 1/. 2 Both are perhaps equally insecure. 
 
 We have now to give some brief account of one of the 
 most remarkable cometary apparitions on record, and with the 
 single exception of that identified with the name of Halley the 
 most instructive to astronomers. The lessons learned from it 
 were as varied and significant as its aspect was splendid ; 
 although from the circumstance of its being visible in general 
 only before sunrise, the spectators of its splendour were com- 
 paratively few. 
 
 The discovery of a great comet at Rio Janeiro, September 
 II, 1882, became known in Europe through a telegram from 
 M. Cruls, director of the observatory at that place. It had, 
 however (as appeared subsequently), been already seen on the 
 8th by Mr. Finlay of the Cape Observatory, and at Auckland as 
 early as September 3. A later, but very singularly conditioned 
 detection, quite unconnected with any of the preceding, was 
 effected by Dr. Common at Baling. Since the eclipse of May 
 17, when a comet named "Tewfik" in honour of the Khedive 
 of Egypt was caught on Dr. Schuster's photographs, entangled, 
 one might almost say, in the outer rays of the corona, he had 
 scrutinised the neighbourhood of the sun on the infinitesimal 
 chance of intercepting another such body on its rapid journey 
 thence or thither. We record with wonder that, after an 
 interval of exactly four months, that infinitesimal chance turned 
 up in his favour. 
 
 On the forenoon of Sunday, September 17, he saw a great 
 comet close to, and rapidly approaching the sun. It was, in 
 fact, then within a few hours of perihelion. Some measures of 
 position were promptly taken; but a cloud-veil covered the 
 interesting spectacle before midday was long past. Mr. Finlay 
 at the Cape was more completely fortunate. Divided from his 
 fellow-observer by half the world, he unconsciously finished, 
 
 1 J. J. Parsons, Am. Jour, of Science, vol. xxvii., p. 34. 2 Astr. Nach., 
 No. 2441. 
 
 28 
 
434 HISTORY OF ASTRONOMY. PART IK 
 
 under a clearer sky, his interrupted observation. The comet f 
 of which the silvery radiance contrasted strikingly with the 
 reddish-yellow glare of the sun's margin it drew near to, was 
 followed "continuously right into the boiling of the limb"- 
 a circumstance without precedent in cometary history. 1 Dr. 
 Elkin, who watched the progress of the event with another 
 instrument, thought the intrinsic brilliancy of the nucleus 
 scarcely surpassed by that of the sun's surface. Nevertheless 
 it had no sooner touched it than it vanished as if annihilated. 
 So sudden was the disappearance (at 4h. 5om. 583. Cape mean 
 time), that the comet was at first believed to have passed 
 behind the sun. But this proved not to have been the 
 case. The observers at the Cape had witnessed a genuine 
 transit. Nor could non-visibility be explained by equality of 
 lustre. For the gradations of light on the sun's disc are amply 
 sufficient to bring out against the dusky background of the 
 limb any object matching the brilliancy of the centre ; while 
 an object just equally luminous with the limb must inevitably 
 show dark at the centre. The only practicable view, then, is 
 that the bulk of the comet was of too filmy a texture, and its 
 presumably solid nucleus too small, to intercept any notice- 
 able part of the solar rays a piece of information worth re- 
 membering. 
 
 On the following morning, the object of this unique ob- 
 servation showed (in Dr. Gill's words) " an astonishing bril- 
 liancy as it rose behind the mountains on the east of Table Bay, 
 and seemed in no way diminished in brightness when the sun 
 rose a few minutes afterward. It was only necessary to shade 
 the eye from direct sunlight with the hand at arm's length, 
 to see the comet, with its brilliant white nucleus and dense 
 white, sharply-bordered tail of quite half a degree in length." 2 
 All over the world, wherever the sky was clear during that day, 
 September 18, it was obvious to ordinary vision. Since 1843 
 nothing had been seen like it. From Spain, Italy, Algeria, 
 
 1 Observatory, vol. v., p. 355. The transit had been foreseen by Mr. Tebbutt, 
 but it occurred after sunset in New South Wales. - Observatory, vol. v., 
 P- 354- 
 
PLATE II. 
 
 THE GREAT COMET OF SEPTEMBER 1882. 
 Photographed at the Royal Observatory, Cape of Good Hope. 
 
CHAP. xi. RECENT COMETS. 435 
 
 Southern France, despatches came in announcing the extra- 
 ordinary appearance. At Cordoba, in South America, the 
 "blazing star near the sun" was the one topic of discourse. 1 
 Moreover and this is altogether extraordinary the records of 
 its daylight visibility to the naked eye extend over three days. 
 At Reus, near Tarragona, it showed bright enough to be seen 
 through a passing cloud when only three of the sun's diameters 
 from his limb, just before its final rush past perihelion 011 
 September 17 ; while at Carthagena in Spain, on September 19, 
 it was kept in view during two hours before and two hours after 
 noon, and was similarly visible in Algeria on the same day. 2 
 
 But still more surprising than the appearance of the body 
 itself were the nature and relations of the path it moved in. 
 The first rough elements computed for it by Mr. Tebbutt, Dr. 
 Chandler of Harvard, and Mr. White, assistant at the Melbourne 
 Observatory, showed at once a striking resemblance to those 
 of the twin comets of 1843 an( ^ 1880. This suggestive fact 
 became known in this country, September 27, through the 
 medium of a Dunecht Circular. It was fully confirmed by 
 subsequent inquiries, for which ample opportunities were 
 luckily provided. The likeness was not, indeed, so absolutely 
 perfect as in the previous case ; it included some slight, though 
 real differences ; but it bore a strong and unmistakable stamp, 
 broadly challenging explanation. 
 
 Two hypotheses only were really available. Either the comet 
 of 1882 was an accelerated return of those of 1843 an( ^ 1880, or 
 it was a fragment of an original mass to which they also had 
 belonged. For the purposes of the first view the "resisting 
 medium " was brought into full play ; the opinion invoking it 
 was for some time both prevalent and popular, and formed the 
 basis, moreover, of something of a sensational panic. For a 
 comet which, at a single passage through the sun's atmosphere, 
 encountered sufficient resistance to shorten its period from 
 thirty-seven to two years and eight months, must, in the 
 immediate future, be brought to rest on his surface ; and the 
 
 1 Gould, Astr. Nach., No. 2481. 2 Flammarion, Comptes Rendus, t. xcv. 
 p. 558. 
 
436 HISTORY OF ASTRONOMY PART n. 
 
 solar conflagration thence ensuing was represented in some 
 quarters, with more license of imagination than countenance 
 from science, as likely to be of catastrophic import to the in- 
 habitants of our little planet. 
 
 But there was a test available in 1882 which it had not been 
 possible to apply either in 1843 or i n 1 880. The two bodies 
 visible in those years had been observed only after they had 
 already passed perihelion ; * the third member of the group, on 
 the other hand, was accurately followed for a week before that 
 event, as well as during many months after it. Mr. Finlay's 
 and Dr. Elkin's observation of its disappearance at the sun's 
 edge formed, besides, a peculiarly delicate test of its motion. 
 The opportunity was thus afforded, by directly comparing the 
 comet's velocity before and after its critical plunge through 
 the solar surroundings, of ascertaining with some approach to 
 certainty whether any considerable retardation had been experi- 
 enced in the course of that plunge. The answer distinctly given 
 was that there had not. The computed and observed places on 
 both sides of the sun fitted harmoniously together. The effect, 
 if any were produced, was too small to be perceptible. 
 
 This result is, in itself, a memorable one. It seems to give 
 the coup de grace to Encke's theory discredited, in addition, by 
 Backlund's investigation of a resisting medium growing rapidly 
 denser inwards. For the perihelion distance of the comet of 
 1882, though somewhat greater than that of its predecessors, 
 was nevertheless extremely small. It passed at less than 300,000 
 miles of the sun's surface. But the ethereal substance long 
 supposed to obstruct the movement of Encke's comet would 
 there be nearly 2000 times denser than at the perihelion of the 
 smaller body, and must have exerted a conspicuous retarding 
 influence. That none such could be detected seems to argue 
 that no such medium exists. 
 
 Further evidence of a decisive kind was not wanting on the 
 question of identity. The "Great September Comet" of 1882 
 was in no hurry to withdraw itself from curious terrestrial 
 
 1 Captain Ray's sextant-observation of the comet of 1843, a few hours before 
 perihelion, was too rough to be of use. 
 
CHAP. xi. RECENT COMETS. 437 
 
 scrutiny. It was discerned with the naked eye at Cordoba as 
 late as March 7, 1883, and still showed in the field of the great 
 equatoreal on June I as an " excessively faint whiteness." 1 It 
 was then about 480 millions of miles from the earth a distance 
 to which no other comet not even excepting the peculiar one 
 of 1729 had been pursued. 2 Moreover, an arc of 340 out of 
 the entire 360 degrees of its circuit had been described under 
 the eyes of astronomers ; so that its' course came to be very well 
 known. That its movement is in a very eccentric ellipse, tra- 
 versed in several hundred years, was very soon ascertained. 3 
 The later and exhaustive inquiries of Dr. H. Kreutz, of the 
 Kiel observatory, 4 have demonstrated that the period may 
 be as long as 1000, but cannot fall short of 772 years. This 
 unexpectedly wide range of doubt resulted from the impossi- 
 bility of determining the comet's true centre of gravity. It 
 closed every prospect of recognising its earlier apparitions. 
 
 The conclusion for a period to be counted by many centuries 
 assures us positively that the comet of 1882 was not a return of 
 any of the three bodies so singularly connected with it. But it 
 has little or no further application to them. 5 For very slightly 
 differing disturbances of each would suffice to develop a marked 
 variety in their periods. A loss of velocity at perihelion, for 
 instance, of only 49 metres a second (less than Yo-Juu" of the 
 whole), would bring a comet revolving in 175 years back in 37. 6 
 Yet the orbit would remain otherwise almost unchanged. A 
 body moving in it would perceptibly diverge from its former 
 track only at a considerable distance from the sun. Hence each 
 of these four comets (1668-1882) has doubtless if they be in 
 truth individually distinct a period of its own. 
 
 A fifth has since come to be associated with them. On the 
 1 8th of January, 1887, M. Thome discovered at Cordoba a comet 
 reproducing with curious fidelity the lineaments of that observed 
 
 1 A str. Nach., No. 2538. 2 Nature, vol. xxix., p. 135. 3 Astr. Nach., No. 
 2482. 4 Vierteljahrsschrift Astr. Ges., Jahrg. xxiv., p. 308 ; Bull. Astr., t. vii. f 
 p. 513. 5 The attention of the author was kindly directed to this point by 
 Professor Young of Princeton (N.J.)- 6 Rebeur-Paschwitz, /Sirius, Bd. xvi., 
 p. 233. This reasoning was designed to show the possible identity of the 
 comets of 1668, 1843, ancl 1880. 
 
438 HISTORY OF ASTRONOMY. PART n. 
 
 in the same latitudes seven years previously. The narrow 
 ribbon of light, contracting towards the sun, and running out- 
 ward from it to a distance of thirty-five degrees ; the unsub- 
 stantial head a veiled nothingness, as it appeared, since no 
 distinct nucleus could be made out ; the quick fading into 
 invisibility, were all accordant peculiarities, and they were con- 
 firmed by some rough calculations of its orbit, shewing geometri- 
 cal affinity to be no less unmistakable than physical likeness. 
 The observations secured were indeed, from, the nature of the 
 apparition, neither numerous nor over-reliable ; and the earliest 
 of them dated from a week after perihelion, passed, almost by a 
 touch-and-go escape, January II. On January 27, this mys- 
 terious object could barely be discerned telescopically at Cordoba. 1 
 That it belonged to the series of " southern comets " can scarcely 
 be doubted ; but the inference that it was an actual return of 
 the comet of 1880 is at present unsupported by any cogent 
 evidence. It is worth noting that M. Meyer appends the 
 " eclipse-comet " of 1882 to this extraordinary group. 2 
 
 The idea of cometary systems was first suggested by Thomas 
 Clausen in i83i. 3 It was developed by the acute inquiries of 
 the late M. Hoek, director of the Utrecht Observatory, in 1865 
 and some following years. 4 He found that in quite a consider- 
 able number of cases, the paths of two or three comets had a 
 common point of intersection far out in space, indicating with 
 much likelihood a community of origin. This consisted, accord- 
 ing to his surmise, in the disruption of a parent mass during its 
 sweep round the star latest visited. Be this as it may, the fact 
 is undoubted, that numerous comets fall into groups, in which 
 similar conditions of motion betray a pre-existent physical 
 connection. Never before, however, had geometrical relation- 
 ship been so notorious as between the three comets now under 
 consideration; and never before, in a comet still, it might be 
 said, in the prime of life, had physical peculiarities tending to 
 account for that affinity been so obvious as in the chief member 
 of the group. 
 
 1 Oppenheim, A sir. Nach., No. 2902. 2 Astr. Nack., No. 2717. 3 Gruithui- 
 sen's Analekten, Heft vii., p. 48. Month. Not., vols. xxv., xxvi., xxviii. Cf. 
 Plummer, Observatory, vol. xiii., p. 263. 
 
CHAP. xi. RECENT COMETS. 439 
 
 Observation of a granular structure in cometary nuclei dates 
 far back into the seventeenth century, when Cysatus and 
 Hevelius described the central parts of the comets of 1618 and 
 1652 respectively as made up of a congeries of minute stars. 
 Analogous symptoms of a loose state of aggregation have of 
 late been not unfrequently detected in telescopic comets, besides 
 the instances of actual division offered by those connected with 
 the names of Biela and Liais. The forces concerned in pro- 
 ducing these effects seem to have been peculiarly energetic in 
 the great comet of 1882. 
 
 The segmentation of the nucleus was first noticed in the 
 United States and at the Cape of Good Hope, September 30. 
 It proceeded rapidly. At Kiel, on October 5 and 7, Professor 
 Kriiger perceived two centres of condensation. A definite and 
 progressive separation into three masses was observed by Pro- 
 fessor Holden, October 13 and i/. 1 A few days later, M. Tempel 
 found the head to consist of four lucid aggregations, ranged 
 nearly along the prolongation of the caudal axis ; 2 and Dr. 
 Common, January 27, 1883, saw Jive nuclei in a line "like 
 pearls on a string." 3 This remarkable character was preserved 
 to the- last moment of the comet's distinct visibility. It was a 
 consequence, according to Dr. Kreutz, of violent interior action 
 in the comet itself while close to the sun. 
 
 There were, however, other curious proofs of a marked ten- 
 dency in this body to disaggregation. On October 9, Schmidt 
 discovered at Athens a nebulous object 4 south-west of the 
 great comet, and travelling in the same direction. It remained 
 visible for a few days, and, from Oppenheim's and Hind's cal- 
 culations, there can be little doubt that it was really the 
 offspring by fission of the body it accompanied. 4 This is 
 rendered more probable by the unexampled spectacle offered, 
 October 14, to Mr. E. E. Barnard, then of Nashville, Tennessee, 
 of six or eight distinct cometary masses within 6 south by west 
 of the comet's head, none of which reappeared on the next 
 opportunity for a search. 5 A week later, however, one similar 
 
 1 Nature, vol. xxvii., p. 246. * Astr. Nach. t No. 2468. 3 Athenaeum, 
 Feb. 3, 1883. 4 Astr. Nack., Nos. 2462, 2466. 5 Ibid., No. 2489. 
 
440 HISTORY OF ASTRONOMY. PART n. 
 
 object was discerned by Mr. W. E. Brooks, in the opposite 
 direction from the comet. Thus, space appeared to be strewn 
 with the filmy debris of this extraordinary body all along the 
 track of its retreat from the sun.* 
 
 Its tail was only equalled (if it were equalled) in length by 
 that of the comet of 1843. It extended in space to the vast 
 distance of two hundred millions of miles from the head ; but, 
 so imperfectly were its proportions displayed to terrestrial 
 observers, that it at no time covered an arc of the sky of more 
 than 30. This apparent extent was attained, during a few 
 days previous to September 25, by a faint, thin, rigid streak, 
 noticed only by a few observers by Elkin at the Cape Observa- 
 tory, Eddie at Grahamstown, and Cruls at Eio Janeiro. It 
 diverged at a low angle from the denser curved train, and was 
 produced, according to Bredichin, 1 by the action of a repulsive 
 force twelve times as strong as the counter-pull of gravity. It- 
 belonged, that is, to type I ; while the great forked appendage, 
 obvious to all eyes, corresponded to the lower rate of emission 
 characteristic of type 2. This was remarkable for the perfect 
 definiteness of its termination, for its strongly-forked shape, and 
 for its unusual permanence. Down to the end of January 1883, 
 its length, according to Schmidt's observations, was still 93 
 million miles ; and a week later it remained visible to the naked 
 eye, without notable abridgment. 
 
 Most singular of all was an anomalous extension of the 
 appendage towards the sun. During the greater part of October 
 and November, a luminous tube or sheath, of prodigious 
 dimensions, seemed to surround the head, and project in a 
 direction nearly opposite to that of the usual outpourings of 
 attenuated matter. (See Plate II.) Its diameter was computed by 
 Schmidt to be, October 15, no less than four million miles, and it 
 was described by Cruls as a " truncated cone of nebulosity," 
 stretching 3 or 4 sunwards. 2 This, and the entire anterior part 
 of the comet, were again surrounded by a thin, but enormously 
 voluminous paraboloidal envelope, observed by Schiaparelli for 
 
 1 Annales, Moscow, t. ix., pt. ii., p. 52. - Comptes Renclus, t xcvii., 
 P- 797- 
 
CHAP. xi. RECENT COMETS. 441 
 
 a full month from October 19.* There can be little doubt that 
 these abnormal effluxes were a consequence of the tremendous 
 physical disturbance suffered at perihelion ; and it is worth remem- 
 bering that something analogous was observed in the comet of 
 1680 (Newton's), also noted for its excessively close approach to 
 the sun. The only plausible hypothesis as to the mode of their 
 production is that of an opposite state of electrification in the 
 particles composing the ordinary and extraordinary appendages. 
 
 The spectrum of the great comet of 1882 was, in part, a 
 repetition of that of its immediate predecessor, thus confirming 
 the inference that the previously unexampled sodium-blaze was 
 in both a direct result of the intense solar action to which they 
 were exposed. But the D line was, this time, not seen alone. 
 At Dunecht, on the morning of September 18, Drs. Copeland 
 and J. G. Lohse succeeded in identifying six brilliant rays in 
 the green and yellow with as many prominent iron-lines ; 2 a 
 very significant addition to our knowledge of cometary constitu- 
 tion, and one which goes far to justify Bredichin's assumption of 
 various kinds of matter issuing from the nucleus with velocities 
 inversely as their atomic weights. All the lines equally showed 
 a slight displacement, indicating a recession from the earth of 
 the radiating body at the rate of 37 to 46 miles a second. A 
 similar observation, made by M. Thollon at Nice on the same 
 day, gave emphatic sanction to the spectroscopic method of esti- 
 mating movement in the line of sight. Before anything was as 
 yet known of the comet's path or velocity, he announced, from 
 the position of the double sodium-line alone, that at 3 P.M. on 
 September 18 it was increasing its distance from our planet by 
 from 6 1 to 76 kilometres per second. 3 M. Bigourdan's subse- 
 quent calculations showed that its actual swiftness of recession 
 was at that moment 73 kilometres. 
 
 Changes in the inverse order to those seen in the spectrum 
 of comet Wells, soon became apparent. In the earlier body, 
 carbon bands had died out with approach to perihelion, and had 
 been replaced by sodium-emissions ; in its successor, sodium 
 
 1 Astr. Nacli., No. 2966. ' 2 Copernicus, vol. ii., p. 235. 3 Comptes Rendus, 
 t. xcvi., p. 371. 
 
442 HISTORY OF ASTRONOMY. PART n. 
 
 emissions became weakened and disappeared with retreat from 
 perihelion, and found their substitute in carbon-bands. Pro- 
 fessor Kicco was, in fact, able to infer, from the sequence of 
 prismatic phenomena, that the ' comet had already passed the 
 sun ; thus establishing a novel criterion for determining the 
 position of a comet in its orbit by the varying quality of its 
 radiations. 
 
 Recapitulating what has been learnt from the five conspicuous 
 comets of 1880-82, we find that the leading facts acquired to 
 science were these three. First, that comets may be met with 
 pursuing each other, after intervals of many years, in the same, 
 or nearly the same, track ; so that identity of orbit can no longer 
 be regarded as a sure test of individual identity. Secondly, 
 that at least the outer corona may be traversed by such 
 bodies with perfect apparent impunity. Finally, that their 
 chemical constitution is a highly complex one, and that they 
 possess, in some cases at least, a metallic core resembling the 
 meteoric masses which occasionally reach the earth from 
 planetary space. 
 
 A group of five comets, including Halley's, own a sort of 
 cliental dependence upon the planet Neptune. They travel out 
 from the sun just to about his distance from it, as if to pay 
 homage to a powerful protector, who gets the credit of their 
 establishment as periodical visitors to the solar system. The 
 second of these bodies to effect a looked-for return was a comet 
 the sixteenth within ten years discovered by Pons, July 20, 
 1812, and found by Encke to revolve in an elliptic orbit, with a 
 period of nearly 71 years. It was not, however, until Septem- 
 ber i, 1883, that Mr. Brooks caught its reappearance; it passed 
 perihelion January 25, and was last seen June 2, 1884. At its 
 brightest, it had the appearance of a second-magnitude star 
 furnished with a poorly-developed double tail, and was fairly 
 conspicuous to the naked eye in southern Europe from Decem- 
 ber to March. One exceptional feature distinguished it : its 
 fluctuations in form and luminosity were unprecedented in 
 rapidity and extent. On September 21, Dr. Chandler 1 observed 
 1 Astr. Nach., No. 2553. 
 
CHAP. xi. RECENT COMETS. 443 
 
 it at Harvard as a very faint, diffused nebulosity, with slight 
 central condensation. On the next night there was found in its 
 place a bright star of the eighth magnitude, scarcely marked out, 
 by a bare trace of environing haze, from the genuine stars it 
 counterfeited. The change was attended by an eight-fold 
 augmentation of light, and was proved by Schiaparelli ? s confir- 
 matory observations x to have been accomplished within a few 
 hours. The stellar disguise was quickly cast aside. The comet 
 appeared on September 23, as a wide nebulous disc, and soon 
 after faded down to its original dimness. Its distance from the 
 sun was then no less than 200 million miles, and its spectrum 
 showed nothing unusual. These strange variations recurred 
 slightly on October 15, and with marked emphasis January i, 
 when they were witnessed with amazement, and photometrically 
 studied by Dr. Miiller of Potsdam. 2 The entire cycle this time 
 was run through in less than four hours the comet having, in 
 that brief space, condensed, with a vivid outburst of light, into 
 a seeming star, and the seeming star having expanded back 
 again into a comet. Scarcely less transient, though not 
 altogether similar, changes of aspect were noted by M. Perrotin, 
 January 13 and 19, i884- 3 On the latter date, the continuous 
 spectrum given by a reddish-yellow disc surrounding the true 
 nucleus, seemed intensified by bright knots corresponding to the 
 rays of sodium. 
 
 A comet discovered by Mr. Sawerthal at the Koyal Observatory, 
 Cape of Good Hope, February 19, 1888, distinguished itself by 
 blazing up, on May 19, to four or five times its normal brilliancy, 
 at the same time throwing out from the head two lustrous lateral 
 branches. 4 These had, on June I, spread backward so as to join 
 the tail, with an effect like the playing of a fountain ; ten or 
 eleven days later, they had completely disappeared, leaving the 
 comet in its former shape and insignificance. Its abrupt 
 display of vitality occurred no less than two months after 
 perihelion. 
 
 1 Astr. Nach., No. 2553. ~ Ibid., No. 2568. 3 Annales de V Observatoire 
 de Nice, t. ii., c. 53. 4 Fenyi, Astr. Nach., No. 2844; Kamruermann, Ibid., 
 No. 2849. 
 
444 HISTORY OF ASTRONOMY. PART n. 
 
 On the morning of July 7, 1889, Mr. W. E. Brooks, of 
 Geneva, New York, next to Barnard of Lick the most successful 
 comet-hunter of our time, secured one of his customary trophies. 
 The faint object in question was moving through the constella- 
 tion Cetus, and turned out to be a member of Jupiter's numerous 
 family of comets, revolving round the sun in a period of seven 
 years. Its past history came then, to a certain extent, within 
 the scope of investigation, and proved to have been singularly 
 eventful ; nor had the body escaped scatheless from the vicissi- 
 tudes to which it had been exposed. Observing from Mount 
 Hamilton, August 2 and 5, Professor Barnard noticed this 
 comet (1889, v -) to ^ e attended in its progress through space by 
 four outriders. " The two brighter companions " (the fainter pair 
 survived a very short time) "were perfect miniatures," Professor 
 Barnard tells us, 1 " of the larger cornet, each having a small, 
 fairly-defined head and nucleus, with a faint, hazy tail, the more 
 distant one being the larger and less developed. The three 
 comets were in a straight line, nearly east and west, their tails 
 lying along this line. There was no connecting nebulosity 
 between these objects, the tails of the two smaller not reaching 
 each other, or the large comet. To all appearance they were 
 absolutely independent comets." Nevertheless, Spitaler, at 
 Vienna, in the early days of August, perceived, as it were, a 
 thin cocoon of nebulosity woven round the entire trio. 2 One of 
 them faded from view September 5 ; the other actually outshone 
 the original comet on August 31, but was plainly of inferior 
 vitality. It was last seen by Barnard on November 25, with the 
 thirty-six inch refractor, while its primary afforded an ob- 
 servation for position with the twelve-inch, March 20, i89O. 3 A 
 cause for the disruption it had presumably undergone had, before 
 then, been plausibly assigned. 
 
 The adventures of Lexell's comet have long served to exemplify 
 the effects of Jupiter's despotic sway over such bodies. Although 
 bright enough in 1770 to be seen with the naked eye, and 
 ascertained to be circulating in five and a half years, it had never 
 
 1 PuU. Astr. Pac. Soc., vol. i., p. 72. - Annuaire, Paris, 1891, p. 301. 
 Astr. Nach., No. 2989. 
 
CHAP. xi. RECENT COMETS. 445 
 
 previously been seen, and failed subsequently to present itself. 
 The explanation of this anomaly, suggested by Lexell, and fully 
 confirmed by the analytical inquiries both of Laplace and 
 Leverrier, 1 was that a very close approach to Jupiter in 1767 
 had completely changed the character of its orbit, and brought 
 it within the range of terrestrial observation; while in 1779, 
 after having only twice traversed its new path (at its second 
 return it was so circumstanced as to be invisible from the earth), 
 it was, by a fresh encounter, diverted into one entirely different. 
 Yet the possibility was not lost sight of that the great planet, 
 by inverting its mode of action, might undo its own work, and 
 fling the comet once more into the inner parts of the solar 
 system. This possibility, it now seems, has been actually 
 realised. The identification of Brooks's with Lexell's comet is 
 due to the acumen of Dr. Chandler. 2 He found that the former 
 body had spent eight months in 1886 under Jupiter's immediate 
 control had, in fact, barely escaped being reduced to the position 
 of his satellite and had issued from the proximity with all the 
 elements of its motion turned, so to speak, topsy-turvy. More- 
 over, its previous orbit proved singularly like that calculated by 
 Leverrier for Lexell's comet after 1779 ; and the pursuance back- 
 ward from 1886 of its career in that orbit brought out the striking 
 fact that it had encountered Jupiter in 1779 at the same time 
 and place with the lost comet of the last century. The inference 
 that the two were one was irresistible, and was quickly ratified 
 by Mr. Charles Lane Poor's more rigorous computations. 3 They 
 showed three revolutions, rendered of unequal length by Saturn's 
 perturbing influence, to have been performed between 1779 and 
 1886; and the approach to the Jovian surface in the latter 
 year to have been within a single Jovian radius. So that the 
 comet passed inside the orbit of the new inner satellite ! The 
 scene of encounter, moreover, coincides, according to Bredichin's 
 determinations, 4 with the point of intersection between the paths 
 of the subordinate comets discovered by Barnard and that of the 
 chief body, rendering it all but certain that the three had, until 
 
 Comptes fiendus, t. xxv., p. 564. 2 Astr. Jour., No. 205. 3 Ibid., Nos. 
 228, 244. 4 Chandler, Ibid., No. 231. 
 
446 HISTORY OF ASTRONOMY. PART n. 
 
 then, formed one mass. The agent of disruption was, by 
 Chandler's conjecture, the fourth satellite of Jupiter. The next 
 meeting of the Lexell-Brooks comet with the formidable vice- 
 gerent of the sun will be in 1 9^ I, 1 when the event of 1779 is 
 likely to be repeated, with such modifications as the varied 
 conditions may bring about. 
 
 On the morning of March 7, 1892, Professor Lewis Swift, 
 director of the Warner Observatory at Rochester, N.Y., dis- 
 covered the brightest comet visible to northern observers since 
 1882. About the time of perihelion, which occurred on' April 6, 
 it was conspicuous, as it crossed the celestial equator from 
 Aquarius towards Pegasus, with a nucleus equal to a third 
 magnitude star, and a tail twenty degrees long. This tail was 
 multiple, and multiple in a most curiously variable manner. It 
 divided up into many thin nebulous streaks, the number and 
 relative lustre of which underwent rapid and marked changes. 
 Their permanent records on Professor Barnard's plates is one of 
 the latest achievements of celestial photography. Plate iii. re- 
 produces two of his pictures, taken with a six-inch camera, on 
 April 5 and 7 respectively, with, in each case, an exposure of 
 about one hour. The tail is in the first composed of three main 
 branches, the middle one having sprung out since the previous 
 morning, and the branches are, in their turn, split up into finer 
 rays, to the number of perhaps a dozen in all. In the second, a 
 very different state of things is exhibited. " The southern com- 
 ponent," Professor Barnard remarks, " which was the brightest 
 on the 5th, had become diffused and fainter, while the middle 
 tail was very bright and broad. Its southern side, which was 
 the best defined, was wavy in numerous places, the tail appear- 
 ing as if disturbing currents were flowing at right angles to it. 
 At 42 from the head the tail made an abrupt bend towards the 
 south, as if its current was deflected by some obstacle. In the 
 densest portion of the tail, at the point of deflection, are a couple 
 of dark holes, similar to those seen in some of the nebulas. The 
 middle portion of the tail is brighter, and looks like crumpled 
 silk in places. The width of the tail at 2 from the head is 
 
 1 Barnard, Publ. Astr. Pac. Soc., vol. ii., p. 24. 
 
PLATE III. 
 
 1. 2. 
 
 PHOTOGRAPHS OF SWIFT'S COMET, 
 By Professor E. E. BARNARD. 
 
 XT- -i T,,*,,,,. A j.*,;j A -i on--) r?^A.. -n. 
 
CHAP. xi. RECENT COMETS. 447 
 
 54'." 1 Next morning the southern was the prominent branch, 
 and it was loaded, at i 42' from the head, with a strange excres- 
 cence, suggesting the budding-out of a fresh comet in that in- 
 congruous situation. 2 Some of these changes, Professor Barnard 
 thought, might possibly be explained by a rotation of the tail on 
 an axis passing through the nucleus ; but moonlight conspired 
 with clouds to prevent anything definite being learned on the 
 point. 
 
 Seven comets were detected in 1892, and all, strange to say, 
 were visible together towards the close of the year. 3 Among 
 them was a faint object, which unexpectedly left a trail on a 
 plate, exposed by Professor Barnard to the stars in Aquila 4 
 on October 1 2. This was the first comet actually discovered by 
 photography, the Sohag comet having been simultaneously seen 
 and pictured. It has a period of about six years. Holmes's 
 comet is likewise periodical, in rather less than seven years. Its 
 path, which is wholly comprised between the orbits of Mars and 
 Jupiter, is less eccentric than that of any other known comet. 
 Since its discovery, on November 6, it has undergone some curious 
 vicissitudes. At first bright and condensed, it expanded rapidly 
 with increasing distance from the sun (to which it had made its 
 nearest approach June 1 3), until, by the middle of December, 
 "it was merely a great and feebly luminous mist on the face of 
 the sky," 5 barely discernible with powerful telescopes. But on 
 January 16, 1893, observers in Europe and America were be- 
 wildered to find, as if substituted for it, a yellow star of the 
 seventh magnitude, slightly enveloped in a minute nebulous 
 mist, within which lay the faint miniature of a tail. 6 This 
 condensation and recovery of light did not, however, last in its 
 full intensity more than a couple of days. The spectrum of 
 Holmes's comet was almost purely continuous. A mere trace of 
 the carbon-bands was discernible in it, 
 
 The origin of comets has been long and eagerly inquired into, 
 
 3 Astr. and Astro-Physics, May, 1892, p. 387. 2 See Knowledge, Dec. 1892. 
 :} H. C. Wilson, Astr. and Astro- Physics, Feb. 1893, p. 121. 4 Observatory, 
 vol. xvi., p. 92. 5 Barnard, Astr. and Astro- Physics, Feb. 1893, P- !8o. 
 6 Palisa, Astr. Nach., No. 3147 ; Denning, Observatory, vol. xvi., p. 142. 
 
448 HISTORY OF ASTRONOMY. PART n. 
 
 not altogether apart from the cheering guidance of ascertained 
 facts. Sir William Herschel regarded them as fragments of 
 nebulae l scattered debris of embryo worlds ; and Laplace ap- 
 proved of and adopted the ielea. 2 But there was a difficulty. 
 No comet has yet been observed to travel in a decided hyperbola. 
 The typical cometary orbit, apart from disturbance, is parabolic ; 
 that is to say, it is indistinguishable from tan enormously long 
 ellipse. But this circumstance could only be reconciled with the 
 view that the bodies thus moving were casual visitors from outer 
 space, by making, as Laplace did, the tacit assumption that the 
 solar system was at rest. His reasoning was indeed thereby 
 completely vitiated, as Gauss pointed out in 1 8 1 5 ; 3 and his 
 objections were reiterated by Schiaparelli, 4 whe demonstrated in 
 1 87 1 that a large preponderance of well-marked hyperbolic orbits 
 should result in comets picked up en route by a swiftly-advanc- 
 ing sun. The fact that their native movement is practically 
 parabolic, shows it to have been wholly imparted from without. 
 They passively obeyed the pull exerted upon them. In other 
 words, their condition previous to being attracted by the sun 
 was one of relative repose. 5 They shared, accordingly, the 
 movement of translation through space of the solar system. 
 
 This significant conclusion had been indicated, on other 
 grounds, as the upshot of researches undertaken independently 
 by Carrington 6 and Mohn 7 in 1 860, with a view to ascertain- 
 ing the anticipated existence of a relationship between the 
 general lie of the paths of comets and the direction of the 
 sun's journey. It is tolerably obvious that, if they wander at 
 haphazard through interstellar regions, a preponderance of 
 their apparitions should seem to arrive from the vicinity of 
 the constellation Hercules ; that is to say, we should meet 
 considerably more comets than would overtake us, for the very 
 same reason that falling stars are more numerous, after than 
 before midnight. Moreover, the comets met by us should be 
 
 1 Phil. Trans, vol. ci., p. 306. " Conn.des Temps. 1816, p. 213. 3 (Euvres, 
 t. vi., p. 581. 4 Mem. dell' Istit. Lombardo, t. xii., p. 164; fiendiconti, t. vii., 
 p. 77, 1874. 5 W. Forster, Pop. Mitth. 1879, p. 7. 6 Mem. R. A. Soc., 
 vol. xxix., p. 335. 7 Month. Not., vol. xxiii., p. 203. 
 
CHAP. xi. RECENT COMETS. 449 
 
 apparently swifter-moving objects than those coming up with 
 us from behind ; because, in the one case, our own real move- 
 ment would be added to, in the other, subtracted from theirs. 
 But nothing of all this can be detected. Comets approach the 
 sun indifferently from all quarters, and with velocities quite 
 independent of direction. 
 
 We conclude then that the " cosmical current " which bears 
 the solar system towards its unknown goal, carries also with it 
 nebulous masses of undefined extent, and at an undefined re- 
 moteness, fragments detached from which, continually entering 
 the sphere of the sun's attraction, flit across our skies under the 
 form of comets. These are, however, almost certainly so far 
 strangers to our system that they had no part in the long 
 processes of development by which its present condition was 
 attained. They are, perhaps, survivals of an earlier, and by us 
 scarcely and dimly conceivable state of things, when the chaos 
 from which sun and planets were, by a supreme edict, to 
 emerge, had not as yet separately begun to be. 
 
 29 
 
CHAPTER XII. 
 
 STARS AND NEBULA. 
 
 THAT a science of stellar chemistry should not only have become 
 possible, but should already have made material advances, is 
 assuredly one of the most amazing features in the swift progress 
 of knowledge our age has witnessed. Custom can never blunt 
 the wonder with which we must regard the achievement of 
 compelling rays emanating from a source devoid of sensible 
 magnitude through immeasurable distance, to reveal, by its 
 peculiarities, the composition of that source. The discovery of 
 revolving double stars assured us that the great governing force 
 of the planetary movements, and of our own physical existence, 
 sways equally the courses of the farthest suns in space ; the 
 application of prismatic analysis certified to the presence in the 
 stars of the familiar materials, no less of the earth we tread, 
 than of the human bodies built up out of its dust and circum- 
 ambient vapours. 
 
 We have seen that, as early as 1823, Fraunhofer ascertained 
 the generic participation of stellar light in the peculiarity by 
 which sunlight, spread out by transmission through a prism, 
 shows numerous transverse rulings of interrupting darkness. 
 No sooner had Kirchhoff supplied the key to the hidden mean- 
 ing of those ciphered characters, than it was eagerly turned to 
 the interpretation of the dim scrolls unfolded in the spectra of 
 the stars. Donati made at Florence, in 1 860, the first efforts 
 in this direction; but with little result, owing to the imper- 
 fections of the instrumental means at his command. His 
 comparative failure, however, was a prelude to others' success. 
 Almost simultaneously, in 1862, the novel line of investigation 
 
CHAP. xii. STARS AND NEBULAE. 45* 
 
 was entered upon by Huggins and Miller near London, by 
 Father Secchi at Eome, and by Lewis M. Rutherfurd in New 
 York. Fraunhofer's device of using a cylindrical lens for the 
 purpose of giving a second dimension to stellar spectra, was 
 adopted by all, and was indeed indispensable. For a luminous 
 point, such as a star appears, becomes, when viewed through a 
 prism, a variegated line, which, until broadened into a band by 
 the intervention of a cylindrical lens, is all but useless for 
 purposes of research. This process of rolling out involves, it is 
 true, much loss of light a scanty and precious commodity, as 
 coming from the stars ; but the loss is an inevitable one. And 
 so fully is it compensated by the great light-grasping power of 
 modern telescopes, that important information can now be gained 
 from the spectroscopic examination of stars far below the range 
 of the unarmed eye. 
 
 The effective founders of stellar spectroscopy, then (since 
 Eutherfurd shortly turned his efforts elsewhither), were Father 
 Secchi, the eminent Jesuit astronomer of the Collegio Romano, 
 where he died, February 26, 1878, and Dr. Huggins, with 
 whom the late Professor W. A. Miller was associated, The 
 work of each was happily directed so as to supplement that of 
 the other. With less perfect appliances, the Roman astronomer 
 sought to render his extensive rather than precise ; at Tulse 
 Hill, searching accuracy over a narrow range was aimed at and 
 attained. To Father Secchi is due the merit of having executed 
 the first spectroscopic survey of the heavens. Above 4000 stars 
 were in all passed in review by him, and classified according to 
 the varying qualities of their light. His provisional establish- 
 ment (1863-67) of four types of stellar spectra 1 has proved a 
 genuine aid to knowledge through the facilities afforded by it 
 for the arrangement and comparison of rapidly accumulating 
 facts. Moreover, it is scarcely doubtful that these spectral 
 distinctions correspond to differences in physical condition of a 
 marked kind. 
 
 1 Report Brit. Ass., 1868, p. 166. Eutherford gave a rudimentary sketch of 
 a classification of the kind in December 1862, but based on imperfect observa- 
 tions. See Am. Jour, of /Sc., vol. xxxv., p. 77. 
 
452 HISTORY OF ASTRONOMY. PART n. 
 
 The first order comprises more than half the visible stars, and 
 a still larger proportion of those eminently lustrous. Sirius, 
 Crucis, Vega, Regulus, Altair, are among its leading mem- 
 bers. Their spectra are distinguished by the breadth and 
 intensity of the four dark bars due to the absorption of 
 hydrogen, and by the extreme faintness of the metallic lines, of 
 which, nevertheless, hundreds are disclosed % careful examina- 
 tion. The light of these " Sirian" orbs is white or bluish ; and 
 it is found to be rich in ultra-violet rays. 
 
 Capella and Arcturus belong to the second, or solar type of 
 stars, which is about one-sixth less numerously represented than 
 the first. Their spectra are closely similar to that of sunlight, 
 in being ruled throughout by innumerable fine dark lines ; and 
 they share its yellowish tinge. 
 
 The third class includes most red and variable stars (com- 
 monly synonymous), of which Betelgeux in the shoulder of 
 Orion, and "Mira" in the Whale, are noted examples. Their 
 characteristic spectrum is of the "fluted" description. It 
 shows like a strongly illuminated range of seven or eight 
 variously tinted columns seen in perspective, the light falling 
 from the red end towards the violet. This kind of absorption 
 is produced by the vapours of metalloids or of compound 
 substances. 
 
 To the fourth order of stars belongs also a colonnaded spec- 
 trum, but reversed; the light is thrown the other way. The 
 three broad zones of absorption which interrupt it are sharp 
 towards the red, insensibly gradated towards the violet end. 
 The individuals composing Class IV. are few, and apparently 
 insignificant, the brightest of them not exceeding the fifth 
 magnitude. They are commonly distinguished by a deep red 
 tint, and gleam like rubies in the field of the telescope. Father 
 Secchi, who detected the peculiarity of their analysed light, 
 ascribed it to the presence of carbon in some form in their atmo- 
 spheres ; and this was confirmed by the researches of H. C. 
 Vogel, 1 director of the Astro-physical Observatory at Potsdam. 
 The hydro-carbon bands, in fact, seen bright in comets, are dark 
 
 1 Publicationen, Potsdam, No. 14, 1884, p. 31. 
 
CHAP. xii. STARS AND NEBULAE. 453 
 
 in these singular objects the only ones in the heavens (save 
 one bright line star and a rare meteor) l which display a 
 cometary analogy of the fundamental sort revealed by the 
 spectroscope. 
 
 The members of all four orders are, however, emphatically 
 suns. They possess, it would appear, photospheres radiating all 
 kinds of light, and differ from each other mainly in the varying 
 qualities of their absorptive atmospheres. The principle that the 
 colours of stars depend, not on the intrinsic nature of their light, 
 but on the kinds of vapours surrounding them, and stopping out 
 certain portions of that light, was laid down by Huggins in i864. 2 
 Moreover, the phenomena of double stars seem to indicate a 
 connection between the state of the investing atmospheres by 
 the action of which their often brilliantly contrasted tints are 
 produced, and their mutual physical relations. A remarkable 
 tabular statement put forward by Professor Holden in June 
 i88o 3 made it, at any rate, clear that inequality of magnitude 
 between the components of binary systems accompanies unlike- 
 ness in colour, and that stars more equally matched in one 
 respect, are pretty sure to be so in the other. Besides, blue and 
 green stars of a decided tinge are never (so far as is certainly 
 known) solitary ; they invariably form part of systems. So 
 that association has undoubtedly a predominant influence upon 
 colour. 
 
 Nevertheless, the crude notion thrown out by Zollner in 1 865,* 
 that yellow and red stars are simply white stars in various stages 
 of cooling, obtained for a time undeserved currency. D' Arrest 
 indeed protested against it, and Angstrom, in i868, 5 substituted 
 for the mere criterion of colour 6 as regards age and tem- 
 
 1 Von Konkoly once derived from a slow-moving meteor a hydro-carbon 
 spectrum. A. 8. Herschel, Nature, vol. xxiv., p. 507. 2 Phil. Trans., vol. 
 cliv., p. 429. 3 Am. Jour, of Sc., vol. xix., p. 467. 4 Photom. Unters., p. 243. 
 s Spectre Solaire, p. 38. 6 Mr. J. Birmingham, in the Introduction to his 
 Catalogue of Ked Stars, adduces sundry instances of colour-change in a 
 direction the opposite to that assumed by Zollner to be the inevitable result of 
 time. Trans., B. Irish Acad. } vol. xxvi., p. 251. A learned discussion by Dr. 
 T. J. J. See of Chicago, moreover, leaves little doubt that Sirius was genuinely 
 red eighteen hundred years ago. Astr. and Astro-Physics, April and May, 1892. 
 
454 HISTORY OF ASTRONOMY. PART n. 
 
 perature, that of atmospheric quality and composition. His 
 lead was followed by Lockyer in I873, 1 and by Vogel in I8/4. 2 
 The scheme of classification due to the Potsdam astro-physicist 
 differed from Father -Secchi's ojily in presenting his third and 
 fourth types as subdivisions of the same order, and in inserting 
 three subordinate categories; but their variety was "ration- 
 alised" by the addition of the seductive idea of progressive 
 development. Thus, the white Sirian stars were represented as 
 the youngest because the hottest of the sidereal family ; those of 
 the solar pattern as having already wasted much of their store 
 by radiation, and being well advanced in middle life ; while the 
 red stars with banded spectra figured as effete suns, hastening 
 rapidly down the road to final extinction. 
 
 Vogel's scheme is, however, incomplete. It traces the down- 
 ward curve of decay, but gives no account of the slow ascent to 
 maturity. The present splendour of Vega, for instance, was 
 prepared, according to all creative analogy, by almost endless 
 processes of gradual change. What was its antecedent con- 
 dition ? The question has been variously answered. Dr. John- 
 stone Stoney advocated, in 1867, the comparative youth of red 
 stars; 3 A. Eitter, of Aix-la-Chapelle, divided them in i883 4 
 into two squadrons, posted, the one on the ascending, the other 
 on the descending branch of the temperature curve, and corre- 
 sponding, presumably, with Secchi's third and fourth orders of 
 stars with banded spectra. Whether, in the interim, they 
 should display spectra of the Sirian or of the solar type, was made 
 to depend on their greater or less massiveness. 5 This relation 
 is, however, now certainly known to be non-existent. Indeed, 
 Mr. Maunder has lately brought forward some evidence in 
 favour of the opinion that the average solar star is a weightier 
 body than the average Sirian star. 6 
 
 On November 17, 1887, Professor Norman Lockyer communi- 
 cated to the Royal Society the first of a series of papers em- 
 bodying his "Meteoritic Hypothesis" of cosmical constitution, 
 
 1 Phil Trans,, vol. clxiv., p. 492. 2 Astr. Nach., No. 2000. 3 Proc. B. 
 Soc., vols. xvi., p. 31, xvii., p. 48. 4 Annalen der Physik, Bd. xx. p. 155. 
 6 Ibid., p. 153. 6 Knowledge, vol. xiv., p. 101. 
 
CHAP. xii. STARS AND NEBULA. 455 
 
 stated and supported more at large in a separate work bearing 
 that name, published in 1890. The fundamental proposition 
 wrought out in it was that " all self-luminons bodies in the celestial 
 space are composed either of swarms of meteorites, or of masses 
 of meteoritic vapour produced by heat." 1 On the basis of this 
 supposed community of origin, sidereal objects were distributed in 
 seven groups along a temperature-curve ascending from nebulsa 
 and gaseous or bright-line stars, through red stars of the third 
 type, and a younger division of solar stars, to the high Sirian 
 level ; then descending through the more strictly solar stars to 
 red stars of the fourth type (" carbon-stars "), below which lay 
 only the caput mortuum entitled Group vii. The groundwork 
 of this classification has not, it is true, proved solid. Certain 
 spectroscopic coincidences, avowedly only approximate, suggest- 
 ing that stars and nebulse of every species might be formed out 
 of variously aggregated meteorites, have not, on more critical 
 inquiry, proved exact. And spectroscopic coincidences admit of 
 no compromise. Either they are absolute, or they are worthless. 
 The attendant evolutionary order is, however, capable of being 
 supported on its own merits. 
 
 In his Presidential Address at the Cardiff Meeting of the 
 British Association in 1891, Dr. Huggins adhered in the main 
 to the line of advance traced by Vogel. The inconspicuousness 
 of metallic lines in the spectra of the white stars, he attributed, 
 not to the paucity, but to the high temperature of the vapours 
 producing them, and the consequent deficiency of contrast 
 between their absorption-rays and the continuous light of the 
 photospheric background. " Such a state of things would more 
 probably," in his opinion, "be found in conditions anterior to 
 the solar stage," while " a considerable cooling of the sun would 
 probably give rise to banded spectra due to compounds." 
 
 The strongest evidence for the primitive state of white stars 
 is found in their nebular relations. The components of mixed 
 groups, such as the Pleiades, show, perhaps invariably, spectra 
 of the first type, occasionally crossed by bright rays. It is 
 indeed not unlikely that all stars so circumstanced pass through 
 
 1 Meteoritic Hypothesis, p. 380. 
 
456 HISTORY OF ASTRONOMY. PART n. 
 
 a bright-line stage in condensing from the surrounding nebulous 
 matter into suns resembling Sirius and Vega. Relative density 
 furnishes another important test of comparative age, and Sirian 
 stars appear, on the whole, to b& more bulky proportionately to 
 their mass than solar stars. But this is far from being an 
 invariable rule; hence the change from one kind of spectrum 
 to the other is not inevitably connected with the attainment of a 
 particular degree of condensation. Nor is it to be supposed 
 that all stars are identical in constitution, and present identical 
 life-histories. 1 Stellar types may be the badges of species 
 originally distinct, and destined to remain so. Their specialities 
 of distribution in the heavens favour this view ; nor is there any 
 sign that the smaller, and therefore more rapidly cooling and 
 condensing members of physical systems are any nearer 
 to the solar stage than their large companions. From these, 
 and other corresponding facts, Mr. Maunder infers "that 
 spectrum type does not primarily or usually denote epoch of 
 stellar life, but rather a fundamental difference of chemical 
 constitution." 2 
 
 The first-known examples of the class of gaseous stars 
 j8 Lyrae and y Cassiopeige were noticed by Father Secchi at the 
 outset of his spectroscopic inquiries. Both show bright lines of 
 hydrogen and helium, so that the peculiarity of their condition 
 probably consists in the intense ignition of their chromospheric 
 surroundings. Their entire radiating surfaces might be described 
 as faculous. That is to say, brilliant formations, such as have 
 been photographed by Professor Hale on the sun's disc, 3 cover 
 perhaps the whole, instead of being limited to a small portion of 
 the photospheric area. But this state of things is inconstant. The 
 brilliant rays indicative of it fade and flash out again with very 
 singular alternations. Dr. VogePs observations at Bothkamp in 
 1871-2 already afforded him a suspicion of such vicissitudes ; 4 but 
 their ascertainment is due to M. Eugen von Gothard. After 
 the completion of his new astrophysical observatory at Hereny in 
 
 1 See the author's System of the /Stars, p. 84. 2 Jour. Brit. Astr. Ass., 
 vol. ii., p. 39; Astr. and Astr o-Phy sties, Feb. 1892, p. 150. 3 See ante, p. 245. 
 4 Bothkamp Beobaclitungen, Heft ii., p. 146. 
 
CHAP. xii. STARS AND NEBULAE. 457 
 
 the autumn of 1 88 1, he repeatedly observed the spectra of both 
 stars without perceiving a trace of bright lines ; and was thus 
 taken quite by surprise when he caught a twinkling of the 
 crimson C in y Cassiopeise, August 13, I883- 1 A few days later, 
 the whole range (including D 3 ) was lustrous. Duly apprised of 
 the recurrence of a phenomenon he had himself vainly looked 
 for during some years, M. von Konkoly took the opportunity of 
 the great Vienna refractor being placed at his disposal to 
 examine with it the relighted spectrum on August 2/. 2 In its 
 wealth of light C was dazzling ; D 3 , and the green and blue 
 hydrogen rays, shone somewhat less vividly ; D and the group b 
 showed faintly dark ; while three broad absorption-bands, 
 sharply terminated towards the red, diffuse towards the violet, 
 shaded the spectrum near its opposite extremities. They thus 
 agreed with the zones of " carbon-stars " in the plan of their 
 structure, though not at all in position ; but proved significantly 
 what the spectrum of T Coronas had already rendered apparent 
 the compatibility in stellar atmospheres of fluted absorption 
 with a high state of incandescence. 
 
 The previous absence of bright lines from the spectrum of this 
 star was, however, by no means so protracted or complete as M. von 
 Gothard supposed. At Dunecht, C was " superably visible " to 
 Lord Lindsay, Drs. Copeland, and J. G. Lohse, December 20, 
 1 879 ; 3 F was seen bright on October 28 of the same year, and 
 frequently at Greenwich in 1880- 1, indeed much more con- 
 spicuously so than three years later. The curious fact has more- 
 over been adverted to by Dr. Copeland, that C is much more 
 variable than F. To Vogel, June 18, 1872, the first was invisible, 
 while the second was bright ; at Dunecht, January 1 1, 1887, the 
 conditions were so far inverted that C was resplendent, F com- 
 paratively dim. 
 
 No spectral fluctuations were detected by Keeler in 1889; but 
 even with the giant telescope of Mount Hamilton, the helium- 
 ray was completely invisible. 4 Nor is there any record of its 
 
 1 Astr. Nach., No. 2539. 2 Ibid., No. 2548; Observatory, vol. vi., p. 332. 
 s Month. Not., vol. xlvii., p. 92. 4 Publ. Astr. Pac. Soc., vol. i., p. 80; Obser- 
 vatory, vol. xiii., p. 46. 
 
458 HISTORY OF ASTRONOMY. PART n. 
 
 appearance in this star since 1883, save on September 19, 1884, 
 when it was faintly seen by Mr. Maunder. 
 
 The spectrum of |3 Lyrse is subject to analogous transitions. 
 Perfectly continuous, "as observed at Hereny, June 17 and July 
 24, it was interrupted by dark lines of hydrogen, September 5> 
 I882. 1 A year later (August 26, 1883), Von Gothard first saw 
 them in bright relief. The helium-ray was, diowever, found to 
 vary independently of, and even more strikingly than, the 
 hydrogen-lines. During 1884 it was followed through several 
 complete cycles from dazzling brilliancy to total extinction, in a 
 period of a few days. 2 Now /3 Lyrso is a " short-period variable." 
 Its light changes with great regularity from 3.4 to 4.4 magnitude 
 every twelve days and twenty-two hours, during which time it 
 attains a twofold maximum, with an intervening secondary 
 minimum. The question then is of singular interest, whether 
 the changes of luminous quality visible in this object correspond 
 to its changes in luminous quantity ? A distinct answer in the 
 affirmative has been supplied by photographic means. Mrs. M. 
 Fleming, in examining the Harvard plates of the star's spectrum 
 in 1891, found recorded upon them diverse complex changes of 
 bright and dark lines, obviously connected with the phases of 
 luminous variation, and obeying, in the long run, precisely the 
 same period. 3 Something more will be said presently as to the 
 import of this discovery. 
 
 Bright hydrogen lines have so far been detected for the 
 most part photographically at Harvard College in at least 
 twenty-five stars, including Pleione, conjecturally identified by 
 Pickering with the lost Pleiad, P Cygni, noted for instability of 
 light in the seventeenth century, and the extraordinary southern 
 variable, r] Argus. In some of, if not in all these objects, other 
 vivid rays are associated with those due to hydrogen. A blaze 
 of hydrogen, moreover, accompanies the recurring outbursts of 
 about fifty " long-period variables," giving banded spectra of the 
 third type. Professor Pickering discovered the first example of 
 this class, towards the close of 1886, in Mira Ceti ; further de- 
 
 1 Astr. Nach., No. 2581. " Ibid., Nos. 2651-2. 3 Ibid., No. 3051 ; Astr. 
 and Astro-Physics, Jan. 1892, p. 25 ; Belopolsky, Astr. Nach., No. 3129. 
 
CHAP. xii. STARS AND NEBULA. 459 
 
 tections were made visually by Mr. Espiii ; and the conjunction 
 of bright hydrogen-lines with dusky bands has of late led to the 
 recognition, through the Harvard photographs, of numerous stars 
 subject to variations of lustre accomplished in some months. 
 
 A third variety of gaseous star is named after MM. Wolf and 
 Rayet, who discovered, at Paris in I86/, 1 its three typical repre- 
 sentatives, close together in the constellation Cygnus. The chief 
 part of their light is concentrated in three or four brilliant lines 
 or bands of unknown origin, united by a faint continuous spec- 
 trum. Vogel examined them at Bothkamp in 1873, and again 
 after ten years with the twenty-seven inch Vienna equatoreal, 
 but found no sign of change. 2 The green line of hydrogen, 
 however, identified by him in one of the objects in question, was 
 vainly looked for by Dr. and Mrs. Huggins in iSpo, 3 and may 
 not improbably be variable. The latter observers established the 
 curious fact that a diffuse, lustrous blue band, conspicuously 
 present in all spectra of this description, is in some notably 
 more refrangible than in others. Their measurements disproved 
 its suggested origin from carbon-radiations, and its chemical 
 relationship remains accordingly obscure. Six Wolf-Rayet stars 
 were discovered by Dr. Copeland, five of them in the course of a 
 trip for the exploration of visual facilities in the Andes in 1883 ; 4 
 and a large number have been made known through spectral photo- 
 graphs taken in both hemispheres under Professor Pickering's 
 direction. At the close of 1892, forty-five such objects had in 
 all been registered, 5 their magnitudes ranging from the sixth to 
 the eleventh ; with the single exception of y Argus, a second- 
 magnitude star, the resplendent continuous spectrum of which, 
 first examined by Eespighi and Lockyer in 1871, is embellished 
 with the yellow and blue rays distinctive of the type. Here then 
 we have a stellar globe apparently at the highest point of sun- 
 like incandescence, 6 sharing the peculiarities of bodies verging 
 towards the nebulous state. Professor Pickering indeed finds 
 
 1 Comptes JRendus, t. Ixv., p. 292. 2 Potsdam Pvibl., No. 14, p. 17. 3 Proc. 
 R. Soc., vol. xlix., p. 33. 4 Copernicus, vol. iii., p. 207. 5 E. C. Pickering, 
 Forty-seventh Ann. Report, p. 6 ; Mrs. Fleming, Astr. and Axtro-Physics, Nov. 
 1892, p. 765. 6 System of the Stars, p. 70. 
 
460 HISTORY OF ASTRONOMY. PART n. 
 
 the resemblance so close between the photographic spectra of 
 the Wolf -Bay et stars on the one side, and of planetary nebulae 
 on the other, that he proposes to erect both species together into 
 a single "fifth type." 1 But this scarcely seems advisable in 
 view of the marked difference in the visual quality of their light. 
 The mode of distribution of the Wolf-Rayet stars is very re- 
 markable. They all, without exception, lie nefer the central line 
 of the Milky Way. They tend also to gather into groups. 
 Seven of them in Argo, eight in Cygnus, are met within a circle 
 of four degrees radius. 
 
 The first spectroscopic star-catalogue was published by Dr. 
 Vogel at Potsdam in 1 883.2 It included 4051 stars, distributed 
 over a zone of the heavens extending from 20 north to 2 south of 
 the celestial equator. 3 More than half of these were white stars, 
 while red stars with banded spectra occurred in the proportion 
 of about one-thirteenth of the whole. To the latter genus, M. 
 Duner, then of Lund, now director of the Upsala Observatory, 
 devoted a work of standard authority, issued at Stockholm in 
 1884. This was a catalogue with descriptive particulars of 352 
 stars showing banded spectra, 297 of which belonged to Secchi's 
 third, 55 to his fourth class (Vogel's iii. a and iii. &). Since 
 then, discovery has progressed so rapidly, mainly through the 
 telescopic reviews of Mr. Espin, and the photographic survey 
 carried on at Harvard College, that considerably over one thousand 
 stars are at present recognised as of the family of Betelgeux and 
 Mira, while about 1 30 have so far exhibited the spectral pattern of 
 19 Piscium. (Secchi's Class IV.) One fact well ascertained as re- 
 gards both species, is the invariability of the type. The prismatic 
 flutings of the one and the broader zones of the other, are as if 
 stereotyped they undergo, in their fundamental outlines, no 
 modification in passing from star to star. They are always 
 accompanied by, or superposed upon, a spectrum of dark lines, 
 in producing which sodium and iron have an obvious share ; 
 but particular examination has not been carried very far as 
 
 1 Astr. Nach., No. 3025. 2 Potsdam Pull, No. ii. 3 The results of Von 
 Konkoly's extension of Vogel's work to 15 of S. declination, were published 
 in Bd. viii., Th. ii., of Gyalla Beobaclitungen (1887). 
 
CHAP. xii. STARS AND NEBULA. 461 
 
 regards the first variety, and is totally wanting as regards the 
 second. 
 
 A fairly complete answer to the question, What are the stars 
 made of? was given by Dr. Huggins in 186/j.. 1 By laborious 
 processes of comparison between stellar dark lines and the bright 
 rays emitted by terrestrial substances, he made quite sure of his 
 conclusions, though at much cost of time and pains. He averred, 
 indeed, that taking into account restrictions by weather and 
 position the thorough investigation of a single star-spectrum 
 would be the work of some years. Of two, however those of 
 Betelgeux and Aldebaran he was able to furnish detailed and 
 accurate drawings. The dusky flutings in the prismatic light of 
 the first of these stars have not been identified with the absorp- 
 tion of any particular substance ; but associated with them are 
 dark lines telling of the presence of sodium, iron, calcium, mag- 
 nesium, and bismuth. Signs of absorption by silver, manganese, 
 thallium, and tin, were also detected though not with certainty 
 by Vogel in 1871 ; and lines of antimony, mercury, and cad- 
 mium, were still more doubtfully recognised. 2 Hydrogen rays 
 are inconspicuously present. That an exalted temperature reigns, 
 at least in the lower strata of the atmosphere, is certified by the 
 vaporisation .there of matter so refractory to heat as iron. 3 
 
 Nine elements those identified in Betelgeux, with the addi- 
 tion of tellurium, antimony, and mercury were recognised by 
 Dr. Huggins as having stamped their signature on the spec- 
 trum of Aldebaran ; while the existence in Sirius, and nearly all 
 the other stars inspected, of hydrogen, sodium, iron, and mag- 
 nesium was rendered certain or highly probable. This was 
 admitted to be a bare gleaning of results ; nor is there reason to 
 suppose any of his congeners inferior to our sun in complexity 
 of constitution. Chemical interpretations of a positive kind 
 have not yet, however, been carried in them much beyond the 
 point to which they were brought by Dr. Huggins's pioneer- 
 
 1 Phil Trans., vol. cliv., p. 413. Some preliminary results were embodied 
 in a "note" communicated to the Eoyal Society, February 19, 1863 (Proc. 
 Eoy. Soc., vol. xii., p. 444). 2 Boihkamp Eeob., Heft i., p. 25. * Phil. Trans., 
 p. 429, note. 
 
<62 HISTORY OF ASTRONOMY. PART n. 
 
 ing efforts. Far greater accuracy of measurement has been 
 attained, but with results, in this respect, chiefly negative. .It 
 has rarely been possible to substitute genuine for certain spurious 
 identifications most usefully collected. Yet from ground so 
 carefully prepared, a further crop of definite knowledge can 
 scarcely fail to spring. The future of the science was indeed 
 assured by the introduction of the photographic method. 
 
 In this, as in so many other directions, Dr. Huggins led the 
 way. In March 1 863 he obtained with his coadjutor, Dr. Miller, 
 microscopic prints of the spectra of Sirius and Capella. 1 But 
 they told nothing. No lines were visible in them. They were 
 mere characterless streaks of light. Nine years later Dr. Henry 
 Draper of New York got an impression of four lines in the 
 spectrum of Vega. Then Dr. Huggins attacked the subject 
 again in 1 876, when the 1 8-inch speculum of the Eoyal Society 
 had come into his possession, using prisms of Iceland spar, and 
 lenses of quartz ; and this time with better success. A photo- 
 graph of the spectrum of Vega showed seven strong lines. 2 
 Still he was not satisfied. He waited and worked for three 
 years longer. At length, on December 18, 1879, he was able to 
 communicate to the Royal Society 3 results answering to his ex- 
 pectations. The delicacy of eye and hand needed to attain them 
 may be estimated from the single fact, that the image of a star 
 had to be kept, by continual minute adjustments, exactly pro- 
 jected upon a slit ^ of an inch in width during nearly an hour, 
 in order to give it time to imprint the characters of its analysed 
 light upon a gelatine plate raised to the highest pitch of sensi- 
 tiveness. 
 
 The ultra-violet spectrum of the white stars of which Vega 
 was taken as the type was by this means shown to be a very 
 remarkable one. Twelve strong lines, arranged at intervals 
 diminishing regularly upwards, intersected it. They belonged 
 presumably to one substance ; and since the two least refran- 
 gible were known hydrogen rays, that substance could scarcely 
 be any other than hydrogen. This was rendered certain by 
 
 1 Month. Not., vol. xxiii., p. 180. 2 Proc. E. Soc., vol. xxv., p. 446. 
 3 Phil Trans., vol. clxxi., p. 669. 
 
CHAP. xn. STARS AND NEBULA. 463 
 
 direct photographs of the hydrogen-spectrum taken by H. W. 
 Vogel at Berlin a few months earlier. 1 In them seven of the 
 white-star series of grouped lines were visible ; and the full 
 complement of twelve appeared on Cornu's plates in i886. 2 
 
 In yellow stars, such as Capella and Arcturus, the same 
 rhythmical series was partially represented, but associated with a 
 great number of other lines their state, as regards ultra-violet 
 absorption, thus approximating to that of the sun ; while the 
 redder stars betrayed so marked a deficiency in actinic rays, 
 that from BetelgeuXj with an exposure forty times that required 
 for Sirius, only a faint spectral impression could be obtained, and 
 from Aldebaran, in the strictly invisible region, almost none at all. 
 
 Thus, by the means of stellar light-analysis, acquaintance was 
 first made with the ultra-violet spectrum of hydrogen ; 3 and its 
 harmonic character, as expressed by " Balmer's Law," supplies a 
 sure test for discriminating, among newly discovered lines, those 
 that appertain from those that are unrelated to it. Deslandres's 
 five additional prominence-rays, for instance, were at once seen 
 to make part of the series, 4 which is continued to its utmost 
 known limit, by two still more refrangible dark rays, in the 
 spectrum of the brilliant solar star Canopus. 5 A group of six 
 dusky bands, on the other hand, photographed by Dr. and Mrs. 
 Huggins, April 4, iSpo, 6 near the extreme upper end of the spec- 
 trum of Sirius, were pronounced without hesitation, from their 
 want of conformity to its law of radiation, to have nothing to do 
 with hydrogen. Their true relationship must be sought elsewhere. 
 
 Dr. Schemer's spectrographic researches at Potsdam ex- 
 emplify the immense advantages of self-registration. In a 
 restricted section of the spectrum of Capella, he was enabled to 
 determine nearly three hundred lines with more precision than 
 has yet been attained in the measurement of terrestrial spectra. 
 This star appears to be virtually identical with the sun in 
 physical constitution, although it emits, according to the best 
 available data, about 250 times as much light, and is hence 
 
 1 Astr. Nach., No. 2301; Monatsb., Berlin, 1879, p. 119, 1880, p. 192. 
 2 Jour, de Physique, t. v., p. 98. 3 System of the /Stars, p. 39. 4 See ante, 
 
 p. 247. 5 W. H. Pickering, Astr. and Astro-Physics, Feb. 1893, p. 171. 
 Proc. B. Soc., vol. xlviii., p. 314. 
 
464 HISTORY OF ASTRONOMY. PART n. 
 
 presumably 4000 times more massive. An equally close ex- 
 amination of the spectrum of Betelgeux showed the pre- 
 dominance in it of the linear absorption of iron ; l but the bands 
 differentiating this star from te near relations of our sun do 
 not extend to the photographic region. Spectra of the second 
 and third orders are for this reason not easily distinguished on 
 the sensitive plate. K 
 
 A spectrographic investigation of all the brighter northern 
 stars was set on foot in 1886 at the observatory of Harvard 
 College, under the form of a memorial to Dr. H. Draper, whose 
 promising work in that line was brought to a close by his pre- 
 mature death in 1882. No individual exertions could however 
 have realised a tithe of what has been and is being accomplished 
 under Professor Pickering's able direction, with the aid of the 
 Draper and other instruments, supplemented by Mrs. Draper's 
 liberal provision of funds. A novel system was adopted, or 
 rather, an old one originally used by Fraunhofer was revived. 2 
 The use of a slit was discarded, as unnecessary for objects, like 
 the stars, devoid of sensible dimensions, and giving hence a 
 naturally pure spectrum ; and a large prism, placed in front of 
 the object-glass, analysed at once, with slight loss of light, the 
 rays of all the stars in the field. Their spectra were taken, as it 
 were, wholesale. As many as two hundred stars, down to the 
 eighth magnitude, were occasionally printed on a single plate 
 with a single exposure. No cylindrical lens was employed, 
 the movement of the stars themselves being turned to account 
 for giving the desirable width to their spectra. The star was 
 allowed by disconnecting, or suitably regulating the clock to 
 travel slowly across the line of its own dispersed light, so broaden- 
 ing it gradually into a band. Excellent results were thus secured. 
 About fifty lines, for example, appeared in the photographed spec- 
 trum of Aldebaran, and eight in that of Vega. On January 26, 
 1 886, with an exposure of thirty-four minutes, a simultaneous im- 
 pression was obtained of the spectra (among many others) of close 
 upon forty Pleiades. With few and doubtful exceptions, they 
 all proved to belong to the same type. The hydrogen-lines were 
 
 Die Spectralanalyse, p. 314. 2 Henry Draper Memorial, First Ann. Report, 
 1887. 
 
CHAP. xii. STARS AND NEBULA. 465 
 
 predominant in all, alone in most. An additional argument for 
 the common origin of the stars forming this beautiful group 
 was thus provided. 1 
 
 The "Draper Catalogue" of stellar spectra was published in 
 iSQO. 2 It gives the results of a rapid analytical survey of the 
 heavens north of 25 of southern declination, and includes 
 10,351 stars, down to about the eighth magnitude. The tele- 
 scope used was of eight inches aperture, and forty-five focus, its 
 field of view owing to the "portrait-lens," or "doublet" form 
 given to it embracing with fair definition no less than one 
 hundred square degrees. An objective prism eight inches square 
 was attached, and exposures of a few minutes were given to the 
 most sensitive plates that could be procured. In this way the 
 sky was twice covered in duplicate, each star appearing, as a 
 rule, on four plates. The registration of their spectra was 
 sought to be made more distinctive than had previously been 
 attempted Secchi's first type being divided into four, his 
 second into five subdivisions; but the differences regarded in 
 them could be confidently established only for stars above the 
 sixth magnitude. The work supplies none the less valuable 
 materials for general inferences as to the distribution and rela- 
 tions of the spectral types. The labour of its actual preparation 
 was borne by a staff of ladies under the direction of Mrs. Fleming. 
 Materials for its completion to the southern pole are in course of 
 accumulation with the identical instrument used in the north, 
 transferred for the purpose in 1889 to Peru. The progress of 
 the Draper Memorial researches was marked by discoveries of 
 an unexampled kind. 
 
 The principle upon which " motion in the line of sight " can 
 be, detected and measured with the spectroscope has already 
 been explained. 3 It depends, as our readers will remember, 
 upon the removal of certain lines, dark or bright (it matters not 
 which), from their normal places by almost infinitesimal amounts. 
 The whole spectrum of the moving object, in fact, is very 
 slightly shoved hither or thither, according as it is travelling 
 
 1 Mem. Amer, Acad., vol. xi., p. 215. 2 Harvard Annals, vol. xxvii. 
 3 See ante, p. 250. 
 
 30 
 
466 HISTORY OF ASTRONOMY. PART n. 
 
 towards or from the eye ; but, for convenience of measurement, 
 one line is usually picked out from the rest, and attention con- 
 centrated upon it. The application of this method to the stars, 
 however, is fraught with difficulties. It needs a powerfully 
 dispersive spectroscope to show line-displacements of the minute 
 order in question ; and powerful dispersion involves a strictly 
 proportionate enfeeblement of light. This, where the supply is 
 already to a deplorable extent niggardly, can ill be afforded ; 
 and it ensues that the operation of determining a star's approach 
 or recession is, even apart from atmospheric obstacles, an exces- 
 sively delicate one. 
 
 It was first successfully executed by Dr. Huggins early in 
 I868. 1 The brightest star in the heavens was selected as the 
 most promising subject of experiment, and proved amenable. 
 In the spectrum of Sirius, the F-line was considered to be just 
 so much displaced towards the red as to indicate (the orbital 
 motion of the earth being deducted) recession at the rate of 
 twenty-nine miles a second ; and the reality and direction of the 
 movement were ratified by Vogel and Lohse's observation. 
 March 22, 1871, of a similar, but even more considerable dis- 
 placement. 2 The inquiry was resumed by Dr. Huggins with 
 improved apparatus in the following year, when the velocities 
 of thirty stars were approximately determined. 3 The retreat of 
 Sirius seemed now slackened to about twenty miles per second, 
 and it was announced to be shared, at rates varying from twelve 
 to twenty-nine miles, by Betelgeux, Rigel, Castor, Regulus, and 
 five of the principal stars in the Plough. Arcturus, on the 
 contrary, gave signs of rapid approach, as well as Pollux, Vega, 
 Deneb in the Swan, and the brightest of the Pointers. 
 
 Numerically, it is true, these results were entitled to little 
 confidence. Thus, Arcturus is now fully ascertained to be 
 travelling towards the sun at the comparatively slow pace of less 
 than five miles a second ; and Sirius is actually approaching, 
 instead of retreating from him. The Greenwich observations 
 from 1874 to 1886 seemed, indeed, to give evidence of regular 
 
 1 Phil. Trans., vol. clviii., p. 529. 2 Schellen, Die Spectralanalyse, Bd. ii., 
 p. 326 (ed. 1883). 3 Proc. Roy. Soc., vol. xx., p. 386. 
 
CHAP. xn. STARS AND NEBULA. 467 
 
 and alternating change in the radial movement of the Dog-star ; 
 yet possibly only through a chance combination of observational 
 errors. The great difficulty of measuring so distended a line as 
 the Sirian F might well account for some apparent anomalies. 
 The scope of Dr. Huggins's achievement was not, however, to 
 provide definitive data, but to establish as practicable the method 
 of procuring them. In this he was thoroughly successful, and 
 his success was of incalculable value. Spectroscopic investiga- 
 tions of stellar movements may confidently be expected to play 
 a leading part in the unravelment of the vast and complex 
 relations which we can dimly detect as prevailing among the 
 innumerable orbs of the sidereal world ; for it supplements the 
 means which we possess of measuring by direct observation 
 movements transverse to the line of sight, and thus completes 
 our knowledge of the courses and velocities of stars at ascer- 
 tained distances, while supplying for all a valuable index to the 
 amount of perspective foreshortening of apparent movement. 
 Thus some, even if an imperfect, knowledge may at length be 
 gained of the revolutions of the stars of the systems they 
 unite to form, of the paths they respectively pursue, and of the 
 forces under the compulsion of which they travel. 
 
 The applicability of the method to determining the orbital 
 motions of double stars was pointed out by Fox Talbot in 1 87 1 ; l 
 but its use for their discovery revealed itself spontaneously 
 through the Harvard College photographs. In " spectrograms n 
 of Urs89 Majoris (Mizar), taken in 1887, and again in 1889, 
 the K-line was seen to be double ; while on other plates it 
 appeared single. A careful study by Miss A. C. Maury of a 
 series of seventy impressions proved that the doubling recurred 
 in a period of fifty-two days, and was shown though less 
 evidently, owing to their less distinct nature by all the remain- 
 ing lines in the spectrum. 2 The only available, and no doubt 
 the true explanation of the phenomenon was that two similar 
 and nearly equal stars are here merged into one telescopically 
 indivisible ; their combined light giving a single or double 
 
 1 System of the Stars, p. 199. - Pickering, Amer. Jour, of Science, vol. 
 xxxix., p. 46; Vogel, Astr. Nach., No. 3017. 
 
468 HISTORY OF ASTRONOMY. PART n. 
 
 spectrum, according as their orbital velocities are directed across 
 or along our line of sight. The movements of a revolving pair 
 of stars must always be opposite in sense and proportionate 
 in amount. That is, they at aH times travel with speeds in 
 the inverse ratio of their masses. Hence, unless the plane of 
 their orbits be perpendicular to a plane passing through the eye, 
 there must be two opposite points where thek velocities in the 
 line of sight reach a maximum, and two diametrically opposite 
 points where they touch zero. The lines in their common spec- 
 trum would thus appear alternately double and single twice in 
 the course of each revolution. In the case of Mizar, this is 
 evidently accomplished in about 104 days, the relative velocity 
 of the components coming out roughly one hundred miles a 
 second. Assuming their paths to be circular, and to coincide 
 with the line of sight, their distance apart is just 143,000 miles, 
 or nearly that of Mars and the sun. The corresponding joint 
 mass of the circulating bodies would be forty times the solar mass . 
 And this is a minimum estimate. For if the orbital plane be 
 inclined, much or little, to the line of sight, the dimensions and 
 mass of the system should be proportionately increased. 
 
 An analogous discovery was made by Miss Maury in 1889. 
 But in the spectrum of Aurigae, the lines open out and close 
 up on alternate days, indicating a relative orbit 1 with a radius 
 of less than eight million miles, traversed in about four days. 
 This implies a rate of travel for each star of sixty-five miles a 
 second, and a combined mass 4.7 times that of the sun. The 
 components are approximately equal, both in mass and light, 2 
 and the system formed by them is transported towards us with 
 a speed of some sixteen miles a second. The line-shiftings so 
 singularly communicative proceed, in this star, with perfect 
 regularity. In f Ursae, on the other hand, anomalies have 
 become conspicuous, and suggest a considerably eccentric orbit, 
 or possibly disturbance by otherwise unrecognised bodies. 3 
 
 1 The " relative orbit " of a double star is that described by one round the 
 other as a fixed point. Micrometrical measures are always thus executed. 
 But in reality, both stars move in opposite directions, and at rates inversely 
 as their masses, round their common centre of gravity. 2 Vogel, Astr. Nach., 
 Nos. 3017, 3039. 3 Harvard Annals, vol, xxvi., pt. i., p. xvii. 
 
CHAP. xii. STARS AND NEBULAE. 469 
 
 This new class of " spectroscopic binaries " could never have 
 been visually disclosed. The distance of ft Aurigae from the 
 earth, as determined by Professor Pritchard, is nearly three and 
 a third million times that of the earth from the sun (parallax = 
 0.06") ; whence it has been calculated that the greatest angular 
 separation of the revolving stars is only five thousandths of a 
 second of arc. 1 To make this evanescent interval perceptible, a 
 telescope of eighty feet in aperture would be required. 
 
 The beautiful star, Spica (a Virginis), was announced by Dr. 
 Vogel, April 24, iSQO, 2 to belong to the novel category, with 
 the difference, however, of possessing a nearly dark, instead of a 
 brilliantly lustrous companion. In this case, accordingly, the 
 tell-tale spectroscopic variations consist merely in a slight 
 swinging to and fro of single lines. No second spectrum 
 leaves a legible trace on the plate. Spica revolves in four days 
 at the rate of fifty-seven miles a second, or quicker, in pro- 
 portion as its orbit is more inclined to the line of sight, round a 
 centre at a minimum distance of three millions of miles. But 
 the position of the second star being unknown, the mass of the 
 system remains indeterminate. Now it is evident that if the 
 plane of its motion made a very small angle with the line of 
 sight, Spica would be a variable star. For, during a few hours 
 of each revolution, some at least of its light should be cut off by a 
 transit of its dusky companion. Such " eclipse-stars " are actually 
 found in the heavens. 
 
 The best and longest-known member of the group is Algol in 
 the Head of Medusa, the "Demon-star" of the Arabs. 3 This 
 remarkable object, normally of the second magnitude, loses and 
 regains three-fifths of its light once in 68.8 hours, the change 
 being completed in about ten hours. Its definite and limited 
 nature, and punctual recurrence, suggested to Goodricke of 
 York, by whom the periodicity of the star was discovered in 
 
 1 Huggins, Pres. Address, 1891 ; Cornu, Sur la Methode Doppler-fizeau, 
 p. D. 38. 2 Sitzungsb., Berlin, 1890, p. 401 ; Astr. NacTi., No. 2995. 3 The 
 derivation of the name Algol, or "El Ghoul," leaves little doubt that the Arab 
 astronomers were acquainted with this star's variability. E. M. Clerke, 
 Observatory, vol. xv., p. 271. 
 
470 HISTORY OF ASTRONOMY. PART n. 
 
 I783, 1 the interposition of a large dark satellite. But the condi- 
 tions involved by the explanation were first seriously investigated 
 by Pickering in i88o. 2 He found that the phenomena could be 
 satisfactorily accounted for by supposing an obscure body 0.764 
 of the bright star's diameter to revolve round it in a period 
 identical with that of its observed variation. This theoretical 
 forecast was verified with singular exactitude at Potsdam in 1 889. 3 
 A series of spectral photographs taken there showed each of 
 Algol's minima to be preceded by a rapid recession from the 
 earth, and succeeded by a rapid movement of approach towards 
 it. They take place, accordingly, when the star is at the 
 furthest point from ourselves of an orbit described round an 
 invisible companion, the transits of which across its disc betray 
 themselves to notice by the luminous vicissitudes they occasion. 
 The diameter of this orbit, traversed at the rate of twenty-six 
 miles a second, is just two million miles ; and it is an easy 
 further inference from the duration and extent of the phases 
 exhibited, that Algol itself must be (in round numbers) one 
 million, its attendant, 830,000 miles in diameter. Assuming 
 both to be of the same density, Vogel found their respective 
 masses to be four-ninths and two-ninths that of the sun, and 
 their distance apart to be 3,230,000 miles. 
 
 This singularly assorted pair of stars probably form part of a 
 larger system. Their period of revolution is shorter now by six 
 seconds than it was in Goodricke's time ; and Dr. Chandler has 
 shown, by an exhaustive discussion, that its inequalities are 
 comprised in a cycle of about 130 year?. 4 They arise, in his 
 view, from a common revolution, in that period, of the close 
 couple about a third distant body, emitting little or no light, in 
 an orbit inclined 20 to our line of vision, and of approximately 
 the size of that described by Uranus round the sun. The time 
 spent by light in crossing this orbit causes an apparent delay in 
 the phases of the variable, when Algol and its eclipsing satellite 
 
 1 Phil. Trans., vol. Ixxiii., p. 484. 2 Proc. Amer. Acad. vol. xvi., p. 17 ; 
 Observatory, vol. iv., p. 116. For a preliminary essay by T. S. Aldis, see 
 Phil. AIag. t vol. xxxix., p. 363, 1870. 3 Astr. Nach., No. 2947. 4 Astr. Jour. 
 Nos. 165-6, 255-6. See also Knowledge, vol. xv., p. 186. 
 
CHAP. xii. STARS AND NEBULA. 471 
 
 are on its further side from ourselves, balanced by acceleration 
 while they traverse its hither side. Dr. Chandler derives con- 
 firmation for his plausible and ingenious theory from a supposed 
 undulation in the line traced out by Algol's small proper motion ; 
 but the fact cannot yet be taken as ascertained. It is remark- 
 able, however, that the parallax for the star which it would 
 imply (0.07") agrees very nearly with that measured photographi- 
 cally by Professor Pritchard. Algol is, on this showing, so 
 remote that light takes nearly forty-seven years to travel thence 
 to our eyes. Moreover, it radiates sixty-three times more power- 
 fully than the sun ; and since it presents only once and a third 
 the sun's radiating surface, it must be forty-seven times more 
 brilliant, area for area. 
 
 The variable in the Head of Medusa is the exemplar of a 
 class including ten recognised members, all of which doubtless 
 represent occulting combinations of stars. But their occulta- 
 tions result merely from the accident of their orbital planes 
 passing through our line of sight ; hence, the heavens must 
 contain numerous similarly constituted systems, some of which, 
 like Spica Virginis, will probably eventually become known 
 through their spectroscopic changes, while others, because 
 revolving in planes nearly tangent to the sphere, or at right 
 angles to the visual line, may never disclose to us their true 
 nature. No exception has so far been met with to the rule that 
 spectroscopic doubles, whether occulting or non-occulting, belong 
 to the first spectral order. In two as yet imperfectly investi- 
 gated examples, however, a "white star" seems to be associated 
 with a gaseous companion. One is the variable /3 Lyrse, already 
 mentioned, the alternating line-displacements in the spectrum 
 of which suggest circulatory velocities of no less than three 
 hundred miles a second. The other is 1 1 Monocerotis, a fourth- 
 magnitude star of steadfast lustre, but showing analogous spec- 
 tral changes. 
 
 Gore's "Catalogue of Known Variables" 1 included, in 1884, 
 190 entries, and the number was augmented to 243 on its 
 
 1 Proc. R. Irish. Ac., July 1884. 
 
472 HISTORY OF ASTRONOMY. PART n. 
 
 revision in IS88. 1 Chandler's list of 225 such objects 2 was pub- 
 lished about the same time, and many more have since been 
 detected. Indeed, Dr. Gould is of opinion that most stars 
 fluctuate slightly in brightness through surface-alternations 
 similar to, but on a larger scale than those of the sun. The 
 solar analogy might, perhaps, be pushed somewhat further. It 
 may be found to contain a clue to much that is perplexing in 
 stellar behaviour. Wolf 3 pointed out in 1852 the striking 
 resemblance in character between curves representing sun-spot 
 frequency, and curves representing the changing luminous 
 intensity of many variable stars. There were the same steep 
 ascent to maximum and more gradual decline to minimum, 
 the same irregularities in heights and hollows, and, it may be 
 added, the same tendency to a double maximum, and complexity 
 of superposed periods. 4 It is impossible to compare the two sets 
 of phenomena thus graphically portrayed, without reaching the 
 conclusion that they are of closely related origin. But the 
 correspondence indicated is not, as has often been hastily 
 assumed, between maxima of sun-spots and minima of stellar 
 brightness, but just the reverse. The luminous outbursts, not 
 the obscurations of variable stars, obey a law analogous to that 
 governing the development of spots on the sun. Objects of the 
 kind do not then gain light through the closing-up of dusky 
 chasms in their photospheres, but by an immense growth of 
 those brilliant formations prominences and faculee which, in 
 the sun, accompany, or are appended to spots. A comparison of 
 light-curves with curves of spot-frequency leaves no doubt on 
 this point, and the strongest corroborative evidence is derived 
 from the kindling of bright lines in the spectra of long-period 
 variables on the rise towards their recurring maxima. 
 
 Every kind and degree of variability is exemplified in the 
 heavens. At the bottom of the scale are stars like the sun, of 
 which the lustre is tried by our instrumental means sensibly 
 steady. At the other extreme are ranged the astounding 
 apparitions of " new," or " temporary " stars. Within the last 
 
 1 Proc. Roy. Irish Ac., vol. i., p. 97. 2 Astr. Jour., Nos. 179-180. 3 See 
 ante, p. 159. 4 System of the Stars, p. 125. 
 
CHAP. xii. STARS AND NEBULA. 473 
 
 twenty-seven years five of these stellar guests (as the Chinese 
 call them) have presented themselves, and we meet with a sixth 
 no farther back than April 27, 1848. But of the "new star'* 
 in Ophiuchus found by Mr. Hind on that night, little more 
 could be learnt than of the brilliant objects of the same kind 
 observed by Tycho and Kepler. The spectroscope had not then 
 been invented. Let us hear what it had to tell of later arrivals. 
 
 Between thirty and fifteen minutes before midnight of May 
 12, 1866, Mr. John Birmingham, of Millbrook, near Tuam, in 
 Ireland, saw with astonishment a bright star of the second mag- 
 nitude unfamiliarly situated in the constellation of the Northern 
 Crown. Four hours earlier, Schmidt, of Athens, had been sur- 
 veying the same part of the heavens, and was able to testify 
 that it was not visibly there ; that is to say, a few hours, or 
 possibly a few minutes, sufficed to bring about a conflagration, 
 the news of which may have occupied hundreds of years in 
 travelling to us across space. The rays which were its mes- 
 sengers, admitted within the slit of Dr. Huggins's spectroscope, 
 May 1 6, proved to be of a composition highly significant as to 
 the nature of the catastrophe. The star which had already 
 declined below the third magnitude showed what was described 
 as a double spectrum. To the dusky flutings of Secchi's third 
 type four brilliant rays were added. 1 The chief of these agreed 
 in position with lines of hydrogen ; so that the immediate cause 
 of the outburst was plainly perceived to have been the eruption, 
 or ignition, of vast masses of that subtle kind of matter, the 
 universal importance of which throughout the cosmos is one of 
 the most curious facts revealed by the spectroscope. 
 
 T Coronas (as the new star was called) quickly lost its adven- 
 titious splendour. Nine days after its discovery it was again 
 invisible to the naked eye. It is now a pale yellow, slightly 
 variable star near the tenth magnitude, and finds a place as such 
 in Argelander's charts. It was thus obscurely known before it 
 made its sudden leap into notoriety. 
 
 The next "temporary" discovered by Dr. Schmidt at Athens, 
 November 24, 1 876, could lay no claim to previous recognition 
 
 1 Proc. Hoy. Soc., vol. xv., p. 146 
 
474 HISTORY OF ASTRONOMY. PART n. 
 
 even in that modest rank. It was strictly a parvenn. There 
 was no record of its existence until it made its appearance as a 
 star of nearly the third magnitude, in the constellation of the 
 Swan. Its spectrum was examined December 2, by Cornu at 
 Paris, 1 and a few days later by Vogel and 0. Lohse at Potsdam. 2 
 It proved of a closely similar character to that of T Corona. 
 A range of bright lines, including those of hydrogen, helium (or 
 sodium), and perhaps of coronium, stood out from a continuous 
 background strongly "fluted "by absorption. It may be pre- 
 sumed that in reality the gaseous substances, which, by their 
 sudden incandescence, had produced the apparent conflagration, 
 lay comparatively near the surface of the star, while the screen 
 of cooler materials rhythmically intercepting large portions of 
 its light, was situated at a considerable elevation in its atmo- 
 sphere. 
 
 The object, meanwhile, steadily faded. By the end of the 
 year it was of no more than seventh magnitude. After the 
 second week of March 1877, strengthening twilight combined 
 with the decline of its radiance to arrest further observation. 
 It was resumed, September 2, at Dunecht, with a strange result. 
 Practically the whole of its scanty light (it had then sunk below 
 the tenth magnitude) was perceived to be gathered into a single 
 bright line in the green, plausibly identified with the most 
 characteristic line of gaseous nebulae. 3 The star had, in fact, so 
 far as outward appearance was concerned, become transformed 
 into a planetary nebula, many of which are so minute as to be 
 distinguishable from small stars only by the quality of their 
 radiations. The nebular phase, however, seems to have been 
 transient. In the course of 1880, Professor Pickering found 
 that Nova Cygni gave an ordinary stellar spectrum of barely 
 perceptible continuous light; 4 and his observation was negatively 
 confirmed at Dunecht, February I, 1881. 
 
 This enigmatical object has now dropped to about the four- 
 teenth magnitude, being thus entirely beyond the reach of spec- 
 troscopic scrutiny. The lesson learnt from its changes appears 
 
 1 Comptes Rendus, t. Ixxxiii., p. 1172. 2 Monatsb., Berlin, 1877, pp. 241, 826. 
 3 Copernicus, vol. ii. , p. 101. 4 Annual Report, 1880, p. 7. 
 
CHAP, xii STARS AND NEBULA. 475 
 
 to be no less than this : That no clear dividing-line can be drawn 
 between stars and nebulae ; but that in what are called " planetary 
 nebulae " on the one side, and in " gaseous stars " (those giving 
 a spectrum of bright lines) on the other, we meet with transi- 
 tional forms, serving to bridge the gap between such vast and 
 highly finished orbs if we may be permitted the expression as 
 Capella, and the inchoate, faintly-lucent stuff which curdles 
 round the trapezium of Orion. 
 
 A supposed new star in Orion's Club (near ^' Orionis), observed 
 by Mr. J. E. Gore, December 13, 1885, proved to be a till then 
 unknown and highly interesting variable, closely akin to Mira. 
 By the end of April it had declined from the sixth to below the 
 ninth magnitude ; x but rose to a second maximum (determined 
 by Dr. Miiller) on December 12, 1886. Since then it has con- 
 formed to an average period of about 378 days, 2 or 45 days 
 longer than that of the "wonderful" star in Cetus; and it 
 shows a precisely similar colonnaded spectrum of great splendour 
 and beauty, 3 vivified by the same bright lines. 
 
 Perhaps none of the marvellous changes witnessed in the 
 heavens has given a more significant hint as to their construction 
 than the stellar blaze kindled in the heart of the great Andro- 
 meda nebula some undetermined number of years or centuries 
 before its rays reached the earth in the month of August 1885. 
 The first published discovery was by Dr. Hartwig at Dorpat on 
 August 3 r ; but it was found to have been already seen, on the 
 19th, by Mr. Isaac W. Ward of Belfast, and on the i;th by M. 
 Ludovic Gully of Eouen. The negative observations, on the 
 1 6th, of Tempel 4 and Max Wolf, limited very narrowly the 
 epoch of the apparition. Nevertheless, it did not, like most 
 temporaries, attain its maximum brightness all at once. When 
 first detected, it was of the ninth ; by September I it had risen 
 to the seventh magnitude, from which it so rapidly fell off that 
 in March it touched the limit of visibility (sixteenth magnitude) 
 with the Washington 26-inch. Its light bleached very percep- 
 
 1 Miiller, Astr. Nack., No. 2734. 2 Porro, Sulla Stella variabile U Orionis, 
 p. 6. 3 C. Wolf, Comptes Rendus, t. ci., p. 1444. 4 Astr. Nach., No. 2682. 
 
476 HISTORY OF ASTRONOMY. PART n. 
 
 tibly as it faded. 1 During the earlier stages of its decline, the 
 contrast was striking between the sharply denned, ruddy disc of 
 the star, and the hazy, greenish-white background upon which it 
 was projected, 2 and with which* it was inevitably suggested to be 
 in some sort of physical connection. 
 
 Let us consider what evidence was really available on this 
 point. To begin with, the position of the star was not exactly 
 central. It lay sixteen seconds of arc to the south-west of the 
 true nebular nucleus. Its appearance did not then signify a 
 sudden advance of the nebula towards condensation, nor was it 
 attended by any visible change in it save the transient effect of 
 partial effacement through superior brightness. 
 
 Equally indecisive information was derived from the spectro- 
 scope. To Vogel, Hasselberg, and Young, the light of the 
 "Nova" seemed perfectly continuous; but Dr. Huggins caught 
 traces of bright lines on September 2, confirmed on the 9th ; 3 
 and Dr. Copeland succeeded, on September 30, in measuring 
 three bright bands with a special acute-angled prism constructed 
 for the purpose. 4 A shimmer of F was suspected, and had also 
 been perceived by Mr. 0. T. Sherman of Yale College. Still the 
 effect was widely different from that of the characteristic blazing 
 spectrum of a temporary star, and prompted the surmise that 
 here too a variable might be under scrutiny. The star, however, 
 was certainly so far " new " that its rays, until their sudden 
 accession of strength, were too feeble to affect even our reinforced 
 senses. Not one of the 1283 small stars recorded in charts of 
 the nebula could be identified with it ; and a photograph taken 
 by Dr. Common, August 16, 1884, on which a multitude of 
 stars down to the fifteenth magnitude had imprinted themselves, 
 showed the uniform, soft gradation of nebulous light to be 
 absolutely unbroken by a stellar indication in the spot reserved 
 for the future occupation of the " Nova." 5 
 
 So far then the view that its relation to the nebula was a 
 merely optical one, might be justified ; but it became altogether 
 
 1 A. Hall, Am. Jour, of Sc., vol. xxxi,, p. 301. 2 Young, Sid. Messenger, 
 vol. iv., p. 282; Hasselberg, Astr. Nach., No. 2690. 3 Report Brit. Ass., 
 1885, p. 935. 4 Month. Not., vol. xlvii., p. 54. 5 Nature, vol. xxxii., p. 522. 
 
CHAP. xii. STARS AND NEBULA. 477 
 
 untenable when it was found that what was taken to be a chance 
 coincidence had repeated itself within living memory. On the 
 2 1 st of May 1 860, M. Auwers perceived at Konigsberg a seventh 
 magnitude star shining close to the centre of a nebula in Scorpio, 
 numbered 80 in Messier's Catalogue. 1 Three days earlier it 
 certainly was not there, and three weeks later it had vanished. 
 The effect to Mr. Pogson (who independently discovered the 
 change, May 28) 2 was as if the nebula had been replaced by a 
 star, so entirely were its dim rays overpowered by the concen- 
 trated blaze in their midst. Now it is simply incredible that 
 two outbursts of so uncommon a character should have acci- 
 dentally occurred just on the line of sight between us and the 
 central portions of two nebulae; we must then conclude that 
 they showed on these objects because they took place in them . 
 The most favoured explanation is that they were what might be 
 called effects of overcrowding that some of the numerous 
 small bodies, presumably composing the nebulas, jostled together 
 in their intricate circlings, and obtained compensation in heat 
 for their sacrifice of motion. But this is scarcely more than a 
 plausible makeshift of perplexed thought. Mr. W. H. S. Monck, 
 on the other hand, has suggested that new stars appear when 
 dark bodies are rendered luminous by rushing through the 
 gaseous fields of space, 3 just as meteors kindle in our atmosphere. 
 The idea, although ingenious, does not (nor was it designed to) 
 apply to our present case, Neither of the objects distinguished 
 by the striking variations just described is of gaseous constitution. 
 That in Scorpio appears under high magnifying powers as a 
 "compressed cluster"; that in Andromeda is perhaps, as Sir J. 
 Herschel suggested, "optically nebulous through the smallness 
 of its constituent stars " 4 if stars they deserve to be called. 
 
 On the 8th of December 1891, Dr. Max Wolf took a photo- 
 graph of the region about ^ Aurigae. No stranger so bright as 
 the ninth magnitude was among the stars depicted upon it. On 
 the loth, nevertheless, a stellar object of the fifth magnitude, 
 situated a couple of degrees to the north-east of /3 Tauri, and 
 
 1 Astr. Nack., Nos. 1267, 2715. 2 Month. Not., vol. xxi., p. 32. 3 Obser- 
 vatory, vol. viii., p. 335. 4 Ibid., vol. viii., p. 325 (Maunder). 
 
478 HISTORY OF ASTRONOMY. PART n. 
 
 previously unrecorded, where eleventh-magnitude stars appeared, 
 stamped itself upon a Harvard negative. Subsequent photo- 
 graphs taken at the same place showed it to have gained about 
 half a magnitude by the 2Oth*; but the plates were not then 
 examined, and the discovery was left to be modestly appropriated 
 by an amateur, the Rev. Dr. Anderson of Edinburgh, by whom 
 it was announced, February I, 1892, through. the medium of an 
 anonymous postcard, to Dr. Copeland, the Astronomer Eoyal 
 for Scotland. 1 By him and others, the engines of modern re- 
 search were promptly set to work. And to good purpose. Nova 
 Aurigse was the first star of its kind studied by the universal 
 chemical method. It is the first, accordingly, of which authentic 
 records can be handed down to posterity. They are of a most 
 remarkable character. The spectrum of the new object was pho- 
 tographed at Stonyhurst 2 and South Kensington on February 3 ; 
 a few days later, at Harvard and Lick in America, at Pots- 
 dam and Hereny on the continent of Europe. But by far the 
 most complete impression was secured February 22, with an ex- 
 posure of an hour and three-quarters, by Dr. and Mrs. Huggins, 
 through whose kindness it is reproduced in Plate IV., Fig. I. The 
 range of bright lines displayed in it is of astonishing vividness 
 and extent. It includes all the hydrogen-rays dark in the spec- 
 trum of Sirius (printed below for comparison), besides many 
 others still more refrangible, as yet unidentified. Very signifi- 
 cant, too, is the marked character of the great prominence-lines, 
 H and K. The visual spectrum of the Nova was splendidly 
 effective. A quartette of brilliant green rays caught the eye ; and 
 they had companions too numerous to be easily counted. The 
 hydrogen-lines were broad and bright ; C blazed, as Mr. Espin 
 said, " like a danger-signal on a dark night " ; the sodium pair 
 were recognised at Tulse Hill, and the helium-ray was suspected 
 to lurk close beside them. Carbon-bands, on the other hand, 
 were certainly, and magnesium-lines probably absent ; nor could 
 a strong band near the place of the chief nebular line be brought 
 into agreement with it. Fig. 2 shows the spectrum as it was 
 
 1 Trans. E. Soc. of Edinburgh, vol. xxvii., p. 51; Astr. and Astro-Physics, 
 Aug. 1892, p. 593. 2 Rev. W. Sidgreaves, Memoirs It. Astr. Soc., vol. li., p. 29. 
 
B8 
 
 > * 
 
 o 
 
 -\ 
 
 PL, g 
 
 O rs 
 
 0^5 
 
 W -a 
 PM g p 
 w * S3 
 
 c^ a ^ 1 
 
 H-l ^ ^ 
 
 ^ ^ "^5 
 
 Q S * 
 
 
 1 1 
 
CHAP. xii. STARS AND NEBULA. 479 
 
 seen and mapped by Mrs. Huggins, February 2-6, together with 
 the spectra employed to test the nature of the emissions dispersed 
 in it. One striking feature will be at once remarked. It is that 
 of the pairing of bright with dark lines. Both in the visible and 
 the photographic regions, this singular peculiarity was unmistak- 
 able ; and since the two series plainly owned the same chemical 
 origin, their separate visibility implied large displacement. 
 Otherwise, they would have been superposed, not juxtaposed. 
 Measurements of the bright rays, accordingly, showed them to 
 be considerably pushed down towards the red, while their dark 
 companions were nearly as much pushed up towards the blue 
 end. Thus the spectrum of Nova Aurigge, like that of j3 Lyrae, 
 with which it had many points in common, appeared to be 
 really double. It combined, by a reasonable supposition, the 
 light of two distinct bodies, one, of a gaseous nature, moving 
 rapidly away from the earth, the other, giving a more sun-like 
 spectrum, approaching it with comparable speed. The relative 
 velocity determined at Potsdam for these oppositely flying 
 masses amounted to 550 miles a second. 1 And this prodigious 
 rate of separation was maintained during fully six weeks ! It 
 did not then represent a mere periastral rush-past. 3 To the 
 bodies exhibiting its effects, and parting company for ever under 
 its stress, it must have belonged, with slight diminution, in 
 perpetuity. The luminous outburst by which they became 
 visible was explained by Dr. Huggins, in a lecture delivered at 
 the Royal Institution, May 13, 1892, on the tidal theory of 
 Klinkerfues and Wilsing. Disturbances and deformations due 
 to the mutual attraction of two bulky globes at a close approach 
 would, he considered, " give rise to enormous eruptions of the 
 hotter matters from within, immensely greater, but similar in 
 kind, to solar eruptions, and accompanied, probably, by large 
 electrical disturbances." The multiple aspect, and somewhat 
 variable character of both bright and dark lines, were plausibly 
 referred to processes of "reversal," such as are nearly always in 
 progress above sun-spots ; but the long duration of the star's 
 
 1 Vogel, Astr. Nach., No. 3079. 2 Observatory, vol. xv., p. 287 ; Seeliger,, 
 Astr. Nach., No. 3118; Astr. and Astro-Physics, Dec. 1892, p. 906. 
 
480 HISTORY OF ASTRONOMY. PART 11. 
 
 suddenly acquired lustre did not easily fit in with the adopted 
 rationale. A direct collision, on the other hand, was out of the 
 question, since there had obviously been little, if any, sacrifice 
 of motion ; and the substitution of a nebula for one of the 
 " stars" 1 compelled recourse to scarcely conceivable modes of 
 action for an explanation of the perplexing peculiarities of the 
 compound spectrum. t 
 
 An unexpected denouement, however, threw all speculations 
 off the track. The Nova retained most of its brightness, fluc- 
 tuations notwithstanding, until March 9 ; after which date it 
 ran swiftly and uniformly down towards what was apprehended 
 to be its total extinction. No marked change of spectrum at- 
 tended its decline. When last examined at Tulse Hill, March 24, 
 all the more essential features of its prismatic light were 
 still faintly recognisable. 2 The object was steadily sinking on 
 April 26, when a (supposed) final glimpse of it was caught with 
 the Lick thirty-six inch. 3 It was then of about the sixteenth 
 magnitude. But on August 17 it had sprung up to the tenth, 
 as Professors Holden, Schaeberle, and Campbell perceived with 
 amazement on turning the same instrument upon its place. 
 And to Professor Barnard it appeared, two nights later, not only 
 revived, but transformed. 4 No longer a star, it had assumed the 
 guise of the nucleus of a planetary nebula, 3" across, and sur- 
 rounded (on that night only) by a faint glow perhaps ten times 
 as extensive. Its nebulous aspect was not indeed recognised 
 elsewhere except at Pulkowa by Herr Kenz. 5 Mr. Newall, 
 especially, found the object perfectly stellar with the Cambridge 
 twenty-five inch refractor. The absence, however, of nebulosity 
 in long-exposure photographs taken by Mr. Roberts and Dr. 
 Max Wolf is inconclusive, its limits being overlapped by the 
 distended nuclear image. 
 
 The rekindled Nova was detected in this country by Mr. H. 
 Corder, on whose communication Mr. Espin, on August 21, ex- 
 amined its nearly monochromatic spectrum. 6 The metamorphosis 
 
 1 Ranyard, Knowledge, vol. xv., p. no; Seeliger, Astr. Nach., No. 3118. 
 z Proc. Eoy. Soc., vol. li., p. 492. 3 Burnham, Mouth. Not., vol. liii., p. 58. 
 4 Astr. Nach., No. 3118, 3143. 5 Ibid., No. 3119. 6 Ibid., No. 3111. 
 
CHAP. xii. STARS AND NEBULAS. 481 
 
 of Nova Cygni seemed repeated. 1 The main part of the light of 
 the new object, like that of its predecessor, was concentrated in 
 a vivid green band, identified with the chief nebular line by 
 Copeland, 2 Von Gothard, 3 and Campbell. 4 The second nebular 
 line was also represented. Indeed, the last-named observer 
 recognised nearly all the eighteen lines measured by him in the 
 Nova as characteristic of planetary nebulae. 5 Of particular 
 interest is the registration in the star-spectrum photographed by 
 Von Gothard of an ultra-violet line originally discovered by Dr. 
 and Mrs. Huggins in the Orion nebula. It is also very strong 
 in the Lyra annular nebula. 
 
 But before drawing the inference that Nova Aurigae now 
 shines as a genuine planetary, we must take into consideration 
 some remarkable observations made by the eminent astro- 
 physicists of Tulse Hill early in February, i893. 6 They succeeded 
 in resolving the so-called first nebular line into a great group 
 with a maximum of intensity near its blue end ; and the second 
 line appeared to be similarly composed, only with an inverted 
 gradation of luminosity. The character of both is then totally 
 different from that of the true nebular lines under ordinary con- 
 ditions. The conditions, however, under which the Nova-spec- 
 trum originated in both its stages were evidently extraordinary, 
 its constituent rays being, not only immensely broadened, but 
 doubled or trebled. Their shiftiiigs were thus difficult to de- 
 termine. Professor Campbell deduced from those affecting the 
 autumn-spectrum an approaching velocity of 128-miles a second 
 on August 20, which increased to 192, then progressively 
 diminished, after the first week in September, to less than one 
 hundred miles a second in November. Orbital motion was 
 thought to be concerned in producing these changes ; but no 
 orbital motion could have turned the recession at the rate of 300 
 miles a second, ascribed to the gaseous apparition in February 
 and March, into the swift approach determined in August. 
 Either the bodies emitting the rays analysed at the two epochs 
 
 1 Belopolsky, Astr. Nach., No. 3120. 2 Nature, Sept. 15, 1892. 3 Astr. 
 Nach.,Nos. 3122, 3129. 4 Ibid., No. 3133, Astr. and Astro-Physics, Oct. 1892, 
 p. 715. 5 Publ. Astr. Pac. Soc., vol. iv., p. 244. 6 Observatory, vol. xvi.. p. 149. 
 
 31 
 
482 HISTORY OF ASTRONOMY. PART n. 
 
 were not the same, or they did not possess the opposite velocities 
 imputed to them. The latter alternative seems favoured by 
 some recently published experiments. 1 M. Victor Schumann of 
 Leipsic alleges evidence to showjbhat hydrogen-lines derived from 
 vacuum-tubes vary most curiously with changes of temperature 
 and pressure, and in the mode of producing electrical excitement. 
 They vary, according to him, in position no l^ss than in aspect, 
 and even transiently assume a multiple character. 
 
 There seems at any rate no doubt that the physical constitu- 
 tion of Nova Aurigae became profoundly modified during the 
 four months of its invisibility. The spectrum of February was 
 or appeared compound ; that of August was simple ; it could 
 reasonably be associated only with a single light-source. Many 
 of the former brilliant lines, too, had vanished, and been re- 
 placed by others, previously inconspicuous or absent. As a 
 result, the solar-prominence type, to which the earlier spectrum 
 had seemed to conform, was completely effaced in the later. The 
 cause, however, remains mysterious of alterations which remain 
 for the present virtually suspended. Nova Aurigae has secured a 
 tolerably permanent footing among celestial objects. It is- 
 situated, like nearly all its congeners, in the full stream of the 
 Milky Way. Hence, we learn without surprise that micro- 
 metrical measures by Burnham and Barnard 2 failed to elicit from 
 it any sign of parallactic shifting. It is certain then that the 
 development of light, of which the news reached the earth in 
 December, 1891, must have been on a vast scale and of ancient 
 date. Nova Aurigae at its maximum assuredly exceeded the sun 
 many times in brightness ; and its conflagration can scarcely 
 have occurred less, and may have occurred much more, than a 
 hundred years ago. 
 
 We have been compelled somewhat to anticipate our narra- 
 tive as regards inquiries into the nature of nebulae. The ex- 
 cursions of opinion on the point were abruptly restricted and 
 defined by the application to them of the spectroscope. On 
 August 29, 1864, Dr. Huggins sifted through his prisms the 
 
 1 Astr. and Astro-Physics, Feb. 1893, p. 159. - Astr. Nacli., No. 3143. 
 
CHAP. xii. STARS AND NEBULAE. 483 
 
 rays of a bright planetary nebula in Draco. 1 To his infinite 
 surprise, they proved to be mainly of one colour. In other 
 words, they avowed their origin from a mass of glowing vapour. 
 As to what kind of vapour it might be by which Herschel's 
 conjecture of a "shining fluid" variously diffused throughout 
 the cosmos was thus unexpectedly verified, an answer only 
 partially satisfactory could be afforded. The conspicuous bright 
 line of the Draco nebula, although nearly accordant in position 
 with one belonging to nitrogen, has since proved to be 
 distinct from it ; of its two fainter companions, one was un- 
 mistakably the F line of hydrogen, while the other, intermediate 
 in situation, still remains unidentified. 
 
 By 1 868, Dr. Huggins had satisfactorily examined the spectra 
 of about seventy nebulae, of which one-third displayed a gaseous 
 character. 2 In all of these (and the rule has hitherto proved 
 without exception) the simulated nitrogen line appeared; 
 though in some cases as "the Dumb-bell" nebula in Vulpecula 
 it appeared alone. On the other hand, a fourth line, the dark 
 blue of hydrogen in addition to the normal three, was sub- 
 sequently detected in the light of the great Orion nebula. All 
 bodies of this class, 3 however, may be taken to be fundamentally 
 alike in composition. Minor varieties have been disclosed by 
 persevering inquiry ; but the differences in their radiations at 
 first perceived were mostly of intensity, not of kind. All 
 planetary and annular nebulae are gaseous, as well as those 
 termed "irregular" which frequent the region of the Milky 
 Way. Thus the signs of resolvability noted at Parsonstown 
 and Cambridge (U.S.) in Orion and the "Dumb-bell," proved 
 fallacious, so far, at least, as they had been taken to indicate 
 a stellar constitution ; though they may have quite faithfully 
 corresponded to the existence in discrete masses of the glowing 
 vapours elsewhere more equably diffused. An approximate 
 coincidence between the chief nebular line and a "fluting" of 
 magnesium having been alleged by Professor Lockyer in support 
 
 1 Phil. Trans., vol. cliv., p. 437. 2 Ibid., vol. clviii., p. 540. 3 With the 
 possible exception of the "looped" nebula in the southern constellation 
 Dorado. Pickering, Forty-seventh Ann. fieport, p. 7. 
 
484 HISTORY OF ASTRONOMY. PART n. 
 
 of his meteoritic hypothesis of nebular constitution, it became 
 of interest to ascertain its reality. The task was accomplished 
 by Dr. and Mrs. Huggins in 1889 and I89O, 1 and by Professor 
 Keeler, with the advantages of the Mount Hamilton apparatus 
 and atmosphere, in 1 890-1. 2 The upshot was to show a slight 
 but sure discrepancy as to place, and a marked diversity as to 
 character, between the two qualities of light. ?. The nebular ray 
 (wave-length 5006 millionths of a millimetre) is slightly more 
 refrangible than the magnesium fluting-edge, and it is sharp 
 and fine with no trace of the unilateral haze necessarily clinging 
 even to the last " remnant " of a banded formation. 
 
 The well-known nebula in Andromeda, and the great spiral 
 in Canes Venatici are among the more remarkable of those 
 giving a continuous spectrum ; and, as a general rule, the 
 emissions of all such nebulae as present the appearance of star- 
 clusters grown misty through excessive distance, are of the 
 same kind. It would, however, be eminently rash to conclude 
 thence that they are really aggregations of sun-like bodies. 
 The improbability of such an inference has been greatly en- 
 hanced by the occurrence, at an interval of a quarter of a cen- 
 tury, of stellar outbursts in the midst of two of them. For it 
 is practically certain that, however distant the nebulas, the 
 stars were equally remote ; hence, if the constituent particles 
 of the former be suns, the incomparably vaster orbs by which 
 their feeble light was well-nigh obliterated must, as was argued 
 by Mr. Proctor, have been on a scale of magnitude such as the 
 imagination recoils from contemplating. 
 
 Among the ascertained analogies between the stellar and 
 nebular systems is that of variability of light. On October 
 II, 1852, Mr. Hind discovered a small nebula in Taurus. 
 Chacornac observed it at Marseilles in 1854, but was con- 
 founded four years later to find it vanished. D'Arrest missed 
 it October 3, and re-detected it December 29, 1861. It was 
 easily seen in 1865-66, but invisible in the most powerful 
 
 1 Proc. Boy. Soc., vols. xlvi., p. 40, xlviii., p. 202. a PubL Astr. Pac. Soc., 
 vol. ii., p. 265 ; Proc. Boy. /Soc., vol. xlix., p. 399. 
 
CHAP. xii. STARS AND NEBULAE. 485 
 
 instruments 1 877-80. l Barnard, however, made out an almost 
 evanescent trace of it, October 15, 1890, with the great Lick 
 telescope. 2 This was the first undisputed instance of nebular 
 variability. It has been confirmed by others of the same nature. 
 Within sixteen years of the emergence to view of Hind's, two 
 more temporary nebulae were detected in the same region of the 
 sky ; 3 and many less startling instances of light-change in such 
 objects have been more or less plausibly alleged. 4 Professor 
 Holden, moreover, having co-ordinated in his admirable "Mono- 
 graph of the Nebula of Orion " 5 the results of all the more 
 prominent inquiries since 1758 into the structure of that mar- 
 vellous object, reached the conclusion that, while the figure of its 
 various parts has (with only one possible exception) remained 
 the same, their brightness has been and is in a state of continual 
 fluctuation. This accords precisely with the conviction expressed 
 by 0. Struve in i85/, 6 and awaits only the ratification which it 
 may be expected to obtain from a comparison of photographs taken 
 at some years' interval, to be accepted as an ascertained fact. 
 
 Probably analogous is the case of the "trifid" nebula in 
 Sagittarius, investigated by Professor Holden in i877- 7 What 
 is certain is, that a remarkable triple star, centrally situated, 
 according to the observations of both the Herschels, 1784-1833, 
 in a dark space between the three great lobes of the nebula, is 
 now, and has been since 1839, densely involved in one of them ; 
 and since the hypothesis of relative motion is on many grounds 
 inadmissible, the change that has apparently taken place must 
 be in the distribution of light. One no less conspicuous has been 
 adduced by Mr. H. C. Russell, director of the Sydney Observa- 
 tory. 8 A particularly bright part of the great Argo nebula, as 
 drawn by Sir John Herschel, has, he asserts, totally disappeared. 
 He noticed its absence in 1871, using a seven-inch telescope, 
 failed equally later on to find it with an 1 1 \ inch, and his long- 
 
 ] Astr. Nach., Nos. 1366, 1391, 1689; Chambers, Descriptive Astr. (3rd ed.), 
 p. 543; Flammarion, L'Uhivers Slderal, p. 818. >z Month. Not., vol. li., p. 94. 
 s System of the /Stars, p. 292. 4 Dreyer, Month. Not., vol. lii., p. 100. 5 Wash. 
 Obs., vol. xxv., App. i. 6 Month. Not., vol. xvii., p. 230. 7 Am. Jour, of Sc., 
 vol. xiv., p. 433. Cf. Dreyer, Month. Not., vol. xlvii., p. 419. 8 Month. 
 Not., vol. li., p. 496. 
 
486 HISTORY OF ASTRONOMY. PART n. 
 
 exposure photographs show no vestige of it. The same structure 
 is missing from a splendid picture of the nebula x taken by Dr. 
 Gill in twelve hours distributed over four nights in March 1892. 
 An immense gaseous expanse hscfe then, it would seem, sunk out of 
 sight. Materially it is no doubt there; but the radiance has left it. 
 Nebulae have no ascertained proper motions. No genuine 
 change of place in the heavens has yet been recorded for any 
 one of them. All equally hold aloof, so far as telescopic 
 observation shows, from the busy journeyings of the stars. 
 This seeming immobility is partly an effect of vast distance. 
 Nebular parallax has, up to the present, proved evanescent, and 
 nebular parallactic drift, in response to the sun's advance 
 through space, remains likewise imperceptible. 2 It may hence 
 be presumed that no nebulae occur within the sphere occupied 
 by the nearer stars. But the difficulty of accurately measuring 
 such objects must also be taken into account. Displacements 
 which would be conspicuous in stars might easily escape recogni- 
 tion in ill-defined, hazy masses. Thus the measures executed 
 by d' Arrest in i857 3 have not yet proved effective for their 
 designed purpose of contributing to the future detection of 
 proper motions. Some determinations made by Mr. Burnham 
 with the Lick refractor in 1891 4 will ultimately afford a more 
 critical test. He found that nearly all planetary nebulae show a 
 sharp stellar nucleus ; the position of which with reference to 
 neighbouring stars could be fixed no less precisely than if it 
 were devoid of nebulous surroundings. Hence, the objects 
 located by him cannot henceforward shift, were it only to the 
 extent of a small fraction of a second, without the fact coming 
 to the knowledge of astronomers. 
 
 The spectroscope, however, here as elsewhere, can supplement 
 the telescope ; and what it has to tell, it tells at once, without 
 the necessity of waiting on time to ripen results. Dr. Huggins 
 made, in i874, 5 the earliest experiments on the radial movements 
 
 1 ^Reproduced in Knowledge for April 1893. 2 Unless an exception be found in 
 the Pleiades nebulas, which may be assumed to share the small apparent move- 
 ment of the stars they adhere to. a Abhandl. Akad. der Wiss., Leipzig, 1857, 
 Bd .iii., p. 295. 4 Month. Not., vol. lii.,p. 31. 5 Proc. Roy. Soc., 1874, p. 251. 
 
CHAP. xii. STARS AND NEBULA. 487 
 
 of nebulae. But with only a negative upshot. None of the six 
 objects examined gave signs of spectral alteration, and it was 
 estimated that they must have done so had they been in course 
 of recession from or approach towards the earth by as much as 
 twenty-five miles a second. His inference has in only one case 
 proved fallacious. Aided by far more powerful appliances, Pro- 
 fessor Keeler renewed the attempt at Lick in 1890-1 ; and this 
 time with unequivocal success. Ten planetary nebulae yielded 
 perfectly satisfactory evidence of line-of-sight motion, 1 the swift- 
 est traveller being the well-known shimmering globe in Draco, 2 
 found to be hurrying towards the earth at the rate of forty miles 
 a second. For the Orion nebula, a recession of about eleven miles 
 was determined, 3 the whole of which may, however, very well 
 belong to the solar system itself, which, by its translation in the 
 direction of Hercules or Lyra, is certainly leaving the great 
 nebula pretty rapidly behind. The anomaly of seeming nebular 
 fixity has nevertheless been removed ; and the problem of 
 nebular motion has begun to be solved through the demonstrated 
 possibility of its spectroscopic investigation. 
 
 Professor Keeler's were the first trustworthy results of their 
 kind obtained visually. That the similar work on the stars 
 begun at Greenwich in 1874, and carried on for sixteen years, 
 remained comparatively unfruitful, was only what might have 
 been expected, the instruments available there being altogether 
 inadequate for the attainment of a high degree of accuracy. 
 
 The various obstacles in the way of securing it were overcome 
 by the substitution of the sensitive plate for the eye. Air- 
 tremors are thus rendered almost innocuous; and measure- 
 ments of stellar lines displaced by motion with reference 
 to fiducial lines from terrestrial sources, photographed on 
 the same plates, can be depended upon within vastly reduced 
 limits of error. Studies for the realisation of the " spectro- 
 graphic " method were begun by Dr. Vogel and his able assistant 
 Dr. Schemer at Potsdam in 1887. Their preliminary results, 
 
 1 Publ. Astr. Pac. Soc,, vol. ii., p. 278. 2 The first gaseous nebula examined 
 by Huggins in 1864. See System of the Stars, p. 257. 3 Proc. Roy. /Soc., vol. 
 xlix., p. 399. 
 
488 HISTORY OF ASTRONOMY. PART n. 
 
 communicated to the Berlin Academy of Sciences, March 15, 
 1888, already showed that the requirements for effective research 
 in this important branch were at last about to be complied with. 
 An improved instrument was erected in the autumn of the same 
 year, and the fifty-one stars, bright enough for determination 
 with a refractor of eleven inches aperture, were promptly taken 
 in hand. A list of their motions in the line of sight, published 
 in I892, 1 was of high value, both in itself and for what it 
 promised. One noteworthy inference from the data it collected 
 was that the eye tends, under unfavourable circumstances, to 
 exaggerate the line-displacements it attempts to estimate. The 
 velocities photographically arrived at were of much smaller 
 amounts than those visually assigned. The average speed of 
 the Potsdam stars came out only 10.4 miles a second, the 
 quickest among them being Aldebaran with a recession of thirty 
 miles a second. The hoped-for completion of a twenty-eight 
 inch refractor will enable Dr. Yogel to investigate spectrographi- 
 cally some hundreds of stars fainter than the second magnitude ; 
 and the data thus accumulated ought to provide the means 
 for a definite and complete solution of the more than secular 
 problem of the sun's advance through space. The solution 
 should be complete, because including a genuine determination 
 of the sun's velocity, apart from assumptions of any kind. 
 M. Homann's attempt in i885, 2 to extract some provisional 
 information on the subject from the radial movements of visually 
 determined stars, gave a fair earnest of what might be done 
 with materials of a better quality. He arrived at a goal for the 
 sun's way shifted eastward to the constellation Cygnus a result 
 congruous with the marked tendency of recently determined 
 apexes to collect in or near Lyra ; and the most probable cor- 
 responding velocity seemed to be about nineteen miles a second, 
 or just that of the earth in its orbit. 
 
 The first successful photograph of the nebular spectrum was 
 taken by Dr. Huggins, March 7, i882. 3 Five lines in all 
 
 1 Potsdam Pull. Bd. vii., Th. i. 2 Astr. Nach., No. 2714 ; Schonfeld, 
 V. J. S. Astr. Ges., Jahrg. xxi., p. 58. s Proc. JR. 8oc., vol. xxxiii., p. 425 ; 
 Report Brit. Ass., 1882, p. 444. The same feat was almost simultaneously 
 
CHAP. xii. STARS AND NEBULAE. 4^9 
 
 stamped themselves upon the plate during forty-five minutes of 
 exposure to the rays of the strange object in Orion. Of these, 
 four were the known visible lines, and a fifth, high up in the 
 ultra-violet, at wave-length 3724, was apparently recorded in 
 1 892 by Von Gothard l in the spectra of the annular nebula in 
 Lyra and of several planetaries, as well as in that of Nova 
 Aurigse in its later phase. Two additional hydrogen lines, 
 making six in all, were photographed by Dr. and Mrs. Huggins 
 from the Orion nebula in 1 890 ; 2 and the presence of two of 
 the ultra-violet series agrees with the appearance of the yellow 
 helium-ray detected by Dr. Copeland in i886, 3 in suggesting 
 that the tenuous matter composing this prodigious formation is 
 in a high state of thermal or electrical excitement. Of its phy- 
 sical connection with the stars visually involved in it, there is 
 convincing spectroscopic evidence. Dr. and Mrs. Huggins 
 found a plate exposed February 5, 1888, impressed with four 
 groups of fine bright lines, originating in the continuous light 
 of two of the trapezium-stars, but extending some way into the 
 surrounding nebula. 4 And Dr. Scheiner 5 obtained proof of a 
 wider relationship in the common possession, by the nebula and 
 the chief stars in the constellation Orion, of a peculiar line, 
 bright in the one case, dark in the others. 
 
 By direct photographic means, the structural unity of the 
 stellar and nebular orders in this extensive region of the sky, 
 has been independently made plain. 
 
 The first promising autographic picture of the Orion nebula 
 was obtained by Draper, September 30, i88o. 6 The marked 
 approach towards a still more perfectly satisfactory result shown 
 by his plates of March 1881 and 1882, was unhappily frustrated 
 by his death. Meanwhile, M. Janssen was at work in the same 
 field from 1881, with his accustomed success. 7 But Dr. A. 
 Ainslie Common left all competitors far behind with an admirable 
 
 performed by Dr. Draper, who however failed to get the fifth line. Comptes 
 Rendus, t. xciv., p. 1243. 1 Astr. awl Astro-Physics, Jan. 1893, P- 5 1 - " Proc. 
 Boy. Soc., vol. xlviii., p. 213. 3 Month. Not., vol. xlviii., p. 360. 4 Proc. 
 Roy. /Sbc., vol. xlvi., p. 40; System of the /Stars, p. 79. 5 Sitzunysb., Berlin, 
 Feb. 13, 1890. 6 Wash. 06*., vol. xxv., App. i., p. 226. 7 Comptes Hendus, 
 t xcii., p. 261. 
 
490 HISTORY OF ASTRONOMY. PART 11. 
 
 picture, taken January 30, 1883, by means of an exposure of 
 thirty-seven minutes in the focus of his three-foot silver-on- 
 glass mirror. 1 Photography may thereby be said to have 
 definitely assumed the office of ; "historiographer to the nebulae, 
 since this one impression embodies a mass of facts hardly to be 
 compassed by months of labour with the pencil, and affords a 
 record of shape and relative brightness in the various parts of 
 the stupendous object it delineates, which must prove invaluable 
 to the students of its future condition. Its beauty and merit 
 were officially recognised by the award of the Astronomical 
 Society's Gold Medal in 1884. 
 
 A second picture of equal merit, obtained by the same means, 
 February 28, 1883, with an exposure of one hour, is reproduced 
 in the frontispiece. The vignette includes two specimens of 
 planetary photography. The Jupiter, with the great red spot 
 conspicuous in the southern hemisphere, is by Dr. Common. 
 It dates from September 3, 1879, and was accordingly one of the 
 earliest results with his 36-inch, the direct image in which 
 imprinted itself in a fraction of a second, and was subsequently 
 enlarged on paper about twelve times. The exquisite little 
 picture of Saturn was taken at Paris by MM. Paul and Prosper 
 Henry, December 21, 1885, with their I2j-inch photographic 
 refractor. The telescopic image was in this case magnified eleven 
 times previous to being photographed, an exposure of about five 
 seconds being allowed; and the total enlargement, as it now 
 appears, is nineteen times. A trace of the dusky ring percep- 
 tible on the original negative, is lost in the print. 
 
 A photograph of the Orion nebula taken by Mr. Roberts in 
 67 minutes, November 30, 1886, made a striking disclosure of 
 the extent of that prodigious object. More than six times the 
 nebulous area depicted on Dr. Common's plates is covered by it, 
 and it plainly shows an adjacent nebula, separately catalogued 
 by Messier, to belong to the same vast formation. 
 
 This disposition to annex and appropriate has come out more 
 strongly with every increase of photographic power. Plates 
 exposed at Harvard College in March 1888, with an eight-inch 
 
 1 Month. Not., vol. xliii., p. 255. 
 
CHAP. xii. STARS AND NEBULAE. 49* 
 
 portrait-lens (the same used in the preparation of the Draper 
 Catalogue) showed the old established, "Fish-mouth" nebula, 
 not only to involve the stars of the sword-handle, but to be in 
 tolerably evident connexion with the most easterly of the three 
 belt-stars, from which a remarkable nebulous appendage was 
 found to proceed. 1 A still more curious discovery was made by 
 W. H. Pickering in 1889.* Photographs taken in three hours 
 from the summit of Wilson's Peak in California, revealed the 
 existence of an enormous, though faint spiral structure, enclosing 
 in its span of nearly seventeen degrees the entire stellar and 
 nebulous group of the Belt and Sword, from which it most likely, 
 although not quite traceably, issues as if from a nucleus. A 
 positively startling glimpse is thus afforded of the cosmical 
 importance of that strange " hiatus " in the heavens which 
 excited the wonder of Huygens in 1656. The inconceivable 
 attenuation of the gaseous stuff composing it has been virtually 
 demonstrated by Mr. Kanyard. 3 
 
 In March 1885, Sir Howard Grubb mounted for Mr. Isaac 
 Roberts, at Maghull, near Liverpool (his observatory has since 
 been transferred to Crowborough in Sussex), a silver-on-glass 
 reflector of twenty inches aperture, constructed expressly for use 
 in celestial photography. A series of nebula-pictures, obtained 
 with this fine instrument, have proved highly instructive both 
 as to the structure and extent of these wonderful objects. 
 Above all, one of the great Andromeda nebula, to which an 
 exposure of three hours was given on October I, i888. 4 In it, 
 a convoluted structure replaced and rendered intelligible the 
 anomalously rifted mass seen by Bond in i84?. 5 The effects of 
 annular condensation appeared to have stamped themselves upon 
 the plate, and two attendant nebulas presented the aspect of 
 satellites already separated from the parent body, and presumably 
 revolving round it. The ring-nebula in Lyra was photographed 
 at Paris in 1886, and shortly afterwards by Von Grothard with a 
 ten-inch reflector, 6 and he similarly depicted in 1888 the two 
 
 1 Harvard Annals, vol. xviii., p. 116. 2 Sid. Mess., vol. ix., p. i. 3 Know- 
 ledge, vol. xv., p. 191. 4 Month. Not., vol. xlix.,p. 65. 5 System of the /Stars, 
 p. 269. fi Astr. Xach., Nos. 2749, 2754. 
 
492 HISTORY OF ASTRONOMY. PART n. 
 
 chief spiral and other nebulas. 1 His representations are on a 
 small scale, yet show a wonderful amount of detail. Photographs 
 of the Lyre nebula taken at Algiers in iSgo, 2 and at the Vatican 
 observatory in 1892,* are remarkable for the strong develop- 
 ment of a central star, difficult of telescopic discernment, 
 but evidently of primary importance to the surrounding annular 
 structure. t. 
 
 The uses of photography in celestial investigations become 
 every year more manifold and more apparent. The earliest 
 chemical star-pictures were those of Castor and Vega, obtained 
 with the Cambridge refractor in 1850 by Whipple of Boston 
 under the direction of W. C. Bond. Double-star photography 
 was inaugurated under the auspices of G. P. Bond, April 27, 
 I %57> with an impression, obtained in eight seconds, of Mizar, 
 the middle star in the handle of the Plough. A series of 
 measures, from sixty-two similar images, gave the distance 
 and position angle of its companion with about the same 
 accuracy attainable by ordinary micrometrical operations ; and 
 the method and upshot of these novel experiments were de- 
 scribed in three papers remarkably forecasting the purposes 
 to be served by stellar photography. 4 The matter next fell 
 into the able hands of Eutherfurd, who completed in 1864 a 
 fine object glass (of nj inches) corrected for the ultra-violet 
 rays, consequently useless for visual purposes. The sacrifice 
 was recompensed by conspicuous success. A set of measure- 
 ments from his photographs of nearly fifty stars in the Pleiades, 
 Taken mT872 and iS/zj^enabled Dr. Gould in 1 866 to ascertain, 
 comparison with Vessel's places for the same stars, that 
 during the intervening third of a century no changes of im- 
 portance had occurred in their relative positions. 5 Mr. Harold 
 jJacoby's^jreduction 6 to the epoch 1873 f seventy-five stars 
 c\ originally measured from the saaass platesVunder Kutherfurd's 
 directions, and his comparison of their relative positions with 
 
 1 Vogel, Astr. Naclt., 2854. ~ Nature, vol. xliii., p. 419. 3 L' Astronomic, t. 
 xl., p. 171. 4 Astr. NacJi., Bande xlvii., p. i, xlviii., p. I, xlix., p. 81. Picker- 
 ing, Mem. Am. Ac., vol. xi., p. 180. 5 Gould on Celestial Photography, Ob- 
 servatory, vol. ii., p. 16, 8 Annals' N. Y. Acad. of Sciences, vol. vi., p. 239, 
 1892 ; Elkin, Publ. Astr. Pac. Soc., vol. iv., p. 134. 
 
CHAP. xii. STARS AND NEBULA. 493 
 
 those derived from Dr. Elkin's heliometric triaugulation of the 
 cluster in I886, 1 proved still more cogently the full adequacy of 
 photographic observations to the most arduous tasks of exact 
 astronomy. This is, however, only one among many results of 
 the application of modern methods to that antique star-group. 
 
 A splendid photograph of 1421 stars in the Pleiades, taken 
 by the MM. Henry with three hours' exposure, November 16, 
 1885, showed one of the brightest of them to have a small spiral 
 nebula, somewhat resembling a strongly curved comet's tail, 
 attached to it. The reappearance of this strange appurtenance 
 on three subsequent plates left no doubt of its real existence, 
 visually attested at Pulkowa, February 5, 1886, by one of the 
 first observations made with the 3O-inch equatoreal. 2 Much 
 smaller ' apertures, however, sufficed to disclose the " Maia 
 nebula," once it was Jcnoivn to be there. Not only did it appear 
 greatly extended in the Vienna 27-inch, 3 but MM. Perrotin and 
 Thollon saw it with the Nice 1 5-inch, and M. Kamrnermann of 
 Geneva, employing special precautions, with a refractor of only 
 ten inches aperture. 4 The advantage derived by him for bring- 
 ing it into view, from the insertion into the eye-piece of an 
 uranium film, gives, with its photographic intensity, valid proof 
 that a large proportion of the light of this remarkable object is 
 of the ultra-violet kind. 
 
 This is not the only nebula in the Pleiades. On October 
 19, 1859, Wilhelm Tempel, who, drawn by an overmastering 
 impulse, had then recently exchanged his graver's tools for a 
 small telescope, discovered an elliptical nebulosity, originating 
 and stretching far to the southward from the star Merope. 
 It has since been pretty constantly observed, though its extreme 
 susceptibility to unfavourable aerial influences has led to a 
 probably unfounded suspicion of variability. Nothing corre- 
 sponding to this delicate object appeared on any of the Henry 
 plates; but in its stead some streaky nebulous patches in the 
 same vicinity, never seen except by Dr. Common with his great 
 reflector, February 8, i88o. 5 A further mass near Alcyone, 
 
 1 Trans. Astr. Observatory of Yale Univ., vol. i., pt. i. 2 Astr. Nach. t No. 
 2719. 3 Ibid., No. 2726. 4 Ibid., No. 2730. 5 Month. Not., vol. xl., p. 376. 
 
494 HISTORY OF ASTRONOMY. PART n. 
 
 perceived on the same occasion, received photographic confirma- 
 tion elsewhere. 
 
 A picture of the Pleiades procured at Maghull in eighty-nine 
 minutes, October 23, -1 886, revealed nebulous surroundings to 
 no less than four leading stars of the group, namely, Alcyone, 
 Electra, Merope, and Maia ; and a second impression, taken in 
 three hours on the following night, showed further " that the 
 nebulosity extends in streamers and fleecy masses, till it seems 
 almost to fill the spaces between tho stars, and to extend far 
 beyond them." l The structural unity of the entire agglomeration 
 was, moreover, placed beyond doubt by the visibly close relation- 
 ship of the stars to the nebulous formations surrounding them in 
 Mr. Eoberts's striking pictures. Thus Goldschmidt's notion that 
 all the clustered Pleiades constitute, as it were, a second Orion 
 trapezium in the midst of a huge formation of which Tempel's 
 nebula is but a fragment, 2 has been to some extent verified. Yet 
 it seemed fantastic enough in 1863. 
 
 The next advance in this investigation was due to the MM. 
 Henry. Early in 1888 they gave exposures of four hours each 
 to several plates, upon which were accordingly exhibited a crowd 
 of 2326 stars, and some new features of the entangled nebulae. 
 The most curious of these was the threading together of stars 
 by filmy processes. In one case, seven aligned stars appeared 
 strung on a nebulous filament, "like beads on a rosary." 3 The 
 " rows of stars," so often noticed in the sky, may then be con- 
 cluded to have more than an imaginary existence. 
 
 The history of celestial photography at the Cape of Good Hope 
 began with the appearance of the great comet of 1882. No 
 special apparatus was at hand ; so Dr. Gill called in the services 
 of a local artist, Mr. Allis of Mowbray, with whose camera, 
 strapped to the Observatory equatoreal, pictures of conspicuous 
 merit were obtained. But their particular distinction lay in the 
 multitude of stars begemming the background. (See Plate II.) 
 The sight of them at once opened to Dr. Gill a new prospect. 
 He had already formed the project of extending Argelander's 
 
 1 Month. Not., vol. xlvii., p. 24. 2 Les Mondes, t. iii., p. 529. 3 Mouchez, 
 Compies Eendus, t. cvi., p. 912. 
 
CHAP. xii. STARS AND NEBULA. 495 
 
 " Durchniusterung " from the point where it was left by 
 Schonfeld to the southern pole ; and his ideas regarding the 
 means of carrying it into execution crystallised at the needle- 
 touch of the cometary experiments. He resolved to employ 
 photography for the purpose. The exposure of plates was 
 accordingly begun, under the care of Mr. Kay Woods, in 1885 ; 
 and in four years, the sky, from 19 of south latitude to the pole, 
 had been covered in duplicate. Their measurement, and the 
 preparation of a catalogue of the stars imprinted upon them, were 
 generously undertaken by Professor Kapteyn of Groningen, and 
 his laborious task is now nearly completed. The Southern Durch- 
 musterung will most likely be in the hands of astronomers in 
 the course of a year. This first photographic census of the 
 heavens will be fuller and surer than Argelander's. It will 
 contain some 350,000 stars, nearly to the tenth magnitude; and 
 their positions, derived from more than a million observations, 
 can be depended upon to about one second of arc. 
 
 The production of this important work was thus a result of 
 the Cape comet-pictures; yet not the most momentous one. 
 They turned the scale in favour of recourse to the camera when 
 the MM. Henry encountered, in their continuation of Chacornac's 
 half-finished enterprise of ecliptical charting, sections of the 
 Milky Way defying the enumerating efforts of eye and hand. 
 The perfect success of some preliminary experiments made with 
 an instrument constructed by them expressly for the purpose, 
 was announced to the Academy of Sciences at Paris, May 2, 
 1885. By its means, stars estimated as of the sixteenth 
 magnitude clearly recorded their presence and their places ; and 
 the enormous increase of knowledge involved may be judged of 
 from the fact that, in a space of the Milky Way in Cygnus 
 2 15' by 3, where 170 stars had been mapped by the old 
 laborious method, about five thousand stamped their images on 
 a single Henry plate. 
 
 These results suggested the grand undertaking of a general 
 photographic survey of the heavens, and Dr. Gill's proposal, 
 June 4, 1886, of an International Congress for the purpose of 
 setting it on foot, was received with acclamation, and promptly 
 
496 HISTORY OF ASTRONOMY. PART n. 
 
 acted upon. Fifty-six delegates of seventeen different nation- 
 alities met in Paris, April 16, 1887, under the presidentship of 
 Admiral Mouchez, to discuss measures and organise action. 
 They resolved upon the construction of a Photographic Chart of 
 the whole heavens, including stars of the fourteenth magnitude, 
 to the surmised number of twenty millions ; to be supplemented 
 by a Catalogue, framed from plates of comparatively short 
 exposure, giving stars to the eleventh magnitude. These will 
 probably amount to about one million and a quarter. Both sets 
 of plates, it was agreed, should be exposed with instruments pre- 
 cisely similar to that of the MM. Henry, which is a photographic 
 refractor thirteen inches in aperture, and of eleven feet focus, 
 attached to a guiding telescope of eleven inches aperture, corrected 
 of course for the visual rays. Each plate will cover an area of 
 four square degrees, and since the series must be duplicated to 
 prevent mistakes, about 22,000 plates will be needed for the Chart 
 alone. The task of procuring them has been apportioned among 
 eighteen observatories in all parts of the world, 1 except the 
 United States, and ought to be completed in less than five years. 
 The Atlas embodying the collected data will consist of copies on 
 glass of the original negatives. It will form one of the most 
 valuable bequests ever made by a generation of mankind to pos- 
 terity. One of the most ardent promoters of the scheme it may 
 be expected to realise was Admiral Mouchez, the successor of 
 Leverrier in the direction of the Paris observatory. But it was 
 not granted to him to see the fruition of his efforts. He died 
 {suddenly June 25, :892. 2 Although not an astronomer by 
 profession, he had been singularly successful in pushing forward 
 the cause of the science he loved, while his genial and open 
 nature won for him wide personal regard. He has been replaced 
 by M. Tisserand, whose mathematical eminence fits him to 
 continue the traditions of Delaunay and Leverrier. 
 
 The sublime problem of the construction of the heavens has 
 not been neglected amid the multiplicity of tasks imposed upon 
 the cultivators of astronomy by its rapid development. But 
 
 1 Miss A. Everett, Jour. Brit. Astr. Ass., vol. iii., p. 24. (See Appendix, 
 Table vi. ). 2 D. Klumpke, Observatory, vol. xv. , p. 305. 
 
CHAP. xii. STARS AND NEBULAE. 497 
 
 data of a far higher order of precision, and indefinitely greater 
 in amount, than those at the disposal of Herschel or Strove, 
 must be accumulated before any definite conclusions on the 
 subject are possible. The first organised effort towards realising 
 this desideratum was made by the German Astronomical Society 
 in 1865, two years after its foundation at Heidelberg. The 
 scheme, as originally proposed, consisted in the exact determination 
 of the places of about 100,000 of Argelander's stars from the 
 re-observation of which, say, in the year 1950, astronomers of 
 two or three generations hence may gather a vast store of 
 knowledge directly of the apparent motions, indirectly of the 
 mutual relations binding together the suns and systems of space. 
 Thirteen observatories in Europe and America joined in the 
 work, now virtually terminated. Its scope was, after its incep- 
 tion, widened to include southern zones as far as the Tropic of 
 Capricorn ; this having been rendered feasible by Schonf eld's 
 extension (1875-1885) of Argelander's survey. Thirty thousand 
 additional stars, thus taken in, were allotted in zones to five 
 observatories. Another important undertaking of the same class 
 is the re-observation of the 47,300 stars in Lalande's Histoire 
 Celeste. Begun under Arago in 1855, its upshot, so far, has 
 been the publication, in 1887 and 1891, of four volumes of the 
 great Paris Catalogue, comprising nearly 15,000 stars. The 
 proper motions of some hundreds of these have, moreover, been 
 ascertained by diligent comparisons with the places assigned to 
 them elsewhere. 
 
 Through Dr. Gould's unceasing labours during his fifteen 
 years' residence at Cordoba, a detailed acquaintance with southern 
 stars was brought about. His Uranometria Argentina (1879) 
 enumerates the magnitudes of 8198 out of 10,649 stars visible 
 to the naked eye under those transparent skies; 73,160 down to 
 9^ magnitude are embraced in his "zones"; and the Argentine 
 General Catalogue of 32,448 southern stars was published in 
 1 886. Valuable work of the same kind has been done at Virginia 
 by Professor 0. Stone ; while the present Eadcliffe observer's 
 " Cape Catalogue for 1880 " affords an aid to the practical astro- 
 nomer south of the line, of which it would be difficult to over- 
 
 32 
 
498 HISTORY OF ASTRONOMY. PART n. 
 
 estimate the importance. Moreover, the gigantic task entered 
 upon in 1860 by the late Dr. C. H. F. Peters, director of the 
 Litchfield Observatory, Clinton (N.Y.), and of which a large 
 instalment was finished in 1882? deserves honourable mention. 
 It was nothing less than to map all stars down to, and even 
 below, the fourteenth magnitude, situated within 30 on either 
 side of the ecliptic, and so to afford "a sure basis for drawing 
 conclusions with respect to the changes going on in the starry 
 heavens." 1 
 
 It is tolerably safe to predict that no work of its kind, and for 
 its purpose, will ever again be undertaken. In a fraction of 
 one night more stars can now be got to register themselves, and 
 more accurately, than the eye and hand of the most skilled 
 observer could accomplish the record of in a year. Fundamental 
 catalogues, constructed by the old time-honoured method, will 
 continue to furnish indispensable starting-points for measure- 
 ment ; 2 but the relative places of the small crowded stars the 
 sidereal ol iro\\oi will henceforth be derived from their 
 signatures on the sensitive plate. Even the secondary purpose 
 that of asteroidal discovery served by detailed stellar enumer- 
 ation, is more surely attained by photography than by laborious 
 visual comparison. For planetary movement betrays itself 
 tolerably quickly by turning the imprinted image of the object 
 affected by it from a dot into a trail. 
 
 In the arduous matter of determining star distances, too, much 
 progress has been made. Together, yet independently, Drs. Gill 
 and Elkin carried out, at the Cape Observatory in 1882-83, an 
 investigation of remarkable accuracy into the parallaxes of nine 
 southern stars. One of these was the famous a Centauri, the 
 distance of which from the earth was ascertained to be just one- 
 third greater than Henderson had made it. The parallax of 
 Sirius, on the other hand, was doubled, or its distance halved ; 
 while Canopus was discovered to be quite immeasurably remote 
 a circumstance which, considering that, among all the stellar 
 
 1 Gilbert, Sidereal Messenger, vol. i., p. 288. 2 Mouchez, Comptes Rendus, 
 t. cii., p. 151. 
 
CHAP. xii. STARS AND NEBULA. 499 
 
 multitude, it is outshone only by the radiant Dog-star, gives a- 
 stupendous idea of its real splendour and dimensions. 
 
 Inquiries of this kind were, during many years subsequent to 
 1867, successfully pursued at the observatory of Dunsink, near 
 Dublin. Annual perspective displacements were by Dr. Briinnow 
 detected in several stars, and in others re-measured with a care 
 which inspired just confidence. His parallax for a Lyras (o. 1 3") 
 was confirmed by Hall in 1886 from an extended series of observa- 
 tions (giving TT = o. 1 34") ; and the received value (TT^O.OQ") for 
 the parallax of the remarkable star " Groombridge 1830" the 
 swiftest traveller, so far known, in the sidereal heavens is that 
 arrived at by him in 1871. His successor as Astronomer-Koyal 
 for Ireland, Sir Eobert Stawell Ball (now Lowndean Professor 
 of Astronomy in the University of Cambridge), has done good 
 service in the same department. For besides verifying approxi- 
 mately Struve's parallax of half a second of arc for 6 1 Cygni, he 
 refuted, in 1881, by a sweeping search for (so-called) "large" 
 parallaxes, certain baseless conjectures of comparative nearness 
 to the earth, in the case of red and temporary stars. 1 Of 450 
 'objects thus cursorily examined, only one star of the seventh 
 magnitude, numbered 1618 in Groombridge's Circumpolar Cata- 
 logue, gave signs of measurable vicinity. 
 
 A second campaign in stellar parallax was undertaken by Drs. 
 'Gill and Elkin in 1887. But this time, the two observers were 
 in opposite hemispheres. Both used heliometers. Dr. Elkin 
 had charge of the fine instrument then recently erected in Yale 
 College Observatory ; Dr. Gill employed one of seven inches, 
 just constructed under his directions, in first-rate style, by the 
 Repsolds of Hamburg. His results with it have not yet been 
 made public. Dr. Elkin, however, completed in 1888 his share 
 >of the more immediate joint programme, which consisted in the 
 'determination, by direct measurement, of the average parallax 
 -of first magnitude stars. It came out, for the northern ones, 
 '0.089", equivalent to a light-journey of more than thirty-six 
 years. The deviations from this average were, indeed, exceed- 
 ingly wide. Four of the stars, Betelgeux, Arcturus, Vega, and 
 
 1 Nature, \>ol. xxiv., p. 91 ; Dunsink Observations, pt. v., 1884. 
 
500 HISTORY OF ASTRONOMY. PART n, 
 
 a Cygni, gave no certain sign of any perspective shifting ; of the 
 rest, Procyon, with a parallax of o".266, proved the nearest to 
 our system. At the mean distance concluded for these ten 
 brilliant stars, the sun would show as of only 6.5 magnitude ; 
 hence it claims a very subordinate rank among the suns of space. 
 Further investigations of the same objects enabled Dr. Elkin to* 
 publish, in 1 892, a revised list of their parallaxes, with greatly 
 reduced probable errors. 1 The upshot was, on the whole,, 
 thoroughly reassuring. An important change was, however, 
 made by the assignment of a well-assured parallax of o."O92 to* 
 Vega, while that of Procyon was increased to o".34i. Another 
 valuable contribution to knowledge on this subject was 
 Professor Kapteyn's determination, by meridian observations 
 made from 1885 to 1889, of the parallaxes of fifteen stars. 2 
 His method seems capable of wide application. 
 
 The great merit of having rendered photography available 
 for the sounding of the celestial depths belongs to Professor 
 Prit chard. The subject of his initial experiment was 6 1 Cygni. 
 From measurements of 200 negatives taken in 1886 he derived 
 for that classic star a parallax of 0.438", in satisfactory agree- 
 ment with Ball's of 0.468". A detailed examination convinced the 
 Astronomer-Royal (Mr. Christie) of its superior accuracy to Bessel's 
 result with the heliometer. Since then the Savilian Professor 
 has successfully executed his project of determining all second- 
 magnitude stars, to the number of about thirty, 3 conveniently 
 observable at Oxford, obtaining as the general outcome of the- 
 research an average parallax, for objects of that rank, of 0.056"- 
 This is in exact correspondence with Elkin's average for stars of 
 the first magnitude. That is to say, it gives the " distance-ratio ' r 
 theoretically derivable from the " light-ratio " of the two classes. 
 
 Observers of double stars are among the most meritorious,, 
 and need to be among the most patient and painstaking workers- 
 in sidereal astronomy. They are scarcely as numerous as could 
 be wished. Dr. Doberck, distinguished as a computer of stellar 
 
 1 Report Yale Coll. Obs., 1891-2. " 2 Annalen der Sternwarte in Leiden, 
 Bd. vii. 3 Researches in Stellar Parallax, pt. ii. 1892. 
 
CHAP. xii. STARS AND NEBULA. 501 
 
 orbits, complained in I882 1 that data sufficient for the purpose 
 had not been collected for above 30 or 40 binaries out of be- 
 tween five and six hundred certainly or probably existing. The 
 progress since made is illustrated by Mr. Gore's useful Catalogue 
 of Computed Binaries, including fifty-nine entries, presented to 
 the Eoyal Irish Academy, June 9, 1890.2 Few have done more 
 towards supplying the deficiency of materials than the late 
 Baron Ercole Dembowski of Milan. He devoted the last 
 thirty years of his life, which came to an end January 19, 1 88 1, 
 to the revision of the Dorpat Catalogue, and left behind him a 
 :store of micrometrical measures as numerous as they are precise. 
 Of living observers in this branch, Mr. S. W. Burnham is be- 
 yond question the foremost. While pursuing legal avocations 
 .at Chicago, he diverted his scanty leisure by exploring the skies 
 with a six-inch telescope mounted in his back-yard ; and had 
 discovered, in May 1882, one thousand close and mostly very 
 difficult double stars. 3 Summoned as chief assistant to the new 
 Lick Observatory in 1888, he resumed the work of his predilec- 
 tion with the thirty-six and twelve-inch refractors of that 
 establishment. But although devoting most of his attention to 
 much-needed remeasurements of known pairs, he incidentally 
 divided no less than 274 stars, the majority of which lay beyond 
 the resolving power of less keen and powerfully-aided eyesight. 
 One of his many interesting discoveries was that of a minute 
 companion to a Ursee Majoris (the first Pointer), which already 
 gives unmistakable signs of orbital movement round the shining 
 orb it is attached to. Another pair, K Pegasi, detected in 1880? 
 was found in 1 892 to have more than completed a circuit in the 
 interim. 4 Its period of a little over eleven years is probably the 
 shortest attributable to a visible binary system ; although S 
 Equulei, discovered by Otto Struve in 1852, appears to be nearly 
 as rapid a couple. Mr. Burnham brought his astronomical 
 career to a pause in 1892, having accepted an official appoint- 
 ment of the legal kind at Chicago. But his abilities as an 
 observer will no doubt be turned to account in the new observa- 
 
 1 Nature, vol. xxvi., p. 177. 2 Proc. 7?. Irish Acad., vol. i., p. 571, ser. iii. 
 3 Mem. R. A. Soc., vol. xlvii., p. 178. 4 Astr. Xach., No. 3142 
 
502 HISTORY OF ASTRONOMY. PART n. 
 
 tory there, just now in course of being equipped with instru- 
 ments of unrivalled magnitude. 
 
 There is as yet no certainty that the stars of 61 Cygni form a 
 true binary combination. It is true that orbits were calculated 
 for them, in I883, 1 by Mr. Mann of Rochester, N.Y., and in 
 i885, 2 by Dr. C. F. W. Peters (now director of the Konigsberg 
 Observatory), giving periods of 1 159 and 783 years respectively ; 
 while the former concluded in 1 890 for one of 462 years. Never- 
 theless, Mr. Burnham holds the components of the supposed 
 pair to be in course of definitive separation ; 3 and Professor 
 Hall's observations at Washington, 1879 to 1891, although 
 favouring their physical connection, are far from decisive on the 
 point. 4 If their movements prove to be really devoid of relative 
 curvature, the stars must form part of a more extended system, 
 circulating in an indefinitely long period round some remote 
 centre of attraction. 
 
 Important series of double-star observations were made by 
 Perrotin at Nice in 1883-4 ; 5 by Hall, with the 26-inch Washing- 
 ton equatoreal, 1874 to 1891 ; 6 above all, by Schiaparelli from 
 1875 to the present time, Burnham's most difficult and inter- 
 esting pairs being picked out by preference for remeasurement 
 with the exquisite achromatics of the Brera Observatory. 7 
 
 A research of striking merit into the origin of binary stars was 
 published in 1892 by Dr. T. J. J. See, of the Chicago University, 
 in the form of an Inaugural Dissertation for his doctor's degree 
 in the University of Berlin. 8 The main result was to show the 
 powerful effects of tidal friction in prescribing the course of their 
 development from double nebulae, revolving almost in contact, to 
 double suns, far apart, yet inseparable. The high eccentricities, 
 of their eventual orbits were shown to result necessarily from 
 this mode of action, which must operate with enormous strength 
 on closely conjoined, nearly equal masses, such as the rapidly 
 
 1 Sidereal Messenger, vol. ii., p. 22. 2 Astr. Nach., Nos. 2708-9, 3157. 3 Sid. 
 Mess., Jan. 1891. 4 Astr. Jour., No. 258. 5 Annales de I'Obs. de Nice, t. ii. 
 6 Wash. Observations, 1888, App. i. 7 Milan Observations, vol. xxxiii., &c. 
 8 See also Observatory, vol. xiv., pp. 92, 115; Astr. and Astro-Physics, April 
 1893, p. 289. 
 
CHAP. xii. STARS AND NEBULJR. 503 
 
 revolving pairs disclosed by the spectroscope. That these are 
 still in an early stage of their life-history is probable in itself, 
 and is further indicated by their invariable display so far as is 
 yet known of the white-star pattern of spectrum. 
 
 Stellar photometry, initiated by the elder Herschel, and pro- 
 vided with exact methods by his son at the Cape, by Steinheil 
 and Seidel at Munich, has of late years assumed the importance 
 of a separate department of astronomical research. Two monu- 
 mental works on the subject, compiled on opposite sides of the 
 Atlantic, were thus appropriately coupled in the bestowal 
 of the Koyal Astronomical Society's Gold Medal in 1886. 
 Harvard College Observatory led the way under the able direc- 
 tion of Professor E. C. Pickering. His photometric catalogue 
 of 4260 stars, 1 constructed from nearly 95,000 observations 
 during the years 1879-82, constitutes a record of incal- 
 culable value for the detection and estimation of stellar 
 variability. It was succeeded in 1885 by Professor Pritchard's 
 " Uranometria Nova Oxoniensis," including photometric deter- 
 minations of the magnitudes of all naked-eye stars, from the 
 pole to ten degrees south of the equator, to the number of 2784. 
 The instrument employed was the " wedge photometer," which 
 measures brightness by resistance to extinction. A wedge of 
 neutral tint-glass, accurately divided to scale, is placed in the 
 path of the stellar rays, when the thickness of it they have 
 power to traverse furnishes a criterion of their intensity. Pro- 
 fessor Pickering's " meridian photometer," on the other hand, is 
 based upon Zollner's principle of equalisation effected by a 
 polarising apparatus. After all, however, as Professor Pritchard 
 observed, " the eye is the real photometer," and its judgment 
 can only be valid over a limited range. 2 Absolute uniformity 
 then, in estimates made by various means, under varying con- 
 ditions, and by different observers, is not to be looked for ; and 
 it is satisfactory to find substantial agreement attainable and 
 attained. Only in an insignificant fraction of the stars common 
 to the Harvard and Oxford catalogues, discordances are found 
 exceeding one-third of a magnitude ; a large proportion (7 1 per 
 
 3 Harvard Annals, vol. xiv., pt. i., 1884. 2 Observatory, vol. viii., p. 309. 
 
504 HISTORY OF ASTRONOMY. PART n. 
 
 cent.) agree within one fourth, a considerable minority (31 per 
 cent.) within one-tenth of a magnitude. 1 
 
 Photographic photometry has meanwhile risen to an import- 
 ance if anything exceeding that.of visual photometry. For the 
 usefulness of the great international star-chart now being pre- 
 pared would be gravely compromised by systematic mistakes 
 regarding the magnitudes of the stars registered upon it. No 
 entirely trustworthy means of determining them have, however, 
 yet been found. There is no certainty as to the relative times 
 of exposure needed to get images of stars representative of 
 successive photometric ranks. All that can be done is to 
 measure the proportionate diameters of such images, and to 
 infer, by the application of a law learned from experience, the 
 varied intensities of light to which they correspond. The law is 
 indeed neither simple nor constant. Different investigators 
 have arrived at different formulae, which, since they are purely 
 empirical, probably vary their nature with the conditions of 
 experiment. Perhaps the best expedient for overcoming the 
 difficulty is that devised by Pickering, of simultaneously photo- 
 graphing a star and its secondary image, reduced in brightness 
 by a known amount. 2 The results of its use will be exhibited in 
 a catalogue of precisely ascertained ninth-magnitude stars, one 
 for each square degree of the heavens. A photographic photo- 
 metry of all the lucid stars, modelled on the visual photometry 
 of 1884, is promised from the same copious source of novelties. 
 The magnitudes of the stars in the Draper Catalogue were deter- 
 mined, so to speak, spectrographically. The quantity measured 
 in all cases was the photographic brightness belonging to a part 
 of the spectrum adjacent to the hydrogen-line G. By the 
 employment of this definite and uniform test, results were 
 obtained, of special value indeed, but in strong disaccord with 
 those given by less exclusive determinations. 
 
 Thought cannot, however, be held aloof from the great subject 
 upon the future illustration of which so much patient industry 
 is being expended. Nor are partial glimpses denied to us of 
 
 1 Month. Not., vol. xlvi., p. 277. 2 Carte Phot, du Ciel. Reunion du ComiU 
 Permanent, Paris, 1891, p. 100. 
 
CHAP. xii. STARS AND NEBULAS. 505 
 
 relations fully discoverable perhaps only by the slow efflux of 
 time. Some important points in cosmical economy have, indeed, 
 become quite clear within the last thirty years, and scarcely 
 any longer admit of a difference of opinion. One of these is 
 that of the true status of nebulae. 
 
 This was virtually settled by Sir J. HerschePs description in 
 1 847 of the structure of the Magellanic clouds ; but it was not 
 until Whewell in 1853, and Herbert Spencer in I858, 1 enforced 
 the conclusions necessarily to be derived therefrom, that the 
 conception of the nebulae as remote galaxies, which Lord Rosse's 
 resolution of many into stellar points had appeared to support, 
 began to withdraw into the region of discarded and half-for- 
 gotten speculations. In the Nubeculae, as Whewell insisted, 2 
 " there co-exists, in a limited compass, and in indiscriminate 
 position, stars, clusters of stars, nebulas, regular and irregular, 
 and nebulous streaks and patches. These, then, are different 
 kinds of things in themselves, not merely different to us. There 
 are such things as nebulae side by side with stars and with 
 clusters of stars. Nebulous matter resolvable occurs close to 
 nebulous matter irresolvable." 
 
 This argument from co-existence in nearly the same region of 
 space, reiterated and reinforced with others by Mr. Spencer, 
 was urged with his accustomed force and freshness by Mr. 
 Proctor. It is unanswerable. There is no maintaining nebulae 
 to be simply remote worlds of stars in the face of an agglomera- 
 tion like the Nubecula Major, containing in its (certainly capa- 
 cious) bosom both stars and nebulae. Add the evidence of the 
 spectroscope to the effect that a large proportion of these 
 perplexing objects are gaseous, with the intimate relation 
 obviously subsisting between the mode of their scattering and 
 the lie of the Milky Way, and it becomes impossible to resist 
 the conclusion that both nebular and stellar systems are parts 
 of a single scheme. 3 
 
 As to the stars themselves, the presumption of their approxi- 
 mate uniformity in size and brightness has been effectually 
 
 1 Essays (2nd ser.), Tlie Nebular Hypothesis. a On the Plurality of Worlds, 
 p. 214 (2nd ed.). 3 Proctor, Month. Not., vol. xxix., p. 342. 
 
506 HISTORY OF ASTRONOMY. PART n. 
 
 dissipated. Differences of distance can no longer be invoked 
 to account for dissimilarity in lustre. Minute orbs, altogether 
 invisible without optical aid, are found to be indefinitely nearer 
 to us than such radiant objects as Canopus, Arcturus, or Rigel. 
 Moreover, intensity of light is perceived to be a very imperfect 
 index to real magnitude. Brilliant suns are swayed from their 
 courses by the attractive power of massive $et faint luminous 
 companions, and suffer eclipse from obscure interpositions. 
 Besides, effective lustre is now known to depend no less upon 
 the qualities of the investing atmosphere, than upon the extent 
 and radiative power of the stellar surface. Ked stars must be 
 far larger in proportion to the light diffused by them than 
 white or yellow stars. 1 There can be no doubt that our sun 
 would at least double its brightness were the absorption suffered 
 by its rays to be reduced to the Sirian standard ; and, on the 
 other hand, that it would lose half its present efficiency as a 
 light-source, if the atmosphere partially veiling its splendours 
 were rendered as dense as that of Aldebaran. 
 
 Thus, variety of all kinds is seen to abound in the heavens ; 
 and it must be admitted that the inevitable abolition of all 
 hypotheses as to the relative distances of individual stars 
 singularly complicates the question of their allocation in space. 
 Nevertheless, something has been learnt even on that point ; 
 and the tendency of modern research is, on the whole, strongly 
 confirmatory of the views expressed by Herschel in 1802. He 
 then no longer regarded the Milky Way as the mere visual 
 effect of an enormously extended stratum of stars, but as an 
 actual aggregation, highly irregular in structure, made up of 
 stellar clouds and groups and nodosities. All the facts since 
 ascertained fit in with this conception ; to which Mr. Proctor 
 added reasons tending to show that the stars forming the 
 galactic stream are not only crowded more closely together, but 
 are also really, as well as apparently, of smaller dimensions than 
 the lucid orbs studding our skies. By the laborious process of 
 isographically charting the whole of Argelander's 324,000 stars, 
 
 1 This remark was first made by J. Michel], 1-hil. Trans., vol. Ivii., p. 253 
 
 (1767). 
 
CHAP. xii. STARS AND NEBULAE. 507 
 
 he brought out in 1 87 1 1 unmistakable signs of relationship 
 between the distribution of the brighter stars and the complex 
 branchings of the Milky Way. The stars, indeed, congregate 
 rather towards a great circle inclined some twenty degrees to- 
 the galactic plane than towards that plane itself; and they 
 were supposed by Gould to form with the sun a subordinate 
 cluster, of which the components are seen projected upon the 
 sky as a zone of stellar brilliants. 2 But the zone displays obvious, 
 galactic affinities ; and many . circumstances render it barely 
 credible that the sun has any organic connection with it. Pro- 
 fessor Kapteyn associates him with a very differently constituted 
 group. 3 An elaborate comparison of the spectra and proper 
 motions of 2357 stars elicited relations demonstrative, in his. 
 view, of the existence of a highly-compressed spherical cluster 
 nearly concentric with the Milky Way. The sun is deeply 
 immersed in the collection, the members of which show, in a 
 large majority of cases, spectra of the solar type, But the rate 
 and manner of their movements appear inconsistent with their 
 dynamical coherence. 4 
 
 A most suggestive delineation of the Milky Way, completed 
 in 1889, after five years of toil, by Dr. Otto Boeddicker, Lord 
 Kosse's astronomer at Parsonstown, was published by litho- 
 graphy in 1892. It shows a curiously intricate structure, 
 composed of dimly luminous streams, and shreds, and patches, 
 intermixed with dark gaps and channels. Ramifications from 
 the main trunk run out towards the Andromeda nebula and 
 the "Bee-hive" cluster in Cancer, involve the Pleiades and 
 Hyades, and winding round the constellation of Orion, just 
 attain the Sword-handle nebula. The last delicate touches 
 had scarcely, however, been put to the picture, when the laborious 
 eye-and-hand method was, in this quarter, as already in so 
 many others, superseded by a more expeditious process. Pro- 
 fessor Barnard took the first photographs ever secured of the 
 true Milky Way, July 28, August I and 2, 1889, at the Lick 
 
 ] Month. Not., vols. xxxi., p. 175 ; xxxii., p. I. 2 System of the Stars, 
 p. 384 ; Old and New Astronomy, p. 749 (Kanyard). 3 Amsterdam Acad. of 
 Sciences, Jan. 28, 1893. 4 Ranyard, Knowledge, April 1893, P- 68 - 
 
5o8 HISTORY OF ASTRONOMY. PART n. 
 
 Observatory. Special conditions were required for success ; 
 above all, a wide field and a strong light-grasp, both complied 
 with through the use of a six-inch portrait-lens. Even thus, 
 the sensitive plate needed some ; -4iours to pick out the exceedingly 
 faint stars collected in the galactic clouds. These cannot be 
 photographed under the nebulous aspect they wear to the eye ; 
 the camera takes note of their real nature, and registers their 
 constituent stars rank by rank. Hence the difficulty of dis- 
 closing them. " In the photographs made with the six-inch 
 portrait lens," Professor Barnard wrote, "besides myriads of 
 stars, there are shown, for the first time, the vast and wonderful 
 cloud-forms, with all their remarkable structure of lanes, holes, 
 and black gaps, and sprays of stars. They present to us these 
 forms in all their delicacy and beauty, as no eye or telescope 
 can ever hope to see them." l In Plate V. one of these strange 
 galactic landscapes is reproduced. It occurs in the Bow of 
 Sagittarius, not far from the Trifid nebula, where the aggre- 
 gations of the Milky Way are more than usually varied and 
 characteristic. One of their distinctive features comes out with 
 particular prominence. It will be noticed that the bright mass 
 near the centre of the plate is tunnelled with dark holes and 
 furrowed by dusky lanes. Such interruptions recur perpetually 
 in the Milky Way. They are exemplified on the largest scale 
 in the great rift dividing it into two branches all the way from 
 Cygnus to Crux ; and they are reproduced in miniature in many 
 clusters. 
 
 Mr. H. C. Russell, at Sydney in 1890, successfully imitated 
 Professor Barnard's example. 2 His photographs of the southern 
 Milky Way have many points of interest. They show the great 
 rift, so black to the eye, as densely star-strewn to the perception 
 of the chemical retina ; while the " Coal-sack " appears abso- 
 lutely dark only in its northern portion. His most remarkable 
 discovery, however, was that of the spiral character of the two 
 Nubeculge. With an effective exposure of four and a half hours, 
 
 1 Pull. Astr. Pac. Soc.,\ol. ii., p. 242. 2 Month. Not., vol. li., pp. 40, 97. 
 For reproductions of some of the photographs in question, see Knowledge, 
 vol. xiv., p. 50. 
 
PLATE V. 
 
 REGION OF THE MILKY WAY IN SAGITTARIUS, 
 
 SHOWING A DOUBLE BLACK APERTURE. 
 
 Photographed by Proi'essor E. E. BARNARD. 
 
CHAP. xii. STARS AND NEBULA. 509 
 
 the Greater Cloud came out as " a complex spiral with two 
 centres " ; while the similar conformation of its minor companion 
 developed only after eight hours of persistent actinic action. 
 The revelation is full of significance. 
 
 Scarcely less so, although after a different fashion, is the 
 disclosure on plates exposed by Dr. Max Wolf with a five-inch 
 lens, in June 1891, of a vastly extended nebula, condensing 
 about a and y Cygni, and bringing them into apparently organic 
 connection with the galactic collections also involved by it. 1 
 The inference thus seems compulsory, that the Milky Way is in 
 reality a prodigious mixed system, resembling that of the 
 Pleiades in point of composition, though differing widely from 
 it in plan of structure. Of corroborative testimony, moreover, 
 is the discovery, independently resulting from Dr. Gill's and 
 Professor Pickering's photographic reviews, that stars of the 
 first type of spectrum largely prevail in the galactic zone of 
 the heavens. 2 For Sirian stars exhibit unquestionable nebulous 
 affinities. 
 
 The first step towards the unravelment of the tangled web of 
 stellar movements was taken when Herschel established the 
 reality and indicated the direction of the sun's journey. But 
 the gradual shifting backward of the whole of the celestial 
 scenery amid which we advance, accounts for only a part of the 
 observed displacements. The stars have motions of their own 
 besides those reflected upon them from ours. All attempts, 
 however, to grasp the general scheme of these motions, have 
 hitherto failed. Yet they have not remained wholly fruitless. 
 The community of slow movement in Taurus, upon which 
 Madler based his famous theory, has proved to be a fact, and 
 one of very extended significance. 
 
 In 1870 Mr. Proctor undertook to chart down the directions 
 and proportionate amounts of about 1600 proper motions, as 
 determined by Messrs. Stone and Main, with the result of 
 bringing to light the remarkable phenomenon termed by 
 him " stardrift." 3 Quite unmistakably, large groups of stars, 
 
 1 Astr. Nach., No. 3048 ; Observatory, vol. xiv., p. 301. >J Proc. Roy. Inst., 
 May 29, 1891 (Gill). 3 Proc. Roy Soc., vol. xviii., p. 169. 
 
510 HISTORY OF ASTRONOMY. PART n. 
 
 otherwise apparently disconnected, were seen to be in progress 
 together, in the same direction, and at the same rate, across the 
 sky. An example of this kind of unanimity alleged by him in 
 the five intermediate stars of the Plough, has not indeed fully 
 maintained its authenticity. Dr. Auwers's researches show that 
 &n identical proper motion can be certainly ascribed to only two 
 out of the five objects in question namely, to and Ursae 
 Majoris. That the agreement in thwartwise motion is no casual 
 one, is practically demonstrated by the presence of a correspond- 
 ing agreement in radial motion, the Potsdam measures giving 
 to each star an approaching velocity of just eighteen miles a 
 second. One of them, moreover, Ursae, alias Mizar, carries 
 with it three other stars Alcor, the Arab " Rider " of the 
 horse, visible to the naked eye, besides a telescopic and a 
 spectroscopic attendant. So that the system is, at the very 
 least, quintuple. Now, Professor Pritchard has photographi- 
 cally determined a parallax of 0.08 i" for e Ursae, whence its 
 more than two hundredfold superiority, in point of light, to our 
 sun, can be inferred. Hence, too, a minimum linear value can 
 be assigned to the apparent interval separating it from Mizar. 
 That is to say, if the line joining the two stars runs at right 
 angles with the line of sight, they are (in round numbers) 
 eighteen billions of miles apart, a distance which light would 
 spend more than three years in crossing. But since this special 
 arrangement is unlikely to prevail, the span of the quintuple 
 system is almost certainly wider, and may be enormously, indefi- 
 nitely wider. The law of its organisation must long remain 
 mysterious. 
 
 This is by no means a solitary example. Particular association, 
 indeed as was surmised by Michell six-score years ago appears 
 to be the rule rather than an exception in the sidereal scheme. 
 Stars are bound together by twos, by threes, by dozens, by 
 hundreds. Our own sun is perhaps not exempt from this gre- 
 garious tendency. Many facts have been brought to light inti- 
 mating its union with hundreds of other stars into a subordinate 
 system within the confines of the Milky Way. 1 Such another 
 1 Month. Not., vol. xl., p. 249. 
 
CHAP. xii. STARS AND NEBULAE. 51 r 
 
 would be the Pleiades. The laws and revolutions of such majestic 
 communities lie, for the present, far beyond the range of possible 
 knowledge; centuries may elapse before even a rudimentary 
 acquaintance with them begins to develop ; while the economy 
 of the higher order of association, which we must reasonably 
 believe that they unite to compose, will possibly continue to 
 stimulate and baffle human curiosity to the end of time. 
 
CHAPTER XIII. 
 METHODS OF RESEARCH. 
 
 COMPAKING the methods now available for astronomical inquiries 
 with those in use thirty years ago, we are at once struck with 
 the fact that they have multiplied. The telescope has been 
 supplemented by the spectroscope and the photographic camera. 
 Now this really involves a whole world of change. It means 
 that astronomy has left the place where she dwelt apart in rapt 
 union with mathematics, indifferent to all things on earth save 
 only to those mechanical improvements which should aid her to 
 penetrate further into the heavens, and has descended into the 
 forum of human knowledge, at once a suppliant and a patron, 
 alternately invoking help from and promising it to each of the 
 sciences, and patiently waiting upon the advance of all. The 
 science of the heavenly bodies has, in a word, become a branch 
 of terrestrial physics, or rather a higher kind of integration of 
 all their results. It has, however, this leading peculiarity, that 
 the materials for the whole of its inquiries are telescopically 
 furnished. They are such as the unarmed eye takes no, or a very 
 imperfect cognisance of. 
 
 Spectroscopic and photographic apparatus are simply additions 
 to the telescope. They do not supersede, or render it of less 
 importance. On the contrary, the efficacy of their action depends 
 primarily upon the optical qualities of the instrument they are 
 attached to. Hence the development, to their fullest extent, of 
 the powers of the telescope is of vital moment to the progress 
 of modern physical astronomy, while the older mathematical 
 astronomy could afford to remain comparatively indifferent 
 to it. 
 
CHAP. xiii. METHODS OF RESEARCH. 513 
 
 The colossal Rosse reflector still marks, as to size, the ne plus 
 ultra of performance in that line. A mirror, four feet in 
 diameter was, however, sent out to Melbourne by the late Thomas 
 Grubb of Dublin in 1 870. This is mounted in the Cassegrainian 
 manner ; so that the observer looks straight through it towards 
 the object viewed, of which he really sees a twice-reflected image. 
 The dust-laden atmosphere of Melbourne is said to impede very 
 seriously the usefulness of this originally fine instrument. 
 
 It may be doubted whether so large a speculum will ever 
 again be constructed. A new material for the mirrors of re- 
 flecting telescopes was proposed by Steinheil in 1856, and inde- 
 pendently by Foucault in I857, 1 which has already in a great 
 measure superseded the use of a metallic alloy. This is glass 
 upon which a thin film of silver has been deposited by a 
 chemical process invented by Liebig. It gives a peculiarly 
 brilliant reflective surface, throwing back more light than a 
 metallic mirror of the same area, in the proportion of about 
 sixteen to nine. Resilvering, too, involves much less risk and 
 trouble than repolishing a speculum. The first use of this plan 
 on a large scale was in an instrument of thirty-six inches 
 aperture, finished by Calver for Dr. Common in 1879. To its 
 excellent qualities, turned to account with rare skill, his triumphs 
 in celestial photography were mainly due. A more daring 
 experiment was the construction and mounting, by Dr. Common 
 himself, of a five-foot reflector. But the first glass-disc ordered 
 from France for the purpose proved radically defective. When 
 figured, polished, and silvered, towards the close of 1888, it gave 
 elliptical instead of circular star-images. 2 A new one had to be 
 procured, and was ready for astronomical use in 1891. The 
 satisfactory nature of its performance is vouched for by the 
 observations made with it upon Jupiter's new satellite in 
 December 1892. This instrument, to which a Newtonian form 
 has been given, concentrates more light than any telescope yet 
 built. A refractor, to be its equal in this respect, should 
 have more than fifty inches of aperture. 
 
 1 Comptes JRendus, t. xliv., p. 339. ' 2 A. A. Common, Memoirs It. Astr. Soc., 
 vol. 1., p. 118. 
 
 33 
 
514 HISTORY OF ASTRONOMY. PART 11. 
 
 It is, however, in the construction of refracting telescopes that 
 the most conspicuous advances have recently been made. The 
 Harvard College 15 -inch achromatic was mounted and ready for 
 work in June 1847. A similar instrument had already for 
 some years been in its place at Pulkowa ; but it was long before 
 the possibility of surpassing these masterpieces of German skill 
 presented itself to any optician. For fifteent.years it seemed as 
 if a line had been drawn just there. It was first transgressed in 
 America. A portrait-painter of Cambridgeport, Massachusetts, 
 named Alvan Clark, had for some time amused his leisure with 
 grinding lenses, the singular excellence of which was discovered 
 in England by Mr. Dawes in I853- 1 Seven years passed, and 
 then an order came from the University of Mississippi for an 
 object-glass of the unexampled size of eighteen inches. An 
 experimental glance through it to test its definition resulted, as 
 we have seen, in the detection of the companion of Sirius, 
 January 31, 1862. It never reached its destination in the South. 
 War troubles supervened ; and it was eventually sent to Chicago, 
 where it has served Professor Hough in his investigations of 
 Jupiter, and Mr. Burnham in his scrutiny of double stars. 
 
 The next step was an even longer one, and it was again taken 
 by a self-taught optician, Thomas Cooke, the son of a shoemaker 
 at Allerthorpe, in the East Riding of Yorkshire. Mr. Newall of 
 G-ateshead ordered from him in 1863 a 25-inch object-glass. It 
 was finished early in 1 868, but at the cost of shortening the life 
 of its maker, who died October 19, 1869, before the giant re- 
 fractor he had toiled at for five years was completely mounted. 
 This instrument, the excellent qualities of which had long been 
 neutralised by an unfavourable situation, was presented by Mr. 
 Newall to the University of Cambridge a few weeks before his 
 death, April 21, 1889. It is destined, under the care of his son, 
 Mr. Frank Newall, for use in stellar physics. 
 
 Close upon its construction followed that of the Washington 
 
 26-inch, for which twenty thousand dollars were paid to Alvan 
 
 Clark. Set to work in 1873, the most illustrious point in its 
 
 career, so far, has been the discovery of the satellites of Mars. 
 
 1 Newcomb, Pop. Astr. t p. 137. 
 
CHAP. xiii. METHODS OF RESEARCH. 515 
 
 Once known to be there, these were, indeed, found to be per- 
 ceptible with very moderate optical means (Mr. Wentworth Erck 
 saw Deimos with a 7-inch Clark) ; but the first detection of such 
 minute objects is a feat of a very different order from their sub- 
 sequent observation. 
 
 For a little over eight years the Washington refractor held 
 the primacy. It had to yield the place of honour in December 
 1880 to a giant achromatic, twenty-seven inches in aperture, 
 built by Sir Howard Grubb (son and successor of Thomas Grubb) 
 for the Vienna Observatory. This, in its turn, has been sur- 
 passed by two of respectively 29^ and 30 inches, sent by 
 Gautier of Paris to Nice, and by Alvan Clark to Pulkowa ; and 
 an object-glass, three feet in diameter, was in 1886 successfully 
 turned out by the latter firm for the Lick Observatory in Cali- 
 fornia. The difficulties, however, encountered in procuring discs 
 of glass of the size and purity required for this last venture, 
 seemed to indicate that a term to progress in this direction was 
 not far off. The flint was indeed cast with comparative ease in 
 the workshops of M. Feil at Paris. The flawless mass weighed 
 170 kilogrammes, was over 38 inches across, and cost .2,000. 
 But with the crown part of the designed achromatic combina- 
 tion things went less smoothly, The production of a perfect 
 disc was only achieved after nineteen failures, involving a 
 delay of more than two years ; and the glass for a third lens, 
 designed to render the telescope available at pleasure for photo- 
 graphic purposes, proved to be strained, and consequently went 
 to pieces in the process of grinding. It has been replaced by 
 one of thirty-three inches, with which a series of admirable lunar 
 and other photographs have been taken. 
 
 Nor is the difficulty in obtaining suitable material the only 
 obstacle to increasing the size of refractors. The " secondary 
 spectrum," as it is called, also interposes a barrier trouble- 
 some to surmount. A truly achromatic combination cannot be 
 obtained with ordinary flint and crown glass ; and it has not yet 
 been found possible to procure large castings of Professor Abbe's 
 " Jena glass," by which outstanding colour is reduced to about 
 one-sixth its usual amount. In the Lick telescope, accordingly, 
 
5i6 HISTORY OF ASTRONOMY. PART n. 
 
 the differences of focal length for the various colours are counted 
 by inches, 1 and this not through any lack of skill in the makers, 
 but by the necessity - of the ca^e. Embarrassing consequences 
 follow. Only a small part of the spectrum of a heavenly body, 
 for instance, can be distinctly seen at one time; and a focal 
 adjustment of half-an-inch is required in passing from the obser- 
 vation of a planetary nebula to that of its stellar nucleus. A 
 refracting telescope loses, besides, one of its chief advantages 
 over a reflector when its size is increased beyond a certain limit. 
 That advantage is the greater luminosity of the images given by 
 it. Considerably more light is transmitted through a glass lens 
 than is reflected from an equal metallic surface ; but only so 
 long as both are of moderate dimensions. For the glass neces- 
 sarily grows in thickness as its area augments, and consequently 
 stops a larger percentage of the rays it refracts. So that a 
 point at length arrives fixed by the late Dr. Kobinson at a dia- 
 meter a little short of three feet 2 where the glass and the 
 metal are, in this respect, on an equality ; while above it, the 
 metal has the advantage. And since silvered glass gives back 
 considerably more light than speculum metal, the stage of equali- 
 sation with lenses is reached proportionately sooner where this 
 material is employed. 
 
 The most distinctive faculty of reflectors, however, is that of 
 bringing rays of all refrangibilities to a focus together. They 
 are naturally achromatic. None of the beams they collect are 
 thrown away in colour-fringes, obnoxious both in themselves 
 and as a waste of the chief object of astro-physicists' greed 
 light. Reflectors, then, are in this respect specially adapted to 
 photographic and spectrographic use. But they have a counter- 
 vailing drawback. The penalties imposed by bigness are for 
 them peculiarly heavy. Perfect definition becomes, with in- 
 creasing size, more and more difficult, to attain ; once attained, 
 it becomes more and more difficult to keep ; for the huge 
 masses of material employed to form great object-glasses or 
 mirrors tend with every movement to become deformed by their 
 
 1 Keeler, Puhl. Astr. Pac. Soc., vol. ii., p. 160. - H. Grubb, Trans. Roy. 
 Ditb. Soc., vol. i. (new ser.), p. 2. 
 
CHAP, xiii METHODS OF RESEARCH. 517 
 
 own weight. Now, the slightest bending of a mirror is fatal to 
 its performance, the effect being doubled by reflection ; while, in 
 a lens, alteration of figure is compensated by the equal and con- 
 trary flexures of the opposing surfaces, so that the emergent 
 beams pursue much the same paths as if the curves of the 
 refracting medium had remained theoretically perfect. For this 
 reason, work of precision must remain the province of refracting 
 telescopes, although great reflectors retain the primacy in the 
 portraiture of the heavenly bodies, as well as in certain branches 
 of spectroscopy. 
 
 Ambition, as regards telescopic power, is by no means yet 
 satisfied. Nor ought it to be. The advance of astro-physical 
 researches of all kinds depends largely upon light-grasp. For the 
 spectroscopic examination of stars, for the measurement of 
 their motions in the line of sight, for the discovery and study of 
 nebulae, for stellar and nebular photography, the cry continually 
 is, " more light." There is no enterprising head of an observa- 
 tory but must feel cramped in his designs if he can command no 
 more than fourteen or fifteen inches of aperture, and he aspires 
 to greater instrumental capacity, not merely with a view to the 
 chances of discovery, but for the steady prosecution of some 
 legitimate line of inquiry. Thus projects of telescope-building 
 on a large scale are rife, and some are in course of execution- 
 Sir Howard Grubb has nearly finished a twenty-eight inch 
 achromatic for Greenwich; one at least its equal ought ere long to 
 be erected at Potsdam ; and Mr. Alvan G. Clark, the sole survivor 
 of the celebrated Cambridgeport firm, has undertaken what he 
 expects will be his last arduous commission. 
 
 Mr. Yerkes of Chicago offered, in October 1892, an unlimited 
 sum for the provision of the University of that city with a 
 " superlative " telescope. And it happened fortunately that a 
 pair of glass discs, nearly forty-two inches in diameter, and of 
 perfect quality, were ready at hand. They had been cast by 
 Mantois for the University of Southern California, when the 
 erection of a great observatory on Wilson's Peak was under con- 
 sideration. The Yerkes instrument may then be completed with 
 relatively small delay ; and a programme of work to be done 
 
5i8 HISTORY OF ASTRONOMY. PART n. 
 
 with it lias already been sketched out. 1 And since, among those 
 engaged in carrying it out will be Professor Hale, Dr. See, and 
 Mr. Burnham, we may be sure that the coming implement of 
 research will not be allowed tcr" rust unburnished." 
 
 Much vaguer is the prospect of seeing a gigantic reflector 
 mounted in Paris by way of enhancement to the glories of the 
 Exhibition year 1900. The mirror, of silvered glass, is to be ten 
 feet in diameter, and will be contained in a tube 140 feet long ; 
 while magnifying powers are promised to be applied by which 
 the moon will be brought within an insignificant number of 
 kilometers of the amazed spectator. But the plan, as at present 
 put forward, belongs to the realm rather of sensation than of 
 science. Its realisation, regarded as possible by so high an 
 authority as Dr. Common, would, none the less, be of great in- 
 terest to optical astronomy. 
 
 These emulative efforts have without doubt been spurred on 
 by the brilliant success of the Lick telescope. The results 
 achieved with it are the best testimony to its excellence. The 
 discovery, by Professors Holden and Schaeberle, of the helical 
 forms of certain planetary nebulae, Mr. Burnham 's hair's-breadth 
 star-divisions, Professor Keeler's spectroscopic determinations of 
 nebular motion, Professor Barnard's detections and prolonged 
 pursuit of faint comets, last, not least, his discovery of Jupiter's 
 tiny moon all this could only have been accomplished in four 
 years, even by an exceptionally able and energetic staff, with the 
 aid of an instrument of first-rate optical quality. But there 
 was another condition which should not be overlooked. 
 
 The best telescope may be rendered useless by its situation. 
 The larger it is, indeed, the more powerless is it to cope with 
 atmospheric troubles. These are the worst plagues of all those 
 that afflict the astronomer. No mechanical skill avails to 
 neutralise or alleviate them. They augment with each increase 
 of aperture ; they grow with the magnifying powers applied. 
 The rays from the heavenly bodies, when they can penetrate the 
 cloud-veils that too often bar their path, reach us in an enfeebled, 
 
 1 Hal3, Astr. and Astro- Physics, Nov. 1892, p. 291. - Observatory, vol. 
 xv., p. 391. 
 
CHAP. xin. METHODS OF RESEARCH. 519 
 
 scattered, and disturbed condition. Hence the twinkling of 
 stars, the " boiling " effects at the edges of sun, moon, and 
 planets, hence distortions of bright, effacements of feeble tele- 
 scopic images. Hence, too, the paucity of the results achieved 
 with many powerful light-gathering machines. 
 
 No sooner had the Parsonstown telescope been built, than it 
 became obvious that the limit of profitable augmentation of size 
 had, under climatic conditions at all nearly resembling those 
 prevailing there, been reached, if not overpassed; and Lord 
 Eosse himself was foremost to discern the need of pausing to 
 look round the world for a clearer and stiller air than was to be 
 found within the bounds of the United Kingdom. With this 
 express object Mr. Lassell transported his two-foot Newtonian to 
 Malta in 1852, and mounted there, in 1860, a similar instrument 
 of four-fold capacity, with which in the course of about two 
 years 600 new nebulae were discovered. Professor Piazzi Smyth's 
 experiences during a trip to the Peak of Teneriffe in 1856 in 
 search of astronomical opportunities, 1 gave countenance to the 
 most sanguine hopes of deliverance, at suitably elevated stations, 
 from some of the oppressive conditions of low-level star-gazing ; 
 yet for a number of years nothing effectual was done for their 
 realisation. Now at last, however, mountain observatories are 
 not only an admitted necessity, but an accomplished fact ; and 
 Newton's long forecast of a time when astronomers would be 
 compelled, by the developed powers of their telescopes, to mount 
 high above the " grosser clouds " in order to use them, 2 has been 
 justified by the event. 
 
 Mr. James Lick, the millionaire of San Francisco, had already 
 chosen when he died, October I, 1876, a site for the new 
 observatory, to the building and endowment of which he 
 devoted a part of his large fortune. The situation of the estab- 
 lishment is exceptional and splendid. Planted on one of the 
 three peaks of Mount Hamilton, a crowning summit of the 
 Californian Coast Range, at an elevation of 4200 feet above the 
 sea, in a climate scarce rivalled throughout the world, it com- 
 mands views both celestial and terrestrial which the lover of 
 
 1 Phil. Trans., 'vol. cxlviii., p. 465. " Optice, p. 107 (2nd ed., 1719). 
 
520 HISTORY OF ASTRONOMY. PART H. 
 
 nature and astronomy may alike rejoice in. Impediments to 
 observation are there found to be most materially reduced. 
 Professor Holden, who was appointed in 1885 president of the 
 University of California and .director of the new observatory 
 affiliated to it, stated that during six or seven months of the 
 year an unbroken serenity prevails, and that half the remaining 
 nights are clear. 1 The power of continuous work thus afforded 
 is of itself an inestimable advantage ; and the high visual 
 excellences testified to by Mr. Burnham's discovery, during a 
 two months' trip to Mount Hamilton in the autumn of 1 879, of 
 forty-two new double stars with a 6-inch achromatic, gave hopes, 
 since fully realised, of a brilliant future for the Lick establish- 
 ment. Its advantages, according to the generous design of 
 Professor Holden, are shared by the whole astronomical world. 2 
 A sort of appellate jurisdiction has been, by general consent, 
 accorded to the great equatoreal, and more than one disputed 
 point has been satisfactorily settled by recourse to it. 
 
 It is unlikely that its performances will be surpassed by out- 
 bidding it in size, unless the care expended upon the selection 
 of its site be imitated. Professor Pickering has thus shown his 
 customary prudence in reserving his efforts to procure a great 
 telescope until Harvard College owned a dependent observatory 
 where it could be employed to advantage. This has been found 
 by Mr. W. H. Pickering, after many experiments in Colorado, 
 California, and Peru, at Arequipa, on a slope of the Andes eight 
 thousand feet above the sea-level. Here the post provided for 
 by the " Boyden Fund" was established in 1891, under ideal 
 meteorological conditions. Temperature preserves a "golden 
 mean " ; the barometer is almost absolutely steady ; the yearly 
 rainfall amounts to no more than three or four inches. No 
 wonder then that the "seeing" there is of the extraordinary 
 excellence attested by Mr. Pickering's observations. In the 
 absence of bright moonlight, he tells us, 3 eleven Pleiades can 
 always be counted ; the Andromeda nebula appears to the naked 
 eye conspicuously bright, and larger than the full moon ; third 
 
 1 Observatory, vol. viii., p. 85. " Holden on Celestial Photography, Over, 
 land Monthly, Nov. 1886. 3 Observatory, vol. xv., p. 283. 
 
CHAP. xni. METHODS OF RESEARCH. 521 
 
 magnitude stars have been followed to their disappearance at 
 the true horizon; the zodiacal light spans the heavens as a 
 complete arch, the " Gegenschein" forming a regular part of the 
 scenery of the heavens. Corresponding telescopic facilities are 
 enjoyed. The chief instrument at the station, a thirteen-inch 
 equatoreal by Clark, shows the fainter parts of the Orion 
 nebula, photographed at Harvard College in 1887, by which the 
 dimensions given to it in Bond's drawing are doubled ; stars are 
 at times seen encircled by half a dozen immovable diffraction- 
 rings, no less than twelve of which have been counted round 
 a Centauri; while on many occasions no practicable increase 
 of magnifying power availed to bring out any wavering in the 
 limbs of the planets. Moreover, the series of fine nights is nearly 
 unbroken from March to November. 
 
 In Professor E. C. Pickering's judgment, the advantages 
 conferred, in many researches, by the climate of Arequipa are 
 equivalent to a doubling of telescopic aperture. His appeal, 1 
 then, for a colossal instrument, to be erected in a spot where its 
 utmost powers could be turned to account with trifling inter- 
 ruption, will doubtless not be made in vain. Arequipa has the 
 additional prerogative, owing to its position sixteen degrees 
 below the line, of commanding the entire southern hemisphere. 
 And at present, all the workable great telescopes in the world 
 are located above the parallel of 35 north latitude. The 
 Melbourne reflector is the only instrument of more than fifteen 
 inches aperture south of that limit; and it no longer counts 
 for much, having probably suffered irreparable injury on the 
 long sea-voyage from its place of origin to its destination. 
 
 Vapours and air-currents do not alone embarrass the use of 
 giant telescopes. Mechanical difficulties also oppose a formidable 
 barrier to much further growth in size. But what seems a 
 barrier often proves to be only a fresh starting-point ; and signs 
 are not wanting that it may be found so in this case. It is 
 possible that the monumental domes and huge movable tubes 
 of our present observatories will, in a few decades, be as much 
 
 1 Astr. and Astro-Physics, Nov. 1892, p. 783. 
 
522 HISTORY OF ASTRONOMY. PART n. 
 
 things of the past as Huygens's " aerial " telescopes. It is certain 
 that the thin edge of the wedge of innovation has been driven 
 into the old plan of equatoreal mounting. 
 
 M. Loewy, the present sub-director of the Paris Observatory, 
 proposed to Delaunay in 1871 the erection of a telescope on a 
 novel system. The design seemed feasible, and was adopted ; 
 but the death of Delaunay and the other untoward circumstances 
 of the time interrupted its execution. Its resumption, after 
 some years, was rendered possible by M. Bischoffsheim's gift of 
 25,000 francs for expenses, and the coud6 or "bent" equatoreal 
 has been, since 1882, one of the leading instruments at the Paris 
 establishment. 
 
 Its principle is briefly this. The telescope is, as it were, its 
 own polar axis. The anterior part of the tube is supported at 
 both ends, and is thus fixed in a direction pointing towards the 
 pole, with only the power of twisting axially. The posterior 
 section is joined on to it at right angles, and presents the object- 
 glass accordingly to the celestial equator, in the plane of which 
 it revolves. Stars in any other part of the heavens have their 
 beams reflected upon the object-glass by means of a plane 
 rotating mirror placed in front of it. The observer, meanwhile, 
 is looking steadfastly down the bent tube towards the invisible 
 southern pole. He would naturally see nothing whatever, were 
 it not that a second plane mirror is fixed at the " elbow " of the 
 instrument, so as to send the rays which have traversed the 
 object-glass to his eye. He never needs to move from his place. 
 He watches the stars seated in an arm-chair in a warm room, 
 with as perfect convenience as if he were examining the seeds 
 of a fungus with a microscope. Nor is this a mere gain of 
 personal ease. The abolition of hardship includes a vast acces- 
 sion of power. 1 
 
 Among other advantages of this method of construction are, 
 first, that of added stability, the motion given to the ordinary 
 equatoreal being transferred, in part, to an auxiliary mirror. 
 Next, that of increased focal length. The fixed part of the tube 
 can be made almost indefinitely long without inconvenience, and 
 
 1 Loewy, Bull. Astr., t. i., p. 286 ; Nature, vol. xxix., p. 36. 
 
CHAP. xin. METHODS OF RESEARCH. 523 
 
 with enormous advantage to the optical qualities of a large 
 instrument. Finally, the costly and unmanageable cupola is got 
 rid of, a mere shed serving all purposes of protection required 
 for the coudti. 
 
 The desirability of some such change as that which M. Loewy 
 has realised, had been felt by others. Professor Pickering 
 sketched in 1881 a plan for fixing large refractors in a perma- 
 nently horizontal position, and reflecting into them, by means of 
 a shifting mirror, the objects desired to be observed. 1 The 
 observations for his photometric catalogue were, in fact, made 
 with. a "broken transit," in which the line of sight remains 
 permanently horizontal, whatever the altitude of the star 
 examined. An instrument with " siderostatic " mounting by 
 Sir Howard Grubb has been in use at the Crawford Observa- 
 tory, Cork, since 1882 ; in a paper read before the Royal Society, 
 January 21. 1884, he proposed to carry out the principle 011 a 
 more extended scale ; 2 and shortly afterwards undertook its 
 application to a telescope eighteen inches in aperture for the 
 Armagh Observatory. 3 The chief honours, however, remain to 
 the Paris inventor. None of the prognosticated causes of fail- 
 ure have proved effective. The loss of light from the double 
 reflection is insignificant. The menaced deformation of images 
 is, through the exquisite skill of the MM. Henry io producing 
 plane mirrors of all but absolute perfection, quite imperceptible. 
 The definition of the novel loj-inch equatoreal is admitted to 
 be singularly good. Dr. Gill stated in 1884 that he had never 
 measured a double star so easily as he did y Leonis by its means. 4 
 Professor Lockyer pronounced it to be " one of the instruments 
 of the future " ; and the principle of its construction was imme- 
 diately adopted by the directors of the Besancon and Algiers 
 Observatories, as well as for a 17-inch telescope destined for a 
 new observatory at Buenos Ayres. At Paris, it has since been 
 carried out on a larger scale. A coudd, of 23 J inches aperture 
 and sixty-two feet focal length, was in 1890 installed at the 
 National Observatory, and has served M. Loewy for his inge- 
 
 1 Nature, vol. xxiv., p. 389. - Ibid., vol. xxix., p. 470. 3 Trans. Boy. 
 Dub. Soc., vol. iii., p. 61. 4 Observatory, vol. vii., p. 167. 
 
524 HISTORY OF ASTRONOMY. PART n. 
 
 nious studies on refraction and aberration. A considerable 
 number of orders for similar instruments are being executed ; 
 but the " bent " form, although suitable to reflectors as well as 
 to refractors, 1 has not yet been teied with the former. The inven- 
 tion has, however, achieved a decisive success. 
 
 Celestial photography is not yet fifty years old ; yet its earliest 
 beginnings already seem centuries behind its*. present perform- 
 ances. The details of its gradual yet rapid improvement are of 
 too technical a nature to find a place in these pages. Suffice it 
 to say that the " dry-plate " process, with which such wonderful 
 results have been obtained, appears to have been first made 
 available by Dr. Huggins in photographing the spectrum of 
 Vega in 1876, and was then successively adopted by Common, 
 Draper, and Janssen. Nor should Captain Abney's remark- 
 able extension of the powers of the camera be forgotten. He 
 began his experiments on the chemical action of red and infra- 
 red rays in 1874, and at length succeeded in obtaining a sub- 
 stance the "blue" bromide of silver highly sensitive to these 
 slower vibrations of light. With its aid he explored a vast, 
 unknown, and for ever invisible region of the solar spectrum, 
 presenting to the Koyal Society, December 5, i879, 2 a detailed 
 map of its infra-red portion (wave-lengths 7600 to 10,750), from 
 which valuable inferences may yet be derived as to the condition 
 of the various kinds of matter ignited in the solar atmosphere. 
 Upon plates rendered " orthochromatic " by staining with ery- 
 throsin, or other dye-stuffs, the whole visible spectrum can now 
 be photographed ; but those with their maximum of sensitiveness 
 near G, are found more practically available, except where the 
 results of light-analysis are sought to be completely recorded. 
 And since photographic refractors are corrected for the blue 
 rays, exposures with them of orthochromatic surfaces would be 
 entirely futile. 
 
 The chemical plate has two advantages over the human retina. 3 
 First, it is sensitive to rays which are utterly powerless to produce 
 any visual effect ; next, it can accumulate impressions almost 
 
 1 Loewy, BtdL Astr., t. i., p. 265. 2 Phil. Trans., vol. clxxi., p. 653. 
 3 Janssen, L' Astronomic, t. ii., p. 121. 
 
CHAP. xin. METHODS OF RESEARCH. 525 
 
 indefinitely, while from the retina they fade after one-tenth part 
 of a second, leaving it a continually renewed tabula rasa. 
 
 It is accordingly quite possible to photograph objects so faint 
 as to be altogether beyond the power of any telescope to reveal ; 
 and we may thus eventually learn whether a blank space in the 
 sky truly represents the end of the stellar universe in that direc- 
 tion, or whether farther and farther worlds roll and shine beyond, 
 veiled in the obscurity of immeasurable distance. 
 
 Of many ingenious improvements in spectroscopic appliances 
 the most fundamentally important relate to what are known as 
 " gratings." These are very finely striated surfaces, by which 
 light-waves are brought to interfere, and are thus sifted out, 
 strictly according to their different lengths, into " normal " spec- 
 tra. Since no universally valid measures can be made in any 
 others, their production is quite indispensable to spectroscopic 
 science. Fraunhofer, who initiated the study of the diffraction 
 spectrum, used a real grating of very fine wires ; but rulings on 
 glass were adopted by his successors, and were by Nobert exe- 
 cuted with such consummate skill that a single square inch of 
 surface was made to contain 100,000 hand-drawn lines. Such 
 rare and costly triumphs of art, however, found their way into 
 very few hands, and practical availability was first given to this 
 kind of instrument by the inventiveness and mechanical dexterity 
 of two American investigators. Both Eutherf urd's and Rowland's 
 gratings are machine-ruled, and reflect, instead of transmitting 
 the rays they analyse ; but Rowland's present to them a very 
 much larger diffractive surface, and consequently possess a 
 higher resolving power. The first preliminary to his improve- 
 ments was the production, in 1882, of a faultless screw, those 
 previously in use having been the inevitable source of periodical 
 errors in striation, giving, in their turn, ghost-lines as subjects 
 of spectroscopic study. 1 Their abolition was not one of Row- 
 land's least achievements. With his perfected machine a metallic 
 area of 6J by 4^ inches can be ruled with exquisite accuracy to 
 almost any degree of fineness; he considers, however, 43.000 
 
 1 Kev. A. L. Cortie, Astr. and Astro-Physics, May 1892, p. 400. 
 
526 HISTORY OF ASTRONOMY. PART n. 
 
 lines to the inch to be the limit of usefulness. 1 The ruled surface 
 is moreover concave, and hence brings the spectrum to a focus 
 without a telescope. A slit and an eyepiece are alone needed 
 to view it, and absorption of light by glass lenses is obviated 
 an advantage especially sensible in dealing with the ultra- or 
 infra-visible rays. 
 
 The high qualities of Professor Eowland's great photographic 
 map of the solar spectrum were thus based upon his previous 
 improvement of the instrumental means used in its execution. 
 The amount of detail shown in it is illustrated by the appearance 
 on the negatives of 1 50 lines between H and K ; and many lines 
 depict themselves as double which, until examined with a concave 
 grating, had passed for one and indivisible. The corresponding 
 hand-drawing for which M. Thollon received in 1886 the Lalande 
 Prize, exhibits, not the diffractive, but the prismatic spectrum 
 as obtained with bisulphide of carbon prisms of large dispersive 
 power. About one-third of the visible gamut of the solar radia- 
 tions (A to Z>) is covered by it ; it includes 3200 lines, and is 
 over ten metres long. 2 The grating is an expensive tool in the 
 way of light. Where there is none to spare, its advantages 
 must be foregone. They could not, accordingly, be turned to 
 account in stellar spectroscopy until the Lick telescope was at 
 hand to supply more abundant material for research. By the 
 use of Rowland's grating thus made possible, Professor Keeler 
 was able to apply enormous dispersion to the rays of stars and 
 nebulae, and so to attain a previously unheard-of degree of 
 accuracy in their measurement. His memorable detection of 
 nebular movement in line of sight ensued as a consequence. 
 Professor Campbell, his successor, has since obtained, by the 
 same means, the first satisfactory photographs of stellar diffrac- 
 tion spectra. 
 
 The means at the disposal of astronomers have not multiplied 
 faster than the tasks imposed upon them. Looking back to the 
 year 1 800, we cannot fail to be astonished at the change. The 
 comparatively simple and serene science of the heavenly bodies 
 known to oar predecessors, almost perfect so far as it went, in- 
 
 1 Phil. Mag., vol. xiii., 1882, p. 469. Bull. Astr., t. iii., p. 331. 
 
CHAP. xin. METHODS OF RESEARCH. 527 
 
 curious of what lay beyond its grasp, has developed into a body 
 of manifold powers and parts, each with its separate mode and 
 means of growth, full of strong vitality, but animated by a rest- 
 less and unsatisfied spirit, haunted by the sense of problems 
 unsolved, and tormented by conscious impotence to sound the 
 immensities it perpetually confronts. 
 
 Knowledge might then be said to be bounded by the solar 
 system ; but even the solar system presented itself under an 
 aspect strangely different from what it now wears. It consisted 
 of the sun, seven planets, and twice as many satellites, all circling 
 harmoniously in obedience to an universal law, by the compen- 
 sating action of which the indefinite stability of their mutual re- 
 lations was secured. The occasional incursion of a comet, or the 
 periodical presence of a single such wanderer chained down from 
 escape to outer space by planetary attraction, availed nothing to 
 impair the symmetry of the majestic spectacle. 
 
 Now, not alone the ascertained limits of the system have been 
 widened by a thousand millions of miles, with the addition of 
 one more giant planet and seven satellites to the ancient classes 
 of its members, but a complexity has been given to its constitu- 
 tion baffling description or thought. Three hundred and sixty 
 circulating planetary bodies bridge the gap between Jupiter and 
 Mars, the complete investigation of the movements of any one 
 of which would overtask the energies of a lifetime. Meteorites, 
 strangers apparently to the fundamental ordering of the solar 
 household, swarm, nevertheless, by millions in every cranny of 
 its space, returning at regular intervals like the comets so singu- 
 larly associated with them, or sweeping across it with hyperbolic 
 velocities, brought perhaps from some distant star. And each 
 of these cosmical grains of dust has a theory far more complex 
 than that of Jupiter ; it bears within it the secret of its origin, 
 and fulfils a function in the universe. The sun itself is no 
 longer a semi-fabulous, fire-girt globe, but the vast scene of the 
 play of forces as yet imperfectly known to us, offering a bound- 
 less field for the most arduous and inspiring researches. Among 
 the planets the widest variety in physical habitudes is seen 
 to prevail, and each is recognised as a world apart, inviting 
 
528 HISTORY OF ASTRONOMY. PART n. 
 
 inquiries which, to be effective, must necessarily be special and 
 detailed. Even our own moon threatens to break loose from the 
 trammels of calculation, and commits "errors" which sap the 
 very foundations of the lunar tl^eory, and suggest the formidable 
 necessity for its complete revision. Nay. the steadfast earth has 
 forfeited the implicit confidence placed in it as a time-keeper, 
 and questions relating to the stability of the earth's axis, and 
 the constancy of the earth's rate of rotation, are among those 
 which it behoves the future to answer. Everywhere there is 
 multiformity and change, stimulating a curiosity which the rapid 
 development of methods of research offers the possibility of at 
 least partially gratifying. 
 
 Outside the solar system, the problems which demand a 
 practical solution are all but infinite in number and extent. 
 And these have all arisen and crowded upon our thoughts within 
 less than a hundred years. For sidereal science became a re- 
 cognised branch of astronomy only through Herschel's discovery 
 of the revolutions of double stars in 1802. Yet already it may 
 be, and has been called, "the astronomy of the future": so 
 rapidly has the development of a keen and universal interest 
 attended and stimulated the growth of power to investigate this 
 sublime subject. What has been done is little is scarcely a 
 beginning ; yet it is much in comparison with the total blank of 
 a century past. And our knowledge will, we are easily 
 persuaded, appear in turn the merest ignorance to those who 
 come after us. Yet it is not to be despised, since by it we reach 
 up groping fingers to touch the hem of the garment of the Most 
 High. 
 
APPENDIX. 
 
 34 
 
TABLE I. 
 
 1774, March 4 
 1774 
 1774 -, 
 
 1781, March 13 
 
 1782 . 
 
 1783 - 
 
 1783 - 
 1784 ';. 
 
 1784 . 
 
 1784 . 
 
 1786 . 
 
 1787, January n 
 
 1787, Nov. 19 
 :i 7 8 9 . 
 
 1789 . 
 
 CHRONOLOGY, 1774-1893. 
 
 Herschel's first observation. Subject, the Orion 
 Nebula. 
 
 The depressed nature of sun-spots geometrically 
 proved by Wilson. 
 
 First experimental determination of the earth's 
 mean density by Maskelyne. 
 
 Discovery of Uranus. 
 
 Herschel's first Catalogue of Double Stars. 
 
 Herschel's first investigation of the sun's move- 
 ment in space. 
 
 Goodricke's discovery of Algol's law of varia- 
 tion. 
 
 Analogy between Mars and the Earth pointed 
 out by Herschel. 
 
 Construction of the Heavens investigated by 
 Herschel's method of star-gauging. " Cloven - 
 disk " plan of the Milky Way. 
 
 Discovery of binary stars anticipated by 
 Michell. 
 
 Herschel's first Catalogue of Nebulae. 
 
 Discovery by Herschel of two Uranian moons 
 (Oberon and Titania.) 
 
 Acceleration of the moon explained by Laplace. 
 
 Herschel's second Catalogue of Nebulae, and 
 classification of these objects as sidereal 
 systems in more or less advanced stages of 
 condensation. 
 
 Completion of Herschel's forty-foot reflector. 
 
532 
 
 1 789, August 28 
 and Sept. 17 j 
 1789 . 
 1789 - 
 
 1790 . 
 
 1791 . 
 
 1792 . 
 
 1794 . 
 
 1795 
 
 1796 . 
 
 1796 . 
 
 1797 '. 
 
 1798 . 
 
 1799 . 
 
 1799, May 7 
 1799, Nov. 12 
 
 1800 . 
 
 1800 . 
 
 1 80 1, January i 
 
 1801 . 
 iSoi 
 
 1802, March 28 
 1802 . 
 1802 . 
 1802 
 
 APPENDIX. 
 
 His discovery with it of the two inner Saturniau 
 
 satellites. 
 
 Repeating-circle invented by Borda. 
 Five-foot circle constructed by Ramsden for 
 
 Piazzi. 
 Maskelyne's Catalogue of thirty-six fundamental 
 
 stars. 
 Herschel retracts his opinidn that all nebulae are 
 
 of sidereal nature, and admits the existence of 
 
 a self-luminous fluid in space. 
 Presence of atmospheric refraction in Venus 
 
 announced by Schroter. 
 Rotation-period of Saturn fixed by Herschel at 
 
 loh. i6m. 
 Promulgation of Herschel's theory of the solar 
 
 constitution. 
 Herschel's first measures of comparative stellar 
 
 brightness. 
 Laplace's Nebular Hypothesis published in 
 
 Exposition du Systeme du Monde. 
 Publication of Olbers's method of computing 
 
 cometary orbits. 
 Retrograde motions of Uranian satellites 
 
 announced by Herschel. 
 Publication of first two volumes of Mecanique 
 
 Celeste. 
 
 Transit of Mercury observed by Schroter. 
 Star-shower observed by Humboldt at Gumana. 
 Monatliche Correspondent started by Von Zach. 
 Invisible heat-rays detected in the solar spectrum 
 
 by Herschel. 
 
 Discovery of Ceres by Piazzi. 
 Publication of Lalande's Histoire Celeste. 
 Investigation by Herschel of solar emissive 
 
 variability in connection with spot-develop- 
 ment. 
 
 Discovery of Pallas by Olbers. 
 Herschel's third Catalogue of Nebulae. 
 Herschel's discovery of binary stars. 
 Marks of clustering in the Milky Way noted by 
 
 Herschel. 
 
APPENDIX. 
 
 533 
 
 1802 . 
 
 1802, Nov. 9 
 1804, Sept. 2 
 
 1804 . 
 
 1805 . 
 
 1807, March 29 . 
 
 1811 . . ' . 
 
 1811, Feb. 9 
 
 1811, Sept. 12 
 
 1812 . 4 
 
 1812, Sept. 15 
 
 1814 . 
 
 1815 . ; , 
 
 1818 . . I . 
 
 1819 . . . .. 
 
 1819, June 26 
 
 1820 . 
 
 1821 . 
 
 1821, September . 
 
 1822, May 24 
 1822, August 25 . 
 1823 . 
 
 1823 . 
 
 1824 . 
 
 1824 . 
 1824 . 
 
 1826 . 
 1826, Feb. 27 
 
 Interruption of the solar spectrum by seven 
 
 dark lines observed by Wollaston. 
 Transit of Mercury observed by Herschel. 
 Discovery of Juno by Harding. 
 Foundation of Optical Institute at Munich. 
 Herschel's second determination of the solar 
 
 apex of motion. 
 Discovery of Yesta by Olbers. 
 Theory of the development of stars from nebulse 
 
 advocated by Herschel. 
 Death of Maskelyne. Pond appointed to succeed 
 
 him as Astronomer Royal. 
 Perihelion passage of great comet. 
 Theory of electrical repulsion in comets origi- 
 nated by Olbers. 
 
 Perihelion passage of Pons's comet. 
 Herschel demonstrates the irregular distribution 
 
 of stars in space. 
 Fraunhofer maps 324 dark lines in the solar 
 
 spectrum. 
 
 Publication of Bessel's Fundamenta A stronomice, 
 Recognition by Encke of the first short- period 
 
 comet. 
 
 Passage of the earth through the tail of a comet. 
 Foundation of the Royal Astronomical Society. 
 Foundation of Paramatta Observatory. 
 First number of Astronomische Nachrichten. 
 First calculated return of Encke's comet. 
 Death of Herschel. 
 Bessel introduces the correction of observations 
 
 for personal equation. 
 
 Fraunhofer examines the spectra of fixed stars. 
 Distance of the sun concluded by Encke to be 
 
 95 J million miles. 
 
 Publication of Lohrmann's Lunar Chart. 
 Dorpat refractor mounted equatoreally with 
 
 clock-work motion. 
 Commencement of Schwabe's observations of 
 
 sun-spots. 
 Biela's discovery of the comet named after 
 
 him. 
 
5.34 
 
 1827 . 
 1829 
 
 1829 . 
 
 1830 . 
 
 1832 . 
 
 1833 
 
 1833, Nov. 12-13 
 1833 . 
 
 1834, January 16. 
 
 1835, September . 
 
 1835, Nov. 1 6 
 
 1836, May 15 
 
 1837 - 
 1837 - 
 
 1837 
 1837 
 
 1837, Dec. 16 
 
 1837 
 
 1838 . 
 
 1839, January 9 . 
 
 1839 . 
 
 1839 
 
 1840, March 2 
 
 1840 . 
 
 1842 . 
 1842 . 
 1842, July 8 
 
 APPENDIX. 
 
 Orbit of a binary star calculated by Savary. 
 Completion of the Royal Observatory at the 
 
 Gape of Good Hope. 
 The Konigsjberg heliometer mounted. 
 Publication of Bessel's Tabula Regiomontance. 
 Discovery by JBrewster of " atmospheric lines '' 
 
 in the solar spectrum. 
 The first magnetic observatory established at 
 
 Gottingen. 
 
 Star-shower visible in North America. 
 Completion of Sir J. Herschel's survey of the 
 
 northern heavens. 
 
 Sir J. Herschel's landing at the Cape. 
 Airy appointed Astronomer-Royal in succes- 
 sion to Pond. 
 
 Perihelion passage of Halley's comet. 
 Annular eclipse of the sun, and recognition of 
 
 " Baily's Beads." 
 Direction of the sun's movement determined by 
 
 Argelander. 
 Bessel's application of the heliometer to the 
 
 measurement of stellar parallax. 
 Publication of Beer and Madler's Der Hond. 
 Publication of Struve's Mensurce Micrometriccv. 
 Outburst of 77 Argus observed by Sir J. Herschel. 
 Thermal power of the sun measured by Herschel 
 
 and Pouillet. 
 
 Parallax of 6 1 Cygni determined by Bessel. 
 Parallax of a Centauri announced by Henderson. 
 Completion of Pulkowa Observatory and in- 
 stallation of Struve as its Director. 
 Solidity of the earth concluded by Hopkins. 
 Death of Olbers. 
 First attempt to photograph the moon by J. W. 
 
 Draper. 
 Doppler's principle of the change of refrangi- 
 
 bility of light by motion enounced. 
 Conclusion of Baily's experiments in weighing 
 
 the Earth. 
 Total solar eclipse. Corona and prominences 
 
 observed by Airy, Baily, Arago, and Struve. 
 
APPENDIX. 
 
 535 
 
 1843, Feb. 27 
 
 1845, February . 
 1845, April . 
 1845, April 2 
 
 1845, October 21 . 
 1845, Dec. 8 
 
 1845, Dec. 29 
 
 1846 . 
 
 1846, March 17 . 
 1846, Sept. 23 
 1846, October 10 . 
 
 1847 
 
 1847 . ' . 
 
 1848 , 
 
 1848 . * . 
 
 1848, April 27 
 1848, Sept. 19 . . 
 
 1849 , 
 
 1850, July 17 
 
 1850, Nov. 15 
 1851 . 
 
 1851, July 28 
 
 1851, October 24 . 
 
 1851 . .. . . 
 
 1852, May 6 
 
 1852, October n . 
 
 1852 . 
 
 Perihelion-passage of great comet, visible to the 
 naked eye at noon, February 28. 
 
 Completion of Parsonstown reflector. 
 
 Discovery with it of spiral nebulae. 
 
 Daguerreotype of the sun taken by Foucault and 
 Fizeau. 
 
 Place of Neptune assigned by Adams. 
 
 Discovery of Astrsea by Hencke. 
 
 Duplication of Biela's comet observed at Yale 
 College. 
 
 Melloni's detection of heating effects from moon- 
 light. 
 
 Death of Bessel. 
 
 Discovery of Neptune by Galle. 
 
 Neptune's Satellite discovered by Lassell. 
 
 Publication of Sir J. Herschel's Remits of Obser- 
 vations at the Cape of Good Hope. 
 
 Cyclonic theory of sun-spots stated by him. 
 
 J. R. Mayer's meteoric hypothesis of solar con- 
 servation. 
 
 Motion-displacements of Fraunhofer-lines ad- 
 verted to by Fizeau. 
 
 New Star in Ophiuchus observed by Hind. 
 
 Simultaneous discovery of Hyperion by Bond 
 and Lassell. 
 
 First experimental determination of the velocity 
 of light (Fizeau). 
 
 Vega photographed at Harvard College. 
 
 Discovery by Bond of Saturn's dusky ring. 
 
 0. Struve's first measurements of Saturn's ring- 
 system. 
 
 Total solar eclipse observed in Sweden. 
 
 Discovery by Lassell of two inner Uranian 
 satellites. 
 
 Schwabe's discovery of sun-spot periodicity 
 published by Humboldt. 
 
 Coincidence of magnetic and sun-spot periods 
 announced by Sabine. 
 
 Variable nebula in Taurus discovered by Hind. 
 
 Lassell's two-foot reflector transported to 
 Malta. 
 
536 
 
 1853 
 
 1854 . 
 
 1854 
 
 1856 . 
 
 1857 - 
 
 1857, April 27 
 
 1858 . 
 
 1858, Sept. 30 
 
 1859 , 
 
 1859 . 
 
 1859, March 26 
 1859, Sept. i 
 1859, October 19 
 
 1859, Dec. 15 
 
 1860, Feb. 27 
 1860, May 21 
 1860, July 1 8 
 
 1 86 1, June 30 
 1861-1862 . 
 1862 . 
 
 1862 . 
 
 1862, January 31 
 
 1862 . 
 1862 . 
 1862 
 
 APPENDIX. 
 
 Adams shows Laplace's explanation of the moon's 
 
 acceleration to be incomplete. 
 Hansen infers from lunar theory the necessity 
 of reducing Encke's value for the distance of 
 the sun. 
 
 Helmholtz's " gravitation theory " of solar energy 
 stated. 
 
 Piazzi Smyth's observations on the Peak of 
 Teneriffe. 
 
 Saturn's rings shown by Clerk Maxwell to be of 
 meteoric formation. 
 
 Double-star photography initiated at Harvard 
 College. 
 
 Solar photography begun at Kew. 
 
 Perihelion of Donati's comet. 
 
 Spectrum analysis established on a secure found- 
 ation by KirchhoiF and Bunsen. 
 
 Carrington's discovery of the compound nature 
 of the sun's rotation. 
 
 Lescarbault's supposed transit of Vulcan. 
 
 Luminous solar outburst and magnetic storm. 
 
 Merope nebula discovered by Tempel. 
 
 Chemical constitution of the sun described by 
 Kirchhoff. 
 
 Discovery by Liais of a " double comet." 
 
 New star in Scorpio detected by Auwers. 
 
 Total solar eclipse observed in Spain. Promin- 
 ences shown by photography to be solar appen- 
 dages. 
 
 The earth involved in the tail of a great comet. 
 
 Kirchhoff 's map of the solar spectrum. 
 
 Presence of hydrogen in the sun announced by 
 Angstrom. 
 
 Nasmyth's solar willow-leaves described. 
 
 Discovery by Alvan G. Clark of the companion 
 of Sirius. 
 
 Foucault applies the velocity of light to the 
 determination of the sun's distance. 
 
 Opposition of Mars. Determination of solar 
 parallax. 
 
 Completion of Bonner Durchmusterung. 
 
APPENDIX. 
 
 537 
 
 1863 . 
 
 1863 . 
 1864, March 5 
 
 1864 . 
 
 1864, August 5 
 
 1864, August 29 . 
 1864 . . 
 
 1864 :. . 
 
 1864, Nov. 23 
 
 1865, January 4 . 
 
 1865, January 16 
 
 i865 : . ,. . 
 
 1865 .' . 
 
 1866 
 
 1866 . 
 
 1866, March 4 
 1866, May 12 
 1866, October 
 
 1866, Nov. 13 
 
 1867 . 
 
 1867, August 29 
 1867 . 
 
 Secchi's first classification of stellar spectra. 
 Foundation of the German Astronomical Society. 
 Rotation period of Mars determined by Kaiser. 
 Huggins's first results in stellar spectrum 
 
 analysis. Spectra of Betelgeux and Aldebaran 
 
 investigated. 
 Application of the spectroscope to Tempel's 
 
 comet by Donati. Found to be self-luminous, 
 
 and of gaseous constitution. 
 Discovery by Huggins of gaseous nebulae. 
 Reduction to 91 million miles of the received 
 
 value of the sun's distance. 
 Croll's explanation of glacial epochs. 
 Death of Struve. 
 Spectroscopic observation by Huggins of the 
 
 occultation of e Piscium. 
 
 Faye propounds his theory of the solar constitu- 
 tion. 
 
 First publication of results by Kew observers. 
 Zollner insists upon the high internal tempera- 
 ture of the great planets. 
 Identity of the orbits of the August meteors, and 
 
 of comet 1862 iii., demonstrated by Schia- 
 
 parelli. 
 Delaunay explains outstanding lunar acceleration 
 
 by a lengthening of the day through tidal 
 
 friction. 
 Spectroscopic study of the sun's surface begun 
 
 by Lockyer. 
 New star in Corona Borealis detected by 
 
 Birmingham. 
 Schmidt announces the disappearance of the 
 
 lunar crater Linne. 
 Meteoric shower visible in Europe. Predicted 
 
 by H. A. Newton. 
 Period of November meteors determined by 
 
 Adams. 
 Total solar eclipse. Minimum sun-spot type of 
 
 corona observed by Grosch at Santiago. 
 Discovery of gaseous stars in Cygnus by Wolf 
 
 and Rayet. 
 
538 
 
 1 868, August 1 8 
 
 1868, August 19 
 
 1868, October 26 
 
 1868 . 
 
 1868 . 
 1868 . 
 
 1869, Feb. ii 
 1869, Feb. 13 
 
 1869, August 7 
 1870 . 
 
 1870 . 
 
 1870 . 
 
 1870, Dec. 22 
 
 1871, May ii 
 1871, June 9 
 
 1871, Dec. 12 
 
 1872 . 
 
 1872 . 
 
 1872 . 
 
 1872 . 
 
 APPENDIX. 
 
 Great Indian eclipse. Spectrum of prominences 
 
 observed. 
 First observations (by Janssen) of prominences 
 
 without an eclipse. 
 Announcement by Lockyer and Janssen of 
 
 their independent discovery of a method of 
 
 daylight spectroscopic observation of promin- 
 ences. 
 Doppler's principle applied by Huggins to 
 
 measure stellar movements in the line of 
 
 sight. 
 Publication of Angstrom's map of the normal 
 
 solar spectrum. 
 Analysis of the light of Winnecke's comet by 
 
 Huggins. Spectrum found to agree with that 
 
 of olefiant gas. 
 Lockyer and Frankland infer the extreme 
 
 tenuity of chromospheric gases. 
 Huggins observes a prominence with an " open 
 
 slit." 
 American eclipse. Detection of bright-line 
 
 coronal spectrum. 
 Mounting of Newall's 25-inch achromatic at 
 
 Gateshead. 
 Proctor indicates the prevalence of drifting 
 
 movements among the stars. 
 A solar prominence photographed by Young. 
 Sicilian eclipse. Young discovers reversing layer- 
 Death of Sir J. Herschel. 
 Line displacements due to solar rotation detected 
 
 by Yogel. 
 Total eclipse visible in India. Janssen observes 
 
 reflected Fraunhofer lines in spectrum of 
 
 corona. 
 Conclusion of a three years' series of observations 
 
 on lunar heat by Lord Bosse. 
 Spectrum of Yega photographed by H. Draper. 
 Cyclonic hypothesis of sun-spots enunciated by 
 
 Faye. 
 Young's solar-spectroscopic observations at 
 
 Mount Sherman. 
 
APPENDIX. 
 
 539 
 
 1872 . 
 1872, Nov. 27 
 
 1873 
 
 1873 . . 
 
 1874 . ..." 
 1874 . 
 1874, Dec. 8 
 1876 . 
 1876, Nov. 24 
 1876 . 
 
 1877, May 19 
 1877 . '.,. 
 1877, Aug. 16-17, 
 
 1877, Sept. 23 
 1877 . 
 
 1877 . 
 
 1878, January 
 
 1878 . 
 1878 . 
 1878 . 
 
 1878, May 6 
 
 1878 . 
 1878, July 29 
 
 1878, October 
 1878, Dec. 12 
 
 1879 . 
 
 Cornu's experiments on the velocity of light. 
 
 Meteoric shower connected with Biela's comet. 
 
 Determination of mean density of the earth by 
 Cornu and Bailie. 
 
 Solar photographic work begun at Greenwich. 
 
 Erection of 26-inch Washington refractor. 
 
 Light-equation redetermined by Glasenapp. 
 
 Vogel's classification of stellar spectra. 
 
 Transit of Yenus. 
 
 Publication of Neison's The Moon. 
 
 New star in Cygnus discovered by Schmidt. 
 
 Spectrum of Yega photographed by Huggins. 
 
 Firs!) use of dry gelatine plates in celestial 
 photography. 
 
 Klein observes a supposed new lunar crater 
 (Hyginus N.) 
 
 Measurement by Yogel of selective absorption 
 in solar atmosphere. 
 
 Discovery of two satellites of Mars by Hall at 
 Washington. 
 
 Death of Leverrier. 
 
 Canals of Mars discovered by Schiaparelli. 
 
 Opposition of Mars observed by Gill at Ascension. 
 Solar parallax deduced = 8"' 7 8. 
 
 Stationary meteor-radiants described by Den- 
 ning. 
 
 Publication of Schmidt's Charte der Gebirge des 
 Mondes. 
 
 First observations of Great Red Spot on 
 Jupiter. 
 
 Conclusion of Newcomb's researches on the 
 lunar theory. 
 
 Transit of Mercury. 
 
 Foundation of Selenographical Society. 
 
 Total eclipse visible in America. Yast equatorial 
 extension of the corona. 
 
 Completion of Potsdam Astrophysical Observa- 
 tory. 
 
 Lockyer's theory of celestial dissociation com- 
 municated to the Royal Society. 
 
 Michelson's experiments on the velocity of light. 
 
540 
 
 18 7 9 
 
 1879, November , 
 
 1879, December 5 , 
 1879, Dec. 18 
 
 1879, Dec - l8 
 
 1880, January 31 , 
 
 1880 . 
 1880 . 
 
 1880, Sept. 30 
 
 1880 . 
 
 1 88 1, January- 20 
 
 1881 . 
 
 1881, June 16 
 1 88 1, June 24 
 1 88 1, June . 
 
 1 88 1, August 15 
 
 1 88 1, August 22 
 
 1881 . 
 
 1882 . 
 
 1882 . 
 1882 . 
 
 1882, March 7 
 1882, May 17 
 
 APPENDIX. 
 
 Publication of Gould's Uranometria Argentina. 
 Observations of the spectra of sun-spots begun 
 
 at South Kensington. 
 
 A-bney's map, of the solar spectrum in the infra- 
 red presented to the Royal Society. 
 Ultra-violet spectra of white stars described by 
 
 Huggins to the Royal Society. 
 Communication of G. H. Darwin's researches 
 
 into the early history of the moon. 
 Discovery at Cordoba of a great southern 
 
 comet. 
 Conditions of Algol's eclipses determined by 
 
 Pickering. 
 Pickering computes mass-brightness of binary 
 
 stars. 
 
 Draper's photograph of the Orion Nebula. 
 The bolometer invented by Langley. 
 G. H. Darwin's researches into the effects of tidal 
 
 friction on the evolution of the solar system, 
 
 communicated to the Royal Society. 
 Langley's observations of atmospheric absorption 
 
 on Mount Whitney. 
 Perihelion of Tebbutt's comet. 
 Its spectrum photographed by Huggins. 
 Pnotographs of Tebbutt's comet by Janssen and 
 
 Draper. 
 Retirement of Sir George Airy. Succeeded as 
 
 Astronomer Royal by Christie. 
 Perihelion of Schaeberle's comet. 
 Publication by Stone of the Cape Catalogue for 
 
 1880. 
 
 Struve's second measures of Saturn's ring- 
 system, showing no certain change since 
 1851. 
 Newcomb's determination of the velocity of light. 
 
 Resulting solar parallax = 8". 7 9. 
 Correction by Nyren of Struve's constant of 
 
 aberration. 
 Spectrum of Orion Nebula photographed by 
 
 Huggins. 
 Total solar eclipse observed at Sohag in Egypt. 
 
APPENDIX. 
 
 541 
 
 1882, May 27 
 
 1882, June 10 
 1882, Sept. 17 
 
 1882, Sept. 18 
 1882, September . 
 
 1882, Dec. 6 ;. 
 1882 ;. '. 
 
 1882 >a :- . . 
 1882 . 
 
 1882 . ' .; . 
 
 1882 ij . 
 
 1882 ,; . .. i , 
 
 1883, January 30 . 
 
 1883, May 6 
 1883, June i 
 
 1883, August 13 , 
 
 1883 . ' . 
 
 1883 . . . 
 
 1884, January 25 
 
 1884 . 
 
 1884 . 
 
 Sodium-rays observed at Dunecht in spectrum 
 
 of Comet Wells. 
 Perihelion of Comet Wells. 
 Perihelion of great comet. Daylight detection 
 
 by Common. Disappearance in transiting 
 
 the sun observed at the Cape. 
 Iron-lines identified in spectrum by Copeland 
 
 and J. G. Lohse. 
 
 Photographs of great comet taken with a portrait- 
 lens at the Cape Observatory, showing a back- 
 ground crowded with stars. 
 Transit of Venus. 
 Duplication of Martian canals observed by 
 
 Schiaparelli. 
 Completion by Loewy at Paris of first equatoreal 
 
 Coude. 
 Rigidity of the earth concluded from tidal 
 
 observations by G. H. Darwin. 
 Foundation of the Liverpool Astronomical 
 
 Society. 
 Experiments by Huggins on photographing the 
 
 corona without an eclipse. 
 Publication of Holden's Monograph oj th$ Orion 
 
 Nebula. 
 The Orion Nebula photographed by Common in 
 
 31 minutes. 
 
 Caroline Island eclipse. 
 Great comet of 1882 observed from Cordoba 
 
 at a distance from the earth of 470 million 
 
 miles. 
 Variability in the bright- line spectrum of y Cas- 
 
 siopeise discovered by Von Gothard. 
 Parallaxes of nine southern stars measured by 
 
 Gill and Elkin. 
 Catalogue of the spectra of 4051 stars by 
 
 Vogel. 
 
 Perihelion of Pons's comet. 
 Photometric Catalogue of 4260 stars by 
 
 Pickering. 
 Publication of Gore's Catalogue of Variable 
 
 Stars. 
 
542 
 
 i88 4 . 
 
 1884, October 4 
 
 1884 . 
 
 1884 . 
 
 1885, February . 
 
 1885 . 
 
 1885, August 17 . 
 
 1885, Sept. 5 
 
 1885, Sept. 9 
 1885, Nov. 1 6 
 
 1885, Nov. 27 
 1885 . . . . 
 1885 . . " . 
 
 1885 . 
 1885 . 
 
 1885, Dec. 13 
 
 1886, January 26 . 
 1886, Feb. 5 
 1886, March 
 1886, May 6 
 1886, June 4 
 
 1886, August 29 . 
 1886, October i . 
 
 1886, Dec. 8 
 
 APPENDIX. 
 
 Publication of Faye's Origine du Monde. 
 
 Eclipse of the moon. Heat-phases measured by 
 Boeddicker at Parsonstown. 
 
 Duner's Catalogue of Stars with Banded 
 " Spectra, f* 
 
 Backlund's researches into the movements of 
 Encke's comet. 
 
 Langley measures the lunar- heat-spectrum. 
 
 Publication of Uranometria Nova Oxoniensis. 
 
 New star in Andromeda nebula discerned by 
 Gully. 
 
 Thollon's drawing of the solar spectrum presented 
 to the Paris Academy. 
 
 Solar eclipse visible in New Zealand. 
 
 Photographic discovery by Paul and Prosper 
 Henry of a nebula in the Pleiades. 
 
 Shower of Biela meteors. 
 
 Thirty-inch achromatic mounted at Pulkowa. 
 
 Publication of .Rowland's photographic map of 
 the normal solar spectrum. 
 
 Bakhuyzen's determination of the rotation- 
 period of Mars. 
 
 Stellar photographs by Paul and Prosper 
 Henry. 
 
 Discovery by Gore of a remarkable new variable 
 (U Orionis.) 
 
 Spectra of forty Pleiades simultaneously photo- 
 graphed at Harvard College. 
 
 First visual observation of the Maia nebula 
 with the Pulkowa 3o-inch refractor. 
 
 Photographs by the Henrys of the Pleiades, 
 showing 2326 stars with nebulae intermixed. 
 
 Periodical changes in spectra of sun spots 
 announced by Lockyer. 
 
 An international Photographic Congress pro- 
 posed by Gill. 
 
 Total eclipse of the sun observed at Grenada. 
 
 Roberta's photograph showing annular structure 
 of the Andromeda nebula. 
 
 Roberts's photograph of the Pleiades with dense 
 masses of nebulosity. 
 
APPENDIX. 
 
 543 
 
 1886, Dec. 28 
 
 1886 . 
 1886, May . 
 
 1886 . 
 1886 . I . 
 
 1886 . 
 1886 
 
 1887, January 18 
 
 1887 , 
 1887, April 1 6 
 
 1887 .. 
 1887 ... 
 
 1887 . 
 
 1887 . 
 1887 . 
 
 1887, April 8 
 1887, August 19 
 
 1887, November 
 1887, Nov. 17 
 1887 . 
 1887 . 
 
 Discovery by Copeland of helium-ray in 
 spectrum of the Orion nebula. 
 
 Thirty-inch refractor mounted at Nice. 
 
 Photographic investigations of stellar parallax 
 undertaken by Pritchard. 
 
 Publication of Argentine General Catalogue. 
 
 Completion of Auwers's reduction of Bradley's 
 observations. 
 
 Draper Memorial photographic work begun at 
 Harvard College. 
 
 Photographic detection, at Harvard College, of 
 bright hydrogen-lines in spectra of variables 
 (Mira Ceti and U Orionis.) 
 
 Discovery by Thome at Cordoba of a great 
 comet belonging to the group represented by 
 the comet of 1882. 
 
 Publication of Lockyer's Chemistry of the Sun. 
 
 Meeting at Paris of the International Astro- 
 photographic Congress. Construction of a 
 general Photographic Chart and Catalogue 
 resolved upon. 
 
 Heliometric triangulation of the Pleiades by 
 Elkin. 
 
 L. Struve's investigation of the sun's motion, 
 and re-determination of the constant of pre- 
 cession. 
 
 Von Konkoly's extension to 15 of Vogel's 
 spectroscopic Catalogue. 
 
 Auwers's investigation of the solar diameter. 
 
 Publication of Schiaparelli's Measures of Double 
 Stars (1875-85). 
 
 Death of Thollon at Nice. 
 
 Total eclipse of the sun. Shadow-path crossed 
 Russia. Observations marred by bad weather. 
 
 Langley's Researches on the Temperature of 
 the Moon. 
 
 Lockyer's " Researches on Meteorites " com- 
 municated to the Royal Society. 
 
 Stroobant's discussion of the observations 
 relating to Venus's pseudo-satellite. 
 
 Completion of 36-inch Lick refractor. 
 
544 APPENDIX. 
 
 1888 . . . Kiistner's detection of variations in the latitude 
 of Berlin brought before the International 
 Geodetic Association. 
 
 1888 . . . Chandler's Catalogue of Variable Stars. 
 
 1888 . . . Mean paraHkx of northern first-magnitude stars 
 determined by Elkin. 
 
 1888 . . . Publication of Dreyer's New General Catalogue 
 of 7840 nebulae. K 
 
 1888 . . . "Vogel's first results in the spectrographic de- 
 termination of stellar motion in line of sight. 
 
 1888 . . Presence of carbon in the solar atmosphere 
 
 ascertained by Trowbridge and Hutchins. 
 
 1888, June i . Activity of the Lick Observatory begun. 
 
 1888 . . . Completion of Dr. Common's five-foot reflector. 
 
 j888 . . Heliometric measures of Iris for solar parallax 
 at the Cape, Newhaven (U.S.A.), and Leipsic. 
 
 j888 . . . Loewy's comparative method of determining 
 constant of aberration described. 
 
 1888, January 28 Total eclipse of the moon. Heat-phases mea- 
 sured at Parsonstown. 
 
 j 888 . . Presentation of the Dunecht instrumental outfit 
 
 to the nation by Lord Crawford. Dr. Cope- 
 land appointed Astronomer Royal for Scotland 
 in succession to Professor Piazzi Smyth. 
 
 1888, Feb. 5 . Photograph of the Orion nebula spectrum taken 
 by Dr. and Mrs. Huggins, showing groups of 
 bright lines common to the nebula and the 
 trapezium-stars. 
 
 1888, Sept, 12 . Death 4 of R. A. Proctor. 
 
 -[889 . . . Photograph of the Orion nebula taken by W. H. 
 Pickering showing it to be the nucleus of a 
 vast spiral. 
 
 [889 . . . Discovery at Harvard College of the first known 
 spectroscopic doubles, Ursse Majoris and 
 Aurigse. 
 
 j88 9 . . Eclipses of Algol demonstrated spectrographic- 
 
 ally by Vogel. 
 
 Completion of photographic work at the Cape 
 Observatory for the SouthernDurchmusterung. 
 Completion of Boeddicker's drawing of the 
 Milky Way. 
 
APPENDIX. 
 
 545 
 
 1889 . 
 
 1889 . 
 
 1889 . 
 1889 '. ... 
 
 1889 . . - . 
 
 1889, January i . 
 1889, Feb. 7 
 
 1889, March 
 
 1889, July 7 
 1889, August 2 . 
 
 1889, July- Aug. . 
 1889, Nov. i 
 1889, December 8 
 1889, Dec. 22 
 
 1889 . 
 
 1890 . 
 
 1890 . 
 
 1890 . 
 
 1890 . . . 
 
 1890 . . 
 
 Photographs of southern star spectra taken at 
 Chosica, Peru, in connexion with the Draper 
 Memorial. 
 Pernter's experiments on scintillation from the 
 
 Sonnblick. 
 
 H. Struve's researches on Saturn's satellites. 
 Harkness's investigation of the masses of Mer- 
 cury, Yenus, and the Earth. 
 Heliometric measures of Victoria and Sappho at 
 
 the Gape. 
 
 Total solar eclipse visible in California. 
 Foundation of the Astronomical Society of the 
 
 Pacific. 
 Investigation by Dr. and Mrs. Huggins of the 
 
 spectrum of the Orion Nebula. 
 Re-discovery by Brooks of Lexell's lost comet. 
 Observation by Barnard of four companions to 
 
 Lexell-Brooks comet. 
 First photographs of the Milky Way taken by 
 
 Barnard. 
 Passage of Japetus behind Saturn's dusky ring 
 
 observed by Barnard. 
 Schiaparelli announces synchronous rotation and 
 
 revolution of Mercury. 
 
 Total eclipse of the sun visible in Guiana and on 
 West African coast. Death of Father Perry, 
 December 27. 
 
 Spectrum of Uranus investigated visually by 
 Keeler, photographically by Dr. and Mrs. 
 Huggins. 
 
 Long-exposure photographs of ring-nebula in 
 Lyra bring out nebulous character of the 
 central star. 
 Determinations of the solar translation by L. 
 
 Boss and 0. Stumpe. 
 Schiaparelli finds for Venus an identical period 
 
 of rotation and revolution. 
 Publication of Thollon's map of the solar 
 
 spectrum. 
 Schaeberle's mechanical theorv of the solar 
 
 corona. 
 
 35 
 
546 APPENDIX. 
 
 1890 . . Bigelow's mathematical theory of coronal struc- 
 
 tures. 
 
 1890 . . . Foundation of the British Astronomical Asso- 
 ciation. 
 
 1890 . . . Measurements by Keeler at Lick of nebular 
 movements in line of sight. 
 
 1890 . . . Janssen's ascent of Mont Blanc, by which he 
 ascertained the purely terrestrial origin of the 
 oxygen-absorption in the solar spectrum. 
 
 1890 . . . Newcomb's discussion of the transits of Venus 
 of 1761 and 1769. 
 
 1 890 . . . Spiral structure of Magellanic clouds displayed in 
 photographs taken by H. C. Russell of Sydney 
 with exposures up to eight hours. 
 
 1890 . . . Publication of the Draper Catalogue of Stellar 
 
 Spectra. 
 1890, March . Redetermination of chief nebular line by Dr. 
 
 and Mrs. Huggins 
 1890, April 24 . Spica announced by Vogel to be a spectroscopic 
 
 binary. 
 
 1890, June . . Gore's Catalogue of computed Binaries. 
 1890, November . Study by Dr. and Mrs. Huggins of the spectra 
 
 of Wolf and Rayet's stars in Cygnus. 
 1890, November . Discovery by Barnard of a close nebulous 
 
 companion to Merope in the Pleiades. 
 1890, November . Presentation to the R. Astronomical Society of 
 
 F. McLean's spectrographs of the high and 
 
 low sun. 
 
 1891 . . . Capture-theory of comets developed by Callan- 
 
 dreau, Tisserand, and Newton. 
 1891 . . . Duner's spectroscopic researches on the sun's 
 
 rotation. 
 1891 . . Preponderance of Sirian stars in the Milky 
 
 Way concluded by Pickering, Gill, and 
 
 Kapteyn. 
 1891 . . . Detection by Mrs. Fleming of spectral variations 
 
 corresponding to light-changes in ft Lyrse. 
 1891 . . . Establishment of a mountain-observatory at 
 
 Arequipa in Peru (height, 8000 feet) in 
 
 connexion with Harvard College. 
 1891 . . . Yariations of latitude investigated by Chandler. 
 
APPENDIX. 
 
 547 
 
 1891 , 
 1891, April . 
 
 1891, May 9 
 1891, August 19 
 
 1891, Dec. 10 
 1891, Dec. 20 
 1891, Dec. 22 
 
 1892 . 
 1892 . 
 1892 . . 
 
 1892 . 
 
 1892, January 2 
 1892, January 21 
 1892, Feb. i 
 
 1892, Feb. 5 
 
 1892, March 6 
 1892, May 13 
 
 1892, June 29 
 
 1892, August 4 
 1892, August 17 
 1892, Sept. 9 
 
 1892, Oct. 12 
 1892, Nov. 6 
 
 Prominence-photography set on foot by Hale 
 at Chicago. 
 
 Meeting at Paris of the Permanent Committee 
 for the Photographic Charting of the 
 Heavens. 
 
 Transit of Mercury. 
 
 Presidential Address by Dr. Huggins at the 
 Cardiff Meeting of the British Association. 
 
 Nova Aurigse photographed at Harvard College. 
 
 Photographic maximum of Nova Aurigaa. 
 
 First photographic discovery of a minor planet 
 by Max Wolf at Heidelberg. 
 
 Commencement of international photographic 
 charting work. 
 
 Photographic determination by Scheiner of 833 
 stars in the Hercules Cluster (M 13.) 
 
 Publication by Yogel of spectrographic de- 
 terminations of motions in the line of sight 
 for 51 stars. 
 
 Publication of Pritchard's photographic paral- 
 laxes for the second magnitude stars visible 
 at Oxford. 
 
 Death of Sir George Airy. 
 
 Death of Professor Adams. 
 
 Announcement by Anderson of the outburst of 
 a new star in Auriga. 
 
 Appearance of the largest sunspot ever photo- 
 graphed at Greenwich. 
 
 Discovery of a bright comet by Swift. 
 
 Lecture by Dr. Huggins on Nova Aurigse at 
 the Royal Institution. 
 
 Death of Admiral Mouchez. Succeeded by 
 Tisserand as director of the National 
 Observatory, Paris. 
 
 Favourable Opposition of Mars. 
 
 Rediscovery at Lick of Nova Aurigse. 
 
 Discovery by Barnard of Jupiter's inner satel- 
 lite. 
 
 First photographic discovery of a comet by 
 Barnard. 
 
 Discovery of Holmes's comet. 
 
548 
 
 1892, Nov. 23 
 
 1892 . 
 1892 . 
 
 1892 . . > 
 
 1892 .' 
 
 1892 . . .. 
 
 1892, March I . 
 1892 * ,iv . 
 
 1892 . - ' . , 
 
 1893, January 28 
 1893, March 10 . 
 
 1893 ~. 
 1893, April 1 6 
 
 APPENDIX. 
 
 Shower of Andromede meteors visible in 
 
 America. 
 Photographic investigation by Deslandres of 
 
 -the spectra^ of prominences. 
 Photographs of the sun with faculse and chro- 
 
 mospheric surroundings taken by Hale with 
 
 a single exposure. 
 Investigation by T. J. J. 'See of the ancient 
 
 colour of Sirius. 
 Publication of T. J. J. See's Thesis on the 
 
 Evolution of Binary Systems. 
 Establishment of an observatory at Abastouman, 
 
 Tiflis, 4600 feet above sea-level. Glasenapp 
 
 appointed director. 
 Photograph of Argo Nebula taken by Dr. Gill 
 
 at the Cape in twelve hours. 
 Chandler accounts for the inequalities of 
 
 Algol's light-period, by the revolution of the 
 
 eclipsing pair round an obscure centre of 
 
 attraction. 
 Nebulae in Cygnus photographically discovered 
 
 by Max Wolf, involving both bright stars 
 
 and " star-dust." 
 Kapteyn's investigation of the structure of the 
 
 stellar universe. 
 Dr. Gill announces to the Royal Astronomical 
 
 Society his results from the Opposition of 
 
 Victoria, among them a solar parallax = 
 
 8-"8o 9 . 
 
 Observations begun at BischofFsheim's Observa- 
 tory on Mount Mounier, at an elevation of 
 
 9000 feet in the Maritime Alps. 
 Total solar eclipse observed in South America 
 
 and West Africa. 
 
APPENDIX. 
 
 549 
 
 TABLE II. 
 
 CHEMICAL ELEMENTS IN THE SUN (ROWLAND, 1891). 
 
 Arranged according to the number of their representative Lines in the 
 Solar Spectrum. 
 
 i 
 
 Iron (2000 +). Neodymium, Cadmium. 
 
 Nickel. Lanthanum. Rhodium. 
 
 Titanium. Yttrium. Erbium. 
 
 Manganese. Niobium. Zinc. 
 
 Chromium. Molybdenum. Copper (2). 
 
 Cobalt. Palladium. Silver (2). 
 
 Carbon (200+ ). Magnesium (20 4- ). Glucinum (2). 
 
 Vanadium. Sodium (11). Germanium. 
 
 Zirconium. Silicon. Tin. 
 
 Cerium. Strontium. Lead (i). 
 
 Calcium (75 + ). Barium. Potassium (i). 
 
 Scandium. Aluminium (4). 
 
 Doubtful Elements. Iridium, osmium, platinum, ruthenium, tantalum, 
 thorium, tungsten, uranium. 
 
 Not in Solar Spectrum. Antimony, arsenic, bismuth, boron, nitrogen 
 vacuum tube), caesium, gold, iridium, mercury, phosphorus, rubidium, 
 selenium, sulphur, thallium, praseodymium. 
 
 TABLE III. 
 
 EPOCHS OF SUNSPOT MAXIMUM AND MINIMUM FROM 1610 
 TO 1893 (R- WOLF). 
 
 MINIMA. 
 
 MAXIMA. 
 
 MINIMA. 
 
 MAXIMA. 
 
 MINIMA. 
 
 MAXIMA. 
 
 1610 .8 
 
 1615 -5 
 
 1712 .0 
 
 1718 .2 
 
 1810 .6 
 
 1816 .4 
 
 1619 .0 
 
 1626 .0 
 
 1723 -5 
 
 1727 -5 
 
 1823 .3 
 
 1829 .9 
 
 1634 .0 
 
 1639 -5 
 
 1734 -0 
 
 1738 -7 
 
 1833 -9 
 
 1837 -2 
 
 1645 .0 
 
 1649 .0 
 
 1745 - 
 
 175 -3 
 
 1843 -5 
 
 1848 .1 
 
 1655 -o 
 
 1660 .0 
 
 1755 - 2 
 
 1761 .5 
 
 1856 .0 
 
 1860 .1 
 
 1666 .0 
 
 1675 .0 
 
 1766 .5 
 
 1769 .7 
 
 1867 .2 
 
 1870 .6 
 
 1679 .5 
 
 1685 .0 
 
 J 77S -5 
 
 1778 .4 
 
 1878 .9 
 
 1884 .0 
 
 1689 .5 
 
 1693 .0 
 
 1784 .7 
 
 1788 .1 
 
 1890 .2(1) 
 
 1893 (?) 
 
 1698 .9 
 
 1705 .5 
 
 1798 -3 
 
 1804 .2 
 
 
 
APPENDIX. 
 
 TABLE IV. 
 
 CONCLUSIONS AS TO THE ; .*DIRECTION OF THE SUN'S 
 MOVEMENT IN SPACE, 1783-1892. 
 
 AUTHORITY. 
 
 DATE. 
 
 NUMBER 
 
 OF 
 
 STARS 
 USED. 
 
 ^POSITION" OP APEX. 
 
 Eight 
 Ascension. 
 
 Declination. 
 
 Sir W. Herschel . 
 
 1783 
 
 H 
 
 260 6' 
 
 + 26 3' 
 
 ... 
 
 1805 
 
 6 
 
 245 52' 
 
 + 49 38' 
 
 Prevost .... 
 
 1783 
 
 
 
 2 3 
 
 + 25 
 
 Gauss .... 
 
 
 7i 
 
 259 10' +30 50' 
 
 Argelander . 
 
 1838 
 
 390 
 
 259 5 1' +32 30' 
 
 Lundahl .... 
 
 1840 
 
 H7 
 
 252 34' 
 
 + 14 26' 
 
 O. Struve .... 
 
 1841 
 
 400 
 
 261 22' 
 
 + 37 3 6 ' 
 
 Galloway 
 
 1847 
 
 81 
 
 260 
 
 + 34 23' 
 
 Madler. 
 
 1848 
 
 2163 
 
 261 38' 
 
 + 3Q ^4/ 
 
 Airy ..... 
 
 1859 
 
 J 
 
 H3 
 
 261 29' 
 
 1 3s ^T- 
 
 + 24 44' 
 
 
 1859 
 
 in 
 
 256 54' 
 
 + "3Q 2Q' 
 
 Dunkin .... 
 
 1863 
 
 ** 
 
 1167 
 
 263 44' 
 
 ' Ds ~s 
 
 + 25 
 
 DeBall .... 
 
 1877 
 
 67 
 
 269 33' 
 
 + 23 n' 
 
 Rancken .... 
 
 1882 
 
 1 06 
 
 275 48' 
 
 + 31 52' 
 
 Plummer .... 
 
 1883 
 
 274 
 
 276 48' 
 
 + 26 1 8' 
 
 Bischof .... 
 
 1884 
 
 480 
 
 285 2' 
 
 + 48 5' 
 
 Ubaghs .... 
 
 1886 
 
 464 
 
 260 50' 
 
 + 27 25' 
 
 L. Struve .... 
 
 1887 
 
 2509 
 
 273 21' 
 
 + 27 19' 
 
 Lewis Boss . . . 
 
 1890 
 
 i35 . 
 
 280 
 
 + 43 
 
 u 5; 
 
 ?j 
 
 144 
 
 286 
 
 + 45 
 
 
 
 ,, 
 
 253 
 
 289 
 
 + 51 
 
 O. Stumpe .... 
 
 ?> 
 
 55i 
 
 287 24' 
 
 + 42 
 
 i> > 
 
 > 
 
 340 
 
 279 42' 
 
 + 40 30' 
 
 J5 J5 
 
 
 !05 
 
 287 54' 
 
 + 32 6' 
 
 * 
 
 
 58 
 
 285 12' 
 
 + 3O 2A' 
 
 5 J> 
 
 5? 
 
 5 
 
 1054 
 
 285 
 
 T jvj 4^ 
 
 + 38 
 
 J. G. Porter 
 
 1892 
 
 576 
 
 281 54' 
 
 + 53 42' 
 
 J5 5> . . 
 
 J? 
 
 533 
 
 280 42' 
 
 + 40 6' 
 
 5) }1 
 
 1) 
 
 142 
 
 285 12' 
 
 + 34 
 
 J? )J ' . . 
 
 Ristenpart .... 
 
 ?> 
 5 J 
 
 70 
 
 277 
 280 
 
 -(-34 54' 
 + 30 
 
 Bakhuyzen, from movements 
 
 
 
 
 
 in R.A. of galactic stars 
 
 )J 
 
 
 
 273 24' 
 
 + 32 
 
 From their movements in 
 
 
 
 
 
 P.D. . 
 
 
 
 
 227 18' 
 
 + ^2 
 
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 APPENDIX. 
 
 TABLE VI. 
 
 & 
 
 List of Observatories employed in the Construction of the Photo- 
 graphic Chart of the Heavens. All are provided with perfectly 
 similar instruments, consisting of a thi*teen-mch photographic 
 coupled with an eleven-inch visual refractor. 
 
 NAME OF OBSERVATORY. 
 
 CONSTRUCTORS OF INSTRUMENTS. 
 
 Optical Part. 
 
 Mechanical Part. 
 
 Paris . 
 
 
 Henrys 
 
 Gautier 
 
 Algiers 
 
 
 
 
 
 Bordeaux 
 
 
 
 
 
 Toulouse 
 
 
 
 
 
 San Fernando (Spain) 
 
 
 
 
 
 Vatican 
 
 
 
 
 
 La Plata 
 
 
 
 
 
 Eio Janeiro . 
 
 
 
 
 
 Santiago 
 
 
 
 
 
 Helsingfors . 
 Potsdam 
 
 
 Steii 
 
 aheil 
 
 Kepsold 
 
 Catania 
 
 
 . 
 
 Salmoiraghi 
 
 Greenwich . 
 
 
 Sir H.' Grubb 
 
 Sir H. Grubb 
 
 Oxford . 
 
 
 
 
 
 
 The Cape . 
 
 
 p 
 
 , 
 
 Melbourne . 
 
 
 M 
 
 > 
 
 Sydney 
 
 
 ,, 
 
 i 
 
 Tacubaya (Mexico) 
 
 
 ) 
 
 " ' 
 
INDEX. 
 
 ABBE, Cleveland, corona of 1878, 
 220-1 
 
 Aberdour, Lord, solar chromosphere, 
 8 3 
 
 Aberration, discovered by Bradley, 4, 
 1 8 ; an uranographical correction, 
 37 ; distance of sun determined by, 
 285, 297 
 
 Abney, daylight coronal photographs, 
 225 ; infra-red photography, 262, 
 277, 524 
 
 Absorption, terrestrial atmospheric, 
 166, 262-3, 266-7, 268, 277, 339 ; 
 solar, 167, 215-6, 265, 278 ; corre- 
 lative with emission, 168-9, J 7 2 
 
 Adams, elements of Neptune, 97-8 ; 
 lunar acceleration, 332 ; orbit of 
 November meteors, 401 
 
 Aerolites during star-showers, 411 
 
 Airy, solar translation, 47 ; promi- 
 nences, 77 ; sierra, 85 ; Astronomer 
 Eoyal, 97 ; search for Neptune, 98, 
 100 ; corona of 1851, 219; solar 
 parallax, 280, 291 ; transit of Venus, 
 287 ; Mercurian halo, 302 ; lunar 
 atmosphere, 324 
 
 Albedo, of Mercury, 303 ; of Venus, 
 314 ; of Mars, 346; of minor planets, 
 350; of Jupiter, 354; of Uranus, 
 
 37i 
 
 Alexander, spiral nebulas, 147 ; obser- 
 vation during eclipse of 1860, 302 
 
 Algol, variability of light, 12, 469; 
 eclipses, 470 ; a ternary system, 
 470-1 
 
 Altitude and azimuth instrument, 149 
 note, 151 
 
 Amici, comet of 1843, I2 8 
 
 Anderson, discovery of Nova Aurigas, 
 478 
 
 Andrews, conditions of liquefaction, 
 o 188 
 
 Angstrom, C. J., Optical Researches, 
 172; solar spectroscopy, 261,263; 
 
 age and temperature of stars, 453 
 
 Angstrom, Knut, infra-red solar 
 spectrum, 262 
 
 Arago, eclipse of 1842, 75, 78-9 ; pro- 
 minences, 78, 84; polarisation in 
 comets, 128 ; magnetic relations of 
 auroras, 161 ; nature of photo- 
 sphere, 1 88 ; meteor-systems, 399 
 
 Aral, photographs of corona of 1887, 
 
 233 
 
 Arcturus, spectrum, 452, 463 ; radial 
 movement, 466 ; remoteness, 499, 
 506 
 
 Argelander, Bonn Durchmusterung, 
 39, 506 ; solar motion, 47 ; centre 
 of Milky Way, 49 ; comet of 1811, 
 
 I2 5 
 
 Aristotle, description of a comet, 423 
 Asten, movements of Encke's comet, 
 
 116 
 Asteroids, minor planets so designated 
 
 by Herschel, 92 
 Astronomical circles, 151 
 Astronomical physics, 9, 175-7, 512 
 Astronomical Society, founded, 7 ; 
 
 Herschel first President, 16 
 Astronomy, classification, i ; rapid 
 
 progress, 6 ; in United States, 8 ; 
 
 in Germany, 33 ; practical reform, 
 
 34 ; of the invisible, 50 ; physical, 
 
 175 
 
 Atmosphere, solar, 117, 229,239, 274 ; 
 of Venus, 291, 311-3; of Mercury, 
 302-3, 305 ; of the moon, 323-5 ; 
 of Mars, 339 ; of minor planets, 
 
556 
 
 INDEX. 
 
 Aurora?, periodicity, 160-1, 201-2 ; 
 excited by meteors, 407 
 
 Auwers, reduction of Bradley's obser- 
 vations, 47 ; system of Procyon, 51 ; 
 opposition of Victoria, 294 ; solar 
 parallax, 296 ; new star in Scctfpio, 
 477 ; proper motions, 509 
 
 BABINET, objection to nebular hypo- 
 thesis, 382 
 
 Backlund, movements of Encke's 
 comet, 1 1 6, 436 
 
 Baily, early life and career, 71-3 ; 
 observations of eclipses, 74-7 ; den- 
 sity of the earth, 73, 321 
 
 Baily 's Beads, 74-5, 290 
 
 Bakhuyzen, rotation of Mars, 337-8 
 
 Ball, Sir Eobert, parallaxes of stars, 
 43 note, 499 ; contacts in transits, 
 
 295 
 
 Balmer's Law, 247, 463 
 
 Barkowski, daguerreotype of eclipsed 
 sun in 1851, 207 
 
 Barnard, photographs of solar corona, 
 235 ; halo round Venus, 313 ; dis- 
 covery of a fifth Jovian satellite, 
 357, 517 ; red spot, 361 ; eclipse of 
 Japetus, 365 ; attendants on comet 
 of 1882, 439 ; on Lexell-Brooks 
 comet, 443 ; observations of Swift's 
 comet, 446 ; photographic discovery 
 of a comet, 447 ; observations and 
 measurements of Nova Aurigse, 
 480, 482 ; discernment of Hind's 
 variable nebula, 485 ; photographs 
 of Milky Way, 507-8 
 
 Bartlett, photograph of a partial 
 eclipse, 207 
 
 Basic lines, 256-7 
 
 Baxendell, meteors of 1866, 402 
 
 Becker, drawings of solar spectrum, 
 263 
 
 Beckett, Sir E. (Lord Grimthorpe), 
 value of solar parallax, 286 
 
 Beer and Miidler, survey of lunar 
 surface, 325, 326, 327 ; studies of 
 Mars, 337 
 
 Belopolsky, photographs of corona 
 (1887), 233 
 
 Berberich, mass of asteroids, 350 ; 
 orbit of Holmes's comet, 409 
 
 Bessel, biographical sketch, 34-6 ; 
 reduction of Bradley's observations, 
 38 ; zone observations, 39 ; parallax 
 of 61 Cygni, 43 ; disturbed motion 
 of Sirius and Procyon, 50 ; trans- 
 Uranian planet, 97 ; Halley's comet, 
 127 ; theory of instrumental errors, 
 
 152; personal equation, 153 ; rota- 
 tion of Mercury, 304 ; lunar atmo- 
 sphere, 324 ; opposite polarities in 
 comets, 394 ; mathematical theory 
 of cometary emanations, 417 ; mul- 
 tiple tails, 420 ; comet of 1807, 425 
 Betelgeux, 452, 461, 464 ; radial move- 
 ment, 466 ; indefinite remoteness, 
 
 499 
 
 Bianchini ? rotation of Venus, 308 
 Biela, discovery of comet, 117 
 Bigelow, theory of solar corona, 238-9 
 Bigourdan, eclipse of 1893, 2 37 > 
 
 movement of 1882 comet, 441 
 Bird's quadrants, 4, 151 
 Birmingham, colours of stars, 453 
 
 note ; discovery of T Coronse, 473 
 Birt, rotation of a sun-spot, 179 ; 
 
 Selenographical Society, 326 
 " Black Ligament," 290-1 
 Bode, popular writings, 6 ; solar con- 
 stitution, 68 ; a planet missed and 
 found, 87, 90 
 
 Bode's Law, 87-8, 90, 102. 348 
 Boeddicker, heat-phases during lunar 
 eclipses, 330, 331 ; drawings of 
 Jupiter, 361 ; of the Milky Way, 
 
 507 
 
 Bohm, solar observations, 182, 184 
 
 Boguslawski, centre of sidereal 
 revolutions, 49 ; observation of 
 Halley's comet. 127 
 
 Bolometer described, 275-6 
 
 Bond, G. P., his father's successor, 
 1 06 ; light of Jupiter, 352 ; fluidity 
 of Saturn's rings, 363 ; Donati's 
 comet, 393-5 ; rifts in Andromeda 
 nebula, 491 ; double-star photo- 
 graphy, 492 
 
 Bond, W. C., observation of Nep- 
 tune's satellite, 103 ; discovery of 
 Hyperion, 105 ; of Saturn's dusky 
 ring 106 ; resolution of nebulae, 
 celestial photography, 190, 
 
 satellite-transit on Jupiter, 
 
 148 
 
 492 
 
 Bonn Durchmusterung, 39 
 
 Borda, repeating circle, 151 
 
 Boss, solar translation, 48 ; observa- 
 tions on comets, 426, 43 1^ 
 
 Bouguer, solar atmospheric absorp- 
 tion, 274 
 
 Boulliaud, period of Mira, 12 
 
 Bouquet de la Grye, photographs of 
 Venus on the Sun, 314 
 
 Bouvard, Tables of Uranus, 96 ; ob- 
 servation of Encke's comet, 112 
 
 Bradley, observational faculties, 3 ; 
 
INDEX. 
 
 557 
 
 discoveries, 4 ; solar translation, 
 n ; star distances, 12, 18 ; observa- 
 tion on Castor, 20 ; instruments, 
 33, 150 ; observations reduced by 
 Bessel, 38 ; by Auwers, 47 
 Brahe, Tycho, star of 1572, 29 
 Brandes, Andromede star-shower, 405 
 Brandes and Benzenberg, heights of 
 
 meteors, 398 
 
 Braun, prominence-photography, 244 
 Brayley, meteoric origin of planets, 
 
 37? 
 
 Bredichin, theory of comets' tails, 
 124, 421 ; repulsive forces, 418-9 ; 
 chemical constitution, 420; types 
 of formation, 425, 426, 430, 440 ; 
 structure of chromosphere, 248 ; 
 red spot on Jupiter, 359 ; spectrum 
 of Coggia's comet, 416 ; Lexell- 
 Brooks and subordinate comets, 
 
 445 
 
 Bremiker, star-maps, 100 
 Brester, Theorie du Soleil, 189 
 Brewster, diffraction-theory of corona, 
 8 1 ; telluric lines in solar spec- 
 trum, 1 66 ; absorption - spectra, 
 169 
 
 Brinkley, illusory stellar parallaxes, 39 
 Brisbane, observatory at Paramatta, 
 
 8, 112 
 
 Brooks, fragment of 1882 comet, 440 ; 
 rediscovery of Pons's comet, 442 ; of 
 Lexell's, 444 
 
 Brtinnow, stellar parallax determina- 
 tions, 499 
 
 Bruno, Giordano, motion of stars, 1 1 
 Buff ham, rotation of Uranus, 369 
 Buffon, internal heat of Jupiter, 352 
 Bunsen, discovery of spectrum analy- 
 sis, 164 
 
 Burnham, coronal photographs, 235 ; 
 measures of Nova Aurigse, 482 ; of 
 planetary nebulae, 486 ; discoveries 
 of double stars, 501, 514, 518, 520; 
 6 1 Cygni, 502 
 
 Burton, canals of Mars, 342 ; rotation 
 of Jupiter's satellites, 356 
 
 CALANDRELLI, stellar parallaxes, 39 
 Callandreau, capture-theory of comets, 
 
 122 
 
 Campani, Saturn's dusky ring, 107 
 
 Campbell, Lieutenant, polarisation of 
 corona, 212 
 
 Campbell, Professor, spectrum of 
 Nova Aurigae, 481 ; photographs of 
 stellar diffraction-spectra, 526 
 
 Canals of Mars, 341-3 
 
 Canopus, spectrum, 463 ; immeasur- 
 able remoteness, 498 
 
 Cape Durchmusterung, 495 
 
 Capella, spectrum and light-power, 
 452, 463 
 
 Carbon, absorption by, in solar atmo- 
 sphere, 264 ; in stars, 452 
 
 Carbonelle, origin of meteorites, 412 
 
 Carrington, astronomical career, 179- 
 8 1 ; sunspot observations, 181-2 ; 
 solar rotation, 183 ; spot-distribu- 
 tion, 184 ; luminous outburst on 
 sun, 198-9 ; Jovian and sunspot 
 periods, 202 ; origin of comets, 
 44? 
 
 Cassini, Domenico, discoveries of 
 Saturnian satellites, 104 ; of divi- 
 sion in ring, 105 ; solar rotation- 
 period, 182 ; solar parallax, 282 ; 
 rotation of Venus, 308; of Mars, 
 336 ; of Jupiter, 353, 359 ; satellite 
 of Venus, 315; satellite-transit on 
 Jupiter, 355 
 
 Cassini, J. J., stellar proper motions, 
 ii ; sun's limb notched by a spot, 64; 
 theory of corona, 80 ; rotation of 
 Venus, 308 ; structure of Saturn's 
 rings, 364 
 
 Cavendish experiment, 73, 321 
 
 Ceres, discovery, 89-91 ; diameter, 
 93 ; phases, 350 
 
 Chacornac, observation on a sunspot, 
 194 ; star-maps, 347, 495 ; variable 
 nebula, 484 
 
 Challis, search for Neptune, 100-1 ; 
 duplication of Biela's comet, 119 
 
 Chandler, variation of latitude, 318; 
 changes in Pons's comet, 443 ; 
 identification of Lexell-Brooks' 
 comet, 445 ; system of Algol, 
 470-1 ; Catalogue of Variables, 
 472 
 
 Charlois, discoveries of minor planets, 
 
 347 
 
 Charropin, coronal photographs, 235 
 Chladni, origin of meteors, 398, 404 
 Christie, Mercurian halo, 301 
 Chromosphere, early indications, 83 ; 
 distinct recognition, 84, 208 ; 
 depth, 217, 218; metallic injec- 
 tions, 242 ; eruptive character, 248 
 Clark, Alvan, large refractors, 141, 
 
 5H-5 
 
 Clark, Alvan G., discovery of Sirian 
 companion, 50, 513; forty-inch re- 
 fractor, 517 
 
 Clarke, Colonel, figure of the earth, 
 322 
 
558 
 
 INDEX. 
 
 Clarke, F. W., celestial dissociation, 
 
 2 55 
 
 Clausen, period of 1843 comet, 131 ; 
 cometary systems, 438 
 
 Clerihew, secondary tail of 1843 
 comet, 129 J 
 
 Coggia, discovery of comet, 415 ' 
 
 Comet, Halley's, return in 1759, 5, 
 109 ; orbit computed by Bessel, 35 ; 
 capture by Neptune, 122, 442 ; re- 
 turn in 1835, 126-8, 417; type of 
 tail, 419, 425 ; Newton's, 109, 441 ; 
 Encke's, 112; changes of volume, 
 115 ; of brightness, 117 ; accelera- 
 tion. 115-7; possible capture by 
 Mercury, 122 ; Lexell's, 121, 132*; 
 rediscovered by Brooks, 444 ; ap- 
 proaches to Jupiter, 445 ; Win- 
 necke's, 116, 414 ; Biela's, 117-20 ; 
 star showers in connection with, 
 405-9 ; Faye's, 121 ; Brorsen's, 121 ; 
 Vico's, 121 ; of 1811, 122-5, 419 ; of 
 1807, 124, 420, 425, 427 ; of 1819, 
 125, 128; of 1843, 128-131; type of 
 tail, 419, 425 ; relationships, 422-4, 
 435 ; Tewfik, 224,433,438; Donati's, 
 392-5 ; type of tail, 419, 420 ; of 
 1861, 395-7, 419; of the August 
 meteors, 397, 403 ; of the November 
 meteors, 397, 403, 405, 415 ; Klin- 
 kerf ues's, 400 ; Holmes's, 409, 415, 
 447 ; Coggia' s, 415, 418, 419 ; of 
 1880, 421-2, 424, 425 ; Aristotle's, 
 423 ; Tebbutt's, 425-9 ; Schae- 
 berle'r, 430 ; Wells' s, 421-3 ; of 
 September 1882, 433-7, 439-42 ; 
 Thome's, 437-8 ; Pons-Brooks, 
 442-3 ; Sawerthal's, 444 ; Swift's, 
 446 
 
 Comets, subject to law of gravitation, 
 109 ; of short period, 112-3 '> con - 
 tractions and expansions, 114-5, 
 127, 443, 447; translucency, 118, 
 131-2, 426 ; small masses, 120, 132; 
 capture by planets, 122, 372, 442 ; 
 polarisation of light, 128, 428, 429 ; 
 refraction by, 132, 426 ; photo- 
 graphs, 224, 427-8, 446, 447, 494 ; 
 relation to meteor-systems, 403-5, 
 408-9 ; disintegration, 404, 409, 
 439 ; spectra, 414-6, 428-9, 43 1-2, 
 441 ; luminous by electricity, 416, 
 429, 432 ; systems, 427, 429, 437, 
 438, 442 ; origin, 447-9 
 
 Comets' tails, repulsive forces pro- 
 ducing, 123-4, I2 7>4 1 7~ 21 5 velocity 
 of projection, 124, 129, 419 ; multi- 
 ple, 124, 393, 417, 419-20, 426, 430, 
 
 440,446; coruscations, 130; three 
 types, 419-20, 425, 430 
 
 Common, reflectors for eclipse photo- 
 graphy, 236 ; observations of 
 Jupiter's inner satellite, 358 ; day- 
 light discovery of great comet, 433 ; 
 five nuclei, 439 ; photograph of 
 Andromeda nebula, 476 ; of Orion 
 nebula and Jupiter, 489-90 ; nebu- 
 lous patches in the Pleiades, 493; 
 great reflectors, 513 
 
 Common, Miss, drawing of eclipsed 
 sun, 236 
 
 Comte, celestial chemistry, 174 ; 
 astronomy, 177 
 
 Cooke, 25-inch refractor, 514 
 
 Copeland, comets of 1843 an( ^ 1880, 
 422; spectrum of comet of 1882, 
 441 ; of 7 Cassiopeia?, 457 ; of Nova 
 Andromedse, 476 ; of Orion nebula, 
 489 ; gaseous stars, 459 ; Nova 
 Aurigae, 478 
 
 Copernicus, stellar parallax, 18 
 
 Cornu, telluric lines in solar spectrum, 
 251 ; movements in prominences, 
 255 ; ultra-violet solar spectrum, 
 262, 267; solar parallax by light 
 velocity, 286, 297 ; spectrum of 
 hydrogen, 463 ; of Nova Cygni, 
 
 474 
 Cornu and Bailie, density of the earth, 
 
 321 
 Corona, of 1842, 76, 78 ; early records 
 
 and theories, 79-82 ; photographs, 
 
 207, 217, 224, 228, 232-6 ; spectrum, 
 
 212, 217, 222-3, 228 ; constitution, 
 
 213, 217, 222-3, 239-40; varying 
 types, 218, 223, 228, 232-3 ; of 1878, 
 218-22; of 1867, 221; of 1882, 
 223-4 ; of 1869, 230; of 1886, 232 ; 
 of 1889, 233-4, 236 ; photographing 
 without an eclipse, 224-6 ; glare - 
 theory, 229 ; mechanical theory, 
 237-8 ; mathematical theory, 238-9 
 
 Coronium, 213, 217, 240 
 
 Cortie, movements within sunspots, 
 
 195 ; changes in spectra, 258 
 Cotes, corona of 1715, 221 
 Croll, secular changes of climate, 
 
 319-20; source of solar- energy, 
 
 38i 
 
 Crookes, analysis of chemical ele- 
 ments, 260 
 
 Crova, solar constant, 278 
 
 Cruls, great comet of 1882, 433, 440 
 
 Cusa, solar constitution, 68 
 
 Cysatus, Orion nebula, 25 ; comet of 
 1652, 439 
 
INDEX. 
 
 559 
 
 DAMOISEAU, theory of Halley's comet, 
 126 
 
 D'Arrest, orbits of minor planets, 347 ; 
 Biela meteors, 405 ; ages of stars, 
 453 ; variable nebula, 484 ; measures 
 of nebulas, 486 
 
 Darwin, G. H. , rigidity of the earth, 
 317; Saturn's ring-system, 366-7; 
 origin of the moon, 384-6 ; develop- 
 ment of solar system, 387, 391 ; 
 solar tidal friction, 388 
 
 Daubree, falls of aerolites, 411 
 
 Davidson, satellite-transit on Jupiter, 
 
 355 
 
 Dawes, prominences in 1851, 85 ; 
 Saturn's dusky ring, 106 ; a star 
 behind a comet, 132 ; solar observa- 
 tions, 178, 204; colour of Mars, 
 339; drawings, 341; ice-island, 343 ; 
 satellite-transit on Jupiter, 355 
 
 De Ball, markings on Mercury, 305 
 
 Delambre, Greenwich observations, 3 ; 
 solar rotation, 182; light-equation, 
 285 
 
 De la Koche, Newton's law of cooling, 
 269 
 
 De la Kue, celestial photography, 
 I90-i 329 ; solar investigations, 
 192 ; expedition to Spain, 207-8 
 
 De la Tour, experiments on liquefac- 
 tion, 1 88 
 
 Delaunay, tidal friction, 332, 334 ; 
 Coude telescope, 522 
 
 Delisle, diffraction-theory of corona, 
 81; method of observing transits 
 of Venus, 288, 295 
 
 Dembowski, measurements of double 
 stars, 501 
 
 Denning, rotation of Mercury, 304 ; 
 mountains of Venus, 311 ; rotation 
 of Jupiter, 353 ; red spot, 359, 360 ; 
 rotation of Saturn, 368 ; meteors of 
 1885, 408-9; of 1892, 410; with 
 stationary radiants, 412 
 
 Denza, meteors of 1872, 406 
 
 Derham, volcanic theory of sunspots, 
 63 ; ashen light of Venus, 314 
 
 Deslandres, daylight coronal photo- 
 graphy, 226 ; eclipse expedition, 
 237; prominence-photography, 246; 
 hydrogen spectrum in prominences, 
 
 247, 463 
 Diffraction, corona explained by, 81, 
 
 85, 228; spectrum, 173, 261, 276,525 
 Dissociation, in the sun, 189, 255-60; 
 
 in space, 380 
 Doberck, orbits of double stars, 46, 
 
 500 
 
 Dollond, discovery of achromatic 
 
 telescope, 4, 139 
 Donati, discovery of comet, 392 ; 
 
 spectra of comets, 414; of stars, 
 
 450 
 Doppler, refrangibility of light 
 
 changed by motion, 249 
 Draper, H., ultra-violet solar spectrum, 
 
 262 ; oxygen in sun, 265 ; photo- 
 
 fraphs of the moon, 329 ; of 
 upiter's spectrum, 354; of Tebbutt's 
 comet, 428 ; of spectrum of Vega, 
 462 ; of Orion nebula, 489 
 
 Draper, J. W., lunar photographs, 
 190 ; distribution of energy in 
 spectrum, 276 note 
 
 Draper Memorial, 464-5 
 
 Dreyer, New General Catalogue of 
 nebula3, 60 
 
 Dulong and Petit, law of radiation, 
 270, 272 
 
 Duner, spectra of sunspots, 195 ; 
 spectroscopic determination of solar 
 rotation, 252 ; spectroscopic star- 
 catalogue, 460 
 
 Dunkin, solar translation, 47 
 
 Duponchel, sunspot period, 202 
 
 EAETH, mean density, 73, 321 ; body 
 of science regarding, 316; rigidity, 
 317-9; variation of latitude, 318; 
 figure, 321-2 ; rotation checked by 
 tidal friction, 333 ; possible in- 
 equalities, 335 ; bodily tides, 384 ; 
 primitive disruption, 386 
 
 Eclipse, solar, of 1836, 74 ; of 1842, 
 75-9, 82, 86 ; of 1851, 84-6, 207 ; 
 of 1860, 207-8 ; of 1868, 209-10 ; of 
 1869, 212; of 1870, 213; of 1871, 
 216-7 ; of 1878, 218-22 ; of 1882, 
 222-4; of 1883, 226-8; of 1885, 
 231 ; of 1886, 231-2 ; of 1887, 233 
 of Jan. i, 1889, 233-5; of Dec. 22, 
 1889, 235 ; of 1893, 237 
 
 Eclipses, lunar, observations of heat- 
 phases, 330-1 
 
 Eclipses, solar, importance, 71 ; 
 different classes, 73; results, 217; 
 ancient, 334 
 
 Eddie, comet of 1880, 421 ; of 1882, 
 440 
 
 Edison, tasimeter, 222 
 
 Egoroff, telluric lines in solar spec- 
 trum, 266, 313 
 
 Elements, chemical, dissociation in 
 sun, 255, 259-61 
 
 Elkin, star parallaxes, 44, 498, 499- 
 500; transit of great comet, 434, 
 
56o 
 
 INDEX. 
 
 436 ; secondary tail, 440 ; measure- 
 ment of Pleiades, 493 
 Elliot, opinions regarding the sun, 
 
 69 
 
 Elvins, red spot on Jupiter, 361 
 Encke, star-maps, 95-; a pupil .^of 
 Gauss, in; identification of short- 
 period comet, 112 ; resisting 
 medium, 115 ; distance of the sun, 
 283, 287 ; period of Pons's comet, 
 442 
 Engelmann, rotation of Jupiter's 
 
 satellites, 356 
 
 Ericsson, solar temperature, 271 
 Erman, meteoric rings, 400 ; method 
 
 of computing meteoric orbits, 401 
 Ertborn, mountain in Venus, 311 
 Espin, bright lines in spectra of vari- 
 able stars, 459 ; stars with banded 
 spectra, 460 ; spectrum of Nova 
 Aurigse, 478 ; rediscovery, 480 
 Euler, resisting medium, 115 
 Evolution of solar system, 374-6, 
 381-3, 391 ; of earth-moon system, 
 384-7 ; of stellar systems, 502 
 
 FABRICIUS, David, discovery of Mira 
 Ceti, ii 
 
 Fabricius, John, discovery of sun- 
 spots, 62 
 
 Faculas, solar, relation to spots, 63, 
 193 ; photographed, 245, 456 ; solar 
 rotation from, 252 
 
 Faye, nature of prominences, 85 ; dis- 
 covery of a comet, 121 ; cyclonic 
 theory of sunspots, 179, 195-6 ; 
 solar constitution, 186-90 ; maxi- 
 mum of 1883-4, 203 ; solar absorp- 
 tion, 215 ; cometary appendages, 
 221 ; velocities in prominences, 
 255 ; distance of the sun, 296 ; 
 planetary evolution, 383-4, 390 
 
 Feilitsch, solar appendages, 85 
 
 Ferrel, tidal friction, 334 
 
 Ferrer, nature of corona, 81 ; pro- 
 minences, 84 
 
 Finlay, comet discovered by, 121 ; 
 transit of 1882 comet, 433, 436 
 
 Fizeau, daguerreotype of the sun, 191 ; 
 Doppler's principle, 250 ; velocity 
 of light, 286 
 
 Flammarion, canals of Mars, 342 ; 
 trans-Neptunian planet, 372 
 
 Flamsteed, nature of the sun, 68; 
 distance, 282 
 
 Flaugergues, detection of 1811 comet, 
 122 ; transit of Mercury, 301 
 
 Fleming, Mrs., spectrum of /3 Lyrse, 
 
 458 ; preparation of Draper Cata- 
 logue, 465 
 
 Fontana, mountains of Venus, 310 ; 
 satellite, 315 ; spots on Mars, 336 
 
 Forbes, Prof. G., trans-Neptunian 
 planets, 372-3 
 
 Forbes, James D., solar spectrum 
 during annular eclipse, 167 ; solar 
 constant, 278 
 
 Foucault, spectrum of voltaic arc, 
 171; firs I. photograph of the sun, 
 191; velocity of light, 286 ; silvered 
 glass reflectors, 513 
 
 Fraunhofer, early accident, 40 ; im- 
 provement of refractors, 41 ; death, 
 42 ; gave clockwork motion to tele- 
 scopes, 150 ; spectra of flames, 163 ; 
 of sun and stars, 165-6, 450 ; objec- 
 tive prism, 464 ; diffraction gratings, 
 
 525 
 
 Fraunhofer lines, mapped, 165; origin, 
 168-70, 216, 252 ; reflected in coro- 
 nal spectrum, 216, 222, 223 ; in 
 cometary spectra, 428, 432 ; a cri- 
 terion of radial motion, 250 
 Fritz, auroral periodicity, 201 
 Frost, solar heat-radiation, 275 
 
 GALILEO, originated descriptive as- 
 tronomy, 2 ; double-star method of 
 parallaxes, 19 ; discovery of sun- 
 spots, 62 ; solar rotation, 182 ; 
 planets and sun-spots, 202 ; darken- 
 ing at sun's edge, 274 
 
 Galle, discovery of Neptune, 100, 101 ; 
 Saturn's dusky ring, 107 ; distance 
 of the sun, 293 ; Biela's comet and 
 meteors, 405, 406 
 
 Galloway, solar translation, 47 
 
 Gambart, discovery of Biela's comet, 
 117 
 
 Gauss, orbits of minor planets, 90-1 ; 
 Theoria Motus, 95 ; magnetic obser- 
 vations, 157 ; cometary orbits, 448 
 
 Gautier,sunspot and magnetic periods, 
 1 56,: 1 59 5 sunspots and weather, 160 
 
 German Astronomical Society, 7, 497 ; 
 
 Gill, star parallaxes, 44, 51, 498, 
 499 ; motion of Toucanas, 47 ; 
 expedition to Ascension, 292 ; dis- 
 tance of the sun, 292-3, 294, 296, 
 298; measurements of minor planets 
 293-4, 296 ; great comet of 1882, 
 434, 494 ; photograph of Argo neb- 
 ula, 486 ; photographic survey of 
 the heavens, 495 ; actinic intensity 
 of Milky Way stars, 509 ; Coude 
 telescope, 523 
 
INDEX. 
 
 561 
 
 Gladstone, Dr. J. H., spectrum analy- 
 sis, 1 66, 169 
 
 Glaisher, occultation by Halley's comet, 
 132 
 
 Glasenapp, coronal photographs, 233 ; 
 light equation, 285, 297 
 
 Glass, optical, excise duty on, 139, 
 143; Guinand's, 140-1; Jena. 515 
 
 Gledhill, spot on Jupiter, 359 
 
 Goldschmidt, nebulas in the Pleiades, 
 
 494 
 
 Goodricke, periodicity of Algol, 469 
 Gore, catalogue of variable stars, 471 ; 
 of computed binaries, 501 ; disco- 
 very of U Orionis, 475 
 Gotha, astronomical congress at, 7 
 Gothard, bright-line stellar spectra, 
 456-7, 458 ; spectra of planetary 
 nebulas, 489 : photographs of ne- 
 bulas, 491 
 
 Gould, variation of latitude, 318 ; 
 first photograph of Mars, 344 ; 
 comets of 1881 and 1807, 425 ; fluc- 
 tuations in stellar brightness, 472 ; 
 measures of photographed Pleiades, 
 492 ; Uranometria Argentina, 497 ; 
 solar cluster, 507 
 Graham, discovery of Metis, 94 
 Grant, solar envelope, 85, 208 ; lumi- 
 nous effects attending transits of 
 Venus, 312 
 Green, N. E., observations of Mars, 
 
 343 
 
 Greenwich observations, 3, 33, 38 
 
 Gregory, David, achromatic lenses, 
 139 note 
 
 Gregory, James, double-star method 
 of parallaxes, 19 ; reflecting tele- 
 scopes, 135-6 
 
 Groombridge, star catalogue, 37 
 
 Grosch, corona of 1867, 221 
 
 Grubb, Sir Howard, photographic 
 reflector, 491 ; Vienna and Green- 
 wich refractors, 515, 517 ; side- 
 rostat, 523 
 
 Grubb, Thomas, Melbourne reflector, 
 
 5.13 
 
 Griinwald, theory of spectra, 242 
 Gruithuisen, snow-caps of Venus, 314 ; 
 
 lunar inhabitants, 326 
 Gully, Nova Andromedas, 475 
 Guthrie, nebulous glow round Venus, 
 
 312 
 
 HADLEY, Saturn's dusky ring, 107 ; 
 
 reflecting telescope, 135 
 Haerdtl, Winnecke's comet, 116 
 Hale, luminous outburst on sun, 
 
 200 ; daylight coronal photography, 
 226 ; spectrum of prominences, 242, 
 246 ; prominence - photography, 
 244-6 
 
 Hall, Asaph, parallax of the sun, 298 ; 
 discovery of Martian satellites, 
 345 ; rotation of Saturn, 367 ; 
 parallax of Vega, 499 ; 61 Cygni, 
 502 ; double star measurements, 
 502 
 
 Hall, Chester More, invention of 
 achromatic telescope, 139 
 
 Hall, Maxwell, rotation of Neptune, 
 
 371 
 
 Halley, stellar proper motions, n ; 
 nebulas, 26 ; eclipse of 1715, 80, 83 ; 
 predicted return of comet, 109 ; 
 magnetic theory of auroras, 161 ; 
 transits of Venus, 288 ; lunar 
 acceleration, 332 ; origin of 
 meteors, 397 
 Hansen, solar parallax from lunar 
 
 theory, 284 
 
 Harding, discovery of Juno, 92 ; celes- 
 tial Atlas, 95 
 
 Harkness, spectrum of corona, 212 ; 
 corona of 1878, 219 ; shadow of the 
 moon in solar eclipses, 229 ; light- 
 equation, 285 ; distance of the sun 
 292, 296, 297, 298 
 
 Harrington, variability of Vesta, 351 
 Harriot, observations on Halley's 
 
 comet, 35 
 
 Hartwig, Nova Andromedas, 475 
 Hasselberg, metallic spectra, 262 
 spectra of comets, 414, 432 ; of 
 Nova Andrornedae, 476 
 Hastings, composition of photo 
 sphere, 189 ; reversing stratum, 215 ; 
 observations at Caroline Island, 
 228 ; Saturn's dusky ring, 365 
 Hegel, number of the planets, 88 
 Heis, radiant of Andromedes, 405 
 Heliometer, 41, 288, 296, 499 
 Helium, a constituent of prominences, 
 241, 247 ; absence of absorptive 
 action, 265 ; present in gaseous 
 stars, 456 ; in Orion nebula, 489 
 Helmholtz, gravitational theory of 
 
 sun-heat, 378-9, 381 
 Hencke, discoveries of minor planets, 
 
 94 
 
 Henderson, parallax of a Centauri, 
 43 ; observation of chromosphere, 
 
 83' 
 
 Henry, Paul and Prosper, lunar twi- 
 light, 325 ; minor planets, 347 ; 
 markings on Uranus, 369 ; photo- 
 
5 62 
 
 INDEX. 
 
 graph of Saturn, 490 ; photographic 
 discovery of nebulas in Pleiades, 
 493, 494 ; plane mirrors, 523 
 
 Herrick and Bradley, duplication of 
 Biela's comet, 119 
 
 Herschel, Alexander S., cometary an 
 meteoric orbits, 403 
 
 Herschel, Caroline, her brother's 
 assistant, 14 ; observation of 
 Encke's comet, 112 
 
 Herschel, Sir John, life and work, 
 54-61 ; expedition to the Cape, 56 ; 
 Magellanic clouds, 56, 505 ; sun- 
 spots, 70-1, 179 ; solar flames, 82 ; 
 anticipated discovery of Neptune, 
 100 ; status of Hyperion, 105 ; 
 Biela's comet, 118 ; "Halley's, 127; 
 comet of 1843, I2 9 > sixth star in 
 trapezium, 141 ; grinding of 
 specula, 144 ; spectrum analysis, 
 164; solar photography, 181 ; 
 solar constitution, 188 ; shadow 
 round eclipsed sun, 229 ; actino- 
 metrical experiments, 268 ; solar 
 heat, 269 ; climate and eccentricity, 
 319 ; lunar atmosphere, 324 ; sur- 
 face of Mars, 338 ; Andromeda 
 nebula, 477 ; observations of ne- 
 bulas, 485 
 
 Herschel, Colonel, spectrum of pro- 
 minences, 209 ; of corona, 217 
 
 Herschel, Sir William, discovery of 
 Uranus, 5 ; popular interest ex- 
 cited by his astronomical career, 
 5-6; founder of sidereal astronomy, 
 10 ; biographical sketch, 12-16 ; 
 sun's motion in space, 17, 47, 509; 
 revolutions of double stars, 21, 528 ; 
 structure of Milky Way, 22-4, 506 ; 
 study of nebulae, 25-30 ; results of 
 astronomical labours, 30 ; centre of 
 sidereal system, 49 ; theory of the 
 sun, 65-7", 86 ; asteroids, 92, 93 ; 
 discoveries of Saturnian and 
 Uranian satellites, 104, 107-8, 137 ; 
 comet of 1811, 123; reflecting 
 telescopes, 135-8 ; sunspots and 
 weather, 160 ; transit of Mercury, 
 301 ; refraction on Venus, 311 ; 
 lunar volcanoes, 327 ; similarity of 
 Mars to the earth, 336-7 ; Jovian 
 trade-winds, 352 ; rotation of 
 Jupiter's satellites, 356 ; ring of 
 Saturn, 363 ; rotation of Saturn, 368 
 
 Herz, comets' tails, 421 
 
 Hevelius, acquaintance with " Mira," 
 ii ; contraction of comets, 114 ; 
 granular structure of a comet, 439 
 
 Higgs, photographs of solar spectrum, 
 
 263 
 
 Hind, solar flames, 84 ; Iris and Flora 
 discovered by, 94 ; distortion of 
 Biela's comet, 119; transit of a 
 comet, 125 ; earth in a comet's 
 tail, 396 ; comets of 1843 and 1880, 
 422 ; computation of Schmidt's 
 comet, 439 ; new star, 473 ; variable 
 nebula, 484 
 
 Hirn, solar ^temperature, 273 ; re- 
 sistance in space, 421 
 Hodgson, outburst on the sun, 199 
 Hoek, cometary systems, 438 
 Holden, Uranian satellites, 108 ; 
 eclipse-expedition, 227 ; coronal 
 extensions, 233, 235 ; solar rotation, 
 252 ; transit of Mercury, 301 ; 
 intra-Mercurian planets, 308 ; 
 drawing of Venus, 310 ; canals on 
 Mars, 342 ; surface of Mars, 344 ; 
 transits of Jupiter's satellites, 355 ; 
 markings on Uranus, 370 ; dis- 
 integration of great comet, 439 : 
 colours of double stars, 453 ; Orion 
 and Trifid nebulas, 485 ; redis- 
 covery of Nova Aurigas, 486 ; di- 
 rector of Lick Observatory, 520 
 Holden and Schaeberle, helical 
 
 nebulas, 518 
 
 Holmes, discovery of a comet, 409 
 Homann, solar translation, 489 
 Hooke, solar translation, n ; stellar 
 parallax, 18 ; repulsive force in 
 comets, 127 note; automatic move- 
 ment of telescopes, 150; spots on 
 Mars, 336, 338 
 
 Hopkins, solidity of the earth, 316 
 Horrebow, sunspot periodicity, 155 ; 
 
 satellite of Venus, 316 
 Hough, red spot on Jupiter, 358, 
 
 360 
 
 Houzeau, solar parallax, 296 
 Hubbard, period of 1843 comet, 130, 
 
 424 
 
 Huggins, Dr., spectroscopic observa- 
 tions of prominences, 212, 243 ; 
 daylight coronal photography, 
 224-6 ; hydrogen-spectrum in stars, 
 246 ; stellar motions in line of 
 sight, 250, 466-7; transit of Mer- 
 cury, 301 ; occultation of e Piscium, 
 324 ; snowcaps on Mars, 338 ; 
 spectrum of Mars, 340 ; of Jupiter, 
 354; Jovian markings and sun- 
 spots, 362 ; spectrum of Saturn's 
 ring, 368 ; of Uranus, 370 ; of 
 comets, 414-6 ; photographs, 428, 
 
INDEX. 
 
 563 
 
 432 ; stellar spectroscopy, 451 ; 
 colours of stars, 453 ; Presidential 
 Address at Cardiff, 455 ; chemical 
 composition of stars, 461 ; photo- 
 graphs of stellar spectra, 462-3, 524 ; 
 spectra of new stars, 473, 476 ; theory 
 of Nova Aurigae, 479 ; spectra of 
 nebulae, 482-3 ; nebular radial move- 
 ment, 486 
 
 Huggins, Dr. and Mrs., photograph of 
 Uranian spectrum, 371 ; spectra of 
 Wolf-Eayet stars, 459 ; ultra-violet 
 spectrum of Sirius, 463 ; spectrum 
 of Nova Aurigas, 478, 481-2 ; of 
 Orion nebula, 484, 489 
 
 Humboldt, sunspot period, 156 ; mag- 
 netic observations, 157, 158; star- 
 shower, 400 
 
 Hussey, search for Neptune, 97 
 
 Huygens, stellar parallax, 19 ; Orion 
 nebula, 25 ; discovery of Titan, 104 ; 
 Saturn's ring, 105, 366 ; spot on 
 Mars, 337 
 
 Hydrogen, a constituent of promin- 
 ences, 209, 241, 247 ; spectrum, 
 246-7, 462-3 ; absorption in stars, 
 246, 452, 461, 462 ; in sun, 263, 462 ; 
 supposed material of first-type 
 cometary tails, 420; bright lines 
 of, in stellar spectra, 456-8, 473, 474, 
 478 ; in nebular spectra, 483, 489 
 
 JACOBY, measurements of Euther- 
 furd's plates, 492 
 
 Janssen, photographs of the sun, 205 ; 
 spectroscopic observations of pro- 
 minences, 210-1 ; escape from Paris 
 in a balloon, 213 ; coronal spec- 
 trum, 216-7, 221 > photographs of 
 corona of 1883, 228 ; rarefaction of 
 chromospheric gases, 229 ; oxygen- 
 absorption in solar spectrum, 266 ; 
 ascent of Mont Blanc, 267 ; transit 
 of 1874, 289 ; spectrum of Venus, 
 313 ; of Saturn, 368 ; photographs 
 of Tebbutt's comet, 427-8 ; of 
 Orion nebula, 489 
 
 Japetus, eclipse of, 365 ; variability, 
 368 
 
 Joule, heat and motion, 376 
 
 Jupiter, mass corrected, 95, 1 14 ; sup- 
 posed influence on sunspot period, 
 202; physical condition, 351-3; 
 spectrum, 354 ; satellite-transits, 
 355 ; discovery of inner satellite, 
 357 ; red spot, 357-61 ; photo- 
 graphs, 361-2, 490 ; periodicity of 
 markings, 362 
 
 KAISER, rotation of Mars, 337 ; map 
 of Mars, 341 
 
 Kammermann, observation of Maia 
 nebula, 493 
 
 Kant, position of nebulae, 16 ; Sirius 
 the central sun, 48 ; planetary in- 
 tervals, 87 ; tidal friction, 333 ; 
 condition of Jupiter, 352 ; cosmo- 
 gony, 374 
 
 Kapteyn, preparation of Cape Durch- 
 musterung, 495 ; stellar parallaxes, 
 500 ; solar cluster, 507 
 
 Kayser and Eunge, carbon in sun, 264 
 
 Keeler, red spot on Jupiter, 361 ; spec- 
 trum of Uranus, 371 ; of nebulas, 
 484 ; radial movements of nebulae, 
 48?, 518, 526 
 
 Kepler, star of 1604, 29 ; solar corona, 
 79 ; missing planets, 87 ; cometary 
 decay, 113, 411 ; comet of 1618, 
 119; physical astronomy, 175 
 
 Kiaer, comets' tails, 421 
 
 Kirchhoff, foundation of spectrum 
 analysis, 164, 167-168, 450 ; map of 
 solar spectrum, 169 ; solar consti- 
 tution, 1 86, 1 88, 215 
 
 Kirkwood, distribution of minor 
 planets, 349 ; grouped orbits, 350 ; 
 divisions in Saturn's rings, 367 ; 
 origin of planets, 382 ; direction of 
 their rotation, 390 ; comets and 
 meteors, 404, 411 
 
 Kleiber, Perseid radiants, 413 
 
 Klein, Hyginus N., 328-9 
 
 Klinkerf ues, comet predicted by, 407, 
 410 ; apparitions of southern comet, 
 
 423 
 
 Konkoly, spectrum of 7 Cassiopeiae, 457 
 Kreil, lunar magnetic action, 161 
 Kreutz, orbit of 1861 comet, 397 ; 
 
 period of great comet of 1882, 437 ; 
 
 multiple nucleus, 439 
 Kriiger, segmentation of great comet, 
 
 439 
 
 Kiistner, variation of latitude, 318 
 Kunowsky, spots on Mars 337 
 
 LACAILLE, southern nebulas, 26 
 
 Lagrange, gravitational theory of 
 solar system, 3 ; planetary disrup- 
 tion, 93 
 
 Lahire, diffraction-theory of corona, 
 81 ; distance of the sun, 282 ; moun- 
 tains of Venus, 310 
 
 Lalande, popularised astronomy, 5 ; 
 revolving stars, 21 ; Histoire Celeste, 
 37> 497 '> nature of sunspots, 63 ; 
 observations of Neptune, 102 
 
564 
 
 INDEX. 
 
 Lambert, solar motion, n ; construc- 
 tion of the universe, 16, 48 ; miss- 
 ing planets, 87 
 
 Lament, magnetic period, 157-8 
 Lamp, ashen light of Venus, 315 
 Landerer, character of the light .pf 
 
 Venus, 314 
 
 Langdon, mountains of Venus, 311 
 Langley, solar granules, 204 ; corona 
 of 1878, 220 ; spectroscopic effects 
 of solar rotation, 251 ; experiments 
 at Pittsburg, 273 ; bolometer, 275 ; 
 distribution of energy in solar spec- 
 trum, 277-8 ; atmospheric absorp- 
 tion, 277-9, 339 ? colour of unveiled 
 sun, 278 ; solar constant, 279 ; lunar 
 heat-spectrum, 330 ; temperature 
 of lunar surface, 331 ; age of the 
 sun, 379 
 
 Laplace, lunar acceleration, 2, 332 ; 
 Exposition du Systeme du Monde, 6 ; 
 nebular hypothesis, 30, 374-6, 381-3, 
 391 ; stability of Saturn's rings, 
 105, 363 ; Lexell's comet, 132, 445 ; 
 solar atmospheric absorption, 274 ; 
 solar distance by lunar theory, 
 284 ; origin of meteors, 398 ; of 
 comets, 448 
 
 Lassell, search for Neptune, 98 ; 
 discovery of satellite, 103 ; of 
 Hyperion, 105 ; dusky ring of Saturn, 
 106; observations at Malta, 108, 
 519 ; reflectors, 142 ; equatoreal 
 mounting, 150 
 Latitude, variation of, 318-9 
 Laugier, period of 1843 comet, 131 ; 
 
 solar rotation, 182 
 Le Chatelier, temperature of the sun, 
 
 272 
 
 Ledger, orbit of Aethra, 348 
 Lescarbault, pseudo - discovery of 
 Vulcan, 306 ; halo round Venus in 
 transit, 313 
 Lespiault, orbits of minor planets, 
 
 348 
 
 Le Sueur, spectrum of Jupiter, 354 
 Leverrier, discovery of Neptune, 99, 
 100, 101 ; Lexell's comet, 121, 445 ; 
 distance of sun, 284, 296; move- 
 ments of Mercury, 305 ; prediction 
 of Vulcan, 306 ; supposed transits, 
 307 ; mass of asteroids, 350 ; orbit 
 of November meteors, 403 ; Perseids 
 and Leonids, 405 
 Lexell, comet of 1770, 121, 445 
 Liais, supposed transit of Vulcan, 306 ; 
 comet of 1 86 1, 396; division of a 
 comet, 410 
 
 Librations, of Mercury, 305 ; of Venus, 
 310 ; of the moon, 327 
 
 Lick, foundation of observatory, 519 
 
 Light, velocity, 45, 285-6, 297 ; ex- 
 tinction in space, 54 ; refrangibility 
 changed by movement, 249 
 
 Light-equation, 285, 297 
 
 Lindsay, Lord, expedition to the 
 Mauritius, 289 
 
 Line of sight, movements in, 249 ; 
 spectroscfcpically deterrainable, 250, 
 
 465 ; of solar limbs, 250-2 ; within 
 prominences, 253, 258-9 ; of stars, 
 466-7 ; serves for detection of close 
 binaries, 467-70 
 
 Listing, dimensions of the globe, 322 
 
 Littrow, chromosphere, 85 ; sunspot 
 periodicity, 156 
 
 Liveing and Dewar, numerical ratios 
 of wave-lengths, 241 ; line-displace- 
 ments in prominences, 259 ; spec- 
 trum of liquid oxygen, 266 
 
 Lockyer, solar spectroscopy, 194, 
 263 ; theory of sunspots, 197-8, 
 203 ; daylight observations of pro- 
 minences, 211, 241, 254 ; eclipse of 
 1870, 213 ; reversals in solar atmo- 
 sphere, 216, 242; slitless spectro- 
 scope, 217; corona of 1878, 219; 
 coronal spectrum, 223 ; glare- theory 
 of corona, 229 ; eclipse of 1886, 
 231 ; classification of prominences, 
 244 ; motion- displacements in pro- 
 minences, 252-3 ; celestial dissocia- 
 tion, 255-60 ; chemistry of sunspots, 
 257-8 ; carbon in sun, 264 ; spots 
 on Mars, 337 ; meteoritic hypothesis, 
 4S4~5 J chief nebular line, 483 ; 
 equatoreal Coud^, 523 
 
 Loewy, constant of aberration, 298 ; 
 equatoreal Coude, 522 
 
 Lohrmann, lunar chart, 325; Linne, 
 328 
 
 Lohse, J. Gr. , spectrum of great comet, 
 441 ; of 7 Cassiopeiae, 457' 
 
 Lohse, O., daylight coronal photo- 
 graphy, 224 note; red spot on 
 Jupiter, 351 ; periodicity of Jupiter's 
 markings, 362 ; recession of Sirius, 
 
 466 ; spectrum of Nova Cygni, 474, 
 Louville, nature of corona,, Si ; chro- 
 mosphere, 83 
 
 Lyman, atmosphere of Venus, 312 
 
 McCLEAN, photographs of solar 
 
 spectrum, 262, 263 
 Macdonnell, luminous ring round 
 
 Venus, 313 
 
INDEX. 
 
 565 
 
 Maclaurin, eclipse of 1737, 78 
 
 Maclear, Admiral, observations during 
 eclipses, 214, 230 
 
 Maclear, Sir Thomas, maximum of 17 
 Argus, 59 ; observation of Halley's 
 comet, 127 
 
 Miidler, central sun, 49 ; observations 
 of Venus, 311, 312 ; lunar rills, 323 ; 
 -aspect of Linne, 328 ; community 
 of proper motions, 509 
 
 Magellanic clouds, 56, 505 ; spiral 
 character, 508-9 
 
 Magnetism, terrestrial, international 
 observations, 157 ; periodicity, 158- 
 9 ; solar relations, 159, 199-200, 
 203,254; lunar influence, 161 
 
 Mann, N. M., period of 61 Cygni, 
 502 
 
 Mann, W., last observation of Donati's 
 comet, 394 
 
 Maraldi, solar corona, 81 ; rotation of 
 Mars, 336 ; satellite -transits on 
 Jupiter, 355 ; spot on Jupiter, 359 
 
 Marius, Simon, Andromeda nebula, 
 25 ; sunspots, 62 ; origin of comets, 
 63 note 
 
 Mars, oppositions, 281 ; solar parallax 
 from, 282, 285, 292, 296 ; polar spots, 
 S3 6 , 338, 34; general stability of 
 markings, 337, 344; rotation, 337-8 ; 
 atmosphere, 338-40 ; climate, 340 ; 
 canals, 341-3 ; photographs, 344 ; 
 satellites, 344-6, 389-90, 514 
 
 Marth, revolutions of Neptune's 
 satellite, 372 
 
 Maskelyne, components of Castor, 
 21 ; Astronomer Eoyal, 33 ; experi- 
 ment at Schehallien, 321 ; comets 
 and meteors, 403 
 
 Maunder, photographs of 1886 corona, 
 232 ; stellar types, 454, 456 ; spec- 
 trum of 7 Cassiopeise, 458 
 
 Maury, duplication of Biela's comet, 
 119 
 
 Maury, Miss A. C., discoveries of 
 spectroscopic binaries, 467, 468 
 
 Maxwell, J. Clerk, structure of 
 Saturn's rings, 364, 366 
 
 Mayer, Kev. C., star satellites, 20 
 
 Mayer, Julius K., tidal friction, 334 ; 
 meteoric sustentation of sun's heat, 
 376-7 
 
 Mayer, Tobias, stellar motions, 1 1 ; 
 solar translation, 17 ; repeating 
 circle, 151 ; solar distance "by lunar 
 theory, 284 ; satellite of Venus, 
 316; lunar surface, 323 
 
 Mazapil meteorite, 411 
 
 Meldrum, sunspots and cyclones, 204 
 
 Melloni, lunar heat, 329 
 
 Melvill, spectra of flames, 162 
 
 Mercury, mass, 114; luminous phe- 
 nomena during transits, 301-2 ; 
 mountainous conformation, 303-4 ; 
 rotation, 303-5 ; theory of move- 
 ments, 305-6, 308 
 
 Mersenne, principle of reflecting 
 telescope, 135 
 
 Messier, catalogue of nebulae, 26 
 
 Meteoric hypothesis of solar susten- 
 tation, 376, 377 ; of planetary 
 formation, 378 
 
 Meteorites, origin, 398 ; relation to 
 shooting stars, 411-12 
 
 Meteoritic hypothesis of cosmical 
 constitution, 454-5, 484 
 
 Meteors, origin, 397-8 ; relationship to 
 comets, 397, 403-5, 414 ; Leonids, 
 398-400, 401-2 ; Perseids, 400, 403, 
 405, 413 ; Andromedes, 405 ; of 
 1872, 406-7; of 1885, 408-9; of 
 1892, 410 ; stationary radiants, 
 
 412-3 
 
 Meunier, canals of Mars, 342 
 
 Meyer, divisions of Saturn's rings, 
 
 367; comet of 1880,424; refraction 
 
 by a comet, 426; comet Tewfik, 438 
 
 Michell, double stars, 20; torsion 
 
 balance, 321 ; star systems, 510 
 Michelson, velocity of light, 296 
 Milky Way, grindstone theory, 16 ; 
 clustering power, 23, 31 ; structure, 
 24, 50, 54, 56, 5 6 -7; centre of 
 gravity, 48-9 ; frequented by tem- 
 porary stars, 482; drawings and 
 photographs, 507-8 
 Miller, W. A., spectrum analysis, 164, 
 
 169, 170; stellar chemistry , 45 1 
 Mira, light-changes, n ; spectrum, 
 
 452, 458 
 
 Mitchel, lectures at Cincinnati, 8 
 Mohn, origin of comets, 448 
 Moller, theory of Faye's Comet, 121 
 Moll, transit of Mercury, 301, 302 
 Monck, Perseid meteors, 413; new 
 
 stars, 477 
 
 Moon, the, acceleration, 3, 332, 334 ; 
 magnetic influence, 161 ; photo- 
 graphs, 190, 329 ; solar parallax 
 from inequality of movement, 284, 
 296 ; study of surface, 323 ; atmo- 
 sphere, 323-5 ; charts, 325-6, 328 ; 
 librations, 327 ; supposed changes, 
 328-9 ; thermal radiation*?, 329-31 ; 
 mode of rotation, 333 ; errors of 
 tables, 334-5 ; origin, 384-6 
 
5 66 
 
 INDEX. 
 
 Morinus, celestial chemistry, 174 
 Morstadt, Biela meteors, 404 
 Mouchez, photographic survey of the 
 
 heavens, 496 
 Muller, phases of minor planets, 350, 
 
 351 ; variability of Ndptune, 37! ; 
 
 of Pons's comet, 443 
 Munich, Optical Institute, 34, 40 
 Myer, description of solar eclipse, 230 
 
 NAEGAMVALA, Mercurian halo, 302 
 
 Nasmyth, assisted in construction of 
 Lassell's reflector, 103 ; solar willow- 
 leaves, 204 ; comparative lustre of 
 Mercury and Venus, 313 ; condition 
 of Jupiter, 352 
 
 Nasmyth and Carpenter, The Moon, 
 326 
 
 Nebula, Andromeda, early observa- 
 tions, 25 ; new star in, 475-6 ; 
 photographs, 476, 491 ; structure, 
 477, 491 ; visibility at Arequipa, 520 
 
 Nebula, Orion, observed by Herschel, 
 14 ; mentioned by Cysatus, 25 ; 
 fallacious signs of resolvability, 
 148, 483 ; spectrum, 483, 484, 489 ; 
 monograph of, 485 : movement of 
 recession, 487 ; photographs, 489-91 
 
 Nebulae, first discoveries, 25-6 ; cata- 
 logues, 26, 55, 60 ; distribution, 27, 
 58, 505 ; composition, 29, 57, 483 ; 
 resolution, 57, 146, 148 ; double, 
 58 ; spiral, 146-7, 484, 491-2 ; new 
 stars in, 475~7 484 ; spectra, 482-4, 
 489 ; variability in light, 484-5 ; 
 radial movements, 486-7 ; photo- 
 graphs, 489-92, 509 
 
 Nebular hypothesis, Herschel's, 29- 
 30 ; Laplace's, 374-6, 391 ; objec- 
 tions, 381-3 
 
 Neison, atmosphere of Venus, 312 ; 
 of the moon, 324; work on The 
 Moon, 326 
 
 Neptune, discovery, 96-102 ; satellite, 
 103; density, 104 ; comets captured 
 by, 122, 372, 442 ; mode of rotation, 
 
 372, 383. 391 
 Newall, F., stellar aspect of Nova 
 
 Aurigae, 470 
 
 Newall, E. S., 25-inch refractor, 514 
 Newcomb, origin of minor planets, 
 94 ; telescopic powers, 148 ; dis- 
 tance of the sun, 284, 287 ; velocity 
 of light, 297 ; variation of latitude, 
 318 ; lunar atmosphere, 324 ; lunar 
 theory, 334-5 ; disturbance of Nep- 
 tune's satellite, 372 ; formation of 
 planets, 382 
 
 Newton, H. A., capture of comets by 
 planets, 122 ; falls of aerolites, 378 ; 
 November meteors, 401-2 ; meteors 
 of 1885, 408, 409; orbits of aero- 
 lites, 412 
 
 Newton, Sir Isaac, founder of theo- 
 retical astronomy, i, 175 ; law of 
 gravitation obeyed by comets, 109 ; 
 first speculum," 135 ; solar radia- 
 tions, 268 ; law of cooling, 269, 271, 
 272 
 
 Niesten, volume of asteroids, 350 ; 
 red spot on Jupiter, 358 
 
 Nobert, diffraction-gratings, 525 
 
 Noble, observations of Mercury, 304 ; 
 secondary tail of Schaeberle's 
 comet, 430 
 
 Nolan, origin of the moon, 386 ; period 
 of Phobos, 389 
 
 Norton, corona of 1869, 220 note : ex- 
 pulsion-theory of solar appendages, 
 240 note; comets' tails, 417, 420 
 
 Nova Andromedae, 475-6 
 
 Nova Aurigae, 478-82 
 
 Nova Cygni, 473-4, 481 
 
 Nutation, discovered by Bradley, 4, 
 1 8 ; an uranographical correction, 
 37 
 
 OBSERVATORY, Greenwich, 3, 33 ; 
 Cape of Good Hope, 8, 44 ; Para- 
 matta, 8, 112; Harvard College, 8, 
 1 06, 464 ; Konigsberg, 36 ; Dorpat, 
 52 ; Pulkowa, 53 ; Palermo, 89 ; 
 Berlin, 112; Anclam, 185; Pots- 
 dam, 185 ; Kew, 191 ; Arequipa, 
 325, 520-1 ; Lick, 519-20 
 
 Occupations, of stars by comets, 118, 
 131-2, 426; by the moon, 324 ; by 
 Mars, 338 ; of Jupiter by the moon, 
 
 325 
 
 Olbers, Bessel's first patron, 35-6 ; 
 discoveries of minor planets, 91-2 ; 
 origin by explosion, 91-4 ; career, 
 no-i ; Biela's comet, 118 ; comet 
 of 1811, 123; electrical theory of 
 comets, 123-4, 130, 393, 417 ; mul- 
 tiple tails, 124, 393, 420 ; comet of 
 1819, 125 ; cometary coruscations, 
 130; a star behind'a comet, 131 ; 
 November meteors, 400 
 
 Olmsted, radiant of Leonids, 399 ; 
 orbit, 400 
 
 Oppenheim, calculation of Schmidt's 
 comet, 439 
 
 Oppolzer, E. von, theory of sunspots, 
 198 
 
 Oppolzer, Th. von, Winnecke's comet, 
 
IXDEX. 
 
 567 
 
 116; position of Vulcan, 308; 
 comet of 1843, 423 
 
 Oxygen, no evidence of presence in 
 sun. 265, 267 ; telluric absorption, 
 
 2 ': - j 
 
 PAIISA, search for Vulcan, 308 ; dis- 
 coveries of minor planets, 347 
 
 Pallas, discovery, 91 ; inclination of 
 orbit, 92, 348 ; diameter, 93, 350 
 
 Pape, tails of Donari" s comet, 417 
 
 Parallax, annual, of stars, 12, 18-19, 
 498-500 ; illusory results, 39 ; of 61 
 Qygni, 43- Soo J of Vega, 43, 499, 500 ; 
 of a Centauri, 44, 498; of Sinus, 
 51, 498; horizontal, of sun, 280; 
 Encke's result from transits of 
 Venus, 283, 287 ; found too large, 
 284; improved value from opposi- 
 tions of Mars, 285, 292-3; from 
 light-velocity, 285-6, 297-8; from 
 recent transits, 291-2, 296; from 
 observations of minor planets, 293- 
 4 ; general result, 298 
 
 Paris Catalogue of Stars, 497 
 
 Pastorff, drawings of the sun, 125 
 
 Peirce, structure of Saturn's rings, 363 
 
 Perrotin, rotation of Venus, 310; 
 canals of Mars, 342; clouds on 
 Mars, 344 ; striation of Saturn's 
 rings, 364; rotation and compres- 
 sion of Uranus, 369, 370; changes 
 of Pons's comet, 443 ; Maia nebula, 
 493 ; measurements of double stars, 
 SM 
 
 Perry, Father, eclipse of 1886, 216; 
 eclipse of Dec. 1889 and death, 236 
 
 Personal equation, 153. 290 
 
 Peters, C. A. F., parallax of 61 Cygni, 
 43 ; disturbed motion of Sinus, 50 
 
 Peters, C. F. W., orbit of November 
 meteors and Tempers comet, 403 ; 
 orbit of 6 1 Cygni, 502 
 
 Peters, C. H. F., sunspot observations, 
 183, 184; discoveries of minor 
 planets, 347 ; star-maps, 347, 498 
 
 Peytal, description of chromosphere. 
 
 Phobos, rapid revolution, 346, 382, 
 389 ; mode of origin, 390 
 
 Photography, solar, 181, 191-2, 205 ; 
 of corona, 207, 216. 224, 228, 230, 
 232-36 ; without an eclipse, 224-6 ; 
 of prominences, 208, 244-6; of 
 coronal spectrum, 223 ; of promin- 
 ence spectrum, 242, 246; of solar 
 spectrum, 262-3, 524. S 2 ^ ; * elec- 
 tric-arc spectrum, 264 ; of Uranian 
 
 spectrum, 371 ; of cometary spectra 
 428, 432 ; of stellar and nebular 
 spectra, 462-5, 478, 481, 487-9; 
 lunar, 190, 329 ; detection of comets 
 by, 224, 447 ; of asteroids, 347 ; 
 use of, in transits of Venus, 288, 
 291, 294, 314; Mars depicted by, 
 344 ; Jupiter, 361 ; comets, 427-8, 
 446, 494 ; nebulae, 486, 489-92, 494, 
 509 ; Milky Way. 508 ; star-charting 
 b 7t 495-6; star-parallaxes deter- 
 mined by, 500 ; rapid improvement, 
 
 5*4 
 
 Photometry, stellar, 59, 503; of 
 planetary phases, 303, 350; of 
 Saturn's rings, 364-5 ; photo- 
 graphic, 504 
 
 Photosphere, named by Schroter, 66 ; 
 structure, 198, 204-5" 
 
 Piazzi, star-catalogues, 37 ; parallaxes, 
 39 ; proper motion of 61 Cygni, 
 42 ; birth and training, 88-9 ; dis- 
 covery of Ceres, 89 ; five-foot circle, 
 89,151 
 
 Picard, Saturn's dark ring, 107 ; sun's 
 distance, 282 
 
 Pickering, E. C., satellites of Mars 
 photometrically measured, 345 ; 
 minor planets, 350 ; variability of 
 Japetus, 368 ; of Neptune, 371 ; the 
 lost Pleiad, 458 ; gaseous stars, 459 ; 
 spectrographic results, 464 ; eclipses 
 of Algol, 470 ; spectrum of Nova 
 Cygni, 474 ; photometric cata- 
 logue, 503 ; photographic photo- 
 metry, 504 ; preponderance of 
 Sirian stars in the Milky Way, 
 509; climate of Arequipa, 520; 
 horizontal telescope, 523 
 
 Pickering, W. H., corona of 1886, 
 232-3 ; photographs of corona, Jan. 
 i, 1889, 235 ; lunar twilight, 325 ; 
 lunar volcanic action, 328 ; melting 
 of snow on Mars, 340 ; snow-fall, 
 344 ; photograph of Orion nebula, 
 491 ; establishment of observatory 
 at Arequipa, 520 
 
 Pingre, phenomena of comets, 114, 
 119 
 
 Planets, influence on sunspots, 202-3 ; 
 periods and distances, 281 ; in- 
 ferior and superior, 351 ; origin, 
 375, 3?8; relative ages, 382-4; 
 predictions of intra-Mercurian, 306 ; 
 pseudo-discoveries, 306-8 ; trans- 
 Neptunian, 372-3 
 
 Planets, minor, existence anticipated, 
 87-8; discoveries, 89, 91-2, 94, 
 
568 
 
 INDEX. 
 
 346-7 ; solar parallax from, 293-4, 
 296 ; distribution of orbits, 347-50 ; 
 collective volume, 350 ; atmo- 
 spheres, 351 
 
 Plantade, halo round Mercury, 301, 302 
 
 Pleiades, community of movement, 
 49 ; photographed spectra, 464 ; 
 photographs, 492-4 ; measurements, 
 492-3 ; nebulae, 493-4 ; systemic 
 union, 511 
 
 Pliicker, hydrogen in sun, 263 
 
 Plummer, solar translation, 47 ; 
 Encke's comet, 122 
 
 Plutarch, solar corona, 79 
 
 Pogson, prominence-spectrum, 209 ; 
 reversing layer, 214; discovery of a 
 comet, 407, 410 ; new star in 
 Scorpio, 477 
 
 Pond, errors of Greenwich quadrant, 
 33 ; controversy with Brinkley, 39 
 
 Pons, discoveries of comets, 112, 116, 
 442 
 
 Pontecoulant, return of Halley's 
 comet, 126 
 
 Poor, C. Lane, identification of Lex- 
 ell's comet, 445 
 
 Pouillet, solar constant, 268, 278 ; 
 solar temperature, 270 ; temperature 
 of space, 331 
 
 Poynting, mean density of the earth, 
 321 
 
 Prince, glow round Venus, 312 
 
 Pritchard, parallax of Algol, 471 ; 
 photographic determinations of 
 stellar parallax, 500 ; photometric 
 catalogue, 503 ; parallax of e Ursas, 
 5io 
 
 Pritchett, corona of Jan. i, 1889, 
 234 ; red spot on Jupiter, 358 
 
 Proctor, glare theory of corona, 229 ; 
 velocity of projection in sun, 254 ; 
 transit of Venus, 287 ; distance of 
 sun, 292 ; atmosphere of Venus, 
 312; rotation of Mars, 338; map 
 and canals of Mars, 341, 342 ; con- 
 dition of great planets, 353 ; Nova 
 Andromedas, 484 ; status of nebulas, 
 505 ; structure of Milky Way, 506 ; 
 star-drift, 509 
 
 Procyon, invisible companion, 50, 
 51 ; parallax, 500 
 
 Prominences, observed in 1842, 76-7, 
 84 ; described by Vassenius, 83 ; 
 observed 1851, 85; photographed 
 during eclipse, 208 ; without an 
 eclipse, 244-6 ; spectrum, 209, 223, 
 241-2,246-7; spectroscopic method 
 of observing, 210-12,241-3 ; white, 
 
 232, 245 ; two classes, 243 ; chemis- 
 try, 247 ; distribution, 247-8 ; 
 cyclonic movements in, 253; velo- 
 cities, 254-5 
 
 QUETELET, periodicity of August 
 meteors, 400 
 
 RANYARD, drawing of a sunspot, 125 ; 
 volume on. eclipses, 219 ; coronal 
 types, 233 ; Jupiter's markings, 
 362 ; meteors from fixed radiants, 
 413 ; production of comets' tails, 
 421 ; tenuity of nebulae, 491 
 
 Rayet, spectrum of prominences, 209, 
 212 ; gaseous stars, 459 
 
 Red spot on Jupiter, 358-61 
 
 Reduction of observations, 37 ; 
 Bessel's improvements, 38 ; Baily's, 
 72 
 
 Reed, observations of Jupiter's new 
 satellite, 358 
 
 Refraction, atmospheric, 37 ; in 
 comets, 132, 426 ; in Venus, 291, 
 
 311-13 
 Reichenbach, foundation of Optical 
 
 Institute, 34, 40, 151 
 Repsold, astronomical circles, 50, 151 ; 
 
 Cape heliometer, 499 
 Resisting medium, 115-7, 435-6 
 Respighi, slitless spectroscope, 217 ; 
 
 prominences and chromosphere, 
 
 241, 244, 247-8 ; solar uprushes, 
 
 255 ; spectrum of 7 Argus, 459 
 Reversing layer, detected, 213-4; 
 
 nature and extent, 215-6, 231 
 Riccioli, secondary light of Venus, 314 
 Ricco, distribution of prominences, 
 
 248 ; spectrum of Venus, 313 ; red 
 
 spot on Jupiter, 359 ; spectrum of 
 
 great comet, 442 
 Richer, distance of sun, 281 
 Ritter, development of stars, 454 
 Roberts, photographs of Nova Aurigas, 
 
 480 ; of Orion nebula, 490 ; of 
 
 Andromeda nebula, 491 ; of the 
 
 Pleiades, 494 
 Roberval, structure of Saturn's rings, 
 
 364 
 Robinson, reflectors and refractors, 
 
 5i6 
 
 Roche, inner limit of satellite-forma- 
 tion, 367 ; modification of nebular 
 hypothesis, 390 
 
 Romer, star-places, n ; invention of 
 equatoreal and transit-instrument. 
 149 ; of altazimuth, 151 ; satellite - 
 transit on Jupiter, 355 
 
INDEX. 
 
 569 
 
 Rosenberger, return of Halley's comet, 
 126 
 
 Rosetti, temperature of the sun, 
 271-2 
 
 Rosse, third Earl of, biographical 
 sketch, 142 ; great specula, 143-6 ; 
 discovery of spiral nebulae, 146 ; 
 resolution of nebulas, 147 ; climate 
 and telescopes, 519 
 
 Rosse, fourth Earl of, experiments on 
 lunar heat, 330 
 
 Rost, nature of sunspots, 64 
 
 Rowland, photographic maps of solar 
 spectrum, 262, 526 ; carbon in sun, 
 264 ; metallic elements, 265 ; con- 
 cave gratings, 525-6 
 
 Rtimker, observation of Encke's 
 comet, 112 
 
 Russell, red spot on Jupiter, 359 ; 
 change in Argo nebula, 485 ; photo- 
 graphs of Milky Way and nubeculse, 
 508 
 
 Rutherfurd, lunar photography, 329 ; 
 star-spectra, 451 ; photographs of 
 the Pleiades, 492 ; diffraction-grat- 
 ings, 5 2 5 
 
 SABINE, magnetic and sunspot 
 periods, 1 58 ; pendulum-experi- 
 ments, 321 
 
 Saf arik, secon dary light of Venus, 315; 
 compression of Uranus, 370 
 
 Satellites, discoveries, 103-5, i7-8, 
 137, 345> 357 ; transits, 355 ; 
 variability, 355-6, 368 ; origin, 375, 
 
 3*7 
 
 Saturn, low specific gravity, 362-3 ; 
 rotation, 367-8 ; spectrum, 368 
 
 Saturn's rings, first disclosure of, 
 105 ; dusky ring, 106-7 5 stability, 
 IO 5> 3^3. 366; meteoric constitu- 
 tion, 364 ; eventual dispersal, 367 
 
 Savary, orbits of double stars, 55 
 
 Sawerthal, discovery of a comet, 
 
 443 
 
 Schaeberle, coronal photographs, 
 235 ; theory of corona, 237-8 ; 
 discovery of a comet, 430 
 
 Schaeberle and Campbell, observa- 
 tions of Jupiter's satellites, 356, 
 
 Scheiner, nature of sunspots, 62, 64 ; 
 helioscope, 181 ; solar rotation, 
 182 ; darkening of sun's edge, 
 274 
 
 Scheiner, Dr. J., spectrographic re- 
 searches, 463, 487 ; stars and 
 nebulas in Orion, 489 
 
 Schiaparelli, rotation of Mercury, 
 304-5 ; of Venus, 309-10 ; spots on 
 Mars, 338 ; snow-cap, 340 ; canals, 
 341-3 ; compression of Uranus, 
 370 ; comets and meteors, 397, 
 402-3, 410 ; anomalous tail of great 
 comet, 440 ; Pons's comet, 443 ; 
 origin of comets, 448 ; measures of 
 double stars, 502 
 
 Schmidt, sunspot period, 156 ; lunar 
 rills, 323 ; lunar maps, 326 ; dis- 
 appearance of Linne, 328 ; discovery 
 of comet, 439 ; appendages to great 
 comet, 440 ; new stars, 473 
 
 Schonfeld, extension of Bonn Durch- 
 musterung, 497 
 
 Schrader, construction of reflectors, 
 299 
 
 Schroter, a follower of Herschel, 6 ; 
 biographical sketch, 299-300 ; ob- 
 servations on Mercury, 300-1, 
 303-4; on Venus, 308, 311, 314; 
 on the moon, 323 ; a lunar city, 
 326 ; Linne", 328 ; spots on Mars, 
 337, 338 ; halo round Vesta, 351 ; 
 Jovian markings, 353 
 
 Schulen, perspective effects in sun- 
 spots, 64 
 
 Schumann, spectrum of hydrogen, 
 482 
 
 Schuster, spectrum of corona, 223 ; 
 photographs, 224, 232; spectra of 
 oxygen, 266 
 
 Schwabe, discovery of sunspot peri- 
 odicity, 155-7 
 
 Secchi, chromosphere, 85 ; Biela's 
 comet, 120 ; cyclonic movements in 
 sunspots, 179; solar constitution, 
 1 88 ; depth of spots, 192 ; nature, 
 194, 197 ; eclipse-observations, 
 207-8 ; reversing layer, 214 ; ob- 
 servations of prominences, 241, 
 244, 247 ; absence of absorption by 
 helium, 265 ; temperature of sun, 
 270 ; solar atmospheric absorption, 
 274 ; Martian canals, 342 ; spec- 
 trum of Uranus, 370 ; of Coggia's 
 comet, 416 ; stellar spectral types, 
 45 1 ; carbon-stars, 452 ; gaseous 
 stars, 456 
 
 See, colour of Sirius, 453 note ; evo- 
 lution of stellar systems, 502 
 
 Seeliger, photometry of Saturn's rings, 
 
 364-5 
 
 Seidel, stellar photometry, 503 
 Sherman, spectrum of Nova Andro- 
 
 medae, 476 
 Short, reflectors, 4, 135, 143, 150 ; 
 
570 
 
 INDEX. 
 
 chromosphere, 83 ; satellite of 
 
 Venus, 316 ; striation of Saturn's 
 
 rings, 364 
 Sidereal science, foundation, 10, 528 ; 
 
 condition at opening, of present 
 
 century, 12 ; progress, 61 ** 
 
 Siemens, regenerative theory of the 
 
 sun, 379-80 
 Simony, photographs of ultra-violet 
 
 spectrum, 267 
 Sirius, a binary star, 50; mass, 51; 
 
 parallax, 51, 498 ; spectrum, 165, 
 
 452, 463 ; former redness, 453 note; 
 
 radial movement, 466-7 
 Smyth, Admiral, Donati's comet, 393 
 Smyth, Piazzi, lunar radiations, 329 ; 
 
 expedition to Teneriffe, 519 
 Solar constant, 268, 278-9 
 Solar spectrum, purified by use of a 
 
 slit, 165 ; fixed lines, 165-9 ; maps 
 
 byFraunhofer, i65; Kirchhoff, 169; 
 
 Lockyer, 256 ; Angstrom, 261 ; 
 
 Cornu, 262 ; Kowland, 262, 526 ; 
 
 Thollon, 262, 526 ; McClean, 262 ; 
 
 Higgs, 263 ; distribution of energy, 
 
 276-7 
 Solar System, translation through 
 
 space, 17, 47-8, 489 ; development, 
 
 374-6, 381-3, 391 ; complexity, 527 
 Soret, solar temperature, 270 
 South, Sir James, observations of 
 
 double stars, 55 ; 12-inchlens, 141 ; 
 
 Kosse reflector, 146 ; occultation by 
 
 Mars, 338 
 
 Spectroscopic binaries, 467-70, 471 
 Spectrum analysis, defined, 161 ; first 
 
 experiments, 162-4 > applied to the 
 
 sun, 162-4; an( i stars, 165, 450-1 ; 
 
 Kirchhoffstheorem, 168; elementary 
 
 principles, 172-4; effects on science, 
 
 175-6 ; radial motion determined 
 
 by, 250, 465 ; comets examined by, 
 
 414-6 ; new stars, 473 ; nebulas, 
 
 482-3 
 
 Spencer, position of nebulae, 505 
 Spitaler, attendants on Lexell-Brooks 
 
 comet, 444 
 Spitta, transits of Jupiter's satellites, 
 
 355 
 
 Sporer, solar rotation, 184, 185 ; 
 chromosphere, 248 
 
 Stannyan, early observation of chro- 
 mosphere, 83 
 
 Star-catalogues, 33, 37, 39, 72, 497 ; 
 spectroscopic, 460, 465 ; photo- 
 graphic, 495-6; photometric, 503 
 
 Star-drift, 509 
 
 Star-gauging, 23, 56 
 
 Star-maps, 95, 100, 347, 498 ; photo- 
 graphic, 495-6 
 
 Stars, movements, u, 42, 47-8, 497, 
 509 ; in line of sight, 486-8 ; photo- 
 metric estimates, 15, 59-60, 503 ; 
 distances, 42-6,498-500; chemistry, 
 450, 461 ; spectroscopic orders, 
 452 ; colours, 453 ; stages of devel- 
 opment, 453-6 ; wide range of real 
 magnitude,, 506; gregarious ten- 
 dency, 510 
 
 Stars, double, physical connexion 
 anticipated, 20 ; proved, 21-2, 528 ; 
 masses, 46, 51 ; catalogues, 52, 55, 
 60, 501 ; orbits, 55, 500; discoveries, 
 56, 57, 501, 520 ; photographs, 492 ; 
 evolution, 502 
 
 Stars, gaseous, 456-60 
 
 Stars, temporary, 29, 472-82 
 
 Stars, variable, first discoveries, 11-12; 
 rj Argus, 58-9, 458 ; sunspot ana- 
 !ogy, I59> 472 ; spectra, 458 ; Algol 
 class, 469-71 ; catalogues, 471-2 
 
 Stefan, law of cooling, 272 
 
 Steinheil, stellar photometry, 503 ; 
 silvered glass reflectors, 513 
 
 Stewart, Balfour, Kirchhoff 's principle 
 1 68 note; solar investigations, 192-3, 
 204 
 
 Stewart, Matthew, solar distance by 
 lunar theory, 284 
 
 Stokes, anticipation of spectrum 
 analysis, 171 
 
 Stone, E. J., reversal of Fraunhofer 
 spectrum, 214; distance of the sun, 
 285, 287, 291-2 ; transit of Venus, 
 295 ; Cape Catalogue, 497 
 
 Stone, 0., observation of Jupiter's 
 inner satellite, 358; cataloguing 
 stars, 497 
 
 Stoney, status of red stars, 454 
 
 Stroobant, satellite of Venus, 316 
 
 Struve, F. G-. W. , stellar parallax, 40, 
 43; career and investigations, 51-4; 
 occultation by Halley's comet, 131 ; 
 Russo-Scandinavian arc, 321 
 
 Struve, Ludwig, solar translation, 48 
 
 Struve, Otto, solar velocity, 48 ; his 
 father's successor at Pulkowa, 54 ; 
 eclipse of 1842, 75, 78 ; Neptune's 
 satellite, 103 ; changes in Saturn's 
 rings, 365-6 ; in Orion nebula, 485 
 
 Stumpe, solar translation, 48 
 
 Sun, the, Herschel's theory, 65-7, 86, 
 185 ; atmospheric circulation, 70-1 ; 
 chemical composition, 168, 263-5 > 
 mode of rotation, 182-4 ; Kirchhoff's 
 theory, 186 ; Faye's, 186-90 ; con- 
 
INDEX. 
 
 57i 
 
 vection currents, 187, 190, 205 ; 
 dissociation in, 189, 255-60 ; lumi- 
 nous outbursts, 198-200 ; explo- 
 sions, 254-5 ; heat emission, 268-9, 
 274, 278-9 ; temperature, 270-3 ; 
 problem of distance, 280 ; results 
 from transits of last century, 283. 
 287 ; from oppositions of Mars, 285, 
 293 ; from transit of 1874, 291-2; 
 from measurements of minor planets, 
 293-4, 296 ; from transit of 1882, 
 296 ; from light velocity, 297-8 ; 
 concluded value, 298 ; maintenance 
 of heat supply, 376-81 ; past and 
 future duration, 379 
 
 Sunspots, first speculations regarding, 
 62-3 ; Wilson's demonstration, 64 ; 
 distribution, 70, 184-5 > cyclonic 
 theory, 70, 179, 196 ; periodicity, 
 156, 159, 201-3 5 magnetic relations, 
 I 5^? 199-200, 202 ; meteorological, 
 160, 203-4; auroral, 161, 199, 201-2; 
 photographs, 181, 191-2 ; spectra, 
 174-5,257-8; volcanic hypothesis, 
 197 ; Lockyer's theory, 198 ; plane- 
 tary influence, 202 ; relation to 
 Jovian markings, 362 
 
 Swan, chromosphere, 85 ; sodium- 
 line, 163 
 
 Swift, supposed detection of Vulcan, 
 227, 307-8 ; discovery of a comet, 
 446 
 
 TACCHINI, spectrum of corona, 223 ; 
 eclipse of 1883, 227 ; white pro- 
 minences, 232 ; prominences and 
 chromosphere, 248 ; spectrum of 
 Venus, 313 
 
 Talbot, Fox, spectrum analysis, 163 ; 
 spectroscopic method of determin- 
 ing movements of binaries, 467 
 
 Tarde, nature of sunspots, 62 
 
 Taylor, eclipse- expedition, 236; spec- 
 trum of Uranus, 371 
 
 Tebbutt, comet of 1861, 396; comet 
 discovered by, 425 
 
 Telescope, achromatic, 139 ; Eosse, 
 144-8, 513 ; equatoreal, 149-50 ; 
 Common's five-foot, 513; Newall's 
 25-inch, 514 ; Lick 36-inch, 515, 
 518 ; Yerke's4O-inch, 517 ; proposed 
 Paris ten-foot, 518 ; Coude, 522-4 
 
 Telescopes, reflecting, Short's, 4, 143, 
 150; Herschel's, 14, 135-8; Lassell's, 
 103, 142, 150; Newtonian, 135, 136; 
 Cassegrainian, 136; Gregorian, 136; 
 silvered glass, 513; achromatism, 
 516 ; refractors, development of 
 
 power, 34, 517 ; difficulties, 515, 
 518,521; Fraunhofer' 5,40-1; Clark's, 
 141, 514-5; distribution in latitude, 
 52i 
 
 Tempel, red spot on Jupiter, 358 ; 
 comets discovered by, 397, 414 ; 
 observations on comets, 426, 439 ; 
 of Andromeda nebula, 475 ; dis- 
 covery of Merope nebula, 493 
 
 Temperature, of the moon, 331 ; of 
 space, 331 ; on Mars, 340 
 
 Teneriffe, Peak of, experimental ob- 
 servations from, 267, 330, 519 
 
 Tennant, eclipse-observations, 209, 
 211,217 
 
 Terby, surface of Mars, 341, 344; 
 secondary tail of comet, 430 
 
 Thalen, basic lines, 256 ; map of solar 
 spectrum, 261 ; solar elements, 263 
 
 Thollon, coronal spectrum, 223 ; line- 
 displacements by motion, 251, 441 ; 
 atlas of solar spectrum, 262, 526 ; 
 death, 263 ; lunar atmospheric ab- 
 sorption, 324 
 
 Thome, comet discovered by, 437 
 
 Thomson, Sir William (Lord Kelvin), 
 solar chemistry, 171 ; tidal strains, 
 316; rotation of the earth, 335; 
 dynamical theory of solar heat, 
 377-8, 380 
 
 Thraen, period of Wells's comet, 433 
 
 Tidal friction, effects on moon's 
 rotation, 333-4, 3 8 4 ; in lengthening 
 month, 384-5 ; effects on planets, 
 388-9, 391 ; on development of bi- 
 nary systems, 502 
 
 Tietjen, asteroidal orbits, 347 
 
 Tisserand, capture of comets, 122; 
 lunar acceleration, 334, 335 ; revo- 
 lutions of Neptune's satellite, 372 ; 
 director of Paris observatory, 496 
 
 Titius, law of planetary intervals, 87-8, 
 
 I0 5 
 
 Todd, eclipse of 1887, 233 ; solar 
 distance, 292, 297 ; trans-Neptunian 
 planet, 373 
 
 Tornaghi, halo round Venus, 313 
 
 Transit-instrument, 149 
 
 Trepied, reversal of Fraunhofer spec- 
 trum, 214 
 
 Troughton, method of graduation, 151 
 
 Trouvelot, veiled spots, 184 ; chromo- 
 sphere, in 1878,220; intra-Mercurian 
 planets, 227, 308 ; observations of 
 prominences, 231, 244, 253 ; of 
 Mercury, 301, 304 ; rotation of 
 Venus, 310; mountains, 314; red 
 spot on Jupiter, 361 
 
572 
 
 INDEX. 
 
 Trowbridge and Hutchins, carbon in 
 
 sun, 264 
 
 Tschermak, origin of meteorites, 411 
 Tupman, transit -expedition, 290 ; 
 
 results, 291 
 Turner, observations during eclipses/* 
 
 216, 233 
 
 ULLOA, eclipse of 1778, 83 
 
 United States, observatories founded 
 in, 8 
 
 Uranus, discovery, 5, 88, 138 ; unex- 
 plained disturbances, 96-7, 373 ; 
 satellites, 107-8, 369; equatoreal 
 markings, 370 ; spectrum, 370-1 ; 
 retrograde rotation, 381, 391 
 
 VALERIUS, darkening of sun's limb, 274 
 
 Valle, corona of 1886, 235 
 
 Vassenius, first description of pro- 
 minences, 83 
 
 Venus, transits, 5, 282-3, 287 ; transit 
 of 1874, 287-92; of 1882, 294-6; 
 atmosphere, 291, 311-3; rotation, 
 308-10; mountains, 310-1, 314; 
 spectrum, 313; albedo, 314; secon- 
 dary light, 315 ; pseudo-satellite, 
 315-6; effects upon, of solar tidal 
 friction, 389 
 
 Very, lunar heat, 331 
 
 Vesta, discovery, "92 ; diameter, 93, 
 350 ; spectrum, 351 
 
 Vicaire, solar temperature, 271 
 
 Vico, comet discovered by, 121 ; ro- 
 tation of Venus, 309, 310 ; moun- 
 tain, 311 
 
 Violle, solar temperature, 270, 271; 
 solar constant, 278 
 
 Vogel, H. C., spectroscopic effects of 
 solar rotation, 250 ; solar atmo- 
 spheric absorption, 275, 278 ; spec- 
 trum of Mercury, 303 ; of Venus, 
 313 ; of Vesta, 351 ; of Jupiter, 
 354 ; of Jupiter's satellites, 356 ; 
 of Uranus, 370 ; rotation of Venus, 
 310 ; secondary light, 315 ; come- 
 tary spectra, 414, 416, 429, 432 ; 
 carbon in stars, 452 ; stellar devel- 
 opment, 454 ; spectrum of 7 Cassio- 
 peias, 456 ; of Betelgeux, 461 ; of 
 Nova Cygni, 474 ; of Nova Andro- 
 medae, 476 ; spectroscopic star- 
 catalogue, 460 ; radial motion of 
 Sirius, 466 ; eclipses of Algol, 470 ; 
 components of Nova Aurigae, 479 ; 
 spectrographic determinations of 
 stellar radial motions, 487-8 
 
 Vogel, H. W., spectrum of hydrogen, 
 256 note, 463 
 
 Vulcan, existence predicted, 306 ; 
 supposed discoveries, 306-8 
 
 WAED, Nova Andromedas, 475 
 
 Wartmann, occultation by a comet, 
 132 
 
 Waterston, solar temperature, 270 ; 
 meteoric infalls, 377 
 
 Watson, supposed discovery of Vul- 
 can, 227, 307-8 
 
 Webb, comet of 1861, 395-6 
 
 Weber, illusory transit of Vulcan, 307 
 
 Weinek, discovery of a lunar crater, 
 
 3 2 9 
 Weiss, comets and meteors, 403, 405, 
 
 406 
 
 Wells, comet discovered by, 431 
 Wesley, drawings of corona, 219 
 Wheatstone, spectrum of electric 
 
 arc, 164 ; method of determining 
 
 light-velocity, 286 
 Whewell, stars and nebulas, 506 
 Williams, A. Stanley, canals of Mars, 
 
 342 ; markings on Jupiter, 362 
 Williams, Mattieu, condition of major 
 
 planets, 353 
 Wilsing, solar rotation from faculae, 
 
 193 ; mean density of the earth, 321 
 Wilson, Alexander, perspective effects 
 
 in sunspots, 64, 192 
 Wilson, H. C., compression of Uranus, 
 
 37 
 Winnecke, comet discovered by, 116 ; 
 
 albedo of Mercury, 303 ; Donati's 
 
 comet, 393, 420 
 Wisniewski, last glimpse of 1811 
 
 comet, 122 
 Wolf, C., objections to Faye's 
 
 cosmogony, 384 ; origin of Phobos, 
 
 390 ; gaseous stars, 459 
 Wolf, Max, photographic discovery of 
 
 minor planets, 347 ; Nova Andro- 
 
 medse, 475 ; Nova Aurigas, 477, 
 
 480 ; nebulas in Cygnus, 508 
 Wolf, K., sunspot and magnetic 
 
 periodicity, 159, 201-2 ; sunspots 
 
 and variable stars, 159, 472; 
 
 auroras, 160 ; suspicious transits, 
 
 307 
 
 Wollaston, ratio of moonlight to sun- 
 light, 60 ; flame-spectra, 163 ; lines 
 in solar spectrum, 165 
 
 Woods, C. Ray, coronal photography, 
 225, 227 ; Cape Durchmusterung, 
 
 495 
 
 Wrangel, auroras and meteors, 407 
 Wright, G. F., Ice Age in North 
 
 America, 320 
 
INDEX. 
 
 573 
 
 Wright, Prof., polarisation of comet- 
 
 ary light, 429 
 Wright, Thomas, theory of Milky 
 
 Way, 1 6 ; structure of Saturn's 
 
 rings, 364 
 
 YEKKES, donation of a superlative 
 telescope, 517 
 
 Young, C. A., spectrum of sunspots, 
 195 ; origin, 196 ; spectrum of 
 corona, 213 ; reversing layer, 
 213-4 ; corona of 1878, 220, 222 ; 
 eclipse of 1887, 233 ; prominences 
 and chromosphere, 241-2 ; photo- 
 graph of a prominence, 244 ; 
 spectroscopic measurement of sun's 
 rotation, 251 ; solar cyclones and 
 explosions, 253-4 ; basic lines, 256 ; 
 spectrum of Venus, 313 ; red spot 
 on Jupiter, 359 ; observations on 
 Uranus, 369-70 ; Andromedes of 
 1892, 410 ; spectrum of Tebbutt's 
 comet, 429 ; of Nova Aurigse, 476 
 
 Young, Thomas, absorption-spectra, 
 169 
 
 ZACH, Baron von, promotion of 
 
 astronomy, 6, 33 ; Baily's beads, 
 
 74 ; search for missing planet, 88 ; 
 
 re-discovery of Ceres, 91 ; use of a 
 
 heliostat, 150 
 Zantedeschi, lines in solar spectrum, 
 
 1 66 ; lunar radiation, 329 
 Zenger, observations on Venus, 311, 
 
 315 
 
 Zenker, mode of production of comet's 
 tails, 421 
 
 Zezioli, observation of Andromedes, 
 406 
 
 Zodiacal light and resisting medium, 
 117; relation to solar corona, 220, 
 239 ; meteoric constitution, 377 
 
 Zollner, electrical theory of comets, 
 123, 416, 418, 420 ; solar constitu- 
 tion, 197 ; observations of promin- 
 ences, 241, 243 ; classification, 244; 
 reversion- spectroscope, 256 ; solar 
 temperature, 273 ; Mercurian 
 phases, 303 ; condition of Venus, 
 315; of great planets, 352 ; albedo 
 of Mars, 346 ; of Jupiter, 354 ; of 
 Uranus, 371; Jovian markings, 
 362 ; ages of stars, 453 
 
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