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C2t UNIFORM BDITION. ——_ SS THE PRACTICAL ASTRONOMER, COMPRISING ILLUSTRATIONS OF LIGHT AND COLOURS; PRACTICAL DESCRIPTIONS OF ALL KINDS OF TELESCOPES ; THE USE OF THE EQUATORIAL, TRANSIT, CIRCULAR, AND OTHER ASTRONOMICAL INSTRUMENTS}; A PARTICULAR ACCOUNT OF Che Earl of Rosse’s Large Celescopes, AND OTHER TOPICS CONNECTED WITH ASTRONOMY BY THOMAS DICK, LL.D. AUTHOR OF ‘‘ THE CHRISTIAN PHILOSOPHER,” “ CELESTIAL SCENERY,” “HE SIDEREAL HEAVENS,” ETC. ILLUSTRATED WITH ONE HUNDRED ENGRAVINGS. VOL. IX. PHILADELPHIA: H. C. & J. BIDDLE, No. 39 SOUTH FOURTH STRERT, 1854. 6 PREFACE. light—of the laws by which it is refracted and reflected when passing through different mediums, and of the effects it produces in the system of nature—in order to prepare the way for a clear understanding of the principles on which optical instruments are constructed, and the effects they produce. As this, as well as every other physical subject, forms a part of the arrangements of the Creator throughout the material system, the author has occasionally taken an op- portunity of directing the attention of the reader to the Wisdom and Beneficence of the Great First Cause, and of introducing those moral reflections which naturally flow from the subject. The present is the ninth volume which the author has presented to the public, and he indulges the hope that it will meet with the same favourable reception ‘which his former publications have uniformly experienced. It was originally intended to conclude the volume with a few remarks on the utility of astronomical studies, and their moral and religious tendency, but this has been prevented, for the present, in consequence of the work having swelled to a greater size than was anticipated. Should he again appear before the public as an author, the subject of dis- cussion and illustration will have a more direct bearing than the present on the great objects of religion and a future world. Broughiy Ferry, near Dundee, August, 1845. CONTENTS. PART I. iN sd: Gee T. INTRODUCTION. Necussity of Light to the Knowledge and Happiness of all Sentient Bemgu—Its beautiful and enlivening Effects—An Emblem of tne Deity -Provision made for its universal Diffusion.........Page 17-21 CHAPTER I. GENERAL PROPERTIES OF “LIGHT. Interesting Nature of this Study—Different Hypotheses which have been formed respecting the Nature of Light—It radiates in straight Lines— Moves with amazing Velocity—F lows in all Directions from luminous Bodies—Duration of its Impressions on the Eye—Supposed to have a certain Degree of Force or Momentum—Experiments in relation to this Point—Its Intensity diminished in Proportion to the Square of the Distance—Its Reflection from opaque Bodies renders Objects visible— Intensity of reflected Light—Subject to the Law of Attraction—Forms a constituent Part of certain Bodies—Solar Phosphori, and the Pheno- mena they exhibit—Produces certain Effects on Plants and Flowers, exemplified in a Variety of Instances—Supposed to have an Influence on the Propagation of Sound: ..+sss000eresseeecenes seessiece 21-41 Reflections on the Nature of Light, and the multifarious Effects it pro- duces throughout the Universe—A Representation of the Divinity— Wisdom and Goodness of God displayed in its Formation..... 41-43 CHAPTER | II. ON THE REFRACTION OF LIGHT. Nature of Refraction—Illustrated by Experiments—Angle of Refraction —Familiar Experiments illustrative of Refraction—Refraction explains the Causes of many curious and interesting Phenomena—Its Effect on the heavenly Bodies—On the 'T'wilight—lIllustrated by Figures 43-45 EXTRAORDINARY CASES OF REFRACTION IN RELATION TO TERRESTRIAL OBJECTS, Extraordinary Appearance of the Coast of France from Hastings—Ap- pearance of a Ship seen by Captain Colby, beyond the Coast of Caith- ness—Scoresby’s View of his Father’s Ship when beyond the Horizon —Phenomenon near the Himalaya Mountains—Bell Rock Light-house —Summary Statement of the diversified Effects of Refraction—Reflec- tions on the beneficent and diversified Effects produced by the Law of 7 8 CONTENTS. Refraction—It increases the Length of the Day, particularly in the Polar Regions—Is the Cause of that Splendour which appears in the Objects around us—Quantity of Refraction in respect to terrestrial Objects, and its Utility—Its Effects may be more diversified in other Wold. occ decccccccccccevevesecccevesdceccecess cscs PF Age O1-OF CHAPTER III. ON THE REFRACTION OF LIGHT THROUGH SPHERICAL TRANSPARENT SUB- STANCES, OR LENSES. Refraction the Foundation of optical Instruments—Various Forms of Lenses—Parallel, converging, and diverging Rays—Illustrated by Dia- rams—Concave Lenses, their Effects, and how to find their Focal Thataabec-aTsenoic formed by convex Lenses—Illustrated by Experi- ments—Principles in relation to Images formed by Lenses—Their magnifying Powers, &C....ccccceeceeeccccvsesesevesescescee: 58-60 REFLECTIONS DEDUCED FROM THE PRECEDING SUBJECT. Property of the Rays of Light in forming Images of Objects—W onderful Results and Discoveries which have flowed from this Property in rela- tion to our Knowledge of the Scenery of the Heavens and the minute Parts of Nature, and of our Views of the Attributes of Deity... 66-69 CHAPTER IV. ’ ON THE REFLECTION OF LIGHT. Nature of Reflection—Plane, convex, and concave Speculums—Angle ~ of Reflection—Reflection of Objects from Plane Mirrors, illustrated by Figures—Reflection by Convex and Concave Mirrors—Properties of Convex Mirrors, and the purposes to which they are applied—Proper- ties of coneave Speculums, and their Utility—Of the Images formed by concave Speculums—Illustrated by a Variety of Figures and Ex- periments—Their Power of magnifying and burning—Amusing De- ceptions produced by—Resemblance between the Properties of convex Lenses and concave Mirrors—Quantity of Light reflected by polished PATIO COS 15 pave oo bishop bia wed wip wp bee's WS 6 pags v SPURS. ofc PRI FOCO UNCOMMON APPEARANCES OF NATURE PRODUCED BY THE COMBINED INFLU- ENCE OF REFLECTION AND REFRACTION. Fata Morgana—The Mirage—Inverted Images of Ships seen in the Horizon—Appearance of Dover Castle at Ramsgate—Spectre of the Brocken—Scenes in the Highlands of Scotland—Large Cross seen at Migné in France—Dr. Wollaston’s Illustrations of such Phenomena— Utility of Science in dissipating superstitious Fears........... 88-96 REMARKS AND REFLECTIONS IN REFERENCE TO THE PHENOMENA DESCRIBED ABOVE. Light, the Beauty of the Universe, and a Symbol of the Divinity—In other orlds it may produce an infinite Variety of sublime Scenery. .96-98 vHAPTER V. SECT. I.—ON THE COLOURS OF LIGHT. Colours, the Beauty of Nature—Opinions which were formerly enter- tained respecting their Cause—Sir I. Newton’s Experiments with the CONTENTS. | 9 Prism—Colours and Phenomena produced by the Prism—Imperfection of optic Lenses—Various Lllustrations—Differently coloured Rays haye not the same illuminating Power—Heating and chemical Properties of some of the Rays of the solar Spectrum—Property of communicating the magnetic Power—Fraunhofer, and his Discoveries in reference to the Spectrum—Experiments on white and coloured Light. . Page 98-108 SECT. Il.—ON THE COLOURS OF NATURAL OBJECTS. Colours not in the Objects themselves, but in the Light which falls upon them—lllustrations of this Position—Atmosphere the Source of a Va- riety of Colours—Various natural Phenomena, in relation to Colour, exp MINOU aie v ese asic c cd oP wh oc aetbie aVilGhWAs ofc eve cet Tepe vee oe 108-112 SECT. II.—PHENOMENA OF THE RAINBOW. Rainbow described—Experiments to illustrate its Cause—Descriptions of its various Phenomena, and optical Explanations of their Causes— Rainbows exhibiting complete Circles—Their Appearance in different Countries—Summary View of the principal Facts respecting the Rain- bow—Lunar Rainbows—Scriptural Allusions to the Rainbow—W he- ther there was any Rainbow before the Deluge...........+- 112-122 SECT. IV.—REFLECTIONS ON THE BEAUTY AND UTILITY OF COLOURS. Beauty and Variety derived from Colours in the Scenery of Nature— Colours produced by the Atmosphere in different Countries—W hat would be the Aspect of Nature, in Heaven and on Earth, were there only one Colour—How it would affect the common Intercourse and Employments of. Society—Wisdom and Beneficence of the Creator displayed in the Diversity of Colours—Throughout all the Systems of the Universe a Diversity of Colours prevails—This Subject has a Ten- dency to inspire us With Gratitude... ssecescccceccseeerese 122-129 PARLE. Ih. ON TELESCOPES. CHAPTER I. HISTORY OF THE INVENTION OF TELESCOPES. The Telescope a noble Instrument—Effects it produces—Whether known to the Ancients—Friar Bacon’s Ideas respecting Telescopes—First constructed in Holland—The Invention claimed by different Persons— Galileo’s Account of the Construction of his Muheisdpesuibiecerateh which he made with this Instrument—How his Discoveries were re- ceived by the Learned—Specimens of learned Nonsense brought for- ward by pretended Philosophers—Supposed Length of Galileo’s Tele- scope—Various Claimants to the Invention of this Instrument.129-138 CHAPTER IL, OF THE CAMERA OBSCURA. Appearance of Objects in a Camera Obscura—The dark Chamber— This Instrument serves to explain the Nature of a refracting Telescope 10 CONTENTS. —Particulars to be attended to in exhibiting Objects with the Camera —lIt illustrates the Nature of Vision—Revolving Camera Obscura—- Portable Camera os sesesvccccccsevcevceteeceecvesessbage 139-147 THE DAGUERREOTYPE. An important Discovery for fixing the Images produced by the Camera— Description of the Daguerreotype Process—Preparation of the Plate, fixing the Impression, &c.—Preparation of photogenic Paper—Bene- ficial Effects which this Art may produce—Representations of Objects in the Heavens; &6. Fe seiec eve siee'ec ce casccececceseseceleee 147153 = CHAPTER III. ON THE OPTICAL ANGLE, AND THE APPARENT MAGNITUDE OF OBJECTS. Various Illustrations of the apparent Magnitude of Objects—Fallacies in relation to apparent Magnitudes—Apparent Magnitude in the Heavens —Difference between absolute and apparent Magnitudes......154-159 CHAPTER IV. ON THE DIFFERENT KINDS OF REFRACTING TELESCOPES. SECT. I.—THE GALILEAN TELESCOPE. Construction and peculiar Properties of this Instrument.....-- 159-161 SECT. Il.—-THE COMMON ASTRONOMICAL REFRACTING TELESCOPE. Description of its Nature and Construction—How its magnifying Power is determined—Table of the linear Aperture, magnifying Powers, &c., of astronomical Telescopes from 1 to 120 Feet in Length—Summary View of the Properties of this Telescope.......sseseeeeeees 161-166 SECT. II.—THE AERIAL TELESCOPE. This Telescope is used without a Tube—Description of the Apparatus connected with it, illustrated with Figures—Huygens’s, Hartsocker’s, and Cassini’s large Telescopese«sseeesseeseccceccceseseess 166-168 SECT. IV.—THE COMMON REFRACTING TELESCOPE FOR TERRESTRIAL OBJECTS. Arrangement of its Lenses—Magnifying Power—Manner in which the Rays of Light are refracted through the Telescopes now described 168-171 SECT. V.—TELESCOPE FORMED BY A SINGLE LENS. Various Experiments in relation to this Point—Experiments with a Lens 26 focal Distance, and 113 Inches Diameter..........-+++++ 171-173 SECT. VI.—THE ACHROMATIC TELESCOPE. Imperfections of common refracting Telescopes—Dollond’s Discovery— Newton’s Error—Explanation of the Principle of Achromatic Tele- scopes—Combination of Lenses—Difficulties in the Construction of such Instruments—Difficulty in procuring large Disks of flint Glass— Guinaud’s Experiments + cesscccesvccccceseesscssceceecees 173-182 NOTICES OF SOME LARGE ACHROMATIC TELESCOPES ON THE CONTINENT, AND IN GREAT BRITAIN. The Dorpat Telescope—Sir J. South’s Telescope—Captain Smyth’s— Rev. Dr. Pearson’s—Mr. Lawson’s—Mr. Cooper’s—Mr. Bridges’s, &e.—Achromatics in Cambridge and Paris Observatories... 182-185 CONTENTS. 11 ACHROMATIC TELESCOPES OF A MODERATE SIZE, WITH THEIR PRICES, AS » SOLD BY LONDON OPTICIANS. The 2$ Feet Achromatic—The 3} Feet—The Powers applied to it, and the Views it gives of the heavenly Bodies—The 5 Feet Achromatic— Stands for Telescopes, illustrated by Engravings.........+. 185-193 PROPORTIONS OF CURVATURE OF THE LENSES WHICH FORM AN ACHROMATIC OBJECT-GLASS, . Various Tables and Explanations wovccccccacccccescnsesvecse 1997196 ACHROMATIC TELESCOPES COMPOSED OF FLUID LENSES. Blair’s fluid Telescope, with an Account of its Performance—Barlow’s large refracting Telescope, with a fluid concave Lens—Its Construc- tion, and the Effect it produces on double Stars, &c.—Rogers’s Achro- matic 'T'elescope ona new Plan—Wilson’s Telescope &c...+. 196-206 CHAPTER V ON REFLECTING TELESCOPES. SECT. I.—HISTORY OF THE INVENTION, AND A GENERAL DESCRIPTION OF THE CONSTRUCTION OF THESE INSTRUMENTS. Gregory’s Reflector—Newtonian Reflector—Cassegrainian Reflector— Magnifying Powers of Reflectors—Short’s Reflectors—Their Powers and Prices—General Remarks on Gregorian Reflectors—Apertures and magnifying Powers of Newtonian Telescopes—Prices of Reflecting Telescopes. occecsecccssccccccccccsecsssccssesssesesssese QO-218 SECT. II.—THE HERSCHELIAN TELESCOPE, Description of Sir W. Herschel’s 40 Feet Telescope, with its Machinery, Apparatus, and the Discoveries made by it—Sir J. Herschel’s 20 Feet Reflector eee eee ae eseeeeee eee eeee ee eeeeeeseeseeeeeeeeaseseeeee 218-223 SECT. Il.—RAMAGE’S LARGE REFLECTING TELESCOPE... 223-224 SECT. IV.—THE AERIAL REFLECTOR—CONSTRUCTED BY THE AUTHOR. Construction of this Telescope, and the Manner of using it—Illustrated by Figures—Its Properties and Advantages—Tube not necessary in reflecting Telescopes—How a large Reflector might be constructed without a Tube—How the Form af a ‘Telescope may be used for view- ing Perspectives... sccsesecsecccseccccvcteveseccecsccsees Q24—I34 SECT. V.—EARL OF ROSSE’S REFLECTING TELESCOPES. His Mode of forming a large Speculum, &c.: see also Appendix 234-237 SECT. VI.—REFLECTING TELESCOPES WITH GLASS SPECULA. Various Experiments on this Subject, with their Results...... 237-238 SECT. VII.—A REFLECTING TELESCOPE WITH A SINGLE MIRROR AND NO EYEPIECE. Experiments illustrative of this Construction.........+sseee0s 238-241 ON THE EYEPIECES OF TELESCOPES—ASTRONOMICAL EYEPIECES. Huygenian Eyepieces—Ramsden’s Eyepiece—Aberration of Lenses-- Celestial Eyepieces with variable Powers—Diagonal Eyepieces—Va 12 CONTENTS. rious Forms of them described—Various Aspects in which Objects may be viewed by them bec ece eee ee ee | 241-249 SECT, II.-—-TERRESTRIAL EYEPIECES. Eyepieces with four Lenses—Proportions of the focal Lengths of these Lenses—Dimensions and Powers of several Eyepieces stated 249-253 DESCRIPTION OF AN EYEPIECE, &c., OF AN OLD DUTCH ACHROMATIC TELE- SCOPE. This telescope supposed to have been inverted in Holland before Dol- lond’s discovery was known—Peculiarity of its Eyepiece ... 253-256 ¢ SECT. UI,—DESCRIPTION OF THE PANCRATIC EYETUBE--... 256-258 CHAPTER VI. “MISCELLANEOUS REMARKS IN RELATION TO TELESCOPES. 1. Adjustments requisite to be attended to in the Use of Telescopes.—2. State of the Atmosphere most proper for observing terrestrial and ce- lestial Objects.—Average Number of Hours in the Year fit for celestial ‘Observations.—3. On the magnifying Powers requisite for observing the Phenomena of the different Planets.—Comets.—Double Stars, &c. —Illustrated at large from p. 285-294.—4. Mode of exhibiting the solar Spots.—Eyepieces best adapted for this Purpose.—How they may be exhibited to a large Company.—Mode in which their Dimensions may be determined.—5. On the space-penetrating Power of 'Telescopes.— Herschel’s Observations on space-penetrating Powers.—Comparison of Achromatic and Gregorian Reflectors.—6. On choosing Telescopes, and ascertaining their Properties.—Various Modes of ascertaining the Goodness of Telescopes. —General Remarks and Cautions on this Point. —A Circumstance which requires to be attended to in using Achro« matics.—7. On the Mode of determining the magnifying Power of Tele- scopes.—Various Experiments in relation to this Point.—8. On clean- ing the Lenses of Telescopes «-...seececssceceececsevesees DO-289 ON MEGALASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS. Mode of adapting a Telescope for this Purpose.—Objects to which they may be applie CoH HOHE REHOME HO HEH HOH EH EHH RO HO HAH EHS 289-292 REFLECTIONS ON LIGHT AND VISION, AND ON THE NATURE AND UTILITY OF TELESCOPES. Wonderful and mysterious Nature of Light.—The Organ of Vision, and its expansive Range.—Wonderful Nature of the Telescope, and the Objects it has disclosed to View.—No Boundaries should be set to the Discoveries of Science and the Improvement of Art.—The Telescope is a Machine which virtually transports us to the distant Regions of Space.—It enlarges our Views of the sublime Scenes of. Creation.—It has tended to amplify our Conceptions of the Empire and the Attri- butes of the Deity.—Various Uses of this Instrument im relation to Science and common Life eevee oeveeweseeeceeseoseoa eee ease one 292-305 CHAPTER VII. ON THE METHOD OF GRINDING AND POLISHING OPTICAL LENSES AND SPECULA. 1. Directions for grinding Lenses for Eyeglasses, Microscopes, &¢.— 2. Method of casting and grinding the Specula of reflecting ‘Telescopes, CONTENTS. 13 Compositions for speculum Metal.—To try the figure of the Metal. —'T'o adjust the Eyehole of Gregorian Reflectors.—T’o centre the Spec- ula.—T'o centre Lenses,e+.oececactsccceede ues onaneapheamsOOrole PART III. ON VARIOUS ASTRONOMICAL INSTRUMENTS. CHAPTER I. ON MICROMETERS. Various Descriptions of Micrometers.—Cavallo’s Micrometer described. —To ascertain the Value of its Divisions.—Practical Uses of this Mi- crometer.—Problems which may be solved by it.—T ables for facilitat- MA its Uses eveevel ives She Sue Caled Aeiele! sie ols se Wousl Ver8asd19 CHAPTER II. ON THE EQUATORIAL TELESCOPE, OR PORTABLE OBSERVATORY. History of Equatorials.—Description of one of the simplest Construction of these Instruments.—To adjust the Equatorial for Observation.—To adjust the line of Sight.—Description of the Nonius.—To find the Me- ridian Line by one Observation.—Manner of observing Stars and Plan- ets in the Daytime.... eevee soeee we ttt dS oles estén nate cg Wa eu ae LOO eO OBSERVATIONS, BY THE AUTHOR, ON THE FIXED STARS AND PLANETS, MADE IN THE DAYTIME, BY THE EQUATORIAL. Object of these Observations.—Stars of the first and second Magnitudes. —General Deductions from these Observations......+.+++++ 326-330 OBSERVATIONS ON THE PLANETS IN THE DAYTIME. Series of Observations on Venus, when near the Sun.—Seen at the Time of her superior Conjunction in 1843.—Conclusions deduced from these Observations.—Phenomena observed during these Observations.—Re- _ markable Phenomena during an Eclipse of the Sun........+ 330-338 OBSERVATIONS ON JUPITER AND OTHER PLANETS. General Conclusions, &c5. ooo c'vsccsecesoaccsecsccncsncssves 300-541 UTILITY OF CELESTIAL DAY OBSERVATIONS....++ 341-345 ON THE ASTRONOMICAL QUADRANT «+eeseeee 345-348 THE ASTRONOMICAL CIRCLE sooecsecseess 348-352 THE TRANSIT INSTRUMENT +++e+-- ¢cmoeve 352-359 CHAPTER III. ON OBSERVATORIES. Leading Features of a Spot adapted for celestial Observations.—Public Vox. IX 2 14 CONTENTS. and private Observatories.—Greenwich Observatory.—Instruments with which an Observatory should be furnished —The Author’s pri- vate Observatory.—Revolving domes for Observatories.—Cautions to be attended to in celestial Observations. .......s+e+eeessees 305-362 CHAPTER IV. ON ORRERIES OR PLANETARIUMS. History of such Machines.—Sphere of Archimedes and Posidonius.—Dr. Long’s Uraniwm.—Wheel-work of the common Planetarium.—Figure representing this Machine.—Problems which may be prio by : 362-369 DR. HENDERSON’S PLANETARIUM. Section of its Wheel-work.—Number of Teeth in the Wheels and Pin- ions which move the different Planets —Extreme accuracy of these Movements Coe eorPeo ere ee ese oversee eee He eeeBEHeeTEFesEeHTesZH ee eH Ree 370-376 ON THE VARIOUS OPINIONS WHICH WERE ORIGINALLY FORMED OF SA- TURN’S RING, ILLUSTRATED WITH 13 VIEWS. When and by whom its true Figure was discovered...+.++.+++ 376-379 ON THE SUPPOSED DIVISION. OF THE EXTERIOR RING OF SATURN. Kater’s, Short’s, Quetelet’s, and Decuppis’s Observations .... 379-382 APPENDIX. 1. DESCRIPTION OF THE EARL OF ROSSE’S LARGEST TELESCOPE. Composition of the Speculum, and the Process of casting it.—Mode of grinding and polishing it—Manner in which it is filled up.—Expenses incurred in its Construction.—Results of Observations which have been made with it—'T wo Views representing this Instrument and the Buildings connected with it.—Sir J. South’s Remarks and Anticipa- tions CeCe o eee se eeeeeses ese oeseseeeeeseesEeeeseeceseesereseoese 883-392 2. HINTS TO AMATEURS IN ASTRONOMY RESPECTING THE CONSTRUCTION OF TELESCOPES; 0's ste gees seb ees ces 395-596 LIST OF ENGRAVINGS. FIGURE PAGE 1. Representation of the diminution of the intensity of light....... 31 2. Illustrative of the refraction of light....+esceceseseccecsecseese 44 3. Representing the angles of incidence and refraction....++++-.+- 45 4. The refraction of the ecipgs a uh Wa os nie Sarees chiveoue et. pe HO 5. Various forms of lenses . wiitalevlnh Jeieds ovasis'e ae cgreigenee ds US 6, 7, 8. Parallel, converging, “and diverging TAYS slo Sec esccceses se 60 9,10, 11. Passage of ER Sree and converging rays through convex lenses. . datpibie ware ait'a ele bio elaine YOO 12. Passage of parallel rays ‘through « concave lenses...esecosecsess 62 13. Images formed by convex lensese++sercecsesccescovcccescoess G4 14. Angle of incidence and reflection..... ed acewVelo edt aenegeayeds Ob 15. Images as reflected from a plane MIITOFr «-.eeceeecsccesceceess FL 16. Illustrative of reflections from a plane MITTOr.++++eeeeeeeeeeees 72 17. Showing how the image in a plane mirror is twice the length of the RUGEEE sess dees davone at doe ve se eo dnwiea We biplonia asus t's 1Nee 18. Reflection from concave MILTOTS «+ ++seeseeeeeeeeceeseeeseeees Th 19. Reflection from conver Mirr0rs’s.esscevccerdcccdisscvsvocscoes, VO 20. Parallel rays as reflected from concave MIIrOrs.-+sseeceeeeeees 77 21. Diverging rays as reflected from concave MirrOrs.+++eeeeeeeees 77 22. Images formed before concave Mirrors. +++. eeeeeeeeeseeeeeeees 79 23. Images formed behind concave Mirrors. .+.sesececeecceseesess 80 24. Illustrating the magnifying power of concave mirrors.+-+-++++- 81 25. Inverted images formed in the front of concave MIITOTS+++eee+- 82 26. Llustrative of deceptions produced by concave mirrors.......-- 84 27, 28. Experiment with a bottle half filled with water.......++. 84, 85 0 29. Effect of extraordinary refraction on ships at S€a...+eeeseecees 30. Experiment for illustrating “s causes ae uncommon refraction. - SIS PURO ONS CIA is «re. hs he BS Lao ths ssl dloleas dew ovlesines 32. Different foci of coloured rays in convex lenses. .-seccseccccoes 33. Experiment to show the different foci of red and violet rays .... 34. Illustrative of the prismatic COlOUTS.....eeeeeececccevececcccs 35. Explanatory of refraction and reflection from drops of rain... +. 36. Explanatory of the rainbow....-.e+seseeees Sin. 5 iat Sha octane Raith 37. Images of objects formed ina oe chamber emokateiin k's taguieah a 88. The revolving camera obscura - a Mason's haven cee week atetE 40. The portable camera obscura « ele view a WON Ualdw cue oat aay 40*, 41, 42. Illustrative of the-angle of v vision, and the BE PaHDS mag- nitude OP ODIO oo oc olsen ned atiadipiend die cin o's atte cee meee LD 43. The Galilean telescope.. Sede a eee Pema ap eens coe wea dee wet 44. The astronomical telescope ..... Cesecve 6 O00 on td ee edestewave 45. The aerial refracting telescope «+ 0 isso. cece sens sccecegvcces 47. The common refracting telescope...... eheenddwSaehe sans 6 48, 49, 50. Manner in which the rays of light + are refracted in tele- scopes - ee ewr eee ee ere eer ee eee eee eee eaeereeeweeeeeeeeeeeeeea 15 w ge 95 100 102 103 107 114 116 140 145 147 4, 155 160 162 167 169 170 16 LIST OF ENGRAVINGS. FIGURE » PAGE 51. 52. Telescopes with a single lens eeeeweeoeweveeoeeevp eeeeeevp ee ee eeeeere 172 Illustrative of spherical aberration. ....eececeseccscccsscecese 174 53. Illustrative of the principle of achromatic telescopes-«..+.++++ 177 54, 55. Double and treble achromatic object-glass ...«+++seeeeee+ 178 57. 58. 59. 60. 61. Common stand for achromatic telescopes. ...e.ecoeceseeceees 190 Equatorial stand for achromatic telescopes..e.scccseocsseeces 191 Dollond’s stand for achromatic telescopes ..-seeesecceseseees 192 Blair’s fluid achromatic object-glass ...eseeseccceeccececsecs 197 Barlovy' a fluid telescope o-os- v6 e+00cn ec ranencdaseaesngesece 109 62, 63, 64, 65,66. Various forms of reflecting telescopes...-.++++- 209 . Gregorian reflecting telescope... .e.cssccccccsecccccccccvnse IZ » "Whe: aerial retlectOrs 2s 010 eves.s oe aisha siehe eit bcavaleieis lob gis elee a aise Kee eO . Front view of the aerial reflector o oe o-0.d6 50 sed Savleieec eg vioce sev’ ele’ 247,248 9. Terrestrial eyepiece with four lenses....+csceecceccceccsseee 250 . Eyepiece of an old Dutch achromatic telescope «.+..eeseeeee 255 « APBNOVRIOTOY OPIC COE 2's 05's obs LRU c Se cle wel de Seles ocdaee ODT . Manner of canes the solar SPpOtse..sesesossccccsceccoees B13 istances from one station ...-.eeceessoes 30D eeeeepeeeeeeeenee eee 315 Mode of measuring Cavallo’s micrometer........ . The equatorial telescope, or portable observatory ...-++seeee+ 321 . Figure to illustrate the principle of the quadrant........+.++. 345 . The: astronomical: quadrant s's0i0¥ ieee Meee ICS IY aide ene s «346 Phe pstrouonii¢alicirclet ssi (erties 6 45s, ies is Sia g's alee BAG , EL RO APARSIE ANSTFEMONE vos vo siSVT Owned sca veed es va deiled eek SOR « Plan of a private observatory. oes \autuis yeu see eet deeeeedde 359 . Rotatory dome. for an observatOrys+..ceceseccsecceccesecsee 360 . Wheel-work of a planetarium... ......ccseccscccetvcccecees 365 . Perspective view of a planetarium... .ssecseccseceeteesecesee 306 . Apparatus for exhibiting the retrograde motions of the planets 368 . Section of the wheel-work of Dr. Henderson’s planetarium... 370 . Thirteen views of the supposed form of Saturn’s ring.......+- 377 ; Karl of Rosse’s great telescope. os. 0. oes ieescsiecseccseseee 390 . Section of the machinery connected with the telescope.-....+ 391 . Perspective view of the author’s observatory. ‘To front the title. THE PRACTICAL ASTRONOMER. PART I. ON LIGHT INTRODUCTION. Lieut is that invisible ethereal matter which renders objects perceptible by the Fitiaed oagacte: It appears to be distributed throughout the immensity of the universe, and is essentially requisite to the enjoyment of every rank of percep- tive existence. Itis by the agency of this mysterious substance that we become acquainted with the beauties and sublimities of the universe, and the wonderful operations of the Almighty Creator. Without its universal influence, an impenetrable veil would be thrown over the distant scenes of creation; the sun, the moon, the planets, and the starry orbs would be shrouded in the deepest darkness, and the variegated surface of the globe on which we dwell would be almost unnoticed andunknown. Creation would disappear, a mysterious gloom would surround the mind of every intelligence, all around would appear a dismal waste and an undistinguished chaos. To whatever quarter we might turn, no form nor comeliness would be seen, and scarcely a trace of the perfections and agency of an All Wise and Almighty Being could be per- ceived throughout the universal gloom. In short, without the influence of light, no world could be inhabited, no animated being could subsist in the manner it now does, no knowledge could be acquired of the works of God, and happiness, even in the lowest degree, could scarcely be enjoyed by any or- ganized intelligence. We have never yet known what it is to live in a world deprived of this delightful visitant; for in the darkest night we enjoy a share of its beneficial agency, and even in the Q* 17 18 INTRODUCTION. deepest dungeon its influence is not altogether unfelt.* The blind, indeed, do not directly enjoy the advantages of light, but its influence is reflected upon them, and their knowledge is promoted through the medium of those who enjoy the use of their visual organs. Were all the inhabitants of the world deprived of their eyesight, neither knowledge nor happiness, such as we now possess, could possibly be enjoyed. There is nothing which so strikingly displays the beneficial and enlivening effects of light as the dawn of a mild morning after a night of darkness and tempest. All appears gloom and desolation in our terrestrial abode till a faint light begins to whiten the eastern horizon. Every succeeding moment brings along with it something new and enlivening. The crescent of light towards the east now expands its dimen- ‘sions, and rises upwards towards the cope of heaven; and objects, which a little before were immersed in the deepest gloom, begin to be clearly distinguished. At length the sun arises, and all nature is animated by his appearance; the magnificent scene of creation, which a little before was in- volved in obscurity, Dba Westley to view, and every object around excites sentiments of wonder, delight, and adoration. The radiance which emanates from this luminary displays before us a world strewed with blessings, and embellished with the most beautiful attire. It unveils the lofty mountains and the forests with which they are crowned; the fruitful fields, with the crops that cover them; the meadows, with the rivers which water and refresh them; the plains adorned with verdure; the placid lake, and the expansive ocean. It removes the curtain of darkness from the abodes of men, and shows us the cities, towns, and villages, the lofty domes, the olittering spires, and the palaces and temples with which the landscape is adorned. ‘Ihe flowers expand their buds and put forth their colours, the birds awake to melody, man goes forth to his labour, the sounds of human voices are heard, and all appears life and activity, as if a new world had emerged from the darkness. of Chaos. The whole of this splendid scene, which light produces, may be considered as a new creation, no less grand and bene- ficent than the first creation, when the command was issued, * Those unfortunate individuals who have been confined in the darkest dungeons have declared, that though, on their first entrance, no object could be perceived, perhaps for a day or two, yet, in the course of time, as the pupils of their eyes expanded, they could readily perceive mice, rats, and other animals that infested their cells, and likewise the walls of their apartments; which shows that, éven in such situations, light is present, and produces a certain degree of influence. INTRODUCTION. 19 ‘Let there be light, and light was.” The aurora and the rising sun cause the earth, and all the objects which adorn its surface, to arise out of that profound darkness and apparent desolation which deprived us of the view of them, as if they had been no more. It may be affirmed, in full accofdance with truth, that the efflux of light in the dawn of the morn- ing, after a dark and cloudy night, is even more magnificent and exhilarating than at the first moment of its creation. At that period there were no spectators on earth to admire its glorious effects; and no objects, such as we now behold, to be embellished with its radiance. The earth was a shapeless chaos, where no beauty or order could be perceived; the mountains had not reared their heads ; the seas were not col- lected into their channels; no rivers rolled through the val- leys; no verdure adorned the plains; the atmosphere was not raised on high to reflect the radiance, and no animated beings existed to diversify and enliven the scene. But now, when the dawning of the morning scatters the darkness of the night, it opens to view a scene of beauty and magnifi- cence. ‘I'he heavens are adorned with azure, the clouds are tinged with the most lively colours, the mountains and plains are clothed with verdure, and the whole of this lower creation stands forth arrayed with diversified scenes of beneficence and grandeur, while the contemplative eye looks round and | wonders. Such, then, are the important and beneficent effects of that light which every moment diffuses its blessings around us. It may justly be considered as one of the most essential sub- stances connected with the system of the material universe, and which gives efficiency to all the other principles and arrangements of nature. Hence we are informed, in the sacred history, that light was the first production of the Al- mighty Creator, and the first born of created beings; for without it the universe would have presented nothing but an immense blank to all sentient existences. Hence, likewise, the Divine Being is metaphorically represented under the idea of light, as being the source of knowledge and felicity to all subordinate intelligences: “God is light, and in Him is no darkness at all;’’ and he is exhibited as “ dwelling in light unapproachable and full of glory, whom no man hath seen or can see.” In allusion to these circumstances, Milton, in his Paradise Lost, introduces the following beautiful apostrophe : * Hail, holy light! offspring of heaven first born, Or of the eternal co-eternal beam! May I express thee unblamed? since God is light, 20 INTRODUCTION. And never but in unapproached light Dwelt from eternity ; dwelt then in thee, Bright effluence of bright essence increate. Sp eee War efore the sun, Before the heavens thou wert, and at the voice Of God, as with a mantle, didst invest The rising world of waters dark and deep, Won from the void and formless infinite.’’ - As light is an element of so much importance and utility in the system of nature, so we find that arrangements have been made for its universal diffusion throughout all the worlds in the universe. The sun is one of the principal sources of light to this earth on which we dwell, and to all the other planetary bodies; and, in order that it may be equally dis- tributed over every portion of the surfaces of these globes, to suit the exigencies of their inhabitants, they are endowed with a motion of rotation, by which every part of their sur- faces is alternately turned towards the source of light; and when one hemisphere is deprived of the direct influence of the solar rays, its inhabitants derive a portion of light from luminaries in more. distant regions, and have their views directed to other suns and systems, dispersed, in countless numbers, throughout the remote spaces of the universe. Around several of the planets, satellites or moons have been arranged for the purpose of throwing light on their surfaces in the absence of the sun, while, at the same time, the pri- mary planets themselves reflect an effulgence of light upon their satellites. All the stars which our unassisted vision can discern in the midnight sky, and the millions more which the telescope alone enables us to descry, must be considered as so many fountains of light, not merely to illuminate the voids of immensity, but to irradiate with their beams surrounding worlds with which they are more immediately connected, and to diffuse a general lustre throughout the amplitudes of infi- nite space; and, therefore, we have every reason to believe that, could we fly, for thousands of years, with the swiftness of a seraph, through the spaces of immensity, we. should - ever approach a region of absolute darkness, but should find ourselves every moment encompassed with the emanations of light, and cheered with its benign influences. That Al- mighty Being who inhabiteth immensity and “dwells in light inaccessible,’’ evidently appears to have diffused light over the remotest spaces of his creation, and to have thrown a radiance upon all the provinces of his wide and eternal empire, so that every intellectual being, wherever existing, may feel its beneficent effects, and be enabled, through its GENERAL PROPERTIES OF LIGHT. 21 agency, to trace his wonderful operations, and the glorious attributes with which he is invested. As the science of astronomy depends solely on the in- fluence of light upon the organ of vision, which is the most noble and extensive of all our senses ; and as the construc- tion of telescopes and other astronomical instruments is founded upon our knowledge of the nature of light and the laws by which it operates, it is essentially requisite, before proceeding to a description of such instruments, to take a cursory view of its nature and properties, in so far as they have been ascertained, and the effects it produces when ob- structed by certain bodies, or when passing through differ- ent mediums. | CHAPTER I. GENERAL PROPERTIES OF LIGHT. Ir is not my intention to discuss the subject of light in minute detail, a subject which is of considerable extent, and which would require a separate treatise to illustrate it in all its aspect and bearings. All that I propose is to offer a few illustrations of its general properties, and the laws by which it is refracted and reflected, so as to prepare the way for explaining the nature and construction of telescopes and other optical instruments. There is no branch of natural science more deserving of our study and investigation than that which relates to light, whether we consider its beautiful and extensive effects, the magnificence and grandeur of the objects it unfolds to view, the numerous and diversified phenomena it exhibits, the optical instruments which a knowledge of its properties has enabled us to construct, or the daily advantages we de- rive, as social beings, from its universal diffusion. If air, which serves as the medium of sound and the vehicle of speech, enables us to carry on an interchange of thought and affection with our fellow-men, how much more exten- sively is that intercourse increased by light, which presents the images of our friends and other objects as it were im- mediately before us, in all their interesting forms and aspects —the speaking eye, the rosy cheeks, the benevolent smile, and the intellectual forehead. The eye, more susceptible of multifarious impressions than the other senses, “takes in 22 THEORIES OF LIGHT. at once the landscape of the world,’’ and enables us to dis- tinguish, in a moment, the shapes and forms of all its ob- jects, their relative positions, the colours that adorn them, their diversified aspect, and the motions by which they are transported from one portion of space to another. Light, through the medium of the eye, not only unfolds to us the persons of others, in all their minute modification and pecu- liarities, but exhibits us to ourselves. It presents to our own vision a faithful portrait of our peculiar features behind re- flecting substances, without which property we should remain entirely ignorant of those traits of countenance which cha- racterize us in the eyes of others. But what is the nature of the substance we call light, which thus unfolds to us the scenes of creation? On this subject two leading opinions have prevailed in the philo- sophical world. One of those opinions is, that the whole sphere of the universe is filled with a subtle matter, which receives from luminous bedies an agitation which is inces- santly continued, and which, by its vibratory motion, enables us to perceive luminous bodies. According to this opinion, light may be considered as analogous to sound, which is con- veyed to the ear by the vibratory motions of the air. This was the hypothesis of Descartes, which was adopted, with some modifications, by the celebrated Euler, Huygens, Frank- lin, and other philosophers, and has been admitted by several scientific gentlemen of the present day. The other opinion is, that light consists of the emission or emanation of the par- ticles of luminous bodies, thrown out incessantly on all sides, in consequence of the continued agitation it experiences. This is the hypothesis of the illustrious Newton, and has been most generally adopted by British philosophers. To the first hypothesis it is objected that, if true, “light would not only spread itself in a direct line, but its motion would be transmitted in every direction like that of sound, and would convey the impression of luminous bodies in the regions of space beyond the obstacles that intervene to stop its pro- gress.” No wall or other opaque body could obstruct its course, if it undulated in every direction like sound ; and it would be a necessary consequence, that we should have no night, nor any such phenomena as eclipses of the sun or moon, or of the satellites of Jupiter and Saturn. This objec- tion has never been very satisfactorily answered. On the other hand, Euler brings forward the following objections against the Newtonian doctrine of emanation. 1. That, were the sun emitting continually, and in all directions, such floods RADIATION OF LIGHT. 293 of lummous matter, with a velocity so prodigious, he must speedily be exhausted, or at least some alteration must, after the lapse of so many ages, be perceptible. 2. That the sun is not the only body that emits rays, but that all the stars have the same quality; and as everywhere the rays of the sun must be crossing the rays of the stars, their collision must be violent in the extreme, and that their direction must be changed by such a collision.* To the first of these objections it is answered, that so vast is the tenuity of light, that it utterly exceeds the power of con- ception ; the most delicate instrument having never been cer- tainly put i motion by the impulse of the accumulated sun- beams. It has been calculated that in the space of 385,130,000 Egyptian years (of 360 days,) the sun would lose only the partazoth of his bulk from the continual efflux of his light. And, therefore, if in 885 millions of years the sun’s diminu- tion would be so extremely small, it would be altogether in- sensible during the comparatively short period of five or six thousand years. ‘To the second objection it is replied, that the particles of light are so extremely rare that their distance from one to another is incomparably greater than their dia- meters ; that all objections of this kind vanish when we attend to the continuation of the impression upon the retina, and. to the small number of luminous particles which are on that account necessary for producing constant vision. For it ap- pears, from the accurate experiments of M. D’Arcy, that the impression of light upon the retina continues eight thirds, and as a particle of light would move through 26,000 miles in that time, constant vision would be maintained by a suc- cession of luminous particles twenty-six thousand miles dis- tant from each other. Without attempting to decide on the merits of these two hypotheses, I shall leave the reader to adopt that opinion which he may judge to be attended with the fewest difficul- ties, and proceed to illustrate some of the properties of light and in the discussion of this subject I shall generally adhere to the terms employed by those who have adopted the hypo- thesis of the emanation of light. 1. Light emanates or radiates from luminous bodies in a straight line. ‘This property is proved by the impossibility of seeing light through bent tubes, or small holes pierced in metallic plates placed one behind another, except the holes be placed in a straight lme. If we endeavour to look at the * Letters to a German Princess, vol. i., p. 68, 69, &c. 24 VELOCITY OF LIGHT. sun or a candle through the bore of a bended pipe, we cannot perceive the object, nor any light proceeding from it, but through a straight pipe the object may be perceived. ‘This is likewise evident from the form of the rays of light that es a dark room, which proceed straight-forward in ines proceeding from the luminous body ; and from the form of the shadows which bodies project that are bounded by right lines passing from the luminous body, and meeting the lines which terminate the interposing body. This property may be demonstrated to the eye by causing light to pass through small holes into a dark room filled with smoke or dust. It is to be understood, however, that in this case the rays of light are considered as passing through the same medium; for when they pass from air into water, glass, or other media, they are bent at the point where they enter a different medium, as we shall afterward have occasion to explain. 2. Light moves with amazing velocity. The ancients believed that it was propagated from the sun and other luminous bodies instantaneously ; but the observations of modern astronomers have demonstrated that this is an erro- neous hypothesis, and that light, like other projectiles, occu- pies a certain time in passing from one part of space to another. Its velocity, however, is prodigious, and exceeds that of any other body with which we are acquainted. It flies across the earth’s orbit, a space 190 millions of miles in extent, in the course of sixteen and a half minutes, which is at the rate of 192,000 miles every second, and more than a million of times swifter than a cannon ball flying with its greatest velocity. It appears from the discoveries of Dr. Bradley, respecting the aberration of the stars, that light flies from those bodies with a velocity similar, if not exactly the same; so that the light of the sun, the planets, the stars, and every luminous body in the universe is propagated with wniform velocity.* But, if the velocity of light be so very great, it may be asked, how does it not strike against all objects with a force equal to its velocity? If the finest sand were thrown against our bodies with the hundredth part of this velocity, each grain would pierce us as certainly as the sharpest and swiftest arrows fron a bow. It is a principle in mechanics that the force with which all bodies strike is in proportion to the size of these bodies, or in the quantity of matter they contain, multi- * 'The manner in which the motion of light was discovered is explained in the author’s work, entitled ‘‘ Celestial Scenery,’’ p. 369-371; and the circumstances which led to the discovery of the aberration of light are stated and illustrated in his volume on the ‘‘ Sidereal Heavens,”’ p. 7h 74, and p. 284-292. PARTICLES OF LIGHT. 25 plied by the velocity with which they move. Therefore, if the particles of light were not almost infinitely small, they would, of necessity, prove destructive in the highest degree. If a particle of light were equal in size to the twelve hundred thousandth part of a small grain of sand—supposing light to be material—we should be no more able to withstand its force than we should that of sand shot point blank from the mouth ofacannon. Every object would be battered and perforated by such celestial artillery, till our world were laid in ruins, and every living being destroyed. And herein are the wis- dom and benevolence of the Creator displayed in making the particles of light so extremely small as to render them in some degree proportionate to the greatness of the force with which they are impelled; otherwise, all nature would have been thrown into ruin and confusion, and the great globes of the universe shattered to atoms. We have many proofs, besides the above, that the particles of light are next to infinitely small. We find that they pene- trate with facility the hardest substances, such as crystal, glass, various kinds of precious stones, and even the diamond itself, though among the hardest of stones; for such bodies could not be transparent, unless light found an easy passage through their pores. When a candle is lighted in an ele- vated situation, in the space of a second or two it will filla cubical space (if there be no interruption) of two miles around it, in every direction, with luminous particles, before the least sensible part of its substance is lost by the candle: that is, it will in a short instant fill a sphere four miles in diameter, twelve and a half miles in circumference, and containing thirty-three and a half cubical miles, with particles of light ; for an eye placed in any part of this cubical space would pesrcire the light emitted by the candle. It has been calcu- ated that the number of particles of light contained in such a space cannot be less than four hundred steptillions—a num- ber which is six billions of times greater than the number of grains of sand which could be contained in the whole earth considered. as a solid globe, and supposing each cubic inch of it to contain ten hundred thousand grains. Such is the in- conceivable tenuity of :aat substance which emanates from all luminous bodies, and which gives beauty and splendour tothe universe! _ This may also be evinced by the following experiment. Make a small pin-hole in a piece of black paper, and hold the paper upright facing a row of candles placed near each other, and at a little distance behind the black paper place a piece of white pasteboard. On this Vox, IX. 3 96 DIFFUSION OF LIGHT. pasteboard the rays which flow from all the candles through the small hole in the black paper, will form as many specks of light as there are candles, each speck being as clear and distinct as if there were only one speck from a single candle. This experiment shows that the streams of light from the dif- ferent candles pass through the small hole without confusion, and, consequently, that the particles of light are exceedingly small. For the same reason we can easily see through a small hole not more than ;}5th of an inch in diameter, the sky, the trees, houses, and nearly all the objects in an exten- sive landscape, occupying nearly an entire hemisphere, the light of all which may pass through this small aperture. 3. Light is sent forth in all directions from every visible point of luminous bodies. If we hold a sheet of paper be- fore a candle, or the sun, or any other source of light, we shall find that the paper is illuminated in whatever position we hold it, provided the light is not obstructed by its edge or by any other body. Hence, wherever a spectator is placed with regard to a luminous body, every point of that part of its surface which is toward him will be visible, when no inter- vening object intercepts the passage of the light. Hence, likewise, it follows that the sun illuminates not only an im mense plane extending along the paths of the planets, from the one side of the orbit of Uranus tothe other, but the whole of that sphere, or solid space, of which the distance of Uranus is the radius. The diameter of this sphere is three thousand six hundred millions of miles, and it consequently contains about 24,000,000,000,000,000,000,000,000,000, or twenty- four thousand quartillions of cubical miles, every point of which immense space is filled with the solar beams. Not only so, but the whole cubical space which intervenes between the sun and the nearest fixed stars is more or less illuminated by his rays. For, at the distance of Sirius, or any other of the nearest stars, the sun would be visible, though only as a small, twinkling orb ; and, consequently, his rays must be diffused, however faint, throughout the most distant spaces whence he is visible, The diameter of this immense sphere of light can- not be less than forty billions of miles, and its solid contents 33, 500, 000, 000, 000, 000,000,000,000,000,000,000,000,000, or thirty-three thousand five hundred seatillions of cubical miles. All this immense and incomprehensible space is filled with the radiations of the solar orb; for were an eye placed in any one point of it, where no extraneous body interposed, the sun would be visible either as a large luminous orb, or as a small twinkling star. But he can be visible only by the DURATION OF THE IMPRESSION OF LIGHT. 27 rays he emits, and which enter the organs of vision. How inconceivably immense, then, must be the quantity of rays which are thrown off in all directions from that luminary which is the source of our day! Every star must likewise be considered as emitting innumerable streams of radiance over a space equally extensive; so that no point in the universe can be conceived where absolute darkness prevails, unless in the inferior regions of planetary bodies. 4. The effect of light upon the eye is not instantaneous, but continues for a short space of time. This may be proved and illustrated by the following examples: if a stick, or a ball connected with a string, be whirled round in a cir- cle, and a certain degree of velocity given it, the object will appear to fill the whole circle it describes. If a lighted fire- brand be whirled round in the same rapid manner, a complete circle of light will be exhibited. This experiment obviously shows that the impression made on the eye by the light froin the ball or the firebrand, when in any given point of the cir- cle, is sufficiently lasting to remain till it has described the whole circle, and again renews its effect as often as the cir- cular motion is continued. The same is proved by the fol- lowing considerations : we are continually shutting our eyes, or winking ; and, during the time our eyes are shut on such occasions, we should lose the view of surrounding objects if the impression of light did not continue a certain time while the eyelid covers the pupil; but experience proves that during such vibrations of the eyelids, the light from surrounding ob- jects is not sensibly intercepted. If we look for some time steadily at the light of a candle, and particularly if we look directly at the sun, without any interposing medium, or if we look for any considerable time at this luminary through a telescope with a coloured glass interposed—in all these cases, if we shut. our eyes immediately after viewing such objects, we shall still perceive a faint image of the object by the i im pression which its light has made upon our eyes. « With respect to the duration of the i impression of light, it has been observed that the teeth of a cog-wheel in a clock were still visible in succession, when the velocity of rotation brought 246 teeth through a given fixed point in a second. In this case it is clear that if the impression made on the eye by the light reflected from any tooth had lasted without sen- sible diminution for the 246th part of a second, the teeth would have formed one unbroken line, because a new tooth would have continually arrived in the place of the interior one before its image could have disappeared. Ifa live coal be 28 MOMENTUM OF LIGHT. whirled round, it is observed that the luminous circle is com- plete when the rotation is performed in the 81th of a second. In this instance we see that the impression was much more durable than the former. Lastly, if an observer sitting in a room, direct his sight, through a window, to any particular object out of doors, for about half a minute, and then shut his eyes and cover them with his hands, he will still continue to see the window, together with the outline of the terrestrial objects bordering on the sky. This appearance will remain for near a minute, though occasionally vanishing and changing colour in.a. manner that brevity forbids our minutely describ- ing. From these facts we are authorized to conclude that all impressions of light on the eye last a considerable time ; that the brightest objects make the most lasting impressions ; and that, if the object be very bright, or the eye weak, the im- pression may remain for a time so strong as to mix with and confuse the subsequent impressions made by other objects. In the last case the eye is said to be dazzled by the light.’”* The following experiment has likewise been suggested as a proof of the impression which light makes upon the eye: If a card, on both sides of which a figure isdrawn, for ex- ample, a bird and a cage, be made to revolve rapidly on the straight line which divides it symmetrically, the eye will per- ceive both figures at the same time, provided they return successively to the same place. M. D’Arcy found by vari- ous experiments that, in general, the impression which light produces on the eye lasts about the eighth of a second. M. Plateau, of Brussels, found that the impression of different colours lasted the following periods, the numbers here stated being the decimal parts of a second: flame, 0-242, or nearly one-fourth of a second; burning coal, 0-229; white, 0-182, or a little more than one-sixth of a second; blue, 0-186; yellow, 0-173; red, 0-184. : ) 5. Light, though extremely minute, 7s supposed to have a certain degree of force or momentum. In order to prove this, the late ingenious Mr. Mitchell contrived the following expe- riment: He constructed a small vane in the form of a com- mon weathercock, of a very thin plate of copper, about an inch square, and attached to one of the finest harpsicord wires about ten inches long, and nicely balanced at the other end of the wire by a grain of very small shot. The instrument had also fixed to it in the middle, at right angles to the length of the wire, and in a horizontal direction, a small bit of a very slender sewing needle, about half an inch long, which * Nicolson’s Introduction to Natural Philosophy, vol. i. QUANTITY OF MATTER. IN THE SUN’S RAYS. 29 was made magnetical.._In this state the whole instrument might weigh about ten grains. ‘The vane was supported in the manner of the needle in the mariner’s compass, so that it could turn with the greatest ease ; and, to prevent its being affected by the vibrations of the air, it was enclosed in a glass case or box. The rays of the sun were then thrown upon the broad part of the vane, or copper plate, from a concave mirror of about two feet diameter, which, passing through the front glass of the box, were collected into the focus of the mirror upon the copper plate. In consequence of this, the plate began to move with a-slow motion of about an inch in a second of time, till it had moved through a space of about two inches and a-half, when it struck against the back of the box. The mirror being removed, the instrument returned to its former situation, and the rays of the sun being again thrown wpon it, it again began to move, and struck against the back of the box as before. This was repeated three or four times with the same success. On the above experiment the following calculation has been founded: if we impute the motion produced in this ex- periment to the impulse of the rays of light, and suppose that the instrument weighed ten grains, and acquired a velocity of one inch in a second, we shall find that the quantity of matter contained in the rays falling upon the ifstrument in that time amounted to nomore than one twelve hundredth- millionth part of a grain, the velocity of light exceeding the velocity of one inch in a_second in the proportion of about 12,000,000,000 to 1. The light in this experiment was col- - lected from a surface of about three square feet, which reflect- ing only about half what falls upon it, the quantity of matter contained in the rays of the sun incident upon a foot and a half of surface in one second of time, ought to be no more than the twelve hundred-millionth part of a grain. But the den- sity of the rays of light at the surface of the sun is greater than that at the earth in the proportion of 45,000 to 1; there ought, therefore, to issue from one square foot of the sun’s surface, in one second of time, in order to supply the waste by light, zz 45th part of a grain of matter, that is, a little more than two grains a day, or about 4,752,000 grains, or 670 pounds avoirdupois, nearly, in 6000 years ; a quantity which would have shortened the sun’s diameter no more than about ten feet, if it were formed of the density of water only. | if the above experiment be considered as having been ac- curately performed, and if the calculation founded upon it be 3% 30 THE INTENSITY OF LIGHT. correct, it appears that there can be no grounds for appre- hension that the sun can ever be sensibly diminished by the immense and incessant radiations proceeding from his body on the supposition that light is a material emanation. For the diameter of the sun is no less than 880,000 miles; and, before this diameter could be shortened by the emission of light, one English mile, it would require three millions one hundred and sixty-eight thousand years, at the rate now stated; and, before it could be shortened ten miles, it would require a period of above thirty-one millions of years. And although the sun were thus actually diminished, it would pro- duce no sensible effect or derangement throughout the plane- tary system. We haveno reason to believe that the system, in its present state and arrangements, was intended to en- dure for ever; and before that luminary could be so far re- duced, during the revolutions of eternity, as to produce any irregularities in the system, new arrangements and modifica- tions might be introduced by the hand of the All Wise and Omnipotent Creator. Besides, it is not improbable that a sys- tem of means is established by which the sun and all the lu- minaries in the universe receive back again a portion of the light which they are continually emitting, either from the planets from whose surfaces it is reflected, or from the millions of stars whose rays are continually traversing the immense spaces of creation, or from some other sources to us un- known. 6. The intensity of light is diminished in proportion to the square of the distance from the luminous body. ‘Thus, a person at two feet distance from a candle, has only the fourth part of the light he would have at one foot; at three feet dis- tance, the ninth part; at four feet, the sixteenth part; at five feet, the twenty-fifth part; and so on for other distances. Hence the light received by the planets of the solar system decreases in proportion to the squares of the distances of these bodies fromthe sun. This may be illustrated by the follow- ing figure : Suppose the ught which flows from a point, A, and pcesses through a square hole, B, is received upon a plane, C, paral- lel to the plane of the hole—or, let the figure C be considered as the shadow of the plane B. When the distance of C is double of B, the length and breadth of the shadow, C, will be each double of the length and breadth of the plane B, and _ treble when A D is treble of A B, and so on, which may be easily examined by the light of a candle placed at A. There- fore the surface of the shadow C, at the distance A C—double THE INTENSITY OF LIGHT. 31 Figure 1. of A B, is divisible into four squares, and, at a treble distance, into nine squares, severally equal to the square B, as repre- sented in the figure. The light, then, which falls upon the plane B being suffered to pass to double that distance, will be uniformly spread over four times the space, and, conse- quently, will be four times thinner in every part of that space. And, at a treble distance, it will be nine times thinner, and, at a quadruple distance, sixteen times thinner than it was at first. Consequently, the quantities of this rarified light re- ceived upon a surface of any given size and shape, when re- moved successively to these several distances, will be but one fourth, one-ninth, one-sixteenth of the whole quantity received by it at the first distance, A B. In conformity with this law, the relative quantities of light on the surfaces of the planets may be easily determined when their distances from the sun are known. Thus, the distance of Uranus from the sun is 1,800,000,000 miles, which is about nineteen times greater than the distance of the earth from the same luminary. The square of 19 is 361; conse- quently, the earth enjoys 361 times the intensity of light, when compared with that of Uranus; in other words, this distant planet enjoys only the ,3, part of the quantity of light which falls upon.the earth. This quantity, however, is equivalent to the light we should enjoy from the combined effulgence of 348 full moons ; and if the pupils of the eyes of the inhabitants of this planet be much larger than ours, and the retina of the eye be endued with a much greater degree of nervous sensibility, they may perceive objects with as great a degree of splendour as we perceive on the objects which surround us in this world. Following out the same princi- ple, we find that the quantity of light enjoyed by the planet Mercury is nearly seven times greater than that of the Earth, and that of Venus nearly double of what we enjoy ; that Mars 82 REFLECTION FROM OPAQUE BODIES: has less than the one half; Jupiter the one twenty-seventh part; and Saturn only the one ninetieth part of the light which falls upon the Earth. ‘That the light of these distant planets, however, is not so weak as we might at first imagine, appears from the brilliancy they exhibit, when viewed in our nocturnal sky, either with the telescope or with the unassisted eye; and likewise from the circumstance that a very small portion of the Sun—such as the one fortieth or one fiftieth part—diffuses a quantity of light sufficient for most of the purposes of life, as is found in the case of total eclipses of the Sun, when his western limb begins to be visible, only like a fine luminous thread, for his light is then sufficient to, render distinctly visible all the parts of the surrounding landscape. 7. SItis by light reflected from opaque bodies that most of the objects around us are rendered visible. _When a lighted candle is brought into a dark room, not only the candle, but all other bodies in the room_become visible. Rays of the sun, passing into a dark room, render luminous a sheet of paper on which they fall, and this sheet, in its turn, enlightens, to a certain extent, the whole apartment, and renders objects in it visible so long as it receives the rays of the sun. In like manner, the moon and the planets are opaque bodies, but the light of the sun falling upon them, and being reflected from their surfaces, renders them visible. Were no light to fall on them from the sun, or were they not.endued with a power of reflecting it, they would be altogether invisible to our sight. When the moon comes between us and the sun, as in a total eclipse of that luminary, as no solar light is reflected from the surface next the earth, she is invisible, only the curve or out- line of her figure being distinguished by her shadow. In this case, however, there is a certain portion of reflected light on the lunar hemisphere next the earth, though not distin- guishable during a solar eclipse. ‘The earth is enlightened by the sun, and a portion of the rays which fall upon it is re- flected upon the dark hemisphere of the moon which is then towards the earth. ‘This reflected light from the earth is dis- tinctly perceptible, when the moon appears as a slender crescent, two or three days after new moon—when the earth reflects its light back on the moon, in the same manner as the full moon reflects her light on the earth. Hence, even at this period of the moon, her whole face becomes visible to us, but its light is not uniform or of equal intensity. The thin crescent on which the full blaze of the solar light falls, is ver brilliant and distinctly seen, while the other part, on med falls only a comparatively feeble light from the earth, appears REFLECTION FROM OPAQUE BODIES. 33 very faint, and is little more than visible to the naked eye; but with a telescope of moderate power—if the atmosphere be very clear—it appears beautifully distinct, so that the relative positions of many of the lunar spots may be distin- guished. The intensity of reflected light is very small, when com- pred with that which proceeds directly from luminous bodies. M. Bouguer, a French philosopher, who made a variety of experiments to ascertain the proportion of light emitted by the heavenly bodies, concluded, from these experiments, that the light transmitted from the sun to the earth is at least 300,000 times as great as that which descends to us from the full moon, and that, of 300,000 rays which the moon receives, from 170,000 to 200,000 are absorbed. Hence we find that, however brilliant the moon may appear at night, in the day- time she appears as obscure as a small portion of dusky cloud to which she happens to be adjacent, and reflects no more ight than a portion of whitish cloud of the same size. And as the full moon fills only the ninety thousandth part of the sky, it would require at least ninety thousand moons to pro- duce as much light as we enjoy in the day-time under a cloudy sky. As the moon and the planets are rendered visible to us only by light reflected from their surfaces, so it is in the same way that the images of most of the objects around us are conveyed to our organs of vision. We behold all the objects which compose an extensive landscape—the hills and vales, the woods and lawns, the lakes and rivers, and the habita- tions of man—in consequence of the capacity with which they are endued of sending forth reflected rays to the eye, from every point of their surfaces and in all directions.. In connection with the reflection of light, the following curious observation may be stated: Baron Funk, visiting some silver mines in Sweden, observed, that, “in a clear day, it was as dark as pitch under ground in the eye of a pit, at sixty or seventy fathoms deep; whereas, in a cloudy or rainy day, he could see to read even at one hundred and six fathoms deep. Inquiring of the miners, he was informed that this is always the case; and reflecting upon it, he imagined it arose from this circumstance, that, when the atmosphere is full of clouds, light is reflected from them into the pit in all directions, and that thereby a considerable proportion of the rays are reflected perpendicularly upon the earth: whereas, when the atmo- sphere is clear, there are no opaque bodies to reflect the light in this manner, at least in a sufficient quantity; and rays 84. DISCOVERY OF PHOSPHORESCENCE. from the sun himself can never fall perpendicularly in that country.” The reason here assigned is, in all probability, the true cause of the phenomenon now described. 8. It is supposed by some philosophers that ight is sub- ject to the same laws of attraction that govern all other material substances, and that zt 1s imbibed and forms a con stituent part of certain bodies. ‘This has been inferred from the phenomena of the Lolognian stone, and what are gene- rally called the solar phosphorit. The Bolognian stone was first discovered about the year 1630, by Leascariolo, a shoe- maker of Bologna. Having collected together some stones of a shining appearance at the bottom of Monte Paterno, and being in quest of some alchemical secret, he put them into a crucible to calcine them; that is; to reduce them to the state of cinders. Having taken them out of the crucible, and ex- posed them to the light of the sun, he afterward happened to carry them into a dark place, when, to his surprise, he ob- served that they possessed a self-illuminating power, and continued to emit faint rays of light for some hours afterward. In consequence of this discovery, the Bolognian spar came into considerable demand among natural philosophers, and the curious in general; and the best way of preparing it seems to have been hit upon by the family of Zagoni, who supplied all Europe with Bolognian phosphorus till the dis- covery of more powerful phosphoric substances put an end to their monopoly. In the year 1677, Baldwin, a native of Misnia, observed that chalk, dissolved in aquafortis, exactly resembled the Bolognian stone in its property of imbibing light, and emitting it after it was brought into the dark ; and hence it has obtained the name of Baldwin’s phosphorus. In 1730 M. du Fay directed his attention to this subject, and observed that all earthy substances susceptible of calci- nation, either by mere fire, or when assisted by the previous action of nitrous acid, possessed the property of becoming more or less luminous, when calcined and exposed for a short time in the light; that the most perfect of these phosphori were limestones, and other kinds of carbonated lime, gypsum, and particularly the topaz, and that some diamonds were also observed to be luminous by simple exposure to the sun’s rays. Some time afterward Beccaria discovered that a great variety of other bodies were convertible into phosphor by exposure to the mere light of the sun, such as organic animal remains, most compound salts, nitre and borax—all the fari- naceous and oily seeds of vegetable substances, all the gums and several of ‘he resins—the white woods and vegetable CANTON’S PHOSPHORUS. 35 fibre, either in the form of paper or linen; also starch and loaf-sugar proved to be good phosphori, after being made thoroughly dry, and exposed to the direct rays of the sun. Certain animal substances by a similar treatment were also converted into phosphori ; particularly bone, sinew, glue, hair, horn, hoof, feathers, and fish-shells. The same property was communicated to rock crystal and some other of the gems, by rubbing them against each other so as to roughen their surfaces, and then placing them for some minutes in the focus of a lens, by which the rays of light were concentrated upon them at the same time that they were also moderately heated. In the year 1768 Mr. Canton contributed some important facts in relation to solar phosphori, and communicated a method of preparing a very powerful one, which, after the inventor, is usually called Canton’s phosphorus. He affirms that his phosphorus, enclosed in a glass flask, and hermeti- cally sealed, retains its property of becoming luminous for at least four years, without, any apparent decrease of activity. It has also been found that, if a common box smocthing~iron, heated in the usual manner, be placed for half a minute. on a sheet of dry, white paper, and the paper be then exposed to the light, and afterward examined in a dark closet, it will be found that the whole paper will be luminous, that part, however, on which the iron had stood being much more shining than the rest. From the above facts it would seem that certain bodies have the power of imbibing light and again emitting it, in certain circumstances, and that this power may remain for a considerable length of time. It is observed that the light which such bodies emit bears an analogy to that which they have imbibed. In general, the illuminated phosphorus is reddish ; but when a weak light only has been admitted to it, or when it has been received through pieces of white paper, the emitted light is pale or whitish. Mr. Morgan, in the seventy-fifth volume of the Philosophical Transactions, treats the subject of light at considerable length ; and as a founda- tion for his reasoning, he assumes the following data: 1. That light is a body, and, like all others, subject to the laws of attraction. 2. That light is a heterogeneous body, and that the same attractive power operates with different degrees of force on its different parts. ‘To the principle of attraction, likewise, Sir Isaac Newton has referred the most extraordi- nary phenomena of light, Refraction and Inflection. He has also endeavoured to show that light is not only subject to the law of attraction, but of repulsion also, since it is repelled or 36 EFFECT OF LIGHT ON PLANTS. reflected from certain bodies. If such principles be admitted, then it is highly probable that the phosphorescent bodies to which we have adverted have a power of attracting or im- bibing the substance of light, and of retaining or giving it out under certain circumstances, and that the matter of light is incorporated, at least, with the surface of such bodies; but on this subject, as on many others, there is a difference of opinion among philosophers.* | 9. Light is found to produce a remarkable effect on plants and flowers, and other vegetable productions. Of all the phenomena which living vegetables exhibit, there are few that appear more extraordinary than the energy and constancy with which their stems incline towards the light. Most of the discous flowers follow the sun in his course. They attend him to his evening retreat, and meet his rising lustre in the morning with the. same unerring law. They unfold their flowers on the approach of, this luminary ; they follow his course by turning on their stems, and close them as soon as he disappears. Ifa plant, also, is shut up in a dark room, and a small hole afterwards opened by which the light of the sun may enter, the plant will turn towards that hole, and even alter its own shape in order to get near it; so that though it was straight before, it will in time become crooked, that it may * Light of a phosphoric nature is frequently emitted from various pu trescent animal substances, which, in the ages of superstition, served to astonish and affright the timorous. We learn from Fabricius, an Italian, that three young men, residing at Padua, having bought a lamb, and eaten part of it on Easter Day, 1592, several pieces of the remainder, which they kept till the following day, shone like so many candles when they _Awere casually viewed in the dark. ‘The astonishment of the whole city was excited by this phenomenon, and a part of the flesh was sent to Fa- bricius, who was professor of anatomy, to be examined by him. » He ob- served that those parts which were soft to the touch and transparent in candle-light were the most resplendent; and also that some pieces of kid’s flesh which had happened to have lain in contact with them were luminous, as well as the fingers and other parts of the bodies of those persons who touched them. Bartholin gives an account of a similar phe- nomenon, which happened at~Montpelier in 1641. A poor woman had bought a piece of flesh in the market, intending to make use of it the fol- lowing day; but happening not to be able to sleep well that night, and her bed and pantry being in the same room, she observed so much light come from the flesh as to illuminate all the place where it hung. We may judge of the terror and astonishment of the woman herself, when we find that a part of this luminous flesh was carried as a very extraordinary curiosity to Henry, duke of Condé, the governor of the place, who viewed it several hours with the greatest astonishment. ‘The light was as if gems had been scattered over the surface, and continued till the flesh began to putrefy, when it vanished, which it was believed to do in the form of across. Henes the propriety of instructing the mass of the com- munity in the knowledge of the facts connected with the material sys- tem, and the physical causes of the various phenomena of nature. EFFROT OF LIGHT ON PLANTS. 87 get near the light. Vegetables placed in rooms where they receive light only in one direction, always extend themselves in that direction. If they receive light in two directions, they direct their course towards that which is strongest. It is not the heat, but the light of the sun which the plant thus covets; for, though a fire be kept in the room, capable of giving a much stronger heat than the sun, the plant will turn away from the fire in order to enjoy the solar light. ‘Trees growing in thick forests, where they only receive light from above, direct their shoots almost invariably upward, and therefore become much taller and less spreading than such as stand single. The green colour of plants is likewise found to depend on the sun’s light being allowed to shine on them; for without the influence of the: solar light they are always of a white colour. It is found by experiment that, if a plant which has been reared in darkness be exposed to the light of day, in two or three days it will acquire a green colour perceptibly similar to that of plants which have grown in open daylight. If we expose to the light one part of the plant, whether leaf or branch, this part alone will become green. If we cover any part of a leaf with an opaque substance, this place will remain white, while the rest becomes green. The whiteness of the inner leaves of cabbages is a partial effect of the same cause, and many other examples of the same kind might easily be produced., M. Decandolle, who seems to have paid particu- lar attention to this subject, has the following remarks: «It is certain, that between the white state of plants vegetating in darkness, and complete verdure, every possible intermediate degree exists, determined by the intensity of the light. Of this any one may easily satisfy himself by attending to the colour of a plant exposed to the full daylight; it exhibits in succession all the degrees of verdure. I had already seen the same phenomenon, in a particular manner, by exposing plants reared in darkness to the light of lamps. In these experi- ments, I not only saw the colour come on gradually, accord- ing to the continuance of the exposure to light, but I satisfied myself that a certain intensity of permanent light never gives to a plant more than a certain degree of colour. The same fact readily shows itself in nature, when we examine the plants that grow under shelter or in forests, or when we ex- amine in succession the state of the leaves that form the heads of cabbages.’’* It is likewise found that the perspiration of vegetables 1s * Memoires de la Soc. d’ Aroncil. vol. ii. Vou. IX. 38 LIGHT EMITTED FROM PLANTS. increased or diminished in a certain measure by the degree of light which falls upon them. The experiments of Mr. P. Miller and others prove that plants uniformly perspire most in the forenoon, though the temperature of the air in which they are placed should be unvaried. M. Guettard likewise informs us that a plant exposed to the rays of the sun has its perspiration increased to a much greater degree than if it had been exposed to the same heat under the shade. Vegetables are likewise found to be indebted to light for their smell, taste, combustibility, maturity, and the resinous principle, which equally depend upon this fluid.. The aromatic substances, resins, and volatile oil are the productions of southern climates, where the light is more pure, constant, and intense. In fine, another remarkable property of light on the vegetable king- dom is that, when vegetables are exposed to open daylight, or to the sun’s rays, they emit oxygen gas, or vital air. It has been proved that, in the production of this effect, the sun does not act as a body that heats. The emission of the gas is determined by the licht: pure air is therefore separated by the action of light, and the operation is stronger as the light is more vivid. By this continual emission of vital air, the Almighty incessantly purifies the atmosphere, and repairs the loss of pure air occasioned by respiration, combustion, fer- mentation, putrefaction and numerous other processes which have a tendency to contaminate this fluid, so essential to the vigour and comfort of animal life ; so that, in this way, by the agency of light, a due equilibrium is always maintained be- tween the constituent parts of the atmosphere. In connection with this subject the following curious phe- nomenon may be stated, as related by M. Haggern, a lecturer on Natural History in Sweden. One evening he perceived a faint flash of light repeatedly dart from a marigold. Sur- prised at such an uncommon appearance, he resolved to ex- amine it with attention; and, to be assured it was no decep- tion of the eye, he placed a man near him, with orders to make a signal at the moment when he observed the light. They both saw it constantly at the same moment. The hght was most brilliant on marigolds of an crange or flame colour, but scarcely visible on pale ones. The flash was frequently seen on the same flower two or three times in quick succes- sion, buc more commonly at intervals of several minutes ; and when several flowers in the same place emitted their light together, < could be observed at a considerable distance. The phenomenon was remarked in the months of July and August at sunset, and for half an hour when the atmosphere was clear; EFFECTS OF LIGHT ON SOUND. 39 but after a rainy day, or when the air was loaded with va- pours, nothing of it was seen. 'Fhe following flowers emitted flashes more or less vivid, in this order: 1. The Marigold. 2. Monk’s Hood. 3.'The Orange Lily. 4. The Indian Pink. As to the cause of this phenomenon, different opinions may be entertained. From the rapidity of the flash and other cir- cumstances, it may be conjectured that electricity is concerned in producing this appearance. M. Haggern, after having ob- served the flash from the orange lily, the anthere of which are at considerable distance from the petals, found that the light proceeded from the petals only ; whence he concludes that this electrical light is caused by the pollen, which, in fly- ing off, is scattered on the petals. But, perhaps, the true cause of it still remains to be ascertained. 10. Light has been supposed to produce a certain degree of influence on the PROPAGATION OF souND? M. Parolette, in a long paper in the “Journal de Physique,”’ vol. 68, which is copied into “ Nicholson’s Philosophical Journal,” vol. 25, p- 28—39, has offered a variety of remarks, and detailed a number of experiments on this subject. . The author states the following circumstances as having suggested the connec- tion between light and sound: “ In 1803 I lived in Paris, and being accustomed to rise before day to finish a work on which I had long been employed, I found myself frequently disturbed by the sound of carriages, as my windows looked into one of the most frequented streets in that city. This circumstance, which disturbed me in my studies every morning, led me to remark that the appearance of daybreak peculiarly affected the propagation of the sound; from duu and deep, which it was before day, it seemed tome to acquire a more scnorous sharpness in the period that succeeded the dissipation of dark- ness. ‘The rolling of the wheels seemed to announce the fric- tion of some substances grown more elastic ; and my ear, on attending to it, perceived this difference diminish in propor tion as the sound of wheels was confounded with those ex- cited by the tumult of objects quittmg their nocturnal silence. Struck with this observation, I attempted to discover whether any particular causes had deceived my ears. I rose several times before day for this purpose alone, and was every time confirmed in my suspicion that light must have a peculiar in- fluence on the propagation of sound. This variation, how- ever, in the manner in which the air gave sounds, might be the effect of the agitation of the atmosphere produced by the rarefaction the presence of the sun occasioned; but the situ- 40 EFFECTS OF LIGHT ON SOUND. ation of my windows, and the usual direction of the morning breeze, militated against this argument.” The author then proceeds to give a description of a very delicate instrument, and various apparatus for measuring the propagation and intensity of sound, and the various experi- ments both in the dark and in daylight, and likewise under different changes of the atmosphere, which were made with his apparatus, all of which tended to prove that light had a sensible influence in the propagation of sound. But the de- tail of these experiments and their several results would be too tedious to be here transcribed. The night has generally been considered as more favourable than the day for the trans- mission of sound. “That this is the case,’”’ says Parolette, ‘“‘ with respect to our ears cannot be doubted; but this argues nothing against my opinion. We hear farther by night, on account of the silence, and this always contributes to it, while the noise of a wind favourable to the propagation of a sound may prevent the sound from being heard.’’ In reference to the cause which produces the effect now stated, he proposes the following queries: “Is the atmospheric air more dense on the appearance of light than in darkness? Is this greater density of the air, or of the elastic fluid that is subservient to the propagation of sound, the effect of aeriform substances kept in this state through the medium of light?”” He is dis- posed, on the whole, to conclude that the effect in question is owing to the action of light upon the oxygen of the atmo- sphere, since oxygen gas is found by experiment to be best adapted to the transmission of sound. Our author concludes his communication with the follow- ing remarks: “Light has a velocity 900,000 times as rapid as that of sound. Whether it emanate from the sun and reach to our earth, or act by means of vibrations agitating the particles of a fluid of a peculiar nature, the particles of this fluid must be extremely light, elastic, and active. Nor does it appear to me unreasonable to ascribe to the mechanical action of these particles set in motion by the sun the effects its presence occasions in the vibrations that proceed from sono- rous bodies. 'The more deeply we investigate the theory of light, the more we must perceive that the powers by which the universe is moved reside in the imperceptible particles of bedies ; and that the grand results of nature are but an assemblage of an order of actions that take place in its infi- nitely small parts ; consequently, we cannot institute a series of experiments more interesting than those which tend te ilevelop the properties of light. Our organs of sense are sc GENERAL REMARKS ON LIGHT. 41 immediately connected with the fluid that enlightens us, that the notion of having acquired an idea of the mode of action of this fluid, presents itself to our minds as the hope of a striking advance in the knowledge of what composes the organic mechanism of our life, and of that of beings which closely follow the rank assigned to the human species.” Such is a brief description of some of the leading proper- ties of light. Of all the objects that present themselves to the philosophic and contemplative mind, light is one of the noblest and most interesting. ‘The action it exerts on all the combinations of matter, its extreme divisibility, the rapidity of its propagation, the sublime wonders it reveals, and the office it performs in what constitutes the life of organic beings, lead us to consider it as a substance-acting the first part in the economy of nature. The magic power which this emanation from the heavens exerts on our organs of vision, in exhibiting to our view the sublime spectacle of the universe, cannot be sufficiently admired. Nor is its power confined to the organs of sight; all our senses are, in a greater or less degree, sub- jected to the action of light, and all the objects in this lower creation—whether in the animal, the vegetable, or the mineral kingdoms—are, to a certain extent, susceptible of -its imflu- ence. Our globe appears to be little more than an accumu- lation of terrestrial materials introduced into the boundless ocean of the solar light, as a theatre on which it may display its exhaustless power and energy, and give animation, beauty, and sublimity to every surrounding scene, and to regulate all the powers of nature, and render them subservient to the purposes for which they were ordained. This elementary substance appears to be universal in its movements and in its influence. It descends to us from the solar orb, It wings its way through the voids of space, along a course of ninety-five millions of miles, till it arrives at the outskirts of our globe ; it passes freely through the surrounding atmosphere; it strikes upon the clouds, and is reflected by them; it irra- diates the mountains, the vales, the forests, the rivers, the seas, and all the productions of the vegetable kingdom, and adorns them with a countless assemblage of colours. It scatters and disperses its rays from one end of creation to another, diffus- ing itself throughout every sphere of the universe. It flies without intermission from star to star, and from suns to planets, throughout the boundless sphere of immensity, forming a con- necting chain and a medium of communication among all the worlds and beings within the wide empire of Omnipotence. 4* 42 GENERAL REMARKS ON LIGHT. When the sun is said “to rule over the day,’’ it is inti- mated that he acts as the vicegerent of the Almighty, who has invested him with a mechanical power of giving light, life, and motion to all the beings susceptible of receiving impres- sions from his radiance. As the servant of his Creator, he distributes blessings without number among all the tribes of sentient and intelligent existence. When his rays illumine the eastern sky in the morning, all nature is enlivened with his presence. When he sinks beneath the western horizon, the flowers droop, the birds retire to their nests, and a mantle of darkness is spread over the landscape of the world. When he approaches the equinox in spring, the animal and vegeta- ble tribes revive, and nature puts on a new and a smiling aspect. When he declines towards the winter solstice, dreari- ness and desolation ensue, and a temporary death takes place among the tribes of the vegetable world. ‘This splendid lumi- nary, whose light embellishes the whole of this lower crea- tion, forms the most lively representation of Him who is the source and the centre of all beauty and perfection. ** God is a sun,” the sun of the moral and spiritual universe, from whom all the emanations of knowledge, love, and felicity descend. -* He covereth himself with light as with a gar- ment,’ and “ dwells in light inaccessible and full of glory.” The felicity and enjoyments of the future world are adum- brated under the ideas of light and glory. “'The glory of God enlightens the celestial city ;”’ its inhabitants are repre- sented as “the saints in light ;’’ it is declared that “their sun shall no more go down,”’ and that “the Lord God is their ever- lasting light.’ So that ight not only cheers and enlivens all beings throughout the materia] creation, but is the emblem of the Eternal Mind, and of all that is delightful and trans- porting in the scenes of a blessed immortality. In the formation of light, and the beneficent effects it pro- duces, the wisdom and goodness of the Almighty are conspi- cuously displayed. Without the beams of the sun and the influence of light, what were all the realms of this world but an undistinguished chaos and so many dungeons of darkness? In vain should we roll our eyes around to behold, amid the universal gloom, the flowery fields, the verdant plains, the flowing streams, the expansive ocean, the moon walking in brightness, the planets in their courses, or the innumerable host of stars. All would be lost to the eye of man, and the ‘¢blackness of darkness’? would surround him for ever. And with how much wisdom has every thing been arranged in relation to the motions and minuteness of light! Were it REFRACTION OF LIGHT. 43 capable of being transformed into a solid substance, and retain its present velocity, it would form the most dreadful and .ap- palling element in nature, and produce universal terror and destruction throughout the universe. ‘That this is not impos- sible, and could easily be affected by the hand of Omnipotence, appears from such substances as phosphorus, where light is supposed to be concentrated in a solid state. But in all its operations and effects, as it is now directed by unerring wis- dom and beneficence, it exhibits itself as the most benign and delightful element connected with the constitution of the ma- terial system, diffusing splendour and felicity wherever its influence extends. CHAPTER II. ON THE REFRACTION OF LIGHT. REFRACTION is the turning or bending of the rays of light out of their natural course. Light, when proceeding from a luminous body—without being reflected from any opaque substance or inflected by passing near one—is invariably found to proceed in straight lines without the least deviation. But if it happens to pass obliquely from one medium to another, it always leaves the direction it had before, and assumes a new one. This change of direction, or bending of the rays of light, is what is called Refraction—a term which probably had its origin from the broken appearance which a staff or a long pole exhibits when a portion of it is immersed in water—the word, derived from the Latin He be literally signifying breaking or bending. When light is thus refracted, or has taken a new direction, it then proceeds invariably in a straight line till it meets with a different medium,* when it is again turned out of its course. It must be observed, however, that though we may by this means cause the rays of light to make any number of angles in their course, it is impossible for us to make them describe a curve, except in one single case, namely, where they pass through a medium, the density of which either uniformly increases or diminishes. ‘This is the case with the light of * By a medium, in optics, is meant the space in which a ray of hight moves, whether pure space, air, water, glass, diamond, or any other transparent substance through which the rays of light can pass in straight lines. 44 REFRACTION OF LIGHT. the celestial bodies, which passes downward through our at- mosphere, and likewise with that which is reflected upward through it by terrestrial objects. Im both these cases it de- scribes a curve of the hyperbolic kind ;_ but at all other times it proceeds in straight lines, or in what may be taken for straight lines without any sensible error. . There are two circumstances essential to refraction. 1. That the rays of light shall pass out of one medium into another of a different density, or of a greater or less degree of resistance. 2. That they pass in an oblique direction. ‘The denser the refracting medium, or that into which the ray enters, the greater will be its refracting power; and of two refracting mediums of the same density, that which is of an oily or inflammable nature will have a greater refracting power than the other. The nature of refraction may be more particularly explained and illustrated by the following figure and descrip- tion : Let ADHI, fig. 2, be a body of water, AD its surface, © a point in which a ray of light, B C, enters from the air into the water. This ray, by the greater den- sity of the water, instead of passing straight for- ward in its first direction to K, will be bent at the point C, and pass along . in the direction C E, which is called the refracted ray. Let the line FG be drawn perpendicular to the surface of the water in C, then it is evi- dent that the ray BC, in passing out of air, a rare medium, into a dense medium, as water, is refracted. into a ray C EK, which is nearer to the perpendicular CG than the incident ray BC, and, on the contrary, the ray E.C, passing out of a denser medium into a rarer, will be refracted into C B, which is farther from the perpendicular. The same thing may be otherwise illustrated as follows: Suppose a hole made in one of the sides of the vessel, as at a, and a lighted candle placed within two or three feet of it, when empty, so that its flame may be at L, a ray of light pro- ceeding from it will pass through the hole, a, in a straight line, LBC K, till it reach the bottom of the vessel at K, where it will form a small circle of light. Having put a I ee the Got ois Figure 2. REFRACTION OF LIGHT. 45 mark at the point K, pour water into the vessel till it rise to the height A D, and the round spot that was formerly at K will appear at E; that is, the ray which went straight forward, when the vessel was empty, to K, has been bent at the point C, where it falls into the water, into the line C E. In this ex- periment it is necessary that the front of the vessel should be of glass, in order that the course of the ray may be seen; and if a little soap be mixed with the water so as to give it a little mistiness, the ray © E will be distinctly perceived. If, in place of fresh water, we pour in salt water, it will be found that the ray BC is more bent at C. In like manner, alcohol will refract the ray BC more than salt water, and oil more than alcohol, and a piece of solid glass, of the shape of the water, would refract the light still more than the oil. The angle of refraction depends on the obliquity of the rays falling on the refracting surface being always such, that the sine of the incident angle is to the sine of the refracted angle in a given proportion. .The incident angle is the angle made by a ray of light and a line drawn perpendicular!y to the re- fracting surface, at the point where the light enters the sur- face. The refracted angle is the angle made by the ray in the refracting medium with the same perpendicular produced. The sine of the angle is a line which serves to. measure the angle, being drawn from a point in one leg perpendicular to the other. The following figure (fig. 3) will tend to illustrate these definitions. B Figure 3. In this ficure, BC is the incident ray, C E the refracted ray, DG the perpendicular, A D the sine of the angle of inci- * 46 REFRACTION OF LIGHT. dence AC D, and HR the-sine of the angle of refraction aC KE. Now, it is a proposition in optics, that the sie A D of the angle of incidence BC D is either accurately or very nearly in a given proportion to the sine H R of the angle of refraction GCE. This ratio of the sines is as four to three, when the refraction is made out of air into water, that is, A D is to H R as four to three. When the refraction is out of air into glass, the proportion is about as thirty-one to twenty, or nearly as three to two. If the refraction be out of air into diamond, it is as five to two, thatis, AD: HR::5:2, The denser the medium is, the less is the angle and sine of refrac- tion. Ifa ray of light, MC, were to pass from air into water, or from empty space into air, in the direction MC perpen- dicular to the plane N O, which separates the two mediums, it would suffer no refraction, because one of the essentials to vhat effect is wanting, namely, the obliquity of the incidence. It may be also proper to remark-that a ray of light cannot - pass out of a denser medium into a rarer, if the angle of incidence exceed a certain limit. Thus a ray of light will not pass out of glass into air, if the angle of incidence exceed 40° 11’; or out of glass into water, if the angle of incidence exceed 59° 20’. In such cases refraction will be changed into reflection. The following common experiments, which are easily per- formed, will illustrate the doctrine of refraction: Put a shil- ling, or any other small object which is easily distinguished, into a basin or any other similar vessel, and then retire to such a distance as that the edge of the vessel shall just hide it from your sight. If thén you cause another person to fill the vessel with water, you will find that the shilling is rendered perfectly visible, although you have not in the slightest de- gree changed your position. The reason of this is, that the rays of light, by which it is rendered visible, are bent out of their course. ‘Thus, suppose the shilling to have been placed in the bottom of the basin at E (fig. 2), the ray of light BC which passes obliquely from the air into water at C, instead of continuing its course to K, takes the direction of C E, and, consequently, an object at E would be rendered visible by rays proceeding in that direction, when they would not have touched it had they proceeded in their direct course. ' The same principle is illustrated by the following experi- ment: Place a basin or square box on a table, and a candle at a small distance from it ;.lay a small rod or stick across the sides of the basin, and mark the place where the extremity of the shadow falls, by placing a shilling or other object at PHENOMENA CAUSED BY REFRACTION. ay the point; then let water be poured into the basin, and the shadow will then fall much nearer to the side next the can- dle than before. This experiment may likewise be performed by simply observing the change produced on the shadow of the side of the basin itself. Again, put a long stick obliquely mmto deep water, and the stick will seem to be broken at the point where it appears at the surface of the water, the part which is immersed in the water appearing to be bent upward. Hence every one must have observed that, in rowing a boat, the ends of the oars appear bent or broken every time they are immersed in the water, and their appearance at such times is a representation of the course of the refracted rays. Again, fill a pretty deep jar with water, and you will observe the bottom of the jar considerably elevated, so that it appears much shallower than it did before the water was poured in, in the proportion of nearly a third of its.depth, which 1s owing to the same cause as that which makes the end of a stick immersed in water appear more elevated than it would do if there were no refraction. Another experiment may be just mentioned. Put a-sixpence in a wine-glass, and pour upon it a little water. When viewed in a certain position, “two sixpences will appear in the glass—one image of the six- pence from below, which comes directly to the eye, and an- other which appears considerably raised above the other, in consequence of the rays of light rising through the water, and being refracted. In this experiment the wine-glass should not be more than half filled with water. The refraction of light explains the causes of many curious and interesting phenomena both in the heavens and on the earth. When we stand on the banks of a river, and look ob- liquely through the waters to its bottom, we are apt to think it is much shallower than it really is. If it be eight feet deep in reality, it will appear from the bank to be only six feet; if it be five feet and a half deep, it will appear only about four feet. This is owing to the effects of refraction, by which the bottom of the river is apparently raised by the re- fraction of the light passing through the water into air, so as to make the bottom appear higher than it really is, as in the experiment with the jar of water. This is a circumstance of some importance to be known and attended to in order to personal safety ; for many school-boys and other young per- sons have lost their lives by attempting to ford a river, the bottom of which appeared to be within their reach when thev viewed it from its banks; and even adult travellers on horse- back have sometimes fallen victims to this optical deception ; ~ 48 PHENOMENA CAUSED BY REFRACTION. and this is not the only case in which a knowledge of the laws of nature may be useful in guarding us against dangers and fatal accidents. It is likewise owing to this refractive power in water that a skilful marksman, who wishes to shoot fish under water, is obliged to take aim considerably below the fish as it appears, because it seems much nearer the top of the water than it really is. An acquaintance with this property of light is particularly useful to divers, for, in any of their movements or operations, should they aim directly at the object, they would arrive at a point considerably beyond it; whereas, by having some idea of the depth of the water, and the angle which a line drawn from the eye to the object makes with its surface, the point at the bottom of the water, between the eye and the object at which the aim is to be taken, may be easily determined. For the same reason, a person below water does not see objects distinctly. -~For, as the aqueous humour of the eye has the same refractive power as water, the rays of light from any object under water will undergo no refraction in passing through the cornea and aqueous humour, and will therefore meet in a point far behind the retina. But if any person, accustomed to go below water, should use a pair of spectacles, consisting of two convex lenses, the radius of whose surface is three-tenths of an inch—which is nearly the radius of the convexity of the cornea—he will see objects as distinctly below water as above it. It is owing to refraction that we cannot judge so accurately of magnitudes and distances in water as in air. A fish looks considerably larger in water than when taken out of it. An object plunged vertically into water always appears con- tracted, and the more so as its upper extremity approaches nearer the surface of the water. Every thing remaining in the same situation, if we take the object gradually out of the water, and it be of a slender form, we shall see it become larger and larger, by a rapid development, as it were, of all its parts. ‘The distortion of objects, seen through a crooked pane of glass in a window, likewise arises from its unequal refraction of the rays that pass through it. It has been cal- culated that, in looking through the common glass of a win- dow, objects appear about the one-thirtieth of an inch out of their real place, by means of the refraction. Refraction likewise produces an effect upon the heavenly bodies, so that their apparent positions are generally different from their real. By the refractive power of the atmosphere, the sun is seen before he comes to the horizon in the morn- TWILIGHT. 49 ing, and after he sinks beneath it in the evening; and hence this luminary is never seen in the place in which it really is, except when it passes the zenith at noon, to places within the torrid zone. The sun is visible when actually thirty-two minutes of a degree below the horizon, and when the opaque rotundity of the earth is interposed between our eye and that orb, just on the same principle as, in the experiment with the shilling and basin of water, the shilling was seen when the edge of the basin interposed between it and the sight. The refractive power of the atmosphere has been found to be much greater, in certain cases, than what has been now stated. In the year 1595, a company of Dutch sailors having been wrecked on the shores of Nova Zembla, and having been obliged to remain in that desolate region during a night of more than three months, beheld the sun make his ap- pearance in the horizon about sixteen days before the time in which he should. have risen according to calculation, and when his body was actually more than four degrees below the horizon; which circumstance has been attributed to the great refractive power of the atmosphere in those intensely cold re- gions. ‘This refraction of the atmosphere, which renders the apparent rising and setting of the sun both earlier and later than the real, produces, at least, one important beneficial effect. It procures for us the benefit of a much longer day at all seasons of the year, than we should enjoy did not this property of the atmosphere produce this effect. It is owing to the same cause that the disks of the sun and moon appear elliptical or oval when seen in the horizon, their horizontal diameters appearing longer than their vertical, which is caused by the greater refraction of the rays coming from the lower limb, which is immersed in the densest part of the at- mosphere. The illumination of the heavens which precedes the rising of the sun, and continues some time after he is set, or what is commonly called the morning and evening twilight, is likewise produced by the atmospherical refraction, which cir- cumstance forms a very pleasing and beneficial arrangement in the system of nature. It not only prolongs to us the in- fluence of the solar light, and adds nearly two hours to the length of our day, but prevents us from being transportea all at once from the darkness of midnight to the splendour of noonday, and from the effulgence of day to the gloom and horrors of the night, which would bewilder the traveller and navigator in their journeys by sea or land, and strike the living world with terror and amazement. Vor. IX. 3) 50 PHENOMENA CAUSED BY REFRACTION. The following figure will illustrate the position now stated, and the manner in which the refraction of the atmosphere produces these effects: Let A aC, fig. 4, represent one half of our globe, and the dark space between that curve and Br D, the atmosphere. A person standing on the earth’s sur- face at a would see the sun rise at b, when that luminary was in reality only at c, more than half a degree below the Z SS b LZ | i@sG<“LRS Figure 4. horizon. When the rays of the sun, after having proceeded in a straight line through empty space, strike the upper part of the atmosphere at the point d, they are bent out of their right-lined course by the refraction of the atmosphere, into the direction d a, so that the body of the sun, though actually intercepted by the curve of the earth’s convexity, consisting of a dense mass of land or water, is actually beheld by the spectator at a. The refractive power of the atmosphere gradually diminishes from the horizon to the zenith, and in- creases from the zenith to the horizon, in proportion to the den- sity of its different strata, being densest at its lower extremity next the earth, and more rare towards its higher regions. If a person at a had the sun e, in his zenith, he would see him where he really is; for his rays coming perpendicularly through the atmosphere,-would be equally attracted in alldirections, and would, therefore, suffer no inflection. But, about two in the afternoon, he would see the sun at 7, though, in reality, he was at k, thirty-three seconds lower than his apparent situa- tion. At about four in the afternoon he would see him at m, when he is at n, one minute and thirty-eight seconds from his apparent situation. But at six o’clock, when we shall suppose he sets, he will be seen at o, though he is at that EXTRAORDINARY CASES. 5l time at p, more than thirty-two minutes below the horizon. These phenomena arise from the different refractive powers of the atmosphere at different elevations, and from the obli- quity with which the rays of light fall upon it; for we see every object along that line in which the rays from it are directed by the last medium through which they passed. The same phenomena happen in relation to the moon, the planets, the comets, the stars, and every other celestial body, all of which appear more elevated, especially when near the - horizon, than their true places. The variable and increasing refraction from the zenith to the horizon is a source of con- siderable trouble and difficulty in making astronomical obser- vations, and in nautical calculations; for, in order to deter- mine the real altitudes of the heavenly bodies, the exact de- oree of refraction at the observed elevation must be taken into account. ‘T'o the same cause we are to ascribe a phenomenon that has sometimes occurred, namely, that the moon has been seen rising totally eclipsed, while the sun was still vi- sible in the opposite quarter of the horizon. At the middle of a total eclipse of the moon, the sun and moon are in oppo- sition, or 180 degrees asunder: and, therefore, were no at- mosphere surrounding the earth, these luminaries, in such a position, could never be seen above the horizon at the same time. But by the refraction of the atmosphere near the hori- zon, the bodies of the sun and moon are raised more than 32 minutes above their true places, which is equal, and some- times more than equal, to the apparent diameters of these bodies. Extraordinary Cases of Refraction in Relation to Terres- trial Objects. In consequence of the accidental condensation of certain strata of the atmosphere, some very singular effects have been produced in the apparent elevation of terrestrial objects to a position much beyond that in which they usually appear The following instance is worthy of notice. It is taken from the Philosophical Transactions of London for 1798, and was communicated by W. Latham, Esq., F. R. 8., who observed the phenomenon from Hastings, on the south coast of England : *On July 26, 1797, about five o’clock in the afternoon, as I was sitting in my dining-room in this place, which is situated upon the Parade, close to the seashore, nearly fronting the south, my attention was excited by a number of people rus- ning down to the seaside. Upon inquiring the reason, I was informed that the coast of France was plainly to be distin- 52 PHENOMENA CAUSED BY REFRACTION. suished by the naked eye. { immediately went down to the shore, and was surprised to find that, even without the assist- ance of a telescope, I could very plainly see the cliffs on the opposite coast, which, at the nearest part, are between forty and fifty miles distant, and are not to be discerned from that low situation by the aid of the best glasses. ‘They ap- peared to be only a few miles off, and seemed to extend for some leagues along the coast. I pursued my walk along the shore eastward, close to the water’s edge, conversing with the sailors and fishermen upon the subject. They at first would not be persuaded of the reality of the appearance; but they soon became so thoroughly convinced by the cliffs gradually appearing more elevated, and approaching nearer, as it were, that they pointed out and named to me the different places they had been accustomed to visit, such as the Bay, the Old Head, or Man, the Windmill, &c., at Boulogne, St. Vallery, and other places on the coast.of Picardy, which they after- ward confirmed, when they viewed them through their tele- scopes. ‘Their observations were, that the places appeared as near as if they were sailing, at a small distance, into the har- bours. The day on which this phenomenon was seen was extremely hot; it was high water at Hastings about two o’clock P. M., and not a breath of wind was stirring the whole day.” From the summit of an adjacent hill, a most beautiful scene is said to have presented itself. At one glance the spectators could see Dungeness, Dover Cliffs, and the French coast, all along from Calais to St. Vallery, and, as some affirmed, as far to the westward as Dieppe, which could not be much less than eighty or ninety miles. By the tele- scopes the French fishing-boats were plainly seen at anchor, and the different colours of the land on the heights, with the buildings, were perfectly discernible. This singular phenomenon was doubtless occasioned by an extraordinary refraction, produced either by an unusual ex- pansion or condensation of the lower strata of the atmosphere, arising from circumstances connected with the extreme heat of the season. ‘The objects seem to have been apparently raised far above their natural positions ; for, from the beach at Hastings, a straight line, drawn across towards the French coast, would have been intercepted by the curve of the waters. They seem also to have been magnified by the refraction, and brought apparently, four or five times nearer the eye than in the ordinary state of the atmosphere. The following are likewise instances of unusual refraction : When Captain Colby was ranging over the coast of Caith- EXTRAORDINARY CASES. 53 ness, with the telescope of his great theodolite, on the 21st of June, 1819, at eight o’clock p. m., from Corryhabbie Hill, near Mortlich, in Banffshire, he observed a brig over the land of Caithness, sailing to the westward in the Pentland Frith, between the Dunnet and Duncansby heads. Having satisfied himself as to the fact, he requested his assistants, Lieutenants Robe and Dawson, to look through the telescope, which they immediately did, and observed the brig likewise. It was very distinctly visible for several minutes, while the party continued to look at it, and to satisfy themselves as to its po- sition. The brig could not have been less than from ninety to one hundred miles distant; and, as the station on Corry- habbie is not above 850 yards above the sea, the phenomenon is interesting. ‘The thermometer was at 44°. The night and day preceding the sight of the brig had been continually rainy and misty, and it was not till seven o’clock of the evening of the 21st that the clouds cleared off the hill.* Captain Scoresby relates a singular phenomenon of this kind, which occurred while he was traversing the Polar seas. His ship had been separated by the ice from that of his father for a considerable time, and he was looking out for her every day with great anxiety. At length, one evening, to his utter astonishment, he saw her suspended in the air, in an inverted position, traced on the horizon in the clearest colours, and with the most distinct and perfect representation. He sailed in the direction in which he saw this visionary phenomenon, and actually found his father’s vessel by its indication. He was divided from him by immense masses of icebergs, and at such a distance that it was quite impossible to have seen the ship in her actual situation, or to have seen her at all, if her spectrum had not been thus raised several degrees above the horizon mto the sky by this extraordinary refraction. She was reckoned to be seventeen miles beyond the visible horizon, and thirty miles distant. | Mrs. Somerville states that a friend of hers, while standing on the plains of Hindostan, saw the whole upper chain of the Himalaya Mountains start into view, from a sudden change in the density of the air, occasioned by a heavy shower, after a long course of dry and hot weather. In looking at distant objects through a telescope, over the top of a ridge of hills, about two miles distant, [ have several times observed that some of the more distant objects which are sometimes hid by the interposition of a ridge of hills, are at other times distinctly visible above them. I have sometimes observed that objects * Edinburgh Philosophical ai tes for October, 1819, p. 411. 54 BENEFICIAL EFFECTS OF REFRACTION. near the middle of the field of view of a telescope, which was in a fixed position, have suddenly appeared to descend to the lower part, or ascend to the upper part of the field, while the telescope remained unaltered. I have likewise seen with a powerful telescope, the Bell Rock Lighthouse, at the distance of about twenty miles, to appear as if contracted to less than two-thirds of its usual apparent height, while every part of it was quite distinct and well defined, and in the course of an hour, or less, it appeared to shoot up to its usual apparent elevation—all which phenomena are evidently produced by the same cause to which we have been adverting. Such are some of the striking effects produced by the re- fraction of light. It enables us to see objects in a direction where they are not; it raises, apparently, the bottoms of lakes and rivers ; it magnifies objects when their light passes through dense mediums; it makes the sun appear above the horizon when he is actually below it, and thus increases the leneth of our day ; it produces the Aurora and the evening twilight, which forms, in many instances, the most delightful part of a summer day; it prevents us from being involved in total darkness, the moment after the sun descended beneath the horizon; it modifies the appearances of the celestial bodies, and the directions in which they are beheld; it tinges the sun, moon, and stars, as well as the clouds, with a ruddy hue when near the horizon; it elevates the appearance of terres- trial objects, and, in certain extraordinary cases brings them nearer to our view, and enables us to behold them when be- yond the line of our visible horizon. In combination with the power of reflection, it creates visionary landscapes, and a variety of grotesque and extraordinary appearances, which delight and astonish, and sometimes appal the beholders. In short, as we shall afterward see more particularly, the refrac- tion of light through glasses of different figures forms the prin- ciple on which telescopes and microscopes are constructed, by which both the remote and the minute wonders of crea- tion have been disclosed to view. So that, had there been no bodies capable of refracting the rays of light, we should have remained for ever ignorant of many sublime and august objects in the remote regions of the universe, and of the ad- mirable mechanism and the countless variety of minute ob- jects which le beyond the range of the unassisted eye in our lower creation, all of which are calculated to direct our views, and to enlarge our conceptions of the Almighty Creator. In the operation of the law of refraction in these and nu- merous other instances, we have a specimen of the diversified BENEFICIAL EFFECTS OF REFRACTION. 55 and beneficent effects which the Almighty can produce by the agency of a single principle in nature. By the influence of the simple law of gravitation the planets are retained in their orbits, the moon directed in her course around the earth, and the whole of the bodies connected with the sun preserved in one harmonious system. By the same law the mountains of our globe rest on a solid basis, the rivers flow through the plains towards the seas, the ocean is confined to its prescribed boundaries, and the inhabitants of the earth are retained to its surface, and prevented from flying upward through the voids of space. In like manner, the law by which light is refracted produces a variety of beneficial effects essential to the present constitution of our world and the comfort of its inhabitants. When a ray of light enters obliquely into the atmosphere, in- stead of passing directly through, it bends a little downwards, so that the oreater portion of the rays which thus enter the atmospheric mass descend by inflection to the earth. . We then enjoy the benefits of that light which would otherwise have been totally lost. We perceive the light of day an hour before the solar orb makes its appearance, and a portion of its light is still retained when it has descended nearly eighteen degrees below our horizon. Wethus enjoy, through- out the year, seven hundred and thirty hours of hght which would have been lost had it not been refracted down upon us from the upper regions of the atmosphere. To the inhabit- ants of the polar regions this effect is still more interesting and beneficial. Were it not for their twilight, they would be involved, for a much longer pericd than they now are, in perpetual darkness; but by the powerful refraction of light which takes place in the frigid zones, the day sooner makes its appearance towards spring, and their long winter nights are, In certain cases, shortened by a period of thirty days. Under the poles, where the darkness of night would continue six months without intermission, if there were no refraction, total darkness does not prevail during the one-half of this period. When the sun sets, at the north pole, about the 23d of September, the inhabitants (if any) enjoy a perpetual aurora till he has descended eighteen degrees below the hori- zon. In his course through the ecliptic, the sun is two months before he can reach this point, during which time there is a perpetual twilight. In two months more he arrives again at the same point, namely, eighteen degrees below the horizon, when a new twilight commences, which is con- tinually increasing in brilliancy for other two months, at the end of which the body of this luminary is seen rising in all 56 BENEFICIAL EFFECTS OF REFRACTION. its glory. So that, in this region, the light of day is enjoyed, in a greater or less degree, for ten months without interrup- tion by the effects of atmospheric refraction; and, during the two months when the influence of the solar light is en- tirely withdrawn, the moon is shining above the horizon for two half months without intermission; and thus it happens that no more than two separate fortnights are passed in ab- solute darkness; and this darkness is alleviated by the light of the stars and the frequent coruscations of the Aurora Bo- realis. Hence it appears that there are no portions of our globe that enjoy, throughout the year, so large a portion of the solar light as these northern regions, which is chiefly owing to the refraction of the atmosphere. The refraction of light by the atmosphere, combined with its power of reflecting it, is likewise the cause of that uni- versal light and splendour which appears on all the objects around us. Were the earth disrobed of its atmosphere, and exposed naked to the solar beams, in this case we might see the sun without having day, strictly so called.. His rising would not be preceded by any twilight as it now is. The most intense darkness would cover us till the very moment of his rising; he would then suddenly break out from under the horizon with the same splendour he would exhibit at the highest part of his course, and would not change his bright- ness till the very moment of his setting, when, in an instant all would be black as the darkest night. At noonday we should see the sun like an intensely brilliant globe shining in a sky as black as ebony, like a clear fire in the night seen in the midst of an extensive field, and his rays would show us the adjacent objects immediately around us; but the rays which fall on the objects remote from us would be for ever lost in the expanse of the heavens. Instead of the beautiful azure of the sky, and the colours which distinguish the face of nature by day, we should see nothing but an abyss of dark- ness, and the stars shining from a vault as dark as chaos. Thus there would be no day, suchas we now enjoy, without the atmosphere; since it is by the refraction and reflections connected with this aerial fluid that light is so modified and directed as to produce all that beauty, splendour, and har- mony which appear on the concave of the sky, and on the objects which diversify our terrestrial abode. The effect of refraction, in respect to terrestrial objects, is likewise of a beneficial nature. The quantity of this refrac- tion is estimated by Dr. Maskelyne at one-tenth of the dis- tance of the object observed, expressed in degrees of a great BENEFICIAL EFFCTS OF REFRACTION. 57 circle. Hence, if the distance be 10,000 fathoms, its tenth part, 1000 fathoms, is the sixtieth part of a degree, or one minute, which is the refraction in altitude. Le Gendre . estimates it at one fourteenth, De Lambre at one eleventh, and others at a twelfth of the distance; but it must be sup- posed to vary at different times and places according to the varying state of the atmosphere. This refraction, as_ it makes objects appear to be raised higher than they really are, enlarges the extent of our landscapes, and enables us to perceive distant objects which would otherwise have been invisible. It is particularly useful to the navigator at sea. It is one important object of the mariner, when traversing his course, to look out for capes and headlands, rocks and islands, so as to descry them as soon as they are within the reach of his eye. Now, by means of refraction, the tops of hills and the elevated parts of coasts are apparently raised into the air, so that they may be discovered several leagues farther off on the sea than they would be did no such re- fractive power exist. This circumstance is therefore a con- siderable benefit to the science of navigation, in enabling the mariner to steer his course aright, and to give him the most early warning of the track he ought to take, or of the dangers to which he may be exposed. In short, the effects produced by the refraction and reflec- tion of light on the scenery connected with our globe teach us that these principles, in the hand of the Almighty, might be so modified and directed as to produce the most pictur- esque, the most glorious, and wonderful phenomena, such as mortal eyes have never yet seen, and of which human imagi- nation can form no conception; and in other worlds, more re- splendent and magnificent than ours, such scenes may be fully realized, in combination with the operation of physical princi- ples and agents with which we are at present unacquainted. From what we already know of the effects of the reflection | and the refraction of light, it is not beyond the bounds of probability to suppose that, in certain regions of the universe, light may be reflected and refracted through different me- diums, in such a manner as to present to the view of their inhabitants the prominent scenes connected with distant sys- tems and worlds, and to an extent as shall infinitely surpass the effects produced by our most powerful telescopes. 58 INTRODUCTION TO OPTICS. CHAPTER III. ON THE REFRACTION OF LIGHT THROUGH SPHERICAL TRANS- ° PARENT SUBSTANCES, OR LENSES. Ir is to the refraction of light that we are indebted for the use of lenses or artificial glasses to aid the powers of vision. It lays the foundation of telescopes, microscopes, camera ob- scuras, phantasmagorias, and other optical instruments, by which so many beautiful, useful, and wonderful effects have been produced. In order, therefore, to illustrate the princi- ples on which such instruments are constructed, it is neces- sary to explain the manner in which the rays of light are re- fracted and modified when passing through spherical me- diums of different forms. I do not intend, however, to enter into the minutie of this subject, nor mto any abstract mathe- matical demonstrations, but shall simply offer a few explana- tions of general principles, and several experimental illustra- tions, which may enable the general reader to understand the construction of the optical instruments to be afterward de- scribed. A lens is a transparent substance of a different density from the surrounding medium, and terminatiny im two surfaces, either both spherical, or one spherical and the other plain. | It is usually made of glass, but may also be formed of any other transparent substance, as ice, crystal, diamond, pebbles, or by fluids of different densities and refractive powers, enclosed between concave glasses. Lenses are ground into various forms, according to the purpose they are intended to serve. They may be generally distin- Figure 5. guished as being either alien A. Plins-conver or concave. A convex glass is Ciltice ti tadl ye taicdlé, noel this, yO Pranno-coneave. ner towards the extremities. Of these there are various forms, ¢ ee which are represented in fig. 5. A is a plano-convex lens, which nH ol has one side plane, and the other spherical or convex. B is a s gy plano-concave, which is plane oe i UT on the one side and concave # 7 (oe on-the other. C is a double convex, or one which is sphercial on both sides. D, a double \S/ Double Convex. i Double Concave. (— Meniscus, DEFINITION OF TERMS. 59 concave, or concave on both sides. E is called a meniscus which is convex on one side and concave on the other. F is a concavo-convex, the convex side of which is of a smaller sphere than the concave. In regard to the degree of convexi- ty or concavity in lenses, it is evident that there may be al- most an infinite variety. For every convex surface is to be considered as the segment of a circle, the diameter and radius of which may vary to almost any extent. Hence lenses have been formed by opticians, varying from one-fiftieth of an inch in radius to two hundred feet. When we speak of the length of the radius of a lens, as, for instance, when we say that a lens is two inches or forty inches radius, we mean that the convex surface of the glass is the part of a circle, the radius of which, or half the diameter, is two inches or forty inches ; or, in other words, were the portion of the sphere on which it is ground formed into a globe of corresponding convexity, it would be four inches or eighty inches in diameter. The azis of a lens is a straight line drawn through the centre of its spherical surface ; and, as the spherical sides of every lens are arches of circles, the axis of the lens would pass through the centre of that circle of which its sides are segments. Rays are those emanations of light which pro- ceed from a luminous body, or from a body that is illuminated, The Radiant is that body or object which emits the rays of light, whether it be a self-luminous body, or one that only reflects the rays of light. Rays may proceed from a Radiant in different directions. ‘They may be either parallel, con- verging, or diverging. Parallel rays are those which pro- ceed equally distant from each other through their whole course. Rays proceeding from the sun, the planets, the stars, and distant terrestrial objects are considered as parallel, as in fig. 6. Converging rays are suchas, proceeding from a body, approach nearer and nearer in their progress, tending toa certain point where they all unite. Thus, the rays proceed- ing from the object A B (fig. 7) to the point F, are said to converge towards that point. All convex glasses cause par- allel rays which fall upon them to converge, in a greater or less degree ; and they render converging rays still more con- vergent. If A B, fig. 7, represent a convex lens, and H GI parallel rays falling upon it, they will be refracted, and con- verge towards the point F’, which is called the focus, or burn- ing point; because, when the sun’s rays are thus converged to a point by a large lens, they set on fire combustible sub- stances. In this point the rays meet and intersect each other, Diverging rays are those which, proceeding from any point, 60 REFRACTION OF LIGHT BY LENSES. Fre. 6. Fig. 7. Fig. 8 as A, fig. 8, continually recede from each other as they pass along in their course towards B C. All the rays which pro- ceed from near objects, as a window in a room, or an adjacent house or garden, are more or less divergent. The following figures show the effects of parallel, converging, and diverging rays, in passing through a double convex lens : Fig. 9 shows the effects of parallel rays, K A, DE, LB, falling on a convex glass, AB. The rays which fall near the extremities at A and B are bent or refracted towards C F, the focus, and centre of convexity. It will be observed that they are less refracted as they approach the centre of the lens, and the central ray DEC, which is called the avis of the lens, and which passes through its centre, suffers no re- fraction. Fig. 10 exhibits the course of converging rays when passing through a similar lens. In this case, the rays ‘converge to a focus nearer to the lens than the centre; for a convex lens uniformly increases the convergence of converg- ing rays. The converging rays here represented may be conceived as having been refracted by. another convex lens of a longer focus, and, passing on towards a point of conver- gence, were intercepted by the lens AB. The point D is the place where the rays would have converged to a focus, had they not been thus intercepted. Fig. 11 represents the course of diverging rays when falling on a double convex glass. In this case, the rays DB, DA, &c., after passing through the Jens, converge to a focus ata point considerably farther from the lens than its centre, as at F. Such rays must be cou. sidered as proceeding from near objects, and the fact may b- REFRACTION OF LIGHT BY LENSES. 61 Figure 11. Figure 9. Figure 10. Ko BoD D cai oD ONIVIE SAVY TaTIVvavd aN i nt a pi D Tui UT ny ANY Wine it) iy il a yu aut mT on ii) Tu ny Ai) wo: centeRC YE’ rocus Laas b ‘ 174 ’ ‘ v a : Hy i E Focus PB: Di illustrated by the following experiment: Take a common reading glass, and hold it in the rays of the sun, opposite a sheet of writing paper or a white wall, and observe at what distance from the glass the rays on the paper converge to a small, distinct white spot. This distance gives the focal length of the lens by parallel rays. If now we hold the glass within a few feet of a window, ora burning candle, and re- Ceive its image on the paper, the focal distance of the image from the glass will be found to be longer. If, in the former case, the focal distance was twelve inches, in the latter case it will be thirteen, fifteen, or sixteen inches, according to the distance of the window or the candle from the glass. If the lens A B, fig. 9, on which parallel rays are repre- sented as falling, were a plano-convezx, as represented at A, fiz. 5, the rays would converge to a point P, at double the radius, or the whole diameter of the sphere of which it is a segment. If the thickness of a plano-convex be considered, and if it be exposed on its convex side to parallel rays, as those of the sun, the focus will be at the distance of twice the radius, Vou. TX. 6 62 FOCAL DISTANCE OF CONCAVE LENSES. wanting two-thirds of the thickness of thelens. But if the same lens be exposed with its plane side to parallel rays, the focus will then be precisely at the distance of twice the radius from the glass. The effects of concave lenses are directly opposite to those of convex. Parallel rays, striking one of those glasses, instead of converging towards a point, are made to diverge. Rays already divergent are rendered more so, and convergent rays are made less convergent. Hence objects seen through concave glasses appear considerably smaller and more distant than they really are. The following diagram, fig. 12, repre- sents the course of parallel rays through a double concave lens, where the parallel rays T A, DE, IB, &c., when pass- ing through the concave glass A B, diverge into the rays GL, EC, HP, &c., as if they proceeded from F, a point before the lens, which is the principal focus of the lens: Figure 12. = Pe. a es me WA ee er az -* S, Parallel Rays. The principal focal distance, E F, isthe same as in convex lenses. Concave glasses are used to correct the imperfect vision of short-sighted persons. As the form of the eye of such persons is too convex, the rays are made to converge before they reach the optic nerve; and therefore a concave glass, causing a little divergency, assists this defect of vision, by diminishing the effect. produced by the too great convexity of the eye, and lengthening its focus. ~ These glasses are seldom used, in modern times, in the construction of optical instruments, except as eye-glasses for small pocket perspec- tives, and opera-glasses. To find the focal distance of a concave glass. Takea piece of pasteboard or card paper, and cut a round hole in it, not larger than the diameter of the lens; and on another piece of pasteboard describe a circle whose diameter is just double the diameter of the hole. Then apply the piece with the hole in it to the lens, and hold them in the sunbeams, with the other piece at such a distance behind that the light proceeding from the hole may spread or diverge so as pre- IMAGES FORMED BY CONVEX LENSES. 63 cisely to fill the circle; then the distance of the circle from the lens is equal to its virtual focus, or to its radius, if it bea double concave, and to its diameter, if a plano-concave. Let d, e, (fig. 12,) represent the diameter of the hole, and g, 2, the diameter of the circle, then the distance C, I, is the vir- tual focus of the lens.* The meniscus, represented at E, fig. 5, is like the crystal of a common watch, and, as the convexity is the same as the concavity, it neither magnifies nor diminishes. Sometimes, however, it is made in the form of a crescent, as at F, fig. 5, and is called a concavo-convex lens ; and, when the convexity is greater than the concavity, or when it is thickest in the middle, it acts nearly in the same way as a double or plano- convex lens of the same focal distance. Of the Imaczs formed by Convex Lenses. It is a remarkable circumstance, and which would naturally excite admiration, were it not so common and well known, that when the rays of light from any object are refracted through a convex lens, they paint a distinct and accurate picture of the object before it, in all its colours, shades, and proportions. Previous to experience, we could have had no conception that light, when passing through such substances, and converging to a point, could have produced so admirable an effect—an effect on which the construction and utility of all our optical instruments depend. The following figure will illustrate this position : Let LN represent a double convex lens, A,C, a its axis, and O B an object perpendicular to it. A ray passing from the extremity of the object at O, after being refracted by the lens at F’, will pass on in the direction FI, and form an image of that part of the object at I. This ray will be the axis of all the rays which fall on the lens from the point O, and I will be the focus where they will all be collected. In like man- ner, BC M is the axis of that parcel of rays which proceed from the extremity of the object B, and their focus will be at M; and since all the points in the object between O and B must necessarily have their foci between I and M, a complete * This mode of finding the focus of a concave lens may be varied as ‘ollows: Let the lens be covered with paper, having two small circular yoles; and, on the paper for receiving the light, describe also two small circles, but with their centres at twice the distance from each other of the centres of the circles. Then move the paper to and from, till the middle of the sun’s light, coming through the holes, falls exactly on the middle of the circles: that distance of the paper from the lens will be the focal length required. 64 IMAGES FORMED BY CONVEX LENSES. Figure 13. = (7) ee QOS SSS SSS” \. eins iE y B picture of the points from which they come will be depicted, and consequently an image of the whole object O B. It is obvious, from the figure, that the image of the object is formed in the focus of the lens in an inverted position. It must necessarily be in this position, as the rays cross at C, the centre of the lens; and as it is impossible that the rays from the upper part of the object, O,can be carried by re- fraction to the upper end of the image at M. ‘This is a uni- versal principle in. relation to convex lenses. of every descrip- tion, and requires to be attended to in the construction and use of all kinds of telescopes and microscopes. It is easily illustrated by experiment. Take a convex lens of eight, twelve, or fifteen inches focal distance, such as a reading glass, or the glass belonging to a pair of spectacles, and holding it, at its focal distance from a white wall, in a line with a burning candle, the flame of the candle will be seen depicted on the wall in an inverted position, or turned upside down. The same experiment may be performed with a win- dow-sash, or any other bright object. But the most beautiful exhibition of the images of objects formed by convex lenses is made by darkening a room,and placing a convex lens of a long focal distance in a hole cut out of the window-shutter; when a beautiful inverted landscape, or picture of all the objects before the window, will be painted on a white paper or screen placed in the focus of the glass. The image thus formed exhibits not only the proportions and colours, but also the motions of all the objects. opposite the lens, forming, as it were, a living landscape. 'This property of lenses lays the foundation of the camera obscura, an instrument to be afterward described. The following principles in relation to images formed by > IMAGES FORMED BY CONVEX LENSES. 65 convex lenses may be stated: 1. That the image subtends the same angle at the centre of the glass as the object itself does. Were an eye placed at C, the centre of the lens LN, fig. 13, it would see the object O B and the image I M un- der the same optical angle, or in other words, they would ap- pear equally large; for, whenever right lines intersect each other, as O I and B M, the opposite angles are always equal, that is, the angle M C Lis equal to the angle OC B. 2. The length of the image formed by aconvex lens is, to the length of the object, as the distance of the image is to the distance of the object from the lens; that is, M I is to OBasCa toC A. Suppose the distance of the object C A from the lens to be forty-eight inches, the length of the object O B= sixteen inches, and the distance of the image from the lens six inches, then the length of the image will be found by the following proportion, 48: 16:: 6: 2, that is, the length of the image, in sucha case, is two inches. 3. Jf the object be at an infinite distance, the image will be formed exactly in the focus. 4. If the object be at the same distance from the lens as its focus, the image is removed to an infinite dis- tance on the opposite side; in other words, the rays will pro- ceed in a parallel direction. On this principle, lamps on the streets are sometimes directed to throw a bright light along a footpath where it is wanted, when a large convex glass is placed at its focal distance from the burner; and on the same principle, light is thrown to a great distance from lighthouses, either by a very large convex lens of a short focal distance, or by a concave reflector. 5. /f the object be at double the _ distance of the focus from the glass, the image will also be at double the distance of the focus from the glass. Thus, if a lens of six inches focal distance be held at twelve inches’ distance from a candle, the image of the candle will be formed at twelve inches from the glass on the other side. 6. Jf the object be a little farther from the lens than its focal distance, animage will be formed ata distance from the object, which will be greater or smaller in proportion to the distance. For example, if a lens five inches focus be held at a little more than five inches from a candle, and a wall or screen at five feet six inches distant receive the image, a large and in- verted image of the candle will be depicted, which will be magnified in proportion as the distance of the wall from the candle exceeds the distance of the lens from the candle. Suppose the distance of the lens to be five and a half inches, then the distance of the wall where the image is formed, be- ing twelve times greater, the image of the candle will be 6* 66 REFLECTIONS ON THE LAWS OF REFRACTION. magnified twelve times. If M I (fig. 13) be considered ag the object, then O B will represent the magnified image on the wall. On this principle, the image of the object is formed by the small object glass of a compound microscope. On the same principle, the large pictures are formed by the Magic Lantern and the Phantasmagoria ; and in the same way small objects are represented in a magnified form on a sheet or wall by the Solar microscope. 7. All convex lenses magnify the objects seen through them, in a greater or less degree. The shorter the focal distance of the Jens, the greater is the mag- nifying power. A lens four inches focal distance will mag- nify objects placed in the focus two times in length and breadth; a lens two inches focus will magnify four times; a lens one inch focus eight times; a lens half an inch focus sixteen times, &c., supposing eight inches to be the least dis- tance at which we see near objects distinctly. In viewing ob- jects with small lenses, the object.to be magnified should be placed exactly at the focal distance of the lens, and the eye at about the same distance on the other side cf the lens. When we speak of magnifying power, as, for example, that a lens one inch focal distance magnifies objects eight times, it is to be under- stood of the Zineal dimensions of the object. But asevery object at which we look has breadth as well as length, the surface of the object is in reality magnified sixty-four times, or the square of its lineal dimensions ; and for the same reason a lens half an inch focal distance magnifies the surfaces of objects 256 times. Reflections deduced from the preceding Subject. Such are some of the leading principles which require to be recognised in the construction of refracting telescopes, microscopes, and other dioptric instruments whose performance chiefly depends on the refraction of light. It is worthy of particular notice, that all the phenomena of optical lenses now described depend upon that peculiar property which the Creator has impressed upon the rays of light, that, when they are refracted to a focus by a convex transparent substance, they depict an accurate vmage of the objects whence they proceed. 'This, however common, and however much overlooked by the bulk of mankind, is, indeed, a very wonderful property vith which light has been endued. Previous to experience, we could have had no conception that such an effect would be produced ; and, in the first instance, we could not possibly have traced it to all its consequences. — All the objects in creation might have been illuminated as they now are, for aught we know, without sending forth either direct or reflected rays with REFLECTIONS ON THE LAWS OF REFRACTION. 67 the property of forming exact representations of the objects whence they proceed. But this we find to be a universal law in regard to light of every description, whether as emanating di- rectly from the sun, or as reflected from the objects he illumi- nates, or as proceeding from bodies artificially enlightened. Itis a law or a property of light not only in our own system, but throughout all the systems of the universe to which mortal eyes have yet penetrated. The rays from the most distant star which astronomers have described are endued with this property, otherwise they could never have been perceived by means of our optical instruments ; for it is by the pictures or images formed in these instruments that such distant ob- jects are brought to view. Without this property of light, therefore, we should have had no telescopes, and, conse- quently, we could not have surveyed, as we can now do, the hills and vales, the deep caverns, the extensive plains, the circular ranges of mountains, and many other novel scenes which diversify the surface of our moon. We should have known nothing of the stupendous spots which appear on the surface of the sun—of the phases of Venus—of the satellites and belts of Jupiter—of the majestic rings of Saturn—of the existence of Uranus and his six moons, or of the planets Vesta, Juno, Ceres, and Pallas, nor could the exact bulks of any of these bodies have been accurately determined. But, above all, we should have been entirely ignorant of the wonderful phenomena of double stars—which demonstrate that suns re- volve around suns—of the thousands and millions of stars which crowd the profundities of the Milky Way and other regions of the heavens—of the thousands of nebule or starry systems which are dispersed throughout the immensity of the firmament, and many other objects of sublimity and grandeur, which fill the contemplative mind with admiration and awe, and raise its faculties to higher conceptions than it could otherwise have formed of the omnipotence and grandeur of the Almighty Creator. Without this property of the rays of light we should, like- wise, have wanted the use of the microscope, an instrument which has disclosed a world invisible to common eyes, and has opened to our view the most astonishing exhibitions of Divine mechanism, and of the wisdom and intelligence of the Eternal Mind. We should have been ignorant of those tribes of living beings, invisible to the unassisted eye, which are found in water, vinegar,and many other fluids, many of which are twenty thousand times smaller than the least visible point, and yet display the same admirable skill and contriv- 68 REFLECTIONS ON THE LAWS OF REFRACTION. ance in their construction as are manifested in the formation of the larger animals. We should never have beheld the purple tide of life, and even the globules of the blood rolling with swiftness through veins and arteries smaller than the finest hair; or had the least conception that numberless species of animated beings, so minute that a million of them are less than a grain of sand, could have been rendered visi- ble to human eyes, or that such a number of vessels, fluids, movements, diversified organs of sensation, and such a pro- fusion of the richest ornaments and the gayest colours could have been concentrated in a single point. We should never have conceived that even the atmosphere is replenished with invisible animation, that the waters abound with countless myriads of sensitive existence, that the whole earth is full of life, and that there is scarcely a tree, plant, or flower but affords food and shelter to a species of inhabitants peculiar to itself, which enjoy the pleasures of existence and share in the bounty of the Creator. We could have formed no con- ception of the beauties and the varieties of mechanism which are displayed in the scenery of that invisible world to which the microscope introduces us—beauties and varieties, in point of ornament and delicate contrivance, which even surpass what is beheld in the visible operations and aspect of nature around us. We find joints, muscles, a heart, stomach, en- trails, veins, arteries, a variety of motions, a diversity of forms, and a multiplicity of parts and functions in breathing atoms. We behold in a small fibre of a peacock’s feather, not more than one-eighth of an inch in length, a profusion of beauties no less admirable than is presented by the whole feather to the naked eye, a stem sending out multitudes of lateral branches, each of which emits a number of little sprigs, which consist of a multitude of bright shining globular parts, adorn- ed with a rich variety of colours. In the sections of plants, we see thousands and ten thousands of tubes and pores, and other vessels for the conveyance of air and juices for the sus- tenance of the plant ; in some instances, more than ten hun- dred thousand of these being compressed within the space of a quarter of an inch in diameter, and presenting to the eye the most beautiful configurations. There is not a weed, nor a moss, nor the most insignificant vegetable, which does not show a multiplicity of vessels disposed in the most curious manner for the circulation of sap for its nourishment, and which is not adorned with innumerable graces for its embel- lishment. All these and ten thousands of other wonders which lie beyond the limits of natural vision, in this new INFERENCE FROM THE LAWS OF REFRACTION. 69 and unexplored region of the universe, would have been for ever concealed from our view, had not the Creator endued the rays of light with the power of depicting the images of objects, when refracted by convex transparent substances. In this instance, as well as in many others, we behold a specimen of the admirable and diversified effects which the Creator can produce from the agency of a single principle in nature. By means of optical instruments, we are now enabled to take a more minute and expansive view of the amazing operations of nature, both in heaven and on earth, than former generations could have surmised. These views tend to raise our conceptions of the attributes of that Almighty Bemg who presides over all the arrangements of the material system, and to present them to our contemplation in a new, a more elevated, and expansive point of view. ‘There is, therefore, a connection which may be traced between the apparently ac- cidental principle of the rays of light forming images of objects, and the comprehensive views. we are now enabled to take of the character and perfections of the Divinity. Without the existence of the law or principle alluded to, we could not, in the present state, have formed precisely the same concep- tions either of the Omnipotence, or of the wisdom and intel- ligence of the Almighty. Had no microscope ever been in- vented, the idea never could have entered into the mind of man that worlds of living beings exist beyond the range of natural vision; that organized beings, possessed of animation, exist, whose whole bulk is less than the ten hundred thou- sandth part of the smallest grain of sand; that, descending from a visible point to thousands of degrees beyond it, an in- visible world exists, peopled with tribes of every form and size, the extent of which, and how far it verges towards in- finity downward, mortals have never yet explored, and per- haps will never be able to comprehend. This circumstance alone presents before us the perfections of the Divinity in a new aspect, and plainly intimates that it is the will and the intention of the Deity that we should explore his works, and investigate the laws by which the material world is regulated, that we may acquire more expansive views of his charactet1 and operations. ‘The inventions of man, in relation to art and science, are not, therefore, to be considered as mere acciden- tal occurrences, but as special arrangements in the Divine government, for the purpose of carrying forward the human mind to more clear and ample views of the scenes of the uni- verse, and of the attributes and the agency of Him “ who is wonderful in counsel and excellent in working.” 70 REFLECTION OF LIGHT. CHAPTER IV. ON THE REFLECTION OF LIGHT. Tue reflection of the rays of light is that property by which, after approaching the surfaces of bodies, they are thrown back, or repelled. It is in consequence of this property that all the objects around us, and all the diversified landscapes on our globe, are rendered visible. It is by light reflected from their surfaces that we perceive the planetary bodies and their sa- tellites, the belts of Jupiter, the rings of Saturn, the various objects which diversify the surface of the Moon, and all the bodies in the universe which have no light of their own. When the rays of light fall upon rough and uneven surfaces, they are reflected very irregularly, and scattered in all direc- tions, in consequence of which thousands of eyes, at the same time, may perceive the same objects, in all their peculiar co- lours, aspects, and relations. But when they fall upon cer- tain smooth and polished surfaces, they are reflected with re- gularity, and according to certain laws. Such surfaces, when highly polished, are called Mirrors or Speculums ; and it is to the reflection of light from such surfaces, and the effects it produces, that Iam now to direct the attention of the reader. Mirrors, or specula, may be distinguished into three kinds, plane, concave, and convex, according as they are bounded by plane or spherical surfaces. These are made either of metal or of glass, and have their surfaces highly polished for the purpose of reflecting the greatest number of rays. Those made of glass are foliated or quicksilvered on one side; and the metallic specula are generally formed of a composition of different metallic substances, which, when accurately po- lished, is found to reflect the greatest quantity of light. I shall, in the first place, illustrate the phenomena of reflection produced by plane mirrors. : When licht impinges, or falls upon a polished flat surface, rather more than the half of it is reflected, or thrown back in a direction similar to that of its approach; that is to say, if it fall perpendicularly on the polished surface, it will be perpen- dicularly reflected ; but if it fall obliquely, it will be reflected with the same obliquity. Hence, the following fundamental law regarding the reflection of light has been deduced both from experiment and mathematical demonstration, namely, REFLECTION FROM PLANE SURFACES. 71 that the angle of reflection is, in all cases, exactly equal to the angle of incidence. This is a law which is universal in all cases of reflection, whether it be from plane or spherical surfaces, or whether these surfaces be concave or convex, and which requires to be recognised in the construction of all in- struments which depend on the reflection of the rays of light. The | reiki figure (fig. 14) will illustrate the position now stated : Figure 14, K A B Ty Let A B represent a plane mirror, and C D a line or ray of light perpendicular toit. Let F D represent the incident ray from any object, then DE will be the reflected ray, thrown back in the direction from D to E, and it will make, with the perpendicular C D, the same angle which the incident ray F D did with the same perpendicular; that is, the angle F D C will be equal to the angle EDC, in all cases of obliquity. The incident ray of light may “be considered as rebounding Figure 15. aan = Bi G ‘ war 72 REFLECTION FROM PLANE SURFACES. from the mirror, like a tennis ball from a marble pavement, or the wall of a court. In viewing objects by reflection, we see them in a different direction from that in which they really are, namely, along the line in which the rays come to us last. Thus, if A B (fig. 15) represent a plane mirror, the image of an object, C, ap- pears to the eye at E, behind the mirror, in the direction EG, and always in the intersection G of the perpendicular CG, and the reflected ray EG; and, consequently, at Gas far behind the mirror as the object C is before it. We thereforu see the image in the line EG, the direction in which the re- flected rays proceed. A plane mirror does not alter the figure or size of objects; but the whole image is equal and similar to the whole object, and has a like situation with respect to one side of the plane, that the object has with respect to the other. Mr. Walker illustrates the manner in which we see our faces in a mirror by the following figure (16.) «8B repre- Figure 16. sents a mirror, and oc a person looking into it. If we con- ceive a ray proceeding from the forehead cx, it will be sent to the eye at o, agreeably to the angle of incidence and reflec- tion. But the mind puts cro into one line, and the forehead is seen at u,as if the lines ceo had turned on a hinge at r. It seems a wonderful faculty of the mind to put the two oblique lines cr and of£ into one straight line on, yet it is seen every time we look ata mirror. For the ray has really travelled from ctor, and from £ to 0, and it is that journey which determines the distance of the object ; and hence we see ourselves as far beyond the mirror as we stand from it Though a ray is here taken only from one Se, + a REFLECTION FROM PLANE SURFACES. 73 part of the face, it may be easily conceived that rays from every other part of the face must produce a similar effect. In every plane mirror the image is always equal to the ob- ject, at what distance soever it may be placed; and, as the mirror is only at-half the distance of the image from the eye, it will completely receive an image of twice its own length. Hence, a man six feet high may view himself completely in a looking-glass of three feet in length and half his own breadth ; and this will be the case at whatever distance he may stand from the glass. Thus, the man A C (fig. 17) will Figure 17. see the whole of his own image in the glass a B, which is but one half as large as himself. The rays from the head pass to the mirror in the line A a, perpendicular to the mirror, and are returned to the eye in the same line; consequently, hav- ing travelled twice the length A a, the man must see his head at B. From his feet, C, rays will be sent to the bottom of the mirror, at B; these will be reflected at an equal angle to the eye in the direction Ba, as if they had proceeded in the direction DBA, so that the man will see his foot at D, and, consequently, his whole figure at B D. A person, when looking into a mirror, will always see his own image as far beyond the mirror as he is before it; and as he moves to or from it, the image will, at the same time, move towards or from him on the other side, but apparently with a double velocity, because the two motions are equal and contrary. In like manner, if, while the spectator is at rest, an object be in motion, its image behind the mirror will be seen to move at the same time. And if the spectator moves, the images of objects that are at rest will appear to” approach or recede from him, after the same manner as when he moves towards real objects; plane mirrors reflecting not only the object, but the distance also, and that exactly in its natural dimensions. The following principle is sufficient for Vou. IX, 7 74 REFLECTION FROM CONCAVE MIRRORS. explaining most of the phenomena seen ina plane mirror, namely : That the image of an object seenin a plane mirror is always in a perpendicular to the mirror joining the object and the image, and that the image is as much on one side the mirror as the object is on the other. Reflection by Convex and Concave Mirrors. Both convex and concave mirrors are formed of portions of a sphere. A convex speculum is ground and polished in a concave dish or tool which is a portion of a sphere, and a concave speculum is ground upon a convex tool. The inner surface of a sphere brings parallel rays to a focus at one fourth of its diameter, as represented in the following figure, where Figure 18. C is the centre of the sphere on which the concave speculum A B is formed, and F the focus where parallel rays from a distant object would be united after reflection, that is, at one half the radius, or one fourth of the diameter from the surface of the speculum. Were a speculum of this kind presented to the sun, F' would be the point where the reflected rays would be converged to a focus, and set fire to combustible substances if the speculum be of a large diameter, and of ‘a short focal distance. Were a candle placed in that focus, its PROPERTIES OF CONVEX MIRRORS. 75 licht would be reflected parallel, as represented in the figure. These are properties of concave specula which require to be particularly attended to in the construction of reflecting tele- scopes. It follows, from what has been now stated, that, if we intend to form a speculum of a certain focal distance, for ex- ample, two feet, it is necessary that it should be ground upon a tool whose radius is double that distance, or four feet. Properties of Convex Mirrors. From a convex surface, parallel rays, when reflected, are made to diverge: convergent rays are reflected less conver- gent; and divergent rays are rendered more divergent. It is the nature of all convex mirrors and surfaces to scatter or disperse the rays of light, and in every instance to impede their convergence. The following figure shows the course of parallel rays as reflected from a convex mirror. A EB is the convex surface of the mirror, and K A, I E, L B par- allel rays falling upon it. ‘These rays, when they strike the mirror, are made to diverge in the direction A G, BH, &c., and both the parallel and divergent rays are here represented as they appear in a dark chamber when a convex mirror is presented to the solar rays. The dotted lines denote only the course or tendency of the reflected rays towards the vir- tual focus F, were they not intercepted by the mirror. This virtual focus is just equal to half the radius C E. Figure 19. Ne xO Wt G- \ A a PARA 2 2 A ARALLEL RAYS K “eB + = 47 Ae in = EE r Cc PS. Sy a s Ef —~ = 5 et Le 7B “> is H “Ss > The following are some of the properties of convex mir- rors: 1. The image appears always erect, and behind the reflecting surface. 2. The image is always smaller than the object, and thé diminution is greater in proportion as the object is farther from the mirror; but if the object touch the mirror, the image at the point of contact is of the same size 76 PROPERTIES OF CONCAVE MIRRORS. as the object. 2. The image does not appear so far behind the reflecting surface as ina plane mirror. 4. The image of a straight object, placed either parallel or oblique to the mirror, is seen curved in the mirror, because the different points of the object are not all at an equal distance from the surface of the mirror. 5. Concave mirrors have a read focus where an image is actually formed ; but convex specula have only a virtual focus, and this focus is behind the mirror, no image of any object being formed before it. The following are some of the purposes to which convex mirrors are applied: ‘They are frequently employed by painters for reducing the proportions of the objects they wish to represent, as the images of objects diminish in proportion to the smallness of the radius of convexity, and to the dis- tances of objects from the surface of the mirror.’ They form a fashionable part of modern furniture, as they exhibit a large company assembled in a room, with all the furniture it con- tains, in a very small compass, so that a large hall, with all its objects, and even an extensive landscape, being reduced in size, may be seen from one point of view. They are like- wise used as the small specula of those reflecting telescopes which are fitted up on the Cassegrainian plan, and in the construction of Smith’s Reflecting Microscope. But, on the whole, they are very little used in the construction of optical instruments. Properties of Concave Speculums. Concave specula have properties very different from those which are convex; they are of more importance in the con- struction of reflecting telescopes and other optical instruments, and therefore require more minute description and illustra- tion. Concave mirrors cause parallel rays to converge; they increase the convergence of rays that are already converging ; they diminish the divergence of diverging rays, and in some cases render them parallel, and even convergent; which effects are all in proportion to the concavity of the mirror. The following figures show the course of diverging and par- allel rays as reflected from concave mirrors. Fig. 20 represents the course of parallel rays, and A B the concave mirror on which they fall. In this case, they are re- flected so as to unite at F', which point is distant from its sur- face one-fourth of the diameter of the sphere of the mirror. This point is called the focus of parallel rays, or the true focus of the mirrors; and, since the sunbeams are parallel among themselves, if they are received un a concave mirror, PROPERTIES OF CONCAVE MIRRORS. q7 Figure 20. ord iv) x ‘ a \\ ia m1 o ZN os, o 3 e LM i=} el rete. Sin. ie VET POE: Za PARALLEL RAYS they will all be reflected to that point, and there burn in pro- portion to the quantity of rays collected by the mirror. Fig. 21 shows the direction of diverging rays, or those which proceed __ Figure 21. from a near object. These rays proceeding from an’ object farther from the mirror than the true focal point, as from D to A and. to B, are reflected converging, and meet at a point F, farther dias the mirror than the focal point of parallel rays. Ifthe distance of the radiant, or object D, be equal to the radius C E, then will the focal distance be likewise equal to the radius ; that is, if an object be placed in the centre of a concave speculum, the image will be reflected upon the object, or they will seem to meet and embrace each other in the centre. If the distance of the radiant be equal to half the radius, its image will be reflected to an infinite distance, for W* 78 IMAGES FORMED BY CONCAVE MIRRORS. the rays will then be parallel.» If, therefore, a luminous body be placed at half the radius from a concave speculum, it will enlighten places directly before it at great distances. Hence their use when placed behind a candle in a common lantern ; hence their utility in throwing light upon objects in the Magic Lantern and Phantasmagoria; and hence the vast importance of very Jarge mirrors of this description, as now used in most of our lighthouses, for throwing a brilliant light to great distances at sea, to guide the mariner when direct- ing his course under the cloud of night. When converging rays fall upon a concave mirror, they are reflected more converging, and unite at a point between the focus of parallel rays and the mirror; that is, nearer the mirror than one-half the radius; and their precise degree of convergency will be greater than that wherein they con- verged before reflection. Of the Images formed by Concave Mirrors. If rays proceeding from a distant object fall upon a concave speculum, they will paint an image or representation of the object on its focus before the mirror. This image will be inverted, because the rays cross at the points where the image is formed. We have already seen that a convex glass forms an image of an object behind it; the rays of light from objects pass through the glass, and the picture is formed on the side farthest from the object. But in concave mirrors the images of distant objects—and of all objects that are farther from its surface than its principal focus—are formed before the mir- ror, or on the same side as the object. In almost every other respect, however, the effect of a concave mirror is the same as that of a convex lens, in regard to the formation of images, and the course pursued by the rays of light, except that the effect is produced in the one case by refraction, and in the other by reflection. The following figure represents the manner in which images are formed by concave mirrors: G F represents the reflecting surface of the mirror; O A B, the object; and I a M the image formed by the mirror. The rays proceeding from O will be carried to the mirror in the direction O G, and according to the law that the angle of in- cidence is equal to the angle of reflection, will be reflected to I in the direction GI. In like manner, the rays from B will be reflected from F' to M, the rays from A will be reflected to A,and so of all the intermediate. rays, so that an inverted image of the object O B will be formed at 1M. If the rays proceeded from objects at a very great distance, the image * IMAGES FORMED BY CONCAVE MIRRORS. 79 Figure 22. would be formed in the real focus of the mirror, or at one- fourth the diameter of the sphere from its surface; but near objects, which send forth diverging rays, will have their images formed a little farther from the surface of the mirror. If we suppose a real object placed at I M, then O B will represent its magnified image, which will be larger than the object in proportion to its distance from the mirror. This may be experimentally illustrated by a concave mirror and a candle. Suppose a concave mirror whose focal distance is five inches, and that a candle is placed before it at a little beyond its focus, (as at I M,) suppose at five and a half inches, and that a wall or white screen receives the image, at the distance of five feet six inches from the mirror, an image of the candle will be formed on the wall which will be twelve times longer. and broader than the candle itself. In this way concave mirrors may be made to magnify the images of objects to an indefi- nite extent. This experiment is an exact counterpart of what is effected in similar circumstances by a convex lens, as de- scribed p. 66: the mirror performing the same thing by re- flection as the lens did by refraction. : From what has been stated in relation to concave mirrors, it will be easily understood how they make such powerful burning-glasses. Suppose the focal distance of a concave mirror to be twelve inches, and its diameter or breadth twelve inches. When the sun’s rays fall on such a mirror, they form an image of the sun at the focal point, whose diameter is found to be about one-tenth of aninch. All the rays which 80 IMAGES FORMED BY CONCAVE MIRRORS. fall upon the mirror are converged into this small point, and, consequently, their intensity is in proportion as the square of the surface of the mirror is to the square of the image. ‘T’he squares of these diameters are as 14,400 to 1, and, conse- quently, the density of the sun’s rays, in the focus, is to their density on the surface of the mirror as 14,400 to 1. That is, the heat of the solar rays in the focus of such a mirror-will be fourteen thousand four hundred times greater than before; a heat which is capable of producing very powerful effects in melting and setting fire to substances of almost every descrip- tion. Were we desirous of forming an image by a concave spe- culum which shall be exactly equal to the object, the object must be placed exactly in the centre ; and, by an experiment of this kind, the centre of the concavity of a mirror may be found. In the cases now stated, the images of objects are all formed in the front of the mirror, or between it and the object. But there is a case in which the image is formed behind the mir- ror. ‘This happens when the object is placed between the mirror and the focus of parallel rays, and then the image is larger than the object. -In fig. 23, GF is a concave mirror, whose focus of parallel rays isat E. If an object O B be placed a little within this focus, as at A,a large image I M will de seen behind the mirror, somewhat curved and erect, Figure 23. CENTRE THE which will be seen by an eye looking directly into the front of the mirror. Here the image. appears at a greater distance behind the mirror than the object is before it, and the object IMAGES FORMED BY CONCAVE MIRRORS. 81 appears magnified in proportion to-its distance from the focus and the mirror. If the mirror be one inch focal distance, and the object be placed eight-tenths of an inch from its surface, the image would be five times as large as the object in length and breadth, and, consequently, twenty-five times larger in surface. In this way small objects may be magnified by re- flection, as such objects are magnified by refraction, in the case of deep convex lenses. When such mirrors are large, for example six inches diameter, and eight or ten inches focal distance, they exhibit the human face as of an enormous bulk. This is illustrated by the following figure: Let cn, fig. 24, represent the surface of a concave mirror, and A a human face looking into it, the face will appear magnified as repre- sented by the image behind the mirror, pg. Suppose a ray, A c, proceeding from the forehead, and another, m n, from the chin; these rays are reflected to the person’s eye at o, which, consequently, sees the image of the lines of reflection 0 p, o q, and in the angle p oq, and, consequently, magnified much beyond the natural size, and at a small distance behind the mirror. Figure 24. If we suppose the side rT u to represent a convex mirror, and the figure p q a head of an ordinary size, then the figure A will represent the diminished appearance whicha person’s face exhibits when viewed insucha mirror. It will not only appear reduced, but somewhat distorted ; because, from the form of the mirror, one part of the object is nearer to it than another, and, consequently, will be reflected under a different angle 82 IMAGES FORMED BY CONCAVE MIRRORS. The effect we have now mentioned as produced by concave mirrors will only take place when the eye is nearer the mir- ror than its principal focus, If the spectator retire beyond this focus—suppose to the distance of five or six feet—he will not see the image behind the mirror, but he will see his image in a diminished form, hanging upside down, and suspended in the air, in a line between his eye and the mirror. In this case, his image is formed before the mirror, as represented at IM, fig. 22. In this situation, if you hold out your hand towards the mirror, the hand of the image will come out to- wards your hand, and, when at the centre of concavity, it will be of an equal size with it, and you may shake hands with this aerial image. If you move your hand farther, you will find the hand of the image pass by your hand, and come be- tween it and your body. If you move your hand towards either side, the hand of the image will move towards the other side; the image moving always in a contrary direction to the object. All this while the by-standers, if any, see nothing of the image, because none of the reflected rays that form it can enter their eyes. The following figure represents a phe- nomenon produced in the same manner. AB is a concave Figure 25. mirror of a large size; c represents a hand presented before the mirror, at a point farther distant than its focus. In this case an inverted image of the hand is formed, which is seen hanging in the air ata m. The rays c and p go diverging from the two opposite points of the object, and by the action of the mirror, they are again made to converge to points at o and s, where they cross, form an image, and again proceed divergent to the eye.* . * Small glass mirrors for performing some of the experiments, and il- lustrating some of the principles above alluded to, may be made of the | OPTICAL DECEPTIONS. 83 In consequence of the properties of concave mirrors now described, many curious experiments and optical. deceptions have been exhibited. ‘The appearance of images in the air, suspended between the mirror and the object, have some- times been displayed with such dexterity, and an air of mystery, as to have struck with astonishment those: who were ignorant of the cause. In this way birds, flying angels, spectres, and other objects have been exhibited ; and when the hand attempts to lay hold on them, it finds them to be nothing, and they seem to vanish into air. An apple ora beautiful flower is presented, and when a spectator attempts to touch it, it instantly vanishes, and a death’s head immedi- ately appears, and seems to snap at his fingers. A person with a drawn sword appears before him, in an attitude as if about to run him through, or one terrific phantom starts up after another, or sometimes the resemblances of deceased per- sons are made to appear, as if, by the art of conjuration, they had been forced to return from the world of spirits. In all such exhibitions a very large concave mirror is requisite, a brilliant light must be thrown upon the objects, and every arrangement is made, by means of partitions, &c., to prevent either the light, the mirror, or the object from being seen by the spectators. The following representation (fig. 26) shows one of the methods by which this is effected: a is a large concave mirror, either of metal or glass, placed on the back part of a dark box; pis the performer, concealed from the spectators by the cross partition ¢; E is a strong light, which is likewise concealed by the partition 1, which is thrown upon the actor p, or upon any thing he may hold in his hand. If he hold a book, as represented in the figure, the light reflected from it will pass between the partitions c and 1 to the mirror, and will be reflected from thence to z, where the image of the book will appear so distinct and tangible, that a spectator look- ing through the opening at x will imagine that it is in his power to take hold of it. In like manner, the person situated at D may exhibit his own head or body, a portrait, a painting, a spectre, a landscape, or any object or device which he can strongly illuminate. , flattest kind of common watch-glasses, by foliating or covering with tin leaf and quicksilver the convex surfaces of such glasses. Their focal dis- tances will generally be from one to two inches. Such mirrors afford a very large and beautiful view of the eye, when held within their focal distance of that organ. Such mirrors will also serve the purpose of re- flecting light on the objects viewed by microscopes. Larger mirrors, of from four to eight inches diameter, may be had of the optician at differ- ent prices, varying from five to ten or fifteen shillings. 84 IMAGES FORMED BY CONCAVE MIRRORS. Figure 26. There is another experiment, made witha concave mirror, which has somewhat puzzled philosophers to account for the phenomena. Take a glass bottle, A C (fig. 27), and fill it Figure 27. with water to the point B; leave the upper part, BC, empty, and cork it in the common manner. Place this bottle oppo- site a concave mirror, and beyond its focus, that it may ap- pear reversed, and before the mirror place yourself still farther distant from the bottle, and it will appear in the situation a B c. Now it is remarkable, in this apparent bottle, that the water, which, according to the laws of catoptrics, should ap- pear at a B, appears, on the contrary, at B c, and, consquently, the part AaB appears empty. If the bottle be inverted and placed before a mirror, its image will appear in its natural” OPTICAL DECEPTIONS. 85 erect position, and the water, which is in reality at BC (fig. 28), is seen at a B. If, while the bottle is inverted, it be un- Figure 28. corked, and the water run gently out, it will appear that, while the part B C is emptying, that of a, in the image is filling, and what is remarkable, as soon as the bottle is empty the illusion ceases, the image also appearing entirely empty. The remarkable circumstances in this experiment are, first, not only to see the object where it is not, but also where its image is not; and, secondly, that of two objects which are really in the same place, as the surface of the bottle and the water it contains, the one is seen at one place, and the other at another ; and to see the bottle in the place of its image, and the water where neither it nor its image is. The following experiments are stated by Mr. Ferguson, in his “ Lectures on Select Subjects,” &c.: “Ifa fire be made in a large room, and a smooth mahogany table be placed at a good distance near the wall, before a large concave mirror, so placed that the light of the fire may be reflected from the mirror to its focus upon the table; if a person stand by the table, he will see nothing upon it buta longish beam of light : but if he stands at a distance towards the fire, not directly be- tween the fire and mirror, he will see an image of the fire upon the table, large and erect. And if another person, who knows nothing of the matter beforehand, should chance to come into the room, and should look from the fire towards the table, he would be startled at the appearance, for the table would seem to be on fire, and by being near the wainscot, to endanger the whole house. In this experiment there should be no light in the room but what proceeds from the fire, and the mirror ought to be at least fifteen inches in diameter. If the fire be darkened by a screen, anda large candle be placed at the back of the screen, a person standing by the candle will see the appearance of a very fine large star, or, rather, planct, 8 Vou. IX. 86 GENERAL PROPERTIES OF SPECULUMS. upon the table,as bright as Venus or Jupiter. And if a small wax taper—whose flame is much less than the flame of the candle—be placed near the candle, a satellite to the planet will appear on the table; and if the taper be moved round the candle, the satellite will go round the planet.” Many other illustrations of the effects of concave specula might have been given, but I shall conclude this department by briefly stating some of the general properties of specu- lums. 1. There is a great resemblance between the properties of convex lenses and concave mirrors. ‘They both form an in- verted focal image of any remote object, by the convergence of the pencil of rays. In those instruments whose perform- ances are the effects of réflection, as reflecting telescopes, the concave mirror is substituted in the place of the convex lens. The whole effect of these instruments, in bringing to view remote objects in heaven and on earth, entirely depends on the property of a concave mirror in forming images of ob- jects in its focus. 2. The image of an object placed beyond the centre is less than the object ; if the object be placed be- tween the principal focus and the centre, the image is greater than the object. In both cases the image is inverted. 3. When the object is placed between the focus and the mirror, the image situated behind the mirror is greater than the ob- ject, and it has the same direction; in proportion as the object approaches the focus, the image becomes larger and more distant. ‘These and similar results are proved by placing a lighted candle at different distances from a concave mirror. 4. An eye cannot see an image in the air except it be placed in the diverging rays; but if the image be received ona piece of white paper, it may be seen in any position of the eye, as the rays-are then reflected in every direction. 5. If a picture, drawn according to the rules of perspective, be placed. before a large concave speculum, a little nearer than its principal focus, the image of the picture will appear ex- tremely natural, and very nearly like the real objects whence it was taken. Not only are the objects considerably magnified, so as to approach to their natural size, but they have also dif- ferent apparent distances, as in nature, so that the view of the inside of a church appears very like what it is in reality, and representations of landscapes appear very nearly as they do from the spot whence they were taken. . In this respect a large concave speculum may be made. to serve nearly the same purpose as the Optical Diagonal Machine in viewing perspective prints. 6. The concave speculum is that alone QUANTITY OF LIGHT REFLECTED. 87 ’ which is used as the great mirror which forms the first im- age in reflecting telescopes ; and it is likewise the only kind of speculum used as the small mirror, in that construction of the instrument called the Georgian Reflector. Quantity of Light reflected by polished Surfaces. As this is a circumstance connected with the construction of reflecting telescopes, it may not be improper, in this place, to state some of the results of the accurate experiments of M. Bonguer on this subject. This philosopher ascertained that of the light reflected from mercury, or quicksilver, more than one-fourth is lost, though it is probable that no substances re- flect more light than this. The rays were received at an angle of eleven and a half degrees of incidence, measured from the surface of the reflecting body, and not from the per- pendicular. The reflection from water was found to be al- most as great as that of quicksilver; so that in very small angles it reflects nearly three-fourths of the direct light. This is the reason why so strong a reflection appears on water, when one walks, in still weather, on the brink of a lake opposite tothe sun. The direct light of the sun di- minishes gradually as it approaches the horizon, while the reflected light at the same time growsstronger ; so that there is a certain elevation of the sun in which the united force of the direct and the reflected light will be the greatest possible, and this is when he is twelve or thirteen degrees in altitude. On the other hand, light reflected from water at great angles of incidence is extremely small. When the light was per- pendicular, it reflected no more than the thirty-seventh part which mercury does in the same circumstances, and only the fifty-fifth part of what fell upon it in this case. Using a smooth piece of glass, one line in thickness, he found that, when it was placed at an angle of fifteen degrees with the incident rays, it reflected 628 parts of 1000 which fell upon it; at the same time, a metallic mirror, which he tried in the same circumstances, reflected only 561 of them. At a less angle of incidence much more light was reflected ; so that at anangle of three degrees the glass reflected 700 -parts, and the metal something less, as in the former case The most striking observations made by this experimenter relate to the very great difference in the quantity of light re- flected at different angles of incidence. He found that for 1000 incident rays, the reflected rays, at different angles of incidence, were as follows : 88 THE FATA MORGANA. Angles of incidence. Rays reflected by water. Rays reflected by glass. 5° 501 549 10 333 412 15 211 299 30 65 112 50 22 > nein ee 70 18 25 90 18 25 With regard to such mirrors as the specula of reflecting telescopes, it will be found, in general, that they reflect little more than the one-half of the rays which fall upon them. Uncommon Appearances in Nature produced by the com- bined Influences of Reflection and Refraction. The reflection and refraction of the rays of light frequentl produce phenomena which astonish the beholders, and shiek have been regarded by the ignorant and the superstitious as the effects of supernatural agency. Of these phenomena I shall state a few examples. One of the most striking appearances of this kind is what has been termed the Fata Morgana, or optical appearances of figures in the sea and the air, as seen in the Faro of Mes- sina. The following account is translated from a work of Minasi, who witnessed the phenomenon, and wrote a disser- tation on the subject: “ When the risimg sun shines from that point whence its incident ray forms an angle of about forty-five degrees to the sea of Riggio, and the bright surface of the water in the bay is not disturbed either by the wind or the current, the spectator being placed on an eminence of the city, with his back to the sun and his face to the sea ; on a sudden there appear on the water, as in a catoptric theatre, various multiplied objects, that is to say, numberless series of pilasters, arches, castles well delineated, regular co- lumns, lofty towers, superb palaces, with balconies and win- dows, extended alleys of trees, delightful plains with herds and flocks, armies of men on foot and horseback, and many other strange images, in their natural colours and proper ac- tions, passing rapidly in succession along the surface of the sea, during the whole of the short period of time, while the above-mentioned causes remain. But if, in addition to the circumstances now described, the atmosphere be highly im- pregnated with vapour and dense exhalations, not previously dispersed by the winds or the sun, it then happens that, in this vapour, as in a curtain extended along the channel, at the height of about thirty palms, and nearly down to the sea, the THE MIRAGE. 89 observer will behold the scene of the same objects, not only reflected from the surface of the sea, but likewise in the air, though not so distant or well defined as the former objects from thesea. Lastly, if the air be slightly hazy or opaque, and, at the same time, dewy, and adapted to form the iris, the then above-mentioned objects will appear only at the sur- face of the sea, as in the first case, but all vividly coloured, or fringed with red, green, blue, and other prismatic colours.’’* It is somewhat difficult to account for all the appearances here described, but, in all probability, they are produced by a calm sea,.and one or more strata of superincumbent air differing in refractive, and, consequently, in reflective power. At any rate, reflection and refraction are some of the essen- tial causes which operate in the production of the phenomena. The Mirage, seen in the deserts of Africa, is a phenome- non, in all probability, produced by asimilar cause. M. Monge, who accompanied the French army to Egypt, relates that, when in the desert between Alexandria and Cairo, the mirage of the blue sky was inverted, and so mingled with the sand below as to give to the desolate and arid wilderness an appearance of the most rich and beautiful country. They saw, in all directions, green islands, surrounded with ex- tensive lakes of pure, transparent water. Nothing could be conceived more lovely and picturesque than the landscape. In the tranquil surface of the lakes, the trees and houses with which the islands were covered were strongly reflected with vivid and varied hues, and the party hastened forward to enjoy the cool refreshments of shade and stream which these populous villages proffered to them. When they arrived, the lake on whose bosom they floated, the trees among whose foliage they were embowered, and the people who stood on the shore inviting their approach, had all vanished, and nothing remained but a uniform and irksome desert of sand and sky, with a few naked huts and ragged Arabs. Had they not been undeceived by their nearer approach, there was nota man in the French. army who would not have sworn that the visionary trees and lakes had a real existence in the midst of the desert. Dr. Clark observed precisely the same appearances at Ro- setta, The city seemed surrounded with a beautiful sheet of water; and so certain was his Greek interpreter—who was unacquainted with the country—of this fact, that he was quite indignant at an Arab who attempted to explain to him that it was a mere optical delusion. At length they reached Ro- * Nicholson’s Journal of Natural eee &c., 4to series, p. 225. * 90 AERIAL VISIONS. setta in about two hours, without Figure 29. meeting with any water; and, on looking back on the sand they had just crossed, it seemed to them as if they had waded through a vast blue lake. ' On the Ist of August, 1793, Dr. Vince observed at Ramsgate a ship which appeared as at a, (fig. 29,) the topmast being the only part of it that was seen above the horizon. An in- verted image of it was seen at B, 1m- mediately above the real ship a, and an erect image at c, both of them be- ing complete and well defined.. The sea was distinctly seen between them, as at v w. As the ship rose to the horizon, the image c gradually dis- appeared, and while this was going on, the image B -descended, but the mainmast of p did not meet the mainmast of a. The two images, B c, were perfectly visible when the whole ship was actually below the .horizon. Dr. Vince then directed his telescope to another ship whose hull was just in the horizon, and he observed a complete inverted image of it, the main- mast of which just. touched the mainmast of the ship itself. He sawat the.same time several other ships whose images ‘appeared in nearly a similar manner, in one of which.the two images were visible when the whole.ship was beneath . the horizon. These phenomena must have been produced by the same causes’ which operated in the case formerly mentioned, in relation to Captain Scoresby, when he saw the figure of his father’s ship inverted im the distant horizon. Such cases are, perhaps, not uncommon, especially. in calm and sultry weather, but they are seldom observed, except when a person’s attention is accidentally directed to the phe- nomenon, and, unless he use a telescope, it will not be so distinctly perceived. : _ The following phenomenon, of a description nearly related to the above, has been supposed to be chiefly owing to reflec- tion: On the 18th of November, 1804; Dr. Buchan, when watching the rising sun, about a mile to the east of Brighton, just as the solar disk emerged from the surface of the water, saw the face of the cliff on which he was standing, a windmill, his own figure, and the figure of his friend, distinctly repre- THE SPECTRE OF THE BROCKEN. 91 sented, precisely opposite, at some distance from the ocean. This appearance lasted about ten minutes, till the sun had risen nearly his own diameter above the sea. The whole then seemed to be elevated into the air, and successively dis- appeared. The surface of the sun was covered with a dense fog of many yards in height, which gradually receded from the rays of the sun as he ascended from the horizon. | The following appearance, most probably, arose chiefly from the refraction of the atmosphere: It was beheld at Rams- gate by Dr. Vince, of Cambridge, and another gentleman. It is well known that the four turrets of Dover Castle are seen at Ramsgate, over a hill which intervenes between a full prospect of the whole. On the 2d of August, 1806, not only were the four turrets visible, but the castle itself appeared as though situated on that side of the hill nearest Ramsgate, and so striking was the appearance that for a long time the doctor thought it an illusion; but at last, by accurate observation, was convinced that it was an actual image of the castle. He, with another individual, observed it attentively for twenty minutes, but were prevented by rain from making farther observations. Between the observers. and the land from which the hill rises there were about six miles of sea, and from thence to the top of the hill there was about the same distance ; their own height above the surface of the water was about seventy feet. The cause of this phenomenon was, undoubtedly, unequal refraction. 'The air being more dense near the ground and above the sea than at greater heights, reached the eye of the observer, not in straight, but in curvi- linear lmes. If the rays from the castle had in their path struck an eye at a much greater distance than Ramsgate, the probability is that the image of the castle would have been inverted in the air; but, in the present case, the rays from the turret and the base of the castle had not crossed each other. To similar causes as those now alluded to are to be attri- buted such phenomena as the following: The spectre of the Brocken. This is a wonderful and, at first sight, a terrific phenomenon, which is sometimes seen from the summit of one of the Hartz Mountains in Hanover, which is about 3300 feet above the level of the sea, and over- looks all the country fifteen miles round. From this moun- tain the most gigantic and terrific spectres have been seen, which have terrified the credulous, and gratified the curious, ina very high degree. M. Hawe, who winessed this phe- nomenon, says the sun rose about four o’clock, after he had _ ascended to the summit, in a serene sky, free of clouds; and, 92 AERIAL VISIONS. about a quarter past five, when looking round to see if the sky continued clear, he suddenly beheld, at a little distance, a human-figure of a monstrous size turned towards him, and glaring at him: While gazing on this gigantic spectre, with a mixture of awe and apprehension, a sudden gust of wind nearly carried off his hat, and he clapped his hand to his head to detain it, when, to his great delight, the colossal spectre did the same. He changed his body into a variety of attitudes, all which the spectre exactly imitated, and then suddenly vanished without any apparent cause, and in a short time as suddenly appeared. Being joined by another spec- tator, after the first visions had disappeared, they kept steadily looking for the aerial spectres, when two gigantic monsters suddenly appeared. These spectres had been long considered as preternatural by the inhabitants of the ad- jacent districts, and the whole country had been filled with awe and terror. Some of the lakes of Ireland are found to be susceptible of producing illusions, particularly the Lake of Killarney. 'This romantic sheet of water is bounded on one side by a semicircle ‘of rugged mountains, and on the other by a flat morass; and the vapours generated in the marsh, and broken by the mountains, continually represent the most fantastic objects. Frequently men riding along the shore are seen as if they were moving across the lake, which is supposed to have given rise to the legend of O’Donougho, a magician who is said to be visible on the lake every May morning. There can be little doubt that most of those visionary ap- pearances. which have been frequently seen in the sky and in mountainous regions are phantoms produced by the cause to which I am adverting, such as armies of footman and horse- men which some have asserted to have seen in the air near the horizon. A well-authenticated instance of this kind oc- curred in the Highlands of Scotland: Mr. Wren, of Wetton Hall, and D. Stricket, his servant, in the year 1744, were sitting at the door of the house in a summer evening, when they were surprised to see opposite them, on the side of Son- terfell hill—a place so extremely steep that scarce a horse could walk slowly along it—the figure of a man with a dog pursuing several horses, all running’ at a most rapid pace. Onward they passed, till at last they disappeared at the lower end of the Fell. In expectation of finding the man dashed to pieces by so tremendous a fall, they went early next morn- ing and made a search, but no trace of man or horse, or the prints of their feet on the turf could be found. Some time afterward, about seven in the evening, on the same spot, THE CELESTIAL CROSS. 93 they beheld a troop of horsemen advancing in close ranks and at a brisk pace. ‘The inmates of every cottage for a mile round beheld the wondrous scene, though they had formerly ridiculed the story told by Mr. Wren and his servant, and were struck with surprise and fear. The figures were seen for upward of two hours, till the approach of darkness rendered them invisible. The various evolutions and changes through which the troops passed were distinctly visible, and were marked by all the observers. It is not improbable that these aerial troopers were produced by the same cause which made the Castle of Dover to appear-on the side of the hill next to Ramsgate, and it is supposed that they were the images of a body of rebels, on the other side of the hill, exercising them- selves previous to the rebellion in 1745,* I shall mention only another instance of this description which lately occurred in France, and for a time caused a powerful sensation among all ranks. On Sunday, the 17th of December, 1826, the clergy in the parish of Migné, in the vicinity of Poictiers, were engaged in the exercises of the Jubilee which preceded the festival of Christmas, and a num- ber of persons, to the amount of 3000 souls, assisted in the service. They had planted, as part of the ceremony, a large cross, twenty-five feet high, and painted red, in the open air beside the church. While one of the preachers, about five in the evening, was addressing the multitude, he reminded them of the miraculous cross which appeared in the sky to Constantine and his army, and the effect it produced, when suddenly a similar celestial cross appeared in the heavens just before the porch of the church, about 200 feet above the horizon, and 140 feet in lenoth, and its breadth from three to four feet, of a bright silver colour, tinged with red. The curate and congregation fixed their wondering gaze upon this extraordinary phenomenon, and the effect produced on the minds of the assembly was strong and solemn: they sponta- neously threw themselves on their, knees; and many, who had been remiss in their religious duties, humbly confessed their sins, and made vows of penance and reformation. A commission was appointed to investigate the truth of this ex- traordinary appearance, and a memorial stating the above and | other facts was subscribed by more than forty persons of rank and intelligence, so that no doubt was entertained as to the * There can be little doubt that some of the facts ascribed, in the West- ern Highlands of Scotland, to second sight, have been owing to the unusual refraction of the atmosphere ; as one of the peculiarities attributed to those who possessed this faculty was, that they were enabled to descrv boats and ships before they appeared in the horizon. 94 PHENOMENA EXPLAINED. reality of the phenomenon. By many it was considered as strictly miraculous, as having happened at the time and in the circumstances mentioned. But it is evident, from what we have already stated, that it may be accounted for on phy- sical principles. The large cross of wood painted red was doubtless the real object which produced the magnified image. The state of the atmosphere, according to the description given in the memorial, must have been favourable for the production of such images. The spectrum of the wooden cross must have been cast on the concave surface of some atmospheric mirror, and so reflected back to the eyes of the spectators from an opposite place, retaining exactly the same shape and proportions, but dilated in size; and, what is worthy of attention, it was tinged with red, the very colour of the object of which it was the reflected i image. Such phenomena as we have now described, and the causes of them which science is able to unfold, are worthy of con- sideration, in order to divest the mind of superstitious terrors, and enable it clearly to perceive the laws by which the Al- mighty directs the movements of the material system. When any appearance in nature, exactly the reverse of every thing we could have previously conceived, presents itself to view, and when we know of no material cause by which it could be produced, the mind must feel a certain degree of awe and terror, and will naturally resort to supernatural agency as acting either in opposition to the established laws of the uni- verse, or beyond the range to which they are confined. Be- sides the fears and apprehensions to which such erroneous conceptions give rise, they tend to convey false and distorted impressions of the attributes of the Deity, and of His moral government. Science, therefore, performs an invaluable ser- vice to man, by removing ‘the cause of superstitious alarms, by investigating the laws and principles which operate in the physical system, and by assigning reasons for those occasional phenomena which at first sight appeared beyond the rs of the operation of natural causes. The late ingenious Dr.. Wollaston illustrated the causes of some of the phenomena we have described, in the follow- ing manner: He looked along the side-of a red-hot poker at a word or object ten or twelve feet distant; and at a distance less than three-eighths of an inch from the line of the poker, an inverted image was seen, and within and without that image,an erect image, in consequence of the change produced, by the heat of the poker, in the density of air. He also sug- gested the following experiment as another illustration of the PHENOMENA EXPLAINED. 95 same principle, namely, viewing an object through a stratum of spirit of wine lying above water, or a stratum of water laid above one of syrup. He poured into a square vial a small quantity of clear syrup, and above this he poured an equal quantity of water, which gradually combined with the syrup, as seen at A, fig. 30. The word Figure 30. “Syrup,” on a card held behind the bottle, appeared erect. when seen through the pure spirit, but inverted when seen through the mixture of water and syrup. He afterward put nearly the same quantity of rectified spirit of wine above the water, as seen at B, and he saw the appearance as repre- sented, namely, the true place of the word “ Spirit,” and the inverted and erect images below. These substances, by their gradual incor- poration, produce refracting power, diminishing from the spirit of wine to the water, or from the syrup, to the water; so that, by looking through the mixed stratum, an inverted image of any object is seen behind the bottle. These. experiments show that the mirage and several other atmospherical phe- nomena may be produced by variations in the refractive power of different strata of the atmosphere. It is not unlikely that phenomena of a new and different description from any we have hitherto observed, may be pro- duced from the same causes to which we have adverted. A certain optical writer remarks: “If the variation of the refractive power of the air takes place in a horizontal line perpendicular to the line of vision, that is, from right to left, then we may have a lateral mirage, that is, an image of a ship may be seen on the right or left hand of the real ship, or on both, if the variation of refractive power is the same on each side of the line of vision, and.a fact of this kind was once observed on the Lake of Geneva. If there should happen at the same time both a vertical and a lateral varia- tion of refractive power in the air, and if the variation should be such as to expand or elongate the object in both directions, then the object would be magnified as if seen through a tele- scope, and might be seen and recognised at a distance at which it would not otherwise have been visible. If the re- fracting power, on the contrary, varied so as to construct the 96 REFLECTIONS. object in both directions, the image of it would be diminished as if seen through a concave lens.” Remarks and Reflections in reference to the Phenomena described above. Such, then, are some of the striking and interesting effects produced by the refraction and the reflection of the rays of light. . As the formation of the images of objects by con- vex lenses lays the foundation of the construction of refract-. ing telescopes and microscopes, and of all the discoveries they have brought to light, so the property of concave specula, in forming similar images, is that on which the construction. of reflecting telescopes entirely depends. ‘To this circumstance Herschel was indebted for the powerful telescopes he was en- abled to construct—which were all formed on the principle of reflection—and for all the discoveries they enabled him to make in the planetary system, and in the sidereal heavens. The same principles which operate in optical instruments, under the agency of man, we have reason to believe frequently act on a more expansive scale in various parts. of the system of nature. ‘The magnificent cross which astonished the preacher and the immense congregation assembled at Migné, was, in all probability, caused by a vast atmospherical speculum formed by the hand of nature, and representing its objects on a scale far superior to that of human art; and probably to the same cause is to be attributed the singular phenomenon of the coast of France having been made to appear within two or three miles of the town of Hastings, as formerly described (see p. 52.) Many other phenomena which we have never witnessed, and of which we can form no conception, ‘may be produced by the same cause operating in an infinity of modes. The facts we have stated above, and the variety of modes by which light may be refracted and reflected by different substances in nature, lead us to form some conceptions of the magnificent and diversified scenes which light may produce in other systems and worlds under the arrangements of the all-wise and beneficent Creator. Light, in all its modifica- tions and varieties of colour and reflection, may be considered as the beauty and glory of the universe, and the source of unnumbered enjoyments to all its inhabitants. It is a symbol of the Divinity himself; for «Gop 1s Liew, and in Him is no darkness at all.’ It is a representative of Him who is exhibited in the sacred oracles as “The Sun of Righteous- ness,” and “the Lieut of the world.’ It is an emblem of REFLECTIONS. 97 the glories and felicities of that future world where know- ledge shall be perfected and happiness complete; for its inhabitants are designated “the saints in light ;” and it is declared in sacred history to have been the first-born of created beings. In our lower world, its effects on the objects which surround us, and its influences upon all sensitive beings, are multifarious and highly admirable. While passing from in- finitude to infinitude, it reveals the depth and immeasity of the heavens, the glory of the sun, the beauty of the stars, the arrangements of the planets, the rainbow encompassing the sky with ‘ts glorious circle, the embroidery of flowers, the rich clothing of the meadows, the valleys standing thick with corn, “the cattle on a thousand hills,’ the rivers rolling through the plains, and the wide expanse of the ocean. .But in other worlds the scenes it creates may be far more resplendent and magnificent. This may depend upon the refractive and reflective powers with which the Creator has endowed the atmospheres of other planets, and the peculiar constitution of the various objects with which they are con- nected. It is evident, from what we already know of the reflection of light, that very slight modifications of certain physical principles, and very slight additions to the arrange- ments of our terrestrial system, might produce scenes. of beauty, magnificence, and splendour of which at present we can form no conception. And it is not unlikely that by such diversities of arrangement in other worlds an infinite variety of natural scenery is produced throughout the universe. In the arrangements connected with the planet Saturn, and the immense rings with which it is encompassed, and in the various positions which its satellites daily assume with regard to one another; to the planet itself, and to these rings, there is, in all probability, a combination of refractions, reflections, light, and shadows, which produce scenes wonderfully diver- sified, and surpassing in grandeur what we can now dis- tinctly conceive. In the remote regions of the heavens there are certain bodies composed of immense masses of luminous matter, not yet formed into any regular system, and which are known by the name of nebulez. What should hinder us from supposing that certain exterior portions of those masses form speculums of enormous size, as some parts of our atmo- sphere are sometimes found to do? Such specula may be conceived to be hundreds and even thousands of miles in diameter, and that they may form images of the most distant objects in the heavens, on a scale of immense magnitude and extent, and which may be reflected, in all their grandeur, to Vor. IX, 9 =— 98 THE COLOURS OF LIGHT: the eyes of intelligences at a vast distance. And, if the organs of vision of such-beings be far superior to ours in acuteness and penetrating power, they may thus be enabled to take a survey of an immense sphere of vision, and to descry magni- ficent objects at distances the most remote from the sphere they occupy. Whatever grounds there may be for such suppositions, it must be admitted that all the knowledge we have hitherto acquired respecting the operation of light, and the splendid effects it is capable of producing, is small indeed, and limited to a narrow circle, compared with the immensity of its range, the infinite modifications it may undergo, and the wondrous scenes it may create in regions of creation to which human eyes have never yet penetrated, and which may present to view objects of brilliancy and magnificence such as “ Eye hath not yet seen, nor ear heard, nor hath it entered into the heart of man to conceive.” CHAPTER V. SECT. IL—ON THE COLOURS OF LIGHT. WE have hitherto considered light chiefly as a simple homogeneous substance, as if all its rays were white, and as if they were all refracted in the same manner by the different lenses on which they fall. Investigations, however, into the nature of this wonderful fluid have demonstrated that this is not the case, and that it is possessed of certain additional properties of the utmost importance in the system of nature. Had every ray of light been a pure white, and incapable of being separated into any other colours, the scene of the uni- verse would have exhibited a very different aspect from what we now behold. One uniform hue would have ‘appeared over the whole face of nature, and one object could scarcely have been distinguished from another. 'The different shades of verdure which now diversify every landscape, the brilliant colouring of the flowery fields, and almost all the beauties and sublimities which adorn this lower creation would have been withdrawn. But it is now ascertained that every ray of white _light is composed of an assemblage of colours, whence pro- ceed that infinite variety of shade and colour with which the whole of our terrestrial habitation is arrayed. Those colours are found not to be in the objects themselves, but in the rays of light which fall upon them, without which they would either be invisible, or wear a uniform aspect. In reference ANCIENT OPINIONS OF LIGHT. 99 to this point, Goldsmith has well observed: “The blushing beauties of the rose, the modest blue of the violet, are not in the flowers themselves, but in the light that adorns them. Odour, softness, and beauty of figure are their own ; but it is light alone that dresses them up in those robes which shame the monarch’s glory.”’ ; Many strange opinions and hypotheses were entertained respecting colours by the ancients, and even by many modern writers, prior to the time of Sir Isaac Newton. The Pytha- goreans called colour the superficies of bodies; Plato said that it was a flame issuing from them. According to Zeno, it is the first configuration of matter; and according to Aristotle, it is that which moves bodies actually transparent. Among the moderns, Des Cartes imagined that the difference of colour proceeds from the prevalence of the direct or rotatory motions of the particles of light. -Grimaldi, Dechales, and others, thought the differences of colour depended upon the quick or slow vibrations of a certain elastic medium filling the whole universe. Rohault imagined that the different colours were made by the rays of light entering the eye at different angles with respect to the optic axis ; and Dr. Hook conceived that colour is caused by the sensation of the oblique or uneven pulse of light; and this being capable of no more than two varieties, he concluded that there could be no more than two primary colours. Such were some of the crude opinions which prevailed before the era of the illustrious Newton, by whose enlightened investigations the true theory of colours was at last discovered... In the year 1666 this phi- losopher began to. investigate the subject, and finding the coloured image of the sun, formed by a glass prism, to be of an oblong, and not of a circular form, as, according to the laws of refraction, it ought to be, he was surprised at the great disproportion between its length and breadth, the former being five times the length of the latter; and he began to conjecture that light is not homogeneal, but that it consists of rays, some of which are much more refrangible than others. Prior to this period, philosophers supposed that all light, in passing out of one medium into another of different density, was equally refracted, in the same or like circumstances ; but Newton discovered that this is not the fact; but that there are different species of light, and that each species is disposed both to suffer a different degree of refrangibility in passing out of one medium into another, and to excite in us the idea of a different colour from the rest; and that bodies appear of that colour which arises from the peculiar rays they are 100 SIR ISAAC NEWTON’S DISCOVERY. disposed to reflect. It is now, therefore, universally acknow- ledged that the light of the sun, which to us seems perfectly homogeneal and white, is composed of no fewer than seven different colours, namely, Red, Orange, Yellow, Green, Blue, Indigo, and Violet. A body which appears of a red colour has the property of reflecting the red rays more powerfully than any of the others; a body ofa green colour reflects the green rays more copiously than rays of any other colour, and so of the orange, yellow, blue, purple, and violet. A body which is of a black colour, instead of reflecting, absorbs all, or the greater part of the rays that fall upon it; and, on the contrary, a body that appears white reflects the greater part of the rays indiscriminately, without separating the one from the other. Before proceeding to describe the experiments by which the above results were obtained, it may be proper to give some idea of the form and effects of the Prism by which such experiments are made. ‘This instrument is triangular and straight, and generally about three or four inches long. It is commonly made of white glass, as free as possible from veins and bubbles, and other similar defects, and is solid through- out. Its lateral faces, or sides, should be perfectly plane, and of a fine polish. ‘The angle formed by the two faces, one receiving the ray of light that is refracted in the instrument, and the other affording it an issue on its returning into the air, is called the refracting angle of the prism, as AC B, (fig. 31.) The manner in which Newton performed his experi- Figure 31. Sen, me ~ ~ =~ Pe. ie ~— s ~. ~ = mal x. ~. ~, ~ ~ ~ - ~ Wag, ~—< ~ ~ - ~ ~ = =. - ments, and established the discovery to which we have al- luded, is as follows: In the window-shutter, E G, (fig. 31,) of a dark room, a hole, F; was made, of about one-third of an inch diameter, and behind it was placed a glass prism, A C B, so that the beam of light, S F, proceeding directly from the sun, was THE PRISMATIC SPECTRUM. 101 made to passthrough the prism. Before the interposition of the prism, the beam proceeded in a straight line towards T, where it formed a round white spot; but, being now bent out of its course by the prism, it formed an oblong image, O P, upon the white pasteboard, or screen, L M, containing the seven colours marked in the figure, the red being the least, and the violet the most refracted from the original direction of the solar beam, S T.. This oblong image is called the prismatic spectrum. If the refracting ‘angle of the prism, A © B, be 64 degrees, and the distance of the pasteboard from the prism about 18 feet, the length of the image, O P, will be about 10 inches, and the breadth 2 inches. The sides of the spectrum are right lines distinctly bounded, and the ends are semicircular. From this circumstance, it is evident that it is still the image of the sun, but elongated by the refractive power of the prism. It is evident from the figure that, since some part of the beam, R Q, is refracted much farther out of its natural course, W T, than some other part of the beam, as W P, the rays towards R O have a much greater dispo- sition to be refracted than those towards W P; and that this disposition arises from the naturally different qualities of those rays, is evident from this consideration, that the refracting angle or power of the prism is the same in regard to the superior part of the beam as to the inferior. By making a hole in the screen, L M, opposite any one of the colours of the spectrum, so as to allow that colour alone to pass—and by letting the colour thus separated fall upon a second prism—Newton found that the light of each of the colours was alike refrangible, because the second prism could not separate them into an oblong image, or into any other colour. Hence he called all the seven colours simple or homogeneous, in opposition to white light, which he called compound, or heterogeneous. With the prism which this philosopher used, he found the lengths of the colours and spaces of the spectrum to be as follows: Red, 45; Orange, 27; Yellow, 40; Green, 60; Blue, 60; Indigo, 48; Violet, 80; or 360 in all. But these spaces vary a little with prisms formed of different substances, and, as they are not separated by distinct limits, it is difficult to obtain any thing like an accurate measure of their re- lative extents. Newton examined the ratio between the sines of incidence and refraction of these decompounded rays, (see p. 45,) and found that each of the seven primary colour-making rays had certain limits within which they were confined. Thus, let the sine of incidence in glass be divided into 50 equal parts, the sine of refraction into air of the least refrangi- 9 * 102 REFRANGIBILITY OF COLOURED RAYS. ble, and the most refrangible rays will contain respectively 77 and 78 such parts.. The sines of refraction of all the de- grees of red will have the intermediate degrees of magnitude, from 77 to 77 one-eighth: Orange, from 77 one-eighth to 77 one-fifth; Yellow, from 77 one-fifth to 77 one-third; Green, from 77 one-third to 77 one-half; Blue, from 77 one-half to 77 two-thirds; Indigo, from 77 two-thirds to 77 seven-ninths; and Violet, from '77 seven-ninths to 78. ) From what has been now stated, it is evident that, in pro- portion as any portion of an optic glass bears a resemblance to the form of a prism, the component rays that pass through it must be necessarily separated, and will consequently paint or tinge the object with colours. The edges of every convex lens approach to this form, and it is on this account that the extremities of objects, when viewed through them, are found to be tinged with the prismatic colours. In such a glass, therefore, those different coloured rays will have different foci, and will form their respective images at different dis- tances from the lens. ‘Thus, suppose L N (fig. 32) to rep- resent a double convex lens, and O B an object at some dis- tance from it. If the object O B was ofa pure red colour, the rays proceeding from it would form a red image at Rr; if the object was of a violet colour, an image of that colour would be formed, at V v, nearer the lens; and if the object was white, or any other combination of the colour-making rays, those rays would have their respective foci at different distances from the lens, and form a succession of images in the order of the prismatic colours, between the space R R and V v. Figure 32. B | This may be illustrated in the following manner; Take a card or slip of white pasteboard, as A B E F, (fig. 33,) and paint one-half, A BC D, red, the other half C EF’, violet or indigo, and, tying black threads across it, set it near the flame REFRANGIBILITY OF COLOURED RAYS. 108 of a candle, G; then take a lens, H I, and, holding a sheet of white paper behind it, move it backward and forward upon the edge of a graduated ruler till you see the black threads most distinctly in the image, and you will find the focus of the violet y = much nearer than that of the red a c, which plainly shows that bodies of different colours can never be depicted by convex-lenses without some degree of confusion. Figure 33. FL The quantity of dispersion of the coloured rays in convex levses depends upon the focal length of the glass, the space which the coloured images occupy being about the twenty- eight part. Thus, if the lens be twenty-eight inches focal distance, the space between RR and V v (fig. 32) will be about one inch; if it be twenty-eight feet focus, the: same space will be about one foot, and so on in proportion. Now, when such a succession of images formed by the different coloured rays is viewed through an eyeglass, it will seem to form but one image, and, consequently, very indistinct, and tinged with various colours; and as the red figure, R R, is largest, or seen under the greatest angle, the extreme parts of this confused image will be red, and a succession of the prismatic colours will be formed within this red fringe, as is generally found in common refracting telescopes, constructed with a single object-glass. It is owing to this circumstance that the common refracting telescope cannot be much im- proved without having recourse to lenses of a very long focal distance; and hence, about 150. years ago, such telescopes were constructed of 80, and 100, and 120 feet in length. But still, the image was not formed so distinctly as was de- sired, and the aperture of the object-glass was obliged to be limited. This is a defect which was long regarded as with- out a remedy; and even Newton himself despaired of dis- covering any means by which the defects of refracting tele- scopes might be removed, and their improvement effected. This, however, was accomplished by Dollond to an extent far surpassing what could have been expected, of which a par- ticular account will be given in the sequel. ‘ It was ongmally remarked by Newton, and the fact has 104 HEATING» AND ILLUMINATING RAYS. since been confirmed by the experiments of Sir W. Herschel, that the different-coloured rays have not the same illumi- nating power. ‘The violet rays appear to have the least illu- minating effect ; the indigo more, and the effect increases in the order of the colours, the green being very great; between the green and the yellow the greatest of all; the yellow the same as the green; but the red less than the yellow. Herschel also endeavoured to determine whether the power of the differently-coloured rays to heat bodies varied with their power to illuminate them. He introduced a beam of light into a dark room, which was decomposed by a prism, and then exposed a very sensible thermometer to all the rays in succession, and observed the heights to which it rose in a given time. He found that their heating power increased from the violet to the red. The mercury in the thermometer rose higher when its bulb was placed in the indigo than when it was placed in the violet, still higher in blue, and highest of all at red. Upon placing the bulb of the thermometer below the red, quite out of the spectrum, he was surprised to find that the mercury rose highest of all, and concluded that rays proceed from the sun, which have the power of HEATING, but not of iluminating bodies. 'These rays have been called invisible solar rays; they were about half an inch from the commencement of the red rays; at a greater distance from this point the heat began to diminish, but was very percepti- ble, even at the distance of 13 inch. He determined that the heating power of the red to that of the green rays was 22 to 1, and 32 to 1, in red to violet. He afterward made experi- ments to collect those invisible calorific rays, and caused them to act independently of the light, from which he concluded that they are sufficient to account for all the effects produced by the solar rays in exciting heat; that they are capable of passing through glass, and of being refracted and reflected, after they have been finally detached from the solar beam. M. Ritter of Jena, Wollaston, Beckman, and others, have © found that the rays of the spectrum are possessed of certain chemical properties ; that beyond the least brilliant extremity, namely, a little beyond the violet ray, there are invisible rays which act chemically, while they have neither the power of heating nor illuminating bodies. Muriate of silver, exposed to the action of the red rays, becomes blackish; a greater effect is produced by the yellow; a still greater by the violet, and the greatest of all by the invisible rays beyond the violet. When phosphorus is exposed to the action of the invisible rays beyond the red, it emits white fumes, but the imvisible MAGNETIC POWER OF THE VIOLET RAYS. 105 gays beyond the violet extinguish them. The influence of these rays is daily seen in the change produced upon vege- table colours, which fade when frequently exposed to the direct influence of the sun. What object they are destined to accomplish in the general economy of nature is not yet distinctly known; we cannot, however, doubt that they are ementially requisite to various processes going forward in the material system. And we know that not only the comfort of all the tribes of the living world, but the very existence of the animal and vegetable creation depends upon the unremitting agency of the calorific rays. It has likewise been lately discovered that certain rays of the spectrum, particularly the violet, possess the property of communicating the magnetic power. Dr. Morichini, of Rome, appears to have been the first who found that the violet rays of the spectrum had this property. The result of his experiments, however, was involved in doubt till it was esta- blished by a series of experiments instituted by Mrs. Somer- ville, whose name is’so well known in the scientific world. This lady having covered half a sewing-needle, about an inch long, with paper, she exposed the other half for two hours to the violet rays. The needle had then acquired north polarity. The indigo rays produced nearly the same effect; and the blue and green rays produced it ina still less degree. In the yellow, orange, red, and invisible rays no magnetic influence was exhibited, even though the experiment was continued for three successive days. The same effects were produced by enclosing the needle in blue or green glass, or wrapping it in blue and green ribands, one-half of the needle being always covered with paper. | One of the most curious discoveries of modern times, in reference to the solar spectrum, is that of Fraunhofer, of Munich, one of the most distinguished artists and opticians on the Continent.* He discovered that the spectrum is co- *Fraunhofer was, in the highest sense of the word, an optician, an original discoverer in the most abstruse and delicate departments of this science, a competent mathematician, an admirable mechanist, and a man of a truly philosophical turn of mind. By his extraordinary talents, he was soon raised from the lowest station in a manufacturing establishment to the direction of the optical department of the business, in which he originally laboured as an ordinary workman. ° He then applied the whole power of his mind to the perfection of the achromatic telescope, the de- fects of which, in reference to the optical properties of the materials used, he attempted to remedy; and, by a series of admirable experiments suc- ceeded in giving to optical determinations the precision of astronomical observations, surpassing in this respect all who had gone before him, ex- cept, perhaps, the illustrious Newton. It was in the course of these researches that he was lec to the important discovery of the dark lines 106 DARK LINES IN°THE SOLAR SPECTRUM. vered with dark and coloured lines, parallel to one another, and perpendicular to the length of the spectrum; and. he counted no less than 590 of these lines. In order to observe these lines, it is necessary to use prisms of the most perfect construction, of very pure glass, free of veins, to exclude all extraneous light, and even to stop those rays which form the coloured spaces which we are not examining. It is neces- sary, also, to use a magnifying instrument, and the light must enter and emerge from the prism at equal angles. One of » the important practical results of this discovery is, that those lines are fixed points in the spectrum, or, rather, that they have always the same position in the coloured spaces in which they are found. Fraunhofer likewise discovered, in the spec- trum produced by the light of Venus, the same streaks as in the solar spectrum ; in the spectrum of the light of Sirius he perceived three large streaks, which, according to appear- ance, had no resemblance to those of the light of the sun; one of them was in the green, two in the blue. The stars appear to differ from one another'in their streaks. The electric light differs very much from the light of the sun and that of a lamp in regard to the streaks of the spectrum. “This experiment may also be made, though in an imperfect manner, by view- ing a narrow slit between two nearly closed window-shutters through a very excellent glass prism, held close to the eye, with the refracting angle parallel to the line of light. When the spectrum is formed by the sun’s rays, either direct or in- direct, as from the sky, clouds, rainbow, moon, or planets, the black bands are always found to be in the same parts of the spectrum, and under all circumstances to maintain the seme relative position, breadth, and intensities.” Fyom what has been stated im reference to the solar spec- trum, it will evidently appear that white light is nothing else than a compound of all the prismatic colours; and this may be still farther illustrated by showing that the seven primary colours, when again put together, recompose white light. This may be rudely proved, for the purpose of illustration, by mix- ing tocether seven different powders, having the colours and which occur in the solar spectrum. His achromatic telescopes are scat- tered over Europe, and are ‘the largest and best that have hitherto been constructed. He died at Munich, at a premature age, in 1826; his death, it is said, being accelerated by the unwholesome nature of the processes employed in his glass-house ; leaving behind him a reputation rarely at- tained by one so young. His memoir ‘‘ On the refractive and dispersive Power of different Species of Glass, in reference to the Improvement of Achromatic Telescopes, and an Account of the Lines on the Spectrum,’’ will be found in the ‘‘ Edinburgh Phiiosophical Journal,’’ vol. ix., p. oS 288—299; and rol. x., p. 26—40, for 1823-4. . WHITE, THE UNION OF THE COLOURED RAYS. 107 roportion of the spectrum ; but the best mode, on the whole, is the following: Let two circles be drawn on a smooth round board, covered with white paper, as in figure 34; let the Figure 34. outermost be divided into 360 equal parts; then draw seven right lines, as A, B, ©, &c., from the centre to the outermost circle, making the lines A and B include 80 degrees of that circle. The lines B and C, 40 degrees; C and D, 60; D and E, 60; Eand F, 48; F and G, 27; Gand A,45. Then between these two circles paint the space A G red, inclining to orange near G; G F orange, inclining to yellow near F; F E yellow, inclining to green near E ; ED green, inclining to blue near D; DC blue, inclining to indigo near C; C B indigo, inclining to violet near B; and B A violet, inclining toa soft red near A. This done, paint all that part of the board black which lies within the inner circle; and, putting an axis through the centre of the board, let it be turned swiftly round that axis, so that the rays proceeding from the above colours may be all blended and mixed together in coming to the eye. Then the whole coloured part will appear like a white ring a little grayish—not perfectly white, because no art can prepare or lay on perfect colours, in all their delicate shades, as found in the real spectrum. That all the colours of light, when blended together in their proper proportions, produce a pure white, is rendered certain Py the following experiment: Take a large convex glass, and place it in the room of the paper or screen on which the solar spectrum was depicted (L M, fig. 31 ;) the glass will unite 108 COLOURS OF NATURAL OBJECTS. all the rays which come from the prism, if a paper is placed to receive them, and you will see a circular spot of a pure lively white. ‘The rays will cross each other in the focus of the glass, and, if the paper be removed a little farther from that point, you will see the prismatic colours again displayed, but in an inverted order, owing to the crossing of the rays. SECT. II.—ON THE COLOURS OF NATURAL OBJECTS. From what has been stated above we may learn the true cause of those diversified hues exhibited by natural and arti- ficial objects, and the variegated colouring which appears on the face of nature. It is owing to the surfaces of bodies being disposed to reflect one colour rather than another. When this disposition is such that the body reflects every kind of ray, in the mixed state in which it receives them, that body appears white to us, which, properly speaking, is no colour, but rather the assemblage of all colours. If the body has a fitness to reflect one sort of rays more abundantly than others, by absorbing all the others, it will appear of the colour belong- ing to that species of rays. . Thus, the grass is green, because it absorbs all the rays except the green. It is these green rays only which the grass, the trees, the shrubs, and all the other verdant parts of the landscape reflect to. our sight, and which make them appear green. In the same manner, the different flowers reflect their respective colours; the rose, the red rays; the violet, the blue; the jonquil, the yellow; the marigold, the orange; and every object, whether natural or artificial, appears of that colour which its peculiar texture is fitted to reflect. A great number of bodies are fitted to re- flect at once several kinds of rays, and, of consequence, they appear under mixed colours. It may even happen that of two bodies which should be green, for example, one may re- flect the pure green of light, and the other the mixture of yellow and blue.. This quality, which varies to infinity, occa- sions the different kinds of rays to unite in every possible manner, and every possible proportion ; and hence the inex- haustible variety of shades and hues which nature has dif- fused over the landscape of the world. When a body absorbs nearly all the light which reaches it, that body appears black ; it transmits to the eye so few reflected rays that it is scarcely perceptible in itself, and its presence and form make no im- pression upon us, unless as it interrupts the brightness of the surrounding space. Black is, therefore, the absence of all the coloured rays. : It is evident, then, that all the various assemblages of co- COLOUR NOT INHERENT IN BODIES. 109 lours which we see in the objects around us are not in the bodies themselves, but in the light which falls upon them. There is no colour trherent in the grass, the trees, the fruits, and the flowers, nor even in the most splendid and variegated dress that adorns a lady. All such objects are as destitute of colour, in themselves, as bodies which are placed in the centre of the earth, or as the chaotic materials out of which our globe was formed before light was created; for where there is no light, there is no colour. Every object is black, or without colour, in the dark, and it only appears coloured as soon as light renders it visible. This is farther evident from the following experiment: If we place a coloured body in one of the colours of the:spectrum which is formed by the prism, it appears of the colour of the rays in which it is placed. ‘T'ake for example, a red rose, and expose it first to the red rays, and it will appear of a more brilliant ruddy hue; hold itin the k’ue rays, and it appears no longer red but of a dingy blue colour, and in like manner its colour will appear different when placed in all the other differently-coloured rays. ‘This is the reason why the colours of objects are es- sentially altered by the’ nature of the light in which they are seen. The colours of ribands, and various pieces of silk or woollen stuffs, are not the same when viewed by candle-light as in the day-time. In the light of a candle ora lamp, blue often appears green, and yellow objects assume a whitish as- pect. The reason is, that the light of a candle is not so pure a white as that of the sun, but has a yellowish tinge, and therefore, when refracted by the prism, the yellowish rays are found to predominate, and the superabundance of yellow rays give to blue objects a greenish hue. The doctrine we are now illustrating is one whichia great many persons, especially among the fair sex, find it difficult toadmit. They cannot conceive it possible that there is no colour really inherent in their splendid attire, and no tints of beauty in their countenances. “ What,” said a certain lady, “ are there no colours in my shawl,and in the ribands that adorn my headdress, and are we all as black as negroes in the dark ?. I should almost shudder to think of it.” Such persons, however, need be in no alarm at the idea, but may console themselves with the reflection’ that, when they are stripped of all their coloured ornaments in the dark, they are certain that they will never be seen by any one in that state ; and therefore there is no reason to regret the temporary loss of those beauties which light creates, when they themselves, and all surrounding objects, are invisible. But, to give 1 Vor. IX. 10 110 EFFECT OF A YELLOW LIGHT, still more palpable proof of this position, the following popu- lar experiments may be stated : ‘Take a pint of common spirit and pour it into a soup dish, and then set it on fire; as it begins to blaze, throw a handful of salt into the burning spirit, and keep stirring it witha spoon. Several handfuls may thus be successively thrown in, and then the spectators, standing around the flame, will see each other frightfully changed, their colours being altered into a ghastly blackness, in consequence of the nature of the light which falls upon them, which produces colours very different from those of the solar light. The following expe- riment, as described by Sir D. Brewster, illustrates the same principle : “ Having obtained the means of illuminating any apartment with yellow light, let the exhibition be made in a room with furniture of various bright colours, and with oil or water-coloured paintings on the wall. The party which is to witness the experiment should be dressed in a diversity of the gayest colours, and the brightest coloured flowers: and highly coloured drawings should be placed on thetables. The room being at first lighted with ordinary lights, the bright and gay colours of every thing that it contains will be finely dis- played. Ifthe white lights are now suddenly extineuished, and the yellow lamps hghted, the most appalling metamor- phosis will be exhibited: The astonished individuals will no longer be able to recognise each other. All the furniture of the : room, and all the objects it contains will exhibit only one colour. The flowers will lose their hues; the paintings and drawings will appear as if they were executed in China ink, and the gayest dresses, the brightest scarlets, the purest lilachs, the richest blues, and the most vivid greens, will all be con- verted into one monotonous yellow. The complexions of the parties, too, will suffer a corresponding change. One pallid death-like yellow, _. Like the unnatural hue - Which autumn paints upon the perished leaf, will envelop the young and the old, and the sallow face will alone escape from the metamorphosis. Each individual de- rives merriment from the cadaverous appearance of his neighbour, without being sensible that he is one of the ghastly assemblage.” From such experiments as these we might eonelinde that, were the solar rays of a very different description from what they are now found to be, the colours which embellish the face of nature, and the whole scene of our sublunary crea- EFFECTS OF THE ATMOSPHERE ON LIGHT. 111 tion, would assume a new aspect, and appear very different from what we now behold around us in every landscape. We find that the stars display great diversity of colour, which is doubtless owmg to the different kinds of light which are emitted from those bodies; and hence we may conclude that the colouring thrown upon the various objects of the universe is different in every different system, and that thus, along with other arrangements, an infinite variety of colouring and of scenery is distributed throughout the immensity of crea- tion. 7 The atmosphere, in consequence of its different refractive and reflective powers, is-a source of a variety of colours which frequently embellish and diversify the aspect of our sky. The air reflects the blue rays most plentifully, and must therefore transmit the red, orange, and yellow more copiously than the other rays. When the sun and other heavenly bodies are at a high elevation, their light is trans- mitted without any perceptible change; but when they are near the horizon, their light must pass through a long and dense tract of air, and must therefore be considerably modi- fied before it reach the eye of the observer. ‘The momentum of the red rays being greater than that of the violet, will force their way through the resisting medium, while the violet rays will be either reflected or absorbed. If the light of the setting sun, by thus passing through a long tract of air, be divested of the green, blue, indigo, and violet rays, the re- maining rays which are transmitted through the atmosphere will illuminate the western clouds first, with an orange colour, and then, as the sun gradually sinks into the horizon, the tract through which the rays must pass becoming longer, the yellow and orange are reflected, and the clouds grow more deeply red, till at length the disappearance of the sun leaves them of a leaden hue, by the reflection of the blue light through the air. Similar changes of colour are sometimes seen on the eastern and western fronts of white buildings. St. Paul’s Church, in London, is frequently seen, at sunset, tinged with a very considerable degree of redness; and the same cause occasions the moon to assume a ruddy colour, by the light transmitted through the atmosphere. From such atmospherical refractions and reflections are produced those rich and beautiful hues with which our sky is gilded by the _ setting sun, and the glowing red which tinges the morning and evening clouds, till their ruddy glare is tempered by the purple of twilight, and the reflected azure of the sky. When a direct spectrum is thrown on colours darker than 112 COLOUR OF THE SEA AND SKY. itself, it mixes with them, as the yellow spectrum of the set- ting sun, thrown on the green grass, becomes a greener yel- low. But when a direct spectrum is thrown on colours brighter than itself, it becomes instantly changed into the reverse spectrum, which mixes with those brighter colours. Thus the yellow spectrum of the setting sun, thrown on the luminous sky, becomes blue, and changes with the colour or brightness of the clouds on which it appears. The red part of light being capable of struggling through thick and re- sisting mediums which intercept all other colours, is likewise the cause why the sun appears red when seen through a for ; why distant light, though transmitted through blue or ereen glass, appears red; why lamps at a distance, seen through the smoke of a long street, ‘are red, while those that are near are white. To the same cause it is owing thata diver at the bottom of the sea is surrounded with the red light which has pierced through the superincumbent fluid, and that the blue rays are reflected from the surface of the ocean. Hence Dr. Halley informs us that, when he was in a diving bell at the bottom of the sea, his hand always ap- peared red in the water. The blue rays, as already noticed, being unable to resist the obstructions they meet with in their course through the atmosphere, are either reflected or absorbed in their passage. It is to this cause that most philosophers ascribe the blue colour of the sky, the faintness and obscurity of distant ob- jects, and the bright azure which tinges the mountains of a distant landscape. SECT. III.—-PHENOMENA OF THE RAINBOW. Since the rays of light are found to be decomposed by re- fracting surfaces, and reflected in an infinite variety of modes and shades of colour, we need not be surprised at the changes produced in any scene or object by the intervention of ano- ther, and by the numerous modifications of which the primary colours of nature are susceptible. The vivid colours which gild the rising and the setting sun must necessarily differ from those which adorn its noonday splendour. Variety of atmospheric scenery will thus necessarily be produced, greater than the most lively fancy can well imagine. The clouds will sometimes assume the most fantastic forms, and at other times will be irradiated with beams of light, or, covered with the darkest hues, will assume a lowering aspect, prognostive of the thunder’s roar and the lightning’s flash, al! THE RAINBOW. 118 n accordance with the different rays that are reflected to our eyes, or the quantity absorbed by the vapours which float in the atmosphere. Light, which embellishes with so much magnificence a pure and serene sky, by means of innumerable bright starry orbs which are spread over it, sometimes, in a dark and cloudy sky, exhibits an ornament which, by its pomp, splendour, and variety of colours, attracts the attention of every eye that has an opportunity of beholding it. At certain times, when there is a shower either around us, or at a distance from us in an opposite quarter to that of the sun, a species of arch or bow is seen in the sky, adorned with all the seven primary colours of light. This phenomenon, which is one of the most beau tiful meteors in nature, has obtained the name of the Rain- Bow. ‘The rainbow was, for ages, considered as an inexpli- cable mystery,,and by some nations it was adored as a deity. Even after the dawn of true philosophy, it was a considera- ble time before any discovery of importance was made as to the true causes which operate in the production of this phe- nomenon. About the year 1571, M. Fletcher, of Breslau, | made a certain approximation to the discovery of the true cause, by endeavouring to account for the colours of the rain- bow by means of a double refraction and one reflection. A nearer approximation was made by Antonio de Dominis, bishop of Spalatro, about 160i. He maintained that the double refraction of Fletcher, with an intervening reflection, was sufficient to produce the colours of the bow, and also to bring the rays that formed them to the eye of the spectator, without any subsequent reflection. To verify this hypothe. sis, he procured a small globe of solid glass, and viewing 1 when it was exposed to the rays of the sun, with his back to that luminary, in the same manner as he had supposed the drops of ram were situated with respect to them, he observed the same colours which he had seen in the rainbow, and «in the same order. But he could give no good reason why the bow should be coloured, and, much less, any satisfactory account of the order in which the colours appear. It was not till Sir I. Newton discovered the different refrangibility of the rays of light, that a complete and satisfactory explanation could be given of all the circumstances connected with this phenomenon. As the full elucidation of this subject involves a variety of optical and mathematical investigations, I shall do little more than explain the general principle on which the prominent phenomena of the rainbow may be accounted for, and some 10* 114 COLOURS OF THE RAINBOW. of the facts and results which theory and observation have deduced. We have just now alluded to an experiment with a glass. globe: If, then, we take either a solid glass globe, or a hollow globe filled with water, and suspend it so high in the solar rays above the eye that the spectator, with his back to the sun, can see the globe reds if it be lowered slowly, he will see it orange, then yellow, then green, then blue, then indigo, and then violet; so that the drop, at different heights, shall present to the eye the seven primitive colours in succession. In this case, the globe, from its form, will act in some measure like a prism, and the ray will be separated into its component parts. ‘The following figure will more particularly illustrate Figure 35. A ds this point. Suppose A (fig. 35) to represent a drop of rain —which may be considered as a globe of glass in miniature, and will produce the same effect on the rays of light—and let Sp represent a ray from the sun falling upon the upper part of the drop atp. At the point of entering the drop it will suffer a refraction, and. mstead of going forward to o, it . COLOURS OF THE RAINBOW. 115 will be bent to n. From wn a part of the light will be reflected to a—some part of it will, of course, pass through the drop. By the obliquity with which it falls on the side of the drop at Q, that part becomes a kind of prism, and separates the ray into its primitive colours. It is found by computation that, after a ray has suffered two refractions and one reflection, as here represented, the least refrangible part of it, namely, the red ray, will make an angle with the incident solar ray of 42° 2',as Srq; and the violet, or greatest refrangible ray, will make, with the solar ray, an angle of 40° 17’, as Scq; and thus all the particles of water within the difference of those two angles, namely, 1° 45’, (supposing the ray to pro- ceed merely from the centre of the sun,) will exhibit severally the colours of the prism, and constitute the interior bow of the cloud. This holds good at whatever height the sun may chance to be in a shower of rain. If he be at a high altitude, the rainbow will be low; if he be at a low elevation, the rain- bow must be high; and if a shower happen in a vale, when the spectator is on a mountain, he will sometimes see the bow in the form of a complete circle below him. We have at pre- sent described the phenomena only of a single drop; but it is to be considered that in a shower of rain there are drops at all heights and at all distances, and therefore the eye situated at @ will see all the different colours. All those drops that are ina certain position with respect to the spectator will re- flect the red rays, all those in the next station the orange, those in the next the green, and so on with regard to all the other colours. It appears, then, that the first or primary bow is formed by two refractions and one reflection; but there is frequently a second bow on the outside of the other, which is considerably fainter. This is produced by drops of rain above the drop we have supposed at A. If B-(fig. 35) represent one of these drops, the ray to be sent.to the eye enters the drop near the bottom, and suffers two refractions and two reflections, by which means the colours become reversed, that is, the violet is lowest in the exterior bow, and the red is lowest in the in- terior one, and the other colours are reversed accordingly. The ray T is refracted at rR: a part of it is reflected from s to 7, and at T it suffers another reflection from 7 to uv. At the points s and T part of the ray passes through the drop, on account of its transparency, towards w and x, and therefore we say that part only of the ray is reflected. ..By these losses and reflec- tions the exterior bow becomes faint and ill-defined in compari- son of the interior or primary bow. In this case the upper 116 COLOURS OF THE RAINBOW. part of the secondary bow will not be seen when the sun is above 54° 10’ above the horizon, and the lower part of the bow will not be seen when the sun is 60° 58’ above the horizon. For the farther illustrations of this subject, we may intro- duce the following section of a bow, (fig. 36,) and, in order to prevent confusion in attempting to represent all the different Figure 36. SRE PUES & Be 7, i ey PYUs colours, let us suppose only three drops of rain, and three dif- ferent colours, as shown in the figure. The spectator, O, being in the centre of the two bows here represented—the planes of which must be considered as perpendicular to his view—the drops A, B, and C produce part of the interior bow by two refractions and one reflection, as stated above, and the drops D, E, F will produce the exterior bow by two re- fractions and two reflections, the sun’s rays being represented by 3, 3. It is evident that the angle C O P is less than the angle B O P, and that the angle A O P is the greatest of the three. The largest angle, then, is formed by the red rays, the middle one consists of the green, and the smallest the FORM OF THE RAINBOW. 117 purple or violet. All the drops of rain, therefore, that happer to be in a certain position with respect to the spectator’s eye, will reflect the red rays, and form a band or semicircle of red, and so of the other colours from drops in other positions. If the spectator alters his station, he will see a bow, but not the same as before; and if there be many spectators, they will each see a different bow, though it appears to be the same. The rainbow assumes a semicircular appearance, because it is only at certain angles that the refracted rays are visible to our eyes, as is evident from the experiment of the glass globe formerly allused to; which will refract the rays only in a certain positions We have already stated that the red rays make an angle of 42° 2’, and the violet an angle of 40° 17’. Now, if a line be drawn horizontally from the spectator’s eye, it is evident that angles formed with this line, of a certain dimension, in every direction, will produce a circle, as will appear by attaching a cord of a given length to a certain point, round which it may turn as round its axis; and, in every point, will describe an angle with the horizontal line of a certain and determinate extent. Sometimes it happens that three or more bows are visible, though with different degrees of distinctness. I have more than once observed this phenomenon, particularly in Edin- burgh, in the month of August, 1825, when three rainbows were distinctly seen in the same quatter of the sky, and, if I recollect. right, a fragment of a fourth made its appearance. This happens when the rays suffer a third or fourth reflec- tion ; but, on account of the light lost by so many reflections, such bows are, for the most part, altogether imperceptible. If there were no ground_to intercept the rain and the view of the observer, the rainbow would form a complete circle, the centre of which is diametrically opposite to the sun. Such circles are sometimes seen in the spray of the sea or of a cas- cade, or from the tops of lofty mountains, when the showers happen in the vales below. Rainbows of various descriptions are frequently observed rising amid the spray and exhalations of waterfalls, and among the waves of the sea, whose tops are blown by the wind into small drops. There is one regularly seen when the sun is shining, and the spectator in a proper position, at the fall of Staubbach, in the bosom of the Alps; one near Schaffhausen; one at the cascade of Lauffen, and one at the cataract of Niagara in North America. A still more beautiful one is said to be seen at Terni, where the whole current of the river Velino, rushing from a steep pre- cipice of 1 arly 200 feet high, presents to the spectator below 118 SUMMARY OF ASCERTAINED FACTS. a variegated circle, over-arching the fall, and two other bows suddenly reflected on the right and left. Don Ulloa, in the account of his journeys in South America, relates that circu- lar rainbows are frequently seen on the mountains above Quito in Peru. It is said that a raimbow was once seen near London, caused by the exhalations of that city, after the sun had been below the horizon more than twenty minutes.* A naval friend, says Mr. Bucke, informed me that, as he was one day watching the sun’s effect upon the exhalations near Juan Fernandez, he saw upward of five-and-twenty ires maring animate the sea at the same time. In these marine bows the concave sides were turned upward, the drops of water rising from below, and not falling from above, as in the instances of the aerial arches. Rainbows are also occasionally seen on the grass in the morning dew, and likewise when the hoar-frost is descending. Dr. Langwith once saw a bow lying on the ground, the colours of which were almost as lively as those of acommon rainbow. It was not round, but oblong, and was extended several; hundred yards. The colours took up less space, and were much more lively in those parts of the bow which were near him than in those which were at a distance. When M. Labillardiere was on Mount Teneriffe, he ‘saw the contour of his body traced on the clouds beneath him im all the colours of the solar bow. He had previously witnesséd this phenomenon on the Kesrouan, in Asia Minor. The rainbows of Greenland are said to be frequently of a pale white, fringed with a brownish yellow, arising from the rays of the sun being reflected from a frozen cloud. The following is a summary view of the principal facts which have been ascertained respecting the rainbow: 1. The rainbow can only be seen when it rains, and in that point of the heavens which is opposite to the sun. 2. Both the pri- mary and secondary bows are variegated with all the pris- matic colours—the red being the highest colour in the pri- mary, or brightest bow, and the. violet the highest in the exterior. 3. The primary rainbow can never be a greater arc than a semicircle ; and, when the sun is set, no bow, in ordinary circumstances, can be seen. 4. The breadth of the inner or primary bow—supposing the sun but a point—is 1° 45’, and the breadth of the exterior bow 3° 12’, which is nearly twice as great as that of the other; and the distance between the bows is 8° 55’.. But since the body of the sur. * Philosophical Transactions, vol. 1. p. 294. AN ARTIFICIAL RAINBOW. 119 subtends an angle of about half a degree, by so much will each bow be increased, and their distance diminished ; and therefore the breadth of the interior bow will be 2° 15’, and that of the exterior 3° 42’, and their distance 8° 25’. The greatest semi-diameter of the interior bow, on the same grounds, will be 42° 17’, and the least of the exterior bow 50° 43’. 5. When the sun is in the horizon, either in the morning or evening, the bows will appear complete semi- circles. On the other hand, when the sun’s altitude is equa: to 42° 2’, or to 54° 10’, the summits of the bows will be de- pressed below the horizon. Hence, during the days of summer, within a certain interval each day, no visible rain- bows can. be formed, on account of the sun’s high altitude above the horizon. 6. The altitude of the bows above the horizon or surface of the earth varies according to the eleva- tion of the sun. The altitude, at any time, may be taken by a common quadrant, or any other angular instrument; but if the sun’s altitude at any particular time be known, the height of the summit of any of the bows may be found: by subtracting the sun’s altitude from 42° 2’ for the inner bow, and from 54° 10’ for the outer. Thus, if the sun’s altitude be 26°, the height of the. primary bow would be 16° 2’, and of the secondary, 28° 10’. It follows that the height and the size of the bows diminish as the altitude of the sun increases. 7. If the sun’s altitude is more than 42°, and less than 54°, the exterior bow may be seen, though the interior bow is in- visible. 8. Sometimes only a portion of an arch will be visible, while all the other parts of the bow are invisible. This happens when the rain does not occupy a space of suf- ficient extent to complete the bow; and the appearance of this position, and even of the bow itself, will be various, ac- cording to the nature of the situation, and the space occupied by the rain. The appearance of the rainbow may be produced by artifi- cial means at any time when the sun is shining and not too highly elevated above the horizon. ‘This is effected by means of artificial fountains, or jet d’eaus, which are intended to throw up streams of water to a great height. These streams, when they spread very wide, and. blend together in their upper parts, form, when falling, a shower of artificial rain. If, then, when the fountain is playing, we move be- tween it and the sun, at a proper distance from the fountain, till our shadow point directly towards it,and look at the shower, we shall observe the colours of the rainbow strong and vivid ; and, what is particularly worthy of notice, the bow appears, 120 LUNAR RAINBOWS. notwithstanding the nearness of the shower, to be as large and as far off as the rainbow which we see in a natural shower of rain. 'The same experiment may be made by candlelight, and with any instrument that will form an artifi- cial shower. Lunar Rainbows.—A \unar bow is sometimes formed at night by the rays of the moon strikmg on a rain-cloud, espe- cially when she is about the full. But'such a phenomenon is very rare. Aristotle is said to have considered himself the first who had seen a lunar rainbow. | For more than a hun- dred years prior to the middle of the last century, we find only two or three instances recorded in which such phenomena are described with accuracy. In the Philosophical Trans- actions for 1783, however, we have an account of three hav- ing been seen in one year, and all in the same place, but they are by no means common phenomena. I have had an op- portunity within the last twenty years of witnessing two phenomena of this description, one of which was seen at Perth, on a Sabbath evening in the autumn of 1825, and the other at Edinburgh, on Wednesday, the 9th of-September, 1840, about eight o’clock in the evening, of both which I gave a detailed description in some of the public journals. The moon, in both cases, was within a day or two of the full ; the arches were seen in the northern quarter of the heavens, and extended nearly from east to west, the moon being not far from the southern meridian, The bows appeared distinct and well defined, but no distinct traces of the prismatic co- lours could be perceived on any of them. ‘That which ap- peared in 1825 was the most distinctly formed, and continued visible for more than an hour. ‘The other was much fainter, and lasted a little more than half an hour, dark clouds having obscured the face of the moon. These bows bore a certain resemblance to some of the luminous arches which some- times accompany the-Aurora Borealis, and this latter phe- nomenon has not unfrequently been mistaken for a lunar rainbow ; but they may be always distinguished by attend- ing to the phases and position of the moon. Ifthe moon be not visible above the horizon, if she be in her first or last quarter, or if any observed phenomenon be not-in a direction opposite to the moon, we may conclude with certainty that, whatever appearance is presented, there is no lunar rainbow. The rainbow is an object which has engaged universal attention, and its beautiful colours and form have excited uni- versal admiration. 'The poets have embellished their writings with many beautiful allusions to this splendid meteor; and REFLECTIONS ON THE RAINBOW. 121 the playful schoolboy, while viewing the “ bright enchant- ment,” has frequently run “ to catch the falling glory.”” When its arch rests on the opposite sides of a narrow valley, or on the summits of two adjacent mountains, its appearance is both beautiful and grand. In all probability, its figure first sug- gested the idea of arches, which are now found of so much utility in forming aqueducts and bridges, and for adorning the architecture of palaces and temples. It is scarcely pos- sible seriously to contemplate this splendid phenomenon without feeling admiration and gratitude toward that wise and beneficent Being whose hands have bent it into so graceful and majestic a form, and decked it with all the pride of co- lours. ‘ Look upon the rainbow,’ says the son of Sirach,* “and praise Him that made it: very beautiful it is inthe brightness thereof. It compasseth the heaven about witha glorious circle, and the hands of the Most High have bended it.’ To this grand ethereal bow the inspired writers fre- quently allude as one of the emblems of the majesty and splendour of the Almighty. In the prophecies of Ezekiel, the throne of Deity is represented as adorned with a bright- ness “ like the appearance of the bow that is in the cloud in the day of rain—the appearance of the likeness of the glory of Jehovah.’”’ “And, in the visions recorded in the Book of the Revelation, where the Most High is represented as sitting upon a throne, “there was a rainbow round about the throne, in sight like unto an emerald,” as anemblem of his propitious character, and of his faithfulness and mercy. After the de- luge, this bow was appointed asa sign and memorial of the covenant which God made with Noah and his sons, that a flood of waters should never again be permitted to deluge the earth and its inhabitants, and asa pledge of inviolable fidelity and Divine benignity.t When, therefore, we at any time be- hold “the bow in the cloud,’ we have not only a beautiful * Ecclesiasticus, xliii. 11, 12. t It is a question which has been frequently started, Whether there was any rainbow before the flood? Some have conceived that the rain- bow was something of a miraculous production, and that it-was never seen before the flood. The equivocal sense of the word ‘‘ set,’’ in our translation, has occasioned a mistaken impression of thiskind. The He- brew word, thus translated, signifies more properly ‘‘ I do give,’’ or “I appoint.’’ The whole passage in reference to this circumstance, literally translated, runs thus: ‘‘ I appoint my bow which is in the cloud, that it may be for a sign or token of a covenant between me andthe earth; and it shall come to pass, when I bring acloud over the earth, and the bow shall be seen in the cloud, that I will remember my covenant that is be- tween me and you,’’ &c. As the rainbow is produced by the immuta- ble laws of refraction and reflection, as applied to the rays of the sun, striking on drops of falling rain, the phenomenon must have been occa Vor. IX. 11 122 REFLECTIONS ON COLOUR. and sublime phenomenon presented to the eye of sense, but. also a memorial exhibited to the mental eye, assuring us that, ‘© While the earth remaineth, seed-time-and harvest, and cold and heat, and summer and winter, and day and night, shall not cease.” ‘© On the broad sky is seen A dewy cloud, and in the cloud a bow Conspicuous, with seven listed colours gay, Betokening peace with God and covenant new. He gives a promise never to destroy . The earth again. by flood, nor let the sea ~ Surpass his bounds, nor rain to drown the world.’’ Milton, Par. Lost, book xi. SECT. IV.——REFLECTIONS ON THE BEAUTY AND UTILITY OF COLOURS. Colour is one of the properties of light, which constitutes chiefly the beauty and sublimity of the universe. It is co- Jour, in all its diversified shades, which presents to our view that almost infinite variety of aspect which appears on the scene of nature, which gives delight to the eye and the im- agination, and which adds a fresh pleasure to every new landscape we behold. Every flower which decks our fields and gardens is compounded of different hues ; every plain is covered with shrubs and trees of different degrees of verdure ; and almost every mountain is clothed with herbs and grass of different shade from those which appear on the hills and landscape with which it is surrounded. In the country, dur- ing summer, nature is every day, and almost every hour, varying her appearance by the multitude and variety of her hues and decorations, so that the eye wanders with pleasure over objects continually diversified, and extending as far as the sight can reach. In the flowers with which every land- scape is adorned, what a lovely assemblage of colours, and what a wonderful art in the disposition of their shades! Here a light pencil seems to have laid on the delicate tints; there they are blended according tothe nicest. rules of art. Al- though green is the general colour which prevails over the scene of sublunary nature, yet it is diversified bya thousand different shades, so that every species. of tree, shrub, and sionally exhibited from the beginning of the world; unless we suppose that there was no rain before the flood, and that the constitution of things in the physical system was very different from what it is now. The passage affirms no more than that the rainbow was then appointed to be a symbol of the covenant between God and man; and, papa it may have been frequently seen before, it would serve the purpose ofa sign equally well as if it had been miraculously formed for this purpose, and even better, as its frequent appearance, according to natural laws, is a perpetual memorial to man of the Divine faithfulness and mercy. REFLECTIONS ON COLOUR. 123 herb is clothed with its own peculiar verdure. The dark green of the forests is thus easily distinguished from the lighter shades of corn-fields and the verdure of the lawns. The system of animated nature likewise displays a diversi- fied assemblage of beautiful colours. The plumage of birds, the brilliant feathers of the peacock, the ruby and emerald hues which adorn the little humming-bird, and the various em- bellishments of many species of the insect tribe, present to the eye, in every region of the globe, a scene of diversi- fied beauty andembellishment. Nor is the mineral kmgdom destitute of such embellishments; for some of the darkest and most unshapely stones and pebbles, when polished by the hand of art, display a mixture of the most delicate and va- riegated colours.. All which beauties and varieties in the scene around usare entirely owing to that property, in every ray of light, by which it is capable of being separated into the primitive colours. To the same cause, likewise, are to be ascribed those beau- tiful and diversified appearances which frequently adorn the face of the sky—the yellow, orange, and ruby hues which embellish the firmament at the rising of the sun, and when he is about to descend below the western horizon; and those aerial landscapes, so frequently beheld in tropical climes, where rivers, castles, and mountains are depicted rolling over each other along the circle of the horizon. The clouds, es- pecially in some countries, reflect almost every colour in na- ture. Sometimes they wear the modest blush of the rose; sometimes they appear like stripes of deep vermilion, and sometimes as large, brilliant masses tinged with various hues; now they are white as ivory, and now as yellow as native old. In some tropical countries, according to St. Pierre, the clouds roll themselves up into enormous masses as white as snow, and are piled upon each other like the Cordeliers of Peru, and are moulded into the shapes of mountains, of caverns, and of rocks. When the sun sets behind this mag- nificent and aerial network, a multitude of luminous rays are transmitted through each particular interstice, which produce such an effect that the two sides of the lozenge illuminated by them have the appearance of being begirt with a fillet of gold; and the other two, which are in the shade, seem tinged with a superb ruddy orange. Four or five divergent streams of light, emanating from the setting sun up to the zenith, clothe with fringes of gold the undeterminate summits of this celestial barrier, and proceed to strike with the reflexes of their fires the pyramids of the collateral aerial mountains, 124 REFLECTIONS ON COLOUR. which then appear to consist of silver and vermilion. In short, colour diversifies every sublunary scene, whether on the earth or in the atmosphere; it imparts a beauty to the phenomena of falling stars, of lumimous arches, and the coruscations of the Aurora Borealis, and gives a splendour and sublimity to the spacious vault of heaven. Let us now consider for a moment what would be the as- pect of nature if, instead of the beautiful variety of embellish- ments which now appear on every landscape, and on the concave of the sky, one uniform colour had been thrown over the scenery of the universe. Let us conceive the whole of terrestrial nature to be covered with snow, so that not an ob- ject on earth should appear with any other hue, and that the vast expanse of the firmament presented precisely the same uniform aspect. What would be the consequence? The light of the sun would be strongly reflected from all the objects within the bounds of our horizon, and would produce a lustre which would dazzle every eye. ‘The day would acquire a ereater brightness than it now exhibits, and our eyes might, after some time, be enabled freely to expatiate over the sur- rounding landscape; but every thing, though enlightened, would appear confused, and particular objects would scarcely be distinguishable. A tree, a house, or a church near at hand might possibly be distinguished, on account of its eleva- tion above the general surface of the ground, and the bed of of a river by reason of its being depressed below it. But we should be obliged rather to guess, and to form a conjecture as to the particular object we wished to distinguish, than to ar- rive at any certain conclusion respecting it; and if it lay ata considerable distance, it would be impossible, with any degree of probability, to discriminate any one object from another. Notwithstanding the universal brightness of the scene, the uni- formity of colour thrown on every object would most certainly prevent us from distinguishing a church from a palace, a cot- tage from a knoll or heap of rubbish, a splendid mansion from rugged rocks, the trees from the hills on which they grow, or a barren desert from rich and fertile plains. In sucha case, human beings would be confounded, and even friends and neighbours be at a loss to recognise one another. The vault of heaven, too, would wear a uniform aspect. Neither planets nor comets would be visible to any eye, nor those millions of stars which now shine forth with so much brilliancy, and diversify the noctural sky; for it is by the con- trast produced by the deep azure of the heavens and the white rad‘ance of the stars that those bodies are rendered REFLECTIONS ON COLOUR. 125 visible. Were they depicted on a pure white ground, they would not be distinguished from that ground, and would, con- sequently, be invisible, unless any of them occasionally as- sumed a different colour. Of course, all that beautiful variety of aspect which now appears on the face of sublunary nature —the rich verdure of the fields, the stately port of the forest, the rivers meandering through the valleys, the splendid hues that diversify and adorn our gardens and meadows, the gay colouring of the morning and evening clouds, and all that variety which distinguishes the different seasons, would en- tirely disappear. As every landscape would exhibit nearly the same aspect, there would be no inducement to the poet and the philosopher to visit distant countries to investigate the scenes of nature, and journeyings from one region to another would scarcely be productive of enjoyment. Were any other single colour to prevail, nearly the same results would ensue. Were a deep ruddy hue to be uniformly spread over the scene of creation, it would not only be of- fensive to the eye, but would likewise prevent all distinction of objects. Were a dark blue or deep violet to prevail, it would produce a similar effect, and at the same time present the scene of nature as covered with a dismal gloom. Even if creation were arrayed in a robe of green, which is a more pleasant colour to the eye, were it not diversified with the different shades it now exhibits, every object would be equally andistinguishable. Such would have been the aspect of creation, and the in- conveniences to which we should have been subjected, had the Creator afforded us light without that intermixture of co- lours which now appears over all nature, and which serves to discriminate one object from another. Even our very apartments would have been tame and insipid, incapable of the least degree of ornament, and the articles with which they are furnished almost. undistinguishable, so that, in dis- criminating one object from another, we should have been as much indebted to the sense of touch as to the sense of vision. Our friends and fellow-men would have presented no objects of interest in our daily associations. The spark- ling eye, the benignant smile, the modest blush, the blended hues of white and vermilion in the human face, and the beau- ty of the female countenance, would all have vanished, and we should have appeared to one another as so many moving marble statues, cast nearly in thesame mould. But what would have been, worst of all, the numerous delays, uncertainties and perplexities to which we should have been subjected, ri * 126 REFLECTIONS ON COLOUR. had we been under the necessity every moment of distinguish- ing objects by trains of reasoning, and by circumstances of time, place, and relative position? An artist, when com- mencing his work in the morning, with a hundred tools of nearly the same size and shape around him, would have spent a considerable portion of his time before he could have seiected those proper for his purpose, or the objects to which they were to be applied; and in every department of society, and in all our excursions from one place to another, similar difficulties and perplexities would have occurred. ‘The one- half of our time must thus have been employed in uncertain guesses and perplexing reasonings respecting the real nature and individuality of objects, rather than in a regular train of thinking and of employment; and after all our perplexities and conjectures, we must have remained in the utmost un- certainty as to the thousands of scenes and objects which are now obvious to us, through the instrumentality of colours, as soon as we open our eyes. In short, without colour we could have had no books nor writings: we could neither have corresponded with our friends by letters; nor have known any thing with certainty of the events which happened in former ages. No written revelation of the will of God, and of his character, such as we now enjoy, could have been handed down to us from. re- mote periods and generations. ‘The discoveries of science and the improvements of art would have remained unrecorded. Universal ignorance would have: prevailed throughout the world, and the human mind have remained in a state of de- moralization and debasement. All these, and many other inconveniencies and evils would have inevitably followed, had not God painted the rays of light with a diversity of colours. And hence we may learn that the most important scenes and events in the universe may depend upon the existence of a single principle in nature, and even upon the most minute circumstances, which we may be apt to overlook, in the ar- rangements of the material world. In the existing state of things in the visible creation, we cannot but admire the wisdom and beneficence of the Deity in thus enabling us to distinguish objects by so easy and ex- peditious a mode as that of colour, which in a moment dis- criminates every object and its several relations. We rise in the morning to our respective employments, and our food, our drink, our tools, our books, and whatever is requisite for our comfort, are at once discriminated. Without the least hesi- tation or uncertainty, and without any perplexing process oi _ REFLECTIONS ON COLOUR. 127 reasoning, we can lay our hands on whatever articles we re- uire. Colour clothes every object with its peculiar livery, and infallibly directs the hands in its movements, and the eye in its surveys and contemplations. But this is not the only end which the Divine Being had in view in impressing on the rays of light a diversity of colours. It is evident that he likewise intended to minister to our pleasures as well as to our wants. To every man of taste, and almost to every human being, the combination of colours in flowers, the delicate tints with which they are painted, the diversified shades of green with which the hills and dales, the mountains and the vales are arrayed, and that beautiful variety which appears in a bright summer day on all the objects of this lower creation, are sources of the purest enjoyment and delight. It is colour, too, as well as magnitude, that adds to the sublimity of ob- jects. Were the canopy of heaven of one uniform hue, it would fail in producing those lofty conceptions, and those delightful and transporting emotions, which a contemplation of its august scenery is calculated to inspire. Colours are likewise of considerable utility in the intercourse of general society. They serve both for ornaments, and for distinguish- ing the different ranks and conditions of the community ; they add to the beauty and gracefulness of our furniture and clothing. At a glance, they enable us at once to distinguish the noble from the ignoble, the prince from his subjects, the master from his servant, and the widow, clothed with sable weeds, from the bride adorned with her nuptial ornaments. Since colours, then, are of so much value and importance, they may be reckoned as holding a rank among the noblest natural gifts of the Creator. As they are of such essential service to the inhabitants of our globe, there can be no doubt that they serve similar or analogous purposes throughout all the worlds in the universe. The colours displayed in the solar beams are common to all the globes which compose the planetary system, and must necessarily be reflected, in all their diversified hues, from objects on their surfaces. . The light which radiates from the fixed stars displays a simi'ar diversity of colours. Some of the double stars are found to emit light of different hues; the larger star exhibiting light of a ruddy or orange hue, and the smaller one a radiance which approaches to blue or green. There is, therefore, reason to conclude that the objects connected with the planets which revolve round such stars—being occasionally enlight- ened by suns of different hues—will display a more varie- gated and sp'endid scenery of colouring than is ever beheld Ce REFLECTIONS ON -COLOUR. in the world on which we dwell; and that one of the distin- guishing characteristics of different worlds, in regard to their embellishments, may consist in the splendour and variety of colours with which the objects on their surfaces are adorned. In the metaphorical description of the glories of the New Je- rusalem, recorded in the Book of Revelation, one of the chief characteristics of that city is said to consist in the splendour and diversity of hues with which it is adorned. It is repre- sented as “coming down from heaven, prepared as a bride adorned for her husband,” and as reflecting all the beautiful and variegated colours which the finest gems on earth can exhibit ; evidently indicating that-splendour and variety of colouring are some of the grandest features of celestial scenery. On the whole, the subject of colours, when: seriously con- sidered, is calculated to excite us to the adoration of the good- ness and intelligence of that Almighty Being whose wisdom planned all the arrangements of the universe, and to inspire us with gratitude for the numerous conveniences and pleasures we derive from those properties and laws he has impressed on the material system. He might have afforded us light, and even splendid illumination, without the pleasures and advantages which diversified colours now produce, and man and other animated beings might have existed in such a state. But what a very different scene would the world have pre- sented from what it now exhibits! Of how many thousands of pleasures should we have been deprived! and to what numerous inconveniences and perplexities should we have been subjected! The sublimity and glories of the firma- ment, and the endless beauties and varieties which now embellish our terrestrial system, would have been for ever unknown, and man could have had little or no incitement to study and investigate the works of his Creator. . In this, as well as in many other arrangements in nature, we have a sensible proof of the presence and agency of that Almighty Intelligence “in whom we live, and move, and have our being.’’? None but an infinitely Wise and Beneficent Being, intimately present in all places, could thus so regularly create in us, by means of colour, those exquisite sensations which afford so much delight, and which unite us, as it were, with every thing around us. In the diversity of hues spread over the face of creation, we have as real a display of the Divine presence as Moses enjoyed at the burning bush. The only difference is, that the one was out of the common order of Divine procedure, and the other in accordance with those permanent laws which regulate the economy of the universe. HISTORY OF THE TELESCOPE. 129 In every colour, then, which we contemplate, we have a sen- sible memorial of the presence of that Being “ whose Spirit garnished the heavens and laid the foundations of the earth,” and whose “ merciful visitation’? sustains us every moment in existence. But the revelation of God to our senses, through the various objects of the material world, has become so fami- liar, that we are apt to forget the Author of all our enjoy- ments, even at the moment when we are investigating his works and participating of his benefits.. “O that men would praise Jehovah for his goodness, and for his wonderful works towards the children of men.”’ PART II. ON TELESCOPES. CHAPTER I. HISTORY OF THE INVENTION OF TELESCOPES. Tuer telescope is an optical instrument for viewing objects at a distance. Its name is compounded of two Greek words, ere, Which signifies at a distance or far off; and oxonew, to view or to contemplate. By means of telescopes, remote objects are represented as if they were near, small apparent magnitudes are enlarged, confused objects are rendered dis- tinct, and the invisible and obscure parts of very distant scenes are rendered perceptible and clear to the organ of vision. ‘The telescope is justly considered as a grand and noble instrument. It is not a little surprising that it should be in the power of man to invent and construct an instrument by which objects too remote for the unassisted eye to dis- tinguish, should be brought within the range of distinct vision, as if they were only a few yards from our eye, and that thousands of august objects in the heavens, which had been concealed from mortals for numerous ages, should be brought within the limits of our contemplation, and be as distinctly perceived as if we had been transported many millions of miles from the space we occupy through the celestial regions 130 MAGNIFYING GLASSES OF THE ANCIENTS. The celebrated Huygens remarks, in reference to this instru- ment, that, in his opinion, “the wit and industry of man has not produced any thing so noble and so worthy of his facul- ties as this sort of knowledge, (namely, of the telescope ;) insomuch that if any particular person had been so diligent and sagacious as to invent this instrument from the principles of nature and geometry, for my part, I should have thought his abilities were more than human; but the case is so ‘far from this, that the most learned men ‘have not yet been able sufficiently to explain the reason of the effects of this casual invention.” The persons who constructed the first telescopes, and the exact period when they were first invented, are involved in some degree of obscurity. It does not certainly appear that such instruments were known to the ancients, although we ought not to be perfectly decisive on this point. The cabinets of the curious contain some very ancient gems of admirable workmanship, the figures on which are so small that they appear beautiful through a magnifying glass, but altogether confused and indistinct to the naked eye; and therefore it may be asked, If they cannot be viewed, how could they be wrought, without the assistance of glasses? And as some of the ancients have declared that the moon hasa form like that of the earth, and has plains, hills, and valleys in it, how could they know this, unless by mere conjecture, without the use of a telescope? And how could they have known that the Milky Way is formed by the combined rays of an infinite number of stars? For Ovid states, in reference to this zone, “its groundwork is of stars.” But, whatever knowledge the ancients may have possessed of the telescope or other optical glasses, it is quite evident that they never had telescopes of such size and power as those which we now possess, and that no discoveries in the heavens, such as are now brought to hight, were made by any of the ancient astronomers, other- wise some allusions to them must have been found in their writings. Among the moderns, the illustrious Friar Bacon appears to have acquired some rude ideas respecting the construction of telescopes. ‘Lenses and specula,” says he, ‘may be so figured that one object may be multiplied into many, that those which are situated at a great distance may be made to appear very near, that those which are small may be made to appear very large, and those which are obscure very plain ; and we can make stars to appear wherever we will.” From these expressions, it appears highly probable that this philo- FRIAR BACON, PORTA, AND MBTIUS. 131 sopher was acquainted with the general principle both of tele- scopes and microscopes, and that he may have constructed telescopes of small magnifying power for his own observation and amusement, although they never came into general use. He was a man of extensive learning, and made so rapid a progress in the sciences, when attending the University of Paris, that he was esteemed the glory of that seat of learning. He prosecuted his favourite study of experimental philosophy with unremitting ardour, and in this pursuit, in the course of twenty years, he expended no less than £2000 in experi- ments, instruments, and in procuring scarce books. In con- sequence of such extraordinary talents and such astonishing progress in the sciences in that ignorant age, he was repre- sented, by the envy of his illiterate fraternity, as having dealings with the devil; and, under this pretence, he was restrained from reading lectures, and at length, in 1278, when sixty-four years of age, he was imprisoned in his cell, where he remained.in confinement for ten years. He shone like a single bright star in a dark hemisphere—the glory of our country—and died at Oxford, in the year 1294, in the eightieth year of his age. “Friar Bacon,” says the Rev. Mr. Jones, “ may be considered as the first of English philo- sophers; his profound skill in-mechanics, optics, astronomy, and chemistry would make an honourable figure in the pre- sent age. But he is entitled to farther praise, as he made all his studies subservient to theology, and directed all his writings, as much as could be, to the glory of God. He had the highest regard for the sacred Scriptures, and was per- suaded they contain the principles of all true science.” The next person who is supposed to have acquired a know- ledge of telescopes was Joannes Baptista Porta, of Naples, who flourished in the sixteenth century. He discovered the Camera Obscura, the knowledge of which might naturally have led to the invention of the telescope; but it does not appear that he ever constructed such an instrument. Des Cartes considers James Metius, a Dutchman, as the first con- structor of a telescope, and says that, “as he was amusing himself with making mirrors and burning-glasses, he casually thought of looking through two of his lenses at a time, and found that distant objects appeared very large and distinct.” Others say that this great discovery was first made by John Lippersheim, a maker of spectacles at Middleburg, or, rather. by his children, who were diverting themselves with looking through two glasses at a time, and placing them at different distances from each other. But Borellus, who wrote a book 132 TELESCOPES OF JANSEN AND LIPPERSHEIM. ‘on the invention of the telescope,” gives this honour to Zacharias Jansen, another spectacle-maker in the same town, who, he says, made the first telescope in 1590. Jansen was a diligent inquirer into nature, and, being engaged in such pursuits, he was trying what use could be made of lenses for those purposes, when he fortunately hit upon the construc- tion. Having found the arrangement of glasses which pro- duced the effect desired, he enclosed them in a tube, and ran with his instrument to Prince Maurice, who, immediately conceiving that it might be of use to him in his wars, desired the author to keep ita secret. Such are the rude concep- tions and selfish views of princely warriors, who would apply every invention in their power for the destruction of mankind. But the telescope was soon destined to more noble and honourable achievements, Jansen, it is said, directed his in- strument towards celestial objects, and distinctly saw the spots on the surface of the moon, and discovered many new stars, particularly seven pretty considerable ones in the Great Bear. His son Joannes is said to have noticed the lucid circle near the lower limb of the moon, now named T'ycho from whence several bright rays seem to dart in different directions. In viewing Jupiter, he perceived two, sometimes three, and, at the most, four small stars, a little above or below him, and thought that they performed revolutions around him. This was probably the first observation of the satellites of Jupiter, though the person who made it was not aware of the importance of his discovery.* It is not improbable that different persons about Middleburg hit upon the invention, in different modes, about the same time. Lippersheim seems to have made his first rude tele- scope by adjusting two glasses on a board, and supporting them on brass circles.t Other workmen, particularly Metius and Jansen, in emulation of each other, seem to have made use of that discovery, and, by the new form they gave it, made all the honour of it their own. One of them, consider- ing the effects of light as injurious to distinctness, placed the glasses in a tube blackened within. The other, still more cautious, placed the same glasses within tubes capable of sliding one in another, both to vary the prospects, by length- ening the instrument, according to the pleasure of the ob- * Though Borellus mentions this circumstance, yet there is some rea- son to doubt. the accuracy of this statement, as young Jansen appears to have been at that period not more than six years old; so that it is more probable that Galileo was the first discoverer of Jupiter’s satellites. + The reader may see an engraving of this instrument in the author’s work entitled ‘‘ The Improvement of Society,” p. 176. DISCOVERIES OF GALILEO. 133 server, and to render it portable and commodious. Thus it is probable that different persons had a share in the inven- tion, and jointly contributed to its improvement. At any rate, it is undoubtedly to the Dutch that we owe the original invention. The first telescope made by Jansen did not ex- ceed fifteen or sixteen inches in length, and therefore its magnifying power could not have been very great. The famous Galileo has frequently been supposed to have been the inventor of the telescope, but he acknowledges that he had not the honour of being the original inventor, having first learned from a German that such an instrument had al- ready been made; although, from his own account, it appears that he had actually re-invented this instrument. The follow- ing is the account, in his own words, of the circumstance which led him to construct a telescope: ‘Nearly ten months ago, pees April or May, 1609,) it was reported that a certain utchman had made a perspective through which many dis- tant objects appeared distinct as if they were near. Several effects of this wonderful instrument were reported, which some believed and others denied ; but, having it confirmed to me a few days after bya letter from the noble John Badoverie, at Paris, | applied myself to consider the reason of it, and by what means I might contrive a similar instrument, which I afterward attained to by the doctrine of refractions. And, first, I prepared a leaden tube, to whose extremities I fitted two spectacle-glasses, both of them plane on one side, and on the other side one of them was spherically convex, and the other concave. Then applying my eye to the concave, I saw objects appear pretty large and pretty near me. ‘They appeared three times nearer and nine times larger in surface than to the naked eye; and soon after I made another, which represented ob- jects about sixty times larger, and eight times nearer; and at last, haying spared no labour or expense, I made an instru- ment so excellent as to show things almost a thousand times larger, and above thirty times nearer than to the naked eye.” In another part of his writings, Galileo informs us that « he was at Venice when he heard of Prince Maurice’s instru- ment, but nothing of its construction ; that the first night after he returned to Padua he solved the problem, and made his in- strument the next day, and soon after presented it to the doge at Venice, who, to do him honour for his grand invention, gave him the duca: iviters which settled him for life in his lecture- ship at Padua ; and the Republic, on the 25th of August, in the same year, (1610,) more than tripled his salary as professor.” The following is the account which this philosopher gives Vor. IX, 12 134 DISCOVERIES OF GALILEO. of the process of reasoning which led him to the constraction of a telescope: “ Targued in the following manner: 'The-con- trivance consists either of one glass or more : one is not suffi- cient, since it must be either convex, concave, or plane; the last does not produce any sensible alteration in objects, the concave diminishes them ; it is true that the convex magnifies, but it renders them confused and indistinct ; consequently, one glass is insufficient to produce the desired effect. Proceeding to consider two glasses, and bearing in mind that the plane glass causes no change, I determined that the instrument could not consist of the combination of a plane glass with either of the other two. I therefore applied myself to make experiments on combinations of the two other kinds, and thus obtained that of which I was in search.’ If the true inventor is the person who makes the discovery by reasoning and reflection, by tracing facts and principles to their consequences, and by applying his invention to important purposes, then Galileo may be considered as the real inventor of the telescope. No sooner had he constructed this instrument—before he had seen any similar one—than he directed his tube to the celes- tial regions, and his unwearied diligence and ardour were soonsrewarded by a series of new and splendid discoveries. He described the four satellites of Jupiter, and marked the periods of their revolutions: he discovered the phases of Venus, and thus was enabled to adduce a new proof of the Copernican system, and to remove an objection that had been brought against it. He traced on the lunar orb a re- semblance to the structure of the earth, and plainly perceived the outlines of mountains and vales, casting’ their shadows over different parts of its surface. He observed that, when Mars was in quadrature, his figure varied slightly from a perfect circle, and that Saturn consisted of a triple body, having a small globe on each side, which deception was owing to the imperfect power of his telescope, which was in- sufficient to show him that the phenomenon was in reality a ring. In viewing the sun, he discovered large dark spots on the surface of that luminary, by which he ascertained that that mighty orb performed a revolution round its axis. He brought to view multitudes of stars imperceptible to the naked eye, and ascertained that those nebulous appearances in the heavens which constitute the Milky Way consist of a vast collection of minute stars too closely compacted together to produce an impression on our unassisted vision. The results of Galileo’s observations were given to the world in a small work, entitled “ Nuncius Sidereus,” or OPPONENTS OF GALILEO. 135 « News from the Starry Regions,’ which produced an ex- traordinary sensation among the learned. ‘These discoveries soon spread throughout Europe, and were incessantly talked of, and were the cause of much speculation and debate among the circles of philosophers. Many doubted ; many positively refused to believe so novel and unlooked-for announcements, because they ran counter to the philosophy of Aristotle, and all the preconceived notions which then prevailed in the learned world. It is curious, and may be instructive, to con- sider to what a length of absurdity ignorance and prejudice carried many of those who made pretensions to learning and science. Some tried to reason against the facts alleged to be discovered ; others contented themselves, and endeavoured to satisfy others with the simple assertion that such things were not, and could not possibly be ; and the manner in which they supported themselves in their incredulity was truly ridi- culous. ‘“O my dear Kepler,” says Galileo, in a letter to that astronomer, “ how I wish we could have one hearty laugh to- gether. Here at Padua is the principal professor of philo- sophy, whom I have repeatedly and urgently requested to look at the moon and planets through my glass, which he pertinaciously refuses to do, lest his opinions should be over- turned. Why are you not here? what shouts of laughter we should have at this glorious folly! and to hear the pro- fessor of philosophy at Pisa labouring with the Grand-duke with logical arguments, as if with magical incantations to charm the new planets cut of the sky.’’ Another opponent of Galileo, one Christmann, says, in a book he published, **We are not to think that Jupiter has four satellites given him by nature, in order, by revolving round him, to immortal- ize the Medici who first had notice cf the observation. These are the dreams of idle men, who love ludicrous ideas better than our laborious and industrious correction of the heavens. Nature abhors so horrible a chaos, and to the truly wise such vanity is detestable.” One Martin Horky, a would-be phi- losopher, declared to Kepler, «I will never concede his four new planets to that Italian from Padua, though J should die {" it ;’’ and he followed up this declaration by publishing a ook against Galileo, in which he examines four principal questions respecting the alleged planets: 1. Whether they exist? 2. What they are? 3. What they are like? 4. Why they are? The first question is soon disposed of by declar- ing positively that he has examined the heavens with Ga- lileo’s own glass, and that no such thing as a satellite about Jupiter exists. To the second, he declares solemnly that he 136 OPPONENTS OF GALILEO. does not more surely know that he has a soul in his body than that reflected rays are the sole cause of Galileo’s erroneous cb- servations. In regard to the third question, he says, that th. planets are like the smallest fly compared to.an elephant; and finally concludes, on the fourth, that the only use of them is to gratify Galileo’s ‘thirst of gold,’’ and to afford himself a sub- ject of discussion. Kepler, ima letter to Galileo, when alluding to Horky, says, “ He begged so hard to be forgiven, that I have taken him again into favour, upon this preliminary con- dition, that Iam to show him Jupiter’s satellites, AND HE IS TO SEE THEM, and own that they are there.”’ The following is a specimen of the reasoning of certain pretended philosophers of that age against the discoveries of Galileo. Sizzi,a Florentine astronomer, reasons in this strain: «There are seven windows given to animals in the domicil of the head, through which the air is admitted to the rest of the tabernacle of the body, to enlighten, to warm, and to nourish it ; two nostrils, two eyes, two ears, and a mouth; so in the heavens, or the great world, there are two favourable stars, two unpropitious, two luminaries, and Mercury alone undecided and indifferent. From which, and many other similar phenomena in nature, such as the seven metals, &c., we gather that the number of planets is necessarily seven. Moreover, the satellites are invisible to the naked eye, and therefore can exert no influence on the earth, and therefore would be useless, and therefore do not exist. Besides, as well the Jews as other ancient nations have adopted the divi- sion of the week into seven days, and have named them from the seven planets. Now, if we increase the number of the planets, this whole system falls to the ground.’ The opi- nions which then prevailed in regard to Galileo’s observations on the moon were such as the following: Some thought that the dark shades on the moon’s surface arose from the inter- position of opaque bodies floating between her and the sun, which prevent his light from reaching those parts: others imagined that, on account of her vicinity to the earth, she was partly tainted with the imperfections of our terrestrial and elementary nature, and was not of that entirely pure and refined substance of which the more remote heavens consist ; and a third party looked on her as a vast mirror, and main- tained that the dark parts of her surface were the reflected images of our earthly forests and mountains. Such learned nonsense is a disgrace to our species, and to the rational faculties with which man is endowed, and ex- hibits, in a most ludicrous manner, the imbecility and preju- GALILEO’S TRLESCOPR. 137 dice of those who made bold pretensions to erudition and philosophy. The statement of such facts, however, may be instructive, if they tend to guard us against those prejudices and preconceived opiniovs which prevent the mind from the cordial reception of truth, and from the admission of improve- ments in society which run counter to long-established customs. For the same principles and prejudices, though in a differ- ent form, still operate in society, and retard the improvement of the social state, the march of science, and the progress of Christianity. How ridiculous is it for a man calling himself a philosopher to be afraid to look through a glass to an exist- ing object in the heavens, lest it should endanger his previous opinions! And how foolish is it to resist any improvement or reformation in society because it does not exactly accord with existing opinions and with “the wisdom of our an- cestors !”’ It is not a little surprising that Galileo should have first hit on that construction of a telescope which goes by his name, and which was formed with a concave glass next the eye. This construction of a telescope is more difficult to be under- stood in theory than one which is composed solely of convex glasses; and its field of view is comparatively very small, so that it is almost useless when attempted to be made of a great length. In the present day, we cannot help wondering that Galileo and other astronomers should have made such dis- coveries as they did with such an instrument, the use of which must have required a great degree of patience and address. Galileo’s best telescope, which ‘he constructed with great trouble and expense,’’ magnified the diameters of ob- jects only thirty-three times ; but its length is not stated, which would depend upon the focal distance of the concave eyeglass. If the eyeglass was two inches focus, the length of the instru- ment would be five feet four inches ; if it was only one inch, the length would be two feet eight inches, which is the nee we can allow to it—the object-glass being thirty-three inches focus, and the eyeglass placed an inch within this focus. With this telescope Galileo discovered the satellites of Jupiter, the crescent of Venus, and the other celestial objects to which we have already alluded. ‘The telescopes made in Hollana are supposed to have been constructed solely of convex glasses, on the principle of the astronomical telescope ; and if so, Galileo’s telescope was in reality a new invention. Certain other claimants of the invention of the telescope have appeared, besides those already mentioned. Francis Fontana, in his “celestial observations,” says that he was 12* 138 DIGGES’S INVENTIONS. assured by a Mr. Hardy, advocate of the Parliament of Paris, a person of great learning and undoubted integrity, that, on the death of his father, there was found among his things an old tube by which distant objects were distinctly seen, and that it was of a date long prior to the telescope lately invented, and had been kept by him as asecret. Mr. Leonard Digges, a gentleman who lived near Bristol in the seventeenth cen- tury, and was possessed of great and various knowledge, positively asserts in his * Stratoticos,”’ and in another work, that his father, a military gentleman, had an instrument which he used in the field, by which he could bring distant objects near, and could know a man at the distance of three miles. Mr. Thomas Digges, in the preface to his “ Pantometria,’’ published in 1591, declares, “My father, by his continual painful practices, assisted by demonstrations mathematical, was able, and sundry times hath, by proportional glasses, duly situate in convenient angles, not only discovered things far off, read letters, numbered pieces of money, with the very coin and superscription thereof, cast by some of his friends of purpose, upon downs in open fields, but also, seven miles off, declared what hath been done that instant in private places. He hath also, sundry times, by the sunbeams, fired, powder, and discharged ordnance half a mile and more distant, and many other matters far more strange and rare, of which there are yet living divers witnesses.” It is by no means unlikely that persons accustomed to re- flection, and imbued with a certain degree of curiosity, when handling spectacle-classes, and amusing themselves with their magnifying powers and other properties, might sometimes hit upon the construction of a telescope, as it only requires two lenses of different focal distances to be held at a certain distance from each other, in order to show distant objects magnified. Nay, even one lens, of a long focal distance, is sufficient to constitute a telescope of a moderate magnifying power, as I shall show in the sequel. But such instruments, when they happened to be constructed accidentally, appear to have been kept as secrets, and confined to the cabinets of the curious, so that they never came into general use; and as their magnifying power would probably be comparatively small, the appearance of the heavenly bodies would not be much enlarged by such instruments, nor is it likely that they would be often directed to the heavens. On the whole, there- fore, we may conclude that the period when instruments of this description came into general use, and were applied to use- fui purposes, was when Galileo constructed his first telescopes, THE CAMERA OBSCURA. 139 CHAPTER II. OF THE CAMERA OBSCURA. Berore proceeding to a particular description of the dif ferent kinds of telescopes, I shall first give a brief description of the camera obscura, as the phenomena exhibited by this instrument tend to illustrate the principle of a refracting tele- scope. The term camera obscura literally signifies a darkened vault or roof, and hence it came to denote a chamber, or box, or any other place made dark for the purpose of optical ex- periments. ‘I'he camera obscura, though a simple, is yet a very curious and noble contrivance, as it “naturally and clearly explains the manner in which vision is performed, and the principle of the telescope, and entertains the spectator with a most exquisite picture of surrounding objects, painted in the most accurate proportions and colours by the hand of nature. The manner of exhibiting the pictures of objects in a dark room is as follows: In one of the window-shutters of a room which commands a good prospect of objects not very distant, a circular hole should be cut of four or five inches diameter. In this hole an instrument should be placed called a sctoptric ball, which has three parts, a frame, a ball, anda lens. The ball has a circular hole cut through the middle, in which the lens is fixed, and its use is to tum every way, so as to take in a view of objects on every side» The chamber should be made perfectly dark, and a white screen, or a large sheet of elephant paper, should be placed opposite to the lens, and in its focus, to receive the image. If, then, the objects without be strongly enlightened by the sun, there will be a beautiful living picture of the scene delineated on a white screen, where every object is beheld in its proportions, and with its colours even more vivid than life. Green objects appear in the picture more intensely green; and yellow, blue, red, or white flowers, appear much more beautiful in the picture than in nature. If the lens be a good one, and the room per- fectly dark, the perspective is seen in perfection. The lights and shadows are not only perfectly just, but also greatly heightened ; and, what is peculiar to this delineation, and which no other picture or painting can exhibit, the motions of all the objects are exactly expressed in the picture; the boughs of the trees wave, the leaves quiver, the smoke as- 140 THE CAMERA OBSCURA. cends in a waving form, the people walk, the children at their sports leap and run, the horse and cart move along, the ships sail, the clouds soar and shift their aspects, and all as natural as in the real objects; the motions being somewhat quicker, as they are performed in a more contracted scene. These are the inimitable perfections of a picture drawn by the rays of light as the only pencil in nature’s hand, and which are finished in a moment; for no sensible interval elapses before the painting is completed, when the ground on which it is painted is prepared and adjusted. In comparison of such a picture, the finest productions of the most celebrated artists, the proportions of Raphael, the natural tints and colour- ing of Titian, and the shadowing of the Venetians, are but coarse and sorry daubings, when set in competition with what nature can exhibit by the rays of light passing through a single lens. The camera obscura is at the same time the painter’s assistant and the painter’s reproach. From the picture it forms he receives his best instructions, and is shown what he should endeavour to attaim ; and hence, too, he learns the imperfections of his art, and what it is impossible for him to imitate. As a proof of this, the picture formed in the dark chamber will bear to be magnified to a great extent, without defacing its beauty or injuring the fineness of its parts; but the finest painted landscape, if viewed through a high magnifier, will appear only as a coarse daubing. The following scheme will illustrate what has been now Figure 37. THE CAMERA OBSCURA. 141 ” a stated respecting the dark chamber: EF represents a dark- ened room, in the side of which, IK, is made the circular hole V, in which, on. the inside, is fixed the scioptric ball. At some considerable distance from this hole is exhibited a landscape of houses, trees, and other objects, A BC D, which are opposite to the wmdow. ‘The rays which flow from the different objects which compose this landscape to the iens at V, and which pass through it, are converged to their respec- tive foci on the opposite wall of the chamber, HG, cr on a white movable screen placed in the focus of the lens, where they all combine to paint a lively and beautiful picture of the range of objects directly opposite, and on each side, so far as the lens can take in. Though I have said that a scioptric ball and socket are ex- pedient to be used in the above experiment, yet, where such an instrument is not at hand, the lens may be placed in a short tube, made of pasteboard or any other material, and fixed in the hole made in the window-shutter. The only im- perfection attending this method is, that the lens can exhibit those objects only which lie directly opposite the window. Some may be disposed to consider it as an imperfection in this picture, that all these objects appear in an inverted posi- tion; as they must necessarily do, according to what we formerly stated respecting the properties of convex lenses (p. 64.) There are, however, different modes of viewing the picture as if it were erect; for if we stand before the picture, and hold a common mirror against our breast at an acute an- gle with the picture, and look down upon it, we shall] see all the images of the objects as if restored to their erect position ; and by the reflection of the mirror, the picture will receive such a lustre as will make it still more delightful. Or, if a large concave mirror were placed before the picture at such a distance that its image may appear before the mirror, it will then appear erect and pendulous in the air in the front of the mirror. Or, if the image be received on a frame of paper, we may stand behind the frame, with our face towards the window, and look down upon the objects, when they will appear as if erect. The experiment of the camera obscura may serve to ex- plain and illustrate the nature of a common refracting tele- scope. Let us suppose that the lens in the window-shutter represents the object-glass of a refracting telescope. This glass forms an image in its focus, which is in every respect an exact picture or representation of the objects before it; and consequently the same idea is formed in the mind of the 142 THE CAMERA OBSCURA. nature, form, magnitude, and colour of the object, whether the eye at the centre of the glass views the object itself, or the image formed in its focus; for, as formerly stated, the object and its image are both seen under the same angles by the eye placed at the centre of the lens. Without such an image as is formed in the camera obscura—depicted either in the tube of a telescope or in the eye itself—no telescope could possibly be formed. If we now suppose that, behind the image formed in the dark chamber, we apply a convex lens of a short focal distance to view that image, then the image will be seen distinctly, in the same manner as we view common objects, such as a leaf or a flower, with a magnifying glass; consequently, the object itself will be seen distinct and magnified ; and as the same image is nearer to one lens than the other, it will subtend a larger angle at the nearest lens, and, of course, will appear larger than through the other, and consequently the object will be seen magnified in proportion, For example, let us suppose the lens in the camera obscura, or the object-lens of the telescope, to be five feet, or sixty inches focal distance ; at this distance from the glass an image of the distant objects opposite to it will be formed. If, now, we place a small lens two inches focal distance beyond this point, or five feet two inches from the object-glass, the ob- jects, when viewed through the small Jens, will appear con- siderably magnified, and apparently much nearer than to the naked eye. ‘The degree of magnifying power is in propor- tion to the focal distances of the two glasses; that is, in the present case, in the proportion of two inches, the focus of the small lens, to sixty inches, the focus of the object lens. Divide sixty by two, the. quotient is thirty, which gives the magnifying power of such a telescope; that is, it represents objects thirty times nearer, or under an angle thirty times larger than to the naked eye. If the eyeglass, instead of being two inches, were only one and a half inch focus, the magnifying power would be in the proportion of one and a half to sixty, or forty times. If the eyeglass were three inches focus, the magnifying power would be twenty times ; and so on with regard to other proportions. In all cases, where a telescope is composed of only two convex lenses, the magnifying power is determined by dividing the focal dis- tance of the object-glass by the focal distance of the eyeglass, ‘and the quotient expresses the number of times the object is magnified in length and breadth. This and various other particulars will be more fully illustrated in the sequel. In performing experiments with the camera obscura in a THE CAMERA OBSCURA. 143 darkened chamber, it is requisite that the foliowing particu- lars be attended to: 1. That the lens be well figured, and free from any veins or blemishes that might distort the picture. 2, That it be placed directly against the object whose image, we wish to see distinctly delineated. 3. The lens should be of'a proper size, both as to its breadth and focal distance. It should not be less than three or four feet focal distance, other- wise the picture will be too small, and the parts of objects too minute to be distinctly perceived ; nor should it exceed fifteen or eighteen feet, as in this case the picture will be faint, and of course not so pleasing. The best medium as to focal dis- tance is from five to eight or ten feet. The aperture, too, or breadth of the glass, should not be too small, otherwise the image will be obscure, and the minute parts of it invisible for want of a sufficient quantity of light. A lens of six feet focal distance, for example, will require an aperture of at least two inches. Lenses of a shorter focal distance require less apertures, and those of a longer focal distance larger. But if the aperture be too large, the image will be confused and indistinct, by the admission of too much light. 4. We should never attempt to exhibit the images of objects, unless when the sun is shining and strongly illuminating the objects, except in the case of very near objects placed in a good light. As one of the greatest beauties in the phenomena of the dark chamber consists in the exquisite appearance and contrast of light and shadows, nothing of this kind can be perceived but from objects directly illuminated by the sun. 5. A south window should never be used in the forenoon, as the sun can- not then enlighten the north side of an object; and, besides, his rays would be apt to shine upon the lens, which would make the picture appear with a confused lustre. An east window is best in the afternoon, and a western in the morn- ing; but a north window is in most cases to be preferred, es- pecially in the forenoon, when the sun is shining with his greatest strength and splendour. In general, that window ought to be used which looks to the quarter opposite to that in which the sun is shining. The picture should be received upona very white surface, as the finest and whitest paper, or a painted cloth, bordered with black ; as white bodies reflect most copiously the incum- bent rays, while black surfaces absorb them. If the screen eould be bent into the concave segment of a sphere, of which the focal distance of the double convex lens which is used is the radius, the parts of the picture adjacent to the ex- tremities would appear most distinct. Sir D. Brewster n- 144 THE CAMERA OBSCURA. forms us that, having tried a number of white substances of different degrees of smoothness, and several metallic surfaces on which to receive the image, he happened to receive the picture on the silvered back of a looking-glass, and was sur- prised at the brilliancy and distinctness with which external objects were represented. 'T’o remove the spherical protuber- ances of the tin-foil he ground the surface very carefully witha bed of hones which he had used for working the plane specula of Newtonian telescopes. By this operation, which may be performed without injuring the other aside of the mir- ror, he obtained a surface finely adapted for the reception of images. ‘The minute parts of the landscape were formed with so much precision, and the brilliancy of colouring was so uncommonly fine, as to equal, if not exceed, the images that are formed in the air by means of concave specula. The following additional circumstances may be stated re- specting the phenomena exhibited in the dark chamber:. A more critical idea may be formed of any movement in the picture here presented than from observing the motion of the object itself. For instance, a man walking in a picture ap- pears to have an undulating motion, or to rise up and down every step he takes, and the hands. seem to move almost ex- actly like a pendulum ;. whereas scarcely any thing of this kind is observed in the man himself, as viewed by the naked eye. Again, if an object be placed just twice the focal dis- tance from the lens without the room, the image will be formed at the same distance from the lens within the room, and con- sequently, will be equal in magnitude to the object itself. The recognition of this principle may be of use to those con- - cerned in drawing, and who may wish at any time to form a picture of the exact size of the object. Ifthe object be placed farther from the lens than twice its focal length, the image will be less than the object. If it be placed nearer, the im- age will be greater than the life. In regard to immovable ob- jects, such as houses, gardens, trees, &c., we may. form the images of somany different sizes by means of different lenses, the shorter focus making the lesser picture, and the longer focal distance the largest. The experiments with the camera obscura may likewise serve to illustrate the nature of vision and the functions of the human eye. The frame or socket of the scioptric ball may represent the orbit of the naturaleye. The ball, which turns every way, resembles the globe of the eye, movable in its orbit. The hole in the ball may represent the pupil of the eye; the convex lens corresponds to-the crystalline humour, REVOLVING CAMERA OBSCURA. 145 which is shaped like a lens, and contributes to form the images of objects on the inner part of the eye. 'The dark chamber itself is somewhat similar to the internal part of the eye, which is lined al] around, and under the retina, with a mem- brane, over which is spread a mucus of a very black colour. The white wall or frame of white paper to receive the picture uf objects is a fair representation of the retina of the eye, on which all the images of external objects are depicted. Such are some of the general points of resemblance between the apparatus connected with the dark chamber and the organ of vision; but the human eye is an organ of such exquisite con- struction, and composed of such a number and variety of deli- cate parts, that it cannot be adequately represented by any artificial contrivance. The darkened chamber is frequently exhibited in a man- ner somewhat different from what we have above described, as in the following scheme, (fig. 38,) which is termed the re- Figure 38. volving camera obscura. In this construction, K H repre- sents a plane mirror or metallic reflector, placed at half a right angle to the convex lens HI, by which rays proceeding from objects situated in the direction O are reflected to the lens, which forms an image of the objects on a round white’ Vox. IX. 13 146 PORTABLE CAMERA OBSCURA. table at T, around which several spectators may stand and view the picture as delineated on a horizontal plane. The reflector, along with its case, is capable of being turned round by means of a simple apparatus connected with it, so as to take in, in succession, all the objects which compose the sur- rounding scene. Butas the image here is received on a flat surface, the rays, fm, en, will have to diverge farther than the central rays, dc; and hence the representation of the object near the sides will be somewhat distorted ; to remedy which, the image should be received on a concave surface, asabor PS. This is the general plan of those camera ob- scuras, fitted up in large wooden tents, which are frequently exhibited in our large cities, and removed occasionally from one town to another. Were an instrument of this kind fitted: up on a small scale, a hole might be made in one of the sides, as at E, where the eye could be applied to view the picture. The focal distances of the lenses used in large instruments of this kind are generally from eight to twelve feet, in which case they produce a telescopic effect upon distant objects, so as to make them appear nearer than when viewed with the naked eye. The camera obscura is frequently constructed in a portable (ort so as to be carried about for the purpose of delineating andscapes. ‘The following is a brief description of the in- Figure 39. strument in this form: A C is aconvex lens, placed near the end of a tube or drawer, which is movable in the side of a square box, within which is a plane mirror, D E, reclining backward in an angle of forty-five degrees from the perpen- dicular, pn. The pencils of rays flowing from the object, O B, and passing through the convex lens, instead of proceed- ing forward and forming the image H I, are reflected upward by the mirror, and meet in points, as F G, at the same dis- tance at which they would have met at H and I, if they had “THE DAGUERREOTYPE. 147 not been mtercepted by the mirror. At F G, the ‘mage of the object, O B, is received either on a piece» of oiled paper, or more frequently on a plane unpolished. glass, placed in the horizontal situation F G, which receives the images of all ob- jects opposite to the lens, and on which, or on an oiled paper placed upon it, their outlines may be traced by a pencil. The movable tube on which the lens is fixed serves. to ad- just the focus for near and distant objects, till their images appear distinctly painted on the horizontal glass at F G.. Be- low is shown the most common form of the box of this kind Figure 40. SUI I Si of camera obscura. A is the position of the lens, B C the position of the mirror, D the plane unpolished glass on which the images are depicted,G H a movable top or screen to prevent the light from injuring the picture, and E F the movable tube. The Daguerreotype-—An important and somewhat sur- prising discovery has lately been made in relation to the picture formed by the camera obscura. It is found that the images formed by this instrument are capable of being inde- libly fixed on certain surfaces previously prepared for the purpose, so that the picture is rendered permanent. When acamera is presented to any object or landscape strongly il- luminated by the sun, and the prepared ground for receiving the image is adjusted, and a certain time allowed to elapse till the rays of light produce their due effect, in a few minutes, or even seconds, a picture of the objects opposite to the lens is indelibly impressed. upon the prepared plate, in all the accurate proportions and perspective which distinguish the images formed in a dark chamber, which representations may be hung up in apartments, along with other paintings and 148 THE DAGUERREOTYPE PROCESS. engravings, and will likely retain their beauty and lustre for many years. Alhese are pictures of nature’s own workman- ship, finished in an extremely short space of time, and with the most exquisite delicacy and accuracy. ‘The effect is evidently owing to certain chemical properties in the rays of light, and opens a new field for experiment and investigation to the philosopher. The only defect in the picture is, that it is not coloured; but, in the progress of experiments on this subject, it is not unlikely that even this object may be accom- plished, in which case we should be able to obtain the most accurate landscapes -and representations of all objects which can possibly be formed. This art or discovery goes by the name of the Daguerreotype, from, M. Daguerre, a French- man, who is supposed to have been ‘the first discoverer, and who received a large premium from the French government for disclosing the process, and making the discovery public. Several improvements and modifications, in reference to the preparation of the plates, have been made since the discovery was first announced, about the beginning of 1839; and the pictures formed on this principle are frequently distinguished by the name of Photogenic drawings, and are now exhibited at most of our public scientific institutions. This new science or art has been distinguished by different names. It was first called Photography, from two Greek works, signifying writing by light: it was afterward called the art of Photogenic drawing, or drawing produced by light. M. Daguerre gave it the name of Heliography, or writing by the suns; all which appeliatives are derived from the Greek and are expressive, in some degree, of the nature of the pro- cess. We shall, however, make use of the term Daguerreo- type, derived from the name of the inventor. As it does not fall within our plan to give any minute descriptions of the Daguerreotype process, we shall just give a few general hints in reference to it, referring those who wish for particular details to the separate treatises which have been published respecting it. The first thing necessary to be attended to in this art is the preparation of the plate on which the drawing is to be made. The plate consists of a thin leaf of copper, plated with silver, both metals together not being thicker than a card. The object of the copper is simply to support the silver, which must be the purest that can be pro- cured. But, though the copper should be no thicker than te serve the purpose of support, it is necessary that it-should be so thick as to prevent the plate from being warped, which would produce a distortion of the images traced upon it, This THE DAGUERREOTYPE PROCESS. 149 plate must be polished ; and for this purpose the following articles are required; a vial of olive oil; some very fine cot- ton; pumice-powder, ground till it is almost impalpable; and tied up in a piece of fine muslin, thin enough to let the pow- der pass through without touching the plate when the bag is shaken ; a little nitric acid, diluted with sixteen times, by measure, its own quantity of water; a frame of wire on which to place the plate when being heated; a spirit lamp to make the plate hot; a small box, with inclined sides within, and having a lid to shut it up close; and a square board large enough to hold the drawing, and having catches at the side to keep it steady. To the above prerequisites, a good camera obscura is, of course, essentially necessary. This instrument should be large enough to admit the plate of the largest drawing in- tended to be taken. The lens which forms the image of the object should, if possible, be achromatic, and of a considerable diameter. . In an excellent instrument of this description now before me, the lens is an achromatic about three inches dia- meter, but capable of being contracted to a smaller aperture. Its focal distance is about 17 inches; and the box, exclusive of the tube which contains the lens, is 15 inches long, 13% inches broad, and 11 inches deep. It forms a beautiful and well-defined picture of every well-enlightened object to which it is directed. | Before the plate is placed in the camera, there are certain operations to be performed. 1. The surface of the plate should be made perfectly smooth, or highly polished. For this purpose it must be laid flat, with the silver side upward, apon several folds of paper for a bedding; and having been well polished in the usual way, the surface must be pow- dered equally and carefully with fine pumice enclosed in the muslin bag. Then taking a little cotton wool, dipped in olive ou, it must be rubbed over the plate with rounding strokes, and then crossing them by others which commence at right angles with the first. This process must be repeated fre- quently, changing the cotton, and renewing the pumice- powder every time. A small portion of cotton must now be moistened with the diluted nitric acid, and applied equally to the whole surface. The next thing to be done is to make the plate thoroughly and equally hot, by holding the plate witha pair of pincers by the corner over a charcoal fire, and when the plate is sufficiently hot, a white coating will be observed on the silver, which indicates that that part of the operation is finished. An even cold surface is next wanted, such as a 13* ‘ 150 THE DAGUERREOTYPE PROCESS. metallic plate conled almost to the freezing point by munate of soda, and to this the heated plate must be suddenly trans- ferred. - . 2. The next operation is to give the plate a coating of iodine. This is accomplished by fixing the plate upon a board, and then putting it into a box containing a little dish with iodine divided into small pieces, with its face downward, and supported with small brackets at the corners. In this position the plate must remain till it assume a full gold colour, through the condensation of the iodine on its surface, which process should be conducted in a darkened apartment. The requisite time for the condensation of the iodine varies from five minutes to half an hour. When this process is satisfac- torily accomplished, the plate should be immediately fixed in a frame with catches and bands, and placed in the camera ; and the transference from one receptacle to another should be made as quickly as possible, and with only so much light as will enable the operator to see what he is doing. 3. The next operation is to obtain the drawing. Having placed the camera in front of the scene to be represented, and the lens being adjusted to the proper focus, the ground glass of the camera is withdrawn, and the prepared plate is substi- tuted for it, and the whole is left till the natural images are drawn. by the natural light from the object. . The time neces- sary to leave the plate for a complete delineation of the ob- jects depends upon the intensity of the hight. Objects in the shade will require more time for their delineation than those in the broad light. The full, clear light of the south of Eu- rope, Spain, Italy, and particularly the more glowing bril- hiancy of tropical countries, will effect the object much more speedily than the duller luminosity of a northern clime. Some hours of the day are likewise more favourable than others. Daguerre states that “the most favourable is from 7 a.m. to 3 o'clock p. m., and that a drawing could be effected in Paris in three or four minutes in June and July, which would re- quire five or six in May and August, and seven or eight in April and September.” In the progress of this art, at the _ present time, portraits and other objects are frequently deli- neated in the course of a few seconds. 4. Immediately after removing the plate from the camera, it is next placed over the vapour of mercury, which is placed in a cup at the bottom of a box,\and a spirit lamp applied to its bottom till the temperature rise to 140° of Fahrenheit. This process is mtended to bring out the image, which is not visible when withdrawn from the camera; but in the PHOTOGENIC PAPER. 151 course of a few minutes a faint tracery will begin to appear, and in a very short time the figure will be clearly developed. 5. The next operation is fo fix the impression. In order to this, the coating on which the design was impressed must be removed, to preserve it from being decomposed by the rays of light. For this purpose, the plate is placed in a trough containing common water, plunging and withdrawing it im- mediately, and then plunging it into a solution of salt and water till the yellow coating has disappeared. Such is a very brief sketch of the photogenic processes of Daguerre. Other substances, however, more easily pre- pared, have been recommended by Mr. Talbot, F. R.S., who appears, about the same time, to have invented a process somewhat similar to that of Daguerre. The following are his directions for the preparation of photogenic paper: »The paper is to be dipped into a solution of salt in water, in the proportion of half an ounce of salt to half a pint of water. Let the superfluous. moisture drain off, and then laying the paper upon a clean cloth, dab it gently with a napkin, so as to prevent the salt collecting in one spot more than in another. ‘The paper is then to be pinned down by two of its corners on a drawing board by iueans of common pins, and one side washed or wetted with the photogenic fluid, using the brush prepared for that purpose, and taking care to distribute it equally. Next, dry the paper as rapidly as you can at the fire, and it will be fit for use for most pur- poses. If, when the paper is exposed to the sun’s rays, it should assume an irregular tint, a very thin extra wash of the fluid will render the colour uniform, and, at the same time, somewhat darker. Should it be required to make a more sensitive description of paper, after the first application of the fluid the solution of salt should be applied, and the paper dried at the fire. Apply a second wash of the fluid, and dry it at the fire again: employ the salt a third time, dry it, and one application more of the fluid will, when dried, have made the paper extremely sensitive. When slips of such papers, dif- ferently prepared, are exposed to the action of daylight, those which are soonest affected by the light, by becoming dark. are the best prepared. When photogenic drawings are finished in a perfect way, the designs then taken on the plate or paper are exceedingly beautiful and correct, and will bear to be inspected with a considerable magnifying power, so that the most minute por- tions of the objects delineated may be distinctly perceived. We have seen portraits finished in this way by a London 152 PERFECTION OF PHOTOGENIC PICTURES. artist with an accuracy which the best miniature painter could never attempt, every feature being s¢ distinct as to bear being viewed with a deep magnifier. ~ And in landscapes and buildings, such is the delicacy and accuracy of such repre- sentations, that the marks of the chisel and the crevices in the stones may frequently be seen by applying a magnifying lens to the picture; so that we may justly exclaim, in the words of the poet, “Who can paint like Nature!’ That LigHt—which is the firstborn of Deity, which pervades all space, and illuminates all worlds—in the twinkling of an eye, and with an accuracy which no art can imitate, depicts every object in its exact form and proportions, superior to every thing that human genius can produce. The photogenic art, in its progress, will doubtless be pro- ductive of many highly interesting and beneficial effects. It affords us the power of representing, by an accurate and rapid process, all the grand and beautiful objects connected with our globe, the landscapes peculiar to every country, the lofty ranges of mountains which distinguish Alpine regions, the noble edifices which art has reared, the monumental remains of antiquity, and every other object which it would be inte- resting for human beings to contemplate; so that, in the course of time, the general scenery of our world, in its pro- minent- parts, might be exhibited to almost every eye. ‘The commission of the French Chambers, when referring to this art, has the following remark: “'To. copy the millions upon millions of hieroglyphics which cover even the exterior of the great monuments of Thebes and Memphis, of Carnac, &c., would require scores of years and legions of designers. B the assistance of the Daguerreotype, a single man could finish that immense work.’’ ‘This instrument lays down objects which the visual organs of man would overlook, or might be unable to perceive, with the same minuteness and nicety that it delineates the most prominent features of a iandscape. The time-stained excrescences on a tree, the blades of grass, the leaf of a rose, the neglected weed, the moss on the summit of a lofty tower, and similar objects, are traced with the same accuracy as the larger objects m the surrounding scene. It is not improbable, likewise, that this art, (still in its infancy,) when it approximates to perfection, may enable us to take re- presentations of the sublime objects in the heavens. The sun affords sufficient light for this purpose; and there appears no insurmountable obstacle in taking, in this way, a highly-mag- nified picture of that luminary, which shall be capable of being again magnified by a powerful microscope. It is by no PROSPECTS OF THE SCIENCE. 153 means ‘improbable, from the experiments that have hitherto been made, that we may obtain an accurate delineation of the lunar world from the moon herself. The plated disks pre- pared by Daguerre receive impressions from the action of the lunar rays to such an extent as permits the hope that photo- raphic charts of the moon may soon be obtained ; and, if so, they will excel in accuracy all the delineations of this orb that have hitherto been obtained ; and if they should bear a mi- croscopic power, objects may be perceived on the lunar sur- face which have hitherto been invisible. Nor is it impossible that the planets Venus, Mars, Jupiter, and Saturn may be delineated in this way, and objects discovered which cannot be descried by means of the telescope. It might, perhaps, be considered as beyond the bounds of probability to expect that even distant nebule might thus be fixed, and a delinea- tion of their objects produced which shall be capable of being magnified by microscopes; but we ought to consider that the art is yet only in its infancy, that plates of a more delicate nature than those hitherto used may yet be prepared, and that other properties of light may yet be discovered which shall facilitate such designs. For we ought now to set no boundaries to the discoveries of science, and to the practical applications of scientific discovery which genius and art may accomplish. | In short, this invention leads to the conclusion that we have not yet discovered all the wonderful properties of that lumi- nous agent which pervades the universe, and which unveils to us its beauties and sublimities; and that thousands of admirable objects and agencies may yet be disclosed to our view through the medium of light, as philosophical investiga- tors advance in their researches and discoveries. In the present instance, as well as in many others, it evidently ap- pears that the Creator intends, in the course of his providence, by means of scientific researches, gradually to open to the view of the inhabitants of our world the wonders, the beau- ties, and the sublimities of his vast creation; to manifest his infinite wisdom and his superabundant goodness, and to raise our souls to the contemplation and the love of Him who is the original source of all that is glorious and beneficent in the scene of nature. 154 - THE OPTICAL ANGLE. CHAPTER III. ON THE OPTICAL ANGLE, AND THE APPARENT MAGNITUDE OF OBJECTS. In order to understand the principle on which telescopes represent distant objects as magnified, it may be expedient to explain what is meant by the angle of vision, and the ap- parent magnitudes under which different objects appear, and the same object, when placed at different distances. The optical angle is the angle contained under two right lines drawn from the extreme points of an object to the eye Thus A EB or C ED (fig. 40*) is the optical or visual angle, Figure 40*. F C A j ~E B D G or the angle under which the object A B or C D appears to the eye at E. These two objects, being at different distances, are seen under the same angle, although C D is_ evidently larger than AB. On the retina of the eye, their images are exactly of the same size, and so is the still larger object F G, The apparent magnitude of objects denotes their magni- tude as they appear to us, in contradistinction from their real or true magnitude, and it is measured by the. visual angle; for, whatever objects are seen under the same or equal angles appear equal, however different their real magnitudes. Ifa half crown or ‘half dollar be placed at about 120 yards from the eye, it is just perceptible as a visible point, and its ap- parent magnitude, or the angle under which it is seen, is very small. At the distance of thirty or forty yards, its bulk appears sensibly increased, and we perceive it to be a round body; at the distance of six or eight yards we can see the king or queen’s head engraved upon it; and at the distance of eight or ten inches from the eye it will appear so large that it will seem to cover a large building placed within the listance of a quarter of a mile; in other words, the apparant APPARENT MAGNITUDE. 155 magnitude of the half crown, held at such a distance, will more than equal that of such a building in the picture on the retina, owing to the increase of the optical angle. If we sup- pose A (fig. 41) to represent the apparent size of the half Figure 41, crown at nine yards’ distance, then we say it is seen under the small angle F ED. B will represent its apparent mag- nitude at 43 yards distant under the angle H EG, and the circle C, its apparent magnitude at 3. yards distant, under the large angle KEL This may be otherwise illustrated by the. following figure : Let A B (fig. 42) be an object viewed directly by the eye Q Figure 42. R. From each extremity A and B draw the lines A N,BM, intersecting each other in the crystalline humour in I: then is A I B the optical angle which is the measure of the ap- parent magnitude or length of the object A.B. From an inspection of this figure, it will evidently appear that the ap- parent magnitudes of objects will vary according to their dis- tances. ‘hus A B, C D, E F, the real magnitudes of which are unequal, may be situated at such distances from the eye as to have their apparent magnitudes all equal, and occupy- 156 THE OPTICAL ANGLE. ing the same space on the retina, M N, as here represented. In like manner, objects of equal magnitude, placed at unequal distances, will appear unequal. ‘The objects A B and GH, which are equal, being situated at different distances from the eye,G H will appear under the large angle T I V, or as large as an object T V, situated at the same place as the ob- ject A B, while A B appears under the smaller angle A I B. Therefore the object G H is apparently greater than the ob- ject AB, though it is only equal to it. Hence it appears that we have no certain standard of the true magnitude of objects by our visual perception abstractly considered, but only of the proportions of magnitude. In reference to apparent magnitudes, we scarcely ever judge any object to be so great or so small as it appears to be, or that there is so great a disparity in the visible magni- tude of two equal bodies at different distances from the eye. Thus, for example, suppose two men, each six feet three inches high, to stand directly before us, one at the distance of . a pole, or 51 yards, and the other at the distance of 100 poles, or 550 yards ; we should observe a considerable difference in their apparent size, but we should scarcely suppose, at first sight, that the one nearest the eye appeared a hundred times oreater than the other, or that, while the nearest one appeared six feet three inches high, the remote one appeared only about three-fourths of aninch. Yet such is in reality the case ; and not only so, but the visible bulk or area of the one is to that of the other asthe square of these numbers, namely, as 10,000 to 1; the man nearest us presenting to the eye a magnitude or surface ten thousand times greater than that of the other. Again, suppose two chairs standing in a large room, the one twenty-one feet distance from us, and the other three feet; the one nearest us will appear seven times larger, both in length and breadth, than the more distant one, and, consequently, its visible area forty-nine times greater. If I hold up my finger at nine inches distance from my eye, it appears to cover a large town a mile and a halfin extent, sit- uated at three miles’ distance ; consequently, the apparent magnitude of my finger, at nine inches distance from the or- gan of vision, is greater than that of the large town at three miles’ distance, and forms a larger picture on the retina of the eye, When stand at the distance of a foot from my window, and look through one of the panes to a village less than a quarter of a mile distant, I see, through that pane, nearly the whole extent of the village, comprehending two or three hun- dred houses; consequently, the apparent magnitude of the APPARENT MAGNITUDE. 157 pane is equal to nearly the extent of the village, and all the buildings it contains do not appear larger than the pane of glass in the window, otherwise the houses and other objects which compose the village could not be seen through the single pane. For, if we suppose a line drawn from one end of the village, passing through the one side -of the pane, and another line drawn from the other end, and passing through the other side of the pane to the eye, these lines would form the optical angle under which the pane of glass and the vil- lage appears. If the pane of glass be fourteen inches broad, and the length of the village 2640 yards, or half a mile, this last lineal extent is 6788 times greater than the other, and yet they have the same apparent magnitude in the case sup- osed. : Hence we may learn the absurdity and futility of attempt- ing to describe the extent of spaces in the heavens, by saying that a certain phenomenon was two or three feet or yards dis- tant from another, or that the tail of a comet appeared several yards in length. Such representations can convey no defi- nite ideas in relation to such magnitudes, unless it be specified at what distance from the eye the foot or yard is supposed to be placed. Ifa rod, a yard in length, be held at nine inches from the eye, it will subtend an angle, or cover a space in the heavens equal to more than one-fourth of the circumfe- rence of the sky, or about one hundred degrees. If it be eighteen inches from the eye, it will cover a space equal to fifty degrees; if at three feet, twenty-five degrees, and so on in proportion to the distance from the eye; so that we can form no correct conceptions of apparent spaces or distances in the heavens, when we are merely told that two stars, for example, appear to be three yards distant from each other. The only definite measure we can use in such cases is that of degrees. ‘The sun and moon are about halfa degree in ap- parent diameter, and the distance between the extreme stars in Orion’s belt three degrees, which measures, being made fa- miliar to the eye, may be applied to other spaces of the heavens, and an approximate idea conveyed of the relative distances of objects in the sky. From what has been stated above, it is evident that the magnitude of objects may be considered in different points of view. ‘The true dimensions of an object, considered in itself, give what is called its real or absolute magnitude ; and the opening of the visual angle determines the apparent magni- tude. The real magnitude, therefore, is a constant quantity ; but the apparent magnitude varies continually with the dis- Vou. IX. 14 158 THE OPTICAL ANGLE. tance, real or imaginary ; and, therefore, if we always judged of the dimensions of an object from its apparent magni- tude, every thing around us would, in this respect, be un- dergoing very sensible variations, which might lead us into strange and serious mistakes. = the telescopes we have now described. Fig. 48 represents the rays of light as they pass from the object to the eye in the Galilean telescope. After passing in a parallel direction to the object-glass, they are refracted by that glass, and undergo a slight convergence in passing towards the concave eyeglass, where they enter the eye in a parallel direction, but no im- age is formed previous to their entering the eye till they arrive at the retina. Fig. 49 represents the rays as they pass through the glasses of the astronomical telescope.. The rays, after entermg the object-glass, proceed in a converging direc- tion till they arrive at its focus about A, where an image of the object is formed ; they then proceed diverging to the eye- glass, where they are rendered parallel, and enter the eye in that direction. Fig. 50 represents the rays as they converge and diverge in passing through the four glasses of the com- mon day-telescope described above. After passing through the object-glass, they converge toward B, where the first image is formed. They then diverge towards the first eye- glass, where they are rendered parallel, and, passing through the second eyeglass, they again converge and form a second image at C, from which point they again diverge, and, pass- ing through the first eyeglass, enter the eye in a parallel direction. If the glasses of these telescopes were fixed on long pieces of wood, at their proper distances from each other, and placed in a darkened room, when the sun is shining, the TELESCOPE OF A SINGLE LENS. — ae beam of the sun’s light would pass through them in the same manner as here represented. SECT. V.—TELESCOPE FORMED BY A SINGLE. LENS. This is a species of telescope altogether unnoticed by opti- cal writers, so far as I know; nor has the property of a single lens in magnifying distant objects been generally adverted to or recognised. It may not, therefore, be inexpedient to state a few experiments which I have made in relation to this point. When we hold a spectacle-glass of a pretty long focal distance—say from 20 to 24 inches—close to the eye, and di- rect it to distant objects, they do not appear sensibly magni- fied. But if we hold the glass about 12 or 16 inches from our eye, we shall perceive a sensible degree of magnifying power, as if distant objects were seen at less than half the distance at which they are placed. This property of a spec- tacle-glass I happened to notice when a boy, and on different occasions since that period have made several experiments on the subject, some of which I shall here relate. With the object-glass of a common refracting telescope, 44 feet focal distance, and 21 inches diameter, I looked at distant objects—my eye being at about 3! feet from the lens, or about 10 or 12 inches within its focus—and it produced nearly the same effect as a telescope which magnifies the diameters of objects 5 or 6 times. With another lens, 11 feet focal dis- tance and 4 inches diameter, standing from it at the distance of about 10 feet, I obtained a magnifying power of about 12 or 14 times, which enabled me to read the letters on the sign- posts of a village half a mile distant. Having some time ago procured a very large lens, 26 feet focal distance and 114 inches diameter, I have tried with it various experiments of this kind upon different objects. Standing at the distance of about 25 feet from it, I can see distant objects through it mag- nified about 26 times its diameter, and consequently 676 times in surface, and remarkably clear and distinct, so that I can distinguish the hour and minute hands of a public clock in a village two miles distant. This single lens, therefore, answers the purpose of an ordinary telescope with a power of 26 times. In making such experiments, our eyes must always be within the focus of the lens, at least 8 or 10 inches. The object will, indeed, be seen at any distance from the glass within this limit, but the magnifying power is dimin- ished in proportion as we approach nearer tothe glass. Dif- ferent eyes, too, will require to place themselves at different distances, so as to obtain the greatest degree of magnifying 172 TELESCOPE OF A SINGLE LENS. power with distinctness, according as individuals are long or short-sighted. This kind of telescope stands in no need of a tube, but only of a small pedestal on which it may be placed ona table, nearly at the height of the eye, and that it be capable of a motion in a perpendicular or parallel direction, to bring it in a line with the eye andthe object. The principle on which the magnifying power in this case is produced, is materially the same as that on which the Galilean telescope depends. The eye of the observer serves instead of the concave lens in that instrument; and as the concave lens is placed as much within the focus of the object-glass as is equal to its own focal distance, so the eye, in these experiments, must be placed at least its focal distance within the focus of the lens with which we are experimenting ; andthe magnifying power will be nearly in the proportion of the focal distance of the lens to the focal dis- tance of the eye. If, for example, the focal distance of the eye, or the distance at which we see to read distinctly, be 10.inches, and the focal distance of the lens 11 feet, the magnifying power will be as 11 feet, or 132 inches to 10, that is, about 13 times. Figure 51. JA Let A (fig 51) represent the lens placed on a pedestal; the rays of light passing through this lens from distant objects will converge towards a focus at F. If a person then place his eye at E, a certain distance within the focal point, he will see distant objects magnified nearly in the proportion of the focal distance of the lens to that of the eye; and when the lens is very. broad—such as the 26 feet lens mentioned above —two or three persons may look through it at once, though they will not all see the same object. I have alluded above toa lens made by M. Azout of 600 feet focal distance. Wereit . possible to use such a lens for distant objects, it might repre- sent them as magnified 5 or 600 times. without the applica- PS THE ACHROMATIC TELESCOPE. 173 tion of any eyeglass. In this way the aerial telescope of Huygens would magnify objects above 100 times, which is about half the magnifying power it produced with its eye- piece. Suppose Azout’s lens had been fitted up as a tele- scope, it world not have. magnified above 480 times, as it would have required an eyeglass of 14 or 15 inches focal distance, whereas, without an eyeglass, it would have magni- fied objects considerably above 500 times. It is not unlikely that the species of telescope to which I have now adverted con- stituted one of those instruments for magnifying distant objects which were said to have been in the possession of certain per- sons long before their invention in Holland, and by Galileo in {taly, to which I have referred in p. 137. Were this kind of telescope to be applied to the celestial bodies, it would re- quire to be elevated upon a pole in the manner represented in fig. 45, p. 167. SECT. VI.——THE ACHROMATIC TELESCOPE. This telescope constitutes the most. important and useful improvement ever made upon telescopic instruments, and it is probable it will, ere long, supersede the use of all other telescopes. Its importance and utility will at once appear when we consider that a good achromatic telescope of only 4 or 5 feet in length will bear a magnifying power as great as that of a common astronomical telescope 100 feet long, and even with a greater degree of distinctness, so that they are now come into general use both for terrestrial and celestial observations. ‘here are, indeed, certain obstructions which prevent their being made of a very large size; but from the improvement in the manufacture of achromatie glass which is now going forward, it is to be hoped that the difficulties which have hitherto impeded the progress of opticians will soon be removed. In order to understand the nature of this telescope, it will be necessary to advert a little to the imper- fections connected with common refracting telescopes. The first imperfections to which I allude is this, that sphe- rical surfaces do not refract the rays of light accurately toa point ; and hence the image formed by a single convex lens is not perfectly accurate and distinct. The rays which pass near the extremities of such a lens meet in foci nearer to the lens than those which pass nearly through the centre, which may be illustrated by the following figure: Let P P (fig. 52) be a convex lens, and Ee an object, the point E of which corresponds with the axis, and sends forth the rays EM, EN, EA, &c., all of which reach the surface of the glass, 15* 174 THE ACHROMATIC TELESCOPE. Figure 52. but in different parts. It is manifest that the ray E A, which passes through the middle of the glass, suffers no refraction. The rays E M, EM, likewise, which pass through near to EA, will be converged to a focus at F, which we generally consider as the focus of the lens. But the rays EN, EN, which are nearer to the edge. of the glass, will be differently refracted, and will meet about G, nearer to the lens, where they will form another image, Gg. Hence, it is evident that the first image, F /, is formed only by the union of those rays which pass very near the centre of the lens; but as the rays of light proceeding from every point of an object are very numerous, there is a succession of images formed, according to the parts of the lens where they penetrate, which neces- sarily produces indistinctness. and confusion. This is the imperfection which is distinguished by the name of spherical aberration, or the error arising from. the spherical form of lenses. The second and most important imperfection of single lenses, when used for the object-glasses of telescopes, is, that the rays of compounded light being differently refrangible, come to their respective foci at different distances from the glass; the more refrangible rays, as the violet, converging sooner than those which are less refrangible, as the red. [ have had occasion to illustrate this circumstance, when treat- ing on the colours produced by the prism, (see p. 101, and ~ figures 52 and 33,) and it is confirmed by the experiment of a paper painted red, throwing its image, by means of a lens, at a greater distance than another paper painted blue. From such facts and experiments, it appears that the image of a white object consists of an indefinite number ef coloured images, the violet being nearest, and the red farthest from the lens, and. the images of intermediate colours at interme- diate distances... The aggregate, or image itself, must there- fore be in some degree confused; and this confusion being THE ACHROMATIC TELESCOPE. 175 much increased by the magnifying power, it is found neces- sary to use an eyeglass of a certain limated convexity to a given object-glass. Thus, an object-glass of 34 inches focal length will bear an eyeglass of only one inch focus, and will magnify the diameters of objects 34 times; one of 50 feet focal distance will require an eyeglass of 43 inches focus, and will magnify only 142 times; whereas, could we apply to it an eyeglass of only one inch focus, as in the former case, it would magnify no less than 600 times. And were we to construct an object-glass of 100 feet focal length, we should require to apply an-eyeglass not less than six inches focus, which would produce a power of about 200 times; so that there is no possibility of producing a great power by single lenses without extending the telescope to an immoderate length. Sir Isaac Newton, after having made his discoveries re- specting the colours of light, considered the vircumstance we have now stated as an insuperable barrier to the improve- ment of refracting telescopes, and therefore turned his atten- tion to the improvement of telescopes by reflection. In the telescopes which he constructed and partly invented, the images of objects are formed by reflection from speculums or mirrors ; and being free from the irregular convergency of the various coloured rays of light, will admit of a much larger aperture and the application of a much greater degree of mag- nifying power. ‘The reflector which Newton constructed was only six inches long, but it was capable of bearing a power equal to that of a six feet refractor. It was a long time, how- ever, after the invention of these telescopes, before they-were made of a size fitted for making celestial observations. After reflecting telescopes had been some time in use, Dollond made his famous discovery of the principle which led him to the construction of the achromatic telescope. This inven- tion consists of a compound object-glass formed of two dif- ferent kinds of glass, by which both the spherical aberration and the errors arising from the different refrangibility of the rays of light are in a great measure corrected. For the ex- planation of the nature of this compound object-glass and of the effects it produces, it may be expedient to offer the follow- ing remarks respecting the dispersion of light and its refrac tion by different substances. The dispersion of light is estimated by the variable angle formed by the red and violet rays which bound the ¢olar spectrum, or, rather, it is the excess of the refraction of the most refrangible ray above that of the least refrangible ray 176 THE ACHROMATIC TELESCOPE. The dispersion is not proportional to the refraction, that is, the substances which have an equal mean refraction do not disperse light in the same ratio. For example, if we make a prism with plates of glass, and fill it with oil of cassia, and adjust its refracting angle, A C B, (fig. 31, p. 100,) so that the middle of the spectrum which it forms falls exactly at the same place where the green rays of a spectrum formed by a glass prism would fall, then we shall find that the spectrum formed by the ot/ of cassia prism will be two or three times longer than that of the glass prism. ‘The oil of cassia, there- fore, is said to disperse the rays of light more than the glass, that is, to separate the extreme red and violet rays at O and P more than the mean ray at green, and to have a greater dispersive power. Sir I. Newton appears to have made use of prisms composed of different substances, yet, strange to tell, he never observed that they formed spectrums whose lenoths were different when the refraction of the green ray was the same, but thought that the dispersion was propor- tional to the refraction. This error continued to be overlooked by philosophers for a considerable time, and was the cause of retarding the invention of the achromatic telescope for more than 50 years. Dollond was among the first who detected this error. By his experiments it appears that the different kinds of glass differ extremely with respect to the divergency of colours produced by equal refractions. He found that two prisms, one of white flint glass, whose refracting angle was about 25 degrees, and another of crown glass, whose refracting angle was about 29 degrees, refracted the beam of light nearly alike, but that the divergency of colour in the white flint was considerably more than in the crown glass; so that when they were applied together, to refract contrary ways, and a beam of light transmitted through them, though the emergent continued parallel to the incident part, it was, notwithstanding, separated into component colours. .From this he inferred that, in order to render the emergent beam white, it is ne- cessary that the refracting angle of the prism of crown glass should be increased, and by repeated experiments he dis- covered the exact quantity. By these means he obtained a theory in which refraction was performed without any sepa- ration or divergency of colour, and thus the way was prepared for applying the principle he had ascertained to the con- struction of the object-glasses of refracting telescopes. For the edges of a convex and concave lens, when-placed in con- tact with each other, may be considered as two prisms which THE ACHROMATIC TELESCOPE. 177 refract contrary ways; and if the excess of refraction in the one be such as precisely to destroy the divergency of colour in the other, a colourless image will be formed. Thus, if two lenses are made of the same focal length, the one of flint glass and the other of crown, the length or diameter of the coloured image in the first will be to that produced by the crown glass as three to two nearly. Now if we make the focal lengths of the lenses in this proportion, that is, as three to two, the coloured spectrum produced by each will be equal. But if the flint lens be concave, and the crown convex, when placed in contact they will mutually correct each other, and a pencil of white light refracted by the compound lens will, remain colourless. The following figure may perhaps illustrate what has been now stated. Let L L (fig. 53) represent a convex lens of Figure 53. crown glass, and lla concave lens of flint glass. A ray of the sun, 8, falls at F on the convex lens, which will refract it exactly as the prism A B C, whose faces touch the two sur- faces of the lens at the points where the ray enters and quits it. The solar ray, S F, thus refracted by the lens L L, or prism A BC, would have formed a spectrum, P T, on the wall, had there been no other lens; the violet ray, F, crossing the axis of the lens at V, and going to the upper end, P, of the spectrum; and the red ray, F R, going to the lower end, T. - But as the flint glass lens / J, on the prism A @C, which receives the rays F V, F R, at the same points, is interposed, these rays will be united at f, and form a small circle of white light; the rays S F of the sun being now refracted withaw colour from its primitive direction S F Y into the new a: rection Ff. In like manner the corresponding ray S M wik be refracted tof, and a white and colourless image of the sun will be there formed by the two lenses. In this combination of lenses, it is cbvious that the spherical aberration of the 178 THE ACHROMATIC TELESCOPE. flint lens corrects to a considerable degree that of the crown glass, and by a proper adjustment of the radii of the surfaces, it may be almost wholly removed. This error is still more completely corrected in the ¢riple achroinatic object-glass, which consists of three lenses—a concave’ flint lens placed between convexes of crown glass. Fig. 54 shows the double achromatic lens, and fig. 55 the triple object-glass, as they are fitted up in their cells, and placed at the object-end of Figure 54. Figure 55. the telescope. In consequence of their producing a focal image free of colour, they will bear a much larger aperture and a much greater magnifying power than.common refract- ing telescopes of the same length. While a common tele- scope whose object-glass is 33 feet focal distance will bear an aperture of scarcely one inch, the 33 feet achromatic will bear an aperture of 3; inches, and consequently transmits 103 times the quantity of light. While the one can bear a magnifying power of only about 36 times, the other will bear a magnifying power for celestial objects of more than 200 times. | The theory of the achromatic telescope is somewhat com- plicated and abstruse, and would require a more lengthened investigation than my limits will permit. But what has been already stated may serve to give the reader a general idea of the principle on which it is constructed, which is all I intended. The term achromatic, by which such instruments are now distinguished, was first given to them by Dr. Bevis. It is compounded of two Greek words which signify “free of colour.” And were it not that-even philosophers are not altogether free of that pedantry which induces us to select Greek words which are unintelligible to the mass of mankind, they might have bern contented with selecting the plain THE ACHROMATIC TELESCOPE. 179 English word colourless, which is as significant and expres- sive as the Greek word achromatic. The crown glass, of which the convex lenses of this telescope are made, is the same as good common window-glass; and the flint glass is that species of glass of which wine-glasses, tumblers, de- canters, and similar articles are formed, and is sometimes distinguished by the name of crystal glass. Some opticians have occasionally formed the concave lens of an achromatic object-glass from the bottom of a broken tumbler. This telescope was invented and constructed by Mr. John Dollond about the year 1758. When he began his researches into this subject, he was a silk weaver in Spitalfields, London. The attempt of the celebrated Euler to form a colourless tele- scope, by including water between two meniscus glasses, attracted his attention, and in the year 1753 he addressed a letter to Mr. Short, the optician, which was published in the Philosophical Transactions of London, “concerning a mistake in Euler’s theorem for correcting the aberrations in the object- glasses of refracting telescopes.” -After a great variety of experiments on the refractive and dispersive powers of dif- ferent substances, he at last constructed a telescope in which an exact balance of the opposite dispersive powers of the crown and flint lenses made the colours disappear, while the predominating refraction of the crown lens disposed the achromatic rays to meet ata distant focus. In constructing such object-glasses, however, he had several difficulties to encounter. In the first place, the focal distance as well as the particular surfaces must be very nicely proportioned to the densities or refractive powers of the glasses, which are very apt to vary in the same sort of glass made at different times. In the next place, the centres of the two glasses must be placed truly in the common axis of the telescope, otherwise the desired effect will be in a great measure de- stroyed. ‘l’o these difficulties is to be-added, that there are four surfaces (even in double achromatic object-glasses) to be wrought perfectly spherical; and every person practised in optical operations will allow that there must be the greatest accuracy throughout the whole work. But these and other difficulties were at length overcome by the judgment and perseverance of this ingenious artist. It appears, however, that Dollond was not the only person who had the merit of making this discovery—a private gen- tleman, Mr. Chest, of Chest Hall, a considerable number of years before, having made a similar discovery, and applied it to the same purpose. This fact was ascertained in the course ~ 180 THE ACHROMATIC TRLESCOPE. of a process raised against Dollond, at the instance of Wate kins, optician at Charing Cross, when applying fora patent. But as the other gentleman had kept his invention a secret, and Dollond had brought it forth for the benefit of the public, the decision was given in his favour.. There was no. evi- dence that Dollond borrowed the idea from his competitor, and both were, to a certain extent, entitled to the merits of the invention. One of the greatest obstructions to the construction of large achromatic telescopes is the difficulty of procuring large discs of flint glass of a uniform refractive density, of good colour, and free from veins. It is said that, fortunately for Mr. Dol- lond, this kind of glass was procurable when he began to make achromatic telescopes, though the attempts of ingenious chemists have since been exerted to make it without much success. It is also said that the glass employed by Dollond in the fabrication of his best telescopes was of the same melt- ing, or made at the same time, and that, excepting this par- ticular treasure, casually obtained, good dense glass for achromatic purposes was always as difficult to be procured as it is now. The dispersion of the flint glass, too, is so variable, that, in forming an achromatic lens, trials on each specimen required to be made before the absolute proportional disper- sion ofthe substances can be ascertained. It is owing ina great measure, to these circumstances that a large and good achromatic telescope cannot be procured unless ata very high price. Mr. Tulley, of Islington—who has been long distinguished as a maker of excellent achromatic instruments —showed me, about six years ago, a rude piece of flint glass about five inches diameter, intended for the concave lens of an achromatic object-glass, for which he paid eight guineas. This was before the piece of glass was either figured or pol- ished, and, consequently, he had still to perform the delicate operation of figuring, polishing, and adjusting this concave to the convex lenses with which it was to be combined; and, during the process. some veins or irregularities might be de- tected in the flint glass which did not then appear. Some years before, he procured a disc of glass from the Continent, about seven or eight inches diameter, for which he paid about thirty guineas, with which an excellent telescope, twelve feet focal length, was constructed for the Astronomical Society of London. _ It is obvious, therefore, that large achro- matic telescopes must be charged at a pretty high price. In order to stimulate Ingenious chemists and opticians to make exper ments on this subject, the Board of Longitude, MANUFACTURE OF LENSES. 181 more than half a century ago, offered a considerable reward for bringing the art of making good flint glass for optical pur- poses to the requisite perfection. But considerable difficulties arise in attempting improvements of this kind, as the experi- ments must all be tried on a very large scale, and are ne- cessarily attended with a heavy expense; and, although government has been extremely liberal in voting money for warlike purposes, and in bestowing pensions on those who stood in no need of them, it has thrown an obstruction in the way of such experiments by the heavy duty of excise, which is rigorously exacted, whether the glass be manufactured into saleable articles or not, and has thus been instrumental in re- tarding the progress of improvement and discovery. It would appear that experiments of this kind have been attended with more success in France, Germany, and other places on the Continent than in Britain, as several very large achromatic telescopes have been constructed in those countries by means of flint glass, which was cast for the purpose in different manu- factories, and to which British artists have been considerably indebted, as the London opticians frequently purchase their largest discs of flint glass from Parisian agents. Guinaud, a Continental experimenter, and who was originally a cabinet- maker, appears to have had his labours in this department of art crowned with great success. Many years were employed in his experiments, and he too frequently, notwithstanding all his attention, discovered his metal to be vitiated by strie, specks, or grains, with cometic tails. He constructed a fur- nace capable of melting two ewt. of glass in one mass, which he sawed vertically, and polished one of the sections, in order to observe what had taken place during the fusion. From time to time, as he obtained blocks including portions of good glass, his practice was to separate them by sawing the blocks into horizontal sections, or perpendicular to their axes. A fortunate accident conducted him toa better process. While his men were one day carrying a block of this glass on a hand- barrow to a saw-mill which he had erected at the Fall of the Doubs, the mass slipped from its bearers, and rolling to the bottom of a steep and rocky declivity, was broken to pieces. Guinaud having selected those fragments which appeared perfectly homogeneous, softened them in circular moulds in _ such a manner, that, on cooling, he obtained discs that were afterward fit for working. ‘To this method he adhered, and contrived a way for clearing his glass while cooling, so that the fractures should follow the most faulty parts. When flaws occurred in the large masses, they were removed by cleaving Vou. IX. 16 182 THE DORPAT TELESCOPE. the pieces with wedges; then smelting them again in moulds, which gave them the form of discs. ‘The Astronomical So- ciety of London have made trial of discs made by Guinaud, and have found them entirely homogeneous and free from fault. Of this ingenious artist’s flit glass some of the largest achromatic telescopes on the Continent have been constructed. But it is more than twenty years since this: experimenter took his flight from this terrestrial scene, and it is- uncertain whether his process be still carried on with equal success. Notices of some large Achromatic Telescopes on the Conti- nent and in Great Britain. 1. The Dorpat Telescope.—This is one of the largest and most expensive refracting telescopes ever constructed. — It was made by the celebrated Fraunhofer, of Munich, for the Observatory of the Imperial University of Dorpat, and was re- ceived into the Observatory by Professor Struve, in the year 1825. ‘The aperture of the object-glass of this telescope is 94 English inches, and its solar focal length about 14 feet, the main tube being 13 French feet, exclusive of the tube which holds the eyepieces. The smallest of the four magnifying powers it possesses is. 175, and the largest 700, which, in favourable weather, is said to present the object with the ut- most precision. ‘This instrument,’ says Struve, “ was sold tous by Privy-counsellor Von Urzcunriper, the chief of the optical establishment at Munich, for 10,500 florins, (about £950 sterling,) a price which only covers the expenses which the establishment incurred in making it.” The framework of the stand of this telescope is of oak, inlaid with pieces of mahogany in an ornamental manner, and the tube is of deal vaneered with mahogany and highly polished.. The whole weight of the telescope and its counterpoises is supported at one point, at the common centre of gravity of all its parts ; and though these weigh 3000 Russian pounds, yet we are told that this enormous telescope may be turned in every di- rection towards the heavens with more certainty than any other hitherto in use. When the object end of the telescope is elevated to the zenith, it is sixteen feet four inches, Paris measure, above the floor, and its eye end in this position is two feet nine inches high. This instrument is mounted on an equatorial stand, and clock-work is applied to the equa- torial axis, which gives it a smooth and regular sidereal motion, which, it is said, keeps a star in the exact centre of the field of view, and produces the appearance of a state of rest in the starry regions, which motion can be made solar, or SIR JAMES SOUTH’S TELESCOPE. 183 even lunar, by a little change given to the place of a pointer that is placed as an index on the dial plate. Professor Struve considers the optical powers of this telescope superior to those of Schroeter’s twenty-five-feet reflector, from having observed o Orionis with fifteen companions, though Schroeter observed only twelve that he could count with certainty. Nay, he seems disposed to place it in competition with the late Sir W. Herschel’s forty-feet reflector. The finder of this telescope has a focal distance of 30 French inches, and 2-42 aperture. 2. Sir James South's Telescope-—About the year 1829, Sir J. South, President of the London Astronomical Society, procured of M. Cauchoix, of Paris, an achromatic object-glass of 11 2-10 inches clear aperture, and of 19 feet focal length. The flint glass employed in its construction was the manu- facture of the late Guinaud Je Pere, and was found to be absolutely perfect. The first observation was made with this telescope while ona temporary stand, on February 13, 1830, when Sir J. Herschel discovered with it a sixth star in the trapezium in the nebula of Orion, whose brightness was about one-third of that of the fifth star discovered by Struve, which is as distinctly seen as the companion to Polaris is in a five- feet achromatic. Sir James gives the following notices of the performance of this instrument on the morning of May 14, 1830. «At half-past two placed the 20 feet. achromatic on the Georgium Sidus, saw it with a power of 346, a beautiful planetary disc ; not the slightest suspicion of any ring, either perpendicular or horizontal; but the planet three hours east of the meridian, and the moon within three degrees of the planet. Ata quarter before three, viewed Jupiter with 252 and 346, literally covered with belts, and the diameters of his satellites might have been as easily measured as himself, One came from behind the body, and the contrast of the colour with that of the planet’s limb was striking. At three o’clock, viewed Mars. The contrast of light in the vicinity of the poles very decided. Several spots on his body well and strongly marked; that about the south pole seems to overtake the body of the planet, and gives an appearance not unlike that afforded by the new moon, familiarly known as ‘the old moon in the new moon’s arms.’”’ Saturn has been repeat- edly seen with powers from 130 to 928, under circumstances the most favourable; but not any thing anomalous about the planet or its ring could even be suspected. This telescope is erected on an equatorial stand, at Sir J. South’s observatory, Kensington, 3. Captain Smyth’s Telescope in his private observatory 184 ROYAL OBSERVATORY TELESCOPES. at Bedford.—This achromatic telescope is 83 feet focal length, with a clear aperture of 5 9-10 inches, worked by the late Mr. Tulley, senior, from a disc purchased by Sir James South at Paris. It is considered by Captain Smyth to be the finest specimen of that eminent optician’s skill, and, it is said, will bear with distinctness a magnifying power of 1200. Its distinctness has been proved by the clear vision it gives of the obscure nebule, and of the companions of Polaris, Rigel, a Lyre, and the most minute double stars—the lunar mountains, cavities, and shadows under all powers—the lucid polar re- gions of Mars—the sharpness of the double ring of Saturn— the gibbous aspect of Venus—the shadows of Jupiter’s satel- lites across his body, and the splendid contrast of colours in o Hercules, y Andromede, and other superb double stars. Other larze Achromatics.—Besides the above, the follow- ing, belonging to public observatories and private individuals, may be mentioned. In the Royal Observatory at Greenwich there is an achromatic of 10 feet focal distance, having a double object-glass 5 inches diameter, which was made by Mr. Peter Dollond, and tie only one of that size he ever con- structed. ‘here is also a 46-inch achromatic, with a triple object-glass 3¢ inches aperture, which is said to be the most perfect instrument of the kind ever produced. It was the favourite instrument of Dr. Maskelyne, late astronomer royal, who hada small room fitted up in the observatory for this telescope. The observatory some years ago erected near Cambridge is, perhaps, the most splendid structure of the kind in Great Britam. It is furnished with several very large achromatic telescopes on equatoria] machinery ; but the achromatic telescope lately presented to it by the Duke of Northumberland is undoubtedly the largest instrument of this description which is to be found in this country. The object-class is said to be 25 feet focal distance, and of a cor- responding diameter; but as there was no access to this in- strument at the time I visited this observatory, nearly six years ago, I am unable to give a particular description of it. In the Royal Observatory at Paris, which I visited in 1837, I noticed among other instruments, two very large achromatic telescopes, which, measuring them rudely by the eye, I esti- mated to be from 15 to 18 feet long, and the aperture at the object end from 12 to 15 inches diameter. They were the largest achromatics I had-previously seen ;* but I could * An achromatic telescope is said to be in possession of Mr. Cooper, mth Nie Sligo, which is 26 feet long, and the diameter of the object-glass inches, TELESCOPES IN PRIVATE OBSERVATORIES. 185 find no person in the observatory at that time who could give me any information as to their history, or to their exact dimen- sions or powers of magnifying. The Rey. Dr. Pearson, Treasurer to the Astronomical Society of London, is in possession of the telescope formerly alluded to, made by Mr. Tulley, of twelve feet focal distance and seven inches aperture, which is said to be a very fine one. The small star which accompanies the pole star, with a power of 100, appears through this telescope as distinct and steady as one of Jupiter’s satellites. With a single lens of 6 inches focus, which produced a power of 24 times, ac- cording to the testimony of an observer who noticed it, the small star appeared as it does in an achromatic of 3 inches aperture, which shows the great effect of illuminating power in such instruments. Mr. Lawson, a diligent astronomical observer in Hereford, possesses a most beautiful achromatic telescope of about 7 inches aperture and 12 feet focal dis- tance, which was made by one of the Dollonds, who con- sidered it as his chef d’w@uvre. It is said to bear powers as high as 1100 or 1400, and has been fitted up with mecha- nism, devised by Mr. Lawson himself, so as to be perfectly easy and manageable to the observer, and which displays this gentleman’s inventive talent. . In several of his observa- tions with this instrument, he is said to have had a view of some of the more minute subdivisions of the ring of Saturn. A very excellent achromatic telescope was fitted up some years ago by my worthy friend William Bridges, Esq., Black- heath. Its object-glass is 53 inches diameter, and about 53 feet local length. It is erected upon equatorial machinery, and placed in a circular observatory which moves round with a slight touch of the hand. The object-glass of this instru- ment cost about 200 guineas; the equatorial machinery on which it is mounted cost 150 guineas; and the circular ob- servatory in which it is placed about 100 guineas ; in all 450 guineas. Its powers vary frum 50 to 300 times.* Achromatic Telescopes of a moderate size. Such telescopes as I have alluded to above are among the largest which have yet been made on the achromatic princi- ple; they are, of course, comparatively rare, and can be af- *'This telescope, which was made by Dollond, with a power of 240 -imes, gives a beautiful view of the belts of Jupiter and the double ring of Saturn, and with the power of 50 the stars in the milky way and some of the nebule appear very numerous and brilliant. Its owner is a gentle man who unites science with Christianity. 16* 186 PRICES OF TELESCOPES. forded only at a very high price. Few of the object-glasses in the telescopes to which I have referred would be valued at less than 200 guineas, independently of the tubes, eye- pieces, and other apparatus with which they are fitted up. It is so difficult to procure large discs of flint glass for optical purposes, to produce the requisite curves of the different Jenses, and to combine them together with that extreme ac- curacy which is requisite, that, when a good compound lens of this description is found perfectly achromatic, the optician must necessarily set a high value upon it, since it may happen that he may have finished half a dozen before he has got one that is nearly perfect. ‘The more common sizes of achromatic telescopes for astronomical purposes, which are regularly sold by the London opticians, are the following: 1. The 23 feet Achromatic.—This telescope has an ob- ject-glass 30 inches in focal length, and 2 inches clear aper- ture. It is generally furnished with two eyepieces, one for terrestrial objects, magnifying about 30 or 35 times, and one for celestial objects, with a power of 70 or 75 times. It might be furnished with an additional astronomical eyepiece, if the object-glass be a good one, so as to produce a power of 90 or 95 times. With such a telescope, the belts and satellites of Jupiter, the phases of Venus, and the ring of Saturn may be perceived, but not to so much advantage as with larger tele- scopes. It is generally fitted up either with a mahogany or a brass tube, and is placed upon a tripod brass stand, with a universal joint which produces a horizontal and vertical mo- tion. It is packed, along with the eyepieces and whatever else belongs to it, ina neat mahogany box. Its price varies according as it is furnished with an elevating rack or other apparatus. Det | ' The following are the prices of this mstrument, as marked in the catalogue of Mr. Tulley, Terrett’s Court, Islington, London. 21 feet telescopes, brass mounted on plain pillar and claw stand, with 1 eyepiece for astronomical purposes and 1 for land objects, to vary the magnifying power, packed in a mahogany (ian aie hn shite oem ail ac vues eat nbea ite Bogen cem Sete eg 10 100 hogany bOxX-cessccccesdeveccceavecccescccessescasccscecs -. 12 120 The following prices of the same kind of telescope are from the catalogue of Messrs. W. and 8. Jones, 30 Lower Hol- born, London. PRICES OF TELESCOPES. 187 The improved 2} feet achromatic refractor, on a brass stand, mies mahogany tube, with three eyepieces, two magnifying about 40 ahd 50 times for terrestrial objects, and the, other about 75 times for astronomical purposes, in a mahogany case .-.--+++- 10100 Ditto, ditto, the tube all brass, with three eyepieces...... A eB GF | . Ditto, ditto, with vertical and horizontal rack-work motions. 15 15 0 2. The 3i feet Achromatic Telescope.-—The object-glass of this telescope is from 44 to 46 inches focal length, and 27 inches diameter. It is generally furnished with four eye- pieces, two for terrestrial and two for celestial objects. The lowest power for land objects is generally about 45, which affords a large field of view, and exhibits the objects with great brilliance. ‘The other terrestrial power is usually from 65 to 70. The astronomical powers are about 80 and 130; but such a telescope should always have another eyepiece, to produce a power of 180 or 200 times, which it will bear with distinctness, in a serene state of the atmosphere, if the object-glass be truly achromatic. The illuminating power in this telescope is nearly double that of the 2 feet telescope, or in the proportion of 7.56 to 4, and therefore it will bear about double the magnifying power with nearly equal dis- tinctness. This telescope is fitted up in a manner somewhat similar to the former, with a tripod stand, which is placed upon a table. Sometimes, however, it is mounted on a long mahogany stand, which rests upon the floor, (as in fig. 58,) and is fitted with an equatorial motion; and has generally a small telescope fixed near the eye end of the large tube, called a finder, which serves to direct the telescope to a par- ticular object in the heavens when the higher powers are applied. It is likewise eligible that it should have an ele- vating rack and sliding tubes, for supporting the eye end of the instrument, to keep it steady during astronomical observa- tions, and it would be an advantage, for various purposes which shall be afterwards described, to have fitted to it a diagonal eyepiece magnifying 40 times or upward. The prices of this instrument, as marked in Mr. Tulley’s catalogue, are as follows: The 34 feet achromatic telescope, 2% inches aperture, on plain pillar and claw stand, 2 eyepieces for astronomical purposes and 1 for land objects, to vary the magnifying power, packed ina mahogany. bOX +o s:008 oses odode wwacie avs Cade ovssevdedweeee 2100 Ditto, ditto, with elevating rack and achromatic finder, 2 eye- pieces for astronomical purposes and 1 for day objects, to vary the magnifying power, packed in a mahogany box....++..+++. 2650 The following are the prices as marked in Messrs. W. and S. Jones’s catalogue : 188 PRICES OF TELESCOPES. The 3} feet achromatic, plain mahogany tube...++seeseeees 18 18 Ditto, ditto, ERS NUE ss ee tM em ee a rae woe cee s winds ob 21 0 Ditto, all in brass, with rack-work motions, &c.. oc ase et oe Ditto, the object-glass of the largest aperture, and the rack motions on an improved principle...........-from £37 16s. to 42 0 Ditto, fitted up with equatorial motion, framed mahogany stand, divided altitude and a ee abasic. or piper nr and right ascension circles, &c., &c.. oe --from £60 to 80 00 This is the telescope aS rd I one particularly recom- mend to astronomical amateurs, whose pecuniary resources do not permit them to purchase more expensive instruments. When fitted up with the eyepieces and powers already men- tioned, and with a finder and elevating rack—price 25 guineas—it will serve all the purposes of general observation. By this telescope satisfactory views may be obtained of most of the interesting phenomena of the heavens—such as the spots on the sun—the mountains, vales, and caverns on the lunar surface—the phases of Mercury and Venus—the spots on Mars—the satellites and belts of Jupiter—the ring of Sa- turn—many of the more interesting nebule, and most of the double stars of the second and third classes. When the object-glass of this telescope is accurately figured and per- fectly achromatic, a power of from 200 to 230 may be put upon it, by which the division of Saturn’s ring might occa- sionally be perceived. It is more easily managed, and repre- sents objects considerably brighter than reflecting telescopes of the same price and magnifying power, and it is not so apt to be deranged as reflectors generally are. A telescope of a less size would not, in general, be found satisfactory for view- ing the objects I have now specified, and for general astrono- mical purposes. It may not be improper, for the information of some readers, to explain what is meant in Mr. Tulley’s catalogue, when it is stated that this instrument “ has one eye- piece for day objects, fo vary the magnifying power.” The eyepiece alluded to is so constructed, that by drawing out a tube next the eye you may increase the power at pleasure, and make it to vary say from 40 to 80 or 100 times; so that such a construction of the terrestrial eyepiece (to be afterward explained) serves, in a great measure, the purpose of separate eyepieces. ‘The whole length of the 33 feet telescope, when the terrestrial eyepiece is apphed, is about 43 feet from the object-glass to the first eyeglass. When the aperture of the object-glass ot this telescope ex- ceeds 27 inches, its price rapidly advances. The following is Mr. Tulley’s scale of prices, proportionate to the increase of aperture : PRICES OF TELESCOPES. 189 3. ¢ 3} feet telescopes, 3} inches aperture, with vertical and hori- zontal rack-work motions, achromatic finder, 3 eyepieces for astronomical purposes and one for day objects, to vary the mag- aityie power, packed in a mahogany box .-+---seeceeveeecs 42 00 itto, ditto, 32 inches diameter, mounted as above ........ 68 50 Ditto, with universal equatorial instead of pillar and claw BERT, Be oon «5 cis RaikieyS Resrroe so pape ccamen ens bets qpme ata opto a B40 Here, in the one case, the increase of half an inch in the diameter of the object glass adds about £16 to the expense, and in the other case, no less than £26, 5s. The proportion of light in those two telescopes, compared with that of 27 in- ches aperture, is as follows: The square of the 27 object-glass is 7.56; that of 3+, 10.56; and that of the 37, 14.06; so that the light admitted by the 8; compared with the 27 aperture is nearly as 10 to 7; and the light admitted by the 3z object- glass is nearly double that of the 27 aperture, and will bear nearly a proportional increase of magnifying power. 3. The 5d feet Achromatic Telescope.-—TVhe focal length of the object-glass of this telescope is 5 feet 3 inches, and the diameter of its aperture 3,8, inches. ‘The usual magnify- ing powers applied to it are, for land objects 65 times, and for celestial objects 110, 190, 250, and sometimes one or two higher powers. ‘The quantity of light it possesses is not much larger than that of the 32 feet telescope, with 37 inches aper- ture; but the larger focal length of this telescope is considered to be an advantage, since the longer the focus of the object- glass, the less will be its chromatic and spherical aberrations, and the larger may be the eyeglasses, and the flatter the field of view. The following are the prices of these telescopes, as marked in Mr. Tulley’s catalogue : 5 feet telescopes, 3% inches aperture, on a universal equatorial stand, with achromatic finder, 4 eyepieces for astronomical pur- poses and 1 for day objects, to vary the magnifying power, packed in a mahogany box ..e.seseecesccseves 100 guineas to 157 10 0 7 feet ditto, 5 inches aperture, on a newly improved universal equatorial stand, 6 eyepieces for astronomical purposes and 1 for day objects, to vary the magnifying power, with achromatic finder and Troughton’s micrometer ....+-.se+eeee evgeedeeses DOF DIO The above are all the kinds of achromatic telescopes gene- rally made by the London opticians. Those of the larger kind, as 5 and 7 feet telescopes, and the 33 feet with 37 in- ches aperture, are generally made to order, and are not always to be procured. But the 23 and 3% feet achromatics of 27 inches aperture are generally to be found ready made at most of the optician’s shops in the metropolis. The prices of these 190 TELESCOPE ON PLAIN STAND. instruments are nearly the same in most of the optician’s shops in London. Some of them demand a higher price, but few of them are ever sold lower than what has been stated above, unless in certain cases where a discount is allowed. The stands for these telescopes, and the manner in which they are fitted up for observation, is represented in figures 57, 58, and 59. Fig. 57 represents either the 23 or the 33 Figure 57. i Sa oe OS EM a Ort 814 6.6 oO feet telescopes, mounted on a plain brass stand, to be placed ona table.. A isthe long eyepiece for land objects, and B the small eyepiece for astronomical observation, which is com- posed of two lenses, and represents the object in an inverted position. ‘These eyepieces are screwed on, as occasion re- quires, at E, the eye end of the telescope. The shorter of the two astronomical eye-tubes which accompany this tele- scope produces the highest magnifying power. For adjust- ing the telescope to distinct vision, there is a brass knob or button at a, which moves a piece of rack-work connected with the eye-tube, which must be turned either one way or the other till the object appears distinctly, and different eyes fre- quently require a different adjustment. Fig. 58 represents a 5 feet telescope fitted up for astrono- TELESCOPE WITH EQUATORIAL MOTIONS. 193 mical observations. It is mounted on a mahogany stand, the three legs of which are made to close up together by means of the brass frame aaa, which is composed of three bars, Figure 58. connected with three joints in the centre, and three other joints, connected with the three mahogany bars. It is furnished with an apparatus for equatorial notions. The brass pin is made to move round in the brass socket 6, and may be tight- ened by means of the finger-screw d, when the telescope is directed nearly to the object intended to be viewed. This socket may be set perpendicular to the horizon, or to any other required angle; and the quantity of the angle is ascertained by the divided arc, and the instrument made fast in that posi- tion by the screw e. If this socket be set to the latitude of the place of observation, and the plane of this arc be turned so as to be in the plane of the meridian, the socket b being fixed to the inclination of the pole of the earth, the telescope, when turned in this socket, will have an equatorial motion, so that celestial objects may be always kept in view when this equatorial motion is performed. The two handles at k are connected with rack-work, intended to move the telescope in any required direction. The two sets of brass sliding rods, 192 TELESCOPE ON DOLLOND’S STAND. 2 2, are intended to render the telescope as steady as possible and to elevate and depress it at pleasure, and are so con- structed as to slide into each other with the utmost ease. The finder is placed at A E, either on the top or the left side of the tube of the telescope. When high magnifying powers are applied to any telescope, it is.sometimes difficult, on account of the smallness of the field of view, to direct the main tube of the telescope to the object. But the finder, which is a telescope with a small power, and consequently has a large field of view, when directed to any object, it is easily found, and being brought to the centre of the field, where two cross-hairs intersect each other, it will then be seen in the larger telescope. B is the eye-tube for terrestrial ob- jects, containing 4 glasses, and C one of the astronomical eye- pieces. A socket is represented at g, containing a stained glass, which is screwed to any of the eyepieces, to protect Figure 59. CURVATURE OF ACHROMATIC LENS. 193 the eye from the glare of light, when viewing the spots of the sun. The brass nut above f is intended for the adjustment of the eyepiece to distinct vision. 'The 3% feet telescope is sometimes mounted in this form. | Fig. 59 represents a 5 or 6 feet telescope, mounted ona stand of a new construction by Dollond. It possesses the ad- vantage of supporting the telescope in two places, which ren- ders it extremely steady, a property of great importance when viewing celestial objects with high magnifying powers. It possesses, likewise, the advantage of enabling the observer to continue seated at the same height from the floor, although the telescope be raised to any altitude, the elevation being en- tirely at the ebject end, although it may be changed from the horizon to the zenith. The framework is composed of bars of mahogany, and rests on three castors, two of which are made fast to their respective legs in the usual way, and the third stands under the middle of the lower horizontal bar that connects the two opposite legs, so that the frame has all the advantages of a tripod. As it becomes very inconvenient to stoop to the eye end of a telescope when the altitude of an ob- ject is considerable, and the centre of motion at the middle of the tube, this construction of a stand serves to remedy such inconvenience. Proportions of Curvature of the Lenses which form. an Achromatic Object-glass. As some ingenious mechanics may feel a desire to attempt the construction of a compound achromatic object-glass, I shall here state some of the proportions of curvature of the concave and convex lenses which serve to guide opticians in their construction of achromatic instruments. ~ 'These propor- tions are various; and even when demonstrated to be mathe- matically correct, it is sometimes difficult to reduce them to practice, on account of the different powers of refraction and dispersion possessed by different discs of crown and flint glass, and of the difficulty of producing by mechanical means the exact curves which theory requires. The following table shows the radii of curvature of the different surfaces of the lenses necessary to form a double achromatic object-glass, it being supposed that the sine of refraction in the crown glass is as 1°528 to 1, and in the flint as 1-5735 to 1, the ratio of their dispersive powers being as 1 to 1524. It is also as- sumed that the curvatures of the concave lens are as 1 to 2, that is, that the one side of this lens is ground on a tool, the Vou. IX. 17 194 CURVATURE OF ACHROMATIC LENS. radius of which is double that of the other. The Ist columu expresses the compound focus of the object-glass in inches ; the 2d column states the radius of the anterior surface of the crown, and column 3d its posterior side. Column 4th ex- presses the radius of the anterior surface of the concave lens, and column 5th its posterior surface, which, it will be ob- served, is exactly double that of the other : Focus in | Radius of anterior | Radius of posterior | Radius ofanterior | Radius of posterior inches. | surface, convex. surface. surface, concave. surface. Inc. Dec. Inc. * Dec. "Ine. Dee. Inc. - Dee. 12 3 4 .652 4 .171 8. .342 24 6 9. .304 8 .342 16.684 30 n° 0 It 563 10.428 20° .856 36 9 IB: 1.956 12 .513 25 .027 48 12 18 .608 16.684 33.~ .369 60 15 23. .260 20 .856 Al 712 120 30 46.520 41 .712 83.424 From the above table it will be seen, that to construct, for example, a 30 inch compound object-glass, the radius of the anterior side of the crown must be 73 inches, and that of the posterior side 11°63 inches ; the radius of the anterior surface of the concave 10-428, and that of the posterior 20-856 inches. It may be proper to observe, that in these computations, the radius of the anterior surface of the concave is less than the posterior side of the convex, and consequently admits of its approach, without touching in the centre—a circumstance which always requires to be guarded against-in the combina- tion of achromatic glasses. The following table shows the radii of curvature of the lenses by a friple object-glass, calculated from formula de- duced by Dr. Robinson, of Edinburgh: pL Ae a RE es OE a a EE ST Ee ee ae i De Be eT Focal Convex lens of crown Concave lens of flint Convex lens of crown length. glass. glass. glass. Inches.| inc. Dec. Ine. Dec. Inc. Dec. Ine. Dec. Inc. Dec. | Inc. Dec. 6 54 3240 .36 6 64 9 6 .83 4 .56 4 .56 9. .54 pee! 92 12 2? 4.20 ea! bY aie ate Wg 12. 3 W220 t- wo 18 13°~ 67 go. 2 9.12 195 BS 19. .08 1392 24 18 -.33 12 (25 12. 25 25-50 250350) 2. 56 30 22. th 13.16 15:46 Shiv 479 dh oe 349 3 cue 36 27 433 18 ..25 18. 525 OO ard 38 .17 3. .84 42 31 x87 pk My?! 21 .28 44 .53 44 .53 | 4 A8 48 36 .42 24.333 24 ~.33 50 °.92 50: we 9B 1 sD <12 54 40 .96 at: 86 SB) soe 57 ~ 28 SY aie te. oo. 1b, 60 45 42 30.33 30> 230 63 .58 Bo. 00 | oD. 4 The following table contains the proportions of curvature said to be employed by the London opticians : e CURVATURE OF ACHROMATIC LENS, 195 eee Ect | conver otorym gas. eigefenthtesarteot] conver iis of row Inches.| Ime. Dec. | Inc. Dec. Inc. Dec. Inc. Dec. | Inc. Dec. 6 Dune 4 .49 3 Baas 4 49 9 5 .65 6 .74 221 65 6 .74 12 7 54 8 .99 6 .95 ‘in wwe. 8 .99 18 LILY .SU 13 .48 10.42 11 .30 13.48 24 15 .08 17 .98 13 -.90 15 .08 17 .98 36 22 .61 26. .96 20 .84 22 .61 26 .96 42 26 .38 3140 24e a 26 .38 = ae 4 48 30 .16 35 .96 2. OO 30 .16 35 .96 54 33.91 40 .45 STA 27 33.91 40 .45 60 37 .68 44 ,94 34 7 37 .68 | 44 .94 From this table it appears that the two convex lenses have the same radii of their respective sides, and that the concave flint lens has its two surfaces equally concave, so that a triple object-glass formed according to these proportions would re- quire only three pair of grinding tools. The following are the curves of the lenses of one of the best of Dollond’s achro- matic telescopes, the focal length of the compound object- glass being 46 inches. Reckoning from the surface next the object, the radii of the crown glass were 28 and 40 inches; the concave lens 20-9 inches, and the inner crown glass lens 28°4 and 28-4 inches. This telescope carried magnifying powers of from 100 to 200 times. Although I have inserted the above tables, which might, in some measure, guide an ingenious artist, yet, on the whole, a private amateur has little chance in succeeding in such at- tempts. The diversity of glasses, and the uncertainty of an unpractised workman’s producing the precise curvature he intends, is so great, that the object-glass, for the most part, turns out different from his expectations. The great difficulty in the construction is to find the exact proportion of the dis- persive powers of the crown and flint glass.. The crown is pretty constant, but there are hardly two pots of flint glass which have the same dispersive power. Even: if constant, it is difficult to measure it accurately; and an error in this greatly affects the instrument, because the focal distances of the lenses must be nearly as their.dispersive powers. In the two preceding tables, the sine of incidence in the crown glass is supposed to be to the sine of refraction as 1-526 to 1; and in the flint glass, as 1-604 to 1. Opticians who make great numbers of lenses, both of flint and crown glass, acquire, in time, a pretty good guess of the nature of the errors which may remain after they have finished an object-glass; and having many lenses intended to be of the same form, but unavoidably differing a little from it, they try several of the 196 CONSTRUCTION OF FLUID LENSES. concaves with the two convexes, and finding one better than the rest, they make use of it to complete the set. In this way some of the best achromatic telescopes are frequently formed. I have sometimes found, when supplying a concave flint glass to a telescope where it happened to be wanting, that, of four or five concave lenses which appeared to be the same as to curvature and other properties, only one was found to produce a distinct and colourless image. Should any one, however, wish to attempt the construction of an achromatic lens, the best way for preventing disappointments in the re- sult is to procure a variety of tables of the respective curva- tures founded on different conditions, and which, of course, require the surfaces of the several lenses to be of different curves. Having lenses of different radii at his command, and having glass of different refractive or dispersive powers, when one combination does not exactly suit, he may try another, and ultimately may succeed in constructing a good achromatic telescope ; for, in many cases, it has been found that chance, or a happy combination of lenses by trial, has led to the for- mation of an excellent object-glass. Achromatic Telescopes composed of fluid Lenses. The best achromatic telescopes, when minutely examined, are found to be in some respects defective, on account of that slight degree of colour which, by the aberration of the rays, they give to objects, unless the object-glass be of small di- ameter. When we examine with attention a good achromatic telescope, we find that it does not show white or luminous objects perfectly free from colour, their edges being tinged on one side with a claret-coloured fringe, and on the other with a green fringe. This telescope, therefore, required farther improvement, to get rid of these secondary colours, and Father Boscovieh, to whom every branch of optics is much indebted, displayed much ingenuity in his attempts to attain this ob- ject. But it is to Dr. Blair, professor of astronomy in Edin- burgh, that we are chiefly indebted for the first successful experiments by which this end was accomplished. By a judicious set of experiments, he proved that the quality of dispersing the rays in a greater degree than crown glass is not confined to a few mediums, but is possessed by a great variety of fluids, and by some of these in a most extraordinary degree. Having observed that when the extreme red and violet rays were perfectly united, the green were left out, he conceived the idea of making an achromatic concave lens which should refract the green less than the united red and CONSTRUCTION OF FLUID LENSES. 197 violet, and an achromatic convex lens which should do the same; and as the concave lens refracted the outstanding green fo the axis, while the concave one refracted them from the axis, it followed that, by a combination of these two op posite effects, the green would be united with the red and violet. By means of an ingenious prismatic apparatus, he ex- amined the optical properties of a great variety of fluids. The solutions of metals and semi-metals proved in all cases more dispersive than crown glass. Some of the salts, such as sal ammoniac, greatly increased the dispersive power of water. The marine acid disperses very considerably, and this quality increases with its strength. The most dispersive fluids were accordingly found to be those in which this acid and the metals were combined. ‘The chemical preparation called causticum antimoniale, or butter of antimony, in its most concentrated state, when it has just attracted sufficient humidity to render it fluid, possesses the quality of dispers- ing the rays in an astonishing degree. ‘lhe great quantity of the semi-metal retained in solution, and the highly concen- trated state of the marine acid, are considered as the cause of this striking effect. Corrosive sublimate of mercury, added to a solution of sal ammoniacum in water, possesses the next place to the butter of antimony among the dispersive fluids which Dr. Blair examined. ‘The essential oils were found to hold the next rank to metallic solutions among fluids which possess the dispersive quality, particularly those obtained from bituminous minerals, as native petrolea, pit coal, and amber. The dispersive power of the essen- tial oil of sassafras, and the essential oil of lemons, when genuine, were found to be not much inferior to any of these. But of all the fluids fitted for optical purposes, Dr. Blair. found that the muriatic acid mixed with a metallic solution, or, in other words, a fluid in which the marine acid and metal- line particles hold a due proportion, most accurately suited his purpose. In a spec- trum formed by this fluid, the green were among the most refrangible rays ; and when its dispersion was corrected by that of glass, there was produced an inverted secondary spectrum, that is, one in which the green was above, when it would have been be- low with a common medium. He therefore 17* Figure 60. 198 BARLOW’S FLUID LENS. placed a concave lens of muriatic acid with a metallic solu- tion between the two lenses, as in fic. 60, where A B is the concave fluid lens, C F a plano-convex lens, with its plane side next the object, and E D a meniscus.. With this object- glass the rays of different colours were bent from their recti- lineal course with the same equality and regularity as in reflection. Telescopes constructed with such object-glasses were ex- amined by the late Dr. Robison and Professor Playfair. The focal distance of the object-glass of one of these did not exceed 17 inches, and yet it bore an aperture of 33 inches. They viewed some single and double stars and some common objects with this telescope, and found that, in magnifying power, brightness, and distinctness, it was manifestly supe- rior to one of Mr. Dollond’s of 42 inches focal length. ‘They had most distinct vision of a star, when using an erecting eyepiece, which made this telescope magnify more than 100 times, and they found the field of vision as uniformly distinct as with Dollond’s 42-inch telescope, magnifying 46 times, and were led to admire the nice figuring and centring of the very deep eyeglasses which were necessary for this amplifi- cation. They saw double stars with a. degree of perfection which astonished them. These telescopes, however, have never yet come into general use; and one reason, perhaps, is, that they are much more apt to be deranged than tele- scopes constructed of object-glasses which are solid. If any species of glass, or other solid transparent substance could be found with the same optical properties, instruments might ' perhaps be constructed of a larger size, and considerably superior to. our best achromatic telescopes.* It is said that Mr. Blair, the son of Dr. Blair, some years ago engaged in prosecuting his father’s views, but I have not heard any thing respecting the result of his investigations. Barlow’s Refracting Telescope with a fiuid concave Lens. Professor Barlow, not many years ago, suggested. a new fluid telescope, which is deserving of attention, and about the year 1829 constructed one of pretty Jarge dimensions. The fluid he employs for this purpose is the sulphuret of carbon, which he found to be a substance which possessed every * For a more particular account of Dr. Blair’s instruments and experi- ments, the reader is referred to his Dissertation on this subject in vol. ii. of the ‘Transactions of the Royal Society of Edinburgh,”’ which occu- pies 76 pages, or to Nicholson’s ‘‘ Journal of Natural Philosophy,’’ &c., quarto series, vol. i, April—September, 1797. _ BARLOW’S FLUID LENS. 199 requisite he could desire. Its index is nearly the same as that of the best flint glass, with a dispersive power more than double. It is perfectly colourless, beautifully transparent, and, although very expansible, possesses the same, or very nearly the same, optical properties under all circumstances “to which it is likely to be exposed in astronomical observa- tions, except, perhaps, direct observations on the solar disc, which will probably be found inadmissible. Mr. Barlow first constructed an object-glass with this fluid of 3 inches aper- ture, with which he could see the small star in Polaris with a power of 46, and with the higher powers several stars which are’ considered to require a good telescope, for exam- ple, 70, p Ophinchi, 39 Bootis, the quadruple star « Lyre, ¢ Aquarii, « Herculis, &c. He next constructed a 6-inch object-glass. With this instrument, the small Figure 61. —_ star in Polaris is so distinct and brilliant, with a power of 143, that its transit might be taken with the utmost certainty. As the mode of constructing these telescopes is somewhat novel, it may be expedient to enter somewhat into detail. In the usual construction of achromatic tele- scopes, the two or three lenses composing the object-class are brought into immediate contact ; and in the fluid telescope of Dr. Blair, the con- struction was the same, the fluid having been enclosed in the object-glass itself. But in Mr. Barlow’s telescope, the fluid correcting: lens is placed at a distance from the plate lens equal to half its focal leneth; and it might be carried still farther back, and yet possess dispersive power to render the object-glass achromatic. By this means, the fluid lens, which is the most difficult part of the construction, is re- duced to one-half, or to less than one-half of the size of the plate lens; consequently, to construct EP, a telescope of 10 or 12 inches aperture involves ‘1! no greater difficulty in the manipulation than in making a telescope of the usual description of 5 or 6 inches aperture, except in the simple plate lens itself; and, hence, a telescope of this kind of 10 or 12 feet length will be equivalent in its focal power to one of 16 or 20 feet. By this means the tube may be shortened several feet, and yet possess a focal power more consi aamacaneoocee= eee eee Se ae me ee ee -- =.= S oo bit... 200 BARLOW 8 FLUID LENS. derable than could be conveniently given to it on the usua principle of construction. This. will be better understoo” from the preceding diagram (fig. 61). | In this figure A BC D represent the tube of the 6-inch telescope, C D the plate object-glass, F the first focus of rays, d e the fluid concave lens, distant from the former 24 inches; - the focal length M F being 48, and, consequently, as 48 : 6 : : 24: 3 inches, the diameter of the fluid lens. ‘The result- ing compound focus is 62°5 inches. It is obvious, therefore, that the rays df, ef, arrive at the focus under the same con- vergency, and with the same light as if they proceeded from a lens of 6 inches diameter, placed at a distance beyond the object-glass C D, (as G H,) determined by producing those rays till they meet the sides of the tube in G H, namely, at 625 inches beyond the fluid lens. Hence, it is obvious, the rays will converge as they would do from an object-glass, GH, of the usual kind with a focus of 10 feet 5 inches. We have thus, therefore, shortened the tube 38-5 inches, or have at least the advantage of a focus 38-5 inches longer than our tube ; and the same principle may be carried much farther, so as to reduce the usual length of refracting telescopes nearly one-half, without increasing the aberration in the first glass beyond the least that can possibly belong to a telescope of the usual kind of the whole length. It should likewise be observed, that the adjustment for focus may be made either in the usual way or by a slight movement of the fluid lens, as in the Gregorian Reflectors, by means of the small spec- ulum. Mr. Barlow afterward constructed another larger telescope on the same principle, the clear aperture of which is 7.8 inches. Its tube is 11 feet, which, together with the eye- piece, makes the whole length 12 feet, but its effective focus is, on the principle stated above, 18 feet. It carries a power of 700 on the closest double stars in South’s and Herschel’s catalocue, and the stars are, with that power, round and de- fined, although the field is not then so bright as could be desired.. The telescope is mounted on a revolving stand, which works with considerable accuracy as an azimuth and altitude instrument.. To give steadiness to the stand, it has been made substantial and heavy, its weight by estimation being 400 pounds, and that of the telescope 130 pounds, yet its motions are so smooth, and the power so arranged, that it ray be managed by one person with the greatest ease, the star being followed by a slight touch, scarcely exceeding that of the keys of a piano-forte. The focal length of the plate BARLOW’S FLUID LENS. 201 lens is 78 inches, and of the fluid lens 59-8 inches; which, at the distance of 40 inches, produce a focal length of 104 inches, a total length of 12 feet, and an equivalent focus of 18 feet. The curves of the parallel meniscus checks for containing the fluid are 30 inches and 144 inches, the latter towards the eye. The curves for the plate lens are 56:4 and 144. There is an interior tube 5 inches diameter, and 3 feet 6 inches long, which carries the cell in which the fluid is en- closed, and an apparatus by which it may be moved back- ward and forward, so that the proper adjustment may be made for colour in the first instance, and afterward the focus is obtained by the usual rack-work motion. The following is the mode by which the fluid was enclosed. After the best position has been determined practically for the checks forming the fluid lens, these, with the ring between them, ground and polished accurately to the same curves, are ap- plied together, and taken into an artificial high temperature, exceeding the greatest at which the telescope is ever ex- pected to be used. After remaining here with the fluid some time, the space between the glasses is completely filled, im- mediately closed, cooled down by evaporation and removed into a lower temperature. By this means a sudden conden- sation takes place, an external pressure is brought on the checks, and a bubble formed inside, which is, of course, filled with the vapour of the fluid; the excess of the atmospheric pressure beyond that of the vapour being afterward always acting externally to prevent contact. ‘The extreme edges are then sealed with the serum of human blood, or by strong fish-glue, and some thin, pliable metal surface. By this pro- cess Mr. Barlow says, “I have every reason to believe the lens becomes as durable as any lens of solid glass. At all events, I have the satisfaction of stating, that my first 3-inch telescope has now been completed more than fifteen months, and that no change whatever has taken place in its perform- ance, nor the least perceptible alteration. either in the quan tity or the quality of the fluid. The following are some of the observations which have been made with this telescope, and the tests to which it has been subjected. ‘The very small star which accompanies the pole staris generally one of the first tests applied to tele- scopes. This small point of light appeared brilliant and dis- tinct ; it was best seen with a power of 120, but was visible with a power of 700. The small star in Aldebaran was very distinct with a power of 120. The small star a Lyre, was distinctly visible with the same power. ‘The small star called 202 BARLOW’S FLUID LENS. by Sir J. Herschel Debilissima, between 4 ¢ and 5 Lyre whose existence, he says, could not be suspected in either the 5 or 7 feet equatorial, and invisible also with the 7 and 10 feet reflectors of six and nine inches aperture, but seen double with the 20 feet reflector, is seen very satisfactorily double with this telescope. » Persei, marked as double in South and Herschel’s catalogue, at the distance of 28’, with another small star at the distance of 3’ 67’’, is seen distinctly sixfold, four of the small stars being within a considerably less distance than the remote one of 7 marked in the catalogue. And, rejecting the remote star, the principal and the four other stars form a miniature representation of Jupiter and his satellites, three of them being’ nearly in a line. on one side, and the other on the opposite, Castor is distinctly double with 120, and well opened, and stars perfectly round with 360 and 700. y Leonis and a Piscium are seen with the same powers equally round and distinct. Ine Bootis, the small star is well separated from. the larger, and its blue colour well marked with a power of 360. 1 Corone Borealis is seen double with a power of 360 and 700. 52 Orionis, ¢ Orionis, and others of the same class, are also well defined with the same powers. In regard to the planets which hap- pened to be visible, Venus appeared beautifully white and well defined with a power of 120, but showed some colour with 360. Saturn, with the 120 power, is a very brilliant object, the double rmg and belts being well and satisfactorily defined, and with the 360 power it is still very fine. The moon also is remarkably beautiful, the edges and the sha- dows being well marked, while the quantity of light is such as to brmg to view every minute distinction of figure and shade. The principal objections that may be made to this con- struction of a telescope are such as these: Can the fluid be permanently secured? Will it preserve its transparency and other optical properties? Will it not act upon the sur- face of the glass and partially destroy it? &c. To such in- quiries Mr. Barlow replies, that experience ‘is the only test we have; our spirit levels, spirit thermometers, &c., show that some fluids, at least, may be preserved for many years without experiencing any change, and without producing any in the appearance of the glass tubes containing them, But should any of these happen, except the last, nothing can be more simple than to supply the means of replacing the fluid at any time, and by any person, without disturbing the adjustment of the telescope. He expresses his hope that, | ROGERS’S ACHROMATIC TELESCOPE. 203 should these experiments be prosecuted, an achromatic tele- scope shall ultimately be produced which shall exceed in aperture and power any instruments of the kind hitherto at- tempted. Ifthe prejudice against the use of fluids could be removed, he feels convinced that well-directed practice would soon lead to the construction of the most perfect instruments, on this principle, at a comparatively small expense. “Iam convinced,”’ he says, “judging from what has been paid for large object-glasses, that my telescope, telescope stand, and the building for observation, with every other requisite con- venience, have been constructed for a less sum than would be demanded for the object-glass only, if one could be pro- duced of the same diameter of plate and flint glass; and this is a consideration which should have some weight, and encourage a perseverance in the principle of construction.”’* ROGERS’S ACHROMATIC TELESCOPE ON A NEW PLAN. The object of this construction is to render a small disc of flint glass available to perform the office of compensation to a much larger one of crown glass, and thus to render possible the construction of telescopes of much larger aperture than are now common, without hinderance from the difficulty at present experienced in procuring large discs of flint glass. It is well known to those who are acquainted with telescopes, that in the construction of an ordinary achromatic object-glass, in which a single crown lens is compensated by a single one of flint, the two lenses admit of being separated only by an interval too small to afford any material advantage, in dimin- ishing the diameter of the flint lens, by placing it in a nar- rower part of the cone of rays, the actual amount of their difference in point of dispersive power being such as to ren- der the correction of the chromatic aberration impossible when their mutual distance exceeds a certain limit. This inconvenience Mr. Rogers proposes to obviate by employing, as a correcting lens, not a single lens of flint, but a compound one consisting of a convex crown and concave flint, whose foci are such as to cause their combination to act as a plain glass on the mean refrangible rays. ‘Then it is evident that * A more detailed account of the processes connected with the con- struction of this telescope will be found in a paper presented to the Royal Society in 1827, and published in the Philosophical Transactions of that Society for 1828, and likewise another paper, published in the Transactions for 1829. From these documents chiefly, the precedin account has been abridged. See also the ‘‘ Edinburgh New Philosophica Journal’’ for Jan.—April, 1828, and Brewster’s “ Edinburgh Journal ef Science’’ for October, 1829. 904 ROGERS’S ACHROMATIC. TELESCOPE. by means of the greater dispersive power of flint than of crown glass, this will act as a concave on the violet, and asa convex on the red rays, and that the more powerfully ac- cording as the lenses separately have greater powers or cur- vature. If, then, such a compound lens be interposed be- tween the object-glass of a telescope—supposed to be a single lens of plate or crown glass—and its focus, it will cause no alteration in the focus for mean rays, while it will lengthen the focus for violet, and shorten it for red rays. Now this is precisely what is wanted to produce an achromatic union of all the rays in the focus; and as nothing in this construction limits the powers of the individual correcting lenses, they may therefore be applied anywhere that convenience may dictate ; and thus, theoretically speaking, a disc of flint glass, however small, may be made to correct the colour of one of crown, however large. This construction likewise possesses other and very re- markable advantages: for, first, when the correcting lens is approximately constructed on a calculation founded on its intended aperture, and on the refractive and dispersive in- dices of its materials, the final and complete dispersion of colour may be effected, not by altering the lenses by grinding them anew, but by shifting the combination nearer to, or far- ther from the object-glass, as occasion may require, along the tube of a telescope, by a screw motion, till the condition of achromaticity is satisfied-in the best manner possible; and, secondly, the spherical aberration may in like manner be finally corrected, by slightly separating the lenses of the cor- recting glass, whose surfaces should for this purpose be figured to curvatures previously determined by calculation, to admit of this mode of correction—a condition which Mr. Rogers finds to be always possible. The following isthe rule which he lays down for the determination of the foci of the lenses of the correcting glass: ‘The focal length of either lens 1s to that of the object-glass in a ratio compounded of the ratio of the square of the aperture of the correcting lens to that of the object-glass, and of the ratio of the difference of the dispersive indices of the crown and flint glass to the dispersive index of crown.” For example, to correct the colour of a lens of crown or plate glass of 9 inches aperture and 14 feet focal length (the dimensions of the telescope of Fraunhofer at Dor- pat) by a disc of flint glass 3 inches in diameter, the focus of either lens of the correcting lens will require to be about 9 inches. ‘To correct it by a 4-inch dise will require a focus of about 16 inches each. MR. WILSON’S TELESCOPE. 205 Mr. Rogers remarks, that it is not indispensable to make the correcting glass act as a plane lens.- It is sufficient if it be so adjusted as to have a shorter focus for red rays than for violet. If, preserving this condition, it be made to act as a concave lens, the advantage procured by Mr. Barlow’s con- struction of reducing the length of the telescope with the same focal power is secured, and he considers, moreover, that by a proper adaptation of the distances, foci, &c., of the lenses, we might hope to combine with all these advantages that of the destruction of the secondary spectrum, and thus obtain a perfect telescope. , The above is an abstract of a paper read to the “ Astrono- mical Society of London” in April, 1828, by A. Rogers, Esq. The reader will easily perceive that the principle on which Mr. Rogers proposes to construct his telescope is very nearly similar to that of Professor Barlow, described above, with this difference, that the correcting lens of the professor’s telescope is composed of atransparent fluid, while that of Mr. Rogers is a solid lens consisting of a convex crown and concave flint. The general object intended to be accomplished by both is the same, namely, to make a correcting lens of a compara- tively small diameter serve the purpose of a large disc of flint glass, which has hitherto been very expensive, and very diffi- cult to be procured ; and likewise to reduce the length of the telescope, while the advantage of a long focal power is se- cured. A telescope on this principle was constructed seven or eight years ago by Mr. Wilson, lecturer on Philosophy and Chemistry, Glasgow, before he was aware that Mr. Rogers had proposed a similar plan. I have had an opportunity of particularly inspecting Mr. Wilson’s telescope, and trying its effects on terrestrial objects with high powers, and was, on the whole, highly pleased with its performance. It appeared to be almost perfectly achromatic, and produced a distinct and well-defined image of minute distant objects, such as small letters on signposts, at two, three, and four miles distant; but. I had no opportunity of trying its effects on double stars ‘or any other celestial objects. ‘The instrument is above 6 feet long, the object-lens is a plano-convex of crown glass, 4 feet focal distance and 4 inches diameter, the plain side next the - object. At 26 inches distant from the object-lens is the compound lens of 2 inches in diameter; and the two Jenses of which it is composed are both ground to a radius of 33 inches. That made of crown glass is plano-convez, the other, made of flint glass, is plano-concave, and are placed close together, the Von. IX. 18 206 REFLECTING TELESCOPES. convex side being next to the object, and the concave side next the eye. _ The greater refractive power of the flint. glass renders the compound one slightly concave in its effect, (although the radius of curvature is similar in both,) and lengthens the focus to 6 feet from the object-glass; and this is consequently the length of the instrument. The compound corrector so placed intercepts all those rays which go to form the image in the field of view, producing there an achromatic , image. ‘The concave power of the corrector renders the image larger than if directly produced by a convex lens of the same focus. The concavity of the corrector is valuable also in this respect, that a very slight alteration in its distance from the object-glass changes the focal distance much more than if it were plain, and enables us to adjust the instrument to perfect achromatism with great precision. & CHAPTER V. ON REFLECTING TELESCOPES. SECT. I.—-HISTORY OF THE INVENTION, AND A GENERAL DE- SCRIPTION OF THE CONSTRUCTION OF THESE INSTRUMENTS. Rerieectine telescopes are.those which represent the images of distant objects by reflection, chiefly from concave mirrors. Before the achromatic telescope was invented there were two glaring imperfections in refracting telescopes, which the astronomers of the seventeenth century were anxious to cor- rect. The first was its very great lenoth when a high power was to be applied, which rendered it very unwieldy and diffi- cult to use. The second imperfection was the incorrectness of the image as formed by a single lens. Mathematicians had demonstrated that a pencil of rays could not be collected in a single point by a spherical lens, and also that the image transmitted by such a lens would be in some degree incur- vated. After several attempts had been made to correct this imperfection by grinding lenses to the figure of one of the conic sections, Sir I. Newton happened to commence an ex- amination of the colours formed by a prism; and having, by the means of this simple instrument, discovered the different refrangibility of the rays of light—to which we have several] GREGORY'S SUGGESTIONS. 207 times adverted in the preceding descriptions—he then per- ceived that the errors of telescopes, arising from that cause alone, were some hundred times greater than such as were occasioned by the spherical figure of lenses, which induced this illustrious philosopher to turn his attention to the im- provement of telescopes by reflection. It is generally supposed that Mr. James Gregory—a son of the Rev. John Gregory, minister of Drumoak, in the county of Aberdeen—was the first who suggested the construction of a reflecting telescope. He was a young man of uncommon genius, and an eminent mathematician; and in the year 1663, at the age of only 24, he published in London his trea- tise entitled “ Optica Promota,’”” in which he explained the theory of that species of reflecting telescope which still bears his name, and which he stated as being his own invention. ‘But as Gregory, according to his own account, was endowed with no mechanical dexterity, and could find no workman capable of realizing his invention, after some fruitless attempts to form proper specula, he was obliged to give up the pur- suit, so that this telescope remained for a considerable time neglected. It was several years after Gregory suggested the construction of reflecting telescopes before Newton directed his attention fully to the subject. In a letter directed to the secretary of the Royal Society, dated in February, 1672, he says, “ Finding reflections to be regular, so that the angle of reflection of all sorts of rays was equal to the angle of inci- dence, I understood that, by their mediation, optic instruments might be brought to any degree of perfection imaginable, providing a reflecting substance could be found which would polish as finely as glass, and reflect as much light as glass transmits, and the art of communicating to it a parabolic figure be also obtained. Amid these thoughts I was forced from Cambridge by the intervening plague, and it was more than two years before I proceeded farther.” It was towards the end of 1668, or m the beginning of the following year, when Newton, being obliged to have recourse to reflectors, and not relying on any artificer for making the specula, set about the work himself, and early in the year 1672 completed two small reflecting telescopes. In these he ground the great speculum into a spherical concave, although he approved of the parabolic form, but found himself unable to accomplish it. ‘hese telescopes were of a construction somewhat different from what Gregory had suggested, and though only 6 inches long, were considered as equal to a 6 feet common refracting telescope. It is not a little singular. 208 NEWTON’S REFLECTORS. however, that we hear no more about the construction of. re- flectors till more than half a century afterward. It was not till the year 1723 that any reflectors were known to have been made, adapted to celestial observations. In that year, Mr. Hadley, the inventor of the reflecting quadrant which goes by his name, published, in No. 376 of the Philosophical Transactions, an account of a large reflector on Newton’s plan, which he had just then constructed, the performance “of which left no room to doubt that this invention would remain any longer in obscurity. The large speculum of this instrument was 62g inches focal distance and 5 inches dia- meter, was furnished with magnifying powers of from 190 to 230 times, and equalled in performance the famous aerial telescope of Huygens of 123 feet in length.* Since this pe- riod the reflecting telescope has been in general use among astronomers in most countries of Europe, and has received numerous improvements, under the direction of Short, Mudge, Edwards, and Herschel, the last of whom constructed reflec- tors of '7, 10, 20, and even 40 feet in focal length, which far surpassed, in brightness and magnifying power, all the in- struments of this description which had previously been attempted. I shall now proceed to give a brief sketch of the nature of a reflecting telescope, and the different forms in which they have been proposed to be constructed. Fig. 62 represents the reflecting telescope as originally proposed by Gregory. A BE F represents a tube open at A.F towards the object ; at the other end is placed a concave speculum, B E, with a hole, C D, in its centre, the focus of which is at e. A little beyond this focus, towards the object end of the telescope, A F, is placed another small concave mirror, G, having its polished face turned towards the great speculum, and is supported by an arm, G H, fastened to a slider connected with the tube. At the end of the great tube, B E, is screwed in a small tube, C D K I, containing a small plano-convex lens, I K. Such are the essential parts of this instrument and their relative positions. It will be recollected in our description of the properties of concave mirrors, (see p. 78,) that, when rays proceed from a distant object, and fall upon a concave speculum, they paint an image or representation of the object in its focus before the o * A particular description of this telescope, with the machinery for moving it, illustrated with an engraving, may be seen in Reid and Gray’s ‘*‘ Abridgment of the Philosophical Transactions,”’ vol. vi., part 1. for 1723, p. 147—152. Lo THE GREGORIAN TELESCOPE. 209 os speculum. Now suppose two parallel rays, a d, falling on the speculum B E, in c d; they are reflected to its focus e, where an inverted image of the object is formed at a little more than the focal distance of the small speculum from its surface, and serves, as it were, for an object on which the small mirror may act. By the action of this mirror this first image is reflected to a point about f, where a second image is formed very large and erect. This image is magnified in the proportion of fG to e G, the rays from which are trans- mitted to the eyeglass I K, through which the eye perceives the object clear and distinct, after the proper adjustments have been made. Figure 66. Fig. 63. “7D Fig. 64. Fig. 65. -——=---- Suppose the focal distance of the great mirror was 9 inches, and the focal distance of the small mirror 14 inch—were we to remove the eyepiece of this telescope, and look through the hole of the great mirror, we should see the image of the object depicted upon the face of the small speculum, and magnified in the proportion of 9 to 13, or 6 times, on the same principle as a common convex object-glass 9 inches focal length, with an eyeglass whose focus is 13 inch, mag- nifies 6 times. This may be regarded as the first part of the magnifying power. If, now, we suppose the small speculum 18* 210 THE CASSEGRAINIAN REFLECTOR. placed a little more than 12 inch from the image formed by the great speculum, a second image is formed about f, as much exceeding the first in its dimensions as it exceeds it in distance from the small speculum, on the principle on whic the object-glass of a compound ‘microscope forms a large image near the eyeglass. Suppose this distance to be 9 times greater, then the whole magnifying power will be compounded of 6 multiplied by 9, or 54 times. As a tele- scope it magnifies 6 times, and in the microscope part 9 times. Such is a general idea of the Gregorian telescope, the minute particulars and structure of which can only be clearly perceived by a direct inspection of the instrument. The Newtonian Refiector.—This instrument is somewhat different both in its form and in its mode of operation from that of Gregory. It is represented in fig. 63, where BA EF is the tube, and B E the object concave mirror, which reflects the parallel rays ab to a plane speculum G, placed 45°, or half a right angle to the axis of the concave speculum. This small plane reflector must be of an oval form; the length of the oval should be to the breadth as 7 to 5, on account of the - obliquity of its position. It is supported on an arm fixed to the side of the tube; an eyeglass is placed in a small tube, movable in the larger tube, so as to be perpendicular to the axis of the large reflector, the perpendicular line passing through the centre of the small mirror. The small mirror is situated between the large mirror and its focus, that its dis- tance from this focal pomt may be equal to the distance from the centre of the mirror to the focus of the eyeglass. When the rays a 0 from a distant object fall upon the large specu- lum at c d, they are reflected towards a focus at hs but, being intercepted by the- plane mirror G, they are reflected perpendicularly to the eyeglass at I, in the side of the tube, and the image formed near that position at ¢ is viewed through a small plano-convex lens. The magnifying power of this telescope is in the proportion of the focal distance of the spe- culum to that of the eyeglass. Thus, if the focal distance of the speculum be 36 inches, and that of the eyeglass one-third of an inch, the magnifying power will be 108 times. It was this form of the reflecting telescope that Newton invented, which Sir W. Herschel adopted, and with which he made most of his observations and discoveries. The Cassegrainian Reflector.—This mode of the reflecting telescope, suggested by M. Cassegrain, a Frenchman, is re- presented in fig. 64. It is constructed in the same way as HOOK’S AND MARTIN’S REFLECTORS. 211 the Gregorian, with the exception of a small convex speculum, G, being substituted in the room of the small concave in Gre- gory’s construction. As the focus of a convex mirror is negative, it is placed at a distance from the large speculum gun to the difference of their foci; that is, if the focal length of the large speculum be 18 inches, and that of the small convex 2 inches, they are placed at 16 inches distant from each other, on a principle similar to that of the Galilean tele- scope, in which the concave eyeglass is placed within the focus of the object-glass by a space equal to the focal length of the eyeglass. In this telescope, likewise, instead of two there is only one image formed, namely, that in the focus of the eyeglass; and, on this account, some are of opinion that the distinctness is considerably greater than in the Gregorian. Mr, Ramsden was of opinion that this construction is prefer- able to either of the former reflectors, because the aberrations of the two metals have a tendency to correct each other, whereas in the Gregorian, both the metals being concave, any error in the specula will be doubled. It is his opinion that the aberrations in the Cassegrainian construction to that of the Gregorian is as 3 to.5, The length of this telescope is shorter than that of a Gregorian of equal focal length by twice the focal length of the small mirror, and it shows every thing in.an inverted position, and, consequently, is not adapted for viewing terrestrial objects. Dr. Hook’s Reflector.—Before the reflecting telescope was much known, Dr. Hook contrived one, the form of which is represented fig. 65, which differs in little or nothing from the Gregorian, except that the eyeglass, I, is placed in the hole of the great speculum, B E. Martin's Reflector.—Mr. Benjamin Martin, a distinguished writer on optical and philosophical science, about a century ago described a new form of the reflecting telescope, approxi- mating to the Newtonian structure, which he contrived for his own use. It is represented in fig. 66. A BEF is the tube, in which there is an opening or aperture, O P, in. the upper part. Against this hole, within the tube, is placed a large plane speculum, GH, at half a right angle with the axis or sides of the tubes, with a hole, C D, perforated through its middle. The parallel rays a}, falling on the inclined plane GH, are reflected perpendicularly and parallel on the great speculum B E in the bottom of the tube. From thence they are reflected, converging to a focus, e, through the hole of the plane mirror C D. which being also the focus of the 22 POWER OF TELESCOPES. eyeglass I K, the eye will perceive the object magnified and distinct. In the figures referred to-in the above descriptions, only one eyeglass is represented, to avoid complexity; but in most reflecting telescopes, the eyepiece consists of a combina- tion of two plano-convex glasses, as in fig. 67, which pro- Figure 67. duces a more correct and a larger field of view than a single lens. This combination is generally known by the name of the Huygenian Eyepiece, which shall be described in the section on the eyepieces of telescopes. The following rule has been given for finding the magnify- ing power of the Gregorian telescope: Multiply the focal distance of the great mirror by the distance of the small mir- ror from the image next the eye, and multiply the focal dis- tance of the small mirror by the focal distance of the eyeglass ; then divide the product of the former multiplication by the product of the latter, and the quotient will express the magni- fying power. The following are the dimensions of one of the reflecting telescopes constructed by Mr. Short, who was long distinguished as the most eminent maker of such instruments on a large scale, and whose large reflectors are still to be found in various observatories throughout Europe: The focal distance of the great mirror, 9.6inches; or P m, fig. 67, its breadth, F D, 2.3; the focal distance of the small mirror, Ln, 1.5, or 13 inch; its breadth, g A, 0.6, or =$,ths of an inch; the breadth of the hole in the great mirror, U V, 0.5, or half an inch; the distance between the small mirror and the next eyeglass, L R, 14.2; the distance between the two eyeglasses, S R, 2.4; the focal distance of the eyeglass next the metal, 3.8; and the focal distance of the eyeglass next the eye, S a, 1.1, or =4,th ofan inch. The magnifying power of this telescope was about 60 times. Taking this telescope as a standard, the following table of the dimensions and magnifying powers of Gregorian reflecting telescopes, as constructed by Mr. Short, has been computed : MR. SHORT. 213 | S F a £; |48 | 24.1 8. |48 . | ss ee & 5 o8 la*.| ye | sf | ge S 4 aa ° ag Sgn hfe = 9 oq E 2-8 g. |) 8 g 28 |baS'| ge | 82: | Be 2 [Bes ak wa) Sa, 2's 8 Ere] aS 5° ae ae sé |= si | Se oko By.) Be lee d Bees Sa} a. | 28 | 43 °|e82| 23 | 32) 28, 1.8 [S23 ge] 36 | 88 | 32 |S22| 2] 33 |$e21 & | S82 6 | sa | 2s | 58 |482| £8 | 83 }488 1-8 | Az Pm DF L aU VIL RR S |R 8S Sals#arg§ansalfaps al Garay a 5 .65| 1.54) 1 .10) O .31| 8 .54| 2 .44/ 0 81] 1.68) 39 | 0 41 9 .60| 2 .30, 1 .50) O .39/14 61} 3.13] 1 .04| 2.09} 60 | 0.52 15 .50| 3.30 2 .14| 0.50/23 .81| 3 .94} 1.31) 2.63} 86 | 0 .66 36 .00| 6 .26| 3 .43| 0 .65/41 .16| 5 .12] 1.71] 3.41], 165 | 0 .85 60 .001 9 .21) 5 .00; 0.85/68 .17| 6 .43] 2.14] 4 .28] 243 | 1,07 Mr. Short—who was born in Edinburgh in 1710, and died near London, 1768—was considered as the most accurate constructor of reflecting telescopes during the period which intervened from 1732 to 1768. In 1743 he constructed a reflector for Lord Thomas Spencer, of 12 feet focal length, for which he received 600 guineas. He made several other telescopes of the same focal distance, with greater improve- ments and higher magnifiers; and, in 1752, finished one for the King of Spain, for which, with its whole apparatus, he received £1200. This was considered the noblest instrument of its kind that had then been constructed, and perhaps it was never surpassed till Herschel constructed his twenty and forty feet reflectors. High as the prices of large telescopes now are, Mr. Short charged for his instruments at a much higher rate than opticians now do, although the price of labour, and every other article required in the construction of a telescope, is now much dearer. But he had then scarcely any competitor, and he spared neither trouble nor expense te make his telescopes perfect, and put such a price upon them as properly repaid him. ‘The following table contains a statement of the apertures, powers, and prices of Gregorian telescopes, as constructed by Mr. James Short :* * Miss Short, who has erected and who superintends an observatory on the Calton Hill, Edinburgh, is the descendant of a brother of Mr. Short. She is in possession of a large Gregorian reflector, about 12 feet long, made by Mr. Short, and mounted on an equatorial axis. It was originally placed in a small observatory erected on the Calton Hill about the year 1776, but for many years past it has been little used. 214 REMARKS ON GREGORIAN REFLECTORS. i | ® B.S eee oe a an S54 =| | abe Ale Magnifying powers, oe 3 Seo | ges 28 A B25 /Ags Ay fo 1 3 1.1 | 1 Power of / 18 times 3 2 4} se pee OF SL 2h. 58 vt 3 7 ge ee as 407-*s 6 4 94 2.5 | 2 Powers 40 and 60 ‘* 8 5 12 3.0112 55 and 85 ‘§° 10 6 12 3.0544 « £8, 35, 55, 85) and-110=‘¢ 14 vl 18 ee ee 55, 95, 130, and 200‘ 20 8 24 SB ear 90, 150, 230, and 300 ‘ 35 9 36 En fe A he a 100, 200, 300, and 400 ‘* vo 10 48 7.614 “. 120, 260, 380, and 500 ‘* 100 11 72 12.2 | 4 di 200, 400, 600, and 800 ‘* 300 12 144 18.0-4-4. * ** 300, 600, 900, and 1200 ‘ 800 From this table, it appears that Mr. Short charged 75 guineas for a 3 feet reflector, whereas such an instrument is now marked in the London opticians’ catalogues at £23 when mounted on a common brass stand, and £39 18s. when ac- companied with rack-work motions and other apparatus. It is now generally understood that in the above table Short always greatly overrated the higher powers of his telescopes. By experiment, they were generally found to magnify much less than here expressed. . General Remarks on Gregorian Reflectors.—1. In regard to the hole, U V, of the great speculum, its diameter should be equal, or nearly so, to that of the small speculum, L, fig. 67; for if it be less,no more parallel rays will be reflected than if it were equal to g h, and it may do harm in contract- ing the visible area within too narrow limits; nor must it be larger than the mirror L, because some parallel rays will then be lost, and those of most consequence, as being nearest the centre. 2. The small hole at e, to which the eye is applied, must be nicely adjusted to the size of the cone of rays pro- ceeding from the nearest lens, 8. If it be larger, it will per- mit the foreign light of the sky or other objects to enter the eye, so as to prevent distinct vision; for the eye should re- ceive no light but what comes from the surface of the small mirror, L. If the hole be smaller than the cylinder of rays at e, then some of the necessary light will be excluded, and the object rendered more obscure. The diameter of this hole may be found by dividing the aperture of the telescope in inches by its magnifying power. Thus, if we divide the dia- meter of one of Short’s telescopes, the diameter of whose REMARKS ON. NEWTONIAN TELESCOPES. 215 large speculum is 2.30, by 60, the magnifying power, the quotient will be .0383, which is nearly the 1.th of an inch. Sometimes this hole is made so small as the 4th of an inch. When this hole is, by any derangement, shifted from its pro- per position, it sometimes requires great nicety to adjust it, and, before it is accurately adjusted, the telescope is unfit for accurate observation. 3. It is usual to fix a plate with a hole in it at a b, the focus of the eyeglass S, of such a diameter as will circumscribe the image, so as to exhibit only that part of it which appears distinct, and to exclude the superfluous rays. 4. There is an adjusting screw on the outside of the great tube, connected with the small speculum, by which that spe- culum may be pushed backward or forward to adjust the instrument to distinct vision. 'The hand is applied for this purpose at T’. Newtonian Telescopes.—These telescopes are now more frequently used for celestial observations than during the last century, when Gregorian reflectors were generally preferred. Sir W. Herschel was chiefly instrumental in introducing this form of the reflecting telescope to the more particular atten- tion of astronomers, by the splendour and extent of the disco- veries which it enabled him to make. In this telescope there is no hole required in the middle of the great speculum, as in the Gregorian construction, which circumstance secures the use of all the rays which flow from the central parts of the mirror. The following table contains a statement of the apertures and magnifying powers of Newtonian telescopes, and the focal distances of their eyeglasses. The first column con- tains the focal length of the great speculum in feet; the second, its linear aperture in inches; the third, the focal dis- tance of the single glass in decimals, or in 1000ths of an inch and the fourth column contains the magnifying power. This portion of the table was constructed by using the dimensions of Mr. Hadley’s Newtonian. telescope, formerly referred to, as a standard, the focal distance of the great mirror being 623 inches, its medium aperture 5 inches, and power 208. The fifth, sixth, and seventh columns contain the apertures of the concave speculum, the focal lengths of the eyeglasses, and the magnifying powers, as calculated by Sir D. Brewster, from a telescope. of Mr. Hauksbee, taken as a standard, whose focal length was 3 feet 3 inches, its aperture about 4 inches, and magnifying power 226 times. 216 ADVANTAGES OF REFLECTING TELESCOPES. —- ————_—— Focal Sir D. Brewster’s Numbers. distance Aperture | Focaldistance} Magni- : of con- | of concave of oe pha an a Focal length pain bang metal. eyeglass. power, een of eyeglass. pone Feet. | Inc. Dec. | Inc. Dec “Ine. Dec. |Ine. Dee. ¥ 0 .86- 0 .167 36 1. ..34 0; -. 107 56 1] 1 .44 Or. 490 60 2 cee O= 7139 93 2 2. 49 0° 2236 102 S ete Or" TI2 158 3 on oe 0.. .261 138 5.14 0 .168 214 4 4% 10 0 281 171 6 .36 O- .181 265 5 4.85 0; 4.297, 202 (MG 0. .192 313 6 Sahay Ooi 232 8 .64 0 .200=} 360 7 6 .24 0- 3323 260 S - BT 0 .209 403 8 6 .89 0 .334 287 10.44 0 .218 445 9 y ears” | 0 .344 314 ll. 369 0 ».:222 487 10 & .Ib 0” 353 340 12 .65 0, .228 527 11 8 .76 0 .362 365 13.58 0 .233 566 12 9%36 0 .367 390 14 .50 0 .238 604 13 9 94 0.397 414 15.41 0 .243 642 14 10. .49 0 .384 437 16 ..25 0 .248 677 3) 11 .04 OF. 39F 460 TZ RE 0 .252 713 16 11: 5.59 0 .397 483 YP. .98 0 .256 749 17 12. .14 0 .403 506 18-82 QO .260 784 18 12°67 0 .409 528 19° .b3 0. .264 818 19 13-20 0 .414 550 20 °.45 0.268 852. 20 1S: (e272 0 .420 571 wha > .O4 0 271 885 One great advantage of reflecting telescopes above common refractors is, that they will admit of eyeglasses of a: much shorter focal distance, and, consequently, will magnify so’ much the more, for the rays are not coloured by reflection from a concave mirror, if it be ground to a true figure, as they are by passing through a convex glass, though figured and polished with the utmost exactness. It will be perceived from the above table that the focal length of the eyeglasses is very small, the lowest there stated bemg only about jth of an inch, and the highest little more than 7th of an inch focal distance. Sir W. Herschel obtained the high powers which he sometimes put upon his telescopes by using small double convex lenses for eyeglasses, some of which did not exceed the one-fiftieth of an inch in focal length. When the focal] length of the concave speculum and that of the eyeglass are given, the magnifying power is found by dividing the former by the latter, after having reduced the focal length of the concave speculum to inches. Thus the 6 feet speculum, multiplied by 12, produces 72 inches, which, divided by Brewster’s number for the focus of the eyeglass = 200, or 3th of an inch, produces a quotient of 360 as the magnifying power. It has been calculated that, if the metals of a New- tonian telescope be worked as exquisitely as those in Sir W. Herschel’s 7 feet reflector, the highest power that such a telescope should bear with perfect distinctness will be found PRICrs OF REFLECTING TELESCOPES. 217 by multiplying the diameter of the great speculum in inches by 74, and the focal distance of the single eyeglass may be found by dividing the focal distance of the great mirror by the magnifying power. Thus 6.25—the aperture in inches of Herschel’s 7 feet Newtonian—multiplied by 74, is 4623, the magnifying power; and 7 multiplied by 12, and divided by 462, is 50.182 of an mch, the focal distance of the single eyeglass required. But it is ‘seldom that more than one-half of this power can be applied with effect to any of the planet- ary bodies. For general purposes, the power produced b multiplying the diameter of the speculum by 30 or 40 will be found most satisfactory. The following are the general prices of reflecting telescopes as made by the London opticians. Ls A four feet, seven inch aperture, Gregorian reflector, with the vertical motions upon a new invented principle, as well as appa- ratus to render the tube more steady in observation, according to the additional apparatus of small speculums, eyepieces, microme- SOee er ne te Chae BV rete Sv Puk erase cdrdae anes og cae .. 80 to 120° 0 Three feet long, mounted on a plain brass stand . : 23° 2%, Ditto, with rack-work motions, improved mounting, and metals 39 18 Two feet long, without rack-work, and with 4 magnifying powers, IMPFOVE” or ec cree ccevocceccoeccscecreccsecvcescriags 15 15, Ditto, with rack-work motion. .....ccccecsccccccevcsececses 22 1 Highteen inch, on a plain stand... ++... eeseeeeeeecneceress 9 ¥ Twelve inch, ities aes ss as dierecsitzenn.nell > Secs tan tekiete tated 6. 6 The above are the prices stated in Messrs. W. and 8, Jones’s catalogue. The following list of prices of the various kinds of saflencinh telescopes is from Messrs. Tulley’s (of Islington) catalogue. £L s, 1 foot Gregorian reflector, on pillar and claw stand, metal 2 inches diameter, packed in a mahogany bOX .++.s-seeeeeseceees 6 6 1} foot ditto, on pillar and claw stand, metal 3 inches diameter, packed in a mahogany 1 PERE Re RIE Re 1 RTE FOSS 11 11, 2 feet ditto, metal 4 inches diameter. ...+.eeeeccceccevece caer 16.16 Ditto, ditto, with rack-work Motions «-..-eesedeceeceecesees 25 = 3) feet ditto, metal 5 inches diameter, with rack-work motions. 42 0 Ditto, metal 6 inches diameter, on a ipod ime vis, centre of gravity motion. Ser Waint ss tabs sO 4 feet ditto, metal 7 qndlied diate tees as ‘above: HAR Ate she, LOS 0 6 feet ditto, metal 9 inches diameter, on an improved iron stand 210 0 7 feet Newtonian reflectors, 6 inches aperture, mounted on a new and improved Band. 72 -. cw ce sc cclew cee ccneeceteesecs Pee Li Ditto, ditto, metal 7 inches diameter .-....-.+.ee0. toAa: soa 1e0 <0 9 feet ditto, metal 9 inches diameter..... eapda»« cctt Vea oes 210 @ 10 feet ditto; metal 10 inches diameter... ......eee0eceeas aoe 915.0 12 feet ditto, metal 12 inches diameter.....+..sseseesesseevs 52D O Comparative Brightness of Achromatic and Reflecting Telescopes.—The late astronomer royal, Dr. Maskelyne, from Von. IX. 19 218 HERSCHEL’S TELESCOPE. a comparison of a variety of telescopes, was led to the follow- ing conclusion: “that the aperture of a common reflecting telescope, in order to show objects as bright as the achroma- tic, must be to that of an achromatic telescope as 8 to 5;”’ in other words, an achromatic whose object-glass is 5 inches in diameter, will show objects with as great a degree of bright- ness as a reflector whose large speculum is 8 inches in dia- meter. This result, if correct, must be owing to the small number of rays reflected from a speculum compared with the number transmitted through an achromatic object-glass. SECT. IIl.—THE HERSCHELIAN TELESCOPE. Soon after Sir William Herschel commenced his astrono- mical career, he intreduced a new era in the history of re- flecting telescopes. After he had cast and polished an im- mense variety of specula for telescopes of different sizes, he at length, in the year 1782, finished a 20 feet reflector with a large aperture. Being sensible of the vast quantity of light which is lost by a second reflection from the small speculum, he determined to throw it aside altogether, and mounted this 20 feet reflector on a stand that admitted of being used with- out a small speculum in making front observations ; that is, in sittmg with his back to the object, and looking directly towards the surface of the speculum. Many of his discove- ries and measurements of double stars were made with this instrument, till at length, in the year 1785, he put the finish- ing hand to that gigantic speculum, which soon became the object of universal astonishment, and which was intended for his forty-feet reflecting telescope. He had succeeded so -well in constructing reflecting telescopes of comparatively small aperture, that they would bear higher magnifying powers than had ever previously been applied; but he found that a deficiency of light could only be remedied by an in- creased diameter of the large speculum, which therefore was * his main object when he undertook to accomplish a work which toa man less enterprising would have appeared im- practicable. The difficulties he had to overcome were nume- rous, particularly in the operative department of preparing, melting, annealing, grinding, and polishing a mass of metal that was too unwieldy to be moved without the aid of me- chanical powers. At length, however, all difficulties having been overcome, this magnificent instrument was completed, with all its complicated apparatus, and erected for observa- tion, on the 28th of August, 1789, and on the same day the sixth satellite of Saturn was detected, as a prelude of still HERSCHEL’S TELESCOPE. 919 farther discoverres which were afterward made by this in- strument in the celestial regions. It would be too tedious to attempt a description of all the machinery and apparatus connected with this noble instru- ment. ‘I'he reader'who wishes to pursue a minute descrip- tion of the stairs, ladders, platform, rollers, and of every cir- cumstance relating to joiner’s work, carpenter’s work, smith’s work, and other particulars connected with the formation and erection of this telescope, will find the details recorded in the 85th volume of the Philosophical Transactions of the Royal Society of London for 1795, in which there are sixty-three pages of letter-press, and eighteen plates illustrative of the subject. I shall content myself with giving a short outline of the essential parts belonging to this Istrument. The tube of this telescope is made of rolled or sheet iron, joined together without rivets; the thickness of the sheets is somewhat less than 34th part of an inch, or 14 pounds weight for a square foot. Great care was taken that the cylindrical form should be secured, and the whole was coated over three or four times with paint, inside and outside, to secure it against the damp.. This tube was removed from the place in which it was formed by 24 men, divided into six sets, so that two men on each side, with a pole 5 feet long in their hands, to which was affixed a piece of coarse cloth 7 feet long, going under the tube, and joined to a pole 5 feet long in the hands of two other men, assisted in carrying the tube. The length of this tube is 39 feet 4 inches, the diameter 4 feet 10 inches ; and, on a moderate computation, it was ascertained that a wooden tube of proper dimensions would have ex- ceeded an iron one in weight by at least 3000 pounds. Reckoning the circumference of the tube 15 feet, its length 392 feet, and 14 lib. for the weight of a square foot, it must have contained 590 square feet, and weighed 8260 pounds. Various hoops were fixed within the tube, and longitudinal bars of iron connecting some of them are attached to the two ends of the tube, by way of bracing the sheets, and preserv- ing the shape perfect, when the pulleys are applied to give the necessary elevation at the upper end, and that the specu- lum may be kept secure at the lower end. The lower end of the tube is firmly supported on rollers, that are capable of being moved forward or backward by a double rack, con- nected with a set of wheels and pinions. By an adjustment at the lower extremity of the tube, the speculum is turned to a small inclination, so that the line of collimation may not be coincident with the longitudinal axis of the tube, but may 220 HERSCHEL’S TELESCOPE. cross the tube diagonally, and meet the eye in the air at about two inches from the edge of the tube, which is the peculiarity of the construction that supersedes the necessity of applying a second reflector. Hence no part of the head of the observer mtercepts the incident rays, and the observation is taken with the face looking at the speculum, the back being turned to the object to be observed. The large speculum is enclosed in a strong iron ring, braced across with bars of iron, and an enclosure of iron, and ten sheets make a case for it. It is lifted by three handles of iron attached to the sides of the ring, and is put mto and taken out of its proper place in the tube by the help of a movable crane, running on a carriage, which operation re- quires great care. The speculum is made of a metallic composition, and is 493 imches in diameter; but the concave polished surface is only 48 inches, or 4 feet in diameter. Its thickness is 84 inches; and when it came from tthe cast its weight was 2118 pounds. The metals for its formation were procured at a warehouse in Thames street, London, where they kept ingots of two kinds ready made, one of white and the other of bell-metal; and it was composed of two ingots of bell-metal for one of white. It was not to be expected that a speculum of such large dimensions could have a perfect figure imparted to its surface, nor that the curve, whatever it might be, would remain. identically the same in changes of temperatures. therefore we-are not surprised when we are told that the magnifying powers used with this ‘telescope seldom exceeded 200, the quantity of light collected by so large a surface being the principal aim of the maker. The raising of the balcony, on which the observer stands, and the sliding of the lower end of the tube, in which the speculum rests, are effected by separate tackles, and require only occa- sional motions ; but the elevation of the telescope requires the main tackle to be employed, and the motion usually given in altitude at once was two degrees; the breadth of the zone in which the observations were made, as the motion of the sphere in right ascension. brought the objects into view. A star, however, could be followed for about a quarter of an hour. Three persons were employed in using this telescope, one to work the tackle, another to observe, and a third to mark down the observations. The elevation was pointed out by a small quadrant fixed to the main tube, near the lower end, but the polar distance was indicated by a piece of machinery, worked by a string, which continually indicated the degree and riinute on a dial in the small house adjoming, while the time HERSCHEL’S TELESCOPR. 221 was shown by a clock in the same place, Miss Herschel per- forming the office of registrar. At the upper end the tube is open, and directed to the part of the heavens intended for observation, and the observer, standing on the foot-board, looks down the tube, and per- ceives the object by rays reflected from the speculum through the eyeglass at the opening of the tube. When the telescope is directed to any objects near the zenith, the observer is ne- cessarily at an elevation at least 40 feet from the ground. Near the place of the eyeglass is the end of a tin pipe, into which a mouthpiece may be placed, so that, during an ob- servation, a person may direct his voice into this pipe, while his eye is at the glass. This pipe, which is 13 inch in dia- meter, runs down to the bottom of the tube, where it goes into a turning joint, thence into a drawing tube, and out of this into another turning joint, from whence it proceeds, by a set of sliding tubes, towards the front of the foundation timber. Its use is to convey the voice of the observer to his assistants, for at the last place it divides itself into two branches, one going into the observatory, the other into the workman’s room, ascending in both places through the floor, and termi- nates in the usual shape of speaking trumpets. Though the voice passes in this manner through a tube, with many inflections, and through not less than 115 feet, it requires very little exertion to.be well understood. T’o direct so unwieldy a body to any part of the heavens at pleasure, many mechanical contrivances were evidently ne- _cessary. ‘The whole apparatus rests upon rollers, and care was previously taken of the foundation in the ground. This con- sists. of concentrical brick walls, the outermost 42 feet, the innermost 21 feet in diameter, 2 feet six inches deep under ground, 2 feet 3 inches broad at the bottom, and 1 foot 2 inches at the top, capped with paving stones 3 inches thick, and 12% inches broad. In the centre is a large post of oak, framed together with braces under ground, and walled fast to brickwork to make it steady. Round this centre the whole frame is moved horizontally by means of 20 rollers, 12 upon the outer and 8 upon the inner wall. The vertical motion is given to the instrument by means of ropes and pulleys, passing over the main beam supported by the ladders.. These ladders are 49 feet long, and there is a movable gallery with 24 rollers to ease its motion. There isa staircase intended for persons who wish to ascend into the gallery without being obliged 19* boyy HERSCHEL’S TELESCOPE. to go up the ladder The ease with which the horizonta’ and vertical motions may be communicated to the tube may be conceived from a remark of Sir W. Herschel, that in the ear 1789 he several times observed Saturn, two or three iets before and after its meridian passage, with one single person to continue, at his directions, the necessary horizontal and vertical motions. By this telescope the’ sixth and. seventh satellites of Saturn were discovered, only one of which is within the reach of the 20 feet reflector, or even of a 25 feet instrument. The dis- covery of the satellites of the planet Uranus, however, was made by the 20 feet reflector, but only after it had been con- verted from the Newtonian to the Herschelian construction, ‘which affords a proof of the superiority of the latter construc- tion over the former, when the same speculum is used. Never had the heavens before been observed with so extra- ordinary an instrument as the forty-feet reflector. ‘The nebu- losities which are found among the fixed stars in various re- gions of the heavens appeared almost all to resolve themselves into an innumerable multitude of stars; others, hitherto im- perceptible, seemed to have acquired a distinct light. On the entrance of Sirius into the field of the telescope, the eye was so violently affected that stars of less magnitude could not immediately after be perceived, and it was necessary to wait for 20 minutes before these stars could be observed. The ring of Saturn had always before ceased to be visible when its plane was directed towards the earth; but the feeble hight which it reflects in that position was enough for Her- schel’s instrument, and the ring, even then, still remained visible to him. It has been generally considered that this telescope was capable of carrying a power of 6000 times; and perhaps, for the purpose of an experiment, and for trymg its effect on certain objects, such a power may have been applied, in which case the eyeglass must have been only ,ths of an mch focal distance, or somewhat less than 3th of an inch. But such a power could not be generally-applied with any good effect to the planetary bodies, and I question much whether any power above 1000 times was ever generally used; for it is the quantity of light which the telescope -col- lects, more than the magnifying power, that enables us:to penetrate, with effect, into the distant spaces of the firmament; and hence, as above stated, the power seldom exceeded 200, which on’ account of the large diameter of the speculum, would enable the instrument to penetrate into the distant RAMAGRE’S REFLECTING TEELSCOPE. 223 celestial spaces perhaps farther than if a power of as many thousands of times had been applied. Sir John Herschel, who inherits all the science, skill, and industry of his father, some time ago ground and polished a new speculum for the 20 feet tube, formerly noticed, which is connected with a stand, pulleys, and other appendages similar to those above described, though of smaller dimen- sions. This telescope shows the double stars exceedingly well defined, and was one of the principal instruments used in forming his catalogue of these objects which was pre- sented to the Royal Society, in conjunction with that of Sir James South, about the year 1828. I suppose it is likewise the same telescope with which Sir John lately made his side- real observations at the Cape of Good Hope. SECT. II].—RAMAGE’S LARGE REFLECTING TELESCOPE. The largest front view reflecting telescope in this country, next to Herschel’s 40 feet instrument, is that which was erected at the Royal Observatory at Greenwich in the year 1820, by Mr. Ramage, of. Aberdeen. The diameter of the concave reflector is 15 inches, and its focal length 25 feet. It is erected on machinery which bears a certain resemblance to that of Herschel’s, which we have now described, but the mechanical arrangements are greatly simplified, so that the instrument is manageable by an observer without an assist- ant. The tube is composed of a twelve-sided prism of deal §ths of an inch thick. At the mouth is.a double cylmder of different diameters on the same axis; around this a cord is wound by a winch, and passes up. from the small cylinder, over a pulley, and down through another pulley on to the large cylinder. When the winch, therefore, is turned to raise the telescope, the endless cord is unwound from the smaller cylinder and wound on to the larger, the difference of the size of the two cylinders will be double the quantity raised, and a mechanical force to any extent may thus be obtained, by duly proportioning the diameters of the two cylinders; by this contrivance the necessity of an assistant is superseded. The view through this instrument. first astonished those ob- servers who had not been accustomed to examine a heavenly body with a telescope possessing so much light, and its per- formance was deemed quite extraordinary. But when the first impression had subsided, and different trials had been made in different. states of the atmosphere, it was discovered that the central portion of the speculum was nore perfectly figured than the ring bordering on the extreme edges. When 224 DICK’S AERIAL REFLECTOR. the aperture was limited to ten or twelve inches, the per- formance as to the distinctness in its defining power was greatly improved. and the light was so brilliant that the as- tronomer royal was disposed to entertain an opinion that it might equal that of a good achromatic refractor of the same dimensions. When, however, very small and obscure objects are to be observed, the whole light of the entire aperture may be used with advantage on favourable evenings. The eyepieces adapted to this telescope have powers which magnify the object linearly from 100 to 1500 times, which are competent to fulfil allthe purposes of vision when cleared of aberration. When the telescope is placed in the plane of the meridian, and elevated, together with the gallery, into any re- quired altitude, the meridional sweeps, formerly practised by Sir W. Herschel, and continued by Sir John with great suc- cess, in the examination of double stars and nebula, may be managed with great ease. Mr. Ramage hada telescope of about the same size erected in an open space in Aberdeen, which I had an opportunity of inspecting when I paid a visit to that gentleman in 1833, but cloudy weather prevented my obtaining a view of any celestial bodies through it. He showed me “at that time two or three large speculums, from-12 to 18 inches in diameter, which he had finished some time before, and which appeared most beautifully polished. He told me, too, that he had eround and polished them simply with his hand, without the aid of any machinery or mechanical power: a circumstance which, he said, astonished the opticians of London when it was stated, and which they considered as almost incredible. His experience in casting and polishing metals of various sizes during a period of 15 or 16 years, qualified him to pre- pare specula of great lustre, and with an unusually high polish. It has been asserted that a fifty-feet telescope by Ramage, of 21 inches aperture, was intended to be substituted for the 25 feet imstrument erected at Greenwich, and the speculum, it is understood, was prepared, and ready for use, provided the Navy Board was disposed to defray the expense of carrying the plan into execution; but, unfortunately, this ingenious artist was unexpectedly cut off in the midst of his eareer, about the year 1835. | SECT. IV.—THE AERIAL REFLECTOR CONSTRUCTED BY THE | AUTHOR. A particular description of this telescope was given in the ' Edinburgh New Philosophical Journal’’ for. Aprili—July DICK’S AERIAL REFLECTOR. 225 1826, conducted by Professor Jameson, the greater part of which was copied in the “ London Encyclopedia,” under the article Telescope. From this description I shall endeavour to condense a brief account of this instrument, with a few additional remarks. | About the year 1822, an old speculum, 27 inches in focal length, very imperfectly polished, happened accidentally to come into my possession, and feeling no inclination to fit it wp in the Gregorian form, I adopted the resolution of throw- ing aside the small speculum, and attempting the front view, notwithstanding the uniform assertion of opticians that such an attempt in instruments of a small size is impracticable. I had some ground for expecting success in this attempt from several experiments I had previously made, particularly from some modifications made-in the construction of astronomical eyepieces, which have a tendency to correct the aberration of the rays of light when they proceed somewhat obliquely from a lens or speculum. In the first instance, I placed the speculum at one end of a tube of the form of the segment of a cone, the end next the eye being somewhat wider than that at which the speculum was fixed, and its length about an inch shorter then the focal distance of the mirror. A small tube for receiving the different eyepieces was fixed in the in- side of the large tube at the end next the eye, and connected with an apparatus by which it could occasionally be moved either in a vertical or horizontal direction. With the in- strument fitted up in this manner, I obtained some interesting views of the moon and of terrestrial objects; but, finding that one side of the tube intercepted a considerable portion of light from the object, I determined to throw aside the tube alto- gether, and to fit up the instrument on a different plan. A short mahogany tube, about three inches long, was prepared, to serve as a socket for holding the speculum. To the side of this tube an arm was attached, about the length _of the focal distance of the mirror, at the extremity of which a brass tube for receiving the eyepieces was fixed connected with screws and sockets, by which it might be raised or de- pressed, and turned to the mght hand or to the left, and with adjusting apparatus, by which it might be brought nearer to or farther from the speculum. Fig. 69 exhibits a general representation of the instrument in profile. A B is the short tube which holds the speculum; C D the arm which carries the eyetubes, which consists of two distinct pieces of mahogany ; the part D being capable of sliding along the under side of C, through the biass sockets E F. To the under part of the 226 DICK’S AERIAL REFLECTOR. socket, F', is attached a brass nut with a female screw, in which the male screw, a 0, acts by applying the hand to the knob ec, which serves for adjusting the instrument to distinct vision. Gis the brass tube which receives the eyepieces It is supported by a strong brass wire, d e, which passes through a nut connected with another strong wire, which passes through the arm D. By means of the nut f this tube may be elevated or depressed, and firmly fixed in its proper position ; and by the nut d it may be brought nearer to, or far- ther from, the arm D. Figure 69. ae “ By the same apparatus, it is also rendered capable of being moved either in a vertical or horizontal direction ; but when it is once adjusted to its proper position, it must be firmly fixed, and requires no farther attention. The eyepiece repre- sented in this figure is the one used for terrestrial objects, which consists of the tubes belonging to a pocket achromatic telescope. When an astronomical eyepiece is used, the length of the instrument extends only to the point I. In looking through this telescope, the right eye is applied at the point H, and the observer’s head is understdod to.be uncovered, or, at. least, tightly covered with a thin cap. _ For those who use only the left eye, the arm would require to be placed on the opposite side of the tube, or the arm, along with the tube, be made to turn round 180 degrees. Fig. 70 repres2nts, a front, or, rather an oblique view of the instrument, in which the position of the speculum may be DICK’S AERIAL REFLECTOR. 227 seen, All the spedula which I fitted up in this form, having. been originally mtended for Gregorian reflectors, have holes in their centres. ‘The eyepiece is therefore directed to a point nearly equidistant from the hole to the left-hand edge of the speculum, that is, to the point a. In one. of these in- struments fitted up with a four-feet speculum, the line of vision is directed to the point 6 on the opposite side of the speculum, but in this case the eyetube is removed farther from the arm than in the former case. The hole in the cen- tre of the speculum is obviously a defect in this construction of a reflecting telescope, as it prevents us from obtaining the full advantage of the rays which fall near the centre of the mirror; yet the performance of the instruments, even with this disadvantage, is superior to what we should previously have been led to expect. ) The principal nicety in the construction of this instrumen consists in the adjustment and proper direction of the eyetube. There is only.one position in which vision will be perfectly distinct. It must be neither too high nor too low; it must be fixed at a certain distance from the arm, and must be directed to a certain point of the speculum. This position must. be ultimately determined by experiment when viewing terrestrial Figure 70. 228 DICK’S AERIAL REFLECTOR. objects. A person unacquainted with this construction of the telescope would perhaps find it difficult, in the first instance, to make this adjustment; but were it at any time deranged, through accident or otherwise, I can easily make the adjust- — ment anew in the course of a minute or two. In pointing this telescope to the object intended to be viewed, the eye is applied at K, fio. 69, and looking along the arm, towards the eyepiece, till it nearly coincide with the object, it will, in most cases, be readily found. In this way I can easily point this instrument to Jupiter or Saturn, or to any of the other planets visible to the naked eye, even when a power of 160 or 170 times is applied. When high magni- fying powers, however, are used, it may be expedient to fix, on the upper part. of the shorttube in which the speculum rests, a Finder, such as that whichis used in Newtonian tele- scopes. When the moon is the object intended to be viewed, she may be instantly found by moving the instrument till her reflected image be seen from the eye-end of the telescope on the face of the mirror. I have fitted up several instruments of the above descrip- tion with specula of 16, 27, 35, and 49 inches focal distance. One of these, having a speculum of 27 inches focal length, and an astronomical eyepiece producing a magnifying power of about 90 times, serves as a good astronomical telescope. By this instrument the belts and satellites of Jupiter, the ring of Saturn, and the mountains and cavities of the moon, may be contemplated with great ease and distinctness. With a magnifying power of 35 or 40 times, terrestrial objects appear remarkably bright and well defined. When compared with a Gregorian, the quantity of ight upon the object appears nearly doubled, and the image is equally distinct, although the speculum has several blemishes, and its surface is but imperfectly polished. It represents objects in their natural colours, without that dingy and yellowish tinge which appears when looking through a Gregorian. Another of these instru- ments is about four feet long. The speculum which belongs to itis a very old one: when it came into my possession, it was so completely tarnished as scarcely to reflect a ray of light. After it. was cleaned, it appeared to be scarcely half polished, and its surface is covered with yellowish stains which cannot be erased. Were it fitted up upon the Gre- gorian plan, it would, I presume, be of very little use, unless when a very small magnifying power was applied ; yet in its present form it bears with distinctness a magnifying power of 130 times, and is equal in its performance to a 33 feet achro- DICK’S AERIAL REFLEOTOR. 229 matic. It exhibits distinct and interesting views of the diver- sities of shade, and of the mountains, vales, cavities, and other inequalities of the moon’s surface. With a power of about 50 times, and a terrestrial eyepiece, it forms an excellent telescope for land objects, and exhibits them in a brilliant and novel aspect. ‘The smallest-instrument I have attempted to construct on this plan is only 53 inches focal distance, and 1{ths of an inch in diameter.» With a magnifying power of about 15 times, it shows ‘terrestrial objects with distinctness and brilliancy. But I should deem it inexpedient to fit up any instrument of this description with specula of a shorter focal distance than 20 or 24 inches. The longer the focal distance, the more distinctness may be expected, although the aperture of the speculum should be comparatively small. The following are some of the properties and advantages peculiar to this construction of the reflecting telescope : 1. It is extremely simple, and may be fitted up at a com- parsers small expense. Instead of large and expensive rass tubes, such as are used in the Gregorian and Newto- nian construction, little more is required than a short mahog- any tube, two or three inches long, to serve as a socket for the speculum, with an arm connected with it about the focal length of the speculum. The expense of small specula, either plain or concave, is saved, together with the numerous screws, springs, &c., for centring the two specula, and placing the small mirror parallel to the large one. ‘The only adjust- ment requisite in this construction is that of the eyetube to _ the speculum ; and, by means of the simple apparatus above described, it can be effected im the course of a few minutes. Almost the whole expense of the instrument consists in the price of the speculum and the eyepieces. The expense of fitting up the four feet speculum alluded to above, exclusive of speculum and eyepiece, but including mahogany tube and arm, brass sockets, screws, eyetube, brass joint, and a cast- iron stand, painted and varnished, did not amount to £1 8s. A Gregorian of the same size would have required a brass tube at least 43 feet in length, which would cost five or six guineas, besides the apparatus connected with the small speculum, and the additional expense connected with the itting up of the joint and stand requisite for supporting and steadying so unwieldy an instrument. While the one instru- ment would require two persons to carry. it from one room to :nother, and would occupy a considerable space in an ordi- nary apartment, the other can be moved, with the utmost Vor. IX. 20 230 DICK’S AERIAL REFLECTOR. ease, with one hand, to any moderate distance, and the space it occupies is extremely small. . 2. It is more convenient for viewing celestial objects at a high altitude than other telescopes. When we look through a Gregorian reflector or an achromatic telescope of four or five feet in length, to an object elevated 50 or 60 degrees above the horizon, the body requires to be placed in an uneasy and distorted position, and the eye is somewhat strained while the observation is continued ; but when viewing similar objects by the Aerial Reflector, we can either stand perfectly erect, or sit on a chair, with the same ease as we sit at a desk when reading a book or writing a letter. In this way, the surface of the moon or any of the planets may be contemplated for an hour or two without the least weari- ness or fatigue. A delineation of the lunar surface may be taken with this instrument with more ease and accuracy than with any other instrument, as the observer can sketch the outline of the object by one eye ona tablet placed a little below the eyepiece, while the other eye is looking at the ob- ject... For the purpose of accommodating the imstrument to a sitting or standing posture, a small table was constructed, capable of being elevated or depressed at pleasure, on which the stand of the telescope is placed. When the telescope is four or five feet long, and the object at a very high elevation, the instrument may be placed on the floor of the apartment, and the observer will stand in an erect position. 3. This instrument is considerably shorter than a Grego- rian telescope whose mirror is of the same focal length. When an astronomical eyepiece is used, the whole length of the in- strument is nothing more than the focal length of the specu- lum; but a Gregorian, whose large speculum is four feet focus, will be nearly five feet in length, including the eye- iece. 4. The Aerial Reflector far excels the Gregorian in bright- ness. The deficiency of light in the Gregorians is owing to the second reflection from the small mirror; for it has been proved by experiment that nearly the one-half of the rays of light which fall upon a reflecting surface is lost by a second reflection. ‘The image of the object may also be presumed to be more correct, as it is not liable to any distortion by being reflected from another speculum. 5. There is less tremor in these telescopes than in Gre- gorian reflectors. One cause, among others, of the tremors complained of in Gregorians is, I presume, the formatior of a second image at a great distance from the first, besides DICK’S AERIAL REFLECTOR. P31 that which arises from the elastic tremor of the small specu- lum, when carried by an arm supported only at one end: but as the image formed by the speculum in the aerial tele- scope is viewed directly, without being exposed to any subse- quent reflection, it is not so lable to the tremors which are so frequently experienced in other reflectors. » Notwithstand- ing the length of the arm of the four-feet telescope above men- tioned, a celestial object appears remarkably steady when passing across the field of view, especially when it is at a moderate degree of altitude; and it is easily kept in the field by a gentle motion applied to the arm of the instrument. — In prosecuting my experiments in relation to these instru- ments, [ wished to ascertain what effect might be produced by using a part of a speculum, instead of a whole. For this purpose, I cut a speculum, three feet in focal length, through the centre, so as to divide it into two equal parts, and fitted up each part as a distinct telescope, so that I obtained two telescopes from one speculum. In this case, I found that each half of the speculum performed nearly as well as the whole speculum had done before; at least, there appeared to be no very sensible diminution in the brightness of the object, when viewed with a moderate power, and the image was equally accurate and distinct ; so that if economy were a par- ticular object aimed at in the construction of these instruments, two good telescopes might be obtained from one speculum ; or if a speculum happened to be broken accidentally into large fragments, one or more of the fragments might be fitted up on this principle to serve as a tolerably good telescope. From the experiments I have made in reference to these instruments, it is demonstrable that a tube is not necessary in the construction of a reflecting telescope—at least, on the principle now stated—whether it be used by day or by night, for terrestrial or celestial objects ; for I have frequently used these telescopes in the open air in the daytime, without any inconvenience from extraneous light. - Therefore, were a re- flecting telescope of 50 or 60 feet in length to be constructed, it might be fitted up at a comparatively small expense, after the expense of the metallic substances, and of casting, grind- ing, and polishing the speculum is defrayed. The largest instrument of this description which has hitherto been con- structed is the 40 feet reflector of Sir W. Herschel. This complicated and most unwieldy instrument had a tube of rolled or sheet iron 39 feet 4 inches in length, about 15 feet in cir~ cumference, and weighed about 8000 pounds. Now I con- ceive that such enormous tubes, in instruments of such dimen- 232 DICK’S AERIAL REFLECTOR. / sions, are altogether unnecessary. Nothing more is requisite than a short tube for holding the speculum. - Connected with one side of this tube, (or with both sides, were it found neces- sary,) two strong bars of wood, projecting a few feet. beyond the speculum end, and extending in front as far as the focal length of the mirror, and connected by crossbars of wood, iron, or brass, would be quite sufficient. for a support to the eye- piece, and for directing the motion of the instrument. A tele- scope of 40 or 50 feet in length, constructed on this plan, would not require one-fifth of the expense, nor one-fourth of the apparatus and mechanical power for moving it to any re- quired position, which were found necessary in the construc- tion of Sir W. Herschel’s large reflecting telescope. The idea here suggested will perhaps be more readily appreciated by an. inspection of fig. 71, where A is the short tube, B C and D E the two large bars or arms, connected with crossbars, for the purpose of securing strength and steadiness. At land K, behind the speculum, weights might be applied; if neces- sary, for counterbalancing the lever power of the long arm. F represents the position of the eyepiece, and G H the joint and part of the pedestal on which the instrument is placed. With regard to telescopes of smaller dimensions, as from 5 to 15 feet in focal length—with the exception of the expense of the specula and eyepieces—they might be fitted up for a sum not greater than from 3 to 10 or 15 guineas. | Figure 71. Were any person to attempt the construction of those tele- scopes, it is possible he might not succeed in his first attempts without more minute directions than Ihave yet given. ‘The DICK’S AERIAL REFLECTOR. 233 following directions may perhaps tend to guide the experi- menter in adjusting the eyetube to the speculum, which is a point that requires to be particularly attended to, and on which depends the accurate performance of the instrument. After having fixed the eyepiece nearly in the position it should occupy, and directed the instrument to a particular object, look along the arm of the telescope, from KX (fig. 69) to the extremity of the eyepiece at H, and observe whether it nearly coincides with the object. If the object appear lower than this line of vision, the eyepiece must be lowered, and if higher, it must be raised, by means of the nuts and screws at g d and fe, till the object and the line of vision now stated nearly co- incide. The eyepiece should be directed as nearly perpen- dicular to the front of the speculum as possible, but so that the reflected image of~one’s head from the mirror shall not interfere to obstruct the rays from the object. An object may be seen with an approximate degree of distinctness, but not accurately, unless this adjustment be pretty accurately made. The astronomical eyepieces used for these telescopes are fitted with a brass cap, which slides on the end next the eye, and is capable of being brought nearer to or farther from the first eyeglass. In the centre of this cap, next the eye, is a small hole, about the jth or =4th of an inch diameter, or about as wide as to admit the point of a pin or a moderate-sized needle. The distance of this hole from the lens next the eye must be adjusted by trial, till the whole field of view appear distinct. A common astronomical eyepiece, without this addition, does not answer well. . I find, by experience, that terrestrial eye- pieces, such as those used in good achromatic telescopes, are, on the whole, best adapted to this construction of a reflecting telescope. [have sometimes used these instruments for the purpose of viewing perspective prints, which they exhibit in a beau- tiful and interesting manner. If a coloured perspective be placed at one end of a large room or gallery, and strongly illuminated either by the sun or by two candles, and one of the reflectors, furnished with a small magnifying power, placed at the opposite end of the room, the representation of a street or a landscape will be seen in its true perspective, and will appear even more pleasant and interesting than when viewed through the common optical diagonal machine. — If an inverting eyepiece be used—which is most eligible in this experiment—the print, of course, must be placed in an inverted position. That reflecting telescopes of the descriptions now stated 20* 934 EARL OF ROSSE’S TELESCOPES. are original in their construction, appears from the uniform language of optical writers, some of whom have pronounceé such attempts to be altogether impracticable. Sir David Brewster, one of the latest and most respectable writers on. this subject, in the “ Edinburgh Encyclopedia,” art. Optics, . and in his last edition of his Appendiz to “« Ferguson’s Lec- tures,’ has the following remarks: “If we could dispense with the use of the small specula in telescopes of moderate length, by inclining the great speculum, and using an oblique, and, consequently, a distorted reflection, as proposed first by Le Maire, we should consider the Newtonian telescope as perfect; and on a large scale, or when the instrument. ex- ceeds 20 feet, it has undoubtedly this character, as nothing can be more simple than to magnify, by a single eyeglass, the image formed by a single speculum. As the front view is guite impracticable, and, indeed, has never been attempted in instruments of a small size, it becomes of great practical consequence to. remove as much as possible the evils which arise from the use of a small speculum,” &c. a The instruments now described have effectuated, in some degree, the desirable object alluded to by this distinguished philosopher, and the mode ef construction is neither that of Sir W. Herschel’s front view, nor does it coincide with that proposed by Le Maire, which appears to have been a mere hint that was never realized in the construction of reflecting telescopes of a small size. The simplicity of the construction of these instruments, and the excellence of their performance, have been much admired by several scientific gentlemen and others to whom they have been exhibited. Prior to the de- scription of them in the Edinburgh Philosophical Journal, they were exhibited in the Calton Hill Observatory, Edin- burgh, in the presence of Professor Wallace and another gentleman, who compared their performance with that of an excellent Gregorian. As this instrument is distinguished from every other telescope in being used without a tube, it has been denominated “.The Aerial Reflector.” SECT. V.—EARL OF ROSSE’S REFLECTING TELESCOPES. This nobleman, unlike many of his compeers, has, for a considerable number of years past, devoted his attention to the pursuits of science, and particularly to the improvement of reflecting telescopes. He is evidently possessed of high mathematical attainments, combined with an uncommon de- gree of mechanical ingenuity. About fourteen or fifteen years ago, he engaged in various experiments with the view EARL OF ROSSE’S TELESCOPES. 235 of counteracting the effects of the spherical aberration of the specula of reflecting telescopes, which imperfection, if it could be completely remedied, would render the reflecting telescope almost a perfect instrument, as it is not affected by the different refrangibility of the rays of light. His method, we believe, consisted in forming a large speculum of two or three separate pieces of metal, which were afterward accu- rately combined into one—a central part, which was sur- rounded by one or two rings ground on the same tool. _When the images formed by the separate pieces-were made exactly to coincide, the image of the object towards which the whole speculum was directed was then found to be as distinct as either Image had been when separate; but, at the period re- ferred to, a sufficient number of experiments had not been made to determine that his lordship had completely accom- plished the object he intended. Great interest, however, has of late been excited by the improvements which his lordship has made in the formation of specula. Sir W. Herschel never made public the means by which he succeeded in giving such gigantic development to the reflecting telescope, and therefore the construction of a large reflector has been considered as a perilous adventure ; but, according to a report of Dr. Robinson, of Armagh, to the Irish Academy, the Earl of Rosse has overcome the difficul- ties which have hitherto been met with, and carried to an extent which even Herschel himself did not venture to con- template, the illuminating power of this telescope, along with a sharpness of definition little inferior to that of the achro- matic; and it is scarcely possible, he observes, to preserve the necessary sobriety of language in speaking of the moon’s appearance with this instrument, which Dr. Robinson be- lieves to be the most powerful ever constructed. The diffi- culty of constructing large specula, and of imparting to them the requisite degree of polish, has hitherto been considered so sreat, that from eight to twelve inches diameter has been, in general, their utmost size; indeed, except with the greatest reluctance, London opticians would not accept of orders for specula of more than nine inches in diameter. It appears, however, that the Earl of Rosse has succeeded, by a peculiar method of moulding, in casting object-mirrors of true speculum metal of three feet in diameter, and of a weight exceeding 17 cwt. He is about to construct a telescope, the speculum of which is six feet in diameter, fifty feet focal distance, and of the weight of four tons; and from what he has already accomplished, it is not doubted that he possesses the power 236 EARL OF ROSSE’S TELESCOPES. to carry his design into effect. These great masses of metal which, in the hands of all other makers of specula, would have been as untractable as so much unannealed flint-glass, the Earl of Rosse has farther succeeded in bringing to the highest degree of polish, and the utmost perfection of curva- ture, by means of machinery. The process is conducted under water, by which means those variations of temperature, so fatal to the finest specula hitherto attempted, are effectually guarded against. ‘To convince Dr. Robinson of the efficacy of this machinery, the earl took the three-feet speculum out of its telescope, destroyed its polished surface, and placed it under the mechanical polisher. In. six hours it was taken out with a perfect new surface as bright as the original. Under the old system of hand polishing, it might have. re- quired months, and even years, to effect this restoration. Even before achieving these extraordinary triumphs on the solid substance, his lordship had constructed a six-feet reflector by covering a curved surface of brass with squares of the true speculum metal, which gave an immense quantity of light, though subject to some irregularities, arising from the num- ber of joiings necessary in such a mosaic work. Of the performance of his lordship’s great telescope, mounted with this reflector, those who have seen it speak in terms of high admiration ;~but in reference. to the smaller and more perfect instrument, furnished with the solid three-feet speculum, the language of the Armagh astronomer assumes a tone of enthu- siasm, and even of sublimity. ~By means of this exquisite instrument, Dr. Robinson and Sir J. South, in the intervals of a rather unfavourable night, saw several new stars, and corrected numerous errors of other observers. For example, the planet Uranus, supposed to possess a ring similar to that of Saturn, was found not to have’ any such appendage ; and those nebule, hitherto regarded, from their apparently circu- lar-outline, as ‘coalescing systems,’’ appeared, when tested by the three-feet speculum, to be very far indeed from pre- senting a globular appearance, numerous offshots and ap- pendages, invisible by other telescopes, appearing in all di- rections radiating from their edges. Such discoveries, which reflect great honour on the Earl of Rosse, will doubtless have great effect on the interests of astronomical science.* _ * A particatar account of the Earl of Rosse’s fifty-feet reflector, which is now finished, is given in the Appendiz. : EXPERIMENTS WITH GLASS SPECULA. 237 SECT. VI.—REFLECTING TELESCOPES WITH GLASS SPECULA. After making a variety of experiments with aerial tele- scopes constructed of metallic specula of different focal lengths, _ I constructed a telescope on the same plan with a concave _ glass mirror. Having obtained a fragment of a very large convex mirror which happened accidentally to have been broken, I caused the convex side to be foliated or silverized, and found its focal length to be about 27 inches. This mirror, which was about five inches diameter, I placed in one of the aerial reflectors instead of the metallic speculum, and tried its effects with different terrestrial eyepieces. Witha power of about 35 or 40 times, it gave a beautiful and splendid view of distant terrestrial objects, the quantity of light re- flected from them being considerably greater than when a metallic speculum was used, and they appeared, on the whole, well defined. The only imperfection—as I had fore- seen—consisted in a double image being formed of objects which were remarkably bright and white, such as a light- house whitened on the outside, and strongly illuminated by the sun, One of the images was bright, and the other faint. This was obviously owing tothe two reflections from the two sur- faces of the mirror—one from the convex silverized side, and the other from the concave side next the eye, which produced the faint image—which circumstance has been generally considered as a sufficient reason for rejecting the use of glass specula in telescopes. But, although very bright objects ex- hibited a double image, almost all the other objects in the terrestrial landscape appeared quite distinct and without any secondary image, so that a common observer could scarcely have noticed any imperfection. ._When the instrument, how- ever, was directed to celestial objects, the secondary image was somewhat vivid, so that every object appeared double. Jupiter appeared with two bodies, a little distance from each other, and his four satellites appeared increased to eight. The moon likewise appeared as a double orb, but the prin- cipal image was distinct and well defined. Such a telescope, therefore, was not well adapted for celestial observations, but might answer well enough for viewing terrestrial objects. Considering that the mjurious effects of the secondary im- age arose from the images reflected from the two surfaces being formed near the same point, and at nearly the same focal. distance, I formed a plan for destroying the secondary image, or at least counteracting its effects, by forming the concavity of the mirror next the eye of a portion of a spnere 238 EXPERIMENTS WITH GLASS SPECULA. different from that of the convex side which was silverized, and from which the principal image is formed’; but, for a long time, I could find no opticians possessed of tools of a sufficient length of radii for accomplishing my design. At lengtha London working optician undertook to finish a glass speculum according to my directions, which were, that the convex sur- face of the mirror should be ground on a tool which ‘would produce a focal distance by reflection of about four feet, and that the concave surface should have its focal distance at about three feet three inches, so that the secondary image might ‘be formed at about nine inches within the focal distance of the silverized side, and not interfere to disturb the prin- cipal image; but, either from ignorance or inattention, the artist mistook the radius for the half radius of concavity, and the speculum turned out to be only 23 inches focal distance by reflection. This mirror was fitted up as a telescope on the aerial plan, and I found, as I expected, the secondary image completely destroyed. It produced a very beautiful and brilliant view of land objects, and even the brightest objects exhibited ‘no double image. The mirror was nearly five inches in diameter, but the image was most accurately de- fined when the aperture was contracted to about three inches. It was fitted with a terrestrial eyepiece which produced a magnifying power of about 25 times. When directed to the moon, it gave a very distinct and Juminous view of that orb, without the least appearance of a secondary image; but as the focal distance of the speculum was scarcely half the length I had prescribed, I did not.apply to it any high astro- nomical powers, as I find that these can only be applied with effect, in this construction, to a speculum of a considerable focal length. Happening to have at hand a convex lens ten feet focal length and four inches in diameter, the one side of which had been ground to a certain degree of concavity, I caused the convex side to be foliated, which produced a focus by reflection at 133 inches distant.’ To this mirror I applied terrestrial powers of 15 and 24 with considerable distinctness. The power of 15 produced a very brilliant and distinct view - of land objects. Had the mirror been at least three times the focal length, it would have formed an excellent telescope with the same aperture. SECT. VII.——-A REFLECTING TELESCOPE, WITH A SINGLE MIRROR AND NO EYEPIECE. On the same principle as that by which a refracting tele- scope may be constructed by means of a single lens, as repre- POWER OF A SINGLE MIRROR. 239 sented fig. 51, (page 172,) we may form a telescope by reflec- tion with a single mirror and without an eyepiece. Let A B, fig. 72, represent a large concave speculum, and C its focus: Figure 72. if an eye be placed at D, about eight or ten inches within the focal point C, all the objects in the direction of C, or behind the spectator, will be seen magnified by reflection on the face of the mirror, and strongly illuminated. The magnifying power, in this case, will be nearly in the proportion of the focal length of the mirror to the focal length of the eye for near objects. If, for example, the focal distance of the mirror be eight feet, and the distance from the eye at which we see near objects most distinctly be eight inches, the magnifying power will be in the ratio of 8 to 96, or 12 times. I have a glass mirror of this description, whose focal length is four feet eight inches, and diameter six inches, which magnifies dis- tant objects about seven times, tales in a large field of view, and exhibits objects with great brilliancy. It presents a very distinct picture of the moon, showing the different streaks of light and shade upon her surface, and in some cases shows the larger spots which traverse the solar disc. This mode of viewing objects is extremely easy and pleasant, especially when the mirror is of a large diameter, and the observer is at first struck and gratified with the novel aspect ‘n which the objects appear. Were a concave mirror of this description—whether of glass or of speculum metal—to be formed to a very long focus, the magnifying power would be considerable. One of 50 feet focal length, and of a corresponding diameter, might produce £ 240 PRICES OF REFLECTING TELESCOPES. a magnifying power, to certain eyes, of about 75 times; and, from the quantity of light with which the object would be seen, its effect would be much greater than the same power applied to a common telescope. Sir W. Herschel states that, on one occasion, by looking with his naked eye on the specu- lum of his 40-feet reflector, without the interposition of any lens or mirror, he perceived distinctly one of the satellites of Saturn, which requires the application of a considerable power to be seen by an ordinary telescope. Such an instru- ment is one of the most simple forms of a telescope, and would exhibit a brilliant and interesting view of the moon, or of terrestrial objects. PRICES OF REFLECTING TELESCOPES. 1. Prices as stated by Messrs. W. and 8. Jones, Holborn, London. L. °8. A 4 feet, 7 inch aperture, Gregorian reflector, with the vertical motions’ upon a new invented principle, as well as apparatus to render the tube more steady fr observation, according to the additional apparatus of small speculums, eyepieces, micrometers, SE OM 19 0 bo a Teen do OS dia Bgin we ob saad ee be seehnev, Ee ao Ditto improved, with rack-work MOtionS......sceecceeceeees 22 1 18 inch, on a plain stand ........- EE) 2 ee TT ee | DMN ATE <0 's Bicttrand AicteseaceW ject, is 1gths of an inch focal length and about one inch in diameter, with the plane side next the object. The focal length of the lens B, 25th inches, diameter ;,ths of an inch, with its plane side next A; distance of these lenses from each other, 2;4;th inches ; distance of the field-lens C from the lens B, 53 inches. The small hole or diaphragm between A and B is at the focus of A, and is about one-sixth of an inch in diameter, and about three-eighths of an inch from the lens B. The field-lens C is two inches focal length, and 17 of an inch in diameter, with its plane side next the eye. The lens next the eye, D, is one inch focal distance, half an inch in diameter, and is distant from the field-glass 1zths of an inch, with its plane side next the.eye. The magnifying power of this eyepiece is equivalent to that of a single lens whose focal length is half an inch, and with the 442 inch ob- ject-glass produces a power of about 90 times... The lens next the eye can be changed for another of 1gths of an inch focal length, which produces a power of 65, and the two glasses C D can be changed for another set of a longer focal DUTCH ACHROMATIC TELESCOPE. 253 distance, which produces a power of 45 times. The whole length of this eyepiece is 113 inches. In another eyepiece, adapted to a pocket achromatic, whose object-glass is nine inches focal length, the lens A is one inch focal length and half an inch in diameter; the lens B, 14th of an inch, and half an inch in diameter; their distance, 14 inches; the lens C, 14th of an inch focal length and five- eighths of an inch in diameter; the eye-lens D, five-eighths of an inch focal length and three-eighths of an inch in dia- meter; distance between C and D, 1gth of an inch; the dis- tance between B and C, 17ths of an inch. The whole length of this eyepiece is 43 inches, and its power is nearly equal to that of a single lens of half or ;8ths of an inch focal length, the magnifying power of the telescope being about 16 times. Another eyepiece of much larger dimensions has the lens A of 23 inches focal length and three-fourths of an inch in dia- meter; the lens B, 27th inches focus and five-eighths of an inch in diameter, and their distance 22th inches; the lens C, 28th inches focus and 1th of an inch in diameter: the lens D, 1?ths of an inch focus and three-fourths of an inch in diameter; distance from each other, 23th inches. The dis- tance between the lenses B and C is four inches. The mag- nifying power is equal to that of a single lens 1gth of an inch focal distance. When applied to an achromatic object-glass six feet seven inches focal length, it produces a power of about 70 times. This eyepiece has a movable tube nine inches in length, in which the ‘two lenses next the eye are contained, by pulling out which, and consequently increasing the distance between the lenses B and C, the magnifying power may be increased to. 100, 120, or 140, according to the distance to which this movable tube is drawn out. It has also a second and third set of lenses, corresponding to C and D, of a shorter focal distance, which produce higher magnifying powers on a principle to be afterward explained. Description of an Eyepiece, §c., of an old Dutch Achro- matic Telescope. About twenty or thirty years ago, I purchased, in an opti- cian’s shop in Edinburgh, a small achromatic telescope, made in Amsterdam, which was supposed by the optician to nave been constructed prior to the invention of achromatic tele- scopes by Mr. Dollond. It is mounted wholly of brass, and in all its parts is a piece of beautiful and exquisite worlman- ship, and the utmost care seems to have been taken to have all the glasses and diaphragms accurately adjusted. The Vox. IX. 22 254 DUTCH ACHROMATIC TELESCOPE. object-glass is a double achromatic, 62 inches focal distance and one inch in diameter, but the clear aperture is only seven-eighths of an inch in diameter. It is perfectly achro- matic, and would bear a power of 50 times if it had a suffi- cient quantity of light. The following inscription is engraved on the tube adjacent to the object-glass: “Jan van Deyl en Zoon, Invenit et Fecit, Amsterdam, Ao. 1769." Although Dollond exhibited the principle of an: achromatic telescope eight or ten years before the date here, specified, yet it is not improbable that the artist whose name is here stated may not have heard of Dollond’s invention, and that he was really, as he assumes, one of the inventors of the achromatic telescope ; for the invention of this telescope by Dollond was not very generally known, except among philosophers and the London — opticians, tilla number of years after the date above stated. Euler, in his “ Letters to a German Princess,”’ in which tele- scopes are particularly described, makes no mention of, nor the least allusion to, the invention of Dollond, though this was a subject which particularly engaged his attention. Now these letters were written in 1762, but were not published till 1770. When alluding to the defects in telescopes arising from the different refrangibility of the rays of light, in Letter 43, and that they might possibly be rectified by means of different transparent substances, he says, “ But neither theory nor practice have hitherto been carried to the degree of per- fection necessary to the execution of a structure which should remedy these defects.” Mr. B. Martin, in his “Gentleman and Lady’s Philosophy,’’ published in 1781, alludes to the achromatic telescope, but speaks of it as if it were but very little, if at all, superior to the common refracting telescope ; and therefore I think it highly probable that Jan van Deyl was really an inventor of an achromatic telescope before he had any notice of what Dollond and others had done in this way some short time before. ieee But my principal object in adverting to this telescope is to describe the structure of the eyepiece, which isa very fine one, and which is somewhat different from the achromatic eyepiece above described. It consists of four glasses, two combined next the eye, and two next the object. Each of these combinations forms an astronomical eyepiece nearly similar to the Huygenian. The lens A, next the object, fig. 80, is five-eighths of an.inch focal distance, and ;‘¢ths of an inch in diameter; the lens B, three-eighths of an: inch focus, and one-fifth of an inch in diameter, and the distance between them somewhat. less..than five-eighths of an inch; DUTCH ACHROMATIC TELESCOPE. 255 Figure 80, the diameter of the aperture e about +), th of an inch. This combination forms an excellent astronomical eyepiece, with a large flat field, and its magnifying power is equivalent to that of a single lens five-eighths or six-eighths of an inch foes length. 'The lens C is half an inch focal length, and-.4,ths of an inch in diameter ; the lens D a quarter of an inch focus, and about one-fifth of an inch in diameter ; their- distance about bali an inch, or a small fraction more. .The hole at d is about zi5th or th of an inch in diameter, and the distance between “the lenses B and C about 1; inches. The whole length of the eyepiece-is 3; inches—exactly the same size as represented in the engraving. Its magnifying power is equal to that of a single lens one-fourth of an inch focal length ; and consequently the telescope, though only 93 inches long, mag- nifies twenty-six times with great distinctness, though there is a little deficiency of ight when viewing land objects which are not well illuminated. The glasses of this telescope are all plano-convex, with their convex sides towards the object, except the lens D, which is double convex, but flattest on the side next the eye, and they are all very accurately finished. The two lenses C and D form an astronomical eyepiece nearly similar to that formed by the lenses A and-B. ‘The focus of the telescope is adjusted by a screw, the threads of which are formed upon the outside of a tube into which the eyepiece slides. The eyepiece and apparatus connected with it is screwed into the inside of the main tube when not in use, when the instru- ment forms a compact brass cylinder six inches long, which is enclosed in a fish-skin case, lined with silk velvet, which opens with hinges. The lenses in the eyepieces formerly described, though stated to be plano-convexes, are, for the most part crossed glasses, that is, ground on tools of a long focus on the one side, and toa short focus on the other. ‘The construction of the eyepiece of the Dutch telescope above described is one which might be adopted with a good effect in most of our 256 THE PANCRATIC EYETUBE. achromatic telescopes; and I am persuaded, from the appli- cation I have made of it to various telescopes, that it is even superior in distinctness and accuracy, and in the flatness of Od which it produces, to the eyepiece in common use. he two astronomical eyepieces of which it consists, when applied to large achromatic telescopes, perform with great - accuracy, and are excellently adapted for celestial observa- tions. _ SECT. III.—-DESCRIPTION OF THE PANCRATIC EYETUBE. From what we have stated-when describing the common terrestrial eyepiece now applied to achromatic instruments, (p. 250, fig. 79,) it appears obvious that any variety of mag- nifying powers, within certain limits, may be obtained by removing the set of lenses C D, fig. 79, nearer to or farther from the tube which contains ihe lenses A and’ B, on the — same principle as the magnifyimg power of a compound microscope is increased by removing the eyeglasses to a ereater distance from the object-lens. If, then, the pair of eye-lenses C D be attached to an inner tube that will draw out and increase their distance from the inner pair of lenses, — as the tube a b e d, the magnifying power may be indefinitely increased or diminished by pushing in or drawing out the — sliding tube, and a scale might be placed on this tube, which, if divided into equal intervals, will be a scale of magnifying powers, by which the power of the telescope will be seen at every division, when the lowest power is once determined. Sir David Brewster, in his “Treatise on New Philosophical Instruments,” book i., chap. viil., page 59, published in 1813, has adverted to the circumstance in his description of an «« Hyepiece Wire Micrometer,” and complains of Mr. Ezekiel Walker having in the “ Philosophical Magazine”’ for August, 1811, described such an instrument as an invention of his own. Dr. Kitchener some years afterward described what he called a Pancratic or omnipotent eyepiece, and got one made by Dollond, with a few modifications different from that suggested by Brewster and Walker, which were little else than cutting the single tube into several parts, and giving it the appearance of a new invention. In fact, none of these gentlemen had a right to claim it as his peculiar invention, as the principle was known and recognised long before. I had increased the magnifying powers of telescopes on the same principle several years before any of these gentlemen com- municated their views on the subject, although I never form- ally constructed a scale of powers. Mr. B. Martin, who died THE PANCRATIC. EYETUBE. 257 in 1782, proposed, many years before, such a movable inte- rior tube as that alluded to for varying the magnifying power. In order to give the reader a more specific idea of this contrivance, I shall present him with a figure and description of one of Dr. Kitchener’s pancratic eyepieces, copied, from one lately in my possession. , The following are the exact dimensions of this instrument, with the focal distances, &c., of the glasses, &c., of which it is composed. , : aha In. Tenths. Length of the whole eyepiece, consisting of Fig.81. four tubes, when fully drawn out, or the dis- tance from A to B, fig. 81... 60. eeseeceeeeeees 14y<, 14 _ Length of the three tubes on which the scale is engraved, from the commencement of the di- visions at B to their termination at C......... FT 95 Each division into tens is equal to 3-10ths of an inch. When the three inner tubes are shut up to C, the length of the eyepiece is exactly.........- ies 5 When these tubes are thus shut up, the mag- nifying power for a 3} feet achromatic is 100 times, which is the smallest power. When the inner tube is drawn out one-third of an inch, or to the first division, the power is 110, &c. Focal distance of the lens next the object.... BPO OL NIG co ss cee eee es Shee een Petes The plane side of this glass is next the object. Focal distance of the second glass from the ODJECE ao dn sea rapactccsccscevucnte toe eKeees : 1 This glass is double and equally convex. MRTMOGT AO ts o> 60s 1s 40 EE BRD Sn tee eee oe Distance between these two glasses ....+.-++> Focal distance of the third or field lens, which 0 1 is plane on the side next the eye ...+.-..eseee 1 Breadtly of Gittod os sig Gale cds ole Ri NU be owe O25 0 0 or a Or Focal distance of the lens next the eye........ ROPE Ds @isa.n germ ahs nn 6s mie Rtat sehen sae «be This glass is plane on the side next the eye. Distance between the third and fourth glasses . 1 1 From the figure and description, the reader will be at no loss to perceive how the magnifying power is ascertained by this eyepiece. Ifthe lowest power for a 44-inch telescope be found to be 100 when the three sliding tubes are shut into the larger one, then by drawing out the tube next the eye four divisions, a.power of 140 is produced; by drawing out the tube next the eye its whole length, and the second tube to the division marked 220, a power of 220 times is produced; and drawing out all the tubes to their utmost extent, as repre- sented in the figure, a power of 400 is obtained. These powers are by far too high for such a telescope, as the powers between 300 and 400 can seldom or never be used. Were the scale to begin at 50 and terminate at 200, it would be 22* 258 MODE OF ADJUSTING A TELESCOPE. much better adapted to a 3% feet telescope. Each alteration of the magnifying power requires a new adjustment of the eyepiece for distinct vision. As the magnifying power is in- creased, the distance between the eyeglass and the object- glass must be diminished. Dr. Kitchener says that “the pancratic eyetube gives a better defined image of a fixed star, and shows double stars decidedly more distinct and perfectly separated, than any other eyetube, and that such tubes will probably enable us to determine the distances of these objects from each other ina more perfect manner than has been pos- sible heretofore.’’ These tubes are made by Dollond, London, and are sold for two-guineas each; but Ido not think they excel in distinctness those which are occasionally made by Mr. Tulley and other opticians. CHAPTER VI. MISCELLANEOUS REMARKS IN RELATION TO. TELESCOPES. The following remarks, chiefly in regard to the manner of using telescopes, may perhaps be useful to young observers, who are not much accustomed to the mode of managing these instruments. 1. Adjustments requisite to be attended to in the use of telescopes.—When near objects are viewed with a consider- able magnifying power, the eyetube requires to be removed farther from the object-glass than when very distant objects are contemplated. When the telescope is adjusted for an object 6, 8, or 10 miles distant, a very considerable alteration in the adjustment is requisite in order to see distinctly an object at the distance of two or three hundred yards, espe- cially if the instrument is furnished with a high magnifying power. In this last case, the eyetube requires to be drawn out to a considerable distance beyond the focus for parallel rays. Ihave found that, in a telescope which magnifies '70 times, when adjusted for an object at the distance of two miles, the adjustment requires to be altered’fully one inch in order to perceive distinctly an object at the distance of two or three hundred yards ; that is, the tube must be drawn, in this case, an inch farther from the object-glass, and pushed in the same extent, when we wish to view an object at the distance of two or three miles. ‘These adjustments are made, in pocket perspectives, by gently sliding the eyetube in or EFFECTS OF THE ATMOSPHERE ON VISION. 259 eut, by giving it a gentle circular or spiral motion, till the object appear distinct. In using telescopes which are held in the hand, the best plan is to draw all the tubes out to their full length, and then, looking at the object, with the left hand supporting the main tube near the object-glass, and the right supporting the eyetube, gently and gradually push in the eyepiece till distinct vision be obtained. In Gregorian re- flecting telescopes this adjustment is made by means of a screw connected with the small speculum; and in large achromatics, by means of a rack and pinion connected with the eyetube. When the magnifying power of a telescope is comparatively small, the eyetube requires to be altered only a very little. There is another adjustment requisite to be attended to in order to adapt the telescope to the eyes of different persons. Those whose eyes are too convex, or who are short-sighted, require the eyetube to be pushed in, and those whose eyes are somewhat flattened, as old people, require the tube to be drawn out. Indeed, there are scarcely two persons whose eyes do not require different adjustments in a slight degree. In some cases | have found that the difference of adjustment for two individuals, in order to produce distinct vision in each, amounted to nearly half an mch. Hence the difficulty of exhibiting the sun, moon, and planets through telescopes, and even terrestrial objects, to a company of persons who are un- acquainted with the mode of using. or adjusting such instru- ments, not one-half of whom generally see the object distinctly ; for upon the proper adjustment of a telescope to the eye, the accuracy of vision in all cases depends, and no one except the individual actually looking through the instrument can be certain that it is accurately adjusted to his eye; and even the individual himself, from not being accustomed to the view of certain objects, may.be uncertain whether or not the adjust- ment be correct. I have found by experience that when the magnifying powers are high, as 150 or 200, the difference of adjustment required for different eyes is very slight; but when low powers are used, as 20, 30, or 40, the difference of the requisite adjustments is sometimes very considerable, amounting to a quarter or halfan inch. 2. State of the Atmosphere most proper for observing Terrestrial and Celestial Objects——The atmosphere which is thrown around the globe, while it is essentially requisite to the physical constitution of our world, and the comfort of its inhabitants, is found in many instances a serious obstruc- tion to the accurate performance of telescopes... Sometimes it 260 EFFECTS OF THE ATMOSPHERE ON. VISION. is obscured by mists and exhalations; sometimes it is thrown into. violent undulations by the heat of the sun and the pro- cess of evaporation; and even, in certain cases, where there appears.a pure unclouded azure, there is an agitation among its particles and the substances incorporated with them which prevents the telescope from producing distinct vision either of terrestrial or celestial objects. For viewing distant terres- trial objects, especially with high powers, the best time is early in the morning, a little after sunrise, and from that period till about nine o’clock a.m. in summer, and in the evening about two or three hours before sunset. From about ten o’clock a.m. till four or five in the afternoon, in sum- mer, if the sky be clear and the sun shining, there is gene- rally a considerable undulation in the atmosphere, occasioned by the solar rays and the rapid evaporation, which prevents high powers from being used with distinctness on any tele- scope, however excellent. The objects at such times, when powers of 50, 70, or 100 are applied, appear to undulate like the waves of the sea, and, notwithstanding every effort to adjust the telescope, they appear confused and_ indistinct. Even with very moderate magnifying powers this imperfec- tion is perceptible. In such circumstances, I have sometimes used a power of 200 times on distant land objects with good effect a little before sunset, when, in the forenoon of the same day, I could not have applied a power of 50 with any degree of distinctness. On days when the air is clear and the atmo- sphere covered with clouds, terrestrial objects may be viewed with considerably high powers. When there has been a long-continued drought, the atmosphere is then in a very unfit state for enjoying distinct vision with high magnifying powers, on account of the quantity of vapours with which the atmosphere is then surcharged, and the undulations they produce. . But, after copious showers of rain, especially if accompanied with high winds, the air is purified, and distant objects appear with greater brilliancy and distinctness than at any other seasons. In using telescopes, the objects at which we look should, if. possible, -be nearly in a direction opposite to that of the sun. When they are viewed nearly in the direction of the sun, their shadows are turned towards us, -and they consequently. appear dim and obscure. By not attending to this circumstance, some persons, in trying tele- scopes, have pronounced a good instrument to be imperfect, which, had it been tried on objects properly illuminated, would have been found to be excellent. In. our variable northerly climate the atmosphere is not so clear and. serene EFFECTS OF THE ATMCSPHERE ON VISION. 261 for telescopic observation as in Italy, the south of France, and. in many of the countries which lie within the tropics. The undulations of the air, owing to the causes alluded to above, constitute one of the principal reasons why a telescope mag- nifying above a hundred times can seldom be used with any good effect in viewing terrestrial objects, though I have some- times used a power of nearly 200 with considerable distinct- ness in the stillness of a summer or autumnal evening, when the rays of the declining sun strongly illuminated distant objects. | The atmosphere is likewise frequently a great obstruction to the distinct perception of celestial objects. It is scarcely possible for one who has not been accustomed to astronomical observations to form a conception of the very great difference there is in the appearance of some of the heavenly bodies in different states of the atmosphere. ‘There are certain condi- tions of the atmosphere essentially requisite for making accu- rate observations ‘with powerful telescopes, and it is but seldom, especially in our climate, that, all the favourable cir- cumstances concur. ‘The nights must be very clear and serene—the moon absent—no twilight—no haziness—no violent wind—no sudden change of temperature, as from thaw to frost—and no surcharge of the atmosphere with aqueous vapour. I have frequently found that, on the first and second nights after a thaw, when a strong frost had set in, and when the heavens appeared very brilliant, and the stars vivid and sparkling, the -planets, when viewed with high powers, appeared remarkably undefined and indistinct ; their margins appeared waving and jagged; and the belts of Jupiter, which at other times were remarkably distinct, were so obscured and ill defined that they could with difficulty be traced. This was probably owing to the quantity of aqueous vapour, and perhaps icy particles, then floating in the air, and to the undulations thereby produced. When a hard frost has continued a considerable time, this impediment to distinct observation is in a great measure removed. . But I have never enjoyed more accurate and distinct views of the heavenly bodies than in fresh, serene evenings, when there was no frost and no wind, and only a few fleecy clouds occa- sionally hovering around. On such evenings, and on such alone, the highest powers may be applied. I have used magnifying powers on such occasions with good effect which could not have been applied, so as to ensure distinct vision, more frequently than two or three days in the course of a year. 262 EFFECTS OF THE ATMOSPHERE ON VISION. Sir William. Herschel has observed, in reference to this point, “In beautiful nights, when the outside of our tele- scopes 1s dropping with moisture, discharged from the atmo- sphere, there are now and then favourable hows in which it is hardly possible to put a limit to the magnifying powers ;_ but such valuable opportunities are extremely scarce, and with large instruments it will always be lost labour to observe at other times. In order, therefore, to calculate how long a time it must take to sweep the heavens, as far as they are within the reach of my forty-feet telescope, charged with a magnifying power of 1000, I have had recourse to my journals to find how many favourable hours we may annually hope for in this climate; and, under all favourable circum- stances, lt appears that a year which will afford ninety, or, at most, one hundred hours, is to be called very productive.” “In the equator, with my twenty-feet telescope, I have swept over zones-of two degrees with a power of 157, but an allow- ance of ten minutes in polar distance must be made for lap- ping the sweeps over one another where they jom. As the breadth of the zones may be increased towards the poles, the northern hemisphere may be swept in about 40 zones; to these we must add 19 southern zones; then 59 zones, which, on account of the sweeps lapping over one another about five minutes of time in right ascension, we must reckon of 25 hours each, will give 1475 hours; and allowing 100 hours per year, we find that with the twenty-feet telescope the heavens may be swept in about fourteen years and three- quarters. Now the time of sweeping with different magni- fying powers will be as the squares of the powers; and put- ting p and ¢ for the power and time in the twenty-feet tele- scope, and P1000 for ne power in the forty-feet instru- ment, we shall have p?: t:: P?: = 59840. Then, making the same allowance for 100 hours per year, it appears that it will require not less than 598 years to look with the forty- feet reflector, charged with the above-mentioned power, only one single moment into each point of space; and even then, so much of the southern hemisphere will remain unexplored as wil] take up 213 years more to examine.’’”* From the above remarks of so eminent an observer, the reader will perceive how difficult it is to explore the heavens with minuteness and accuracy, and with how many disap- pointments, arising from the state of the atmosphere, the astronomer must lay his account, when employed in planetary * Philos»phical Transactions for 1800, vol. xc., p. 80, &c. | POWERS NECESSARY FOR OBSERVING STARS. 2638 or sidereal investigation. Besides the circumstances now stated, it ought to be noticed that a star or a planet is only in a situation for a high magnifying power about half the time it is above the horizon. ‘The density of the atmosphere, and the quantity of vapours with which it is charged near the horizon, prevent distinct vision of celestial objects with high powers till they have risen to at least 15 or 20 degrees in altitude, and the highest magnifiers can scarcely be applied with good effect unless the object is near the meridian, and at a considerable elevation above the horizon. If the moon be viewed a little after her rising, and afterward when she comes to her highest elevation in autumn, the difference in her appearance and distinctness will be strikingly perceptible. It is impossible to guess whether a night be well adapted for celestial observations till we actually make the experiment, and instruments are frequently condemned, when tried: at improper seasons, when the atmosphere only is in-fault. A certain observer remarks, “I have never seen the face of Sa- turn more distinctly than in a night when the air has been so hazy that with my naked eye I could hardly discern a star of less than the third magnitude.”? ‘The degree of the trans- parency of the air is likewise varying almost in the course of every minute, so that even in the course of the same half hour planets and stars will appear perfectly defined, and the reverse.. The vapours moving and undulating the_ atmo- sphere, even when the sky appears clear to the naked eye, will in a few instants destroy the distinctness of vision, and in a few seconds more the object will resume its clear and well-defined aspect.’’* x 3. On the magnifying Powers requisite for observing the Phenomena of the different Planets, Comets, Double Stars, &c.—There are some objects connected with . astronomy which cannot be perceived without having recourse to in- struments and to powers of great magnitude; but it is a vul- gar error to'imagine that very large and very expensive tele- scopes are absolutely necessary for viewing the greater part of the more interesting scenery of the heavens. Most of the phenomena of the planets, comets, double stars, and other ob- * In using telescopes within doors, care should generally be taken that there be no fires in the apartment where they are placed for observa- tion, and that the air within be nearly of the same temperature as the air of the surrounding atmosphere ; for if the'room be filled with heated air, when the windows are opened there will be a current of cold air rushing in, and of heated air rushing out, which will produce such an undulation and tremulous motion as will prevent any celestial object from peing distinctly seen. 964 POWERS NECESSARY FOR OBSERVING PLANETS. jects, are visible with instruments of moderate dimensions, s* that every one who has a relish for celestial investigations may at a comparatively small expense, procure a telescope for occa sional observations which will show the principal objects ana. phenomena described in books on astronomy. Many persons have been misled by some occasional remarks which Sir W. Herschel made, in reference to certai very high powers which he sometimes put, by way of experiment, on some of his telescopes, as if these were the powers requisite for view- ing the objects to whichhe refers. For example, it is stated that be once put a power of 6450 times on his seven-feet Newtonian, telescope of 6,3,th inches aperture; but this was only for the purpose of an experiment, and could be of no use whatever when applied to the moon, the planets, and most ob- jects in the heavens. Herschel, through the whole course of his writings, mentions his only having used it twice, namely, on the stars o Lyre and y Leonis, which stars can be seen more distinctly and sharply. defined with a power of 420. ‘To produce a power of .6450 on such a telescope would require a lens of only .A,th of an inch in focal distance ; and it is questioned by some whether Herschel had lenses of so small a size im his possession, or whether it is possible to form them with accuracy. Powers requisite for observing the Phenomena of the Planets.—The planet Mercury requires a considerable magni- fying power in order to perceive its phases with distinctness. I have seldom viewed this planet with a less power than 100 and 150, with which powers its half moon, its gibbous, and its crescent phase may be distinctly perceived. With a power of 40, 50, or even 60 times, these phases can with difficulty be seen, especially as it is generally at a low altitude when such observations aremade. The phases of Venus are inuch more easily distinguished, especially the crescent phase, which is seen to the greatest advantage about a month before and after the inferior conjunction. With a power not exceeding 25 or 30 times, this phase, at such periods, may be easily perceived. It requires, however, much higher powers to perceive distinctly the variations of the gibbous phase; and if this planet be not viewed at a considerably high altitude when in a half-moon or gibbous phase, the obscurity and undulations of the atmosphere near the horizon prevent such phases from being accurately distinguished, even when high powers are applied. Although certain phenomena of the planets may be seen with such low powers as I have now stated, yet in every instance the highest magnifying powers POWERS NECESSARY FOR OBSERVING PLANETS. 265 consistent with distinctness should be preferred, as the eye 1s not then strained, and the object appears with a greater degree of magnitude and splendour. The planet Mars re- quires a considerable degree of magnifying power, even when at its nearest distance from the earth, in order to discern its spots and its gibbous phase. I have never obtained a satis- factory view of the spots which mark the surface, and their relative position, with a less power than 130, 160, or 200 times; and even with such powers, persons not much ac- customed to look through telescopes find a difficulty in dis- tinguishing them. The strongest and most prominent belts of Jupiter may be seen with a power of about 45, which power may: be put upon a twenty-inch achromatic or a one-foot reflector; but a satisfactory view of all the belts, and the relative positions they occupy, cannot be obtained with much lower powers than 80, 100, or 140. The most common positions of these belts are, one dark and well-defined belt to the south of Jupi- ter’s equator; another of nearly the same description tothe north of it, and one about his north and his south polar cir- cles. These polar belts are much more faint, and, con- sequently, not so easily distinguished as the equatorial belts. The moons of this planet, in a very clear night, may some- times be seen with a pocket. one-foot achromatic glass, mag- nifying about 15 or 16 times. Sofme people have pre- tended that they could see some of these satellites with their naked eye; but this is very doubtful, and it is probable that such persons mistook certain fixed stars which happened to be near Jupiter for his satellites. .But, in order to have a clear and interesting view of these, powers of at least 80 or 100 times should be used. In order to perceive their im- mersions into the shadow of Jupiter, and the exact moment of their emersions from it, a telescope not less than a 44-inch achromatic, with a power of 150, should be employed. When these satellites are viewed through large telescopes with high magnifying powers, they appear with well-defined disks, like small planets. The planet Jupiter has generally been considered as a good test by which to try telescopes for celestial purposes. When it is near the meridian and at a hich altitude, if its general surface, its belts, and its margin appear distinct and well defined, it forms a strong presump- tive evidence that the instrument is a good one. The planet Saturn forms one of the most interesting ob jects for telescopic observation. The ring of Saturn may be seen with a vower of 45; but it can only be contemplated Von. IX, 23 266 POWERS NECESSARY FOR OBSERVING PLANETS. with advantage when powers of 100, 150, and 200 are ap- plied to a three or a five-feet achromatic. The belts of Sa- turn are not to be seen distinctly with an achromatic of less than 27th inches aperture, or a Gregorian reflector of less than four inches aperture, nor with a less magnifying power than 100 times. Sir W. Herschel has drawn this planet with five belts across its disk; but it is seldom that above one or two of them can be seen by moderate-sized telescopes and common observers. ‘The division of the double ring, when the planet-is in a favourable position for observation, and in a high altitude, may’ sometimes be perceived with a 44-inch achromatic, with an aperture of 27th inches, and with powers of 150 or 180; but higher powers and larger instruments are generally requisite to perceive this phenomenon distinctly ; and even when a portion of it is seen at the extremities of the ansz, the division cannot, in every case, be traced along the whole of the half-circumference of the ring which is pre- sented to our eye. Mr. Hadley’s engraving of Saturn, in the “Philosophical Transactions’ for 1723, though taken with a Newtonian reflector with a power of 228, represents the division of the ring as seen only on the anse or extremities of the elliptic figure in which the ring appears. The best period for observing this division 1s when the ring appears at its utmost width. In this position it was seen in 1840, and it will appear nearly in the same position in 1855. When the ring appears like a very narrow-ellipse a short time previous to its disappearance, the division, or dark space between the rings, cannot be seen by ordinary instru- ments. Sir W. Herschel very, properly observes, “'There is not, perhaps, another object in the heavens that presents us with such a variety of extraordinary phenomena as the planet Saturn: a magnificent globe, encompassed by a stupendous double ring; attended by seven satellites; ornamented with equatorial belts; compressed. at the poles; turning upon its axis; mutually eclipsing its ring and satellites, and eclipsed by them; the most distant of the rings also turning upon its axis, and the same taking place with the farthest of the satel- lites ; all the parts of the system of Saturn occasionally reflect- ing light on each other; the rings and moons illuminating the nights of the Saturnian, the globe and satellites enlight- ening the dark parts of the ring; and the planet and rings throwing back the sun’s beams upon the moons, when they are deprived of them at the time of. their conjunctions.” This illustrious astronomer states that with a new seven-feet POWERS NECESSARY FOR OBSERVING PLANETS. 267 mirror of extraordinary distinctness he examined this planet, and found that the ring reflects more light than the body, and with a power of 570 the colour of the body becomes yellow- ish, while that of the ring remains more white. On March 11, 1780, he tried the powers of 222, 332, and 440, succes- sively, and found the light of Saturn less intense than that of the ring; the colour of the body turning, with the high powers, to a kind of yellow white, while that of the ring still remained white. Most of the satellites of Saturn are difficult to be perceived with ordinary telescopes, excepting the fourth, which may be seen with powers of from 60 to 100 times. It was discovered by Huygens in 1655 by means of a common refracting tele- scope 12 feet long, which might magnify about 70 times The. next in brightness to this is the fifth satellite, which Cassini discovered in 1671 by means ofa 17-feet refractor, which might carry a power of above 80 times. The third was discovered by the same astronomer in 1672 by a longer telescope ; and the first and second in 1684, by means of two excellent object-glasses of 100 and 136 feet, which might have magnified from 200 to 230 times. They were after- ward seen by two. other glasses of 70 and 90 feet, made by Campani, and sent from Rome to the Royal Observatory at Paris by the king’s order, after the discovery of the third and fifth satellites. It is asserted, however, that all those five satellites were afterward seen with a telescope of 34 feet, with an aperture of 3,3,th imches, which would magnify about 120 times. These satellites, on the whole, except the fourth and fifth, are not easily detected. Dr. Derham, who frequently viewed Saturn through Huygens’s glass of 126 feet focal length, declares, in the preface to his “ Astro- Theology,” that he could never perceive above three of the satellites. Sir W. Herschel observes that the visibility of these minute and extremely faint objects depends more on the penetrating than upon the magnifying power of our telescopes ; and that with a ten-feet Newtonian, charged with a magnifying power of only 60, he saw all the five old satel- lites; but the sixth and seventh, which were discovered and were easily seen with his forty-feet telescope, and were also visible in his twenty-feet instrument, were not discernible in the seven or the ten-feet telescopes, though all that magnify- ing power can do may be done as well with the seven-feet as with any larger instrument. Speaking of the seventh satellite, he says, “ Even in my forty-feet reflector it appears no bigger than a very small lucid point. I see it, however, 268 POWERS NECESSARY FOR OBSERVING PLANETS. very well in the twenty-feet reflector, to which the exquisite figure of the speculum not a little contributes.” A late ob- server asserts that in 1825, with a twelve-feet achromatic, of seven inches aperture, made by Tulley, with a power of 150, the seven satellites were easily visible, but not so easily with a power of 200; and that the planet appeared as bright as brilliantly burnished silver, and the division in the ring anda belt were very plainly distinguished with a power of 200. The planet Uranus, being generally invisible to the naked eye, is seldom an object of attention to common observers. A considerable magnifying power is requisite to make it appear in a planetary form with a well-defined disk. The best pe- riods for detecting it are when it is near its opposition to the sun, or when it happens to approximate to any of the other planets, or to a well-known fixed star. When none of these circumstances occur, its position requires to be pointed out by an equatorial telescope. On the morning of the 25th of Jan- uary, 1841, this planet happened to be in conjunction with Venus, at which time it was only four minutes north of that planet. Several days before this conjunction I made obser- vations on Uranus. On the evening of the 24th, about eight hours before the conjunction, the two planets appeared in the same field of the telescope, the one exceedingly splendid, and the other more obscure, but distinct and well defined. Uranus could not be perceived either with the naked eye or with an opera-glass, but could be distinguished as a very small star by means of a pocket achromatic telescope magnifying about 14 times. It is questionable whether, under the most favour- able circumstances, this planet can ever be distinguished by the naked eye. With magnifying powers of 30 and 70, it appeared as a moderately large star with a steady light, but without any sensible disk. With powers of 120, 180, and 250, it presented a round and pretty well-defined disk, but not so luminous and distinct as it would have done in a higher altitude. The Double Stars require a great variety of powers in order to distinguish the small stars that accompany the larger. Some of them are distinguished with moderate powers, while others require pretty large instruments, furnished with high magnifying eyepieces. [I shall therefore select only a few as a specimen. The star Castor, or qo Geminorum, may be easily seen to be double with powers of from 70 to 100. I have sometimes seen these stars, which are nearly equal in size and colour, with a terrestrial power of 44 on a 44-inch achromatic. The appearance of this star with such powers BEST POWERS FOR OBSERVING NEBULA. 269 is somewhat similar to that of y Corone in a seven-feet achro- matic of five inches aperture, with a power of 500. y An- dromede# may be seen with a moderate power. In a thirty- inch achromatic of two inches aperture and a power of 80, it appears like « Bootis when seen in a five-feet achromatic with a power of 460. ‘This star is said to be visible even in a one-foot achromatic with a power of 35. = Lyrx, which is a quintuple star, but appears to the naked eye as a single star, may be seen to be double with a power of from six to twelve times. y Leonis is visible in a 44-inch achromatic with a power of 180 or 200. Rigel in a 33-feet achromatic, may be seen with powers varying from 130 to 200. The small star, however, which accompanies Rigel, is sometimes difficult to be perceived, even with such powers. « Bootis is seldom distinctly defined with an achromatic of less aper- ture than 3ith inches, or a reflector of less than five inches, with a power of at least 250. These and similar stars are not to be expected to be seen equally well at all times, even when the magnifying and illuminating powers are properly proportioned, as much de- pends upon the state of the weather, and the pureness of the atmosphere. In order to perceive the closest of the double stars, Sir W. Herschel recommends that the power of the telescope should be adjusted upon a star known to be single, of nearly the same altitude, magnitude and colour with the double star which is to be observed, or upon one star above and another below it, Thus the late Mr. Aubert, the astro- nomer, could not see the two stars of y Leonis when the focus was adjusted upon that star itself, but he soon observed the small star after he had adjusted the focus upon Regulus. An exact adjustment of the focus of the instrument is indis- pensably requisite in order to perceive such minute objects. In viewing the Nebule, and the very small and immensely distant fixed stars, which require much light to render them visible, a large aperture of the object-glass or speculum, which admits of a great quantity of light, is of more import- ance than high magnifying powers. It is light chiefly, ac- companied with a moderate magnifying power, that enables us to penetrate into the distant regions of space. Sir W. Herschel, when sweeping the profundities of the Milky Way, and the Hand and Club of Orion, used a ore of the Newtonian form, twenty feet focal length and 18,;th inches in diameter, with a power of only 157. On applying this telescope and power to a part of the Via Lactea, he found nat it completely resolved the whole whitish appearance into 270 BEST POWERS FOR OBSERVING COMETS. stars, which his former telescopes had not light enough to effect, and which smaller instruments with much higher magnifying powers would not have effected. He tells us that, with this power, “the glorious multitude of stars,’ in the vicinity of Orion, “of all possible sizes, that presented themselves to view, was truly astonishing, and that he had fields which contained 70, 90, and 110 stars, so that a belt of fifteen degrees long and two degrees broad, which passed through the field of the telescope im an hour, could not con- tain less than fifty thousand stars that were large enough to be distinctly numbered.”’ In viewing the Milky Way, the Nebule, and small cluster of stars, such as Presepe in Cancer, I generally use a power of 55 times on an achromatic tele- scope six feet six inches in focal length and four inches in diameter. The eyepiece which produces this power—which I formed for the purpose—consists. of two convex lenses, the one next the eye three inches focal length and 1;,ths of an inch diameter, and that next the object 33 inches focus and 1ths of an inch diameter, the deepest convex surfaces being next each other, and their distance a quarter of aninch. With this eyepiece a very large and brilliant field of view is ob- tained ; and I find it preferable to any higher powers in view- ing the nebulosities and clusters of stars. In certain spaces of the heavens it sometimes presents in one field nearly a hundred stars, It likewise serves to exhibit a very clear and interesting view of the full moon. In observing Comets, a very small power should generally be used, even on large instruments. These bodies possess so small a quantity of light, and they are so frequently en- veloped ina veil of dense atmosphere, that magnifying power sometimes renders them more obscure, and therefore the ¢llu- minating power of a large telescope with a small power is in all cases to be preferred. A comet eyepiece should be con- structed with a very large and uniformly distinct. field, and should magnify only from 15 to 30 or 40 times, and the lenses of such an eyetube should be nearly two inches in diameter. The late Rev. F. Wollaston recommended for ob- serving comets ‘a telescope with an achromatic object-glass of 16 inches focal length and 2 inches aperture, witha Rams- den’s eyeglass magnifying about 25 times, mounted on a very firm equatorial stand, the field of view taking in two degrees of a great circle.”’ In viewing the moon, various powers may be applied, ac- cording to circumstances. The best periods of the moon for mspecting the inequalities on its surface are either when it . MODE OF VIEWING THE SOLAR SPOTS. 271 assumes-a crescent or a half-moon phase, or two or three days after the period of half moon. Several days after full moon, and particularly about the third quarter, when this orb is waning, and when the shadows of its mountains and vales are thrown in a different direction from what they are when on the increase, the most prominent and interesting views may be obtained. . The most convenient season for obtaining such views is during the autumnal months, when the moon, about the third quarter, sometimes rises as early as eight o’clock p. m., and may be viewed at a considerably high alti- — tude by ten or eleven. When in the positions now alluded to, and at a high altitude, very high magnifying powers may sometimes be applied with good effect, especially if the at- mosphere be clear and serene. I have sometimes applied a power, in such cases, of 350 times on a 46-inch achromatic with considerable distinctness; but it is only two or three times in a year, and when the atmosphere is remarkably favourable, that such a power can be used. The autumnal evenings are generally best fitted for such observations. _'The full moon is an object which is never seen to advantage with high powers, as no shadows or inequalities on its surface can then be perceived. It forms, however, a very beautiful ob- ject when magnifying powers not higher than 40, 50, or 60 times are used. A power of 45 times, if properly construct- ed, will show the whole of the moon with a margin around it, when the darker and brighter parts of its surface will pre- sent a variegated aspect, and appear somewhat like a map to the eye of the observer. 4. Mode of exhibiting the Solar Spots.—The solar spots may be contemplated with advantage by magnifying powers varying from 60 to 180 times; about 90: times is a good medium power, though they may sometimes be distinguished. with very low powers, such as those usually adapted to a one-foot telescope, or even by means of a common opera-glass. The common astronomical eyepiece given along with achro- matic telescopes, and the sunglasses connected with them, are generally ill adapted for taking a pleasant and comprehensive view of the solar spots. Inthe higher magnifying powers, the first eyeglass is generally at too great a distance from the eye, and the sunglass which is screwed over it removes it to a still greater distance from the point to which the eye is ap- plied, so that not above one-third of the field of view can be taken in. This circumstance renders it difficult to point the instrument to any particular small spot on the solar disk which we wish miautely to inspect ; and besides, it prevents 272 MODE OF SHOWING THE SOLAR SPOTS. us from taking a comprehensive view of the relative positions of all the spots that may at any time be traversing the disk. To obviate this inconvenience, the sunglass would require to be placed so near to the glass next the eye as almost to touch it. But this is sometimes difficult to be attained, and, in high powers, even the thickness of the sunglass itself is sufficient to prevent the eye from taking in the whole field of view. For preventing the inconveniences to which I now allude, I _ generally make use of a terrestrial eyepiece of a considerable power, with a large field ; the sunglass is fixed at the end of a short tube, which slides on the eyepiece, and permits the coloured glass to approach within a line or two of the lens next the eye, so that the whole field of the telescope is com- pletely secured. ‘The eyepiece alluded to carries a magni- fying power of 95 times for a 46-inch telescope, and takes in about three-fourths of the surface of the sun, so that the rela- tive positions of all the spots may generally be perceived at one view. Such a power is, in most cases, quite sufficient for ordinary observations, and I have seldom found any good effect to arise from attempting very high powers when mi- nutely examining the solar spots. But the most pleasant mode of viewing the solar spots, es- pecially when we wish to exhibit them to others, is to throw the image of the sun upon a white screen, placed in a room which is considerably darkened. It is difficult, however, when the sun is at a high altitude, to put this method into practice, on account of the great obliquity with which his rays then fall, which prevents a screen from being placed at any considerable distance from the eye-end of the telescope. The following plan, therefore, is that which I uniformly adopt, as being both the easiest and the most satisfactory. A telescope is placed in a convenient position, so as to be di- rected to the sun. This telescope is furnished with a diago- nal eyepiece, such as that represented fig. 77, (p. 247.) The window-shutters of the apartment are all closed excepting a space sufficient to admit the solar rays; and when the tele- scope is properly adjusted, a beautiful image of the sun, with all the spots which then happen to diversify his surface, is thrown upon the ceiling of the room. This image may be from 12 to 20, or 30 inches or more in diameter, according to the distance of the ceiling from the diagonal eyepiece. The greater this distance is, the larger the image. If the sun is at a very high altitude, the image will be elliptical; if he be at no great distance from the horizon, the image will appear circular, or nearly so; but in either case the spots will be MODE OF MEASURING THE SOLAR SPOTS. 273 distinctly depicted, provided the focus of the telescope be ac- curately adjusted. In this exhibition, the apparent motion of the sun produced by the rotation of the earth, and the pas- sage of thin fleeces of clouds across the solar disk, exhibit a very pleasing appearance. By this mode of viewing the solar spots we may easily ascertain their diameter and magnitude, at least to a near ap- proximation. We have only to take a scale of inches, and measure the diameter of any well-defined and remarkable spot, and then the diameter of the solar image; and, com- paring the one with the other, we can ascertain the number of miles, either lineal or square, comprehended in the dimen- sions of the spot. For example, suppose a spot to measure half an inch in diameter, and the whole image of the sun 25 inches, the proportion between the diameter of the spot and that of the sun will be as 1 to 50; in other words, the one- Jiftieth part of the sun’s diameter. Now this diameter being 880,000 miles, this number divided by 50, produces a quo- tient of 17,600—the number of miles which its diameter measures. Such a spot will therefore contain an area of 243,285,504, or more than two hundred and forty-three mil- lions of square miles, which is 46 millions of miles more than the whole superficies of the terraqueous globe. Again, sup- Figure 82. 274 SPACH-PENETRATING POWER OF TELESCOPES. pose the diameter of a spot measures ths of an inch, and the solar image 23 inches, the proportion of the diameter of the spot to that of the sun is as 3 to 230=the number of tenths in 23 inches. The number of miles in the spot’s dia- — meter will therefore be found by the followmg proportion: 230 : 880,000 :: 3: 11,478; that is, the diameter of sucha spot measures eleven thousand four hundred and seventy- eight miles. Spots of such sizes are not eee seen to transit the solar disk. 7 By this mode of viewing the image of the sun, his spots may be exhibited to twenty or thirty individuals at once without the least stramimg or injury to the eyes; and as no separate screen is requisite, and as the ceilings of rooms are general.y white, the experiment may be performed im half a minute without any previous preparation except screwing on and adjusting the eyepiece. ‘The manner of exhibiting the solar spots in this way is represented in fig. 82. 5. On the Space-penetrating Power of Telescopes.—The power of telescopes to penetrate into the profundity of space is the result of the quantity of light they collect and send to the eye in a state fit for vision. ‘This property of telescopes is sometimes designated by the expression Jlluminating Power. Sir W. Herschel appears to have been the first who made a distinction between the magnifying power and the space- penetrating power of a telescope; and there are many ex- amples which prove that such a distinction ought to be made, especially in the case of large instruments. For example, the small star, or speck of light, which accompanies the pole star, may be seen through a telescope of large aperture with a smaller magnifying power than with a telescope of ‘a small aperture furnished with a much higher power. If the mag- nifying power is sufficient to show the small star completely separated from the rays which surround the large one, this is sufficient in one poimt of view; but, in order that this effect may be produced, so as to render the small star perfectly dis- tinguishable, a certain quantity of light must be admitted into the pupil of the eye, which quantity depends upon the area of the object-glass or speculum of the instrument, or, in other words, on the illuminating power. If we compare a telescope of 27th inches aperture with one of 5 inches aperture, when the magnifying power of each does not exceed 50 times for terrestrial objects, the effect of illuminating power is not so evident; but if we use a power of 100 for day objects, and 180 for the heavenly bodies, the effect of illuminating power SPACE-PENETRATING POWER OF TELESCOPES. 275 _is so clearly perceptible, that objects not only appear brighter and more clearly visible in the larger telescope, but with the same magnifying power they also appear larger, particularly when the satellites of Jupiter and small stars are the objects we are viewing. Sir W. Herschel remarks, that “ objects are viewed in their greatest perfection when, in penetrating space, the mag- nifying power is so low as only to be sufficient to show the object well, and when, in magnifying objects, by way of ex- amining them minutely, the space-penctrating power is no higher than what will suffice for the purpose ; for in the use of either power, the injudicious overcharge of the other will rove hurtful to vision.””. When illuminating power is in too high a degree, the eye is offended by the extreme brightness of the object; when it is in too low a degree, the eye is dis- tressed by its endeavours to see what is beyond its reach; and therefore it is desirable, when we wish to give the eye all the assistance possible, to have the illuminating and the magnifying powers in due proportion. What this proportion is, depends, in a certain degree, upon the brightness of the object. In proportion to its brightness or luminosity, the magnifying power may, to acertain extent, be increased. Sir W. Herschel remarks, in reference to a Lyre, “ This star, I surmise, has light enough to bear being magnified at least a hundred thousand times, with no more than six inches of aperture.”” However beautifully perfect any telescopes may appear, and however sharp their defining power, their per- formance is limited by their illuminating powers, which are as the squares of the diameters of the apertures of the respec- tive instruments. Thus a telescope whose object-glass is four inches diameter will have four times the quantity of light, or illuminating power, possessed by a telescope whose aper- ture is only two inches, or in the proportion of 16 to 4; the square of 4 being 16, and the square of 2 being 4. The nature of the. space-penetrating power to which we are adverting, and the distinction between it and magnifying power, may be illustrated from a few examples taken from Sir W. Herschel’s observations. The first observation which I shall notice refers to the ne- bula between y and ¢ Ophiuchi, discovered by Messier in 1764. The observation was made with a ten-feet reflector, having a magnifying power of 250, and a space-penetrating power of 28.67. His. note 1s dated May 3, 1783. “I see several stars in it, and make no doubt a higher power and more light will resolve it all into stars. ‘This seems tome a 276 SPACE-PENETRATING POWER OF TELESCOPES. good nebula for the purpose of establishing the connection | between nebule and clusters of stars in general.’”’ “ June 18, 1784. The same nebula viewed with a Newtonian twenty- feet reflector; penetrating power 61, anda magnifying power of 157; a very large and a very bright cluster of excessively _ compressed stars. The stars are but just visible, and are of unequal magnitudes. The large stars are red; the cluster is a miniature of that near Flamstead’s forty-second Come Be- renices. Right ascension, 17° 6" 32%. Polar distance, 108° 18”.”’ In this case, a penetrating power of about 28, with a magnifying power of 250, barely showed a few stars ; while in the second instrument the illuminating power of 60, with the magnifying power of only 157, showed them completely. Subsequently to the date of the latter observation, the twenty-feet Newtonian telescope was converted into an Her- schelian instrument by taking away the small speculum, and giving the large one the proper inclination for obtaining the front view; by which alteration the illuminating power was increased. from 61 to 75, and the advantage derived from the alteration was evident in the discovery of the satellites. of Uranus by the altered telescope, which before was incom- petent in the point of penetration, or illuminating power. “March 14, 1798, I viewed the Georgian planet (or Uranus) with a new twenty-five feet reflector. Its penetrating power is 95°85, and having just before also viewed it with my twenty-feet instrument, I found that with an equal magnify- ing power of 300, the twenty-five feet telescope had consider- ably the advantage of the former.” The aperture of the twenty-feet instrument was 18°8 inches, and that of the twenty-five feet telescope 24 inches, so that the superior effect of the latter instrument must have been owing to its oreater illummating power. The following observations show the superior power of the forty-feet telescope, as compared with the twenty-feet. «Feb. 24,1786, I viewed the nebula near F'lamstead’s fifth Serpentis with my twenty-feet reflector, magnifying power 157. ‘The most beautiful, extremely compressed cluster of small ‘stars, the greatest part of them gathered together into one brilliant nucleus, evidently con- sisting of stars, surrounded with many detached gathering stars of the same size and colour. R.A., 15°77 12% P.D., 87° 8”.” “May 27, 1791, I viewed the same object with my forty-feet telescope, penetrating power 191-69, magnify- ing power 370. A beautiful cluster of stars. I counted about 200 of them. The middle of it is so compressed that it is impossible to distinguish the stars.” “ Nov. 5, 1791, I viewed SPACEH-PENETRATING POWER OF TELESCOPES. 277 Saturn with the twenty and forty-feet telescopes: Twenty- feet.—The fifth satellite of Saturn is very small. The first, second, third, fourth, and fifth, and the new sixth satellites, are in their calculated places. Forty-feet.—I see the new sixth satellite much better with this instrument than with the twenty-feet. The fifth is also much larger here than in the twenty-feet, in which it was nearly the same size as a small - fixed star, but here it is considerably larger than that star.” These examples, and many others of a similar kind, ex- plain sufficiently the nature and extent of that species of power that one telescope possesses over another, in conse- quence of its enlarged aperture; but the exact quantity of this power is in some degree uncertain. ‘T'o ascertain practically the illuminating power of telescopes we must try them with equal powers on such objects as the following: the small stars near the pole star, and near Rigel and « Bootis; the division in the ring of Saturn; and distant objects’ in the twilight or towards the evening. These objects are distinctly seen with a five-feet achromatic of 3,8,th inches aperture, and an illu- minating power of 144, while they are scarcely visible in a 32 feet with an aperture of 27th inches, and an illuminating power of 72, supposing the same magnifying power to be ap- plied. The illuminating power of a telescope is best esti- mated, in regard to land objects, when it is tried on minute objects, and such as are badly lighted up; and the advantage of a telescope with-a large aperture will be most obvious when it is compared with another of inferior size in the close of the evening, when looking at a printed bill composed of letters of various sizes. As darkness comes on, the use of illumi- nating power becomes more evident. In a five-feet telescope some small letters will be legible which are hardly discern- ible in the 33 feet, and in the 23 feet are quite undefinable, though the magnifying powers be equal. Sir W. Herschel informs us that,in the year 1776, when he had erected a telescope of twenty feet focal length of the Newtonian con- struction, one of its effects by trial was, that when, towards evening, on account of darkness, the natural eye could not penetrate far into the space, the telescope possessed that power sufficiently to show, by the dial of a distant church steeple, what o’clock it was, notwithstanding the naked eye could no longer see the steeple itself. In order to convey an idea of the numbers by which the de gree of space-penetrating power is expressed, and the general grounds on which they rest, the following statements may be made. ‘The depth to which the naked eye can penetrate Von. IX, 24 278 SPACE-PENETRATING POWER OF TELESCOPES. into the spaces of the heavens is considered as extending to the twelfth order of distances ; in other words, it can perceive a star at a distance twelve times farther than those lumina- ries, such as Sirius, Arcturus, or Capella, which, from their vivid light, we presume to be nearest to us. It has been stated above that Herschel calculated his ten-feet telescope to have a space-penetrating power of 28-67, that is, it could | enable us to descry a star 28 times farther distant than the ~ naked eye can reach. His twenty-feet Newtonian was con- sidered as having a similar power of 61; his 26 feet, nearly 96; and his forty feet instrument, a power of 191.69. If each of these numbers be multiplied by 12, the product will indicate how much farther these telescopes will penetrate into space than the nearest range of the fixed stars, such as those of the first magnitude. For instance, the penetrating power of the forty-feet reflector being 191.69, this number, multiplied by 12, gives a product of 2300, which shows that, were there a series of two thousand three hundred stars ex- tended in a line beyond Sirius, Capella, and» similar stars, each star separated from the one beyond it by a space equal to the distance. of Sirius from the earth, they might be all seen through the forty-feet telescope. In short, the pene- trating power of telescopes is a circumstance which requires to be particularly attended to in our observations of celestial phenomena, and in many cases is of more importance than magnifying power. It is the effect produced by illuminating power that renders telescopes, furnished with comparatively. small magnifying powers, much more efficient in observing comets, and certain nebule and clusters of stars, that when high powers are attempted. Every telescope. may be so ad- justed as to produce different ‘ space-penetrating powers. If we wish to diminish such a power, we have only to con- tract the object-glass or speculum by placing circular rims, or apertures of different degrees of breadth, across the mouth of the great tube of the instrument. But we cannot increase this illuminating power beyond a certain extent, which is limited by the diameter of the object-glass. When we wish illuminating power beyond this limit, we must be furnished with an object-glass or speculum of a larger size; and hence the rapid advance«in price of instruments which have large apertures, and consequently high illuminating powers. Mr. Tulley’s 33 feet achromatics of 22th inches aperture sell at £26 5s.; when the aperture is 3th inches, the price is £42; when 37th inches, £68 5s. The following table contains a statement of the “ comparative lengths, apertures, illuminating HOW TO CHOOSE A TELESCOPE. 279 powers, and prices of achromatic refractors and Gregorian re- flectors,”’ according to Dr. Kitchener. ACHROMATIC REFRACTORS. GREGORIAN, &c., REFLECTORS. ~ s 3 os o5 a2 Se a re ad my 0 ee | Se | £8 | Price |lMityarctnown| EE (| 28 | Pre ev , a ‘ eies | gs | 22 y2 ge | 23 Feet. In. Th eae are Feet In. Th £8 2 re 6 25 4 4/1 pay 62 teers 2k 2 40 | 12 12 || 14 3 90 12,12 3h | 2 .7 72 | 2lto || 2 #5 202 |. 20 ‘| 42 a eo 302 50 5 3, 8 144 '105 to 4 7 490 | 105 1150 7 Newtonian| 7 490 | 126 fs 5 250 250 5 Gregorian | 9 810 | 200 7 6 360 360 10 Newtonian|10 1009 | 315 The illuminating powers stated in the above table are only comparative. Fixing on the number 25 as the illuminating power of a two-feet telescope, 1,%ths of an inch aperture, that of a 22 feet, two inches aperture, will be 40; of a five- feet, 3,8,th inches aperture, 144, &c. If the illuminating power of a Gregorian 13 foot, and three inches aperture, be 90, a five-feet, with nine inches aperture, will be 810, &c. 6. On choosing Telescopes, and ascertaining their Pro- perties.—lIt is an object of considerable importance to every astronomical observer that he should be enabled to form a judgment of the qualities of his telescope, and of any instru- ments of this description which he may intend to purchase. The following directions may perhaps be. useful to the reader in directing him in the choice of an achromatic refracting telescope : Supposing that an achromatic telescope of 32 feet focal length and 3ith inches aperture were offered for sale, and that it were required to ascertain whether the object-glass, on which its excellence chiefly depends, is a good one, and duly adjusted, some opinion may be formed by laying the tube of the telescope in a horizontal position, on a firm support, about the height of the eye, and by placing a printed card ora watch-glass vertically, but in an inverted position, against some wall or pillar at 40 or 50 yards distant, so as to be ex- posed toa clear sky. When the telescope is directed to this object, and accurately adjusted to the eye, should the letters on the card, or the strokes and dots on the watch-glass, appear clearly and sharply defined, without any mistiness or colora- tion, and if very small spots appear well defined, great hopes 280 HOW TO CHOOSE A TELESCOPE. may be entertained that the glass will turn out a good one. But a telescope may appear a good one, when viewing com- mon terrestrial objects, to eyes unaccustomed to discriminate deviations from perfect vision, while it may turn out to be an © indifferent one when directed to certain celestial objects. Instead, therefore, of a printed card, fix a black board, or one half of a sheet of black paper, in a vertical position at the same distance, and a circular disk of white writing paper, about. one-fourth of an inch in diameter, on the centre of the biack ground; then, having directed the telescope to this object, and adjusted for the place of distinct vision, mark with a black-lead pencil the sliding eyetube at the end of the main tube, so that this position can always be known; and if this sliding tube be gradually drawn out or pushed in while the eye beholds the disk, it will gradually enlarge and_ lose its colour till its edges cease to be well defined.. Now if the enlarged misty circle is observed to be concentric with the disk itself, the object-glass is properly centred, as it has reference to the tube ; but if the misty circle goes to one side of the disk, the cell of the object-glass is not at right angles to the tube, and must have its screws removed and its holes elongated by a rat-tailed file small enough to enter the holes. When this has been done, the cell may be replaced, and the disk examined a second time, and a slight stroke on one edge of the cell by a wooden mallet will show by the alteration made in the position of the misty portion of the disk how the adjustment is to be effected, which is known to be right when a motion in the slidmg tube will make the diluted disk en- large in a circle concentric with the disk itself. When the disk will enlarge so as to make a ring of diluted white light round its circumference as the sliding tube holding the eye- piece is pushed in or drawn out, the cell may be finally fixed by the screws passing through its elongated holes. When the object-glass is thus adjusted, it may then be ascertained whether the curves of the respective lenses com- posing the object-glass are well formed, and suitable for each other. Ifa small motion of the sliding tube of about j,th of an inch in a 32 feet telescope from the point of distinct vision will dilute the light of the disk and render the appearance confused, the figure of the object-glass is good, particularly if the same effect will take place at equal distances from the point of distinct vision when the tube is alternately drawn out and pushed in. A telescope that will admit of much motion in the sliding tube without sensibly affecting the dis- tinctness of vision, will not define an object well at any point HOW TO CHOOSE A TELESCOPE. 281 of adjustment, and must be considered as having an imperfect object-glass, masmuch as the spherical aberration of the transmitted rays is not duly corrected. ‘The due adjustment of the convex lens or lenses to the concave one will be judged of by the absence of coloration round the enlarged disk, and is a property distinct from the spherical aberration; the achromatism depending on the relative focal distances of the convex and concave lenses is regulated by the relative dis- petrsive powers of the pieces of glass made use of, but the distinctness of vision depends on a good figure of the com- puted curves that limit the focal distances. When an object- glass is free from imperfection in both these respects, it may be called a good glass for terrestrial purposes. It still, however, remains to be determined how far such an object-glass may be good for viewing a star or a planet, and can only be known by actual observations on the heavenly bodies. When a good telescope is directed to the moon or to Jupiter, the achromatism may be judged of by alternately pushing in and drawing out the eyepiece from the place of distinct vision. In the former case, a ring of purple will be formed round the edge; and in the latter, a ring of light green, which is the central colour of the prismatic spectrum ; for these appearances show that the extreme colours, red and violet, are corrected. Again, if one part of a lens employed have a different refractive power from another part of it, that is, if the flint-glass particularly is not homogeneous, a star of the first and even of the second magnitude will point out the natural defect by the exhibition of an irradiation, or what is called a wing, at one side, which no perfection of figure or of adjustment will banish, and the greater the aperture, the more liable is the evil to happen: hence caps with different apertures are usually supplied with large telescopes, that the extreme parts of the glass may be cut off in observations re- quiring a round and well-defined image of the body observed. Another method of determining the figure and quality of an object-glass is by first covering its centre by a circular piece of paper, as much as one-half of its diameter, and adjusting it for distinct vision of a given object, such as the disk above mentioned, when the central rays are intercepted, and then trying if the focal length remains unaltered when the paper is taken away and an aperture of the same size applied, so that the extreme rays may in their turn be cut off. If the vision remains equally distinct in both cases, without any new adjustment for focal distance, the figure is good, and the spherical aberration cured, and it may be seen by view- 24* ~ 282 HOW TO CHOOSE A TELESCOPE. ing a star of the first magnitude successively in both cases, whether the irradiation is produced more by the extreme or by the central parts of the glass; or, in case the one-half be faulty and the other good, a semicircular aperture, by being | turned gradually round in trial, will detect what semicircle contains the defective portion of the glass; and if such por- tion should be covered, the only inconvenience that. would ensue would be the loss of so much light as is thus excluded. When an object-glass produces radiations in a large star, it is unfit for the nicer observations of astronomy, such as viewing double stars of the first class. The smaller a large star ap- pears in any telescope, the better is the figure of the object- glass; but if the image of the star be free from wings, the size of its disk is not an objection in practical observations.* Some opticians are in the habit of inserting a diaphragm into the body of the large tube, to cut off the extreme rays coming from the object-glass when the figure is not good, instead of lessening the aperture by a cap.. When this is the case, a deficiency of light will be the consequence beyond what the apparent aperture warrants. It is therefore proper to examine that the diaphragm be not placed too near the object-glass, so as to intercept any of the useful rays. Some- times a portion of the object-glass is cut off by the stop in the eyetube. T'o ascertain this, adjust the telescope to distinct vision, then take out the eyeglasses, and put your finger on some other object on the edge of the outside of the object- glass, and look down the tube; if you can see the tip of your finger, or any object in its place, just peeping over the edge of the object-glass, no part is cut off. I once had a 33 feet telescope whose object-glass measured three inches in diame- ter, which was neither so bright, nor did it perform in other respects nearly so well as another of the same length whose object-glass ‘was only 27th inches in diameter; but I found that a diaphragm was placed about a foot within the end of the large tube, which reduced the aperture of the object-glass to less than 23 inches, and when it was removed the tele- scop* was less distinct than before. ‘The powers given along with this instrument were much lower than usual, none of them exceeding 100 times. This is a trick not uncommon with some opticians. : ; Dr. Pearson mentions that,an old Dollond’s telescope of 63 inches focal length and 3zth inches aperture, supposed to be an excellent one, was brought to Mr. Twlley when he was * The above directions and remarks are abridged with some altera- tions from Dr, Pearson’s “ Introduction to Practical Astronomy,’’ vol. ii. { 1 HOW TO CHOOSE A TELESCOPE. 283 present, and the result of the examination was that its achro- matism was not perfect. The imperfection was thus deter- mined by experiment. A small glass globe was placed at forty yards distance from the object-end of the telescope when the sun was shining, and the speck of light seen reflected from this globe formed a good substitute for a large star, as an object to be viewed. When the focal length of the object- glass was adjusted to this luminous object, no judgment could be formed of its prismatic aberrations till the eyepiece had been pushed in beyond the place of correct vision; but when the telescope was shortened a little, the luminous disk occa- sioned by such shortening was strongly tinged with red rays at its circumference. On the contrary, when the eyepiece was drawn out so as to lengthen the telescope too much, the disk thus produced was tinged with a small circle of red at its centre, thereby denoting that the convex lens had too-short a focal length; and Mr. Tulley observed, that if one or both of the curves of the convex lens were flattened till the total length should be about four inches increased, it would render the telescope quite achromatic, provided in doing this the aberration should not be increased. The followmg general remarks may be added: 1. To make any thing like an accurate comparison of telescopes, they must be tried not only at the same place, but as nearly as possible at the same time, and, if the instruments are of the same length and construction, if possible, with the same eyepiece. 2. A difference of eight or ten times in the mag- nifying power will sometimes, on certain objects, give quite a different character to a telescope. It has been found by va- rious experiments that object-glasses of two or three inches tonger focus will produce different vision with the same eye- piece. 3. Care must be taken to ascertain that the eye- glasses are perfectly clean and free from defects. The de- fects of glass are either from veins, specks, scratches, colour, or an incorrect figure. ‘To discover veins in an eye or an object-glass, place a candle at the distance of four or five yards; then look through the glass, and move it from your eye till it appear full of light; you will then see every vein, or other imperfection in it, which may distort the objects and render vision imperfect. Specks or scratches, especially in object-glasses, are not so injurious as veins, for they do not distort the object, but only intercept a portion of the light. 4. We cannot judge accurately of the excellence of any tele- scope by observing objects with which we are not familiarly acquainted, Opticians generally try an instrument at their 284 MAGNIFYING POWER OF TELESCOPES. own marks, such asthe dial-plate of a watch, a finely-eneraved card, a weather-cock, or the moon and the planet Jupiter, when near the meridian. Of several telescopes of the same length, aperture, and magnifying power, that one is generally considered the best with which we can read a given print at the greatest distance, especially if the print consists of figures, such as a table of logarithms, where the eye is not apt to be deceived by the imagination in guessing at the sense of a passage when two or three words are distinguished. There is a circumstance which I have frequently noticed in reference to achromatic telescopes, particularly those of a small size, and which I have never seen noticed by any op- tical writer. Itis this: if the telescope, when we are view- ing objects, be gradually turned round its axis, there is a certain position in which the objects will appear distinct and accurately defined ; and if it be turned round exactly a semi- circle from this point, the same degree of distinctness is per- ceived, but in all other positions there is an evident want of clearness and defining power. © This I find to be the case in more than ten one-foot and two-feet telescopes now in my possession, and therefore [ have put marks upon the object- end of each of them to indicate the positions in which they should be used for distinct observation. This is a circum- stance which requires, in many cases, to be attended to in the choice and the use of telescopical instruments, and in fixing and adjusting them on their pedestals. In some tele- scopes this defect is very striking, but it is in some measure perceptible in the great majority of instruments which I have had occasion to inspect.. Even in large and expensive achro- matic telescopes this defect is sometimes observable. I have an achromatic whose object-glass is 45th inches diameter, which was much improved in its defining power by being unscrewed from its original position, or turned round its axis about one-eighth part of its circumference. This defect is best detected by looking at a large printed bill, or a sign-post at a distance, when, on turning round the telescope or object- glass, the letters will appear much better defined in one posi- tionthan in another. The position in which the object appears least distinct is when the upper part of the telescope is a quadrant of a circle different from the two positions above stated, or at an equal distance from each of them. 7. On the Mode of determining the magnifying Power of Telescopes.—In regard to refracting telescopes, we have already shown that, when a single eyeglass is used, the mag- nifying ‘power may be found by dividing the focal distance : MAGNIFYING POWER OF TELESCOPES. 285 of the object-glass by that of the eyeglass; but when a Huy- genian eyepiece, or a four-glass terrestrial,eyepiece, such as is now Common in achromatic telescopes, is used, the magni- fying power cannot be ascertained in this manner; and in some of the delicate observations of practical astronomy, it is of the utmost importance to know the exact magnifying power of the instrument with which the observations are made, par- ticularly when micrometrical measurements are employed to obtain the desired results. The following is a general method of finding the magnifying powers of telescopes when the in- strument called a dynameter is not employed, and it answers for refracting and reflecting telescopes of every description. Having put up a small circle of paper an inch or two in diameter at the distance of about 100 yards, draw upon a card two black parallel lines, whose distance from each other is equal to the diameter of the paper circle; then view through the telescope the paper circle with one eye, and the parallel lines with the other, and let the parallel lines be moved nearer to or farther from the eye, till they seem exactly to cover the small circle viewed through the telescope: the quotient ob- tained by dividing the distance of the paper circle by the dis- tance of the parallel lines from the eye will be the magnifying power of the telescope. It requires a little practice before this experiment can be performed with accuracy. ‘The one eye must be accustomed to look at an object near at hand, while the other is looking at a more distant object through the telescope. Both eyes must be open at the same time, and the image of the object seen through the telescope must be brought into apparent contact with the real object near at hand. But a little practice will soon enable any observer to perform the experiment with ease and correctness, if the tele- scope be mounted on a firm stand, and its elevation or de- pression produced by rack-work. The following is another method, founded on the same principle: Measure the space occupied by a number of the courses, or rows of bricks in amodern building, which, upon an average, is found to have eight courses in two feet, so that each course or row is three inches. Then cut a piece of paper three inches in height, and of the length of a brick, which is about nine inches, so that it may represent a brick, and fixing the paper against the brick wall, place the tele- scope to be examined at the distance of about 80 or 100 yards from it. Now, looking through the telescope at the paper with one eye, and at the same time, with the other eye, look- ing past the telescope, observe what extent of wall th> mag- 286 MAGNIFYING POWER OF TELESCOPES. nified image of the paper appears to cover; then count the courses of bricks in that extent, and it will give the magnify- ing power of the telescope. It is to be observed, however, that the magnifying power determined in this way will be a fraction greater than for very distant objects, as the focal dis- tance of the telescope is necessarily lengthened in order to obtain distinct vision of near objects. In comparing the magnifying powers of two telescopes, or of the same telescope when different magnifying powers are employed, I generally use the following simple method. The telescopes are placed.at eight or ten feet distance from a win- dow, with their eye-ends parallel to each other, or at the same distance from the window. Looking at a distant ob- ject, I fix upon a portion of it whose magnified image will appear. to fill exactly two or three panes of the window ; then, putting on a different power, or looking through another tele- scope, [ observe the same object, and mark exactly the extent of its image on the window-panes, and compare the extent of the one image with the other.. Suppose, for example, that the one telescope has been previously found to magnify ninety times, and that the image of the object fixed upon ex- actly fills three panes of the window, and that with the other power or the other telescope the image fills exactly two panes, then the magnifying power is equal to two-thirds of the for- mer, or sixty times; and were it to fill only one pane, the power would be about thirty times. A more correct method is to place at one side of the widow a narrow board two or tnree feet long, divided into fifteen or twenty equal parts, and observe how many of these parts appear to be covered by the respective images of the different telescopes. Suppose, in the one case, ten divisions to be covered by the image in a tele- scope magnifying ninety times, and that the image of the same object in another telescope measures six divisions, then its power is found by the following proportion: 10: 90 :: 6 54; that is, this telescope magnifies 54 times. Another mode which I have used for determining, to a near approximation, the powers of- telescopes, is as follows : Endeavour to find the focus of a single lens which is exactly equivalent to the magnifying power of the eyepiece, whether the Huygenian or the common terrestrial eyepiece. This may be done by taking a small lens, and using it as an ob- ject-glass to the eyepiece. Looking through the eyepiece to a window and holding the lens at a proper distance, observe whether the image of one of the panes exactly coincides with the pane as seen by the naked eye; if it does, then the mag- MAGNIFYING POWER OF TELESCOPES. 287 nifying power of the eyepiece is equal to that of the lens. If the lens be half an inch in focal length, the eyepiece will pro- duce the same magnifying power as a single lens when used as an eyeglass to the telescope, and the magnifying power will then be found by dividing the focal distance of the object- glass by that of the eyeglass ; but if the image of the pane of glass does not exactly coincide with the pane as seen by the other eye, then proportional parts may be taken by observing the divisions of such a board as described above, or we may try lenses of different focal distances. Suppose, for example, that a lens two inches focal length had been used, and that the image of a pane covered exactly the space of two panes, the power of the eyepiece is then equal to that of a single lens of one inch focal distance. The following is another mode depending on the same general principle. If a slip of writing-paper one inch long, or a disk of the same material one inch in diameter, be placed on a black ground at from 30 to 50 yards’ distance from the object-end of the telescope, and a staff painted white, and di- vided into inches and parts by strong black lines, be placed vertically near the said paper or disk, the eye that is directed through-the telescope when adjusted’ for vision will see the magnified disk, and the other eye, looking along the outside of the telescope, will observe the number of inches and parts that the disk projected on it will just cover, and as many inches as are thus covered will indicate the magnifying power of the telescope, at the distance for which it 1s adjusted for distinct vision. The solar power, or powers for very distant objects, may be obtained by the following proportion: As the terrestrial focal length at the given distance is to the solar focal length, so is the terrestrial power to the solar power. For example, a disk of white paper one inch in diameter was placed on a black board, and suspended on a wall contiguous to a vertical black staff that was graduated into inches by strong white lines, at a distance of 33 yards 23 feet, and when the adjustment for vision was made with a 42-inch telescope, the left eye of the observer viewed the disk projected on the staff, while the right eye observed that the enlarged image of the disk covered just 582 inches on the staff, which number was the measure of the magnifying power at the distance an- swering to 33 yards 22 feet, which in this case exceeded the solar focus by an inch anda half. Then, according to the above analogy, we have, as 43.5 : 42 :: 58.5 : 56.5 nearly. Hence the magnifying power due to the solar focal length of the telescope in question is 56.5, and the distance, 33 yards 288 MAGNIFYING POWER OF TELESCOPES. 23 feet, is that which corresponds to an elongation of the solar focal distance an inch and a half.* If we multiply the terrestrial and’ the solar focal distances together, and divide the product by their difference, we shall again obtain the distance of the terrestrial object from the telescope. Thus, — 1218 inches=101°5 feet, or 33 yards 23 feet. The magnifying power of a telescope is also determined by measuring the image which the object-glass or the large speculum of a telescope forms at its solar focus. This is ac- complished by means of an instrument called a Dynameter. This apparatus consists of a strip of mother-of-pearl, marked with equal divisions, from the ;45th to the ;g4 th of an inch apart, according to the accuracy required. ‘This measure is attached to a magnifying lens in its focus, in order to make the small divisions more apparent. When the power ofa telescope is required, the person must measure the clear aperture of the object-glass; then, holding the pearl dyna- meter next the eyeglass, let him observe how many divisions the small circle of light occupies when the instrument is di- rected to a bright object; then, by dividing the diameter of the object-glass by the diameter of this circle of light, the power will be obtained.t ‘The most accurate instrument of this kind is the Double Image Dynameter invented by Rams- den, and another on the same principle now made by Dollond, a particular description of which may be found in Dr. Pear- son’s “Introduction to Practical Astronomy.” ‘The advan- tage attending these dynameters is, that they do not require any knowledge of the thickness and focal lengths of any of the lenses employed in a telescope, nor yet of their number or relative positions; neither does it make any difference whether the construction be refracting or reflecting, direct or inverting. One operation includes the result arising from the most complicated construction. I shall only mention farther the following method of dis- covering the magnifying power, which is founded on the same general principle as alluded to above. Let the tele- scope be placed in such a position opposite the sun that the rays of light may fall perpendicularly on the object-glass ; the pencil of rays may be received on a piece of paper, and its diameter measured. Then, as the diameter of the pencil of rays is to that of the. object-glass, so is the magnifying power of the telescope. * Pearson’s ‘‘ Practical Astronomy,”’ vol. ii. t The mother-of-pearl dynameter may be purchased for about twelve shillings. See fig. 57, a, b,c, p. 190. HOW TO CLEAN A TELESCOPE. 289 8. On Cleaning the Lenses of Telescopes.—tIt is neces- sary, in order to distinct vision, that the glasses, particularly the eyeglasses of telescopes, be kept perfectly clean, free of damp, dust, or whatever may impede the transmission of the rays of light; but great caution ought to be exercised in the wiping of them, as they are apt to be scratched or otherwise injured by a rough and incautious mode of cleaning them. They should never be attempted to be wiped unless they really require it; and in this case, they should be wiped care- fully and gently with a piece of new and soft lamb’s-skin leather; if this be not at hand, a piece of fine silk paper, or fine clean linen may be used as a substitute. The lens which requires to be most particularly attended to is the second glass from the eye, or the field-glass; for if any dust or other im- pediment be found upon this glass, it is always distinctly seen, being magnified by the glass next the eye. The next glass which requires attention is the fourth from the eye, or that which is next the object. Unless the glass next the eye be very dusty, a few small spots or grains of dust are seldom perceptible. The object-glass of an achromatic should sel- dom be touched unless damp adheres to it. Care should be taken never to use pocket-handkerchiefs or dirty rags for wiping lenses. From the frequent use of such articles, the glasses of seamen’s telescopes get dimmed and scratched in the course of a few years. If the glasses be exceedingly dirty, and if-greasy substances are attached to them, they may be soaked im spirits and water, and afterward carefully wiped. In replacing the glasses in their socket, care should be taken not to touch the surfaces with the fingers, as they would be dimmed with the perspiration: they should be taken hold of by the edges only, and carefully screwed into the same cells from which they were taken. ON MEGALASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS. It appears to have been almost overlooked by opticians and others, that telescopes may be constructed so as to exhibit a beautiful and minute view of very near objects, and to pro- duce even a microscopic effect, without the least alteration in the arrangement of the lenses of which they are composed. This object is effected simply by making the eyetube of.a telescope of such a length as to be capable of being drawn out twelve or thirteen inches beyond the point of distinct vision for distant objects. The telescope is then rendered capable of exhibiting with distinctness all kinds of sbjects, Vou. IX 25 290 THE MEGALASOOPE. from the most distant to those which are placed within three or four feet of the instrument, or not nearer than double the focal distance of the object-glass. Our telescopes, however, are seldom or never fitted with tubes that slide farther than an inch or two beyond the point of distinct vision for distant objects, although a tube of a longer size than usual, or an additional tube, would cost but a very trifling expense. The following, among many others, are some of the objects on which I have tried many amusing experiments with tele- scopes fitted up with the long tubes to which Iallude. The telescope to which I shall more particularly advert is an achromatic, mounted on a pedestal, having an object-gilass about nineteen inches focal length, and 1gths of an inch in diameter, with magnifying powers for distant objects of thir- teen and twenty times. When this instrument is directed to a miniature portrait 32 inches in length, placed in a good light, at the distance of about eight or ten feet, it appears as large as an oil-painting four or five feet long, and represents the individual as large as life. The features of the face ap- pear to stand out in bold relief; and perhaps there is no representation of the hunfan figure that more resembles the living prototype than in this exhibition, provided the minia- ture is finely executed. In this case the tube requires to be pulled out four or five inches from the point of distmet vision for distant objects, and consequently the magnifying power is proportionally increased. Another class of objects to which such a telescope may be applied is perspective prints, either of public buildings, streets, or landscapes... When viewed in this way they present a panoramic appearance, and seem nearly as natural as life,.just in the same manner as they’ appear in the Optical Diagonal Machine, or when reflected in a large concave mirror, with this advantage, that while in these instruments the left-hand side of the print appears where the right should be, the objects seen through the tele- scope appear exactly in their natural position. In this case, however, the telescope should have a small. magnifying ‘power, not exceeding five or six times, so as to take in the whole of the landscape. If an astronomical eyepiece be used, the print will require to be inverted. _ | ' Other kinds of objects which may be viewed with this Instrument are trees, flowers, and other objects in gardens immediately adjacent to the apartment in which we make our observations. In this way we may obtain a distinct view ef a variety of rural objects, which we cannot easily approach, such as the buds and blossoms on the tops of trees, and the THE MEGALASCOPE, 291 insects with which they may be infested. There are certain objects on which the telescope may be made to produce a powerful microscopical effect, such as the more delicate ane beautiful kinds of flowers, the leaves of trees, and similar ob- jects. In viewing such objects, the telescope may be brought within little more than double the focal distance of the object- glass from the objects to be viewed, and then the magnifying power is very considerably increased. A nosegay composed of a variety of delicate flowers, and even a single flower, such as the sea-pink, makes a splendid appearance in this way. A peacock’s feather, or even the fibres on a common quill, appear very beautiful when placed in a proper light. The leaves of trees, particularly the leaf of the plane-tree, when placed against a window-pane, so that the light may shine through them, appear, in all their internal ramifications, more distinct, beautiful, and interesting, than when viewed in any other way ; and in such views a large portion of the object is at once exhibited to the eye. In this case, the eyepiece of such a telescope as that alluded to requires to be drawn out twelve or fourteen inches beyond the point of distinct vision for objects at a distance, and the distance between these near objects and the object-end of the telescope is only about 33 feet. A telescope having a diagonal eyepiece presents a very pleasant view of near objects in this manner. With an in- strument of this kind I have frequently viewed the larger kind of small objects alluded to above, such as the leaves of shrubs and trees, flowers consisting of a variety of parts, the fibres of a peacock’s feather, and similar objects. In this case, the object-glass of the instrument, which is 103 inches focal length, was brought within 22 inches of the object, and the eye looked down upon it in the same manner as when we view objects in a compound microscope. A common pocket achromatic telescope may be used for the purposes now stated, provided the tube in the eyepiece containing the two lenses next the object be taken out, in which case the two glasses next the eye form an astronomical eyepiece, and the tubes may be drawn out five or six inches beyond the focal point for distant objects, and will produce distinct vision for objects not farther distant than about 20 or 24 inches; but in this case, the objects to be viewed must be inverted, in order that they may be seen in their natural positions when viewed through the instrument. Telescopes of a large size and high magnifying powers may likewise be used with advantage for viewing very near objects in gardens adjacent to the room in 292 REFLECTIONS ON LIGHT. which the instruments are placed, provided the sliding-tub next the eye has a range of two or three inches beyond the point of vision for distant objects. In this case, a magnifying power of 100 times on a 33 or a five feet achromatic produces a very pleasant effect. In making the observations to which I have now alluded, it is requisite, in order to distinct vision and to obtain a pleasing view of the objects, that the instru- ment should be placed on a pedestal, and capable of motion in every direction. 'The adjustment for distinct vision may be made either by the sliding-tube, or by removing the telescope nearer to or farther from the object. REFLECTIONS ON LIGHT AND VISION, AND ON THE NATURE AND UTILITY OF TELESCOPES. Light is one of the most wonderful and beneficial, and, at the same time, one of the most. mysterious agents in the material creation. ‘Though the sun from which it flows to this part of our system is nearly a hundred millions of miles from our globe, yet we perceive it as evidently, and feel its influence as powerfully, as if it emanated from no higher a region than the clouds. It supplies life and comfort to our physical system, and without its influence and operations on the various objects around us, we could scarcely subsist and participate of enjoyment, for a single hour. It is diffused around us on every hand from its fountain, the sun ;, and even the stars, though at a distance hundreds of thousands of times greater than that of the solar orb, transit to our distant re-- gion a portion of this element. It gives beauty and fertility to the earth, it supports. the vegetable and animal tribes, and is connected with the various motions which are going for- ward throughout the system of the universe. It unfolds to us the whole scenery of external nature: the lofty mountains and the expansive plains, the majestic rivers and the mighty ocean; the trees, the flowers, the crystal streams, and the vast canopy of the sky, adorned with ten thousands of shining orbs. In short, there is scarcely an object within the range of our contemplation but what is exhibited to our understand- ing through the medium of light, or at least bears a certain relation to this enlivening and universal agent. When we consider the extreme minuteness of the rays of light, their inconceivable velocity, the invariable laws by which they act upon all bodies, the multifarious phenomena produced by their inflections, refractions, and reflections, while their origi- nal properties remain the same ;_ the endless variety of colours they produce on every part of our terrestrial creation, and the ad REFLECTIONS ON VISION. 293 facility with which millions of rays pass through the smallest apertures, and pervade substances of great density, while every ray passes forward in the crowd without disturbing another, and produces its own specific impression, we can- not but regard this element as the most wonderful, astonish- ing, and delightful part of the material creation. When we consider the admirable’ beauties and the exquisite pleasures of which light is the essential source, and how much its na- ture is still involved in mystery, notwithstanding the profound investigations of modern philosophers, we may well exclaim with the poet, “How then shall I attempt to sing of Hi Wha, light himself, in aL ae light Investe deep, dwells awfully retired From mortal eye or angel’s purer ken; Whose single smile has, from the first of time, Filled, overflowing, all yon lamps of heaven, That beam for ever through the boundless sky.”’ ‘THOMSON. The eye is the instrument by which we perceive the beautiful and multifarious effects of this universal agent. Its delicate and complicated structure ; its diversified muscles; its coats and membranes; its different humours, possessed of different refractive powers; and the various contrivances for performing and regulating its external and internal mo- tions, so as to accomplish the ends intended, clearly demon- strate this organ to be a masterpiece of Divine mechanism— the workmanship of Him whose intelligence surpasses con- ception, and whose wisdom is unsearchable. “Our sight,” says Addison, “is the most perfect and delightful of all our senses. It fills the mind with the largest variety of ideas, converses with its objects at the greatest distance, and con- tinues the longest in action, without being tired or satiated with its proper enjoyments. ‘The sense of | feeling can indeed give us a. notion of extension, shape, and all other ideas that enter the eye except colours ; but, at the same time, it is very “much strained, and confined in its operation to the number, bulk, and distance of its particular objects. Our sight seems designed t* supply all these defects, and may be.considered as a more delicate and diffusive kind of touch, that spreads itself over an infinite multitude of bodies, comprehends the largest figures, and brings into our reach some of the more remote parts of the universe.” Could we suppose an order of beings endued with every human faculty but that of sight, it would appear incredible to such beings, accustomed only to the slow information of 25* 294 REFLECTIONS ON VISION. touch, that by the addition of an organ consisting of a ball ana socket, of an inch in diameter, they might be enabled, in an instant of time, without changing their place, to perceive the disposition of a whole army, the order of a battle, the figure of a magnificent palace, or all the variety of a landscape. Ifa man were by feeling to find out the figure of the Peak of Teneriffe, or even of St. Peter’s Church at Rome, it would be the work of a lifetime. It would appear still more incre- dible to such beings as we have supposed, if they were in- formed of the discoveries which may be made by this little organ in things far beyond the reach of any other sense, that by means. of it we can find our way in the pathless ocean ; that we can traverse the globe of the earth, determine its figure and dimensions, and delineate every region of it; yea, that we can measure the planetary orbs, and make discove- ries in the sphere of the fixed stars. And if they were farther informed that, by means of this same organ, we can perceive the tempers and dispositions, the passions and affections of our fellow-creatures, even when they, want most to conceal them ; that when the tongue is taught most artfully to lie and dissemble, the hypocrisy should appear. in the countenance to a discerning eye; and that by this organ we can often perceive what is straight and what crooked in the mind as well as in the body, would it not appear still more astonishing to beings such as we have now suppesed ?* Notwithstanding these wonderful properties of the organ of vision, the eye, when unassisted by art, is comparatively hmited in the range of its powers. It cannot ascertain the existence of certain objects at the distance of three or four miles, nor perceive what is going forward in nature or art beyond such a limit. - By its natural powers we perceive the moon to be a globe about half a degree in-diameter, and diversified with two or three dusky spots, and that the sun _-is a luminous body of apparently the same size; that the’ planets are luminous points, and that about a thousand stars exist in the visible canopy of the sky. But the ten. thou- sandth part of those luminaries which are within the reach of human vision can never be seen by the unassisted eye. Here the TELEscore interposes, and adds a new power to the organ of vision, by which it is enabled to extend its views to regions of space immeasurably distant, and to objects, the number and magnitude of which could never otherwise have been surmised by the human imagination. By its aid we * Reid’s Inquiry into the Human Mind, chap. iv. THE NATURE OF THE TELESCOPE. 295 ebtain a sensible demonstration that space is boundless; that the universe is replenished with innumerable suns and worlds ; that the remotest regions.of immensity, immeasur- ably beyond the limits of unassisted vision, display the energies of Creating Power; and that the empire of the Creator extends far beyond what eye hath seen or the human imagination can conceive. The telescope is an instrument of a much more wouldenpal nature than what most people are apt to imagine. However popular such instruments now are, and however common a circumstance it is to contemplate objects at a great distance which the naked eye cannot discern, yet, prior to their in- vention and improvement, it would have appeared a thing most mysterious, if not impossible, that objects at the distance of ten miles could be made to appear as if within a few yards of us, and that some of the heavenly bodies could be seen as distinctly as if we had been transported by some ‘superior power hundreds of millions of miles beyond the bounds of our terrestrial habitation. Who could ever have imagined, rea- soning @ priori, that the refraction of light in glass—the same power by which a straight rod appears crooked in water, by which vision is variously distorted, and by which we are liable to innumerable deceptions—that that same power or law of nature, by the operation of which the objects in a landscape appear distorted when seen through certain panes of glass in our windows, that that power should ever be so modified and directed as to extend the boundaries of vision, and enable us clearly to distinguish scenes and objects at a distance a thousand times beyond the natural limits of our visual organs? Yet such are the discoveries which science has achieved, such the powers it has brought to light, that by glasses ground into different forms, and properly adapted to each other, we are enabled, as it were, to contract the bounda- ries of space, to penetrate into the most distant regions, and to bring within the reach of our knowledge the most sublime objects in the universe. When Pliny declared in reference to Hipparchus, the ancient astronomer, “ 4usus rem Deo improbam annumerare posteris stellas,’’—that “ he dared to enumerate the stars for posterity, an undertaking forbidden by God,”—what would that natural historian have said had it been foretold that in less than 1600 years afterward a man would arise who should enable posterity to perceive and to enumerate ten times more new stars than Hipparchus ever beheld—who should point out higher mountains on the moon than on the earth—who 296 THE NATURE OF THE TELESCOPE. should discover dark spots as large as our globe in the sun the fountain of light—who should desery four moons revolv- ing in different periods of time around the planet Jupiter, and could show to surrounding senators the varying phases of Venus? and that another would soon after rise who should point out a double ring of six hundred thousand miles in cir- cumference revolving around the planet Saturn, and ten hundreds of thousands of stars which neither Hipparchus nor any of the ancient astronomers could ever descry? Yet these are only a small portion of the discoveries made by Galileo and Herschel by means of the telescope. Had any one pro- phetically informed Archimedes, the celebrated geometrician of Syracuse, that vision would, in after ages, be thus wonder- fully assisted by art; and, farther, that one manner of im- proving vision would be to place a dark, opaque body directly between the object and the eye; and that another method would be, not to look at the object, but to keep the eye quite in a different, and even in an opposite direction, or to stand with the back directly opposed to it, and to behold all the parts of it, mvisible to the naked eye, most distinctly in. this way; he would doubtless have considered the prophet as an enthusiastic fool or a raving madman. - Yet these things have been realized in modern times in the fullest extent. In the Gregorian reflecting telescope, an opaque body, namely, the small speculum near the end of the tube, interposes di- rectly between the eye and the object. In the Newtonian reflector, and in the diagonal eyepieces formerly described, the eye is directed in a line at right angles to the object, or a deviation of 90 degrees from the direct line of vision. In Herschel’s large telescopes, and in the erial Reflector formerly described (in p. 224-234,) the back is turned to the object, and the eye m an opposite direction. These circumstances should teach us humility and a be- coming diffidence in our own powers; and they should admonish us not to be too dogmatical or peremptory in affirm- ing what is possible or impossible in regard either to nature or art, or to the operations of the Divine Being. Art has accomplished, in modern times, achievements in regard to locomotion, marine and aerial navigation, the improvement of vision, the separation and combinations of invisible gases, and numerous other objects, of which the men of former ages could not have formed the least conception ; and even yet we «am set no boundaries to the future discoveries of science and the improvements of art, but. have every reason to indulge the hope that, in the ages to come, scenes of Divine mechanism THE UTILITY OF THE TELESCOPE. 297 in the system of nature will be unfolded, and the effects of shention? and mechanical powers displayed, of which the human mind, in its present state of progress, cannot form the most imperfect idea. Such circumstances likewise should teach us not to reject any intimations which have been made to us in relation to the character, attributes, and dispensations of the Divine Being, and the moral revelations of his will given in the Sacred Records, because we are unable to com: prehend every truth and to remove every difficulty which relates to the moral government of the Great Ruler of the universe ; for if we meet with many circumstances in secular science, and even in the common operations of nature, which are difficult to comprehend—if even the construction of such telescopes as we now use would have appeared an incompre- hensible mystery to ancient philosophers, we must expect to find difficulties almost insurmountable to such limited minds as ours in the eternal plans and moral arrangements of the “King Immortal and Invisible,’”’ as delineated only in their outlines in the Sacred Oracles, particularly those which relate to the origin of physical and moral evil, the ultimate destiny of man, and the invisible realities of a future world. : The vritiry of the telescope may be considered in relation to the following circumstances : In the first place, it may be considered as an instrument or machine which virtually transports us to the distant regions of space. When we look at the moon through a telescope which magnifies 200 times, and survey its extensive plains, its lofty peaks, its circular ranges of mountains, throwing their deep shadows over the vales, its deep and rugged caverns, and all the other varieties which appear on the lunar surface, we behold such objects in the same manner as if we were standing at a point 238,800 miles from the earth in the di- rection of the moon, or only twelve hundred miles from that orb, reckoning its distance to be 240,000 miles. When we view the planet Saturn with a similar instrument, and obtain a view of its belts and satellites, and its magnificent rings, we are transported, as it were, through regions of space to a point in the heavens more than nine hundred millions of miles from the surface of our globe, and contemplate those august objects as if we were placed within five millions of miles of the surface of that planet.* Although a supernatural power * The distance of Saturn from the sun is 906,000,000 of miles; it js sometimes nearer to, and at other times farther from the earth, accord. ing as it is near the pent: of its opposition to, or conjunction with the sun. Ifthis number be divided by 200, the supposed magnifying power 298 THE UTILITY OF THE TELESCOPE. sufficient to carry us in such a celestial journey a thousand miles every day were exerted, it would require more than — two thousand four hundred and sixty years before we could arrive at such a distant position; yet the telescope, in a few moments, transports our visual powers to that far distant point of space. When we view with such an instrument the minute ~ and very distant clusters of stars in the Milky Way, we are carried, in effect, through the regions of space to the distance of five hundred thousand millions of miles from the earth; — for we behold those luminaries through the telescope nearly as if they were actually viewed from such a distant point in the spaces of the firmament.. These stars cannot be con- ceived as less than a hundred billions of miles from our globe, and the instrument we have supposed brings them within the two hundredth part of this distance. Suppose we were carried forward by a rapid motion towards this point at the rate of a thousand miles every hour, it would require more than fifty-seven thousand years before we could reach that very distant station in space to which the telescope, in effect, transports us: so that this instrument is far more efficient im opening to our view the scenes of the universe, than if we were invested with powers of locomotion to carry us through the regions of space with the rapidity of a cannon ball at its utmost velocity ; and all the while we may sit at ease in our terrestrial apartments. In the next place, the telescope has been the means of enlarging our views of the sublime scenes of creation more than any other instrument which art has contrived. Before the invention of this instrument, the universe was generally conceived as circumscribed within very narrow limits. The earth was considered as one of the largest bodies in creation ; the planets were viewed as bodies of a far less size than what they are now found to be; no bodies similar to our moon were suspected as revolving around any of them; and the stars were supposed to be little more than a number of brilliant lamps hung up to emit a few glimmering rays, and to adorn the canopy of our earthly habitation. Such a wonderful phenomenon as the ring of Saturn was never once suspected, and the sun was considered as only a large ball of fire. It was suspected, indeed, that the moon was diversified with of the telescope, the quotient is 4,530,000, which expresses. the distance in miles at which it enables us to contemplate this planet. If this num- ber be subtracted from 906,000,000, the remainder is 901,470,000, which expresses the number of miles from the earth at which we are supposed to view Saturn with such an instrument. THE UTILITY OF THE TELESCOPE. 299 _ mountains and vales, and that it.might possibly be a habit- able world; but nothing certainly could be determined on this point, on account of the limited nature of unassisted vision. But the telescope has been the means of expanding our views of the august scenes of creation to an almost unli- mited extent: it has withdrawn the veil which formerly interposed to intercept our view of the distant glories of the sky: it has brought to light five new planetary bodies, un- known to former astronomers, one of which is more than eighty times larger than the earth, and seventeen. secondary planets which revolve around the primary: it has expanded the dimensions of the solar system to double the extent which was formerly supposed: it has enabled us to descry hundreds of comets which would otherwise have escaped our unassisted vision, and to determine some of their trajectories and periods of revolution: it has explored the profundities of the Milky Way, and enabled us to perceive hundreds of thousands of those splendid orbs, where scarcely one is visible to the naked eye: it has Jaid open to our view thousands of Vebule, of various descriptions, dispersed through different regions of the firmament, many of them containing thousands of sepa- rate stars: it has directed our investigations to thousands of double, treble, and multiple stars—suns revolving around suns, and systems around systems; and has enabled us to determine some of the periods of their revolutions: it has demonstrated the immense distances of the starry orbs from our globe, and their consequent magnitudes, since it shows us that, having brought them nearer to our view by several hun- dreds or thousands of times, they still appear only as so many shining points: it has enabled us to perceive that mighty changes are going forward throughout the regions of im- mensity—new stars appearing, and others removed from our view, and motions of incomprehensible velocity carrying for- ward those magnificent orbs through the spaces of the firma ment: in short, it has opened a vista to regions of space so immeasurably distant, that a cannon ball impelled with its greatest velocity would not reach tracts of creation so remote in two thousand millions of years; and even light itself, the swiftest body in nature, would require more than a thousand years before it could traverse this mighty interval. It has thus laid a foundation for our acquiring an approximate idea of the infinity of space, and for obtaining a glimpse of the far distant scenes of creation, and the immense extent of the universe. Again, the telescope, in consequence of the discoveries i! 800 THE UTILITY OF THE TELESCOPE. has enabled us to make, has tended to amplify our concep- tions of the attributes and the empire of the Deity. Yhe amplitude of our conceptions of the Divine Being bears a certain proportion to the expansion of our views in regard to his works of creation, and the operations he is incessantly — carrying forward throughout the universe. If our views of the works of God, and of the manifestations he has given of himself to his intelligent creatures, be circumscribed to a nar- row sphere, as to a parish, a province, a kingdom, or a single atti world, our.conceptions of that Great Being will be propor- — tionably limited; for it is chiefly from the manifestation of. God in the material creation that our ideas of his power, his wisdom, and his other natural attributes are derived. But in proportion to the ample range of prospect we are enabled to take of the operations of the Most High, will be our concep- tions of his character, attributes, and agency. Now the tele- scope, more than any other invention of man, has tended to open to our view the most magnificent and extensive prospects of the works of God; it has led us to ascertain that, within the limits of the solar system, there are bodies which, taken torether, comprise a mass of matter nearly two thousand five hundred times greater than that of the earth; thatehese bo- dies are all constituted and arranged_in such a manner as to | fit them for being habitable worlds; and that the sun, the centre of this system, is five hundred times larger than the whole. But, far beyond the limits of this system, it has pre- sented to our view a universe beyond the grasp of finite intel- ligences, and to which human imagination~can assign no boundaries: it has enabled us to descry suns clustering be- hind suns, rismg to view in boundless perspective, in propor- tion to the extent of its magnifying and illuminating powers, the numbers of which are to be estimated, not merely by thousands, and tens of thousands, and hundreds of thousands, but by scores of millions; leaving us no room to doubt. that hundreds of millions more, beyond the utmost limits of human vision, even when assisted by art, lie hid from mortal view in the unexplored and unexplorable regions of immensity. _ Here, then, we are presented with a scene which gives us a display of Omnipotent Power which no other objects can unfold, and which, without the aid of the telescope, we should never have beheld; a scene which expands our conceptions of the Divine Being to an extent which the men of former generations could never have anticipated; a scene which enables us to form an approximate idea of Him who is the ‘King Eternal, Immortal, and Invisible.” who “created all THE UTILITY OF THE TELESCOPE. 301 worlds, and for whose pleasure they are and were created.” Here we behold the operations of a Being whose power is illimitable and uncontrollable, and which far transcends the comprehension of the highest created intelligences; a power, displayed not only in the vast extension of material existence, and the countless number of mighty globes which the uni- verse contains, but in the astonishingly rapid motions with which myriads of them are carried along through the im- measurable spaces of creation, some of those magnificent orbs moving with a velocity of one hundred and seventy thousand miles an hour. Here, likewise, we have a display of the infinite wisdom and intelligence of the Divine Mind, in the harmony and order with which all the mighty movements of the universe are conducted; in proportionating the magni- tudes, motions, and distances of the planetary worlds; in the nice adjustment of the projectile velocity to the attractive power; in the constant proportion between the times of the periodical revolution of the planets and the cubes of their mean distances; in the distances of the several planets from’ the central body of the system, compared with their respec- tive densities; and in the constancy and regularity of their motions, and the exactness with which they accomplish their destined rounds—all which circumstances evidently show that He who contrived the universe is “the only wise God,” who is “ wonderful in counsel and. excellent in working.” Here, in fine, is a display of boundless benevolence; for we cannot suppose, for a moment, that so many myriads of mag- nificent globes, fitted to be the centres of a countless number of mighty worlds, should be nothing else than barren wastes, without the least relation to intelligent existence; and if they are peopled with intellectual beings of various orders, how vast must be their numbers, and how overflowing that Divine Beneficence which has provided for them all every thing requisite to their existence and happiness. In these discoveries of the telescope we obtain a glimpse of the grandeur and the unlimited extent of God’s universal empire. ‘l’o this empire no boundaries can be perceived. The larger and the more powerful our telescopes are, the farther are we enabled to penetrate into those distant and un- known regions; and however far we penetrate into the abyss of space, new objects of wonder and magnificence still con tinue rising to our view, affording the strongest presumption that, were we to penetrate ten thousand times farther into those remote spaces of immensity, new suns, and systems, and worlds would be disclosed to our view. Over all this Von. IX. 26 802 THE UTILITY OF THE TELESCOPE. vast assemblage of material existence, and over all the senst- tive and intellectual beings it contains, God eternally and unchangeably presides; and the minutest movements, either of the physical or the intelligent system, throughout every department of those vast dominions, are at every moment ‘naked and open” to his omniscient eye. What boundless intelligence is implied in the superintendence and arrange- ment of the affairs of such an unlimited empire! and what a lofty and expansive idea does it convey of Him who sits on the throne of Universal Nature, and whose greatness is un- searchable! But without the aid of the telescopic tube we could not have formed such ample conceptions of the great- ness, either of the Eternal Creator himself, or of the universe which he hath brought into existence. Besides the above, the following uses of the telescope, in relation to science and common life, may be shortly noticed : In the business of astronomy, scarcely any thing can be done with accuracy without the assistance of the telescope. 1. It enables the astronomer to determine with precision the transits of the planets and stars across the meridian; and on the accuracy with which these transits are obtained, a variety of important conclusions and calculations depend. The com- putation of astronomical and nautical tables for aiding the navigator in his voyages round the globe, and facilitating his calculations of latitude and longitude, is derived from obser- vations made by the telescope, without the use of which in- strument they cannot be made with precision. 2. The ap- parent diameters of the planets can only be measured by means of this instrument, furnished with a micrometer. By the naked eye no accurate measurements of the diameters of these bodies can be taken; and without knowing their appa- rent diameters in minutes or seconds, their real bulk cannot be determined, even although their exact distances be known. The differences, too, between the polar and equatorial diame- ters cannot be ascertained’ without observations made by powerful telescopes. For example, the equatorial diameter of Jupiter is found to be in proportion to the polar as 14 to 13, that is, the equatorial is*more than 6000 miles longer than the polar diameter, which could never have been determined by observations made by the naked eye. 3. The parallaxes of the heavenly bodies can only be accurately ascertained by the telescope; and it is only from a knowledge of their pa- rallaxes that their distances from the earth.or from the sun can be determined. In the case. of the fixed stars, nothing of tne nature of a parallax could ever be expected to be found rt THE UTILITY OF. THE TELESCOPE. 3038 without the aid of a telescope. It was by searching for the parallax of a certain fixed star that the important fact of the Aberration of Light was discovered. The observations for this purpose were made by means of a telescope 24 feet long, fixed in a certain position. 4. The motions and revolutionary periods of Sidereal Systems can only be determined by ob- servations made by telescopes of great magnifying and illu- minating powers. Without a telescope the small stars which accompany double or treble stars cannot be perceived, and much less their motions or variation of their relative posi- tions. Before the invention of the telescope, such phenomena, now deemed so wonderful and interesting, could never have been surmised. _ 5. The accurate determination of the longi- tude of places on the earth’s surface is ascertained by the telescope, by observing with this instrument the immersions and emersions of the satellites of Jupiter. From such obser- vations, with the aid of a chronometer, and having the time at any known place, the situation of any unknown place is easily determined. But the eclipses of Jupiter’s moons can be perceived only by telescopic instruments of considerable power. 6. By means of a telescope with cross hairs in the focus of the eyeglass, and.attached to a quadrant, the altitude of the sun or of a star, particularly the pole star, may be most accurately taken, and from such observations the latitude of the place may be readily and accurately deduced. Again, in the Surveying of Land, the telescope is particu- larly useful; and for this purpose it is mounted on a stand with a horizontal and vertical motion, pointing out by divi- sions the degrees and, minutes of inclination of the instrument. For the more accurate reading of these divisions, the two limbs are furnished with a nonius, or Vernier’s scale. The object here is to take the angular distances between distant objects on a plane truly horizontal, or else the angular eleva- tion or depression of objects above or below the plane of the horizon. In order to obtain either of those kinds of angles to a requisite degree of exactness, it is necessary that the sur- veyor should have as clear and distinct a view as possible’ of the objects, or station-staves, which he fixes up for his pur- pose, that he may with the greater certainty determine the point of the object which exactly corresponds with the line he is taking. Now, as such objects are generally at too great a distance for the surveyor to be able to distinguish with the naked eye, he takes the assistance of the telescope, by which he obtains, 1, a distinct view of the object to which his atten- tion is directed, and, 2, he is enabled to determine the precise 304 THE UTILITY OF THE TELESCOPE. point of the object aimed at by means of the cross hairs in the focus of the eyeglass. A telescope mounted for this purpose is called a Theodolite, which is derived from two Greek words, deouar, to see, and odos, the way or distance. In the next place, the telescope is-an instrument of special importance in the conducting of Telegraphs, and in the con- veyance of signals of all descriptions. Without its assistance telegraphic despatches could not be conveyed with accuracy to any considerable distance, nor in quadruple the time in which they are now communicated, and the different stations would need to be exceedingly numerous; but, by the assist- ance of the telescope, information may be communicated, by a series of telegraphs, with great rapidity. Twenty-seven telegraphs convey information from Paris to Calais, a distance of one hundred and sixty miles, in three minutes; twenty- two from Paris to Lisle in two minutes ; forty-six from Stras- burg to Paris in four and a half minutes; and eighty from Paris to Brest im ten minutes. In many other cases which occur both on land and sea, the telescope is essentially requi- site for descrying signals. The Bell-Rock Lighthouse, for example, is situated twelve miles from Arbroath, and from every other portion of land, so that the naked eye could not discern any signal which the keepers of that light could have it in their power to make; but by means of a large telescope in the station-house in Arbroath, the hoisting of a ball every morning at 9 a.m., which indicates that “All is well,” may be distinctly recognised. Many other uses of this instrument, in the ordinary trans- actions of life, will readily occur to the reader, and therefore I shall only mention the following purpose to which it may be applied, namely, To measure the Distance of an Object from one Station. —This depends upon the increase of the focal distance of the telescope inthe case of near objects. Look through a tele- scope at the object whose distance is required, and adjust the focus till it appear quite distinct; then slide in the drawer till the object begins to be obscure, and mark that place of the tube precisely ; next draw out the tube till the object begins to be again obscured, and then make another mark as before ; then take the middle point between these two marks, and that will be the point where the image of the object is formed most distinctly, which is to be nicely measured from the object-~ ‘ens, and compared with the solar focus of the lens or tele- scope, so as to ascertain their difference. And the rule for finding the distance is, as the difference between the foca THE UTILITY OF THE TELESCOPE. 305 distance of the object and the solar focal distance is to the solar focal distance, so is the focal distance of the object to its true distance from the object-lens. An example will render this matter more perspicuous : Figure 84. Let A B (fig. 84) be the object-lens, E Y the eyeglass, FC the radius, or focus of the lens A B, and Cf the focal distance of the object O B, whose distance is to be measured. Now suppose C FE’ = 48 inches, or four feet, and that we find by the above method that C f is 50 inches, then Ff is two inches; and the analogy is, as F f = 2is to C F = 48, so is co f =50 toC Q = 1200 inches, or 100 feet. Again, suppose of =49 inches, then will F f =1 inch; and the propor- tion. is, 1; 48 :: 49: 2352 —=QC, or 196 feet.. A telescope of this focal length, however, will measure only small dis- tances. But suppose A B a lens whose solar focus is 12 feet, or 144 inches. and that we find by the above method that Cf, or the focal distance of the object, is 146 inches, then will Ff be two inches, and the proportion will be as 2: 144:: 146: 21,024 inches, or 1752 feet —the distance QC. If with such a large telescope we view an object OB, and find Ff but qipth of an inch, this will give the distance of.the object as 17,292 feet, or nearly 33d miles. . Since the difference between the radius of the object-lens and the focal distance of the object is so considerable as two inches in a tube of four feet, and more than twelve inches in one of twelve feet,a method might be contrived for determin- ing the distance of near objects by the former, and more dis- tant objects by the latter, by inspection only. This may be done by adjusting or drawing a spiral line round the drawer or tube through the fwo-inch space in the small telescope, and by calculation graduate it for every 100 feet and the in- termediate inches, and then, at the same time we view an object, we may see its distance on the tube. In making such experiments, a common object-glass of a long focal length, and a single eyeglass, are all that is requisite, since the in- verted appearance of the object can cause no great inconve nience, 26* 306 METHOD OF GRINDING LENSES. CHAPTER VII. ON THE METHOD OF GRINDING AND POLISHING OPTICAL LENSES AND SPECULA. I or1GINALLY intended to enter into particular details on this subject for the purpose of gratifying those mechanics and others who wish to amuse themselves by constructing tele- scopes and other optical instruments for their own use ; but, having dwelt so long on the subject of telescopes in the pre- ceding pages, I am constrained to confine myself to a very general sketch. Zi 1. Zo grind and polish Lenses for Eyeglasses, Micro- scopes, &c.—First provide an upright spindle, at the bottom of which a pulley is fixed, which must be turned by a wheel by means of a cord and handle. At the top of the spindle make a screw the same as.a lathe-spindle, on which you may screw chocks of different sizes, to which the brass tool in which the lens is to be ground may be fixed. Having fixed upon the breadth and focal length of the lens, and whether it is to be a plano or a double convex, take a piece of tin-plate or sheet copper, and with a pair of compasses draw an arch upon its surface, near one of its extremities, with a radius equal to the focal distance of the lens if intended to be double convex, or with half that distance if it is to be plano-convex. Remove with a file that part of the copper which is without the circular arch, and then a convex gauge is formed. With the same radius strike another arch, and having removed that part of the copper which is within it, a concave gauge will be obtained. The brass tool in which the glass is to be ground is then to be fixed upon a turning-lathe, and turned into a portion of a concave sphere, so as to correspond to the convex gauge. In order to obtain an accurate figure to the concave tool, a convex tool of exactly the same radius is generally formed, and they are ground one upon another with floar of emery, and when they exactly coincide they are fit for use. The convex tool will serve for grinding concave glasses of the same radius; and it should be occasionally ground in the concave tool to prevent it from altering its figure. The next thing to be attended to is to prepare the piece of glass which is to be ground, by chippimg it in a circular shape by means of a large pair of scissors, and removing the roughness from its edges by a common grindstone. ‘The METHOD OF GRINDING ‘LENSES. 307 faces of the glass near the edges should likewise be ground on the grindstone till they nearly fit the concave gauge, by which the labour of grinding in the tool will be considerably saved. The next thing required is to prepare the emery for grinding, which is done in the following manner: Provide four or five clean earthen vessels; fill one of them with water, and put into it a pound or halfa pound of fine emery, and stir it-about with a stick; after which, let it stand three or four seconds, and then pour it into, another vessel, which may stand about ten seconds; then pour it off again into the several vessels till the water is quite clear, and by this means emery of different degrees of fineness is obtained, which must be kept separate from each other, and worked in their proper order, beginning at the first, and working off all the marks of the grindstone ; then take of the second, next of the third, &c., holding the glass upon the pan or tool with a light hand when it comes to be nearly fit for polishing. The glass, in this operation, should be cemented to a wooden handle by means of pitch or other strong cement. After the finest emery has been used, the roughness which remains may be taken away, and a-slight polish given, by grinding the glass with pounded pumice-stone. Before proceeding to the polish- ing, the glass should be ground as smooth as possible, and all the scratches erased, otherwise the polishing will become ia tedious process. The polishing is performed as follows: Tie a piece of linen rag or fine cloth about the tool, and-with fine putty, (calcined tin,) or colcothar of vitriol, (a very fine pow- der, sometimes called the red oxide of iron,) moistened with water, continue the grinding motion, and in.a short time there will be an excellent polish. | In order to grind lenses very accurately for the finest op- tical purposes, particularly object-glasses for telescopes, the concave tool is firmly fixed to.a table or bench, and the glass wrought upon it by the hand with circular strokes, so that its centre may never go beyond the edges of the tool. For every six or seven circular strokes, the glass should receive two or three cross ones along the diameter of the tool, and in different directions ; and, while the operation is going on, the convex tool should, at the end of five minutes, be wrought upon the concave one for a few seconds, in order to preserve the same curvature to the tools and to the glass. The finest polish is generally given in the folowing way: Cover the concave tool with a layer of pitch, hardened by the addition of a little rosin, to the thickness of jth of an inch; then, havmg taken a piece of thin writing paper, press it upon the 308 METHOD OF MAKING SPEOULA. surface of the pitch with the convex tool, and pull the paper quickly from the pitch before it has adhered to it; ; and if the surface of the pitch is marked everywhere with the lines of the paper, it will be truly spherical. If any paper remains on the surface of the pitch, it may be rubbed off by soap and water; and if the marks of the paper should not appear on ~ any part of it, the operation must be repeated till the polisher or bed of pitch is accurately spherical. The glass is then to be wrought on the polisher by circular and cross strokes with the putty or colcothar till it-has received a complete polish. When one side is finished, the glass must be separated from its handle by inserting the point of a knife between it and the pitch, and giving it a gentle stroke. The pitch which remains upon the glass may be removed by rubbing it with a | little oil, or spirits of wine. The operation of polishing on cloth is slower, and the polish less perfect than on pitch ; but it is a mode best fitted for those who have little experience, and who would be apt, in the first instance, to injure the figure of the lens by polishing it on a bed of pitch. 2. On the Method of casting and grinding the Specula of Reflecting Telescopes.—The first thing to be considered in the formation of reflecting telescopes is the composition of the metal of which the specula are made. The qualities re- quired are, a sound, uniform metal, free from all microscopic pores—not liable to tarnish by absorption of moisture from the atmosphere—not so hard as to be incapable of taking a good figure and polish, nor so soft as to be easily scratched, and possessing a high reflecting power. Various composi- tions have been used for this purpose, of which the following are specimens: Take of good Swedish copper 32 ounces, and when melted, add 142 ounces of grain tin to it; then, having taken off the scoria, cast it into an ingot. ‘This metal must be a second time melted to cast a speculum; but it will fuse in this compound state with a small heat, and therefore will not calcine the tin to putty. It should be poured off as soon as it is melted, giving it no more heat than is absolutely necessary. The best’ method for giving the melted metal a good surface is this: the moment before it is poured off, throw into the crucible a spoonful of charcoal-dust ; imme- diately after which, the metal must be stirred with a wooden spatula and poured into the moulds. The following is another composition, somewhat similar: Take two parts of copper as pure as it is possible to procure: this must be melted ina crucible by itself; then put, in another crucible one part d pure grain tin: when they are both melted, mx and su METHOD OF MAKING SPECULA. 309 .them with a wooden spatula, keeping a good flux on the melted surface to prevent oxidation, and then pour the metal ey into the moulds, which may be made of founder’s oam. The composition suggested, more than half a century ago, by the Rev. Mr. Edwards, has often been referred to with peculiar approbation. This gentleman took a» great deal of pains to discover the best composition, and to give his metals a fine polish and the true parabolical figure. His telescopes were tried by Dr. Maskelyne, the astronomer royal, who found them greatly to excel in brightness, and to equal in other respects those made by the best artists. They showed a white object perfectly white, and all objects of their proper colour. He found, after trying various combinations, the fol- lowing to be the best, namely, 32 ounces of copper, with 15 or 16 ounces of grain tin, (according to the purity of the cop- per,) with the addition of one ounce of ‘brass, one of silver, and one ounce of arsenic. ‘This, he affirms, will form a metal capable, when polished in a proper manner, of reflect- ing more light than any other metal yet made public. The Rev. J. Little, in his observations on this subject in the “Irish Transactions,’ proposes the followmg composition, which he found to answer the purpose better than any he had tried, namely, 32 parts of best bar copper, previously fluxed with the black flux of two parts tartar and one of nitre, four parts of brass, 16 parts of tin, and 17 of arsenic. If the metal be granulated, by pouring it, when first melted, mto water, and then fused a second time, it will be less porous than. at first. In this process, the chief object is to hit on the exact point of the saturation of the copper, &c., by the tin; for if the latter be added in too great quantity, the metal will be dull-coloured and soft; if too little, it will not attain the most perfect whiteness, and will certainly tarnish.* When the metal is cast, and prepared by the common grindstone for receiving its proper figure, the gauges and grinding tools are to be formed in the same manner as for- merly described for lenses, with this difference, that the radius of the gauges must always be double the focal length of the speculum, as the focus of parallel rays by reflection is at one- half the radius of concavity. In addition to the concave and convex tools, which should be only a little broader than the metal itself, a convex elliptical tool of lead and tin should be formed with the same radius, so that its transverse should be : Irish Transactions, vol. x., and Nicholgon’s Philosophical Journal, vol. xvi. 310 METHOD OF MAKING SPECULA. to its conjugate diameter as 10 to 9, the latter being exactly equal to the diameter of the metal. The grinding of the speculum is then to be commenced on this tool with coarse emery powder and water, when the roughness is taken off by moving the speculum -across the toolin different directions, walking round the post on which the tool is fixed, holding the speculum by the wooden handle to which it is cemented; it is then to be wrought with great care on the convex brass tool, with circular and cross strokes, and with emery of dif- ferent degrees—the concave tool being sometimes ground upon the convex one, to keep them all of the same radius— and when every scratch is removed from its surface, it will be fit for receiving the final polish. When the metal is ready for polishing, the elliptical tool is to be covered with black pitch about jth of an inch thick, and the polisher formed in the same way as in the case of lenses, either with the concave brass tool or with the metal itself. The colcothar of vitriol should then be triturated be- tween two surfaces of olass, and a considerable quantity of it applied at first to the surface of the polisher. The speculum is then to be wrought in the usual way upon the polishing tool till it has received a brilliant lustre, taking care to use no more of the colcothar, if it can be avoided, and only a small quantity of it, if it should be found necessary... When the metal moves stiffly on the polisher, and the colcothar assumes a dark, muddy hue, the polish advances with great rapidity. The tool will then grow warm, and would probably stick to the speculum if its motion were discontinued for a moment. At this stage of the process, therefore, we must proceed with great caution, breathing continually on the polisher till the friction is so great as to retard the motion of the speculum. When this happens, the metal is to be slipped off the tool at one side, cleaned with soft leather, and placed in a tube for the purpose of trying its performance; and if the polishing has been conducted with care, it will be found to have a true parabolic figure.* It was formeriy the practice, before the speculum’ was brought to the polisher, to smooth it on a bed of hones, ora convex tool made of the best blue stone, suchas clockmakers use in polishing their work, which was made one-fourth part larger than the metal which was to be ground upon it, and turned as true as possible to a gauge; but this tool is not generally considered..as absolutely necessary, except when silver and brass enter into the composition of the metal, in * Brewster’s Appendix to Ferguson’s Lectures. - METHOD OF MAKING SPECULA. 311 order to remove the roughness which remains after grinding with the emery. To try the Figure of the Metal.—In order to this, the speculum must be placed in the tube of the telescope for which it is intended, and at about 20 or 30 yards distant there should be put up a watch-paper, or similar object, on which there are some very fine strokes of an engraver. An annular kind of diagram should be made with card-paper, so as to cover a circular portion of the middle part of the speculum, between the hole and the circumference, equal in breadth to about one-eighth of its diameter. ‘This paper ring should be fixed in the mouth of the telescope, and remain so during the whole experiment. There must likewise be two other cir- cular pieces of card-paper cut out, of such sizes that one may cover the centre of the metal by completely filling the hole in the annular piece now described, and the other such a round piece as shall exactly fill the tube, and so broad as that the inner edge just touches the outward circumference of the middle annular piece. All these pieces together will com- pletely shut up the mouth of the telescope. Let the round piece which covers the centre of the metal be removed, and adjust the instrument so that the image may be as sharp and distinct as possible; then replace the central_piece, and re- move the outside annular one, by which means the circum- ference only of the speculum will be exposed, and the image now formed will be from the rays reflected from the exterior side of the metal. If the two images formed by these two portions of the metal be perfectly sharp and equally distinct, the speculum is perfect and of the true parbolic curve; if, on the contrary, the image from the outside of the metal should uot be distinct, and it should be necessary to bring the little speculum nearer by the screw, the metal is not yet brought to the parabolic figure; but if, in order to procure distinctness, we be obliged to move the small speculum farther off, then the figure of the great speculum has been carried beyond the parabolic, and has assumed the hyperbolic form. To adjust the Eyehole of Gregorian Reflectors.—If there is only one eyeglass, then the distance of the small hole should be as nearly as possible equal to its focal length; but in the compound Huygenian eyepiece, the distance of the eyehole may be thus found: Multiply the difference between the focal distance of the glass next to the speculum, and ihe distance of the two eyeglasses, by the focal distance of the glass nearest the eye; divide the product by the sum of the focal distances of the two lenses, lessened by their distance, and the quotient 312 DIRECTIONS FOR ADJUSTING TELESCOPES. - will be the compound focal distance required. Thus, if the focal distance of the lens next the speculum be three inches, that of the lens next the eye one inch, and their distance two inches, then the compound focal distance from the eyeglass will be ee inch. ~The diameter of the eyehole is always equal to the quotient obtained by dividing the diameter of the great speculum by the magnifying power of the telescope. It is generally from 1th to th of an inch in diameter. It is necessary, In many cases, to obtain from direct experiment an accurate determination of the place and size of the eyehole, as on this circumstance depends, ina certain degree, the accu- rate performance of the instrument. To centre the two Specula of Gregorian Reflectors. —Ex- tend two fine threads or wires across the aperture of the tube at right angles, so as to intersect each other exactly in the axis of the telescope. Before the arm is finally fastened. to the slider, place it in the tube, and through the eyepiece . (without glasses) the intersection of the cross-wires must ‘be seen exactly in the centre of the hole of the arm. When this exactness is obtained, let the arm be firmly riveted and soldered to the slider. | 7 : To centre Lenses.—The centring of lenses is of great im- portance, more especially for the object-glasses of achromatic instruments. The following is reckoned a good method: Let the lens to be centred be cemented on a brass chuck, having the middle turned away so as not to touch the lens except near the edge, which will be hid when mounted. This rim is very accurately turned flat where it is to touch the glass. When the chuck and cement is warm, it is made to revolve rapidly ; while in motion, a lighted candle is brought before it, and its reflected image attentively watched. If this image has any motion, the lens is not flat or central; a piece of soft wood must therefore be applied to it in the manner of a turn- ing tool, till such time as the light becomes stationary. When ‘the whole has cooled, the edges of the lens must be turned by a diamond, or ground with emery. For more particular details in reference to grinding and polishing specula and lenses, the reader is referred to Smith’s “Complete System of Optics,” Imison’s “School of Arts,” Fluygenti Opera, Brewster’s Appendix to “Ferguson’s Lectures,” ‘Irish Transactions,” vol. x., or “Nicholson’s Journal,’ vol. xvi., Nos. 65, 66, for January and February, 1807. CAVALLO’S MICROMETER. 313 PART III. ON VARIOUS ASTRONOMICAL INSTRU- MENTS. CHAPTER I. ON MICROMETERS. ~ A MICROMETER is an instrument attached to a telescope, in order to measure small spaces in the heavens, such as the spaces between two stars, and the diameters of the sun, moon, and planets ; and by the help of which, the apparent magni-. tude of all objects viewed through telescopes may be mea- sured with great exactness. There are various descriptions of these instruments, con- structed with different substances and in various forms, of which the following constitute the principal variety; the Wire Micrometer—the Spider’s-line Micrometer—the Poly- metric Reticle—Divided Object-glass Micrometer—Divided Eyeglass Micrometer—Ramsden’s Catoptric Micrometer— Rochon’s Crystal Micrometer—Maskelyne’s Prismatic Mi- crometer— Brewster’s Micrometrical Telescope—Sir W. Herschel’s Lamp Micrometer—Cavallo’s Mother-of-Pearl Micrometer, and several others; but, instead of attempting even a general description of these struments, I shall con- fine myself merely to a very brief description of Cavallo’s Micrometer, as its construction will be easily understood by the general reader, as it is one of the most simple of these instruments, and is so cheap as to be procured for a few shillings, while some of the instruments now mentioned are so expensive as to cost nearly as much asa tolerably good telescope.* | This micrometer consists of a thin and. narrow slip of mother-of-pearl finely divided, which is placed in the focus of the eyeglass of a telescope, just where the image of the “A particular description of the micrometers here enumerated, and several others, will be found in Dr. Pearson’s “Introduction to Practical Astronomy,’’ vol, ii. Von, IX. 27 é 814 CAVALLO’S MICROMETER. object is formed; and it may be applied either to a reflecting or a refracting telescope, provided the eyeglass be a convex lens. It is about the twentieth part of an inch broad, and of the thickness of common writing paper, divided into equal parts by parallel lines, every fifth and tenth of which is a little longer than the rest. ‘The simplest way of fixing it is to stick it upon the diaphragm, which generally stands within the tube, and in the focus of the eyeglass. When thus fixed, if you look through the eyeglass, the divisions of the micro- metrical scale will appear very distinct, unless the diaphragm is not exactly in the focus of the eyeglass, in which case it must be moved to the proper place; or the micrometer may be placed exactly in the focus of the eye-lens by the interposi- tion of a circular piece of paper, card, or by means of wax. If a person should not like to see always the micrometer in the field of the telescope, then the micrometrical scale, in- stead of being fixed to the diaphragm, may be fitted to a circular perforated plate of brass, of wood, or even of paper, which may be occasionally placed upon the said diaphragm. One of these micrometers, in my possession, which contains 600 divisions in an inch, is fitted up in a separate eyetube, with a glass peculiar to itself, which slides into the eyepiece of the telescope when its own proper glass is taken out. To ascertain the Value of the Divisions of this Micro- meter.—Direct the telescope to the sun, and observe how many divisions of the micrometer measure its diameter ex- actly; then take out of the Nautical Almanac the diameter of the sun for the day on which the observation is made; divide it by the above-mentioned number of divisions, and the quotient is the value of one division of the micrometer. Thus, suppose that 262 divisions of the micrometer measure the diameter of the sun, and that the Nautical Almanac gives for the measure of the same diameter 31’ 22’, or 1882’: divide 1882 by 26.5, and the quotient is 71”, or 1’ 11”, which is the value of one division of the micrometer, the double of which is the value of two divisions, and soon. The value of the divisions may likewise be ascertained by the passage of an equatorial star over a certain number of divisions in a certain time. The stars best situated for this purpose are such as the following: 6 in the Whale, R. A. 37°33’, Dec. 37’ 50 8.; Sin Orion, R. A. 80° 11/42”, Dec. 28’ 40” S.; vin the Lion, R. A. 171° 25’ 21”, Dec. 23’ 22” N.; y in Virgo, R. A. 182° 10’, Dec. 33’ 27” N. But the following is the most easy and accurate method of determining the value of the divisions : = CAVALLO’S MICROMETER. 315 / Mark upon a wall or other place the length of six inches, which may be done by making two dots or lines six inches asunder, or by fixing a six-inch ruler upona stand. Then aes the telescope before it, so that the ruler or six-inch ength may be at right angles with the direction of the tele- scope, and just 57 feet 32 inches distant from the object-glass of the telescope ; this done, look through the telescope at the ruler, or other extension of six inches, and observe how many divisions of the micrometer are equal to it, and that same number of divisions is equal to half a degree, or 30’; and this is all that is necessary for the required determination ; the reason of which is, because an extension of six inches subtends an angle of 30’ at the distance of 57 feet 33 inches, as may be easily calculated from the rules of Plane Trigono- metry. Fig. 85 exhibits this micrometer scale, but shows it four Figure 85. times larger than the real size of one which was adapted toa three-feet achromatic telescope magnifying 84 times. The divisions upon it are the 200ths of an inch, which reach from one edge of the scale to about the middle of it, excepting every fifth and tenth division, which are longer. T'wo divi- sions of this scale are very nearly equal to one minute ; and as a quarter of one of these divisions may be distinguished by estimation, therefore an angle of one-eighth of a minute, or of 72”, may be measured with it. When a telescope mag- nifies more, the divisions of the micrometer must be more minute. When the focus of the eyeglass of the telescope is shorter than half an inch, the micrometer may be divided with the 500ths of an inch; by means of which, and the telescope magnifying about 200 times, one may easily and accurately measure an angle smaller than half a second. On the other hand, when the telescope does not magnify above 30 times, the divisions need not be so minute. In one of Dollond’s pocket telescopes, which, when drawn out for use, is only 14 inches long, a micrometer with the hundredths of an inch is quite sufficient, and one of its divisions is equal to little less than three minutes, so that an angle of a minute may be measured by it. Supposing 113 of those divisions equal to 80’, or 23 to a degree, any other angle measured by 316 CAVALLO’S MICROMETER. any other number of divisions is determined by proportion. Thus, suppose the diameter of the sun, seen through the same telescope, be found equal to 12 divisions, say, as 113 divisions are to 30 minutes, so are 12 divisions to (>) 31.3, which is the required diameter of the sun, Practical Uses of this Micrometer.—This micrometer may be applied to the following purposes: 1. For mea- suring the apparent diameters of the sun, moon, and planets. 2. For measuring the apparent distances of the satellites from their primaries. 3. For measuring the cusps of the moon in eclipses. 4. For measuring the apparent distances between two contiguous stars—between a star and a planet—between a star and the moon—or between a comet and the contiguous stars, so as to determine its path. 5. For finding the dif- ference of declination of contiguous stars, when they have nearly the same right ascension. 6. For measuring the small elevations or depressions of objects above and below the ho- rizon. ‘7. For measuring the proportional parts of buildings, and other objects in perspective drawing. 8. For ascertaining whether a ship at sea, or any moving object, is coming nearer or going farther off; for if the angle subtended by the object appears to increase, it shows that the object is coming nearer, and if the angle appears to decrease, it indicates that the ob- ject is receding from us. 9. For ascertaining the real dis- tances of objects of known extension, and hence to measure heights, depths, and horizontal distances. _ 10. For measuring the real extensions of objects when their distances are known. 11. For measuring the distance and. size of an object when neither of them is known. When the micrometer is adapted to those telescopes which have four glasses in the eyetube, and when the eyetube only is. used, it may be applied to the following purposes: 1. For measuring the real or lineal dimensions of small objects in- stead of the angles; -for if the tube be unscrewed from the rest of the telescope, and applied to small objects, it will serve for a microscope, having a considerable magnifying power, as we have already shown (p. 250); and the micrometer, in that case, will measure the lineal dimensions of the object, as the diameter of a hair, the length of a flea, or the limbs of an insect. In order to find the value of the divisions for this purpose, we need only apply a ruler, divided into tenths of an inch, to the end of the tube, and, looking through the tube, observe how many divisions of the micrometer measure one tenth of an inch on the ruler, which will give the re- quired value. Thus, if 30 divisions are equal to ;4,th of an PROBLEMS SOLVED BY THE MICROMETER. 317 inch, 300 of them must be equal to one inch, and one division is equal to the 300th part of an inch 2. For-measuring the magnifying power of other telescopes. This is done by mea- suring the diameter of the pencil of light at the eye-end of the telescope in question; for, if we divide the diameter of the object-lens by the diameter of this pencil of light, the quotient will express how many times that telescope magnifies in diame- ter. ‘Thus, suppose that 300 divisions of the micrometer are equal to the apparent extension of one inch—that the pencil of light is measured by four of these divisions—and that the diameter of the object-lens measures one inch and two-tenths: Multiply 1.2 by 300, and the product 360, divided by 4, gives 90 for the magnifying power of the telescope. Problems which may be solved by this Micrometer.—t. The angle—not exceeding one degree—which is subtended by an extension of one foot, being given to find its distance from the place of observation: Rule 1. If the angle be ex- pressed in minutes, say, as the given angle is to 60, so is 687.55 to a fourth proportional, which gives the answer in inches. 2. If the angle be expressed in seconds, say as the given angle is to 3600, so is 687.55 to a fourth proportional, which expresses the answer in inches. 3. If the angle be expressed in minutes and seconds, turn it all into seconds, and proceed as above. Example: At what distance is a globe of one foot in diameter when it subtends an angle of two seconds? 2:3600:: 687°55: aaa 1237596 inches, or 1031323 feet = the answer required. II. The angle which is subtended by any known extension being given, to find its distance from the place of obseryation: Rule—Proceed as if, the extension were of one foot, by Problem I., and call the answer B; then, if the extension in question be expressed in inches, say, as 12 inches are to that extension, so is Btoa fourth proportional, which is the answer in inches. But if the extension in question be expressed in feet, then we need only multiply it by B, and the product is the answer in inches. Example: At what distance is a man six feet high when he appears to subtend an angle of 30"? By Problem I., if the man were one foot high, the distance would be 82506 inches; but as he is six feet high, therefore multiply 82506 by 6, and the product is the required distance, namely, 495036 inches, or 41253 feet. / For greater convenience, especially in travelling, when one has not the opportunity of making such calculations, the fol- lowing two tables have been calculated, the first of which shows the distance answering to any angle from one minute * / 318 TABLES FOR ASCERTAINING DISTANCES. to one degree, which is subtended by a man whose height is considered an extension of six feet, because at a mean, such is the height of a man when dressed with hat and shoes on. Angles subtended by an extension of |} Angles subtended by an extension of one foot at different distances. six feet at different distances. Sa} Sas fa O43 Aa O43 Aa ® eh) £2 |28| 22 |lge| 2 | 82] 22 a $ g wal] $5 bo $5 ws $a 48) AT a8) AN SE] AN) ae 1 | 3438 31 | 110.9 1 | 20626.8 | 31 | 665.4 2} 1719 32 | 107.4 2 | 10313 32 | 644.5 3 | 1146 33 | 104.2 3 6875.4 | 33 | 625 4 859.4 | 34 | 101.1 4 5156.5 | 34 | 606.6 5 687.5 } 35 98.2 B) 4125.2 | 35 | 589.3 6 572.9 | 36 95.5 6 3437.7 | 36 | 572.9 7 491.1 | 37 92.9 f 2946.6 | 37 | 557.5 8 429.7 | 38 90.4 8 2578.2 | 38 | 542.8 9 382 39 88,1 9 2291.8 | 39 | 528.9 10 343.7 -| 40 85.9 || 10 2062.6 | 40 | 515.6 11 312.5 | 41 83.8 ||} 11 1875.2 | 41 | 503.1 12 286.5 | 42 81.8 || 12 1718.8 | 42 | 491.1 13 264.4 | 43 79.9 |} 13 1586.7 | 43 | 479.7 14 245.5. | 44 78.1 {| 14 1473.3 | 44 | 468.8 15 229.2 | 45 76.4 || 15 1375 45 | 458.4 16 214.8 | 46 74.7 || 16 1298.1 | 46 | 448.4 17 202.2 | 47 73.1 || 17 1213.3 | -47 | 438.9 18 191 48 71.6 || 18 1145.9 | 48 |} 429.7 < 181 49 70.1 || 19 1085.6 | 49 | -421 20 171.8 | 50 68.7 || 20 1031.4 | 50 | 412.5 21 162.7 | 51 67.4 || 21 982.2 | 51 | 404.4 22 156.2 | 52 66.1 || 22 937.6 | 52 | 396.7 23 149.4 | 53 | 64.8 || 23 896.8 | 53 | 389.2 24 143.2 | 54 63.6 || 24 859.4 | 54 | 381.9 25 137.5 | 55 62.5 || 25 825 55. 375 26 132.2 | 56 61.4 |} 26 793.3 | 56 | 368.3 27 127.3 | 57 60.3 || 27 763.9 | 57 | 361.9 28 122.7 | 58 59.1 || 28 736.6 | 58 | 355.6 29 118.5 | 59 58.2 || 29 711.3: | 59 | 349.6 * 30 | .114.6 | 60 57.3 || 30 687.5 } 60 | 343.7 These tables may be transcribed on a card, and may be kept always ready with a pocket telescope furnished with a micro- meter. Their use is to ascertain distances without any cal- culations; and they are calculated only to minutes, because with a pocket telescope-and micrometer it is not possible to measure an angle more accurately than toa minute. Thus, if we want to measure the extension of a street, let a foot ruler be placed at the end of the street; measure the angular appearance of it, which suppose to be 36’, and in the table we have the required distance against 36’, which is 953 feet. Thus, also, a man who appears to be 49’ high is at the dis- tance of 421 feet. Again: Suppose the trunk of a tree, which is known to be three feet in diameter, be observed to subtend an angle of 93’.. Take the number answering to 9’ THE EQUATORIAL TELESCOPE. 819 out of the table, namely, 382, and subtract from it a propor- tional part for the half minute, namely, 19.1, which, sub- tracted from 382, leaves 362.9. This multiplied by 3,.the diameter of the tree, produces 1087, 7 feet = the distance from the object end. of the telescope. In this way the distance of a considerably remote object, as a town or building at ten or twelve miles’ distance, may be very nearly determined, provided we have the lineal dimen- sions of a house or other object that stands at right angles to the line of vision. The breadth of a river, of an arm of the sea, or the distance of a lighthouse, whose elevation above the sea or any other point is known, may likewise, in this man- ner, be easily determined. CHAPTER II. ON THE EQUATORIAL TELESCOPE, OR PORTABLE OBSERVATORY. Tue equatorial instrument is intended to answer a number of useful purposes in Practical Astronomy, independently of any particular observatory. Besides answering the general purpose of a quadrant, a transit instrument, a theodolite, and an azimuth instrument, it is almost the only instrument adapted for viewing the stars and planets in the daytime, and for following them in their apparent diurnal motions. It may be made use of in any steady room or place, and per- forms most of the useful problems in astronomical science. The basis of all equatorial instruments is a revolving axis,. placed parallel to the axis of the earth, by which an attached telescope is made to follow a star or other celestial body in the arc of its diurnal revolution, without the trouble of re- peated adjustments for changes of elevation, which quadrants and. circles with vertical and horizontal axes require. Such an instrument is not only convenient for many useful and in- teresting purposes in celestial observations, but is essentially requisite In certain cases, particularly in examining and measuring the relative positions of two contiguous bodies, or m determining the diameters of the planets, when the spi- der’s-line micrometer is used. Christopher Scheiner is supposed to have been the first astronomer who, in the year 1620, made use of a polar axis, Sut without any appendage of graduated circles. It was not. however, till the middle of the last century that any instru- 320 THE EQUATORIAL TELESCOPE. ments of this description, worthy of the name, were attempted to be constructed. In 1741, Mr. Henry Hindley, a clock- maker in York, added to the polar axis an equatorial plate, a quadrant of altitude, and declination semicircle; but when this piece\of mechanism was sent to London for sale in 1748, it remained unsold for the space of thirteen years. Mr. Short, the optician, published in the Philosophical Transac- tions for 1750 a “description of an equatorial telescope,” which was of the reflecting kind, and was mounted over a combination of circles and semicircles, which were strong enough to support a tube, and a speculum of the Georgian construction 18 inches in focal length. This instrument con- sisted of a somewhat cumbersome and expensive piece of machinery, a representation cf which may be seen in vol. iil. of Martin’s “ Philosophia Britannica, or System of the New- tonian Philosophy.” Various modifications of this instrument have since been made by Nairne, Dollond, Ramsden, Trough- ton, and other artists ; but even at the present period it has never come into very general use, though it is one of the most pleasant and useful instruments connected with astro- nomical observations. As many of these instruments are somewhat. complicated and very expensive, I shall direct the attention of the reader solely to one which I consider as the most simple, which may be purchased at a moderate expense, and is sufficiently ac- curate for general observations. This instrument consists of the following parts: A hori- zontal circle, E F, (fig. 86,) divided into four quadrants of 90 degrees each. There is a fixed nonius at N ; and the circle is capable of bemg turned round on an axis. In the centre of the horizontal circle is fixed a strong upright pillar, which supports the centre of a vertical semicircle, A B, divided into two quadrants of 90 degrees each. This is called the semicircle of altitude, and may, at any time, serve the pur- pose of a quadrant in measuring either altitudes or depres- sions.’ It has a nonius plate at K. At right angles to the plane of this semicircle, the equatorial circle, MN, is firmly fixed. It represents the equator, and is divided into twice twelve hours, every hour being divided into twelve parts of five minutes. each. Upon the equatorial circle moves ano- ther circle, with a chamfered edge, carrying a nonius, by which the divisions on the equatorial may be read off to sin- gle minutes; and at right angles to this movable circle is fixed the semicircle of declination, D, divided into two quad- rants of 90 degrees each. The telescope, P O, is surmounted ~ (TE EQUATORIAL TELESCOPE. 321 Figure 86. above this circle, and is fixed to an index movable on the semicircle of declination, and carries a nonius opposite to Q. The telescope is furnished with two or three Huygenian eye- pieces, and likewise with a diagonal eyepiece for viewing objects near the zenith. Lastly, there are two spirit levels fixed on the horizontal circle at right angles to each other, by means of which this circle is made perfectly level when ob servations are to be made. To adjust the Equatorial for Observations.—Set the in strument on a firm support; then, fo adjust the levels and the horizontal circle, turn the horizontal circle till the beginning 0 of the divisions coincides with the middle stroke of the nonius, or near it. In this situation, one of the levels will be found to lie either in a right line joining the two foot-screws which are nearest the nonius, or else parallel to such a right Ime. By means of the last two screws, cause the bubble in the level to become stationary in the middle of the glass ; then 322 THE EQUATORIAL TELESCOPE. turn the horizontal circle half round by bringing the other 0 — to the nonius; and if the bubble remains in the middle as © before, the level is well adjusted ; if it does not, correct the position of the level by turning one or both of the screws which pass through its ends till the bubble has moved half the ~ distance it ought to come to reach the middle, and cause it to move the other half by turning the foot-screws already men- tioned; return the horizontal circle to its first position, and if the adjustments have been well made, the bubble will remain in the middle; if otherwise, the process must be repeated till it bears this proof of its accuracy; then turn the horizontal circle till 90° stands opposite to the nonius; and by the foot- screw immediately opposite the other 90°, cause the bubble of the same level to stand in the middle of the glass ; lastly, by its own proper screws set the other level so that its bubble may occupy the middle of its glass. To adjust the Line of Sight.—Set the nonius on the de- clination semicircle at 0, the nonius on the horary circle at VI, and the nonius on the semicircle of altitude at 90; look through the telescope towards some part of the horizon where - there is a diversity of remote objects; level the horizontal circle, and then observe what object appears in the centre of the cross-wires, or in the centre of the field of view if there be no wires; reverse the semicircle of altitude so that the other 90° may apply to the nonius, taking care, at the same time, that the other three noniuses continue at the same parts of their respective graduations as before. If the remote object con- tinues. to be seen. on the centre of the cross-wires, the line of sight is truly adjusted. , To find the Correction to be applied to Observations by the Semicircle of Altitude.—Set the nonius on the declina- tion semicircle to 0, and the nonius on the horary circle to XII; direct the telescope to any fixed .and distant object by moving the horizontal circle and semicircle of altitude, and nothing else; note the degree and minute of altitude or de- pression ; reverse the declination semicircle by directing the nonius on the horary circle to the opposite XIT; direct the telescope again to the same object, by means of the horizontal circle and semicircle of altitude, as before. If its altitude or depression be the same as was observed in the other position, no correction will be required ; but if otherwise, half the dif- ference of the two angles is the correction to be added to all observations made with that quadrant, or half of the semicircle which shows the least angle, or to be subtracted from all the observations made with the other quadrant, or half of the THE EQUATORIAL TELESCOPE. 323 | -semicircle. When the levels and other adjustments are once truly made, they will be preserved in order for a length of time, if not deranged by violence; and the correction to be applied to the semicircle of altitude is a constant quantity. Description of the Nonius.—The nonius—sometimes called she Vernier—is a name given to a device for sub-dividing the _ ares of quadrants and other astronomical instruments. ~ It de- _ pends on the simple circumstance that if any lie be divided into equal parts, the length of each part will be greater the fewer the divisions; and contrariwise, it will be less in pro- portion as those divisions are more numerous. ‘Thus, in the equatorial now described, the distance between the two ex- treme strokes on the nonius is exactly equal to eleven degrees on the limb, only that it is divided into twelve equal parts. Each of these last parts will therefore be shorter than the . degree on the limb in the proportion of 11 to 12, that is to say, it will be ;4,th part, or five minutes shorter; consequently, if the middle stroke be set precisely opposite to any degree, ‘the relative positions of the nonius and the limb must be altered five minutes of a degree before either of the two ad- jacent strokes next the middle on the nonius can be brought to coincide with the nearest stroke of a degree; and so like- wise the second stroke on the nonius will require a change of ten minutes,.the third of fifteen, and so on to thirty, when the middle line of the nonius will be seen to be equidistant between two of the strokes on the limb; after which, the lines on the opposite side of the nonius will coincide in succession with the strokes on the limb. It is clear from this that when- ever the middle stroke of the nonius does not stand precisely opposite to any degree, the odd minutes, or distance between it and the degree immediately preceding, may be known by the number of the stroke marked on the nonius, which coin- cides with any of the strokes on the limb.* In some instru- ments the nonius-plate has its divisions fewer than the num- ber of parts on the limb to which it is equal; but when once a clear idea of the principle of any nonius is obtained, it will be easy to transfer it to any other mode in which this instru- ment is contrived. , To find by this Equatorial the Merwian Linz, and the Time, FROM ONE OBSERVATION OF THE SuN.—lIn order to this, it is requisite that the sun’s declination and the latitude of the place be known. ‘The declination of the sun may be found for every day in the Nautical Almanac, or any other astro- nomical ephemeris ; and the latitude of the place may be found * Adams’s Introduction to Practical Astronomy. ; 824 THE EQUATORIAL TELESCOPE. by means of the semicircle of altitude, when the telescope is directed to the sun or a known fixed star. It is likewise: re- quisite to make the observation when the azimuth and altitude of the sun alter quickly, and this is generally the case the farther that luminary_is from the meridian; therefore, at the distance of three or four hours either before or after noon, (in summer,) adjust the horizontal circle; set the semicircle of altitude so that its nonius may stand at the co-latitude of the place; lay the plane of the last-mentioned semicircle in the meridian by estimation, its 0 being directed towards the de- pressed pole; place the nonius of the declination semicircle to the declination, whether north or south; then direct the telescope towards the sun, partly by moving the declination semicircle on the axis of the equatorial circle, and partly by moving the horizontal circle on its-own axis. ‘There is but one position of these which will admit of the sun being seen exactly in the middle of the field of view. When this posi- tion is obtained, the nonius on the equatorial circle shows the apparent time, and the circle of altitude is in the plane of the meridian. When this position is ascertained, the meridian may be settled by a landmark at a distance. With an equatorial instrument nearly similar to that now described, I formerly made a series of * day observations on the celestial bodies,” which were originally published in vol. xxxvi. of “Nicholson’s Journal of Natural Philosophy,’ and which occupy twenty pages of that journal. Some of these observations I shall lay before the reader, after having explained the manner in which they are made. The instrument was made by Messrs. W. and S, Jones, - opticians, Holborn, London. The telescope which originally accompanied the instrument was an achromatic refractor, its object-glass being 83 inches focal distance, and one inch in diameter, This telescope, not admitting sufficiently high magnifying powers for the observations intended, was _after- ward thrown aside for another telescope having an object- glass 20 inches focal length and 17th inches in diameter, which was attached to the equatorial machinery in place of the small telescope. It was furnished with magnifying powers of 15, 30, 45, 60, and 100 times. The instrument was placed ona firm pedestal about three feet high. The feet of this pedestal had short iron pikes, which slipped into corresponding holes in the floor of the.apartment adjacent to a south window, so that when the direction of the meridian was found, and the circles properly adjusted, the instrument was in no danger of being shifted from this position. Though | THE EQUATORIAL TELESCOPE. $25 this instrument generally stood fronting the southern part of the heavens, yet the equatorial part, along with the telescope, could occasionally be removed to another position fronting the north and north-west, for observing the stars in those quarters. Manner of observing Stars and Planets in the Daytime by the Equatorial.—Before such observations can be made, the semicircle of altitude must be placed in the meridian, and the degree and minute pointed out by the nonius on the hori- zontal circle, when in this position, noted down in a book, so that it may be placed again in the same position, should any derangement afterward happen. ‘The semicircle of altitude must be set #0 the co-latitude of the place, that is, to what the latitude wants of 90°. Suppose the. latitude of the place of - obsem@ftion be 52° 30’ north, this latitude subtracted from 90° leaves 37° 30’ for the co-latitude, and therefore the semi- circle of altitude, on which the equatorial circle is fixed, must be elevated to 37° 30’, and then the equatorial circle on the instrument coincides with the equator in the heavens. Lastly, the telescope must be adjusted on the declination semicircle so as exactly to correspond with the declination of the heavenly body to be viewed. If the body is in the equator, the tele- scope is set by the index at 0 on the semicircle of declination, or at the middle point between the two quadrants, and then, when the telescope, along with the semicircle of declination, is moved from right to left, or the contrary, it describes an are of the equator. If the declination of the body be north, the telescope is elevated to the northern division of the sem1- circle ; if south, to the southern part of it. These adjustments being made, take the difference between the right ascension of the sun and the body to be observed, and if the right ascension of the body be greater than that of the sun, subtract the difference from the time of observation; if not, add to the time of observation.* The remainder in one case, or the sum in the other, will be the hour and minute to which the nonius on the equatorial circle is to be set; which being done, the telescope will point to the star or planet to whose declinatfon the instrument is adjusted. When the heavenly body is thus found, it may be followed, in its diurnal course, for hours, or as long as it remains above the horizon ; foy as the diurnal motion of a star is parallel to the equator, * Or find the sun’s right ascension for the’ given day; subtract this from the star or planet’s right ascension, and the remainder is the ap- prompt time of the star’s coming to the meridian. The difference etween this time and the time of observation will then determine the point to which the telescope is to be directed. Von. IX. 4 326 THE EQUATORIAL TELESCOPE. the motion of the telescope on the equatorial circle will always be in the star’s diurnal arc; and should it have left the field of the telescope for any considerable time, it may be again — recovered by moving the telescope onward according to the — time which elapsed since it was visible in the field of view. We may illustrate what has been now stated by an example or two: Suppose, on the 30th of April, 1841, at 1 -o’clock Pp. M., we wished to see the star Aldebaran: the right ascen- sion of this star is 4" 27™, and the sun’s right ascension for that day at noon, as found in “ White’s Ephemeris” or the «¢ Nautical Almanac,” is 2" 30™; subtract this last number from 4" 27™, and the remainder, 1" 57™, shows that the star comes to the meridian on that day at 57 minutes past.1 o’clock p.M.; and as the time of observation is 1 p.m., the nonius which moves on the equatorial circle must be set to three minutes past XI., as the star is at that hour 57 minutes from the meridian. The declination of Aldebaran is 16° 11/ north, to which point on the semicircle of declination the telescope must be adjusted, and then the star will be visible in the field of view. Again: suppose we wished to observe the planet Venus on the Ist of January, 1842, at 12 o’clock noon: the sun’s right ascension on that day is 18" 46™, and that of Venus 17 41™, from which the sun’s right ascension being subtracted, the remainder is 22% 55™, or 55 minutes past 10 A.M. Here, as the right ascension of Venus is too small to have the sun’s right ascension taken from it, we borrow 24 hours, and reckon the remainder from XJI. at noon. As the planet at 12 noon is one hour and five minutes past the meri- dian, the nonius on the equatorial circle must be set to that point, and the telescope adjusted to 23° 6’ of south declina- tion, which is the declination of Venus for that day, when this planet will appear in the field of view. Observations on the Fixed Stars and Planets, made in the Daytime by the Equatorial. For the purpose of illustrating the descriptions now given, and for affording some information respectin& celestial day observations, I shall select a few of the observations above alluded to, which I formerly published in Nicholson’s Journal, along with a few others which have been since made. These observations were made with a view to determine the follow ing particulars: 1. What stars and planets may be conveni- ently seen in the daytime when the sun is above the horizon? 2. What degrees of magnifying power are requisite for dis- tinguishing them? 3. How near their conjunction with the ~ ——- THE EQUATORIAL TELESCOPE. 827 sun they may be seen? and, 4. Whether the diminution of the aperture of the object-glass of the telescope, or the in- crease of magnifying power, conduces most to render a star or a planet visible in daylight. Having never seen such observations recorded in books of astronomy or in scientific journals, I was induced to continue them, almost every clear day, for nearly a year, in order to determine the points now specified. Some of the results are stated in the following ages. Observations on Fixed Stars of the first Magnitude.— April 23, 1813, at 10" 15" a.m., the sun being 53 hours above the horizon, saw the star Vega or a Lyre, very dis- tinctly with a power of 30 times. Having contracted the | aperture of the object-glass to ;%ths of an inch, saw it ona darker ground, but not more plainly than before. Having contracted the aperture still farther to half an inch, I per- ceived the star, but not so distinctly as before. The sky being very clear, and the star in a quarter of the heavens nearly opposite to the sun, I diminished the magnifying power to 15, and could still perceive the star, but indistinctly ; it was just perceptible. August 23, at 0" 12™ p.m., saw the star Capella, or a Murige, with a power of 60, and immedi- ately afterward with a power of 30, the aperture undimi- nished. With this last power it appeared extremely distinct, but not so brilliant and splendid as with the former power. Having diminished the aperture to ;%ths of an inch, it ap- peared on a darker ground, though in the former case it was equally perceptible. A few minutes afterward, could dis- tinguish it with a power of 15, the aperture being contracted to half an inch. It appeared very small; it was with diffi- culty the eye could fix upon it in the field of the telescope ; but when it was once perceived, its motion across the field of view could be readily followed. It could not be perceived when the diminished aperture was removed. The sun was then shining in meridian splendour. August 10th, 9" 30™ a.m., saw the star Sirius with a power of 60, the aperture contracted to ;®ths of an inch; saw it likewise when the aperture was diminished to half an inch, but not so distinctly as through the aperture of ,9ths of aninch. Having put on a power of 30, could distinguish it distinctly enough through each of the former apertures, and likewise when they were removed, but somewhat. more dis- tinctly with the apertures of nine-tenths and half an inch than without them. At this time the star was 2° 42™ in time of right ascension west of the sun, having an elevation above 328 THE EQUATORIAL TELESCOPE. the horizon of about 17° 10’, the sun shining’ bright, and the sky very much enlightened in that quarter of the heavens where the star appeared. ‘There was also a considerable undulation of the air, which is generally the case in the hot ‘mornings of summer, which renders a star more difficult to be perceived than in the afternoon, especially when it is viewed at a low altitude. June 4th, 1° 30™ p. m., saw Sirius with a power of 30 with great distinctness, the aperture not contracted. The star was then within 1" 50™ in time of right ascension east from the sun. August 24th, 9% 5™ a. m., saw the star Procyon, or «a Canis-Minoris, distinctly with a power of 60, the aperture not contracted. When diminished to ths of an inch, it appeared rather more distinct, as the ground on which it was seen was darker. With a power of 30, and the aperture contracted to ,%ths of an inch, could perceive it, but somewhat indistinctly. When the equatorial motion was performed in order to keep it in the field of view, it- was some time before the eye could again fix upon it. When the aperture was diminished to half an inch, it could not be perceived. Saw it when both the apertures were re- moved, but rather more distinctly with the aperture of .%,ths of aninch. ‘The difference in the result of this observation from that of Capella above stated was owing to the star’s proximity to the sun, and the consequent illumination of the sky in that quarter where it appeared. Its difference in right ascension from that of the sun was then about 2% 5™ of time, and its difference of declination about 4° 50’.* This star may be considered as one of those which rank between the first and second magnitudes. Similar observations to the above were made and frequently repeated on the stars Rigel, Aldebaran, Betelguese, Cor- Leonis, and other stars of the first magnitude, which gave nearly the same results. The stars Altares and Fomalhaut are not so easily distinguished, on account of their great southern declination, and consequent low elevation above the horizon. The following observation on Arcturus may be added. June 3d, observed Arcturus very distinctly a little before seven in the evening, the sun being about 1* 40™ * The right ascensions, declinations, longitudes, &c., stated in these memoranda, which were noted at the time of observation, are only ap- proximations to the truth, perfect accuracy in these respects being of no importance in such observations. They are, however, in general, within a minute or two of the truth. The times of the observations, too, are noted in reference, not to the astronomical, but to the civil day. The astronomical day commences: at 12 noon, and the hours are reckoned, without. interruption, to the following noon. ‘The civil day commences at 12 midnight. OBSERVATIONS BY DAY. 329 above the horizon, and shining bright—with a power of 15, the aperture not contracted. It appeared very small but dis- tinct. This star is easily distinguishable at any time of the day with a power of 30. Observations on Stars of the Second Magnitude.—May 5, 1813, at 6" p. m., the sun being an hour and three quar- ters above the horizon, saw Alphard, or a Hydre, a star of the second magnitude, with a power of 60, the aperture di- minished to ,%ths of an inch. A few minutes afterward could perceive it, but indistinctly, with a power of 30, the aperture contracted as above. It could not be seen very dis- tinctly with this power till about half an hour before sunset. It was then seen rather more distinctly when the aperture was contracted than without the contraction. May 7th, saw the star Deneb, or B Leonis, distinctly with a power of 60, about an hour and a half before sunset. August 20th, saw Ras Alkague, or a Ophiuchi, at 4" 40™ p. m., with a power of 100, the sun being nearly three hours above the horizon, and shining bright. Perceived it about an hour afterward with a power of 60, with the aperture contracted to -% ths of an inch, and also when this contraction was refhoved, The star was seen nearly as distinctly in the last case as in the first. August 27, 5" p. m., the same star appeared quite distinct with a power of 60, the aperture not contracted. It did not appear more distinct when the aperture was contracted to j¥sths of an inch. The sun was then more than two hours above the horizon. August 28th, saw the star Pollux or gp Gemini, two hours after sunrise, with a power of 60, aper- ture undiminished. November 12th, 1" 30’ p. m., saw the star Altair, or a Aquile, with an 8% inch telescope, one inch aperture, carrying a power of 45, the aperture not contracted. Having contracted the aperture a little, it appeared somewhat less distinct. 'This star is reckoned by some to belong to the class of stars of the first magnitude, but in White’s “ Ephe- meris” and other Almanacs it is generally marked as being of the second magnitude. It forms a kind of medium be- tween stars of the first and second magnitude. Similar observations, giving the same results, were made on the stars Ballatrix, Orion’s Girdle; a Andromede, o Pegasi, Alioth, Benetnach, North Crown, or « Coron Borealis, and various other stars of the same magnitude. From the above and several hundreds of similar obser- vations, the following conclusions are deduced. 1. That a.magnifying power of 30 times is sufficient for distinguishing a fixed star of the first magnitude, even at noon 28* 330 THE EQUATORIAL TELESCOPE. day at any season of the year, provided it have a moderate degree of elevation above the horizon, and be not within 30° or 40° of the sun’s body; also that by a magnifying power of 15, a star of this class may be distinguished. when the sun is not more than an hour and a half above the horizon; but, in every case, higher powers are to be preferred. Powers of 45 or 60, particularly the last, were found to answer best in most cases, as with such powers the eye could fix on the star with ease as soon as it entered the field of the telescope. 2. That most of the stars of the second magnitude may be seen with a power of 60 when the sun is not much more than two hours above the horizon; and, any time of the day, the brightest stars of this class may be seen witha power of 100 when the sky is serene, and the star not too near the quarter in which the sun appears. 3. That, in every instance, an increase of magnifying power has the principal effect in rendering a star easily per- ceptible; that diminution of aperture, in most cases, produces a very slight effeci—in some c&ses, none at all; and, when the aperture is contracted beyond a certain limit, it: produces, a hurtful effect. The cases in which a moderate contraction is useful are the two following? 1. When-the star appears in a bright part of the sky, not far from that quarter in which the sun, appears. 2. When an object-glass of a large aper- ture and a small degree of magnifying power is used. » In almost every instance, the contraction of the object-glass of the 82-inch telescope with a power of 45 had a hurtful effect ; but when the 20-inch telescope carried a power of only 15, the contraction served to render the object more perceptible. | Observations on the Planets made in the Daytime. Some of the planets are not so easily distinguished in the daytime as the fixed stars of the first magnitude. The one which is most easily distinguished at all times is the planet Venus. 1. Observations ‘on Venus.—My observations on this planet commenced about the end of August, 1812, about three or four weeks after its inferior conjunction. About that period, between ten and-eleven in the forenoon, with a power of 45, it appeared as a beautiful crescent, quite distinct and well defined, with a lustre similar to that of the moon about sunset, but of a whiter colour. The view of its surface and phase was fully more distinct and. satisfactory than what is obtained in the evening after sunset; for, being at a high ele- vation, the undulation near the horizon did not affect the dis- OBSERVATIONS BY DAY. 331 tinctness of vision. The planet was then very distinctly seen with a power of seven times, when it appeared like a star of the first or second magnitude. I traced the variation of its phases almost every clear day till the month of May, 1813. Asat that time it was not far from its superior conjunc tion with the sun, I wished to ascertain how near its conjunc- tion with that luminary it might be seen, and particularly whether it might not be possible, in certain cases; to see it at the moment of its conjunction. : The expressions of all astronomical writers previous to this period, when describing the phases of Venus, either directly assert, or at least imply, that it is impossible to see that planet, in any instance, at the time of its superior conjunction. | This is the language of Dr. Long, Dr. Gregory, Dr. Brewster, Ferguson, Adams, B. Martin, and most other: writers on the science of Astronomy. How far such language is correct will-appear from the following observations and remarks. April 24, 1813, 10" 50’ a. m., observed Venus with a power of 30, the aperture not contracted. She was then about 31 minutes of time in, right ascension distant from. the sun, their difference of declination 3° 59’. She appeared distinct and well defined. With a power of 100, could dis- tinguish her gibbous phase. May Ist, 10" 20™ a. m., viewed this planet with a power of 60, the aperture not contracted. It appeared distinct. Saw it about the same time with a power of 15, the aperture being contracted to ,%ths: of an inch. Having contracted the aperture to half an mch, saw it more distinctly. When the contracted apertures: were re- moved, the planet could with difficulty be distinguished, on account of the direct rays of the sun striking on the inside of the tube of the telescope. .'The sun was shining bright, and the planet about 25’ of time in R. A. west of his centre, their difference of declination being 3°°7’. May 7th, 10" a. m., saw Venus distinctly with a power of 60, the sun shining bright. . It was then about 19’ of time in R. A., and 4° 27’ in longitude west of the sun, their difference of declination being 2° 18’. I found a diminution of aperture particularly useful when viewing the planet at this time, even when the higher powers were applied. This was the last observation I had an opportunity of making prior to the conjunction of Venus with the sun, which happened on May 25, at 9» 3™ A. M. Its geocentric latitude at that time being about 16’ south, the planet must have passed almost close by the sun’s southern limb. Cloudy weather for nearly a month after the last: observation. prevented any farther views of the planet, 332 THE EQUATORIAL TELESCOPE. ~ when it was in that part of the heavens which was within the range of the instrument. The first day that proved favoura- ble after it had passed the superior conjunction was June 5th. The following is the memorandum of the observation then taken. June 5th, 9" a. m., adjusted the equatorial telescope for viewing the planet Venus, but it could not be perceived on account of the direct rays of the sun entering the tube of the telescope. I contrived an apparatus for screening his. rays, but could not get it conveniently to move along with the tele- scope, and therefore determined to wait till past eleven, when the top of the window of the place of observation would inter- cept the solar rays. At 11" 20" a. m., just as the sun had passed the line of sight from the eye to the top of the win- dow, and his body was eclipsed by it, I was gratified with a tolerably distinct view of the planet, with a power of 60, the aperture being contracted to =%ths of an inch. The distinct- ness increased as the sun retired, till, in two or three minutes, the planet appeared perfectly well defined. Saw it imme- diately afterward with a power of 30,the aperture contracted as before. Saw it also quite distinctly with a power of 15; but it could not be distinguished with this power when the contracted aperture was removed. » At this time Venus was just 3° in longitude, or about.13’ in time of R. A. east of the sun’s centre, and of course only about 27th degrees from his eastern limb; the difference of their declination being 27’, and the planet’s latitude 11’ north. Several years afterward I obtained views of this planet when considerably nearer the sun’s margin than as stated in the above observation, particularly on the 16th of October, 1819, at which time Venus was seen when only six days and nineteen hours past the time of the superior conjunction. At that time its distance from the sun’s eastern limb was only 1° 28’ 42’... A subsequent observation proved that Venus can be seen when only 1° 27’ from the sun’s margin, which I consider as approximating to the nearest distance from the sun at which this planet. is distinctly visible. I shall only state farther the two or three following observations. June 7th, 1813, 10" a. m., saw Venus with a power of 60, the aperture being contracted to ,9ths of an inch, the direct rays of the sun not being intercepted by the top of the win- dow. The aperture having been farther contracted to half an inch, could perceive her, but not quite so distinctly. When the contractions were removed, she could scarcely be seen. She was then 3° 33’ in longitude, and nearly 15 minutes in. OBSERVATIONS BY DAY. 333 time of R. A. distant from the sun’s centre. Some fleeces of clouds having moved across the field of view, she was seen remarkably distinct in the intérstices, the sun at the same time being partly obscured by them. August 19th, 1° 10™ p. M., viewed Venus with a magnifying power of 100. Could perceive her surface and gibbous phase almost as distinctly as when the sun is below the horizon. She appeared bright, steady in her light, and well defined, without that glare. and tremulous appearance she exhibits in the evening when near the horizon. She was then nearly on the meridian. On the whole, such a view of this planet is as satisfactory, if not pre- ferable, to those views we obtain with an ordinary telescope in the evening, when it is visible to the naked eye. | All the particulars above stated have been confirmed by many subsequent observations continued throughout a series of years. I shall state only two recent observations, which show that Venus may be seen somewhat nearer the sun than what is deduced from the preceding observations, and at the point of its superior conjunction. Mareh 10th, 1842, ob- served the planet Venus, then very near the sun, at 19 minutes past 11 a.m. It had passed the point of its superior conjunction with the sun on the 5th of March, at 1" 19™ p.m. The difference of right ascension between the sun and the planet was then about 63 minutes of time, or about 1° 373’, and it was only about 1° 21/ distant from the sun’s eastern limb. It appeared quite distinct and well defined, and might perhaps have been seen on the preceding day, had the obser- vation been then made. The following. observation shows that Venus may be seen still nearer the sun than in the pre- ceding observations, and even at the moment of its superior conjunction. On the 2d of October, 1843, this planet passed the point of its superior conjunction with the sun at 4°15" p.m. At two o’clock p.m., only two hours before the conjunc- tion, I perceived the planet distinctly, and kept it in view for nearly ten minutes, till some dense clouds intercepted the view. It appeared tolerably distinct and well defined, though not brilliant, and with a round, full face, and its apparent path was distinctly traced several times across the field of view of the telescope. I perceived it afterward, about half-past four p. M., only a few minutes after it had passed the point of con- junction, on which occasion it appeared less distinct than in the preceding observation, owing to the low altitude of the planet, being then only a few degrees above the horizon. The observations, in this instance, were made, not with an equatorial instrument, which I generally use in such obser- 334 THE EQUATORIAL TELESCOPE. vations, but with a good achromatic telescope of 442 inches focal distance, mounted on a common tripod, with a terres- trial power of 95 times. A conical tube about ten imches long was fixed on the object-end of the telescope, at the ex- tremity of which an aperture 1; inches in diameter was placed, so as to intercept, as much as possible, the direct ingress of the solar rays. The top of the upper sash of the window of the place of observation was likewise so adjusted as to intercept the greater part of the sun’s rays from enter- ing the tube of the telescope. The sun’s declination at that time was 3° 26’ south, and that of Venus 2° 12’ south; con- sequently, the difference of declination was 1° 14’ = the dis- tance of Venus from the sun’s centre; and as the sun’s dia- meter was about 16’, Venus was then only 58’ from the sun’s northern limb, or 6’ less than two diameters of the sun. This is the nearest approximation to the sun at which I have ever beheld this planet, and it demonstrates that Venus may be seen even within a degree of the sun’s margin; and it is, perhaps, the nearest position to that luminary in which this planet can be distinctly perceived. It shows that the hight reflected from the surface of Venus is far more brilliant than that reflected from the surface of our moon; for no trace of this nocturnal luminary can be perceived, even when at a much greater distance from the sun, nor is there any other celestial body that can be seen within the limit now stated. This is the first observation, so far as my information extends, of Venus having been seen at the time of her superior con- junction.* The practical conclusion from this observation is, that at the superior conjunction of this planet, when its distance from the sun’s margin is not less than 58’, its polar and equatorial diameter may be measured by a micrometer, when it will be determined whether or not Venus be of a spheroidal figure. The Earth, Mars, Jupiter, and Saturn are found to be, not spheres, but spheroids, having their polar shorter than their equatorial diameters. But the true figure of Venus has never yet been ascertained, because it is onl at the superior conjunction that she presents a full, enlight- ened hemisphere, and when both diameters can be measured, except at the time when she transits the sun’s disk, which happens only twice in the course of 120 years.t * This observation is inserted in the ‘‘ Edinburgh Philosophical Jour- nal’’ for January, 1844. t The late Mr. Benjamin Martin, when describing the nature of the solar telescope, in his *‘ Philosophia Britannica,’’ vol. iii, p: 85, gives the OBSERVATIONS BY DAY. 335 The following conclusions are deduced from the observa- tions on Venus: following relation: ‘‘I cannot here omit to mention a very unusual phe- nomenon that I observed about ten years ago in my darkened room. ‘I'he window looked toward the west, and the spire of Chichester Cathedral was before it at the distance of 50 or 60 yards. I used very often to di- vert myself by observing the pleasant manner in which the sun passed behind the spire, and was eclipsed by it for some time; for the image of the sun and of the spire were very large, being made by a lens of 12 feet focal distance; and once, as I observed the occultation of the sun behind the spire, just as the disk disappeared, I saw several small, bright, round bodies or balls running towards the sun from the dark part of the room, even to the distance of 20 inches. I observed their motion was a little irregular, but rectilinear, and seemed accelerated as they approached the sun. ‘These luminous globules appeared also on the other side of the spire, and preceded the sun, running out into the dark room, sometimes more, sometimes less, together, in the same manner as they followed the sun at its occultation. ‘They appeared to be, in general, one-twentieth of an inch in diameter, and Earetore must be very large, luminous globes in some part of the heayens, whose light was extinguished by that of the sun, so that they appeared not in open daylight; but whether of the me- teor kind, or what sort of bodies they might be, I could not conjecture.’’ Professor Hansteen mentions that, when employed in measuring the zenith distances of the polar star, he observed a somewhat similar phe- nomenon, which he describes as ‘‘a luminous body which passed. over the field of the universal telescope ; that its motion was neither perfectly equal nor rectilinear, but resembled very much‘the unequal and some- what serpentine motion of an ascending rocket ;’’ and he concluded that it must have been ‘‘a meteor’’ or ‘‘ shooting star’’ descending from the higher regions of the atmosphere.* Tn my frequent observations on Venus, to determine the nearest posi- tions to the sun in which that planet could be seen, I had several times an Srportany. of witnessing similar phenomenon. I was not a little sur- prised, when searching forthe planet, frequently to perceive a body pass across the field of the telescope, apparently of the same size as Venus, though sometimes larger and sometimes smaller, so that I frequently mistook that body for the planet, till its rapid motion undeceived me. In several instances four or five of these bodies appeared to cross the field of view, sometimes in a perpendicular, and at other times in a horizontal direction. ‘They appeared to be luminous bodies, somewhat resembling the appearance of a planet when viewed in the daytime with a moderate magnifying power. ‘Their motion was nearly rectilinear, but sometimes incliued to a waving or serpentine form, and they appeared to move with considerable rapidity—the telescope being furnished with a power of about 70 times. I was for a considerable time at a loss what opinion to form of the nature of these bodies; but, having occasion to continue these observations almost every clear day for nearly a twelvemonth, I had frequent opportunities of viewing this phenomenon in different as- ects, and was at length enabled to form an opinion as to the cause of at east some of the appearances which presented themselves. In several instances, the bodies alluded to appeared much larger than usual, and to move with a more rapid velocity ; in which case I could plainly perceive that they were nothing else than birds of different sizes, and apparently at different distances, the convex surface of whose bodies in certain posi- tions, strongly reflected the solar rays. | In other instances, when they appeared smaller, their true shape was undistinguishable, by reason of their motion and their distance. * See Edinburgh Philosophical Journal for April, 1825, No. xxiv. 336 THE EQUATORIAL TELESCOPE. 1. That this planet may be seen distinctly, with a moderate degree of magnifying power, at the moment of its superior Having inserted a few remarks on this subject in No. xxv. of the Edin- burgh Pivtoaophicnt Journal for July, 1825, particularly in reference to Professor Hansteen’s opinion, that article came under the review of M. Serres, sub-prefect of Embrun, in a paper inserted in the Annales de Chimie for October, 1825, entitled, ‘‘ Notices regarding fiery meteors seen during the day.’’* In the discussion of this subject, M. Serres ad- mits that the light reflected very obliquely from the feathers of a bird is capable of producing an effect similar to that which I have now described, but that ‘‘ the explanation ought not to be generalized.’’ He remarks, that while observing the sun at the repeating circle, he frequently per- ceived, even through the coloured glass adapted to the eyepiece, large luminous points which traversed the field of the telescope, and which appeared too well defined not to admit them to be distant, and subtended too large angles to imagine them birds. In illustration of this subject, he states the following facts: On the 7th of September, 1820, after having observed for some time the eclipse of the sun which happened on that day, he intended to take a walk in the fields, and on crossing the town, he saw a numerous group of individuals of every age and sex, who had their eyes fixed in the direction of the sun. Farther on, he perceived another group, having their eyes in like manner turned towards the sun, He questioned an intelligent artist who was among them to learn the ob- ject that fixed his attention. He replied, ‘“We are looking at the stars which are detaching themselves from the sun.’’ ‘* You may look your- self; that will be the shortest way to learn the fact.’’ He looked, and saw, in fact, not stars, but balls of fire, of a diameter equal to the largest stars, which were projected in various directions from the upper hem- isphere of the sun, with an incaleulable velocity ; and although this ve- locity of projection appeared the same in all, yet they did not all attain the same distance. ‘These globes were projected at unequal and pretty short intervals. Several were often projected at once, but always di- verging from one another.. Some of them described a right line, and were extinguished in the distance ; some described a parabolic line, and were in like manner extinguished; others, again, after having removed to a certain distance in a right line, retrograded upon the same line, and seemed to enter, still luminous, into the sun’s disk. The ground of this magnificent picture was a sky-blue, somewhat tinged with brown. Such was his astonishment at the sight of so majestic a spectacle, that it was impossible for him to keep his eyes off it till it ceased, which happened gradually as the eclipse wore off and the solar rays resumed their ordi- nary lustre. It was remarked by one of the crowd that “the sun pro- jected most stars at the time when it was palest;’’ and that the circum- stance which first excited attention to this phenomenon was that of a woman, who cried out, “Come here! come and see the flames that are issuing from the sun!’’ I have stated the above facts because they may afterward tend to throw light upon certain objects or phenomena with which we are at present urfacquainted. ‘The phenomenon of “ falling stars’’ has of late years ex- cited considerable attention, and it seems now to be admitted that at least certain species of these bodies descend from regions far beyond the limits of our atmosphere. ‘This may be pronounced as certain with regard to the ‘‘ November meteors.’’ May not some of the phenomena described above be connected with the fall of meteoric stones—the showers of fall- ing stars seén on the 12th and 13th of November, or other meteoric phe- nomena whose causes we have hitherto been unable to explain? Or, may we conceive that certain celestial bodies, with whose nature and * See Edinburgh Philosophical Journal for July, 1826, p. 114. OBSERVATIONS BY DAY. 337 conjunction with the sun, when its goecentric latitude, either north or south, at the time of conjunction, is not less than 1° 14’, or when the planet is about 58’ from the sun’s limb. This conclusion is deduced from the observation of October 2, 1843,* as stated above. 2. Another conclusion is, that during the space of 583 days, or about 19 months—the time this planet takes in moving from one conjunction with the sun to a like conjunc- tion again—when its latitude at the time of its superior con- junction exceeds 1° 14’, it may be seen with an equatorial telescope every clear day without interruption, except about the period of its inferior conjunction, when its dark hemi- sphere is turned towards the earth, and a short time before and after it. When its geocentric latitude is less that 1° 14’, it will be hid only about four days before, and the same time after its superior conjunction, During the same period it will be invisible to the naked eye, and consequently no observa- tions can be made upon it with a common telescope for nearly six months, and sometimes more, according as its declination is north or south, namely, about two or three months before, and the same time after its superior conjunction, except where there is a very free and unconfined horizon. In re- gard to the time in which this planet can be hid about the period of its inferior conjunction, I have ascertained from ob- servation that it can never be hid longer than during a space of 2 days 22 hours, having seen Venus, about noon, like a fine, slender crescent, only 35 hours after she had passed the point of her inferior conjunction ; and in a late instance she was seen when little more than a day from the period of con- junction. The longest time, therefore, that this planet can be hid from view during a period of 583 days, is only about 10 days; and when its latitude at the time of the superior conjunction equals or exceeds 1° 14’, it can be hid little more than two days. This is a circumstance which cannot be affirmed of any other celestial body, the sun only excepted. _ 3. That every variation of the phases of this planet, from a slender crescent to a full, enlightened hemisphere, may, on every clear-day, be conveniently exhibited by means of the equatorial telescope. This circumstance renders this instru- destination we are as yet unacquainted, may be revolving in different courses in the regions around us, some of them opaque and cthers lu- minous, and whose light is undistinguishable by reason of the solar ef- ence? For an explanation of the manner of viewing Venus at her superior conjunction, see “‘ Celestial Scenery,’’ [Vol. 7, Uniform Edition of Dick s Works,} p. 76. Vou. IX. 29 338 THE EQUATORIAL TELESCOPE. ment peculiarly useful in the instruction of the young in the principles of astronomy ; for if the phase which Venus should exhibit at any particular time be known, the equatorial tele- ‘scope may be directed to the planet, and its actual phase in the heavens be immediately exhibited to the astronomical pupil. 4. Since it is only at the period of the superior conjunction that this planet presents a full, enlightened hemisphere, and since it is only when this phase is presented that both its diameters can be measured, it is of some importance that ob- servations be made on it at the moment of conjunction, by means of powerful telescopes furnished with micrometers, so as to determine the difference (if any) between its polar and equatorial diameters. 5. Another conclusion from the observations on Venus is, that a moderate diminution of the aperture of the object-glass of the telescope is useful, and even necessary, in viewing this planet when near the sun. _ Its effect is owing in part to the direct solar rays being thereby more effectually excluded ; for when these rays enter directly into the tube of the tele- scope, it is very difficult, and almost impossible, to perceive this planet, or any other celestial body when in the vicinity of the sun. Observations on Jupiter and other Planets. This planet is very easily distinguished in the daytime with a very moderate magnifying power, when it is not within 30° or 35° of the sun. The following extract from my memorandums may serve as a specimen: May 12, 1813, 1+ 40" p.m., saw Jupiter with a power of 15 times, the aperture not contracted. The planet appeared so distinct with this power that I have reason to believe it would have been perceived with a power of six or seven times. "When the aperture was contracted to =®ths of an inch, and after- ward to half an inch, there was little perceptible difference in its appearance. It was then about 58° in longitude east of the sun. Though Jupiter, when at a considerable distance from the sun, and near his opposition, appears to the naked eye with a brilliancy nearly equal to that of Venus, yet there is a very striking difference between them in respect of lustre when viewed in daylight. Jupiter, when viewed with a high mag- nifying power in the daytime, always exhibits a very dull, cloudy appearance, whereas Venus appears with a moderate degree of splendour. About the end of June, 1813, between OBSERVATIONS BY. DAY. 339 five and six in the evening, having viewed the planet Venus, then within 20° of the sun, and which appeared with a mode rate degree of lustre, I directed the telescope to Jupiter, a that time more than 32° from the sun, when the contrast. be- tween the two planets was very striking, Jupiter appearing so faint as to be just discernible, though his apparent magni tude was nearly double that of Venus. In this observation a power of 65 was used. In his approach towards the sun about the end of July, I could not perceive him when he was within 16° or 17° of his conjunction with that luminary. These circumstances furnish a sensible and popular proof, independently of astronomical calculations, that the planet Jupiter is placed at a much greater distance from the sun than Venus, since. its light is so faint as to be scarcely per ceptible when more than 20 degrees from the sun, while that of Venus is distinctly seen amid the full splendour of the solar rays, when only about a degree from the margin of that luminary. With a power of 65 I have been enabled to dis- tinguish the belts of Jupiter before sunset, but could never perceive any of his satellites till the sun was below.the hon- zon. ‘There are no observations which so sensibly and strikingly indicate the different degrees of light emitted by the different planets as those which are made in the daytime. To.a common observer, during night, Jupiter ‘and Venus ap- pear, ina clear sky, nearly with equal brilliancy, and even Mars, when about the point of his opposition to the sun, appears with a lustre somewhat similar, though tinged with a ruddy hue; but when seen in daylight their aspect is very dissimilar. ‘This circumstance evidently indicates, 1, That these planets are placed at different distances from the sun, and consequently are furnished with different degrees of light proportional to the square of their distances from that lumi- nary; and, 2. That there are certain circumstances connected with the surfaces and atmospheres of the planetary bodies which render the light they emit more or less intense, inde- pendently of their different distances from the central lumi- nary; for Mars, though much nearer to the sun than Jupiter, is not so easily distinguished in the daytime, and even in the night-time appears with a less degree of lustre. My observations on Safurn in daylight have not been so frequent as those on Jupiter. I have been enabled to distin- guish his ring several times before sunset with a power of 65, but his great southern declination, and consequent low altitude, at the periods when these observations were made were unfavourable for determining the degree of his visibility 340 THE’ EQUATORIAL TELESCOPE. in daylight; for a planet or a star is always more distinctly perceptible.in a high than in a low altitude, on account of the superior purity of the atmosphere through which a celestial object is seen when ata high elevation above the horizon. This planet, however, is not nearly so distinctly visible in daylight as Jupiter, and I have chiefly seen it when the sun was not more than an hour or two above the horizon, but never at noonday, although it is probable that with powerful instruments it may be seen even at that period of the day. The planet Mars is seldom distinctly visible in the daytime, except when at no great distance from its opposition to the sun. The following is a memorandum of an observation on Mars, when in a favourable position: October 24, 1836, saw the planet Mars distinctly with a power of about 60, at. 40 minutes past 9 a.m., the sun having been above the horizon nearly three hours. It appeared tolerably distinct, but scarcely so brilliant as a fixed star of the first magnitude, though with apparently as much light as Jupiter generally exhibits when viewed in daylight. It could not be traced longer at the time, so as to ascertain if it could be seen at midday, on account of the interposition of the western side of the window of the place of observation. The ruddy aspect of this planet— doubtless caused by a dense atmosphere with which it is environed—is 6ne of the causes which prevents its appearing with brilliancy in the daytime. With respect to the planet Mercury, 1 have had opportunities of observing it several times after sunrise and before sunset, about 10 or 12 days before and after its greatest elongation from the sun, with a power of 45. I have several times searched for this planet about noon, but could not perceive it. The air, however, at the times alluded to, was not very clear, and I was not cer- tain that it was within the field of the telescope, and therefore Iam not convinced but that, with a moderately high power, it may be seen even at noonday. Such are some specimens of the observations I have made on the heavenly bodies in the daytime, and the conclusions which may be deduced from them. I have been induced to communicate them from the consideration that the ‘most minute facts in relation to any science are worthy of being known, and may possibly be useful. They may at least gratify the astronomical tyro with some information which he will not find in the common treatises on Astronomy, and may perhaps excite him to prosecute a train of similar obser- vations for confirming or correcting those which have been noted above. . OBSERVATIONS BY DAY. 341 Besides the deductions already stated, the following gene- ral conclusions may be noted: 1. That a celestial body may be as easily distinguished at noonday as at any time between the hours of nine in the morning and three in the afternoon, except during the short days in winter. 2. They are more easily distinguished at a high than at a low altitude—in the afternoon than in the morning, especially if their altitudes be low—and in the northern region of the heavens than in the southern. The difficulty of perceiving them at a low altitude is obviously owing to the thick vapours near the horizon. Their being less easily distinguished in the morning than in the afternoon is owing to the undulations of the atmosphere, which are generally greater in the morning than in the after- noon. This may be evidently perceived by looking at distant land objects at those times, in a hot day, through a telescope which magnifies about 40 or 50 times, when they will be found to appear tremulous and distorted in consequence of these undulations, especially if the sun be shining bright. In consequence of. this circumstance, we can seldom use a high terrestrial power with effect on land objects except early in the morning and a short time before sunset. Their being more easily distinguished in the northern region of the heavens is owing to that part of the sky being of a deeper azure, on account of its being less enlightened than the southern with the splendour of the solar rays. Utility of Celestial Day Observations. The observations on the heavenly bodies in the daytime, to which I have now directed the attention of the reader, are not to be considered as merely gratifications of a rational curiosity, but may be rendered subservient to the promotion of astronomical science. As to the planet Venus: when I consider the degree of brilliancy it exhibits, even in daylight, I am convinced that useful observations might frequently be made on its surface in the daytime, to determine some of its physical peculiarities and phenomena. Such observations might set at rest any disputes which may still exist respect- ing the period of rotation of this planet. Cassini, from obser- vations on a bright spot, which advanced 20° in 24% 34™, de- termined the time of its rotation to be 23 hours 20 minutes. On the other hand, Bianchini, from similar observations, con- cluded that its diurnal period was 24 days and 8 hours. The difficulty of deciding between these two opinions arises from the short time in which observations can be made on this planet, either before sunrise auee sunset, which prevents 842 THE EQUATORIAL TELESCOPE. us from tracing with accuracy the progressive motion of its spots for a sufficient length of time; and, although an observer should mark the motion of the spots at the same hour on two succeeding evenings, and find they had moved forward about 15° in 24 hours, he would still be at a loss to determine whether they had moved only 15° in all since the preceding observation, or had finished a revolution and 15° more. If, therefore, any spots could be perceived on the surface of Venus in the daytime, their motion might be traced, when she is in north declination, for 12 hours or more, which would completely settle the period of rotation. ‘That it is not im- probable that spots fitted for this purpose may be discovered on her disk in the daytime, appears from some of the obser- vations of Cassini, who saw one of her spots when the sun was more than eight degrees above the horizon.* The most distinct and satisfactory views I have ever had of this planet were those which I obtained in the daytime, in summer, when it was viewed at a high altitude with a 442 inch achro- matic telescope, carrying a power of 150. I have at such times distinctly perceived the distinction between the shade and colour of its margin and the superior lustre of its central parts, and some spots have occasionally been seen, though not so distinctly marked as to determine its rotation. ~ Such distinct views are seldom to be obtained in the evening after sunset, on account of the undulations of the atmosphere, and the dense mass of vapours through which the celestial bodies are viewed when near the horizon. Nor do I consider it altogether improbable that its satellite (if it have one, as some have supposed) may be detected in the daytime, when this planet is in a favourable position for such an observation, particularly when a pretty large portion of its enlightened surface is turned towards the earth, and when its satellite, of course, must present a similar phase. About the period of its greatest elongation from the sun, and soon after it assumes a crescent phase, in its approach to the inferior conjunction, may be considered as the most eligible ’ times for prosecuting such observations. If this supposed satellite be about one-third or one-fourth of the diameter of its primary, as Cassini, Short, Baudouin, Montbarron, Mon- taigne, and other astronomers supposed, it must be nearly as large as Mercury, which has been frequently seen in daylight. If such a satellite have a real existence, and yet undistin- guisnable in daylight, its surface must be of a very different * See Long’s Astronomy, vol. ii. p. 487, and Encyclopedia Britannica, vol. 1. p. 436, 3d edition. OBSERVATIONS BY DAY. 343 quality for reflecting the rays of light from that of its primary ; for it is obvious to every one who has seen Venus with a high power in the daytime, that a body of equal brilliiancy, though four times less in diameter, would be quite perceptible, and exhibit a visible disk. Such observations, however, would be made with much greater effect in Italy and other southern countries, and particularly in tropical climates, such as the southern parts of Asia and America, and in the West India islands, where the sky is more clear and serene, and where the planet may be viewed at higher altitudes and for a greater length of time, without the interruption of clouds, than in our island. 7 ; aaah Again, the apparent magnitudes of the fixed stars, the quantity of licht they respectively emit, and the precise class of magnitude which should be assigned to them, might be more accurately determined by day observations than by their appearance in the nocturnal sky. All the stars which are reckoned to belong to the first magnitude are not equally distinguishable in daylight. Forexample, the stars 4ldebaran and Procyon are not so easily distinguished, nor do they ap- pear with the same degree of lustre by day, as the stars a Lyre and Capella. In like manner, the stars, Altair, Alphard, Deneb Ras Alkague, considered as belonging to the second magnitude, are not equally distinguishable by the same aperture and magnifying power, which seems to indicate that a different quantity of light is emitted by these stars, arising from a difference either in their magnitude, their distance, or the quality of the light with which they are irradiated. The following are likewise practical purposes to which celestial day observations may be applied. In accurately ad- justing circular and transit instruments, it is useful, and even necessary, for determining the exact position of the meridian, to take observations of certain stars which differ ereatly in zenith distance, and which transit the meridian nearly at the same time. But as the stars best situated for this purpose cannot, at every season, be seen in the evenings, we must, in certain cases, wait for several months before such observations can be made, unless we make them in the day- time, which can very easily be done if the instrument have a telescope adapted to it, furnished with such powers as those above stated, or higher powers if required. I have likewise made use of observations on the stars in the daytime for ad- justing a clock or watch to mean time, when the sun was in a situation beyond the range of the.instrument, or obscured by clouds, and when I did not choose to wait till the evening. 344 THE EQUATORIAL TELESCOPE. This may, at first view, appear to some as paradoxical, since the finding of a star in daylight depends on our knowing its right ascension from the sun, and. this last circumstance de- pends, in some measure, on our knowing the true time. But if a watch or clock is known not to have varied above seven or eight minutes from the time, a star of the first magnitude may easily be found by moving the telescope a little back- ward or forward till the star appear; and when it is once found, the exact variation of the movement is then ascertained. by comparing the calculations which were previously neces- sary with the time pointed out by the nonius on the equato- rial circle ; or, in other words, by ascertaining the difference between the time assumed-and the time indicated by the in- strument when the star appears in the centre of the field of view. All this may be accomplished in five or six minutes. Besides the practical purposes now stated, the equatorial telescope is perhaps the best instrument for instructing a learner in the various operations of practical astronomy, and particularly for enabling him to distinguish the names and positions of the principal stars ; for when the right ascension and declination of any star is known from astronomical tables, the telescope may be immediately adjusted to point to it, which will infallibly prevent his mistaking one star for another. In this way, likewise, the precise position of the planet Jer- cury, Uranus, Vesta, Juno, Ceres, Pallas, a small comet, a nebula, a double star, or any other celestial body not easily distinguishable by the naked eye, may be readily pointed. out, when its right ascension and declination are known to a near approximation. | In conclusion, I cannot but express my surprise that the equatorial telescope is so little known, even by many of the lovers of astronomical science. In several respectable acade- mies in this part of Britain, and, if I am not misinformed, in most of our universities, this instrument is entirely unknown. This is the more unaccountable, as a small equatorial may be purchased for a moderate sum, and as there is no single in- strument so well adapted for illustrating all the operations of practical astronomy. Where very great accuracy is not re- quired, it may occasionally be made to serve the general pur- ‘poses of a transit instrument for observing the passages of the sun and stars across the meridian. It may likewise be made to serve as a theodolite for surveying land and taking horizontal angles—as a quadrant for taking angles of altitude —as a level—as an equal altitude instrument—an azimuth — instrument for ascertaining the sun’s distance from the north THE QUADRANT. 345 or south points of the horizon—and as an accurate universal sun dial, for finding the exact mean or true time on any occa- sion when the sun is visible.. The manner of applying it to these different purposes will be obvious to every one who is in the least acquainted with the nature and construction of this instrument. The price of a small equatorial instrument, such as that de- scribed p. 320, is about 16 guineas, exclusive of some of the eyepieces, which were afterward added for the purpose of making particular observations. Instruments of a larger size, and with more complicated machinery, sell from 50 to 100 guineas and upward. Messrs. W. and 8. Jones, Holborn, London, construct such instruments. ON THE QUADRANT. Every.circle being supposed to be divided into 360 equal parts or degrees, it is evident that 90 degrees, or. the fourth part of a circle, will be sufficient to measure all angles between the horizon of any place and the line perpendicular to it which goes up to the zenith. Thus, in fig. 87, the line C B repre- sents the plane of the horizon. A C BH, the quadrant; A C, the perpendicular to the horizon; and A, the zenith point. If the lines B C and C A represent a pair of compasses with the legs standing perpendicular to each other, and the curved Figure 87. C LL G E B lines A B, D E, and F G, the quarter of as many circles of different sizes, it is evident that although each of these differs from the others in size, yet that each contains the same por- tion of a circle, namely, a quadrant or fourth part; and thus & would be from the smallest to the largest quadrant that could be formed—they would all contain exactly 90 degrees each. By the application of this principle, the comparative measure of angles may be extended to an indefinite distance 346 TILE QUADRANT. By means of an instrument constructed in the form of a quad- rant of a circle, with its curved edge divided into 90 equal parts, the altitude of any object in the heavens can at any time be determined. There are various constructions of this instrument, some of them extremely simple, and others considerably complex and expensive, according to the degree of accuracy which the observations require. ‘The following is a description of the Pillar Quadrant, as it was made by Mr. Bird forthe Obser- vatory of Greenwich, and several Continental observatories. This instrument consists of a quadrant, E E H G L, (fig. 88,) mounted on a pillar, B, which is supported by a tripod, A A, resting on three foot-screws.. The quadrant, the pillar, and the horizontal circle all revolve round a vertical axis. A telescope, H, is placed on the horizontal radius, and is di- rected to a meridian mark previously made on some distant -object for placing the plane of the instrument in the meridian, and also for setting the zero, or beginning of the scale, truly horizontal. ‘This is sometimes done by a level instead of a Figure 88 THE QUADRANT. 347 telescope, and sometimes by a plumb-line, G, suspended from near the centre, and brought to bisect a fine dot made on the limb, where a microscope is placed to examine the bisection. The weight or plummet at the end of the plumb-line is sus- _ pended in the cistern of water 6, which keeps it from being agitated by the air. A similar dot is made for the upper end of the plumb-line upon a piece of brass, adjustable by a screw, d, in order that the line may be exactly at right angles to the telescope when it is placed at 0. The quadrant is screwed by the centre of its frame against a piece of brass, e, with three screws, and this piece is screwed to the top of the pillar B with other three screws. By means of the first three screws the plane of the quadrant can be placed exactly paral- lel to the vertical axis, and by the other screws the telescope H can be placed exactly perpendicular to it. ‘The nut of the delicate screw L is attached to the end of the telescope F by a universal joint. ‘The collar for the other end is jointed in the same manner toaclamp, which can be fastened to any part ofthe limb. A similar clamp-screw and slow motion is seen at n for the lower circle, which is intended to hold the circle fast and adjust its motion. The divisions of the lower, or horizontal circle, are read by verniers, or noniuses, fixed to the arms of the tripod at / and m; and in some cases, three are used to obtain greater accuracy. In using this quadrant, the axis of the telescope H is ad- justed to a horizontal line, and the plane of the quadrant to a vertical line, by the means already stated. The screw of the clamp L is then loosened and the telescope directed to the star or other object whose altitude is required. The clamp- screw being fixed, the observer looks through the telescope, and with the nut of the screw L he brings the telescope into a position where the star is bisected by the intersection of the wires in the field of the telescope. The divisions are then to be read off. upon the vernier, and the altitude of the star will be obtained. By means of the horizontal circle D, all angles in the plane of the horizon may be accurately mea- sured, such as the amplitudes and azimuths of the celestial bodies. ; Quadrants of amore simple construction than the above may be occasionally used, such as Gunter’s, Cole’s, Sutton’s, and others; but none of these is furnished with telescopes or tele- scopic sights, and therefore an altitude cannot be obtained by them with the same degree of accuracy as with that which has been now described. By means of the quadrant, not only the altitudes of the 848 THE ASTRONOMICAL CIRCLE. heavenly bodies may be determined, but also the distances of objects on the earth by observations made at two sta- tions; the altitude of fireballs and other meteors in the atmo- sphere; the height of a cloud, by observation on: its altitude and velocity; and numerous other problems, the solution of which depends upon angular measurements. A Mural Quadrant is the name given to this instrument when it is fixed upon a wall of stone, and in the plane of the meridian, such as the quadrant which was erected by Flamstead in the Observatory at Greenwich. Although the quadrant was formerly much used in astronomical observations, yet it may be proper to state that its use has now been almost completely superseded by the recent introduction of Astronomical Cir- cles, of which we shall now give the reader a very short de- scription, chiefly taken from Troughton’s account of the im- strument he constructed, as found in Sir D. Brewster’s Supplement to Ferguson’s Astronomy. THE ASTRONOMICAL CIRCLE. An astronomical circle is a complete circle substituted in place of the quadrant, and differs from it only in the superior accuracy with which it enables the astronomer to make his observations. The large vertical or declination circle C C (fig. 89) is composed of two complete circles, strengthened by an edge-bar on their inside, and firmly united at their extreme borders by a number of short braces or bars, which stand perpendicular between them, and which keep them at such a distance as to admit the achromatic telescope TT. This double circle is supported by 16 conical bars, firmly united, along with the telescope, to a horizontal axis. The exterior limb of each circle is divided into degrees and parts of a de- gree, and these divisions are divided into seconds by means of the micrometer microscopes m m, which read off the angle on opposite sides of each circle. The cross-wires in each microscope may be moved over the limb till they coincide with the nearest division of the limb, by means of the mi- crometer screws ¢ c,and the space moved through is ascer- tained by the, divisions on the graduated head above ¢, as- sisted by a scale within the microscope. ‘The microscopes are supported by two arms proceeding from a small circle concentric with the horizontal axis, and fixed to the vertical columns. ‘This circle is the centre upon which they can turn round nearly a quadrant for the purpose of employing a new portion of the divisions of the circle, when it is reckoned pru- dent to repeat any delicate observations upon any part of the THE ASTRONOMICAL CIRCLE. 349 Figure 89. limb. At h is represented a level for placing the axis in a true horizontal line, and at é is fixed another level parallel to the telescope for bringing the zero of the divisions to a hori- zontal position. The horizontal axis to which the vertical circle and the telescope are fixed is equal in length to the distance between the vertical pillars, and its pivots are sup- ported by semicircular bearings placed at the top of each pillar. ‘These two vertical pillars are firmly united at their bases to a crossbar, f. ‘To this crossbar is also fixed a verti- cal axis about three feet long, the lower end of which, termi- nating in an obtuse point, rests in a brass conical socket firmly fastened at the bottom of the hollow in the stone pedestal D, which receives the vertical axis. This socket supports the whole weight of the movable part of the instrument. The Vou. IX. 30 850 THE ASTRONOMICAL CIRCLE. upper part of the vertical axis is supported by two pieces of brass, one of which is seen at e, screwed to the ring ?, and containing a right angle, or Y.. At each side of the ring, opposite to the points of contact, is placed a tube containing a heliacal spring, which, by a constant pressure on the axis, keeps it against its bearings, and permits it to turn, in these four points of contact, with an easy and steady motion. The two bearings are fixed upon two*rings capable of a lateral ad- justment ; the lower one by the screw d to incline the axis to the east or west, while the screw b gives the upper one, 2, a motion in the plane of the meridian. By this means the axis — may be adjusted to a perpendicular position as exactly as by the usual method of the tripod with foot-screws. ‘These rings are attached to the centre-piece s, which is firmly connected — with the upper surface of the stone by six conical tubes A, A, A, &c., and brass standards at every angle of the pedestal. Below this frame lies the azimuth circle, E E, consisting of a circular limb, strengthened by ten hollow cones firmly united with the vertical axis, and consequently turning freely along with it. The azimuth circle, EE, is divided and read off in the same manner as the vertical circle. The arms of the microscopes, B B, project from the ring 7, and the micro- scopes themselves are adjustable by screws, to bring them to zero and to the diameter of the circle. ' ' hela is j ry} itd an ‘ eee. fie : ae, 4°) ‘5, OT Sais cman ral , ’, rk | aie ion f te , a ay ha’ oe ty? Dy phi! iL | ia -* 4 , . He as vo UNIVERSITY OF ILLINOIS-URB ANA 522 p55P1854 Practical astro c001 nomer comprising i\lustr 84070 vu