UNIVERSITY OF CALIFORNIA. 
 
 GIFT OF 
 
 Class 
 
3 to 
 
SPECTRUM ANALYSIS 
 
 EXPLAINED. 
 
 ITS USES TO SCIENCE ILLUSTRATED, SHOWING ITS APPLICATION 
 
 IN MICROSCOPICAL RESEARCH, AND TO DISCOVERIES OF THE 
 
 PHYSICAL CONDITION AND MOVEMENTS OF THE 
 
 HEAVENLY BODIES, AND INCLUDING AN 
 
 EXPLANATION OF THE RECEIVED 
 
 THEORY OF 
 
 SOUND, HEAT, LIGHT, AND COLOR. 
 
 COMPILED BY THE EDITOR OF 
 
 " HALF-HOUR RECREATIONS IN POPULAR SCIENCE," 
 
 FROM THE WORKS AND OB ERVATIONS OF 
 
 PROFS. SCHELLEN, ROSCOE, HUGGINS, LOCKYER, YOUNG, 
 AND OTHERS. 
 
 h 5 is 
 
 FULLY 
 
 BOSTON : 
 
 ESTES AND LAURIAT, 
 143 WASHINGTON STREET. 
 
 1872. 
 
Entered, according to Act of Congress, in the year 1872, 
 
 BY DANA ESTES, 
 In the Office of the Librarian of Congress, at Washington. 
 
 Stereotyped at the Boston Stereotype Foundry, 
 19 Spring Lane. 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 55 
 
 4. Spectrum Analysis Explained, and its llses 
 to Science Illustrated* 
 
 INTRODUCTORY. 
 
 THIS work is founded upon a series of lectures deliv- 
 ered by the author during the winter of 1869, before 
 the " Society for Scientific Lectures," in Cologne. Its 
 object is, on the one hand, to give a clear and familiar 
 representation of the nature and phenomena of spectrum 
 analysis, enabling an educated person not previously familiar 
 with physical science to become acquainted with the newest 
 and most brilliant discovery of this century ; and, on the 
 other hand, to show the important position which spectrum 
 analysis has acquired in the pursuit of physics, chemistry, 
 technology, physiology, and astronomy, as well as its adapta- 
 bility to almost every kind of scientific investigation. 
 
 The general reader will be introduced by this book into a 
 new realm of science, the dominion of which has extended 
 in a few years over all terrestrial substances, and even be- 
 yond them to the most distant parts of the universe. He 
 will learn to decipher the new language of light, which by 
 unequivocal signs yields him information not only concerning 
 the nature of terrestrial substances, but also of the physical 
 constitution of the heavenly bodies. 
 
 To facilitate the due appreciation of the results which 
 have been obtained by the application of spectrum analysis 
 to the heavenly bodies, the author has given with each class 
 of objects a summary of the information hitherto furnished 
 by the telescope, and has sought to give a glance in passing 
 at the progressive development and partial transformation 
 of the heavenly bodies. 
 
 The author acknowledges with grateful thanks the val- 
 uable assistance rendered him by various scientific men who 
 
 I 
 
 237471 
 
56 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 have kindly communicated to him the results of their labors, 
 among whom he would especially mention Messrs. Huggins, 
 Secchi, Lockyer, Zollner, Janssen, Morton, and Young. 
 
 ON THE ARTIFICIAL SOURCES OF HIGH DEGREES OF HEAT 
 AND LIGHT. 
 
 The total eclipse of the sun in India of the i8th of Au- 
 gust, 1868, was an event which, it will be remembered, 
 excited extreme interest in the scientific world, and led to a 
 large expenditure of money and labor, in order that a new 
 method of investigation spectrum analysis might be ap- 
 plied to those mysterious phenomena invariably present at a 
 total solar eclipse, the nature and character of which the 
 unassisted powers of the telescope had proved themselves 
 inadequate to reveal. The brilliant results obtained at this 
 eclipse were fully confirmed by the more recent observations 
 made in North America during the total eclipse of the 7th 
 of August, 1869, and the records of those eclipses laid be- 
 fore the various scientific societies clearly assert the triumph 
 of spectrum analysis. On this account, the new method 
 of investigation has excited great interest in all cultivated 
 circles, and therefore a familiar and comprehensive exposi- 
 tion of the details of spectrum analysis, in which is shown 
 the great value of this method of research in every depart- 
 ment of physical science, seems not uncalled for. 
 
 By spectrum is not understood in physics a spectre or 
 ghostly apparition, as the verbal interpretation of the word 
 might well lead one to suppose, but that beautiful image, 
 brilliant with all the colors of the rainbow, which is obtained 
 when the light of the sun, or any other brilliant object, is 
 allowed to pass through a triangular piece of glass a 
 prism. 
 
 The unassisted eye can perceive no difference in the light 
 from the heavenly bodies and that from various artificial 
 sources, beyond a variation in color and brilliancy ; but it is 
 
 2 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 57 
 
 quite otherwise when the light is viewed through a prism. 
 There are then formed very beautiful colored images or 
 spectra, the constitution and appearance of which depend 
 upon the nature of the substance emitting the light. The 
 different appearances presented by these colored images are 
 so entirely characteristic, that to every substance, when 
 luminous in a gaseous form, there corresponds a peculiar 
 spectrum, which belongs only to that particular substance. 
 
 It follows, therefore, that when the spectra of different 
 substances have been determined once for all, by previous 
 researches, and have been recorded in maps or impressed 
 upon the memory, it is easy in any future investigation to 
 recognize at once, from the form of the spectrum which a 
 body of unknown constitution presents, the individual sub- 
 stances of which it is composed. 
 
 This statement presents in general terms the nature of 
 spectrum analysis. It analyzes bodies into their constit- 
 uent parts, not as the chemist, with alembics and retorts, 
 with re-agents and precipitates, but by means of the spectra 
 which these substances give when in a state of intense 
 luminosity. 
 
 Spectrum analysis in no way supplants the methods of 
 chemical analysis hitherto in use ; for its function is neither 
 to decompose nor to combine bodies, but rather to reconnoitre 
 an unknown territory, and to stand sentinel, and signalize to 
 the physicist, the chemist, and the astronomer, the presence 
 of any substance brought beneath its scrutiny. 
 
 With what acuteness, with what delicacy does spectrum 
 analysis accomplish this task ! When the balance, the mi- 
 croscope, and every other means of research at the command 
 of the physicist and the chemist utterly fail, one look in the 
 spectroscope is sufficient, in most cases, to reveal the presence 
 of a substance. If a pound of common salt be divided into 
 500,000 equal parts, the weight of one of these portions is 
 called a milligramme. The chemist is able, by the use of 
 the most delicate scales and the application of special skill, 
 
 3 
 
58 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 to determine the weight of such a particle ; but in doing so, 
 he comes close upon the limits of his power of detecting by 
 chemical means the presence of sodium, the chief element 
 in common salt. But if that small milligramme be subdi- 
 vided into three million parts, we arrive at so minute a par- 
 ticle that all power of discerning it fails, and yet even this 
 excessively small quantity is sufficient to be recognized with 
 certainty in a spectroscope. We have but to strike together 
 the pages of an old dusty book in order to perceive immedi- 
 ately in a spectroscope placed at some distance, the flash of 
 a line of yellow light which we shall presently learn is an 
 unfailing sign of the presence of sodium. 
 
 It was to be expected that so sensitive a means of investi- 
 gation, from which no known substance can escape, would 
 very soon lead to the tracking out and discovery of new ele- 
 ments which, till then, had remained unknown, either be- 
 cause they are scattered very sparingly in nature, or stand 
 out with so little that is characteristic, from some other sub- 
 stances, that the imperfect chemical methods hitherto in use 
 have not been able to distinguish them. 
 
 This expectation was brilliantly realized even by the first 
 steps taken in this direction. The two Heidelberg pro- 
 fessors, Bunsen and Kirchhoff, to whom we are indebted for 
 the discovery of spectrum analysis and its application to 
 practical science, very soon discovered with their new instru- 
 ment, two new metals, Caesium and Rubidium, to which 
 two others, Thallium and Indium, have been since added. 
 
 But all the brilliant and astounding results which spectrum 
 analysis has furnished in the provinces of physics and chem- 
 istry have been far surpassed by its performances in that of 
 astronomy. Newton's law of gravitation has given us the 
 means of calculating the courses of the heavenly bodies, of 
 projecting the orbits of the earth, the planets and comets, 
 and of predicting their relative positions in these orbits, 
 together with the accompanying phenomena of the ebb and 
 flow of the tides, and the eclipses and occultations of the 
 
 4 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 59 
 
 heavenly bodies. But this same gravitation chains man to 
 the earth, and forbids him to leave it. It is therefore only 
 on the wings of light, that news reaches him of the existence 
 of those numberless worlds by which he is surrounded. 
 The light alone, which proceeds from these stars, is the 
 winged messenger which can bring him information of their 
 being and nature ; spectrum analysis has made this light 
 into a ladder on which the human mind can rise billions and 
 billions of miles, far into immeasurable space, in order to 
 investigate the chemical constitution of the stars, and study 
 their physical conditions. 
 
 Until within a few years, the telescope was the only 
 means by which these investigations could be carried on, 
 and the intelligence derived from this source concerning the 
 stars and nebulas was very scant, being confined to but par- 
 tial information of their outward form, size, and color. 
 
 Since the year 1859, spectrum analysis has entered the 
 service of astronomy, and its performances for the short 
 space of eleven years are, in the most widely-differing ways, 
 perfectly astounding. 
 
 It is possible by means of a prism to decompose into its 
 component parts the light of the sun, the planets, the fixed 
 stars, comets and nebulas, and thus obtain their spectra iri 
 the same way as that of earthly luminous substances. By a 
 careful comparison of the spectra of the stars with the well- 
 known spectra of terrestrial substances, it can be determined, 
 from their complete agreement or disagreement, with a cer- 
 tainty almost amounting to mathematical precision, whether 
 these substances do or do not exist in those remote heavenly 
 bodies. 
 
 The foregoing statements present, in general terms, the 
 essence and scope of spectrum analysis. Its starting-point 
 is the spectrum of each individual substance, and in order 
 to obtain this it is requisite that the substance should not 
 only be luminous, but should emit a sufficient quantity of 
 light. Dark bodies are not available for spectrum analysis ; 
 
 5 
 
60 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 if they are to be submitted to its scrutiny, they must first be 
 brought into a state of vivid luminosity. 
 
 That the nature of the analysis of light may be more 
 easily understood, we will first proceed to explain 
 
 THE SOLAR SPECTRUM. 
 
 If a ray of sunshine be allowed to pass through a small, 
 round hole in the window-shutter of a darkened room, as is 
 shown in Fig. I., there will appear a round, white spot of 
 light exactly in the direction of the ray, upon a screen placed 
 opposite the opening, as will be seen indicated by the dotted 
 lines in the figure. A very different appearance will be pre- 
 sented if the ray of light be made to fall upon a prism. 
 The ray is at once deflected from its straight course upwards, 
 that is to say, towards the base of the prism, and away from 
 the sharp edge of the refracting surfaces, which, as repre- 
 sented in the drawing, are turned downwards : on its emer- 
 gence from the prism it no longer remains one single ray, as 
 it entered the window-shutter, but is separated into very 
 many single-colored rays, which, as they continue to diverge, 
 form upon the screen an elongated band of brilliant colors, 
 instead of the former round, white image of the sun. In 
 this brilliant band the individual colors blend gradually one 
 into the other, beginning at thai end lying nearest the direc- 
 tion of the incident ray (the lowest end in the figure), with 
 the least refrangible color, a dark and very beautiful red ; 
 this passes imperceptibly into orange, and orange again into 
 bright yellow ; a pure green succeeds, which is shaded off 
 into a brilliant blue, and this gives place to a rich, deep 
 indigo ; a delicate purple leads finally to a soft violet, by 
 which the range of the visible rays is terminated. C^A faint 
 picture of this magnificent solar image is given in No. i of 
 the Frontispiece ; this is called the spectrum?) In the above- 
 mentioned colors of the solar spectrum, the eye discerns 
 numberless gradations, which pass imperceptibly from one 
 
 6 
 
SCHELLEN.J SPECTRUM ANALYSIS EXPLAINED. 61 
 
 to another ; and since language does not suffice to give 
 separate names to each of these, we must content ourselves 
 with designating only the seven principal groups, which are 
 known as the colors of the spectrum. 
 
 This experiment furnishes conclusive evidence that white 
 
 FIG. I. 
 
 EXHIBITION OF THE SOLAR SPECTRUM. 
 
 light is not simple and indivisible, but composed of innu- 
 merable colored rays, each of which possesses its own pe- 
 culiar degree of refrangibility, and therefore, on refraction, 
 pursues a separate path. The prism analyzes white light ; 
 the result is the separation of all the colored rays of which 
 
 7 
 
62 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 it is composed, and the consequent formation of the colored 
 image called the spectruyi., 
 
 The decomposition of sunliglrTby refraction is shown in 
 various phenomena known to the ancients as well as our- 
 selves, though they were not able, as we are, to trace them 
 back to their true cause. The rainbow, with its pure but 
 delicate colors, the sparkle of the cut jewel in its brilliant 
 flashes, the play of color emitted by cut glass, and the pris- 
 matic facets of crystal lustres as the sun shines upon them, 
 the glow of the clouds and high mountain peaks in the 
 various colored light of the rising and setting sun, all 
 these effects are occasioned by the decomposition of white 
 light by its refraction on passing through glass in a prismatic 
 form, through drops of liquid, or through vapor. 
 
 The colors of the solar spectrum possess a purity and 
 brilliancy to be met with nowhere else ; they are all per- 
 fectly indivisible, and cannot be further decomposed, as may 
 be easily proved on attempting to analyze a colored ray by 
 means of a second prism. If a small, round hole be made 
 in the screen in any portion of the image-of the spectrum, 
 the extreme red, for instance (Fig. I.), a red ray passes 
 through it, and appears upon the opposite wall as a round 
 spot of red light, precisely in the same direction as the red 
 rays left the prism on the other side of the screen. If a 
 second prism be interposed in the path of the ray that has 
 passed through the screen, the ray will suffer a second re- 
 fraction, arid the image be thrown upon another place 
 (higher up in the figure) on the wall ; this new image, how- 
 ever, is simply red, like the incident ray, and by a careful 
 adjustment of the prism shows no elongation, but appeals 
 perfectly round. 
 
 This decomposition of sunlight, or white light, as it is 
 usually termed, is called dispersion, and is caused by refrac- 
 tion, by which is meant the deviation of a ray of light from 
 a straight line when it strikes any transparent substance (as, 
 for instance, glass or water) obliquely. Different colors are 
 
 8 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 63 
 
 said to have different degrees of refrangibility ; and by this 
 we mean, that different colored rays of light, being passed 
 through a triangular piece of glass, or prism, are turned 
 more or less from the direction in which they entered it. 
 Thus the colors of the rainbow have each a different degree 
 of refrangibility, from red, which has the least, to violet, 
 which has the greatest ; and when white light, which is 
 composed of all these colors combined, is analyzed, or sep- 
 arated, these colors are refracted, or turned at different 
 angles, so as to separate, though each is beautifully blended 
 with the one next it. The same is true of light from any 
 source. It is only necessary that any substance should be 
 made sufficiently luminous, to become thoroughly sifted, or 
 analyzed, by means of the spectroscope. 
 
 We will now proceed to explain how all substances, such 
 as gases, metals, earths, etc., may be made sufficiently 
 luminous to be made available for spectrum analyses. It is 
 necessary to have sufficient heat to volatilize or turn into 
 vapor any substance to be analyzed. For this purpose 
 many ingenious contrivances have been invented, one of 
 which was invented by Bunsen, one of the discoverers of 
 spectrum analysis. This is called the Bunsen burner, and 
 consists of a gas burner, arranged with a chamber beneath 
 it, where atmospheric air is mixed with the gas before pass- 
 ing up the tube to feed the burner. The flame from this 
 burner is non-luminous, but its heat is intense, and may be 
 made much greater if the atmospheric air, instead of being 
 left to mix itself with the gas, be forced in by means of a 
 powerful blowpipe. 
 
 In the Bunsen burner, the combustion of coal gas ensues 
 slowly and incompletely ; slowly, because the hydrogen in 
 combination with carbon is supplied only in small quantities ; 
 incompletely, because the gases are not mixed in due pro- 
 portions, and the nitrogen of the air presents a hinderance. 
 If, on the contrary, pure hydrogen gas be previously mixed 
 with as much pure oxygen as will insure its complete com- 
 
 9 
 
64 RECREATIONS IN POPULAR SCIENCE. [SCHELLBK. 
 
 bustion (two volumes of hydrogen with one of oxygen), 
 oxyhydrogen gas is obtained, which, when ignited, explodes 
 with a fearful noise, and occasions sometimes the destruction 
 of the strongest vessels. The heat evolved by this combus- 
 tion is the greatest which can at present be produced by 
 chemical means, and it is sufficient to accomplish the fusion 
 of substances which have borne unchanged the action of the 
 hottest furnaces. 
 
 In order to make the oxyhydrogen flame a source of in- 
 tense light, a cylinder of well-burnt lime is placed upon the 
 socket of the lamp, and the flame directed against the upper 
 part. It begins at once to glow, and soon throws out a 
 dazzling, incandescent, or white-heat light. This is called 
 the oxyhydrogen or Drummond's lime-light. 
 
 THE ELECTRIC SPARK. 
 
 To attain, however, the greatest amount of heat and light 
 which can at present be produced, we must leave the prov- 
 ince of chemistry, with its processes of combustion, and 
 turn to that of electricity, where we are encountered by a 
 host of phenomena, accompanied by an intense degree of 
 light and heat. 
 
 Besides the well-known machines which excite electricity 
 through the friction of a glass disk, there has been added of 
 late a contrivance called an induction machine, which yields 
 a rich supply of electric force, and gives a spark of intense 
 brilliancy. In all electrical motors arranged for exhibiting 
 light, sparks are formed between two metallic poles or pieces 
 of wire, which are placed in contact with those parts of the 
 machine which collect the positive and negative electricity. 
 By the mutual attraction of the two electricities, and the 
 struggle for union, there ensues a tension of electricity at the 
 end of the metal poles when they are separated from each 
 other ; if this be so strong that the obstacle presented by the 
 stratum of air between the metallic conductors is overcome 
 
 10 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 65 
 
 by it, then the electricities are instantly united, and the 
 union takes place in that form of light and heat which is 
 called the electric spark. 
 
 The amount of heat thus generated depends upon the 
 degree of tension and the quantities of electricity by the 
 union of which it is produced ; but in most cases it is so 
 great that small particles of the metal poles are volatilized, 
 and become luminous. The glowing metallic vapor affects 
 the color of the spark, which therefore appears with various 
 kinds of light, according to the nature of the conductors. 
 These phenomena afford us, in aid of our researches with 
 spectrum analysis, a very simple method of volatilizing and 
 raising to a high degree of luminosity most of the metals, 
 and other substances which are conductors of electricity. To 
 obtain the same result with liquids, it is only necessary to 
 place one of the metal poles in the liquid to be examined, 
 and to bring the other sufficiently near the surface for the 
 spark to pass from it to the liquid. By the heat of the spark 
 a small portion of the liquid is volatilized and made luminous. 
 
 THE VOLTAIC ARC. 
 
 It will be well now to turn our attention for a short time to 
 that source of electricity which is able to evolve the highest 
 degree of heat with the most intense light namely, the vol- 
 taic arc, or the electric light. When the poles of a power- 
 ful voltaic battery of fifty or sixty elements are connected, by 
 means of two metal wires, with two pieces of carbon, , b 
 (Fig. II.), and these brought into contact, the electricity gen- 
 erated by the battery is discharged between them through 
 the carbon, which is nearly as good a conductor as the 
 metal. If these pieces of carbon be pointed at the ends, an 
 extraordinarily intense light is emitted on the passage of the 
 current at the points of contact, and they may be separated 
 one or two tenths of an inch without interrupting the 
 discharge. The apparatus is then placed in the lantern, 
 
 ii 
 
66 
 
 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN 
 
 FIG. II. 
 
 THE ELECTRIC LIGHT. 
 
 which is provided with requisite arrangements to keep the 
 carbon points at a proper distance from each other, and also 
 with a small mirror and lens to project the light through a 
 slit in the lantern. The complete arrangement is called 
 
 THE ELECTRIC LAMP. 
 
 To exhibit the spectrum, the room should be darkened, 
 and the electric lamp placed on a table, with the wires con- 
 nected with a battery. (See Fig. III.) The light from the 
 
 12 
 
SCHELLEN.J SPECTRUM ANALYSIS EXPLAINED. 67 
 
 FIG. III. 
 
 THE ELECTRIC LAMP. 
 
 carbon points passes through the narrow, vertical slit in the 
 lamp, and by means of the movable lens, D, a distinct image 
 of the slit is thrown through the prisms E E upon a screen 
 at a distance of about twelve feet, and we behold a magnifi- 
 cent spectrum, about three feet long and sixteen inches wide, 
 exhibiting the whole range of colors, as shown in No. i of 
 
 13 
 
68 RECREATIONS IN POPULAR SCIENCE. [SCHBLLEN. 
 
 the Frontispiece, with this difference. In No. i, Frontis- 
 piece, the colors are crossed with dark lines, this being a 
 feature of the Solar Spectrum which will be explained far- 
 ther on. The colors from the incandescent or white-hot 
 carbon points succeed each other without the slightest inter- 
 ruption. Their limits are not sharply defined ; they rather 
 blend gradually one into the other, and thus form an un- 
 broken or continuous spectrum. 
 
 RECOMBINATION OF THE COLORS OF THE SPECTRUM. 
 
 If white light be actually composed of the colors con- 
 tained in the spectrum, then the recombination of the same 
 colors must reproduce white light. The simplest method of 
 collecting several rays of light into one point is by a convex 
 lens or a burning-glass. If the sun's rays fall perpendic- 
 ularly on such a glass, the refraction they suffer in their 
 passage through it causes them to converge to one point 
 the focus. To accomplish by this means the recombination 
 of the colored rays of the spectrum of the electric light, a 
 cylindrical lens must be interposed between the prism and 
 the screen on which the spectrum of the small line of light 
 issuing from the slit is extended to a length of some six feet : 
 this lens is a convex lens of peculiar form, which possesses 
 the property of recombining in a point all the rays issuing 
 from each point of the line of light passing through the slit 
 after dispersion by the prism, and therefore of representing 
 the whole of the rays of that short line of light again as a 
 small line. When, therefore, this lens is placed at a proper 
 distance behind the prism, the colors of the spectrum disap- 
 pear from the screen, and are replaced by a short line of 
 light, in which all the colored rays issuing from the prism 
 have been recombined, and the white light reproduced out 
 of which they originated. 
 
 "4 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 69 
 
 THE CONTINUOUS SPECTRA OF SOLID AND LIQUID BODIES. 
 
 When the carbon points used for the production of the 
 electric light are carefully prepared, and completely free 
 from all extraneous substances, the light is purely white, 
 being emitted exclusively by solid particles of carbon in a 
 state of incandescence. The spectrum of this light is, there- 
 fore, continuous, like that of incandescent lime; it is un- 
 broken by gaps in the colors, or by sudden transitions from 
 one color to another, and is uninterrupted by either dark 
 or bright bands. 
 
 All other incandescent bodies, whether solid or liquid, 
 give a similar spectrum, the colors being distributed in the 
 order represented in the Frontispiece, No. I. If, instead of 
 the lime-light, the magnesium light, the light of an incandes- 
 cent platinum wire, or the flame of coal gas in which light 
 is produced by incandescent particles of carbon, be analyzed 
 by the prism, continuous spectra are always obtained, but 
 with this difference, that the various groups of color are not 
 always distributed in exactly the same proportion in each 
 individual spectrum ; and therefore, according to the kind 
 of light employed, sometimes red, sometimes yellow, and 
 sometimes violet predominates. Therefore, where there is 
 a continuous spectrum without gaps, and containing every 
 shade of color, the light is derived from an incandescent 
 solid or liquid body. 
 
 THE SPECTRA OF VAPORS AND GASES. 
 
 Very different spectra are obtained when the source of 
 light is not an incandescent solid or liquid body, but a vapor 
 or a gas in a glowing state. Instead of a continuous succes- 
 sion of colors, the spectrum then exhibits a series of distinct 
 bright-colored bands, separated one from another by dark 
 spaces. 
 
 15 
 
70 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 The characteristic feature of spectra obtained from lumi- 
 nous vapors or gases is the want of continuity in the succes- 
 sion of the colors. Such a spectrum is composed of distinct 
 colored bands, irregularly arranged, with dark spaces be- 
 tween them, and is therefore called a discontinuous spec- 
 trum, a spectrum of bright lines, or a gas spectrum. 
 
 The spectra of the vapors of potassium, sodium, caesium, 
 and rubidium are represented in Nos. 2, 3, 4, and 5 of the 
 Frontispiece. 
 
 The colored plate at the beginning shows certain spectra, 
 observed by means of the spectroscope. It contains five 
 specimens of discontinuous or gas spectrums. Fig. I. rep- 
 resents the solar spectrum, which is, as we showed before, 
 simple white light, or sunlight analyzed into its constituent 
 parts. Fig. II. shows the spectrum of potassium. It is con- 
 tinuous, with the exception of the line in the extreme red, 
 and another in the extreme violet, and without these two 
 lines, would show the spectrum of an incandescent solid or 
 liquid body, or, in other words, a body raised to the point of 
 white heat. Fig. III. shows the spectrum of sodium or salt. 
 This spectrum contains neither red, orange, green, blue, nor 
 violet. It-is marked by a very brilliant yellow ray. Of all 
 metals, sodium is that which possesses the greatest spectral 
 sensibility. In fact, it has been ascertained that one two- 
 hundred millionth of .a grain of salt is enough to cause the 
 appearance. of the yellow line of sodium. A very little dust 
 scattered in the room is enough to produce it : a circumstance 
 which shows how abundantly sodium is scattered throughout 
 nature. Kirchhoff has also ascertained that it exists in the 
 sun and the fixed stars. 
 
 Figures IV. and V. show the spectra of ccesium and rubid- 
 ium, metals discovered by Bunsen and Kirchhoff, by means 
 of Spectrum Analysis. The former is distinguished by two 
 blue lines, .the latter by two brilliant red lines, and two less 
 intense violet lines. 
 
 16 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 71 
 
 Two other metals, thallium and indium, have lately been 
 discovered by other scientists by the same means. 
 
 Terrestrial substances must be volatilized, or made into 
 vapor, to be examined. The spectra of incandescent solid 
 and liquid bodies are continuous, and resemble each other 
 so closely, that only in a very few instances can they be dis- 
 tinguished ; spectra of this kind are, therefore, not suitable 
 for the recognition of a substance, though they authorize the 
 conclusion, as a rule, that the substance is either in a solid 
 or liquid state. Only the discontinuous spectra, consisting 
 of colored lines which are obtained from a gas or vapor, are 
 sufficiently characteristic to enable the observer to pronounce 
 with certainty, by the number, position, and relative bright- 
 ness of these lines, the chemical constitution of the vapors 
 by which the light has been emitted. It follows from this 
 circumstance that spectrum analysis deals pre-eminently 
 with the investigation of gas spectra, and that for the exam- 
 ination of a substance which does not exist in nature in the 
 form of gas or vapor, the first step must be to place it in 
 this condition. We explained how this is done by the use 
 of the Electric Lamp and the Drummond Lime-light. 
 
 LIGHT. 
 
 Although the theory of light is now so completely under- 
 stood that we are able to explain the most complicated opti- 
 cal phenomena, yet an elementary reply to the question, 
 What is the nature of light? still presents some difficulty. 
 We perceive the operation of this power of nature in all 
 directions, and in the most manifold ways; the sun, as it 
 stands in full splendor in the heavens, pours forth but a 
 single tone of color over the earth, and yet the individual 
 objects in the landscape appear in the most varied and glo- 
 rious tints. What then are these colors? How are they 
 developed out of the white light which the sun and other 
 luminous bodies emit? 
 
 17 
 
72 RECREATIONS IN POPULAR SCIENCE. [SCHELLRK 
 
 We need not seek to avoid answering this question, if we 
 can succeed in giving a clear insight into the phenomena of 
 spectrum analysis ; for we have already intimated that the 
 world of color is the peculiar province of this new method 
 of investigation. 
 
 The approaches to science are frequently obstructed by 
 strange propositions^ discouraging and apparently contradic- 
 tory, which seem to the uninitiated like those ghosts that 
 haunted the way by which Dante and his heavenly guide 
 descended to the realms of the departed ; with a little cour- 
 age, however, we may easily traverse this dreaded path, 
 seize hold of the harmless apparitions, and make friends 
 first with one and then with another as we approach them. 
 
 We will therefore boldly grasp the proposed inquiry ; if 
 the answer to it cannot be exhaustive, it will at least contain 
 material enough to incite to further reflection, and perhaps 
 also afford the necessary basis for a more easy comprehen- 
 sion of the elaborate theories which are enunciated in phys- 
 ical treatises. 
 
 According to the theory generally received at present, the 
 whole universe is an immeasurable sea of highly attenuated 
 matter, imperceptible to the senses, in which the heavenly 
 bodies move with scarcely any impediment. This fluid, 
 which is called ether, fills the whole of space fills the 
 intervals between the heavenly bodies, as well as the pores 
 or interstices between the atoms of a substance. The small- 
 est particles of this subtle matter are in constant vibratory 
 motion ; when this motion is communicated to the retina of 
 the eye, it produces, if the impression upon the nerves be 
 sufficiently strong, a sensation which we call light. 
 
 Every substance, therefore, which sets the ether in power- 
 ful vibration, is luminous ; strong vibrations are perceived 
 as intense light, and weak vibrations as faint light, but both 
 of them proceed from the luminous object at the extraordi- 
 nary speed of 186,000 miles in a second, and they necessa- 
 
 iS 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 73 
 
 rily diminish in strength in proportion as they spread them- 
 selves over a greater space. 
 
 Light is not therefore a separate substance, but only the 
 vibration of a substance, which, according to its various 
 forms of motion, generates light, heat, or electricity. 
 
 ANALOGY BETWEEN LIGHT AND SOUND. 
 
 This representation of the nature of light ceases to be sur- 
 prising when we come to compare the vibrations of ether 
 with those of atmospheric air, and draw a parallel between 
 light and sound between the eye and the ear. 
 
 A string set in vibration causes a compression and rarefac- 
 tion of the surrounding air ; in front of it the air is pushed 
 together and condensed ; behind it the vacuum it creates is 
 filled up by the surrounding air, which thus becomes rarefied 
 for the moment. This periodic movement of the air is 
 transmitted to our ears at the rate of about 1,100 feet in a 
 second ; it strikes against the tympanum, and occasions, by* 
 its further impulse on the auditory nerves and brain, the sen- 
 sation we call sound. Air in motion, by its influence on the 
 organs of hearing, is the cause of sound ; ether in motion, 
 by its influence on the organs of sight, is the cause of light. 
 Without air, or some other medium whereby the vibration 
 of bodies can be propagated to our ears, no sound is pos- 
 sible. As a sonorous body throws oft' no actual substance 
 of sound, but only occasions a vibration of the air, so lu- 
 minous body sends out no substance of light, but only gives 
 an impulse to the ether, and sets it in vibration. 
 
 A musical sound, in contradistinction to mere noise, is 
 produced v only when the impulses of the air reach the ear at 
 regular intervals ; if the intervals between the impulses are 
 not sufficiently regular, the ear is only conscious of a hissing, 
 a rushing, or a humming noise ; a musical sound requires 
 perfect regularity in the succession of impulses. 
 
 The pitch of a musical note depends on the number of 
 
 '9 
 
/4 RECREATIONS IN POPULAR SCIENCE. [SCHELLEX. 
 
 impulses in a given time as, for instance, in a second ; the 
 greater the number of vibrations in a second, the higher will 
 be the note produced. When the single impulses are fewer 
 than sixteen or more than forty thousand in a second, the 
 ear is no longer sensible of a musical sound : in the first 
 case, it either perceives only an undefined, deep hum, or 
 else it distinguishes the individual strokes upon the tym- 
 panum, and becomes sensible of them as distinct blows ; in 
 the latter case, there is an impression of a sharp, but equally 
 indefinite shrill or hissing noise. The limits of susceptibility 
 of the ear for musical sounds lie between sixteen and forty 
 thousand impulses per second. The number of vibrations 
 in a second given by a normal tuning-fork was determined 
 in the year 1859 * ke 435 * n a temperature of 15 C. 
 
 (59 F.) 
 
 The truth of the foregoing statements may be easily proved 
 in the following manner. A disk of zinc is fastened to an 
 axis which can be set in rapid rotation by means of a cord 
 .working over a large wheel. The disk is perforated with 
 eight series of holes, placed along eight concentric circles ; 
 the holes are of the same size in each circle, and at equal 
 distances from each other, so that their number increases in 
 each ring from the centre to the edge. 
 
 When the disk, by means of the large wheel, is set in uni- 
 form motion at the rate of one revolution in a second, and 
 one circle of the holes is blown upon with considerable force 
 through a glass or metal tube, a note is heard : by blowing 
 upon the next series higher, the note is of a higher pitch ; a 
 lower set of holes gives, on the contrary, a deeper note ; so 
 that if all the rings were blown upon in succession, from the 
 
 * [The number of vibrations of a C tuning-fork is 512. The 
 deepest tone of orchestral instruments is the E of the double bass, 
 with 41^ vibrations. Some organs go as low as C'with 33 vibra- 
 tions, and some pianos may reach A with 27^ vibrations. In height 
 the piano-forte reaches to a iv with 3-520. The highest note of or- 
 chestra is probably d v of the piccolo flute wrth 4752 vibrations.] 
 
 20 
 
SCHKLLEN.] SPECTRUM ANALYSIS EXPLAINED. 75 
 
 lowest upwards, the distinct notes of the complete octave 
 would be heard. 
 
 What is it that here produces the sound? The mere rev- 
 olution of the disk makes no noise ; the motion of the air 
 by the blowing through the tube first elicits the notes. 
 When, by the rotation of the disk, the current of air strikes 
 against an opening, it presses through it, pushing the air 
 before it and condensing it ; this impulse reaches the ear at 
 once, and strikes upon the tympanum : the current of air 
 immediately afterwards comes against the solid part between 
 the holes, by which it is interrupted. If the circle blown 
 upon contain twenty-four openings, the ear would receive 
 twenty-four impulses at every revolution of the disk ; and 
 if the disk made twenty revolutions in a second, the ear 
 would receive 20 X 24 =480 impulses in the same interval. 
 The outside circle has twice as many openings as the inner- 
 most one ; it therefore furnishes with the same speed of rota- 
 tion 20 X 48 = 960 impulses in a second. 
 
 The ear cannot distinguish individual impulses when they 
 exceed sixteen in a second ; the impressions they then pro- 
 duce become blended together, the one following the other 
 so instantly that the sensation in the ear is that of one con- 
 tinuous impulse or sound. 
 
 The pitch of a note is thus seen to depend entirely upon 
 the number of successive impulses following each other at 
 the same uniform rate, its strength upon the force of the 
 impulse. With a stronger blast, the pitch of the note re- 
 mains unchanged, but the 'tone becomes more piercing, 
 while if a ring containing a greater number of holes be 
 blown upon, the pitch rises till in the last circle, with double 
 the number of openings, the octave of the same note is 
 heard that was given by the innermost circle. 
 
 It is true that the cause of sound is not the same in all 
 musical instruments ; sometimes it is the vibration of strings, 
 or elastic prongs, sometimes stretched membranes, or, again, 
 columns of air confined in tubes which create at regular 
 
 21 
 
76 RECREATIONS IN POPULAR SCIENCE. [SCHELLBK. 
 
 periods a condensation and rarefaction of the air ; but in 
 every case a note can only be produced by similar impulses 
 recurring at regular intervals, conveyed by the air to the 
 organs of hearing. 
 
 Savart exhibited the cause of sound in another way which 
 is not less instructive than the one just described. Instead 
 of the perforated disk, he made use of a wheel provided 
 with six hundred teeth, which could be set in very rapid 
 rotation in the same manner as the disk, and as the wheel 
 revolved, the teeth were allowed to press against the edge 
 of a card. . To make this experiment, it is only necessary 
 to substitute a toothed or cog wheel for the perforated disk, 
 and while the wheel is in rapid revolution, to hold a thin 
 card, or a piece of pasteboard against its toothed edge. The 
 card is bent a little by each tooth as it goes by, and springs 
 back to its first position as soon as it is released by the pass- 
 ing of the tooth : the motion of the card is communicated 
 to the surrounding air, and reaches the ear in consequence 
 of the regular revolution of the wheel, in the form of waves 
 of air, or of condensations and rarefactions of the air fol- 
 lowing each other at regular intervals. 
 
 When the wheel is turned slowly, there is heard only a 
 succession of taps, or isolated impulses of the card, distinctly 
 separable one from another, which do not as yet unite to 
 form a musical sound. In proportion, however, as the ra- 
 pidity of the rotation is increased, the number of impulses 
 increases also, and they unite in the ear to produce musical 
 notes rising continually in pitch. A small recording appa- 
 ratus, fixed to the axle of the toothed wheel, gives the num- 
 ber of revolutions in a second ; if this number be multiplied 
 by six hundred, the number of teeth on the wheel, the result 
 gives the number of condensations of air striking the ear in 
 a second. It is easy by this means to determine the number 
 of vibrations the ear receives in a second from a note of any 
 given pitch, and thus to verify the results obtained by the 
 perforated disk. 
 
 22 
 
SCHELLEN.J SPECTRUM ANALYSIS EXPLAINED. 77 
 
 It will now be easier to understand the motion of ether, 
 and its mode of operation on the organs of sight. Ether, as 
 well as air, can be set in regular vibrations, and even in such 
 a manner that the phases of condensation and rarefaction 
 are repeated at regular periods of time. The difference 
 between the vibrations of the air and the ether is occasioned 
 by the remarkable delicacy and elasticity of the latter, which 
 not only permits a greater rapidity in the propagation of 
 motion than is possible with the coarse and heavy particles 
 of air, but also allows the number of vibrations per second 
 to be immensely greater, so that their number has to be 
 reckoned by billions. 
 
 ANALOGY BETWEEN MUSICAL SOUNDS AND COLORS. 
 
 Colors are to the eye what musical tones are to the ear. 
 A certain number of ether impulses in a second against the 
 retina of the eye are necessary to produce the sensation of 
 light : if the number of these waves pass above or below a 
 certain limit, the eye is no longer sensible of them as light. 
 
 The first sensation of these vibrations on the part of the 
 eye commences at about four hundred and fifty billion im- 
 pulses in a second, and the eye ceases to perceive them when 
 they have reached double this number, or about eight hun- 
 dred billion : in the first case, the impression produced is 
 that of dark red ; in the latter, of deep violet. 
 
 The greater the number of vibrations in any given time, 
 the more rapidly must the single impulses succeed each 
 other ; it may be concluded, therefore, that the different 
 colors are only produced by the different degrees of rapidity 
 with which the ether vibrations recur, just as the various 
 notes in music depend upon the rapidity of the succession 
 of vibrations of air. The vibrations which recur most 
 slowly, amounting, however, to at least four hundred and 
 fifty billion in a second, give the sensation of red ; those 
 recurring more rapidly produce that of yellow ; and if the 
 ' 23 
 
78 RECREATIONS IN POPULAR SCIENCE. [SCHELLKN 
 
 rapidity with which the impulses succeed each other con- 
 tinue to increase, the sensation becomes in succession green, 
 blue, and violet, with which last color the human eye be- 
 comes insensible to the ether motion, which, however, is 
 still very far from having attained its limit of rapidity. . 
 
 The gradation of the colors from red through yellow, 
 green, and blue, to violet, is to the eye what the gamut is to 
 the ear ; and it is therefore not without reason that we speak 
 of the tone and harmony of color. To the physicist the 
 words color and tone are only different modes of expression 
 for similar and closely allied phenomena ; they express the 
 perception of regular movements recurring in equal periods 
 of time, in ether, producing colors ; in air, musical sounds ; 
 in the former instance, by means of the organs of sight ; in 
 the latter, by the organs of hearing, movements of extreme 
 rapidity in ether, of more moderate speed in air. 
 
 But it will be asked what becomes of those vibrations 
 which are above and below the limits of the eye's sensibility 
 to light and color? Do they wander about purposeless and 
 unnoticed? By no means: forces are proved to exist in the 
 rays of the sun, and other intensely luminous bodies, which 
 cannot be perceived by the eye. Those slower vibrations 
 which, though they are reckoned by billions in a second, do 
 not yet amount to four hundred and fifty billion, are made 
 apparent to us in the sensation of heat, which is also the 
 result of oscillatory movement radiant heat being, like 
 light, propagated without the aid of foreign bodies. Those 
 vibrations, on the other hand, which have a velocity greater 
 than that by which deep violet is produced at which color 
 the eye's susceptibility to light ceases reveal themselves by 
 their powerful chemical action ; they succeed each other too 
 rapidly for the visual nerves to be any longer conscious of 
 the impulses, but they have the power of working chemical 
 changes, and the decomposition of various substances can 
 be undoubtedly traced to the agency of these invisible rays. 
 An English physicist has succeeded in moderating the ex- 
 
 24 
 
SCHKLLHN.] SPECTRUM ANALYSIS EXPLAINED. 79 
 
 cessive velocity of these vibrations by means of certain sub- 
 stances, and in this way has brought some of the invisible 
 chemical rays within reach of the eye's susceptibility.* 
 
 Dove describes, in his own ingenious manner, the course 
 of the vibrations as they produce successively sound, heat, 
 and light, as follows : 
 
 " In the middle of a large, darkened room let us suppose 
 a rod, set in vibration and connected with a contrivance for 
 continually augmenting the speed of its vibrations. I enter 
 the room at the moment when the rod is vibrating four times 
 in a second. Neither eye nor ear tell me of the presence of 
 the rod, only the hand, which feels the strokes when brought 
 within their reach. The vibrations become more rapid, till 
 when they reach the number of thirty-two in a second,! a 
 deep hum strikes my ear. The tone rises continually in 
 pitch, and passes through all the intervening grades up to 
 the highest, the shrillest note ; then all sinks again into the 
 former grave-like silence. While full of astonishment at 
 what I have heard, I feel suddenly (by the increased velocity 
 of the vibrating rod) an agreeable warmth, as from a fire, 
 diffusing itself from the spot whence the sound had pro- 
 
 * [Fluorescent substances possess this property. The peculiar 
 blue light diffused from a perfectly colorless solution of sulphate of 
 quinine was observed by Sir John Herschel, and the colored light 
 diffused from various vegetable solutions and essential oils was sub- 
 sequently examined by Sir David Brewster. To Professor Stokes, 
 however, is due the true explanation of these phenomena; he 
 shoved that the blue light of the solution of quinine consists of 
 vibrt tions brought within the limits of the power of the eye which 
 were originally too rapid to be visible. If a fresh infusion of the 
 bark of the horse-chestnut be placed beyond the limits of the visible 
 spectrum of sunlight admitted through a slit into a dark room, it 
 becomes beautifully luminous, in consequence of the power which 
 it possesses to lower the invisible ultra-violet vibrations into light 
 which can affect the eye.] 
 
 t That is to say, the tympanum is pressed in sixteen times, and 
 sixteen times withdrawn; therefore sixteen blows are received upon 
 the ear. 
 
 25 
 
So RECREATIONS IN POPULAR SCIENCE. [SCHELLBN. 
 
 ceeded. Still all is dark. The vibrations increase in rapid- 
 ity, and a faint red light begins to glimmer ; it gradually 
 brightens till the rod assumes a vivid red glow, then it turns 
 to yellow, and changes through the whole range of colors 
 up to violet, when all again is swallowed up in night. 
 Thus nature speaks to the different senses in succession ; at 
 first, a gentle word, audible only in immediate proximity, 
 then a louder call from an ever-increasing distance, till 
 finally her voice is borne on the wings of light from regions 
 of immeasurable space." 
 
 THE COLORS OF NATURAL OBJECTS. 
 
 Besides the colors of the spectrum, which are the simple 
 elements composing white light, there is another class of 
 colors apparent in every substance, w T hich are therefore 
 known as the colors of natural objects. When we see that 
 a picture is formed by covering the canvas with various pig- 
 ments, and that leaves and flowers are bright with the most 
 beautiful tints, while white cloth becomes red, green, or blue, 
 according to the color of the liquid into which it is dipped, 
 we are easily led to believe that every substance carries in 
 itself its own color, which is peculiar to it alone, and is 
 inherent in the substance. At most, we might admit that 
 light was requisite to render the color visible. 
 
 And yet this is not so. Were colors really something in- 
 herent in the object, every colored substance would mani- 
 festly appear always of the same color, by whatever light it 
 was illuminated. But this, as every one knows, is not the 
 case. The beautiful violet dress, which in daylight appears 
 of the purest color, seems dull and gloomy by gaslight ; 
 materials which in daylight are a bright blue, are tinged 
 with green in candle or lamp light. And what if the land- 
 scape, or a colored object, be viewed through a tinted glass? 
 All colors then seem changed, without the objects in them- 
 selyes being altered ; if the color of the glass be intense, the 
 
 26 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. Si 
 
 various colors of the objects immediately disappear, and 
 everything seems shaded in the color of the glass. The 
 same thing happens if some common salt be rubbed into the 
 wick of a spirit lamp, and surrounding objects viewed by 
 the yellow light of such a flame ; the colors disappear, or 
 lose much of their brilliancy, and everything seems either 
 in mere light and shade, or else of a dull gray. 
 
 These facts clearly prove that colors are not inherent in 
 objects, that they have no independent existence, but that 
 they are called forth by some extraneous cause. 
 
 On the other hand, these considerations show that there 
 must be something in the objects themselves to help in the 
 formation of color ; for they in no way assume the color of 
 the light illuminating them, but appear, as a rule, of quite 
 a different hue. 
 
 The natural color of an object is that in which it appears 
 when illuminated by the pure white light of the sun, or by 
 daylight ; it is called red or blue when it so appears by day- 
 light. Now if an object be illuminated by white light, and 
 yet appear of another color, the cause of the change must 
 b'e looked for in the influence which the surface of the body 
 exercises on the ether waves constituting white light. The 
 effects of this influence are very different, according to the 
 nature of the coloring matter with which the object is pro- 
 vided ; but they may mostly be reduced to one of two cases 
 either that a portion of the ether motion is entirely 
 stopped, or so considerably diminished in its passage over 
 the ponderable atoms of the substance, as that heat, instead 
 of light, is evolved, or else that the ether waves are irreg- 
 ularly reflected from the surface of the object, as sometimes 
 occurs with the waves of sound. In the first case, the rays 
 of light are said to be absorbed; in the latter, scattered. 
 
 When the surface of a body has the property of absorbing 
 all the colors of the solar spectrum with the exception of 
 one, the red, for example, that body appears red to us 
 by daylight, because this color alone is reflected to the eye. 
 
 27 
 
82 RECREATIONS IN POPULAR SCIENCE. [SCHELLEJI. 
 
 When, on the contrary, it has the power of absorbing some 
 of the rays, the red and orange, for instance, and of 
 reflecting the others, namely, the yellow, green, and blue, 
 the color of the object will then be that produced by the 
 mixture of the unabsorbed the reflected colors. Now 
 as white light contains the whole range of colors visible in 
 the spectrum, it can easily be understood why so many dif- 
 ferent colored objects should be seen in nature with such an 
 infinite variety of tints.* 
 
 When all the colors of white light are reflected from an 
 object in the same proportions as they occur in the solar 
 spectrum, the object appears white by daylight, and brilliant 
 in proportion to the quantity of light it reflects. In propor- 
 tion, however, as it reflects fewer rays of all kinds, the 
 white loses in intensity ; the object appears first gray, then 
 dark, and at last black, when all the rays falling upon it are 
 absorbed, and none reflected. 
 
 Those objects are therefore black the surfaces of which 
 are so constituted as to absorb all the colored rays of white 
 light ; those are white which reflect all the rays which fall 
 upon the surface ; and those are colored which reflect some 
 of the rays and absorb others. 
 
 A white object may therefore appear of all colors : if red 
 light falls upon it, it reflects it to the eye, and appears red ; 
 in blue light, it appears blue ; in green light, green, etc. ; 
 whereas a black object always appears black, whatever may 
 be the color of the light by which it is illuminated. 
 
 We may here further remark that a colored substance 
 assumes a different tint when illuminated by colored light, 
 and then appears of another than its natural, that is to say, 
 daylight color. Vermilion, for example, when placed in 
 red light, becomes of a more fiery red ; in orange or yellow 
 
 * [A certain proportion of the light falling upon colored bodies 
 is usually sent back unchanged by superficial reflection, without 
 undergoing the elective absorption to which the color of the sub- 
 stance is due.] 
 
 28 
 
SCHBLLEN.] SPECTRUM ANALYSIS EXPLAINED. 83 
 
 light, it appears orange or yellow, but deeper in tone ; green 
 rays impart to it something of their own tint, but as the red 
 substance can reflect only a few of the green rays, it appears 
 pale and dull by their light ; it seems still duller and darker 
 in blue light, and with indigo and violet it is almost black. 
 
 These phenomena are explained by the supposition that 
 the surfaces of colored bodies possess the property of re- 
 flecting the rays of one particular color in far greater pro- 
 portion than those of the other colors ; they do not therefore 
 appear black when illuminated by a light differing from their 
 own natural color. Take, for example, a piece of paper 
 half of which is colored a deep blue and half red : the col- 
 ored rays other than the blue and red are not all absorbed : 
 it is true that the blue piece reflects the blue rays pre-emi- 
 nently and in greatest number, as the red part does the red 
 rays, but the red has also the capability of reflecting other 
 rays to a small amount. If the pure yellow light of a spirit 
 flame impregnated with salt be allowed to fall on the paper 
 in a completely dark room, the paper must appear black if 
 the coloring matter reflect only the red and blue rays, 
 because the yellow rays of the burning sodium will be 
 absorbed, and no other light falls upon the paper ; but this 
 is not the case. The paper only appears black on the blue 
 part ; the red half is still visibly colored, though of a de- 
 cidedly yellow shade. We therefore conclude that the blue 
 of the paper does not reflect the yellow rays, but that the red 
 has that power in a small degree. Almost all colored 
 objects act like the red paper ; they reflect pre-eminently 
 one particular color, namely, that one of which they appear 
 by daylight ; but they are able also to reflect in small quan- 
 tities all other, or at least some other colors, and so they 
 vary in tint according to the kind of light in which they are 
 seen. 
 
 The colors of objects are very rarely pure and simple, 
 like those of the spectrum ; most of them are composed of 
 several colors, and can be decomposed into their original 
 
 29 
 
84 RECREATIONS IN POPULAR SCIENCE. [SCHELLE*. 
 
 elements by a prism. As without prismatic decomposition, 
 we are unable merely from the color of an object to say pos- 
 itively which colors are absorbed and which reflected, so it is 
 equally impossible for us to decide, from the color of a flame, 
 what the composition of its light may be, without investiga- 
 tion. The light of the sun, the lime-light, the magnesium 
 light, the light of coal gas, petroleum, and oil, all appear to 
 us more or less white, and yet the spectra of the various 
 lights differ considerably. It is true they all contain the 
 whole range of the colors of the spectrum, from red to 
 violet ; but each color is present in very different proportions. 
 The light from gas, oil, and candles has less blue than that 
 of the sun and the lime-light, and very much less violet. A 
 blue material will therefore reflect less blue by lamp, gas, or 
 candle light than by daylight ; the color will not only be flat 
 and dull, but will have a touch of green in it, on account of 
 the preponderance of yellow light. Blue and violet espe- 
 cially receive a green tinge by candle light, in which these 
 colors appear much duller than in daylight; and indeed 
 sometimes, according to the nature of the coloring matter 
 employed, this tint is so decided that in artificial light many 
 kinds of green cannot be distinguished from blue. 
 
 ABSORPTION OF LIGHT BY SOLID BODIES. 
 
 By the term absorption we have already designated that 
 action by which light, in its passage through certain media, 
 or by its reflection from the surfaces of bodies, is weakened, 
 partially retained, or entirely stopped. We found that those 
 substances called black absorbed rays of every color, and 
 reflected no light from their surfaces, and that most sub- 
 stances absorbed with great avidity rays of certain colors, 
 while they were insensitive to others. The cause of this 
 absorption is probably due to the vibrations of the ether 
 being communicated to the ponderable molecular particles of 
 the substance. 
 
 30 
 
SCHELLEK.] SPECTRUM ANALYSIS EXPLAINED. 85 
 
 Similar phenomena are noticed when light is transmitted 
 through colored glass. When all the objects in a landscape 
 appear red through a red glass, it is because the glass allows 
 only the red rays to pass through, and absorbs every other 
 colored ray : such a glass is transparent only to red light, 
 and is opaque to every other color. But it is rarely the case 
 that colored glass is transparent for one color only; most 
 kinds of glass absorb rays of certain colors, and allow the 
 others to pass through in very different proportions. The 
 naked eye is unable to decide which of the colored rays are 
 transmitted through a colored glass ; this can only be accu- 
 rately determined by analyzing the transmitted light by a 
 spectroscope or simple prism. 
 
 If we examine by a spectroscope the transmitted light of 
 the colored glass that we before made use of for obtaining 
 red, green, and blue light, it will at once be seen that the 
 ruby red glass transmits some orange and even some yel- 
 low rays, as well as the red, but that it entirely absorbs the 
 green, blue, and violet rays ; the cobalt blue glass transmits 
 some violet and green rays, besides the blue, but absorbs all 
 the red rays. If both glasses be laid one over the other, and 
 a gas flame looked at through them, it seems as if scarcely a 
 single ray was transmitted ; the red glass absorbing the 
 green, blue, and violet rays, and the blue glass absorbing the 
 red rays, there pass through only traces of such light as has 
 not been entirely absorbed, and this causes the gas flame to 
 appear of a dull yellow. A combination of several glasses, 
 or indeed any single glass which absorbs all the colored rays 
 composing white light, is opaque, that is to say, black ; 
 glass of perfect transparency, absorbing absolutely none of 
 the transmitted light, does not exist. 
 
 3' 
 
86 RECREATIONS IN POPULAR SCIENCE. [SCHELLEM. 
 
 RELATION BETWEEN THE EMISSION AND THE ABSORPTION 
 OF LIGHT. 
 
 When it is remembered that solid bodies in a state of in- 
 candescence emit a much greater body of light than gases 
 emit in a similar condition, and that they are able to absorb 
 a much greater quantity of the light falling on them, in 
 certain circumstances, even the whole of it, through the 
 transfer of the ether vibrations to their ponderable atoms ; 
 when, further, it is remembered that just those substances 
 \\\d(\. give out heat with the greatest facility, and in the fullest 
 quantity, are also the most capable of receiving heat from 
 without, or absorbing it, the thought is suggested that there 
 must be an intimate connection, a certain reciprocity between 
 the power of a body to emit light (emission) and to absorb 
 it (absorption). That the temperature of the substance has 
 an influence on this relation between its emissive and ab- 
 sorptive powers, is proved by the phenomena of the gas 
 spectra of the first, second, and third order, as well as by the 
 variety of absorption spectra exhibited at different tempera- 
 tures by the same substance. A century ago, the eminent 
 mathematician and physicist, Euler, in his u Theoria lucis et 
 caloris," enunciated the principle that every substance ab- 
 sorbs light of such a wave-length as coincides with the 
 vibrations of its smallest particles. Foucault mentioned in 
 his work on the spectrum of the electric light, published in 
 1849, that owing to the impurity of the carbon points, the 
 intense yellow sodium line appeared, and was changed into a 
 black line when sunlight was transmitted though the electric 
 arc. Angstrom gave expression, as early as the year 1853, 
 to the general law that a gas, when luminous, emits rays of 
 light of the same refrangibility as those 'which it has 
 power to absorb^ or, in other words, that the rays which a 
 substance absorbs are precisely those which it emits when 
 made self-luminous. 
 
 32 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 87 
 
 But all these facts remained isolated, and there was yet 
 wanting the comprehensive grasp of a general physical law, 
 under which the individual phenomena could be arranged. 
 It was reserved to Kirchhoff to discover this law, and to es- 
 tablish triumphantly its truth, not only by mathematical 
 proof, but also, in many striking instances, by experiment. 
 
 In the year 1860, he published his memoir on the relation 
 between the emissive and absorptive powers of bodies for 
 heat, as well as for light, in which occurs the celebrated sen- 
 tence : "The relation between the power of emission and 
 the power of absorption of one and the same class of rays, 
 is the same for all bodies at the same temperature" which 
 will ever be distinguished as announcing one of the most 
 important laws of nature, and which, on account of its ex- 
 tensive influence and universal application, will render 
 immortal the name of its illustrious discoverer. 
 
 REVERSAL OF THE SPECTRA OF GASES. 
 
 From KirchhofY's law it follows as a necessary conse- 
 quence that gases and vapors, in transmitting light, absorb 
 or impair precisely those rays (colors) which they them- 
 selves emit, when rendered luminous, while they remain 
 perfectly transparent to all other colored rays. Luminous 
 sodium vapor, for example, gives, under ordinary circum- 
 stances, a spectrum of one bright yellow double line ; it 
 emits therefore this yellow light only. If the white light of 
 the sun, the electric arc, or the oxyhydrogen lamp be 
 allowed to pass through the vapor of sodium, the vapor will 
 abstract or extinguish from the white light just those yellow 
 rays which it emitted when in a luminous state. While the 
 greater part of these yellow rays are absorbed by the sodium 
 vapor, all the other rays the red, orange, green, blue, and 
 violet pass through unimpaired. 
 
 The important result of these investigations is, therefore, 
 that the characteristic bright lines of sodium, lithium, etc., 
 
 33 
 
88 RECREATIONS IN POPULAR SCIENCE. [SCHKLLBN. 
 
 are changed into dark lines when the intense white light of 
 incandescent solid or liquid bodies passes through the vapor 
 of 'these metals. The spectrum of luminous sodium vapor 
 is a bright yellow (double) line, the rest of the field in the 
 spectroscope remaining dark ; * the spectrum of an incandes- 
 cent solid or liquid body, after it has passed through sodium 
 vapor at a lower temperature than itself, occupies on the 
 contrary the whole field with its brilliant colors, excepting 
 only that one place in which the dark sodium line is found. 
 As therefore the bright lines of gas spectra are converted, in 
 these experiments, into dark lines, while the dark parts of 
 the spectrum are changed into brilliant colors by the contin- 
 uous spectrum of the white light, the entire gas spectrum 
 seems to be reversed in respect of its illumination : for this 
 reason, the phenomenon has been called, after Kirchhoff, 
 " the reversal of the spectrum" 
 
 We can now readily predict what appearance will be pre- 
 sented in the spectroscope, if the light of an incandescent 
 , solid or liquid body, before entering the slit of the instru- 
 ment, pass through a less highly heated atmosphere of any 
 kind of vapor, such as that of sodium, lithium, iron, etc. 
 The incandescent body would have produced a continuous 
 spectrum, if its light had sustained no change on the way ; 
 but in the vaporous atmosphere through which its rays must 
 pass, each vapor absorbs just those rays which it would have 
 emitted if luminous, thereby extinguishing these particular 
 colors, and substituting for them dark bands in those places 
 of the continuous spectrum where it would have produced 
 bright lines. The spectroscope shows therefore a continuous 
 spectrum, extending through the whole range of colors, 
 from red to violet, .but intersected by dark lines; the sodium 
 line, the two lithium lines, the numerous iron lines, etc., 
 appear on the colored ground of the continuous spectrum as 
 BO many dark lines. 
 
 * See No. 3 of Frontispiece. 
 34 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 89 
 
 Spectra of this kind are evidently absorption spectra; 
 they are also called reversed or compound spectra. If a 
 complete coincidence can be established in such a spectrum, 
 by means of either a prism of comparison, or a scale, be- 
 tween the characteristic bright lines of the gas spectrum of 
 a certain substance, with the same number of dark lines, 
 the conclusion may be admitted that in the absorptive atmos- 
 phere which has produced the dark lines, the same substance 
 is contained in a condition of vapor. 
 
 THE SOLAR SPECTRUM AND THE FRAUNHOFER LINES. 
 
 The most brilliant example of a reversed spectrum, that 
 is to say, a continuous spectrum, crossed by dark absorption 
 lines, is afforded by the sun. If an ordinary spectroscope, 
 armed with a telescope of low power, be directed to a bright 
 sky, with a rather wide opening of the slit, a magnificent 
 continuous spectrum will be seen, exhibiting the most beauti- 
 ful and brilliant colors, without either bright or dark lines. 
 But if the slit be narrowed so as to obtain the purest possible 
 spectrum, and the focus of the telescope be very accurately 
 adjusted, the spectrum, now much fainter, will be seen to be 
 crossed by a number of dark lines and cloudy bands. If, by 
 the use of several prisms, the spectrum be lengthened, and a 
 higher magnifying power employed, these thick lines and 
 bands will become resolved into separate fine lines and 
 groups of lines, which are so sharply defined and so charac- 
 teristically grouped that, by the help of a scale, they are 
 easily impressed upon the memory, and distinguished one 
 from another. 
 
 As early as 1802, these dark lines in the solar spectrum 
 had been observed and described by Wollaston ; later, in 
 1814, they were more carefully examined and mapped by 
 Fraunhofer, of Munich ; and later still by Becquerel, Zante- 
 deschi, Matthiessen, Brewster, Gladstone, and others; but 
 their origin and nature remained a mystery, notwithstanding 
 
 35 
 
90 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 the acutest reasoning and most painstaking researches of 
 many able physicists, until Kirchhoff made his splendid 
 discovery in 1859. 
 
 Fraunhofer was able to distinguish with certainty about 
 six hundred lines ; he found also that with the same prism 
 and telescope they always kept the same relative order and 
 position, and were therefore peculiarly adapted to serve as 
 marks for denoting the place of any single set of colored 
 rays, and for determining the refrangibility of any particular 
 cplojv x 
 
 ^^To facilitate reference to any of the innumerable colors of 
 the solar spectrum (Frontispiece, No. i), Fraunhofer, whose 
 drawing is accurately represented in -F-igriV-., selected out 
 of the great number he observed eight characteristic lines, 
 situated in the most important places of the spectrum, which 
 he designated by the letters A, B, C, D, E, F, G, H ; of 
 these lines A and B lie in the red, C in the red near the 
 orange, D in the orange, forming a double line with a high 
 power, E in the yellow, F on the borders of the green and 
 blue, G in the dark blue or indigo, and H in the violet. Be- 
 sides these lines, there is a noticeable group #, of fine lines 
 between A and B, and also a group , consisting of three 
 fine lines, between E and F. It was remarked even by 
 Fraunhofer that the position of the two dark lines in the 
 solar spectrum designated by him D, were coincident with 
 the two bright lines shown by the light of a lamp, now 
 known as the double sodium line. These dark lines of the 
 solar spectrum have been called, after their discoverer, the 
 Fraunhofer lines. 
 
 BInce^*raunHoler counted in the spectrum more than six 
 hundred dark lines, Brewster has counted two thousand, and 
 others, by causing the refracted rays to pass successively 
 through several prisms (some using as many as nine at one 
 observation), the existence of three thousand lines has been 
 ascertained. These lines have been carefully mapped by 
 Angstrom and Thalen, and Kirchhoff, who has carefully 
 
 36 
 
SCHELLEN.] SPECTRUM ANALYSIS EXPLAINED. 
 
 9 1 
 
 studied them, and ascertained 
 wherein they coincide with the 
 bright lines of metals known 
 to us, says, " The most strik- 
 ing coincidences between the 
 spectrum lines of terrestrial 
 elements and the dark lines of 
 the solar spectrum are shown 
 m iron, sodium, potassium, 
 calcium, magnesium, manga- 
 nese, chromium, nickel, and hy- 
 drogen ; the spectrum lines of 
 these substances not only agree 
 exactly with the dark lines in 
 position and breadth, but pro- 
 claim their relationship to them 
 by a similar degree of inten- 
 sity. The brighter, for in- 
 stance, a spectrum line ap- 
 pears, so much the darker will 
 its corresponding line be in the 
 solar spectrum. 
 
 Fig. IV. shows the principal 
 Fraunhofer lines, A, B, C, etc., 
 and many intermediate ones. 
 It also shows the coincidence 
 between sixty-five of the bright 
 lines in the spectrum of iron 
 and the same number of dark 
 lines in the solar spectrum, 
 proving beyond a doubt the 
 existence of iron in the sun. 
 Other observations prove as 
 conclusively the existence of 
 sodium, potassium, and the 
 other metals which are above 
 mentioned. 
 
 37 
 
 FIG. IV. 
 
92 RECREATIONS IN POPULAR SCIENCE. [SCHELLEM 
 
 THE TELLURIC LINES. 
 
 In addition to the Fraunhofer lines, there are other dark 
 lines in the solar spectrum, which are called atmospheric or 
 telluric lines. These are variable, and are caused by the 
 varying condition of the earth's atmosphere. Janssen found 
 them to be darkest at sunrise and sunset, and less intense in 
 the middle of the day, a periodicity of change which at 
 once proves their atmospheric origin, but they were never 
 entirely absent from the spectrum. He instituted a series of 
 experiments by which he found that a large number of the 
 variable lines in the solar spectrum are due to the presence 
 of aqueous vapor in the earth's atmosphere, and also a 
 method secured for detecting the presence of aqueous vapor 
 in the heavenly bodies. 
 
 Angstrom says that nearly all the changes of color ob- 
 served in the red glow of sunrise and sunset find a simple 
 explanation in the phenomena of atmospheric absorption, 
 whereby all the ingenious and elaborate explanations hith- 
 erto attempted are completely set aside. 
 
 It is fully admitted that other heavenly bodies besides the 
 earth maybe surrounded by an atmosphere; Janssen's dis- 
 covery of the spectrum of aqueous vapor furnishes the 
 means of ascertaining whether this vapor, indispensable to 
 the maintenance of all the living organisms f our planet, is 
 also present in the other celestial bodies. Repeated obser- 
 vations undertaken by Janssen on the high mountains of 
 Italy and Greece have already furnished proof that aqueous 
 vapor is present in the atmospheres of the planets Mars and 
 Saturn. 
 
 .SPECTRUM APPARATUS. 
 
 The thought is perhaps rising in the minds of many who 
 have accompanied us thus far, that ithe production of the 
 spectrum of a substance .for the purposes of analytical ex- 
 amination is encumbered with great difficulties and many 
 troublesome details, involving too much labor to be available 
 
 38 
 
SCHELLEN.J SPECTRUM ANALYSIS EXPLAINED. 93 
 
 for the use of the chemist and the physicist. This is, how- 
 ever, not the case ; if, iu our mode of illustration, a powerful 
 galvanic battery, and the electric lamp, with its revolving 
 table and large screen, have been employed, it has been only 
 to show how, by the extraordinary heat and light of the 
 voltaic arc, the simple phenomena on which spectrum anal- 
 ysis is based can be made visible to many hundred spectators 
 at once in a large lecture-room. When, however, the light 
 from the heated vapors need not be greater than is required 
 for a single observer, the whole electric apparatus may be 
 dispensed with, and the simple Bunsen burner substituted ; 
 in place of the large screen of paper that reflected the light, 
 the small, sensitive screen of nerves the retina of the hu- 
 man eye becomes the surface on which the spectrum is 
 received ; and the whole cumbrous contrivance occupying so 
 much space is replaced by a small spectrum apparatus as 
 trustworthy as it is easy to manipulate. 
 
 FIG. V. 
 
 THE COMPOUND SPECTROSCOPE. 
 
 Fig. V. shows a compound spectroscope. With this we 
 are enabled to use two flames, and the apparatus is so ar- 
 ranged that we can see the two spectra, one above the other, 
 and compare them with each other. For instance, putting a 
 small quantity of the substance we know to contain sodium 
 
 39 
 
94 RECREATIONS IN POPULAR SCIENCE. [SCHELLBN. 
 
 into one flame, we place a substance supposed to contain 
 sodium in the other flame, and then, by means of a small 
 reflecting prism placed at the end of the slit, we have the 
 spectra of both sent into the telescope, one above the other, 
 and can tell at once whether the lines coincide with each 
 other. Another arrangement for facilitating the comparison 
 of spectra consists of the illuminated millimetre scale con- 
 tained in the tube g (Fig. V.), a magnified reflection of 
 which is thrown into the telescope^", from the surface of the 
 prism a. The scale is thus seen between the two spectra, 
 and the position of any line can be accurately measured, and 
 ascertained. 
 
 The reader is now in a position to understand the use of 
 the various parts of a complete spectrum apparatus, espe- 
 cially the three tubes directed to the prism at different an- 
 gles, as in that constructed by Kirchhoff and Bunsen. The 
 eye of the observer is placed in the axis of the telescope 
 directed to that surface from which the light emerges in the 
 form of a spectrum ; the opposite surface of the prism re- 
 ceives through the slit and collimating lens the light emitted 
 from the object to be examined ; at the side of the observer 
 is the tube carrying the illuminated scale, or the micrometer 
 screw, so that the mark coinciding with any division of the 
 scale may be placed on any line of the spectrum. 
 
 CONCLUSION. 
 
 If we have given our readers anything approaching an 
 adequate idea of the beauty and utility of this wonderful 
 discovery, we feel sure it will only stimulate them to inves- 
 tigate and study it further. It was our intention to have 
 continued the subject, and closed this article with a resume 
 of the discoveries made by means of spectrum analysis, but 
 we find it impossible to condense the matter sufficiently to 
 bring it into a size appropriate to these papers, and we defer 
 it to the next number, which will be devoted to Spectrum 
 Analysts Discoveries. 
 
 40 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 95 
 
 i 
 
 5, Sputum Analysis 
 
 Showing its Application in Microscopical Research, and to Discov- 
 eries of the Physical Constitution and Movements of 
 
 THE HEAVENLY BODIES. 
 
 IN the preceding article we gave our readers, in general 
 terms, the scope of this great discovery, and its impor- 
 tance to the scientific world, and, to make it plainer, laid 
 down in a condensed form the received laws of the theory of 
 sound, heat, light, and color, a& well as an explanation of 
 the modes in use of detecting minute particles of elements, 
 where all other means had failed. We also showed that, by 
 means of it, many substances, before unknown, had been 
 discovered ; and it is not doubted that others will be in the 
 future, as the discovery is still in its infancy. 
 
 THE MICROSPECTROSCOPE. 
 
 In addition to ordinary spectroscopes, Messrs. Sorby and 
 Browning have devised a combination of the microscope 
 and spectroscope, called the microspectroscope, which ren- 
 ders it possible to examine very minute traces of substances. 
 This application of the spectroscope has been very useful in 
 investigating substances which have special importance in 
 physiology and pathology: thus in examining normal and 
 diseased blood, in detecting albumen in urine, and in ascer- 
 taining the rate at which certain substances pass into the 
 various fluids of the system, as well as investigating sup- 
 posed cases of poisoning. The characteristic absorption 
 bands which certain liquids, such as wine, beer, etc., present 
 in their normal state, compared with those yielded by adul- 
 terated substances, furnish a delicate and certain means of 
 detecting the latter. 
 
96 RECREATIONS IN POPULAR SCIENCE. [SCHELLBN. 
 
 An instance of its use in detecting the cause of impurity in 
 water is related, as follows : The water used by the inhab- 
 itants of a crowded court, amongst whom several cases of 
 typhoid fever had appeared, was drawn from a rather shal- 
 low well, and was highly charged with various unoxidized 
 compounds of nitrogen. It was suspected that, from some 
 defect of drainage, the contents of a public urinal obtained 
 entrance to the well. The fact that the well-water contained 
 seven times as much common salt as the normal water of 
 the vicinity, was some confirmation of the suspicion. Pro- 
 fessor Church obtained absolute proof 'of the fact by the fol- 
 lowing method. He introduced two grammes of a lithium 
 salt into the urinal, and two hours later was enabled readily 
 to detect with the spectroscope the presence of lithium in a 
 litre of the well-water, which by previous examination had 
 shown no traces of this substance. Many other instances of 
 its use for similar purposes might be cited. 
 
 Spectrum analysis has thus opened a wide field of investi- 
 gation to the physiologist, the physician, the botanist, the 
 zoologist, the chemist, and the technologist, and the labors 
 undertaken in these various departments of science have 
 already yielded valuable results. 
 
 It was shown in our last paper that by it we have at last 
 the means of forming a definite idea of the physical consti- 
 tution of the heavenly bodies, and ascertaining the existence 
 of atmospheres around other planets, and other means of 
 supporting life such as exist on this earth. We will proceed 
 to give a resume of the other discoveries regarding the phys- 
 ical constitution and movements of the heavenly bodies. 
 
 KIRCHHOFF'S THEORY OF THE PHYSICAL CONSTITUTION 
 OF THE SUN. 
 
 It had long been assumed that the gaps in the colors of 
 the solar spectrum which form the Fraunhofer dark lines, 
 were due to an absorption of the corresponding colored rays 
 
 2 
 
SCHEILEN.] SPECTRUM ANALYSIS DISCOVERIES. 97 
 
 in the atmosphere of the sun ; but no explanation could be 
 given of this phenomenon. The cause of this absorption 
 was ascertained by Kirchhoff in his discovery that a vapor 
 absorbs from white light just those rays which it emits when 
 luminous, and he proved the whole system of the Fraunhofer 
 lines to be mainly produced by the overlying of the reversed 
 spectra of such substances as are to be found in the earth. 
 He thus arrived at a new conception of the physical consti- 
 tution of the sun which is entirely opposed to the theories 
 held by Wilson and Sir William Herschel in explanation of 
 the solar spots. 
 
 According to Kirchhoff, the sun consists of a solid or par- 
 tially liquid nucleus in the highest state of incandescence, 
 which emits, like all incandescent solid or liquid bodies, 
 every possible kind of light, and therefore would of itself 
 give a continuous spectrum without any dark lines. This 
 incandescent central nucleus is surrounded by an atmospher3 
 of lower temperature, containing, on account of the extreme 
 heat of the nucleus, the vapors of many of the substances 
 of which this body is composed. The rays of light, there- 
 fore, emitted by the nucleus, must pass through this atmos- 
 phere before reaching the earth, and each vapor extinguishes 
 from the white light those rays which it would itself emit in 
 a glowing state. Now it is found, when the sun's light is 
 analyzed by a prism, that a multitude of rays are extin- 
 guished, and just those rays which would be emitted by the 
 vapors of sodium, iron, calcium, magnesium, etc., were they 
 made self-luminous; consequently the vapors of the fol- 
 lowing substances, sodium, iron, potassium, calcium, barium, 
 magnesium, manganese, titanium, chromium, nickel, cobalt, 
 hydrogen, and probably, also, zinc, copper, and gold, must 
 exist in the solar atmosphere, and these metals, therefore, 
 must also be present to a considerable extent in the body of 
 the sun. 
 
 Could the light from the sun's nucleus in any way be set 
 aside, and only that of the incandescent vapors of the sun's 
 
 3 
 
98 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 atmosphere be received through the slit of the spectroscope, 
 a spectrum would then be obtained composed of the actual 
 spectra of these substances, that is to say, the same system 
 of bright colored lines which now appear as the dark Fraun- 
 hofer lines. 
 
 THE SOLAR SPOTS ; THE FACUL^E AND THEIR SPECTRA. 
 
 It would lead us too far from our subject were we to dwell 
 upon the phenomena of the solar spots, important as they 
 are for acquiring a knowledge of the physical constitution 
 of the sun, or enter upon a full description of their form, 
 their mode of formation and disappearance, their motion, 
 their connection with the sun's rotation upon its axis, their 
 periodic occurrence, and the various hypotheses that have 
 been formed as to their nature ; but, on the other hand, we 
 must still less be silent on the subject, since spectrum anal- 
 ysis has investigated these wonderful appearances with a 
 success which has added much to our knowledge of the 
 constitution of the sun. 
 
 Fig. 6 shows a remarkable group of solar spots. These, 
 and indeed a large proportion of solar spots, consist princi- 
 pally of a dark, almost black, central portion, the umbra * 
 surrounded by a space somewhat less dark called the pen- 
 umbra: the umbra has generally an irregular form, while 
 the penumbra exhibits a structure radiating towards the 
 centre. 
 
 If the sun be observed with a high power, the surface pre- 
 sents by no means a uniform appearance ; a multitude of 
 bright and dark stripes cross each other in all directions, and 
 
 * The dark central part of a spot has been distinguished through- 
 out by the name umbra, in accordance with the usual custom of as- 
 tronomers. Mr. Dawes showed that within this part of a spot one 
 or more darker spots may generally be observed, to which he gave 
 the name of nucleus. 
 
 4 
 
FIG. VI. 
 
 fi/tt&'%<5^9KTCT\tS&&' 
 
 Group of Solar Spots observed and drawn by Nasmyth, 5th June, 1864. 
 
TOO RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 the luminous surface appears like a net of bright meshes 
 interwoven with dark threads and small dark pores. Th- 
 brightest portions (Fig. 6) show a more or less elongated 
 form, which suggested to Nasmyth the name of " willow 
 leaves," while Dawes compares them to " bits of straw," 
 and Huggins calls them merely " granules." 
 
 On this uneven and ever-varying bright background the 
 spots make their appearance in the greatest variety of form 
 and size. The penumbra not unfrequently stretches across 
 the black central portion in various places (see right hand 
 spot, Fig. 6), and generally appears much darker at the 
 outer edges, where the spot touches the bright part of the 
 sun's surface, than in other places. Very often the penum- 
 bra is traversed by few or more bright curved bands, stretch- 
 ing from the outer edge towards the nucleus, generally at 
 right angles to the confines of the nucleus and penumbra, 
 which give the spot the appearance as if a number of streams 
 of some luminous matter had broken through the dam formed 
 by the penumbra, to fall into the abyss of the umbra. (See 
 central spot, Fig. 6.) Even the umbra itself is often crossed 
 by one or more broad luminous bands called bridges, by 
 which it is divided into several portions. 
 
 Besides the dark spots, and chiefly in their immediate 
 neighborhood, bright places make their appearance on the 
 sun's surface, which have been called faculcz. They are 
 generally the attendants of solar spots, and are especially to 
 be seen at the extreme edge of the penumbra when the spot 
 has reached the sun's limb, that is, its edge or border ; that 
 they are not the effect of contrast between the dark spot and 
 the neighboring brightness, is proved by the circumstance 
 that every spot is not accompanied by fuculae, and that very 
 frequently isolated faculae are to be seen, which are almost 
 always the precursor of a coming spot. 
 
 The faculae, ' like the spots, vary considerably in form ; 
 generally they are round and concentrated, but often they 
 
 6 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. IOI 
 
 have the appearance of long stripes of light, disposed like 
 veins, converging from all sides towards a spot. 
 
 The wreathed faculae are almost always followed in a few 
 days by the appearance of a group of spots ; among the 
 vein-like waves of light visible in many places, more espe- 
 cially towards the sun's limb, there is first developed a dull 
 scar-like place out of which the spots are formed, sometimes 
 singly, and sometimes in groups ; and not unfrequently the 
 formation of a spot may be predicted from the increased 
 intensity of light at that place on the sun's disk. 
 
 When a spot is observed near the sun's limb or edge in 
 the midst of the surrounding faculae, it is difficult to avoid 
 the impression that the spot lies in a hollow between bright 
 overhanging mountains ; and it was observed by Secchi on 
 the 5th of August, 1865, that the faculae, when they reached 
 the western limb of the sun, appeared like small projections 
 and irregularities upon the sharply defined limb of the sun. 
 4 Although the real connection between the faculae and the 
 spots is not yet fully understood, it may be safely concluded 
 from these observations that the spots lie deeper in the solar 
 surface than do the faculae, and that these faculae are moun- 
 tainous elevations of the luminous matter forming the photo- 
 sphere, by which the spot is surrounded in a wide circuit as 
 by a wall. 
 
 The group of solar spots observed and drawn by Nasmyth 
 on the 5th of June, 1864, is given in Fig. 6, in which all the 
 details characteristic of a spot are to be recognized the 
 black umbra, the penumbra in a variety of forms, composed 
 of the 4k leaves" directed towards the umbra, and the sur- 
 rounding luminous surface of the sun presenting its usual 
 granulated appearance. This surface is called the photo- 
 sphere, a name given without reference to any particular 
 theory as to its physical constitution or structure. The pho- 
 tosphere is entirely covered with pores, or small spots, less 
 luminous than the other parts ; where they congregate, and 
 become conspicuous by forming a black umbra and shaded 
 
 7 
 
102 RECREATIONS IN POPULAR SCIENCE. [SCHELLEK. 
 
 penumbra, they constitute the ordinary solar spot; where 
 the portions of greater brilliancy than the surrounding parts 
 of the photosphere congregate, they form the faculce, and 
 these generally accompany the spots, or precede their for- 
 mation. 
 
 If a solar spot be watched in the telescope from day to 
 day, or from hour to hour, it will soon be seen to change in 
 form ; it increases or diminishes, or completely vanishes 
 away, while new spots make their appearance. In the 
 process of disappearing, the dark umbra first gradually 
 contracts until it becomes invisible, leaving the dusky pen- 
 umbra perceptible for some time longer. Not unfrequently 
 a spot breaks up into several spots, and occasionally a group 
 unites to form one, and sometimes, even, one spot is seen to 
 pass over another, partially covering it, and then with- 
 drawing from it. In all these changes, the spots exhibit an 
 amount of mobility displayed in general only by liquid or 
 vaporous masses. 
 
 The formation and changes in the configuration of a spot 
 may often be watched during the course of observation, and 
 it not unfrequently happens that the appearance of a group 
 of spots is so entirely changed from one day;to another, that 
 it can no longer be recognized in the new form it has as- 
 sumed. 
 
 'On the other hand, there are spots presenting scarcely any 
 change, which preserve nearly the same form for many days 
 together. Spots of this kind are of the highest value to the 
 astronomer, as they afford the only means of ascertaining 
 the time of the revolution of the-sun upon its axis, the posi- 
 tion of this axis, and its inclination to the earth's orbit. 
 
 It not unfrequently happens, that 'the same spot which has 
 been observed to disappear on the western limb has, in the 
 course of about fourteen days, been seen to reappear ort the 
 eastern Hrnb, and in the lapse of another fourteen days has 
 .disappeared a second time on the western limb a phe- 
 nomenon tliat proves beyond a doubt that the spots are con- 
 
 8 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 103 
 
 nected with the surface of the sun, and that the sun itself 
 has a revolution upon its axis. If the time required for the 
 earth's motion round the sun be allowed for in this revolution 
 of the spot, the result will show, according to Sporer, a 
 mean time of rotation for the sun amounting to twenty-five 
 days, five hours, thirty-eight minutes. 
 
 KirchhofT considers these forms to be cloud-like condensa- 
 tions in the sun's atmosphere, which are produced by the 
 loss of the solar heat by radiation, in the same way as the 
 aqueous vapors of the earth's atmosphere are formed into 
 mist and cloud. When such clouds arise over the bright 
 and glowing surface of the sun, they obscure the light of 
 the sun at that spot, and it is but natural that these cloudy 
 masses, so irregularly formed, should also become further 
 condensed, or be dispersed with the same amount of irreg- 
 ularity, according as they come in contact with cooler or 
 warmer streams of gas. 
 
 The results of the spectrum observations of Secchi, Lock- 
 yer, and Young, important and valuable as they are, remain 
 as yet too isolated and unconnected with telescopic observa- 
 tions of the spots and faculae to yield material sufficient for 
 explaining the nature of these forms. This much, however, 
 may be regarded as certain that the phenomena of the in- 
 crease in the width and intensity of the Fraunhofer lines, as 
 well as the appearance of new dark bands in the spectrum 
 of the umbra, are produced by the increased absorptive 
 power exercised by the substances of 'which the spot is 
 formed. 
 
 When the white light of the sun's nucleus which has 
 already suffered absorption from the absorptive stratum 
 passes through the vaporous matter of a spot, it undergoes a 
 yet further absorption from the additional matter which the 
 spot contains. As, therefore, the lines of calcium and iron 
 are considerably affected in the spectrum of a spot, the so- 
 dium lines in a smaller degree, and to some extent those of 
 magnesium, it may be concluded that the substance forming 
 
 9 
 
104 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 the solar spots is composed pre-eminently of vapors of cal- 
 cium, iron, titanium, sodium, barium, and magnesium, and 
 that these substances occur in layers of varying thickness, 
 and in very different proportions. 
 
 That hydrogen gas constitutes an important element in 
 the formation of the spots, is shown in the most unequivocal 
 manner by the spectrum. The hydrogen lines are most 
 affected in the parts that lie close to the umbra, in the bridge, 
 when one is formed, and in the penumbra. 
 
 An explanation of this phenomenon is, that hydrogen gas 
 breaks forth, from time to time, from the interior of the in- 
 candescent solar nucleus. Owing to its extreme lightness, 
 this gas would rise in enormous pillars of flame (promi- 
 nences) over the absorptive vaporous stratum of the photo- 
 sphere, and, in consequence of the cooling ensuing from ex- 
 pansion, would enter into a variety of chemical combina- 
 tions, especially with oxygen ; the uncombined part would 
 then flow to the side, while that in combination with oxygen 
 (steam) and the other solar substances would form gaseous or 
 vaporous masses, which, from their nature as well as from 
 their continued cooling, would be heavier than the hydrogen 
 gas, and would sink down from their greater gravity. It is to 
 be expected that the stream of gas on rising would carry up 
 with it a quantity of those substances that exist in the sun's 
 nucleus and the surounding stratum of absorptive vapor (the 
 photosphere) ; if these substances, themselves incandescent, 
 were present in sufficient quantities in the luminous hydro- 
 gen gas, their characteristic lines would be seen as bright 
 lines in the spectrum of the pillars of flame. During the 
 recent total eclipses, many such lines were in fact observed, 
 together with the bright hydrogen lines, in the prominences, 
 a description of which will be given farther on ; they can 
 now be observed daily, sometimes in great numbers, upon 
 the sun's disk. 
 
 When the force of the gas eruption has somewhat subsided, 
 and the chemical combinations ensue, producing vaporous 
 
 10 
 
SCHELLEN.J SPECTRUM ANALYSIS DISCOVERIES. 105 
 
 precipitations of many kinds, the formation of the spot be- 
 gins. The heavier portions of these precipitations sink 
 down, and form the umbra of a spot at the place of greatest 
 condensation, while the parts which are less dense constitute 
 the penumbra.. The vaporous umbra, however, though 
 apparently quite black, is yet able to transmit a considerable 
 amount of sunlight ; indeed, according to Zollner's meas- 
 urements, the black umbra of a spot emits fottr thousand 
 times as much light as that derived from an equal area of 
 the full moon. This statement is fully confirmed by the 
 results of spectrum' analysis, for even the blackest umbra 
 yields a spectrum exhibiting all the details of full sunlight. 
 
 The various remarkable changes which the lines of hy- 
 drogen, magnesium, sodium, calcium, and iron suffer in the 
 spectrum of the umbra, seem to show that in the cloud-like 
 and vaporous substances constituting the spot, the new com- 
 binations are disposed in layers, according to their specific 
 gravity. Thus hydrogen gas occupies the highest stratum ; 
 aqueous vapor, magnesium, and sodium follow in thinner 
 layers below ; and the heavier vapors of calcium, titanium, 
 and iron form the lowest and densest stratum, the base of 
 the spot. 
 
 The formation of a spot will accordingly immediately fol- 
 low an eruption of hydrogen : the spot itself is a dense, 
 cloudy, luminous mass, probably of a semi-fluid consistency, 
 composed of many constituents according to Zollner, a 
 kind of scoria which sinks by its gravity a certain depth 
 into the photosphere, or outer portion of the sun, and par- 
 tially intercepts the light from the lower stratum of the pho- 
 tosphere, therefore presenting to us the appearance of a dark 
 mass projected upon the disk of the sun, in the same way as 
 the exceedingly intense light of the oxyhydrogen lime-light 
 appears black when seen against the sun. 
 
 The enormous dimensions of these dense masses of vapor, 
 which extend sometimes in all directions, account for the 
 length of time the spots continue visible, not unfrequently 
 
 ii 
 
106 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 remaining during several rotations of the sun. Their disap- 
 pearance is to be explained partly by the substance of the 
 photosphere flowing into the cavity of the spot, partly by 
 the complete subsidence of the vapors into the nucleus of 
 the sun, where, in consequence of the enormous heat, the 
 compound substances which may exist in them are broken 
 up into their original elements. 
 
 These conjectures are by no means intended to afford a 
 complete explanation of all the phenomena of a solar spot. 
 Though it certainly is of the highest interest for us to acquire 
 a knowledge of the physical nature of that heavenly body 
 whence we derive light, heat, motion, and life, we must yet 
 be cautious of receiving for truth what is only the result of 
 speculation, especially as the theories on this subject rest on 
 isolated observations which are too unconnected to point to 
 any certain conclusion. The suggestions here thrown out 
 are only intended, therefore, to throw some light upon the 
 results hitherto obtained by the spectrum observations of 
 Secchi, Huggins, Lockyer, and Young, and by affording an 
 unconstrained interpretation of them to bring them into har- 
 mony with the phenomena observed during the total solar 
 eclipses of 1868, 1869, and 1871. 
 
 TOTAL SOLAR ECLIPSES. 
 
 The reason why our knowledge concerning the nature of 
 the sun is still so imperfect, is that the remarkable phenom- 
 ena occurring on the sun's limb are so completely over- 
 powered by the blinding light of the solar nucleus or photo- 
 sphere, that they remain invisible even in the most powerful 
 telescopes. It is not sufficient to get rid of the sun's rays by 
 the interposition of an opaque screen, because the diffused 
 light of the sky cannot be eliminated by this means, and this 
 light, even, is so intense as to conceal the faint light of the 
 sun's appendages.' It is quite otherwise, however, during a 
 total eclipse of the sun ; then the moon covers the whole of 
 
 12 
 
SCHHLLEN.] SPECTRUM ANALYSIS DISCOVERIES. 107 
 
 the sun's disk, and includes a large tract of the earth's sur- 
 face in the cone of its shadow, revealing to the observer, 
 who is no longer hindered by the light of day, a display of 
 phenomena round the sun which can be seen in no other 
 way, and the study of which is peculiarly fitted to throw 
 light on the nature and physical constitution of the sun. 
 
 We will not suffer ourselves to be detained by a descrip- 
 tion of those changes that pass over the landscape as the 
 darkness advances, nor dwell upon the deep impression 
 which the sudden disappearance of the last rays of the sun, 
 and the equally sudden reappearance of the light, make 
 both upon men and animals. 
 
 The diameter of the cone of the shadow thrown by the 
 moon towards the earth, amounts at the spot where it 
 touches the earth's surface on the equator during the time of 
 totality to about 122 miles: as, however, the moon, which 
 throws the shadow, only completes its course in the heavens 
 round the earth from west to east in one month, and the 
 earth, which receives the shadow, accomplishes its revolu- 
 tion from west to east in one day, it follows that the motion 
 of the moon's shadow is very much slower than that of the 
 earth's surface. It therefore happens that the earth appears 
 to run away from under the moon's shadow, or that the 
 moon's shadow seems to run over the earth from east to west. 
 From an elevated position the shadow of the moon is seen 
 to approach with enormous rapidity, and the sensation as 
 though a material substance, such as a terrific cloud of 
 smoke, were rushing over the earth's surface, fills the unini- 
 tiated spectator with fear and dread. A few minutes before 
 the commencement of the totality, the brightest stars become 
 visible, and the sharply defined black edge of the moon ap- 
 pears surrounded on all sides by a very narrow but very 
 brilliant ring of light, of silver whiteness, which is called 
 the corona. From the corona faint rays of light, irregular 
 in length and breadth, stream out in all directions, surround- 
 
 13 
 
IOS RECREATIONS IN POPULAR SCIENCE. [SCHKLLEK. 
 
 ing the moon's disk like a glory, whence this crown of rays 
 is usually designated the glory or halo. 
 
 When the total darkness has commenced, the prominences 
 make their appearance, which are cloud-like masses of a 
 rose or pale coral color, disposed either singly or in groups, 
 at various places on the moon's limb. 
 
 They pierce the corona in the most wonderful forms, 
 sometimes as single outgrowths of enormous height, some- 
 times as low projections spreading far along the moon's 
 limb. The prominences are generally first seen on the 
 eastern (left) side of the sun, where at the commencement 
 of the totality the moon only grazes the sun's edge, and the 
 space immediately surrounding the sun is yet uncovered; in 
 proportion as the moon advances to the east, the space im- 
 mediately surrounding the western parts of the sun becomes 
 free, and the prominences are then seen also on that side in 
 greater number, and developed with much greater dis- 
 tinctness. 
 
 There remains now no longer any doubt that these re- 
 markable phenomena belong to the sun, and are great accu- 
 mulations of the luminous gaseous material by which the 
 solar body is wholly surrounded ; it cannot therefore greatly 
 astonish us that their forms have been seen to change even 
 during the short duration of the totality ; that which calls 
 much more for wonder is the enormous height to which 
 these pillars of gas extend beyond the limb of the sun, a 
 height which in some instances exceeds ninety thousand 
 miles. 
 
 THE TOTAL SOLAR ECLIPSE OF THE 7 OF AUGUST, 1869. 
 
 This eclipse was invisible in Europe ; the zone of totality 
 stretched from Alaska, where the eclipse began at noon, 
 over British America and the south-west corner of Minne- 
 sota, then crossed the Mississippi near Burlington (Iowa), 
 and passed through Illinois, Western Virginia, and North 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 109 
 
 Carolina, reaching the Atlantic Ocean in the neighborhood 
 of Beaufort. 
 
 The event excited the most lively interest among astrono- 
 mers and photographers throughout the whole of North 
 America, and occasioned the equipment of a number of sci- 
 entific expeditions, which were also supplemented by the 
 valuable labors of many private individuals. The observers 
 were in almost every instance favored with the finest weather, 
 and their efforts were rewarded by a large collection of pho- 
 tographic pictures, and many valuable spectroscopic and 
 other observations. That portion of the zone of totality 
 which traversed the inhabited parts of the United States was 
 studied everywhere with telescopes, spectroscopes, and other 
 instruments of observation, so that the whole of this tract of 
 country became one vast observatory. Although the dura- 
 tion of totality was less than in the eclipse observed in In- 
 dia (1868), yet the phenomenon was attended on the whole 
 with many more favorable circumstances ; the heat was less 
 intense, the places suitable for observation were much more 
 conveniently situated, and the sun's altitude was not so great 
 as in the eclipse of 1868. The most important points of 
 investigation had reference to the scrutiny of the promi- 
 nences by means of photography and the spectroscope, the 
 examination of the nature of the corona, and the search for 
 planets between Mercury and the Sun. 
 
 The most complete expeditions were those sent out from 
 Washington, one from the Nautical Almanac Office, the as- 
 tronomical department being under the charge of Professor 
 Coffin, while the photographic arrangements were conducted 
 by Professor Henry Morton, of Philadelphia : another expe- 
 dition was despatched from the United States Naval Obser- 
 vatory, under the superintendence of Commodore B. F. 
 Sands. 
 
 The first expedition, under the guidance of Professor 
 Morton, selected stations in the State of Iowa, as follows : 
 
 i. Burlington, where the observers were Professor Mayer, 
 
 15 
 
110 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 and Messrs. Kendall, Willard, Phillipps, and Mahoney, to- 
 gether with Dr. C. A. Young, Professor of Dartmouth College 
 (Hanover), well known as an experienced spectroscopist, 
 and Dr. B. A. Gould, to whose charge the photographic 
 department was committed ; 
 
 2. Ottumwa, where Professor Hiines, and Messrs. Zent- 
 mayer, Moelling, Brown, and Baker, were stationed ; 
 
 3. Mount Pleasant, occupied by Professor Morton, and 
 Messrs. Wilson, Clifford, Cremer, Ranger, and Carbutt, as 
 well as by some other Professors, including Pickering, who 
 were desirous of making astronomical observations on the 
 physical phenomena of the eclipse. 
 
 Stations selected by the second expedition : 
 
 1. Des Moines (Iowa), where Professor Newcomb under- 
 took the observation of the corona and the search for in- 
 termercurial planets, Professor Harkness the spectroscopic 
 investigations, and Professor Eastman the meteorological 
 department. Several other gentlemen skilled in solar pho- 
 tography associated themselves with these observers. 
 
 2. Bristol (Tennessee), where Bard well, who undertook 
 the observation of the corona, and other observers were 
 stationed. 
 
 Besides these most important expeditions, furnished with 
 the most admirable and complete means of observation, sev- 
 eral scientific men were engaged at various points in the 
 zone of totality, either in observing the astronomical details 
 of the eclipse, or in investigating the prominences, the coro- 
 na, and their spectra. Among these may be mentioned Dr. 
 Edward Curtis, who at Des Moines obtained no fewer than 
 one hundred and nineteen pictures of the different phases of 
 the eclipse ; W. S. Oilman, by whom some most valuable 
 observations were instituted at St. Paul Junction (Iowa) 
 upon the connection between the solar spots, the faculaB, and 
 the prominences ; J. A. Whipple, who with Professor Win- 
 lock and several assistants procured at Shelbyville (Ken- 
 tucky) eighty photographic pictures, six of which were taken 
 
 16 
 
SCHKLLEN.] SPECTRUM ANALYSIS DISCOVERIES. Ill 
 
 during the totality, one of them exhibiting a complete and 
 magnificent corona ; as well as Professor G. W. Hough, 
 Director of the Dudley Observatory, who in company with 
 nine fellow-observers recorded all the details of the eclipse 
 at Mattoon (Illinois). 
 
 Out of the mass of materials afforded by the observations 
 of this eclipse, it will only come within our province to 
 communicate those results which have reference to the phys- 
 ical constitution of the sun, and were obtained partly by 
 photographic delineation, and partly by the help of the 
 spectroscope. The course of the eclipse and the photo- 
 graphic work carried on at Mount Pleasant, where, the total- 
 ity lasted two minutes, forty-eight seconds, is described by 
 Wilson nearly as follows : 
 
 " For some days prior to the eclipse, the sky was over- 
 cast, and threatened rain ; but the 7th of August was bright, 
 without a cloud, such a day as had not occurred for months, 
 and the sun shone with remarkable clearness and warmth. 
 The moment of first contact arrived ; the first plate was 
 already placed in the tube ; Professor Watson signalled to 
 us the moment for exposure by a motion of the hand ; the 
 instantaneous shutter was opened and closed, and the first 
 picture, was taken. We thus commenced a series of pictures 
 taken at intervals of five or ten minutes till the commence- 
 ment of totality, after which the series was continued on the 
 re-appearance of the sun till the termination of the eclipse. 
 Darkness came on with the totality, but not the darkness of 
 night ; still it rendered reading impossible. The amount of 
 light upon the landscape was scarcely equal to that of bright 
 moonlight, yet it was sufficient for us to pursue our work. 
 An instant before the commencement of totality, the thin 
 crescent of the sun was still quite dazzling ; then the light 
 went out as from an expiring candle. 
 
 " There, between heaven and earth, hung face to face the 
 two great luminaries, sun and moon, a large black round 
 spot encircled by a brilliant ring of deep gold-colored light, 
 
 '7 
 
112 RECREATIONS IN POPULAR SCIENCE. [SCHELLEM. 
 
 interrupted here and there by the brighter spots of the 
 flesh-colored prominences of irregular size and form, and 
 surrounded by the magnificent corona, which shot out rays 
 in every direction, faintest where the prominences were 
 most conspicuous, but enveloping the whole with a glory 
 which was marvellously beautiful, as if the Creator were 
 about to show His omnipotence in this wonder. The phe- 
 nomenon resembled a gigantic image from a magic lantern, 
 received upon the heavens as a screen. Four plates were 
 exposed, when suddenly the full significance of those words 
 was realized, ' Let there be light, and there was light,' for a 
 mighty flood of brilliant light gushed forth, like the rushing, 
 foaming waters of Niagara. The sun came forth like a 
 conqueror from a battle with the Titans, and was greeted 
 with acclamations by the assembled spectators." 
 
 A picture of this magnificent spectacle is given in Fig. VII., 
 showing the prominences and corona after a photograph by 
 Professor Eastman, which was taken at the commencement 
 of the totality. The instant the totality began, the corona 
 made its appearance as a light of silvery whiteness, with an 
 exceedingly tender flush of a greenish-violet hue at the ex- 
 treme edges, and not the slightest change was perceptible 
 during the totality in the color, the outline, or the position 
 of the rays an observation confirmed by Professor Hough 
 at Mattoon (Illinois), by Gill, and by several others. 
 
 The corona appeared to consist of two principal portions : 
 the inner one, next to the sun, was nearly annular, reaching 
 an elevation of about i', and in color of a pure silvery white- 
 ness ; the outer portion consisted of rays, some of which 
 grouped themselves into five star-like points, while the others 
 assumed the appearance of radiations, and were the most 
 sharply defined ; the corona was scarcely visible between the 
 prominences a and b. The star-like rays attained a height 
 equal to half the diameter of the sun. 
 
 The observations made by several astronomers in India 
 during the eclipse of 1868, and those made by many others 
 
 18 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 113 
 
 in America during that of 1869, and still later those by 
 Lockyer, Janssen, and others in and near the island of Cey- 
 lon, in December, 1871, fully justify the conclusion that, in- 
 dependently of the cosmical materials which must exist in 
 the neighborhood of the sun, there exists around this body 
 
 FIG. VII. 
 
 The Corona of the Eclipse of 7th August. 1869, at Des Moines. 
 
 an atmosphere very extensive and excessively rare, with 
 hydrogen for its basis. This atmosphere, which undoubt- 
 edly forms the outside gaseous envelope of the sun, is fed by 
 the material of the protuberances projected with such vio- 
 lence from the bowels of the photosphere, but is distin- 
 
 19 
 
114 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 guished from the chromosphere and the protuberances by 
 a* density enormously less, a lower temperature, and perhaps 
 by the presence of certain different gases. 
 
 The prominences are masses of luminous gas, principally 
 luminous hydrogen gas ; they envelop the entire surface of 
 the solar body, sometimes in a low stratum extending over 
 exceedingly large tracts of the sun's surface, sometimes in 
 accumulated masses rising at certain localities to a height 
 of more than 80,000 miles. 
 
 They are, as respects their first formation, phenomena of 
 eruption. The velocity with which the gaseous matter of 
 the prominences must pass the photosphere must be, in many 
 cases at least, two hundred miles per second, and its initial 
 velocity probably not less than three hundred miles per sec- 
 ond. Dense gaseous matter flung out with the hydrogen 
 would probably retain a velocity of, say two hundred and 
 forty miles per second, and reach a height exceeding that in- 
 dicated by the greatest extension of the radiations observed 
 last December. 
 
 The body of the sun, or its light-giving envelope, the pho- 
 tosphere, is completely surrounded by a gaseous envelope, in 
 which hydrogen constitutes the chief element, and which is 
 called the chromosphere. Its mean thickness is between 
 five thousand and seven thousand miles. 
 
 The prominences are local accumulations of the chromo- 
 sphere, and therefore pre-eminently of hydrogen gas, which 
 appear to break forth from time to time from the interior of 
 the sun in the form of monster eruptions, forcing their way 
 through the photosphere and chromosphere. As this gas, 
 on effecting a passage, rises with great rapidity, it becomes 
 quickly rarefied in a direction away from the sun's limb. 
 
 From the experiments undertaken by Lockyer, Frankland, 
 Wiillner, and Secchi, it appears that even in the lowest stra- 
 tum of this gaseous envelope, the pressure is smaller than 
 that of our atmosphere, therefore that the gas of the chro- 
 mosphere is in a state of greater attenuation. 
 
 20 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 115 
 
 Under the chromosphere lies the luminous cloud-like 
 vaporous or nebulous photosphere, which contains all the 
 substances, the spectrum lines of which appear as absorption 
 lines in the solar spectrum. These substances among 
 which iron, magnesium, and sodium are especially prom- 
 inent often burst forth in a state of incandescence, and are 
 carried up to a certain distance into the chromosphere, and 
 into the basis of the prominences, though not in general to 
 any considerable elevation. 
 
 It is probable that, owing to a continuous decrease in its 
 temperature and density, the chromosphere stretches out 
 into space to a distance far beyond our power of recognition. 
 
 MODES OF OBSERVING THE PROMINENCES IN SUNSHINE. 
 FORM OF THE PROMINENCES. 
 
 As early as 1866, Lockyer attempted to observe the prom- 
 inences in full sunshine by means of a Herschel-Browning 
 spectroscope placed in combination with a telescope. The 
 method he employed, and which he laid before the Royal 
 Society in a special communication, depends on the specific 
 difference between the light of the prominences and that of 
 the sun itself. 
 
 The light of an incandescent solid or liquid body which 
 passes through the slit of a spectroscope will be spread out 
 by the prism into a band of greater or less length, and form 
 a continuous spectrum. 
 
 The light of a gaseous or vaporous body will by the same 
 means, on the contrary, be decomposed into a few only, 
 sometimes even into a very few, bright lines. 
 
 In the first case, the greater the length of the spectrum, 
 the less will be its intensity in comparison with that of the 
 source of light ; in the second case, especially when the 
 spectrum consists only of a couple of lines, the intensity 
 of each line is little less than half that of the light itself. 
 
 If, therefore, an equal amount of light from two self 
 
 21 
 
Il6 RECREATIONS IN POPULAR SCIENCE. [SCHELLEX. 
 
 luminous bodies, one of which is solid or liquid, and the 
 other gaseous or vaporous, enter the slit of the spectroscope 
 at the same time, the bright lines of the latter will be more 
 brilliant than the color of the corresponding portion of the 
 continuous spectrum. 
 
 Now, by increasing the number of prisms, the continuous 
 spectrum may become so elongated, and consequently dimin- 
 ished in light, that the once brilliant solar spectrum may be 
 reduced to the verge of visibility, while the same amount of 
 dispersion produces on a spectrum of lines from glowing 
 gas only an increase in the distance between the lines, and 
 no considerable diminution of their brilliancy. 
 
 The reason why the prominences round the sun's limb 
 cannot be seen through a telescope at any time by screening 
 off the intense light of the sun, is owing to the extreme bril- 
 liancy with which the sun illuminates the earth's atmos- 
 phere, the particles of which scatter so large an amount of 
 light as quite to overpower the fainter light of the prom- 
 inences, and prevent them making any sensible impression 
 on the eye. 
 
 In a total eclipse of the sun, the light of this atmosphere 
 is so considerably reduced as to allow the larger prominences 
 beyond the limb of the sun to be observed by the unassisted 
 eye. The possibility of reducing the glare of sunlight at 
 any other time without extinguishing the light of the prom- 
 inences, rests on the circumstance already mentioned, that 
 the light of the sun consists of rays of every color, and 
 therefore produces in a spectroscope of highly dispersive 
 power a long and faint spectrum, while the light of the 
 prominences, consisting in general of only three or four 
 kinds of rays, remains even after the greatest dispersive 
 power still concentrated into the same number of lines 
 (H,H?,Hy,D 3 ). 
 
 It was on these principles, first announced by Lockyer, 
 that Janssen succeeded, the day after the eclipse of the i8th 
 of August, 1868, in observing the spectrum of the prom- 
 
 22 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 1 17 
 
 inences in sunshine. That the method he employed was no 
 other than that suggested by Lockyer, is evident from his 
 own communication to the French Academy, dated Calcutta, 
 the 3d of October, 1868, in which he expressed himself as fol- 
 lows : " The principle of the new method rests upon the 
 difference between the spectrum peculiarities of the light of 
 the prominences and that of the photosphere. The light of 
 the photosphere, which is derived from incandescent solid or 
 liquid particles, is incomparably stronger than that of the 
 prominences, which is derived from gases. On this account 
 it has been impossible hitherto to see the prominences, ex- 
 cept during a total solar eclipse. By the employment, how- 
 ever, of spectrum analysis, the circumstances of the case 
 may be reversed. In fact, by the process of analyzation, 
 the light of the sun is dispersed over the whole range of 
 the spectrum, and its intensity becomes considerably les- 
 sened. The prominences, on the contrary, furnish only a 
 few detached groups of rays which are bright enough to 
 bear comparison with the corresponding rays of the solar 
 spectrum. It is for this reason that the lines of the prom- 
 inences may be seen easily in the same field of the spectro- 
 scope with the solar spectrum, while the direct images of 
 the prominences are invisible on account of the overpower- 
 ing light of the sun. Another circumstance very favorable 
 to the new method of observation lies in the fact that the 
 bright lines of the prominences correspond with the dark 
 lines of the solar spectrum : they can, therefore, not only be 
 more easily recognized in the field of the spectroscope 
 along the edges of the solar spectrum, but also detected on 
 the solar spectrum itself, and their traces even followed on 
 the very surface of the sun." 
 
 As soon as Janssen and Lockyer had succeeded by this 
 method in observing the spectrum of the prominences inde- 
 pendently of a total eclipse, it became a question whether it 
 would not be possible not merely to see the lines of the 
 
 2 3 
 
IlS RECREATIONS IN POPULAR SCIENCE. [SCHELLEX. 
 
 prominences, but also to make their actual forms visible 
 during sunshine. 
 
 It was on the principles before mentioned that Lockyet 
 based his plan of observing the spectra of the prominences 
 in full sunlight by means of a telespectroscope (Fig. VIII.). 
 
 FIG. VIII. 
 
 LOCKYER'S TELESPECTROSCOPE. 
 
 For this purpose the slit of a highly dispersive spectroscope, 
 d c e /z, firmly attached by the rods a a b to an equatorially 
 mounted telescope L T P, driven by clockwork, is directed 
 perpendicularly on to the edge of the sun's image formed in 
 the telescope. By moving the tube e of the spectroscope 
 
 24 
 
SCHBLLKM.] SPECTRUM ANALYSIS DISCOVERIES. 119 
 
 from end to end of the spectrum, and setting the focus each 
 time, the bright lines of the prominences may be seen as 
 prolongations of the dark lines of the spectrum of the sun's 
 disk on a background of the exceedingly faint spectrum of 
 the earth's atmosphere. In the picture, S is the finder, g a 
 handle for moving the telescope in declination, d the tube 
 containing the slit, h a small telescope for reading the divis- 
 ions on the micrometer screw head, partly concealed by the 
 rod a a. 
 
 The telespectroscope is furnished with seven prisms, and 
 it confirmed, after a few trials, the correctness of this view, 
 and he was the first to succeed, without additional mechan- 
 ical help or the use of colored glasses, in observing the 
 prominences at any time when the sun was visible, and 
 tracing their complete outline. 
 
 By the same means Zollner saw the prominences for the 
 first time on the ist of July, 1869. He has published the 
 results of his observations, and accompanied them by a 
 series of highly interesting drawings of some of the larger 
 prominences, in which their origin, development, and subse- 
 quent disappearance are very clearly exhibited. 
 
 When the spectrum of the earth's atmosphere has disap- 
 peared in consequence of the powerful dispersion of the 
 light, and the portion of the prominence then in the field of 
 view alone is visible through the widely opened slit, the tel- 
 escope or slit is moved slowly forward, and luminous images 
 of the most wonderful forms flit before the eye, being just as 
 easily observed as during a total solar eclipse. In describing 
 some of these shadow forms, Lockyer writes, u Here one is 
 reminded, by the fleecy, infinitely delicate cloud-films, of an 
 English hedgerow with luxuriant elms ; here of a densely 
 intertwined tropical forest, the intimately interwoven branches 
 threading in all directions, the prominences generally ex- 
 panding as they mount upward, and changing slowly, 
 indeed almost imperceptibly. ... As a rule, the attach- 
 ment to the chromosphere is narrow, and is not often. 
 
 25 
 
120 RECREATIONS IN POPULAR SCIENCE. [SCHKLLEN. 
 
 single ; higher up, the stems, so to speak, intertwine, and 
 the prominence expands and soars upward until it is lost 
 in delicate filaments, which are carried away in floating 
 masses." 
 
 The various forms of the prominences may be classified 
 generally into two characteristic groups, very aptly desig- 
 nated by Zollner as vaporous or cloud-like forms, and erup- 
 tive forms. 
 
 Slight changes in the form of the prominences may be 
 watched almost without intermission with an open slit ; 
 great changes, as a rule, take place only very slowly, or 
 quite imperceptibly. In .some cases, however, the change 
 in the form of a prominence is so extraordinary, and occurs 
 with such rapidity, that it can only be ascribed to extremely 
 violent agitation in the upper portions of the solar atmos- 
 phere, compared with which the cyclonic storms occasion- 
 ally agitating the earth's atmosphere sink into insignificance. 
 The observation of such a solar storm has been thus de- 
 scribed by Lockyer : 
 
 "On the I4th of March, 1869, about 9 h. 45m., with a 
 slit tangential to the sun's limb instead of radial, which was 
 its usual position^ I observed a fine dense prominence near 
 the sun's equator, on the eastern limb, in which intense ac- 
 tion was evidently taking place. At 10 h. 50 m., when the 
 action was slackening, I opened the slit ; I saw at once that 
 the dense appearance had all disappeared, and cloud-like 
 filaments had taken its place. At 1 1 h. 5 m., it was about 
 twenty-seven thousand miles high, and the portion on the 
 left resembled a straight column in a slightly leaning posi- 
 tion. I left the Observatory for a few minutes, and on re- 
 turning, at ii h. 15 m., I was astonished to find that part of 
 the straight prominence had entirely disappeared ; not even 
 the slightest rack appeared in its place : whether it was en- 
 tirely dissipated, or whether parts of it had been wafted 
 towards the other part, I do not know, although I think the 
 latter explanation the more probable one, as the other part 
 
 26 
 
SCHELLKN.] SPECTRUM ANALYSIS DISCOVERIES. 
 
 121 
 
 had increased. Fig. IX. shows the prominence as it ap- 
 peared at the last observation. 
 
 FIG. IX. 
 
 STORM OBSERVED BY LOCKYER ON THE 14th MARCH, 1859. 
 
 Professor C. A. Young, of Dartmouth College, Hanover, 
 N. II., has devoted himself especially to the observation of 
 the forms and variability of the prominences. In the col- 
 ored plate accompanying this paper are represented some 
 of the forms and the color of the prominences. The one 
 here shown is of the vaporous or cloud-like form, and was 
 observed by Professor Young, October 7, 1869. 
 
 The change in the form between the first and second ob- 
 servation will serve to illustrate the motions of the hydrogen 
 flames, and will give the reader some slight idea of the dis- 
 turbances which are constantly occurring on the surface of 
 the sun. By means of the accompanying scale their height 
 
 27 
 
122 RECREATIONS IN POPULAR SCIENCE. [SCHELLKM 
 
 can be easily ascertained. Professor Young has since ob 
 served others of a much greater height and magnitude. 
 
 As the meteorologist registers many times in a day the 
 conditions of our atmosphere, in the hope that a comparison 
 of the observations may lead to a discovery of the law gov- 
 erning these changes, so has Respighi, Director of the Uni- 
 versity Observatory at the Campidoglio at Rome, made it 
 his daily task since October, 1869, to observe the entire limb 
 of the sun, when the weather was favorable, including the 
 chromosphere and prominences, and to mark upon a straight 
 line representing the circumference of the sun the position, 
 height, and form of the prominences for each day. By col- 
 lating these lines, or circumferences of the sun one bdow the 
 other, and crossing them with lines indicating the principal 
 positions, a comprehensive picture is afforded of the distri- 
 bution of the prominences round the sun's limb, which 
 shows at a glance those regions in which the prominences 
 abound, and those in which they are least frequently to be 
 met with. 
 
 By a comparison of the maps already constructed, Res- 
 pighi has arrived at the following results : 
 
 1. In the polar regions prominences occur only excep- 
 tionally. The district from which they are absent lies be- 
 tween north and north-east on the one side, and south and 
 south-west on the other ; the portion which is almost entirely 
 without prominences has a semi-diameter of 22^. 
 
 2. The district where the prominences most frequently 
 occur lies between north and north-west, at about 45 north 
 latitude, in a region where solar spots are rarely seen. 
 
 3. The prominences are, therefore, phenomena quite dis- 
 tinct from the spots ; they are probably more intimately con- 
 nected with the formation of faculcc. 
 
 4. The various forms of the prominences show that they 
 are not of the nature of clouds, which float in an atmosphere 
 in which they are produced by local condensations ; they 
 are much more like eruptions out of the chromosphere, 
 
 28 
 
Solar Prominence observed ~byYoun 
 1869-Oor 7 
 
 Time Oct. 7. 2* 
 
 ,*/*> INS L,rn CO IOS.SV** 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 123 
 
 which often spread out of the higher regions, and take the 
 form of bouquets of flowers, some being bent over on one 
 side, and some on the other, and which fall again on to the 
 surface of the chromosphere as rapidly as they rose from it. 
 
 5. It appears that eruptions of hydrogen take place from 
 the interior of the sun ; their form and the extreme rapidity 
 of their motion necessitates the hypothesis of a repulsive 
 power, at work either at the surface or in the mass of the 
 sun, which Respighi attributes to electricity, but Faye simply 
 to the action of the intense heat of the photosphere. 
 
 On the 28th of September, 1870, Professor Young suc- 
 ceeded for the first time in photographing the prominences 
 on the sun's limb in bright sunshine. This he effected by 
 bringing the blue hydrogen line H y near G into the middle 
 of the field of the spectroscope, and placing a small photo- 
 graphic camera in connection with the eye-piece of the tel- 
 escope. As the chemicals employed were those ordinarily 
 used in taking portraits, the requisite time of exposure was 
 three and one half minutes, during which time the image of 
 the prominence suffered a slight displacement on the pre- 
 pared plate, owing to a want of accuracy in the perfect ad- 
 justment of the polar axis. Still, however, the various 
 forms of the prominences could be clearly discerned in the 
 photograph, which was half an inch in diameter, so that the 
 possibility of photographing the prominences has been 
 proved by Young's experiment. 
 
 MEASUREMENT OF THE DIRECTION AND SPEED OF THE 
 GAS-STREAMS IN THE SUN. 
 
 One of the most glorious triumphs of spectrum analysis 
 surpassing, perhaps, in splendor all its other wonderful 
 achievements is the discovery that, by means of accurate 
 measurements, undertaken with the best instruments, of the 
 position, or rather of the small displacement in the position 
 of the spectrum lines of a star or other source of light, a 
 
 29 
 
124 RECREATIONS IN POPULAR SCIENCE. [SCHBLLEN. 
 
 prominence- for instance, it is possible to ascertain whethei 
 this luminous body be approaching us or receding from us, 
 and at what speed it is travelling. 
 
 The pitch of a musical tone depends, as is well known, 
 upon the number of impulses which the ear receives from 
 the air in a given time (p. 74). Now, as a tone rises in pitch 
 the greater the number of air vibrations which strike the 
 tympanum in a second, so must a sound ascend in tone if we 
 rapidly approach it, and fall in pitch if we recede from it. 
 The truth of this supposition may be fully proved by the 
 whistle of a railway engine in rapid motion. To an ob- 
 server standing still, the pitch of the tone rises on the rapid 
 approach of the locomotive, although the same note is 
 sounded, and falls again as the engine travels away. 
 
 As the various tones of sound depend on the rapidity of 
 the air vibrations, so the varieties of color are regulated by 
 the number of ether vibrations (p. 77)- If> therefore, a lu- 
 minous object as, for instance, the glowing hydrogen of a 
 prominence be receding rapidly from us, fewer waves of 
 ether will strike the optic nerve in a second than if it were 
 stationary. If the difference in the number of ether waves 
 be sufficiently great to be perceived by the eye, then each 
 color of the glowing gas must sink in the scale of the spec- 
 trum, that is to say, incline more towards the red. The 
 individual colored rays will not then, in the prismatic de- 
 composition of the light, occur in the same place of the 
 spectrum in which they would have appeared had the light 
 been stationary ; they will all be displaced somewhat 
 towards the red. 
 
 The converse takes place when the luminous body is rap- 
 idly approaching us : the number of ether vibrations re- 
 ceived by the eye is then increased beyond what it would be 
 if the source of light were stationary ; in the prismatic anal- 
 ysis of the light, the colored rays will be found likewise to 
 have changed their place in the scale of the spectrum, and 
 taken a position in accordance with their increased re- 
 
 30 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 
 
 I2 5 
 
 frangibility, suffering a general displacement towards the 
 violet. 
 
 When it is remembered that the number of ether waves in 
 red light is at least 480 billion, and in violet 800 billion in a 
 second, and that moreover the wave length of the greenish- 
 blue light (H ), situated at the spot marked F in the solar 
 spectrum, is only 485 millionth of a millimetre, and that 
 instruments of sufficient delicacy to measure these minute 
 quantities are required for this purpose, there will be little 
 danger of under-estimating the extreme difficulty connected 
 with observations of this displacement in the colors of the 
 spectrum. Indeed, these observations would scarcely be 
 possible, were it not that in the dark lines crossing the spec- 
 tra of the sun and fixed stars, the places of some of which 
 may be accurately ascertained, we have fixed positions in 
 the spectrum, the degree of refrangibility or wave-length of 
 which may be determined beforehand, both for the sun and 
 terrestrial substances, and also for the stars or other sources 
 of light supposed to be at rest. 
 
 Fig. X., which is from a drawing by Lockyer, shows 
 clearly what remarkable changes take place in the dark line 
 
 FIG. X. 
 
 Displacement of the F-line ; Velocity of the Gas-streams in the Sun. 
 
 F when the spectroscope is directed to a solar spot in the 
 middle of the sun. The F-line, which, as a rule, is sharply 
 defined at the edges,, appears in some places not merely as a 
 
 3 1 
 
126 RECREATIONS IN POPULAR SCIENCE. [SCHELLBN. 
 
 bright line, but as a bright and dark line twisted together, in 
 which parts it suffers the greatest displacement towards the 
 red. When this occurs, there is frequently also a bright line 
 to be seen on the violet side. In small solar spots, this line 
 sometimes breaks off suddenly, or spreads out immediately 
 before its termination in a globular form ; over the bright 
 faculae of a spot (the bridges) the line is often altogether 
 wanting, or else it is reversed, and appears as a bright line. 
 
 The same phenomena are exhibited also by the red C-line 
 (H), though as the greenish-blue F-line (H @) is by an 
 equal increase of pressure much more sensitive with regard 
 to expansion than the red line is, and exhibits with greater 
 distinctness the changes that have been already described, it 
 is better adapted to observations of this kind. 
 
 All these expansions, twistings, and displacements of the 
 F-line result from a change in the wave-length of the green- 
 ish-blue light emitted by the moving masses of incandescent 
 hydrogen gas in the sun. The middle of this line, when it 
 is well defined, corresponds to a wave-length of 485 mil- 
 lionth of a millimetre, yet it is possible, by means of Ang- 
 strom's maps of the solar spectrum, to measure a displace- 
 ment of this line when the wave-length has only changed as 
 much, as TUMnrVznjiy of a millimetre, and, inversely, it is also 
 possible to read off at once, by the measured displacement 
 of the F-line, the corresponding amount which the wave- 
 length of the greenish-blue hydrogen light has lengthened or 
 shortened, to ten millionth of a millimetre. In observing a 
 prominence, if the F-line were to be displaced from its nor- 
 mal place in the solar spectrum iwrfuwu f a millimetre, 
 the light would be approaching the eye of the observer, and 
 an eruption of gas be ascending at the spot observed in the 
 middle of the sun, and would be approaching the earth at 
 the rate of thirty-six miles per second. 
 
 If the F-line were to suffer an equal displacement to the 
 left, that is to say, towards the red, the wave-length of the 
 greenish-blue hydrogen light would then be lengthened ; the 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 
 
 127 
 
 gas would therefore be moving away from the earth at the 
 same rate of thirty-six miles in a second, and the stream of 
 gas be sinking down to the surface of the sun. 
 
 Fig. XL shows the displacement which occurs when a 
 cyclone takes place on the sun's surface. Such a circular 
 storm or cyclone was observed by Lockyer on the sun's 
 limb, on the I4th of March, 1869. With the first setting ot 
 
 FIG. XL 
 
 MOVEMENT OF A GAS-VORTEX IN THE SUN. 
 
 the slit, the image of the bright F-line (H |?) in the chromo- 
 sphere appeared in the spectroscope, as in No. I ; a slight 
 alteration of the slit gave in succession the pictures 2 and 3. 
 There occurred also a simultaneous displacement of the 
 bright F-line towards both the red and violet a sign that 
 at that place on the sun a portion of the hydrogen was mov- 
 ing towards the earth, while another portion was going in 
 an opposite direction away from the earth towards the sun, 
 and thus the whole action of the gas in motion resembled 
 that of a whirlwind. 
 
 By means of the distances from the normal dark F-line 
 which are taken from Angstrom's maps, Lockyer found that 
 the furthest displacement of the bright F-line corresponded 
 to a shortening of the wavelength that indicated a velocity 
 in the stream of gas of at least one hundred and forty-seven 
 
 33 
 
128 RECREATIONS IN POPULAR SCIENCE. [SCHEME*. 
 
 miles in a second in the direction from the sun towards the 
 earth. 
 
 These spectroscopic observations receive an additional 
 interest when taken in connection with those made with the 
 telescope. On the 2ist of April, 1869, Lockyer observed a 
 spot in the neighborhood of the sun's limb. At 7 h. 30 m., a 
 prominence showing great activity appeared in the field of 
 view. The lines of hydrogen were remarkably brilliant, 
 and as the spectrum of the spot was visible in the same 
 field, it could be seen that the prominence was advancing 
 towards the spot. The violence of the eruption was so 
 great as to carry up a quantity of metallic vapors out of the 
 photosphere in a manner not previously observed. High up 
 in the flame of hydrogen floated a cloud of magnesium va- 
 por. At 8 h. 30 m. the eruption was over ; but an hour 
 later another eruption began, and the new prominence dis- 
 played a motion of extreme rapidity. Whilst this was tak- 
 ing place, the hydrogen lines at the side of the spot nearest 
 to the earth were suddenly changed into bright lines, and 
 expanded so remarkably as to give undoubted evidence of 
 the occurrence of a cyclonic storm. 
 
 The sun was photographed at Kew on the same day at 
 10 h. 55 m. ; the picture showed clearly that great disturb- 
 ances had taken place in the photosphere in the neighbor- 
 hood of the spot observed by Lockyer. In a second photo- 
 graph, taken at 4 h. i m., the sun's limb appeared as if torn 
 away just at the place where the spectroscope had revealed 
 a rotatory storm. 
 
 Who could have dreamed, ten years ago, that we should 
 so soon attain such an insight into the processes of creation? 
 And yet, great though the results of spectrum analysis al- 
 ready are, they are but a tithe of the numerous questions 
 which this branch of discovery has opened up, questions 
 of such number and magnitude, that many generations of 
 men will pass away before they are all satisfactorily an- 
 swered. 
 
 34 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 129 
 
 SPECTRA OF THE MOON AND PLANETS. 
 
 Since the planets and their satellites do not emit light of 
 their own, but shine only by the reflected light of the 
 sun, their spectra are the same as the solar spectrum, and 
 any differences that may be perceived can arise only from 
 the changes the sunlight may undergo by reflection from the 
 surfaces of these bodies, or by its passage through their 
 atmospheres. 
 
 The observations of Huggins and Miller, as well as Jans- 
 sen, agree in establishing the complete accordance of the 
 lunar spectrum with that of the sun. In all the various por- 
 tions of the moon's disk brought under observation, no dif- 
 ference could be perceived in the dark lines of the spectrum, 
 either in respect of their number or relative intensity. From 
 this entire absence of any special absorption lines, it must be 
 concluded that there is no atmosphere in the moon, a con- 
 clusion previously arrived at from the circumstance that 
 during an occultation no refraction is perceived on the 
 moon's limb when a star disappears behind the disk. 
 
 The spectra of the planets Venus, Mars, Jupiter, and 
 Saturn are also characterized by the Fraunhofer lines pe- 
 culiar to the solar light, but contain, in addition, the absorp- 
 tion lines which are known to be telluric lines, and are evi- 
 dence of the presence of an atmosphere containing aqueous 
 vapor. 
 
 The spectrum of Jupiter, which has been recently exam- 
 ined by Browning with a spectroscope attached to his 1 2- 
 inch reflector, is not of sufficient brilliancy to allow of its 
 being observed or measured with extreme accuracy. Not- 
 withstanding the great brilliancy with which this planet 
 shines in the heavens, its spectrum is not so bright as that 
 of a star of the second magnitude ; this is owing to the 
 brightness being more apparent than real, and arises from 
 the large size of the disk compared with a star, and from 
 the light being reflected, and not original. 
 
 35 
 
130 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 The comparatively faint spectrum of Saturn has been ex- 
 amined by Huggins, who observed in it some of the lines 
 characteristic of Jupiter's spectrum. These lines are less 
 clearly seen in the light of the ring than in that of the ball, 
 whence it may be concluded that the light from the ring 
 suffers less absorption than does the light from the planet 
 itself. The observations of Janssen, which have been sup- 
 ported by Secchi, have since shown that aqueous vapor is 
 probably present both in Jupiter and Saturn. Secchi has 
 further discovered some lines in the spectrum of Saturn 
 which are not coincident with any of the telluric lines, nor 
 with any of the lines of the solar spectrum produced by the 
 aqueous vapor of the earth's atmosphere. It is not improb- 
 able, therefore, that the atmosphere of Saturn may contain 
 gases or vapors which do not exist in that of our earth. 
 
 The spectrum of Uranus, which has been investigated 
 by Secchi, appears to be of a very remarkable character. 
 It consists mainly of two broad black bands, one in the 
 greenish-blue, but not coincident with the F-line, and the 
 other in the green, near the line E. A little beyond the 
 band the spectrum disappears altogether, and shows a 
 blank space extending entirely over the yellow to the red, 
 where there is again a faint re-appearance of light. The 
 spectrum is therefore such a one as would be produced were 
 all the yellow rays extinguished from the light of the sun. 
 The dark sodium line D occurs, as is well known, in the 
 part of the spectrum occupied by this broad, non-luminous 
 space : is this extraordinary phenomenon, therefore, to be 
 ascribed to the influence of this metal, or is the planet Ura- 
 nus, which has a spectrum differing so greatly from that of 
 the sun, self-luminous? Has the planet not yet attained that 
 degree of consistency possessed by the nearer planets, which 
 shine only by the sun's light, and, as the photometric obser- 
 vations of Zollner lead us to suppose is possible, is still in 
 that process of condensation and subsequent development 
 through which the earth has already passed? These are 
 
 36 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 131 
 
 questions to which, at present, we can furnish no reply, and 
 the problem can only be solved by additional observations 
 of the strange characteristics exhibited by this spectrum. 
 
 While Jupiter and his satellites, with a power of 350, give 
 a sharply defined image, the disk of Neptune, with the same 
 power, ceases to be well defined, and appears with a neb- 
 ulous edge. From this it may be inferred that the planet is 
 surrounded by a dense mist of considerable extent, the 
 chemical nature of which has yet to be discovered, or else 
 that, like Jupiter, Saturn, and Uranus, it has not yet attained 
 that degree of density which must necessarily precede the 
 formation of a solid surface. 
 
 SPECTRA OF THE FIXED STARS. 
 
 The fixed stars, though immensely more remote, and less 
 conspicuous in brightness than the moon and planets, yet 
 from the fact of their being original sources of light, fur- 
 nish us with fuller indications of their nature. In all ages, 
 and among every people, the stars have been the object of 
 admiring wonder, and not unfrequently of superstitious ado- 
 ration. The greatest investigators and the deepest thinkers 
 who have devoted themselves to the study of the stars, have 
 felt a longing to know more of these sparkling mysteries. 
 
 The telescope has been appealed to, but in vain, for in the 
 largest instruments the stars remained diskless, never ap- 
 pearing more than as brilliant points. The stars have indeed 
 been represented as suns, each surrounded by a dependent 
 group of planets, but this opinion rested only upon a possible 
 analogy, for of the peculiar nature of these points of light, 
 and of what substances they are composed, the telescope 
 yields us no information. Spectrum analysis alone can dis- 
 close to us this much-coveted knowledge, as it gives us the 
 means of reading, in the light emitted by these heavenly 
 bodies, the indications of their true nature and physical con- 
 stitution. In this light we possess a telegraphic communi- 
 
 37 
 
132 RECREATIONS IN POPULAR SCIENCE. [SCHELLKN. 
 
 cation between the stars and our earth ; the spectroscope is 
 the telegraph, the spectrum lines are individually the letters 
 of the alphabet, their united assemblage as a spectrum forms 
 the telegram. It is not, however, easy to comprehend this 
 language of the stars, but through the indefatigable labors 
 of Secchi, Huggins, and Miller, most of the bright stars, the 
 nebula?, and some of the comets have been investigated by 
 spectrum analysis, and valuable evidence obtained as to their 
 physical constitution. 
 
 As the spectra of the stars bear in general a marked re- 
 semblance to the spectrum of the sun, being continuous, and 
 crossed by dark lines, there is every reason for applying 
 KirchhofF's theory also to the fixed stars, and for accepting 
 the same explanation of these similar phenomena that we 
 have already accepted for the sun. By the supposition that 
 the vaporous incandescent photosphere of a star contains or 
 is surrounded by heated vapors, which absorb the same rays 
 of light which they would emit when self-luminous, we may 
 discover from the dark lines in the stellar spectra the sub- 
 stances which are contained in the photosphere or atmos- 
 phere of each star. In order to ascertain this with certainty, 
 the dark lines must be compared with the bright lines of 
 terrestrial substances volatilized in the electric spark ; and 
 the complete coincidence of the characteristic bright lines 
 of a terrestrial substance with the same number of dark 
 lines in the stellar spectrum, would justify the conclusion 
 that this substance is present in the atmosphere of the star 
 a conclusion that gains all the more in certainty, the 
 greater the number of lines coincident in the two spectra. 
 
 The fact that certain stars possess an atmosphere of aque- 
 ous vapor has been observed both byjanssen and Secchi. 
 They belong, for the most part, to the class of red and yel- 
 low stars, and in their spectra, as might be supposed, the 
 lines of luminous hydrogen are wanting. As early as 1864, 
 Janssen had remarked the existence of an atmosphere of 
 aqueous vapor in the star Antares ; and after a more com- 
 
 33 
 
SPECTRUM ANALYSIS DISCOVERIES. 133 
 
 plete investigation of the spectrum of steam in 1866, and 
 further observations of stellar spectra, made after the total 
 solar eclipse of 1868 in the remarkably dry air of the heights 
 of Sikkim (Himalaya), he could no longer doubt that 
 there are many stars surrounded by a similar atmosphere. 
 Notwithstanding the dry condition of the air, the lines of 
 aqueous vapor were more strongly marked in the spectra of 
 these stars, as seen from the heights of the Himalaya, than 
 had been observed previously a phenomenon which can- 
 not be ascribed to the absorption of the earth's atmosphere, 
 and must therefore be due to that of the star. 
 
 From all the observations thus far made, it may be con- 
 cluded that at least the brightest stars have a physical consti- 
 tution similar to that of our sun. Their light radiates, like 
 that of the sun, from matter in a state of intense incandes- 
 cence, and passes, in like manner, through an atmosphere 
 of absorptive vapors. Notwithstanding this general con- 
 formity of structure, there is yet a great difference in the 
 constitution of individual stars ; the grouping of the various 
 elements- is peculiar and characteristic for each star, and we 
 must suppose that even these individual peculiarities- are in 
 necessary accordance with the special object of the star's 
 existence, and its adaptation to the animal life of the plan- 
 etary worlds by which it is surrounded. 
 
 COLOR OF THE STARS. DOUBLE STARS AXD THEIR 
 SPECTRA. 
 
 In a transparent atmosphere, especially in a southern 
 clime, the stars do not all appear with the white brilliancy 
 of the diamond : here and there the eye discovers richly- 
 colored gems, sparkling on the sombre robe of night, in 
 every shade of red, green, blue, and violet; and the as- 
 tronomer, enabled by his powerful telescope to investigate 
 the faintest objects, is lost in wonder over the variety of 
 these colors, and their remarkable distribution in the starry 
 
 39 
 
134 RECREATIONS IN POPULAR SCIENCE. [SCHELLBN. 
 
 heavens. This play of color is most conspicuous in the 
 double stars, so called from their consisting of two or more 
 suns, kept together by the bond of mutual attraction, and 
 revolving in orbits according to their mass, either one around 
 the other or both round a common centre of gravity. To 
 the naked eye their appearance is that of a single star, on 
 account of their close proximity ; but on the application of 
 sufficient magnifying power, they are found to be constituted 
 of three, four, or more suns in intimate connection : such a 
 system is to be found in the beautiful constellation of Orion 
 (in the Sword), consisting of sixteen stars, where to the un- 
 assisted eye there seems but one. In several of these double 
 stars, the number of which already exceeds six thousand, it 
 has been possible to calculate the time of revolution of the 
 small star. The period of one in the Great Bear has been 
 found to be sixty years ; of another, in Virgo, five hundred 
 and thirteen years ; and of y Leonis twelve hundred years. 
 
 A peculiar interest attaches to double stars from their 
 great diversity of color, which occasioned Sir John Herschel 
 to remark, in describing a cluster in the Southern Cross, that 
 it resembled a splendid ornament composed of the richest 
 jewels. While the majority of single stars shine with a 
 white light, but sometimes with a yellow, and even occasion- 
 ally with a red hue, in double stars the companion is almost 
 always blue, green, or red, thus contrasting with the white 
 light of the larger or central star. 
 
 It has long been a subject of inquiry whence these colors 
 arise. It has been supposed that they were complementary 
 colors, and therefore that they were not inherent in the stars, 
 but dependent on an optical illusion similar to that produced 
 by looking upon a white wall immediately after gazing at 
 the sun, when the wall appears covered with violet spots. 
 But the simple expedient of covering the central star in the 
 telescope suffices to show the incorrectness of this supposi- 
 tion : for the color of the small star remains unaffected by 
 its separation from the light of the larger one. Zollner, to 
 
 40 
 
SCHBLLEN.] SPECTRUM ANALYSIS DISCOVERIES. 135 
 
 whom we are indebted for a masterly work on light and 
 the physical constitution of the heavenly bodies, was the first 
 to express the idea that, as all known substances, in their 
 transition from a state of incandescence to that of a lower 
 temperature, pass through the stage of red heat, so the fixed 
 stars, in their process of development from the condition of 
 glowing gas through the period of an incandescent liquid 
 state, and the subsequent development of floating scoriaB, or 
 gradual formation of a cold, non-luminous surface, must, 
 together with the gradual diminution of their light, be also 
 subject to a change of color. For many colored stars, espe- 
 cially for the so-called new stars, in which the color has 
 been known to sink in the scale from white to yellow and to 
 red, this conjecture of Zollner's has a high degree of prob- 
 ability ; but that other circumstances must exercise an influ- 
 ence also on the color of stars, is proved by a change of 
 color having been observed to take place in the opposite 
 direction, that is, from red to white, of which, among 
 other stars, we have an example in Sirius, regarded by the 
 ancients as a red star, and which is now considered as a type 
 of the white stars, as well as in Capella, which formerly 
 was red, and now shines with a pale blue light. Huggins 
 and Miller have discovered, by means of the spectroscope, 
 that the color of a star not only depends upon the degree of 
 incandescence of the intensely hot liquid or solid nucleus, 
 but also upon the kind of absorptive power its atmosphere 
 may exert upon the light emitted by the glowing nucleus. 
 
 As the source of stellar light, remarks Huggins, is incan- 
 descent solid or liquid matter, it appears very probable that 
 at the time of its emission the light of all stars is alike 
 white. , The colors in which we see them must 'therefore be 
 produced by certain changes which the light has undergone 
 since its emission. It is further obvious that if the dark ab- 
 sorption lines are more numerous, or more strongly marked 
 in some parts of the spectrum than in others, then the pe- 
 culiar colors of those places will be subdued in tone, and in 
 
 4 1 
 
136 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 any case will appear relatively weaker than in those parts 
 of the spectrum where the absorption lines are much less 
 numerous. While in this way certain colors would be par- 
 tially extinguished from the spectrum, the remaining colors, 
 being unaffected, would predominate, and give their own 
 tints to the originally white light of the star. 
 
 The colors of the stars are, therefore, without doubt pro- 
 duced by the vapors of certain substances contained in their 
 atmosphere ; and as the chemical constitution of the atmos- 
 phere of a star depends upon the elements of which the star 
 itself is composed, and upon its temperature, it would be 
 possible to ascertain the chief constituents of these small 
 telescopic worlds, if the position of the dark absorption lines 
 could be determined with accuracy, or if these lines could 
 be compared with the spectrum lines of terrestrial elements. 
 
 VARIABLE STARS. 
 
 Among the fixed stars there are several which vary, from 
 time to time, in brightness, as compared with neighboring 
 stars ; their light increases or diminishes, and alternates, in 
 some cases, from the brilliancy even of a star of the first 
 magnitude to complete invisibility. In some, this change of 
 brightness takes place as a constant, very slow and regular 
 diminution of light ; in others, there appears an almost sud- 
 den increase and decrease of brilliancy ; while with others, 
 again, the change takes place within regularly recurring 
 periods. The period of variability is, therefore, the time 
 elapsing between the two successive seasons of greatest 
 brilliancy. 
 
 Of all variable stars, Mira Ceti is perhaps the most inter- 
 esting, since, at its maximum brightness, it equals a star of 
 the first or second magnitude. Scarcely less interesting is 
 (9 Persei, which, for two days thirteen hours and a half, 
 shines with the brightness of a star of the second magnitude, 
 then suddenly decreases in light, and sinks down, in three 
 
 42 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 137 
 
 hours and a half, to a star of the fourth magnitude ; its light 
 then again increases, and in a similar period of three hours 
 and a half regains its original brilliancy. All these changes 
 recur regularly in the space of less than three days, during 
 which the star always remains visible to the naked eye. 
 
 Whence comes this variation in the light of a star? Zoll- 
 ner, with great acuteness, and supported by numerous obser- 
 vations of these changes of brightness, offers a simple and 
 unconstrained explanation, in supposing the cause to lie in 
 the configuration and distribution of dark masses of scoriae, 
 which form on the red-hot liquid body of the star in the 
 process of cooling, and which, in consequence of the star's 
 rotation on its axis, and the centrifugal force thus arising, 
 would take certain definite courses on the surface of the star 
 in a manner analogous to that which may be observed with 
 floating icebergs on our earth. As a consequence of this 
 peculiar relative motion, the dark masses of scorias would 
 arrange themselves in a fixed order, and would produce 
 on the surface of the star an unequal distribution of red- 
 hot luminous matter, and accumulations of non-luminous 
 scoriaB. 
 
 It has recently been remarked by Secchi that the spectrum 
 of the nucleus of a solar spot bears a close resemblance to 
 that given by several red stars, such as Orionis, Antares, 
 Aldebaran, o Ceti. A series of dark bands and stripes, as 
 represented in the spectrum of Orionis, are present equally 
 in the spectrum of a solar spot as in the spectra of the above 
 named red stars, which leads to the supposition that the red 
 color of these stars arises from the same cause that produces 
 the absorption bands in the spectrum of the solar spot. As 
 nearly all these stars are variable, it is not improbable that 
 they are also subject to spots which occur with a certain de- 
 gree of regularity, as the solar spots have been proved to do. 
 The period of variability in the light would then depend 
 upon the period of the formation of the spots, in the same 
 way as our sun appears as a variable star, of which the 
 
 43 
 
138 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 period of variation in the light coincides with the regular 
 recurrence of the spots. 
 
 NEW OR TEMPORARY STARS. 
 
 Among the variable stars must also be reckoned those 
 which, from time to time, but only at exceedingly long in- 
 tervals, have suddenly flamed forth in the sky and disap- 
 peared again after a longer or shorter interval, and which 
 always excite the greatest wonder and interest, not only from 
 the rarity of their appearance, but also from the mighty rev- 
 olutions in space which they announce. According to Hum- 
 boldt, only twenty-one such stars have been recorded in the 
 space of two thousand years, from 134 B. C. to 1848 A. D., 
 the most remarkable of which was that observed by Ty- 
 cho Brahe (1572) in Cassiopeia?, which surpassed both Sirius 
 and Jupiter, and even rivalled Venus in brilliancy, but dis- 
 appeared after seventeen months, without leaving a trace 
 visible to the naked eye;* and that seen by Kepler (1604) 
 in the right foot of Ophiuchus, which excelled Jupiter, but 
 did not quite equal Venus, in brightness, and at the end of 
 fifteen months was visible only by means of the telescope. 
 Two similar stars, which have appeared in recent times, 
 one observed by Hind, in 1848, and another seen in the 
 Northern Crown, in 1866, though they soon lost their 
 ephemeral glory, still continue visible as stars of the tenth 
 and ninth magnitude. A characteristic peculiarity of these 
 temporary stars is, that they nearly all flash out at once with 
 a degree of brilliancy exceeding, in some cases, even stars 
 of the first magnitude, and that they have not been ob- 
 served, at least with the naked eye, to increase gradually in 
 brightness. 
 
 Are we to suppose that these so-called new stars are really 
 
 * The telescope was not invented until thirty-seven years aftef 
 this date. 
 
 44 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES'. 139 
 
 new creations, as Tycho Brahe believed, and that those that 
 have disappeared are really annihilated or burned out? Can 
 we suppose, with Riccioli, that these heavenly bodies are 
 luminous only on one side, which by a sudden semi-revolu- 
 tion the Creator, at the appointed time, has turned towards 
 us ? The first supposition has been set aside by later obser- 
 vations, which have shown, by the help of maps, that a 
 small star had already existed precisely in the place where 
 the new star burst forth ; the other view is too absurd to de- 
 serve, in these days, any further consideration. The star 
 observed by Tycho, as well as that one seen by Kepler, are 
 still visible. If, therefore, the sudden bursting forth of a 
 star in the heavens does not denote the creation of a new 
 star, nor its gradual disappearance indicate its complete 
 annihilation, we may well suppose that both phenomena are 
 the successive effects of a violent outbreak of fire taking 
 place in the star, either in the form of an eruption of the 
 internal red-hot liquid matter, and its suffusion over the sur- 
 face, or of the ignition of gigantic streams of gas forcing 
 their way from the interior. While such an occurrence 
 would raise the star to a state of extreme incandescence, and 
 cause it to emit an intense light for some time, the cooling 
 subsequent to this combustion would ensue more or less rap- 
 idly, and the brightness consequently diminish in quick pro- 
 gression, until, in certain conditions, the star would cease to 
 be visible. 
 
 Fortunately for science, such an occurrence has taken 
 place since spectrum analysis has been so successfully ap- 
 plied to the examination of the heavenly bodies. On the 
 night of the I2th of May, 1866, a new star, brighter than 
 one of the second magnitude, was observed at Tuam, by 
 Mr. John Birmingham, in the constellation Corona Borealis. 
 On the following night it was seen by the French engineer, 
 Courbebaisse, at Rochefort, and was observed a few hours 
 earlier at Athens by the astronomer Julius Schmidt, who 
 expressly declares that the new star could not have been vis* 
 
 45 
 
140 RECREATIONS IN POPULAR SCIENCE. 
 
 ible before eleven o'clock on the night of the I2th of May, 
 as he had been observing with his comet-seeker the star 
 R Coronae, and while sweeping for some time in its neigh- 
 borhood for meteors, could not have failed to notice the new 
 star, if it had been then visible. On the same night (i3th 
 of May) the light of the star sensibly decreased, and by the 
 1 6th of May it had become only of the fourth magnitude. 
 Its brightness then waned somewhat rapidly : it decreased 
 from 4.9 on the i7th to 5.3 on the iSth, and from 5.7 ou the 
 1 9th to 6.2 on the 2oth, till, by the end of the month, it had 
 become a star of the ninth magnitude. 
 
 Argelander observed the star on the iSth of May, 1855, 
 and on the 3ist of March, 1856, and on both occasions had 
 classed the star as between the ninth and tenth magnitudes. 
 
 Huggins was informed by Birmingham of his discovery 
 on the I4th of May, and was thus enabled, on the I5th inst, 
 in conjunction with Miller, to examine the spectrum of this 
 star when it had not fallen much below the third magnitude. 
 The result of this investigation is as follows. 
 
 The spectrum of the star was very remarkable, and 
 showed clearly that there were two distinct sources of light, 
 each producing a separate spectrum, one a continuous 
 spectrum, crossed by dark lines similar to that given by the 
 sun and other stars, while the other consists of four bright 
 lines, which, from their great brilliancy, stand in bold relief 
 upon the dark background of the first spectrum. 
 
 The principal spectrum traversed by dark lines shows the 
 presence of a photosphere of incandescent matter, probably 
 solid or liquid, which is surrounded by an atmosphere of 
 cooler vapors, giving rise by absorption to the dark lines. 
 This absorption spectrum contains two strong dark bands, 
 of less refrangibility than the D-line of the solar spectrum ; 
 a group of fine lines stretches from them close up to D, 
 while one fine line is quite coincident with D. Up to this 
 point, the constitution of this object is analogous to that of 
 the sun and the stars; but the star has also a spectrum con- 
 
SCHELLEX.] SPECTRUM ANALYSIS DISCOVERIES. 141 
 
 sisting of bright lines, which denotes the presence of a sec- 
 ond source of light, which, from the nature of the spectrum, 
 is undoubtedly an intensely luminous gas. 
 
 Huggins compared the spectrum of the star on the lyth 
 of May with the spectrum of hydrogen gas produced by 
 means of the induction spark through a GeiSvsler's tube, and 
 found that the strongest of the stellar lines 2 was coincident 
 with the greenish-blue line H ^ of hydrogen gas. Appar- 
 ently, also, the line i in the red coincided with the H "-line 
 of hydrogen, but owing to the want of brilliancy of the line, 
 the coincidence could not be ascertained with the same de- 
 gree of certainty. The great brilliancy of these lines, com- 
 pared with the parts of the continuous spectrum where they 
 occur, proves that the luminous gas was at a higher temper- 
 ature than the photosphere of the star. 
 
 These facts, taken in connection with the suddenness of 
 the outburst of light in the star, and the immediate very 
 rapid decline in its brightness from the second down to the 
 eighth magnitude, have led to the hypothesis already alluded 
 to, that, in consequence of some internal convulsion, enor- 
 mous quantities of hydrogen and other gases were evolved, 
 which, in combining with some other elements, ignited on 
 the surface of the star, and thus enveloped the whole body 
 suddenly in a sheet of flame. The ignited hydrogen gas, in 
 its combination with some other element, produced the light 
 characterized by the two bright bands in the red and green ; 
 the remaining bright lines, among which those of oxygen 
 might have been expected, were not coincident with any of 
 the lines of this gas. The burning hydrogen gas must also 
 have greatly increased the heat of the solid matter of the 
 photosphere, and brought it into a state of more intense in- 
 candescence and luminosity, which may explain how the 
 formerly faint star could so suddenly assume such remark- 
 able brilliancy. As the liberated hydrogen gas became ex- 
 hausted, the flame gradually abated, and with the conse- 
 
 47 
 
142 RECREATIONS IN POPULAR SCIENCE. [SCHB-LEH. 
 
 quent cooling the photosphere became less vivid, and the 
 star returned to its original condition. 
 
 Robert Meyer and H. J. Klein have expressed the opinion 
 that the sudden blazing out of a star might be occasioned by 
 the violent precipitation of some great mass, perhaps of a 
 planet, upon a fixed star, by which the momentum of the 
 falling mass would be changed into molecular motion, or, in 
 other words, into heat and light. It might even be supposed 
 that the star in Corona, through its motion in space, may 
 have come in contact with one of the nebulae, which traverse 
 in great numbers the realms of space in every direction, and 
 which, from their gaseous condition, must possess a high 
 temperature. Such a collision would recessarily set the 
 star on a blaze, and occasion the most vehement ignition of 
 its hydrogen. 
 
 It must not be forgotten that light, though an extremely 
 quick messenger, yet occupies a certain time ini coming to 
 us from a star. The speed of light is one hundred and 
 eighty-five thousand miles in a second. The distance of the 
 nearest fixed star (Centauri) is about sixteen billion miles, 
 so that light takes about three years to travel from this star 
 to us. The great physical convulsion which was observed 
 in the star in Corona, in the year 1866, was therefore an 
 event which had really taken place long before that period, 
 at a time, no doubt, when spectrum analysis, to which we 
 are indebted for the information we obtained on the subject, 
 was yet quite unknown. 
 
 The discoveries made by means of spectrum analysis, 
 connected with at least three classes of heavenly bodies, still 
 remain to be reviewed, and will form the basis of another 
 paper of this series, which will be entitled Nebulas, Comets, 
 and Meteors. 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 143 
 
 6, $bula3, {gamuts, 
 
 And the Revelations of the Spectroscope regarding them. 
 
 SPECTRA OF NEBULAE. 
 
 WE now come to treat of the remotest realms of the 
 Universe, those regions of stellar clusters and neb- 
 ulae which can only be reached by means of the most power- 
 ful telescopes. When the starry heavens are viewed through 
 a telescope of moderate power, a great number of stellar 
 clusters and faint nebulous forms are revealed against the 
 dark background of the sky, which might be taken at first 
 sight for passing clouds, but which, by their unchanging 
 forms and persistent appearance, are proved to belong to the 
 heavenly bodies, though possessing a character widely dif- 
 fering from the point-like images of ordinary stars. Sir 
 William Herschel was able, with his gigantic forty-foot tel- 
 escope, to resolve many of these nebulas into clusters of stars, 
 and found them to consist of vast groups of individual suns, 
 in which thousands of fixed stars may be clearly separated 
 and counted, but which are so far removed from us that we 
 are unable to perceive their distance one from the other, 
 though that may really amount to many millions of miles, 
 and their light, with a low magnifying power, seems to come 
 from a large, faintly-luminous mass. But all nebula3 were 
 not resolvable with this telescope, and in proportion as such 
 nebulae were resolved into clusters of stars, new nebulas ap- 
 peared which resisted a power of six thousand, and sug- 
 gested to this astute investigator the theory that, besides the 
 many thousand apparent nebulae which reveal themselves to 
 us as a complete and separate system of worlds, there are 
 also thousands of real nebulae in the Universe, composed of 
 primeval cosmical matter, out of which future worlds 
 were to be fashioned. 
 
144 RECREATIONS IN POPULAR SCIENCE. [SCHELLEH. 
 
 Lord Rosse, by means of a telescope of fifty-two feet focus, 
 of his own construction, was able to resolve into clusters of 
 stars many of the nebulae not resolved by Herschel ; but 
 there were still revealed to the eye, thus carried farther into 
 space, new nebula3 beyond the power even of this gigantic 
 telescope to resolve. 
 
 Telescopes failed, therefore, to solve the question whether 
 the unresolved nebulae are portions of the primeval matter 
 out of which the existing stars have been formed ; they leave 
 us in uncertainty as to whether these nebulae are masses of 
 luminous gas, which, in the lapse of ages, would pass through 
 the various stages of incandescent liquid (the sun and fixed 
 stars), of scoriae or gradual formation of a cold and non- 
 luminous surface (the earth and planets), and finally of com- 
 plete gelation and torpidity (the moon), or whether they 
 exist as a complete and separate system of worlds ; tele- 
 scopes have only widened the problem, and have neither 
 simplified nor solved its difficulties. 
 
 That which was beyond the power of the most gigantic 
 telescopes has been accomplished by that apparently insig- 
 nificant, but really delicate, and almost infinitely sensitive 
 instrument, the spectroscope. We are indebted to it for 
 being able to say with certainty that luminous nebulas ac- 
 tually exist as isolated bodies in space, and that these bodies 
 are luminous masses of gas. 
 
 The spectroscope, in combination with the telescope, af- 
 fords means for ascertaining, even now, some of the phases 
 through which the sun and planets have passed in their pro- 
 cess of development or transition from masses of luminous 
 nebulae to their present condition. 
 
 Great variety is observed in the forms of the nebulae : 
 while some are chaotic and- irregular, and sometimes highly 
 fantastic, others exhibit the pure and beautiful forms of a 
 curve, a crescent, a globe, or a circle. 
 
 The largest and most irregular of all the nebulae is that in 
 the constellation of Orion (Fig. XII.). It is situated rather 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 145 
 
 below the three stars of second magnitude composing the 
 central part of that magnificent constellation, and is visible 
 to the naked eye. It is extremely difficult to execute even a 
 tolerably correct drawing of this nebula ; but it appears, 
 from the various drawings, made at different times, that a 
 
 FIG. XII. 
 South. 
 
 North. 
 
 Central and most brilliant portion of the great Nebula in the Sword-handle of Orion, as ob- 
 served by Sir John Herschel in his 20-foot Reflector at Feldhausen, Cape of Good Hope 
 (1834 to 1837). 
 
 change is taking place in the form and position of the bright- 
 est portions. Fig. XII. represents the central and brightest 
 part of the nebula. Four bright stars, forming a trapezium, 
 are situated in it, one of which only is visible to the naked 
 eye. The nebula surrounding these stars has a flaky appear- 
 
 3 
 
RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 ance, and is of a greenish-white color; single portions form 
 long curved streaks, stretching out in a radiating manner 
 from the middle and bright parts. 
 
 The interest aroused by these irregular and chaotic neb- 
 ulous forms is still further increased by the phenomena of 
 the spiral or convoluted nebula with which the giant tele- 
 scopes of Lord Rosse and Mr. Bond have made us further 
 acquainted. As a rule, there streams out from one or more 
 centres of luminous matter innumerable curved nebulous 
 streaks, which recede from the centre in a spiral form, and 
 finally lose themselves in space. 
 
 Fig. XIII. represents the most remarkable of all the spiral 
 nebulaB, which is situated in the constellation Canes Venatici. 
 
 FIG. XIII. 
 
 SPIRAL NEBULA IN CANES VENATICI. 
 
 It is hardly conceivable that a system of such a nebulous 
 form could exist without internal motion. The bright nu- 
 cleus, as well as the streaks curving round it in the same 
 direction, seem to indicate an accumulation of matter towards 
 
 4 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 
 
 H7 
 
 the centre, with a gradual increase of density, and a rotatory 
 movement. But if we combine with this motion the suppo- 
 sition of an opposing medium, it is difficult to harmonize 
 
 FIG. XIV. 
 
 TRANSITION FROM THE SPIRAL TO THE ANNULAR FORM. 
 
 such a system with the known laws of statics. Accurate 
 measures are, therefore, of the highest interest for the pur- 
 
 FIG. XV. 
 
 ANNULAR NEBULA IN LYRA. 
 
 pose of showing whether actual rotation or other changes 
 are taking place in these nebulae ; but, unfortunately, they 
 
 5 
 
H S RECREATIONS IN POPULAR SCIENCE. [SCHELLEX. 
 
 are rendered extremely difficult and uncertain by the want 
 of outline, and by the remarkable faintness of these nebulous 
 objects. 
 
 The transition state from the spiral to the annular form is 
 shown in such nebulas as the one represented in Fig. XIV. ; 
 and they then pass into the simple or compound annular 
 nebula, of which a type is given in Fig. XV. 
 
 The space within most of these elliptic rings is not per- 
 fectly dark, but is occupied either by a diffused faint nebulous 
 light, as in Fig. XV., or, as in most cases, by a bright nu- 
 cleus, round which sometimes one ring, sometimes several, 
 are disposed in various forms. 
 
 Those nebulas which appear with tolerably sharply-defined 
 
 FIG. XVI. FIG. XVII. 
 
 Planetary Nebula with two Stars. Planetary Nebula. 
 
 edges, in the form of a circle or slight ellipse, seem to belong 
 to a much higher stage of development. From their resem- 
 blance to those planets which shine with a pale or bluish 
 light, they have been called planetary nebulas ; in form, 
 however, they vary considerably, some of them being spiral 
 and some annular. Some of these planetary nebulas are 
 represented in Figs. XVI. and XVII. The first has two 
 central stars or nuclei, each surrounded by a dark space, 
 beyon-d which the spiral streaks are disposed ; the second 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 149 
 
 is without a nucleus, but shows a well-defined ring of 
 light. 
 
 The highest type of nebulae are certainly the stellar neb- 
 ulas, in which a tolerably well defined bright star is sur- 
 rounded by a completely round disk, or faint atmosphere of 
 light, which sometimes fades away gradually into space, at 
 other times terminates abruptly, with a sharp edge. Fig. 
 XVIII. exhibits one of the most striking of these very re- 
 markable stellar nebulas. It is a veritable star of the eighth 
 magnitude, and is not nebulous, but is surrounded by a 
 
 FIG. XVIII. 
 
 STELLAR NEBULA. 
 
 bright luminous atmosphere perfectly concentric. To the 
 right of the star is a small dark space, such as often occurs 
 in these nebulas, indicating, perhaps, an opening in the sur- 
 rounding atmosphere. 
 
 We have now passed in review nearly all that is at present 
 known of the nebulas, so far as their appearance and form 
 have been revealed by the largest telescopes. The informa- 
 tion as yet furnished by the spectroscope on this subject is 
 certainly much less extensive, but is nevertheless of the 
 
 7 
 
150 RECREATIONS IN POPULAR SCIENCE. [SCHKLLEN. 
 
 greatest importance, since the spectroscope has power to 
 reveal the nature and constitution of these remote heavenly 
 bodies. It must here again be remembered that the char- 
 acter of the spectrum not only indicates what the substance 
 is that emits the light, but also its physical condition. If 
 the spectrum be a continuous one, consisting of rays of 
 every color or degree of refrangibility, then the source of 
 light is either a solid or liquid incandescent body ; if, on the 
 contrary, the spectrum be composed of bright lines only, 
 then it is certain that the light comes from luminous gas; 
 finally, if the spectrum be continuous, but crossed by dark 
 lines interrupting the colors, it is an indication that the 
 source of light is a solid or liquid incandescent body, but 
 that the light has passed through an atmosphere of vapors at 
 a lower temperature, which,. by their selective absorptive 
 power, have abstracted those colored rays which they would 
 have emitted had they been self-luminous. 
 
 When Huggins first directed his telespectroscope, in Au- 
 gust, 1864, to one of these objects, a small but very bright 
 nebula, he found, to his great surprise, that the spectrum, 
 instead of being a continuous colored band, such as that 
 given by a star, consisted only of three bright lines. 
 
 This one observation was sufficient to solve the long-vexed 
 question, at least for this particular nebula, and to prove that 
 it is not a cluster of individual, separable stars, but is ac- 
 tually a gaseous nebula, a body of luminous gas. In fact, 
 such a spectrum could only be produced by a substance in a 
 state of gas ; the light of this nebula, therefore, was emitted 
 neither. by solid nor liquid incandescent matter, nor by gases 
 .in a state of extreme density, as may be the case in the sun 
 .and stars, 'but *by luminous gas in a highly rarefied con- 
 dition. 
 
 In order to discover the chemical nature of this gas, Hug- 
 gins followed the usual methods of comparison, and tested 
 the spectrum with the Fraunhofer lines of the solar spectrum, 
 and the bright lines of terrestrial elements. 
 
 8 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 151 
 
 Besides the spectrum containing these three bright lines, 
 the nebula gave also a very faint continuous spectrum, of 
 scarcely perceptible width, which, from its nature, could 
 proceed only from the diffused light of a faintly glowing 
 nucleus, either solid or liquid, or from faintly luminous mat- 
 ter in the form of a cloud of solid or liquid particles. 
 
 All planetary nebulae yield the same spectrum ; the bright 
 lines appear with considerable intensity in the spectroscope, 
 and are of sufficient brilliancy to compare with the bright 
 lines in the spectrum of a candle, although the nebulas may 
 not be brighter in the heavens than stars of the ninth mag- 
 nitude. The reason of this is, that the light of the candle is 
 spread out into a continuous spectrum, while that of the neb- 
 ula remains concentrated into a few lines. The principle is 
 identical with that by which the spectra of the solar prom- 
 inences have been since observed in sunlight simultaneously 
 with the greatly subdued spectrum of daylight. 
 
 During the years 1865 and 1866, more than sixty nebulas 
 were examined by Huggins with the spectroscope, mainly 
 with the intention of ascertaining whether those which were 
 clearly resolvable by the telescope into a cluster of bright 
 points, gave a continuous spectrum, or one composed of 
 bright lines. 
 
 As a result of his observations, Huggins divides the neb- 
 ulas into two groups : 
 
 1. The nebulas giving a spectrum of one or more bright 
 lines. 
 
 2. The nebulas giving a spectrum apparently continuous. 
 About a third of the sixty nebulae observed belong to the 
 
 first group ; their spectrum consists of one, two, or three 
 bright lines ; a few showing at the same time a very narrow, 
 faint, continuous spectrum. 
 
 The great nebula of Orion (Fig. XII.) has been the sub- 
 ject of spectroscopic investigations. Its spectrum consists 
 of three very conspicuous bright lines, one of which again 
 indicates nitrogen, and another hydrogen. 
 
 9 
 
152 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 Huggins has lately repeated his former observations with 
 instruments of much greater power, and compared especially 
 these two lines with those of the terrestrial gases, under cir- 
 cumstances which gave him a spectrum four times the length 
 of the one he obtained in his earlier investigations. The 
 result of these observations, continued for several nights, 
 was to show the complete coincidence, even in this greatly 
 extended spectrum of the nebular lines, with those of both 
 gases, so that there can be no remaining doubt as to the 
 identity of the lines. 
 
 Half of the nebulae giving a continuous spectrum have 
 been resolved into stars, and about a third more are probably 
 resolvable ; while of those yielding a spectrum of lines, not 
 one has been certainly resolved by Lord Rosse. Considering 
 the extreme difficulty attending investigations of this kind, 
 there is scarcely any doubt that there is a complete accord- 
 ance between the results of the telescope and spectroscope ; 
 and therefore those nebula giving a continuous spectrum 
 are clusters of actual stars, while those giving a spectrum 
 of bright lines must be regarded as masses of luminous gas, 
 of which nitrogen and hydrogen form the chief constit- 
 uents. 
 
 COMETS AND THEIR SPECTRA. 
 
 Besides the planets, which, already cold or in process of 
 cooling, derive their light from the incandescent sun, round 
 which they revolve in their appointed orbits, all travelling 
 nearly in one plane among the fixed stars in regular progress 
 from west to east, there appear from time to time certain 
 other wandering stars of peculiar aspect, which, from their 
 rapid change of form and size, their fantastic contour, and 
 their brilliant light, usually excite the greatest attention; 
 these remarkable visitors are comets ; and though their laws 
 of motion have been well ascertained, yet their physical con- 
 stitution has presented greater difficulties to astronomers 
 than even that of the nebulae. When they first become vis- 
 
 10 
 
SCHHLLEN.] SPECTRUM ANALYSIS DISCOVERIES. 153 
 
 ible, their motion is evidently round the un, but frequently 
 in orbits of such great elongation as hardly to be called 
 elliptical, travelling, besides, in all possible planes and direc- 
 tions sometimes, like the planets, from west to east, some- 
 times in the reverse way, from east to west. Several of 
 these extraordinary objects move in closed orbits round the 
 sun, with a regular period of revolution ; others come quite 
 unexpectedly from the regions of space into our system, and 
 retreat again, to be seen no more. The periodic comets are 
 as follows : 
 
 Distance from the Sun. 
 Comet. Period Perihelion. | Aphelion. 
 
 Encke's, . 3^ years. 289,000,000 miles. 350,000,000 miles. 
 
 Winnecke's, 5^ " 69,000,000 " 501,000,000 " 
 
 Brorsen's, . 5f " 55,000,000 " 516,000,000 " 
 
 Biela's, . . 6 u 78,000,000 " 564,000,000 " 
 
 Faye's, . . i\ " 156,000,000 " 543,000,000 " 
 
 Halley's, . 76^ " 52,000,000 " 3,175,000,000 " 
 
 While these comets have but a short period, there are 
 others, such as the comets of 1858, iSir, and 1844, the 
 calculated periods of which amount respectively to 2,100, 
 3,000, and 100,000 years. Differences of quite a proportion- 
 ate magnitude are observable, in relation to the points of 
 nearest approach to and greatest distance from the sun. 
 Encke's comet is twelve times nearer the sun at its perihelion 
 than at its aphelion. Some of them, with an orbit extending 
 beyond Jupiter, approach so close to the sun as almost to 
 graze the surface. Newton estimated that the comet of 1680 
 came so near to the sun, that its temperature must have ex- 
 ceeded by two thousand times that of melted iron. At its 
 nearest approach it was removed from the sun by only a 
 sixth of his diameter. The comet of 1843, also, was so near 
 the sun at its perihelion as to be seen in broad daylight. 
 
 Most comets exhibit a planetary disk, more or less bright, 
 which is called the nucleus, and tTiis is surrounded by a 
 fainter cloudy or nebulous envelope, the coma ; the nucleus 
 
 ii 
 
154 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 and coma form the head of the comet. In almost all comets 
 visible to the naked eye, there streams out from the head a 
 fan of light the tail, consisting of one or more luminous 
 streaks, which vary in width and length, are sometimes 
 straight, sometimes curved, but almost always turned away 
 from the sun, forming the prolongation of a straight line 
 connecting the sun and the comet. While telescopic comets 
 are usually without a tail, which causes them to assume the 
 appearance of a more or less irregularly shaped nebula, 
 possessing a nucleus, an example of which is given in Do- 
 nati's comet, as it appeared when first seen on the 2d of 
 June, 1858, the comet of July, 1861, exhibited two tails, 
 and the comet of 1844 had even six. 
 
 Comets are transparent in every part, and cause no refrac- 
 tion in the light of the stars seen through them. Bessel saw 
 a fixed star through Halley's comet, and Struve one through 
 Biela's comet, when distant only a few seconds from the 
 centre of the nucleus, which passed over the star in both 
 instances without either rendering it invisible or even per- 
 ceptibly fainter ; from accurate measures taken at the time, 
 and the calculated motion of the comet, it was evident that 
 the position of the star had not been changed by any refrac- 
 tion of the light. 
 
 Similar observations were made with respect to Donati's 
 comet of 1858, and the comet of July, 1861. Close to the 
 head of the former, where the tail at its commencement was 
 about 54,000 miles in thickness, Arcturus was seen to shine 
 with undiminished brightness ; while in both comets a 
 number of fixed stars appeared in full brilliancy through 
 even a much thicker portion of the tail. The cornet of 1828 
 possessed a nucleus about 528,000 miles in diameter, and yet 
 Struve saw a star of the eleventh magnitude through it 
 a fact which seems to justify the conclusion of Babinet, 
 drawn from his own observations, that a comet "has no influ- 
 ence upon the light of a star, and that stars of the tenth and 
 eleventh magnitude, and some even fainter, may be seen 
 
 12 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 155 
 
 through their greatest mass, without losing in the smallest 
 degree either their light or their color. 
 
 The tail is a prolongation of the coma, and is in most 
 cases turned away from the sun,, whether the comet be 
 approaching or receding from the sun in the course of its 
 orbit. 
 
 As a comet approaches the sun, the tail regularly in- 
 creases, from which it appears that the sun, whether by the 
 action of heat or other means, contributes essentially to the 
 formation of the tail, and produces a separation of material 
 particles from the head of the comet. The length of the tail 
 is rarely less than 500,000 miles, and in some cases it extends 
 as far as 100,000,000 or 150,000,000 miles. The breadth of 
 the tail of the great comet of 1811, at its widest part, was 
 nearly 14,000,000 miles, the length 116,000,000; and that of 
 the second comet of the same year even 140,000,000 miles. 
 And yet the formation of the tail takes place in a very short 
 space of time, often in a few weeks, or even days. 
 
 The influence exercised on the formation of the tail by its 
 approach to the sun was shown in the comet of 1680, for at 
 its perihelion it travelled at the rate of 1,216,800 miles in an 
 hour, and, as a consequence, put forth a tail in two days 
 54,000,000 miles in length. 
 
 It is easily conceivable that, under such circumstances, the 
 mass of a comet must be exceedingly small. It is very 
 probable that our earth actually passed, on the 3oth of June, 
 1861, through part of the tail of the magnificent comet called 
 the July comet, which suddenly appeared in the heavens, as 
 if by magic, on the 29th of June, and no indication of such 
 a contact was evinced, beyond a peculiar phosphorescence 
 in the atmosphere, which was noticed by Mr. Hind, and also 
 at the Liverpool Observatory. In the same way, the comet 
 of 1776 passed among the satellites of Jupiter without dis- 
 turbing their position in the slightest degree. This was not 
 the case, however, with the comet, for the influence of the 
 planet was so great on its small mass as to send it quite out 
 
 13 
 
156 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 of its course, into an entirely new orbit, which it now ac- 
 complishes in about twenty years. 
 
 We must now consider the remarkable phenomenon of a 
 comet being divided into two parts, each part becoming a 
 separate comet, and pursuing an orbit of its own. Such an 
 occurrence happened to Biela's comet while under observa- 
 tion in the year 1845. When observed on the 26th of No- 
 vember of that year, it appeared as a faint nebulous spot, 
 not perfectly round, with an increased density towards the 
 middle. On the I9th of December it was rather more elon- 
 gated, and ten days later it had become divided into two 
 separate, cloudy masses of equal dimensions, each furnished 
 with a nucleus and tail, and for three months one followed 
 the other at a distance of one tenth, subsequently one fifth, 
 of the moon's diameter. The pair made their appearance 
 again in August, 1852, after having travelled together in one 
 common orbit round the sun for more than six years and a 
 half; but the distance between them had much increased, 
 and from 154,000 miles, it had now reached 1,404,000 miles. 
 Nor is this all : in conformity with its known period, the 
 return of this comet was expected in the year 1859, anc ^ 
 again in 1866, when it must have been visible from the 
 earth, as its path crossed the earth's orbit at the place where 
 the earth was on the 3oth of November. Notwithstanding 
 the most diligent search, however, the comet could not be 
 found, and it would seem that either, like Lexell's comet, 
 it has been drawn out of its orbit by some member of the 
 solar system, or else, as analogy suggests, it has ceased 
 to be a comet, and has passed into some other form of 
 existence. 
 
 We must enter a little further than might seem needful for 
 our purpose into the important phenomena observed in com- 
 ets, partly by the naked eye, but more especially by the tel- 
 escope, in order to obtain some ground for answering queries 
 as to the physical nature of these heavenly bodies, as well as 
 to acquire a standard by which to compare the facts collected 
 
 H 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 157 
 
 by telescopic observation with those gathered by spectrum 
 analysis. 
 
 These questions are directed, in the first place, to the con- 
 sideration of whether comets, like fixed stars and nebulae, 
 are self-luminous, or whether, like planets, they shine by the 
 reflected light of the sun ; in the second place, to the con- 
 sideration of their material composition and physical consti- 
 tution. That the nucleus of a comet cannot be in itself a 
 dark and solid body, such as the planets are, is proved by its 
 great transparency ; but this does not preclude the possibility 
 of its consisting of innumerable solid particles, separated 
 one from another, which, when illuminated by the sun, give 
 by the reflection of the solar light the impression of a homo- 
 geneous mass. It has therefore been concluded that comets 
 are either composed of a substance which, like gas in a state 
 of extreme rarefaction, is perfectly transparent, or of small, 
 solid particles, individually separated by intervening spaces, 
 through which the light of a star can pass without obstruc- 
 tion, and which, held together by mutual attraction, as well 
 as by gravitation towards a central denser conglomeration, 
 moves through space like a cloud of dust. It is not impos- 
 sible that comets without a nucleus are masses of gas at a 
 white heat, of similar constitution to the nebula?, while those 
 possessing a nucleus are composed of disengaged solid par- 
 ticles. In any case, the connection lately noticed by Schi- 
 aparelli between comets and meteor showers seems to 
 necessitate the supposition that in many comets a similar 
 aggregation of particles exists. 
 
 Donati, at Florence, was the first to examine spectroscop- 
 ically the light of comets : he compared the spectrum of the 
 comet I., 1864, with the spectra of metals, in which the dark 
 places were wider than the luminous parts, and he found 
 that the entire spectrum consisted of three bright lines. 
 
 Tempel's comet was observed in January, 1866, by Secchi 
 and Huggins, who found that it yielded a continuous spec- 
 trum exceedingly faint at the two ends, in which three bright 
 
 15 
 
158 RECREATIONS IN POPULAR SCIENCE. [SCHELLKN. 
 
 lines were seen by the former observer, and only one by 
 Huggins. It appears from this, that the nucleus is at least 
 -partially self-luminous, and is composed of gas in a lumi- 
 nous condition. On the other hand, the continuous spec- 
 trum proves that some of the light is reflected sunlight, for 
 it cannot be admitted that the coma is formed of incandes- 
 cent solid or liquid particles. 
 
 The spectroscope gives no information as to the nature or 
 condition of a substance from which we receive only re- 
 flected light ; it is, however, probable that the coma and tail 
 are of the same substance as the nucleus. These observa- 
 tions, therefore, yield no further result than that a gas in a 
 state of luminosity is present in the comet, but that, at the 
 same time, either from this gas or from other portions of the 
 comet which are non-luminous, sunlight is also reflected. 
 
 Secchi's observations have been completely confirmed by 
 those of Huggins. The spectrum of the comet consisted of 
 three broad, bright bands, which were sharply defined at 
 the edge towards the red, but faded away gradually on the 
 opposite side. 
 
 It would be premature to draw decisive results from these 
 comprehensive but as yet isolated observations. The spec- 
 trum of the three bright bands is derived unquestionably 
 from the light of the comet's nucleus, and not from that of 
 the coma, which is far too faint and ill-defined to produce 
 such a spectrum ; it may therefore be assumed that the nu- 
 cleus is self-luminous, and that it is very possibly composed 
 of glowing gas containing carbon. 
 
 By collating these various phenomena, the conviction can 
 scarcely be resisted that the nuclei of comets not only emit 
 their own light, which is that of a glowing gas, but also, 
 together with the coma and the tail, reflect the light of the 
 sun. There seems, therefore, nothing to contradict the the- 
 ory that the mass of a comet may be composed of minute 
 solid bodies, kept apart one from another in the same way 
 as the infinitesimal particles forming a cloud of dust or 
 
 16 
 
SCHBLLEN.] SPECTRUM ANALYSIS DISCOVERIES. 159 
 
 smoke are held loosely together, and that as the comet ap- 
 proaches the sun, the most easily fusible constituents of 
 these small bodies become wholly or partially vaporized, 
 and, in a condition of white heat, overtake the remaining 
 solid particles, and surround the nucleus in a self-luminous 
 cloud of glowing vapor. Spectrum analysis will not be able 
 to afford any more certain evidence regarding the physical 
 nature of comets until the appearance of a really brilliant 
 comet, which can be examined in the various phases it may 
 present. 
 
 It would lead us too far from our purpose, were we to de- 
 scribe more minutely the extremely interesting phenomena 
 which the telescope has revealed of the separation of cometic 
 matter, and the gradual formation of the coma and tail ; nor 
 can we enter more fully here into the causes of the changes 
 produced in the form of a comet by its approach to the sun, 
 or to one of the larger planets ; but we cannot pass over the 
 extremely ingenious hypothesis brought forward by Pro- 
 fessor Tyndall, before the Philosophical Society of Cam- 
 bridge, on the 8th of March, 1869. This admirable investi- 
 gator had already proved, by a series of interesting experi- 
 ments, that concentrated solar light, or the electric light, 
 decomposes the volatile vapors of many liquids, producing 
 almost instantly a precipitate of cloudy matter, in which 
 some very peculiar phenomena of light are displayed. The 
 quantity of vapor may be so small as to escape detection, 
 but the concentrated light falling upon it soon forms a blue 
 cloud from the moving atoms of vapor which now become 
 visible, and appear, according to the nature of the vapor, in 
 a variety of forms, as precipitations of matter on the beams 
 of light. 
 
 It is very striking, in this experiment, to see the astonishing 
 amount of light that an infinitesimal amount of decompos- 
 able vapor is able to reflect. When the electric light is 
 admitted into the tube, nothing is to be seen for the first 
 moment ; but soon a blue cloud shows itself, which is 
 
 '7 
 
160 RECREATIONS IN POPULAR SCIENCE. [SCHELLEK. 
 
 formed of almost infinitely small, particles, either of vapor, 
 or, what is more probable, of the molecules set free by its 
 decomposition, and after some minutes the whole tube is 
 filled with this blue color. The vaporous particles gradually 
 augment in magnitude, and after some time (from ten to 
 fifteen minutes) a dense white cloud fills the tube, which dis- 
 charges so great a body of light that it is scarcely conceiv- 
 able how so small a quantity of matter can possibly reflect 
 so much light. 
 
 " Nothing," says Tyndall, " could more perfectly illustrate 
 that * spiritual texture' which Sir John Herschel ascribes to 
 a comet, than these actinic clouds. Indeed, the experiments 
 prove that matter of almost infinite tenuity is competent to 
 shed forth light far more intense than that of the tails of 
 comets." 
 
 FALLING STARS, METEOR SHOWERS, BALLS OF FIRE, AND 
 THEIR SPECTRA. 
 
 Whoever has observed the heavens on a clear night with 
 some amount of attention and patience, cannot fail to have 
 noticed the phenomenon of a falling star, one of those well- 
 known fiery meteors which suddenly blaze forth in any 
 quarter of the heavens, descend towards the earth, generally 
 with great rapidity, in either a vertical or slanting direction, 
 and disappear after a few seconds at a higher or lower alti- 
 tude. As a rule, falling stars can only be seen of an evening, 
 or at night, owing to the great brightness of daylight ; but 
 many instances have occurred in which their brilliancy has 
 been so great as to render them visible in the daytime, as 
 well when the sky was overcast as when it was perfectly 
 cloudless. It has been calculated that the average number 
 of these meteors passing through the earth's atmosphere, 
 and sufficiently bright to be seen at night with the naked eye, 
 is not less than seven million and a half during the space of 
 twenty-four hours; and this number must be increased to 
 four hundred million, if those be included which a tele- 
 
 18 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. l6l 
 
 scope would reveal. In many nights, however, the number 
 of these meteors is so great, that they pass over the heavens 
 like flakes of snow, and for several hours are too numerous 
 to be counted. Early in the morning of the I2th of No- 
 vember, 1799, Humboldt and Bonpland saw before sunrise, 
 when on the coast of Mexico, thousands of meteors during 
 the space of four hours, most of which left a track behind 
 them of from 5 to 10 in length ; they mostly disappeared 
 without any display of sparks, but some seemed to burst, 
 and others, again, had a nucleus as bright as Jupiter, which 
 emitted sparks. On the 12th of November, 1833, there fell 
 another shower of meteors, in which, according to Arago's 
 estimation, two hundred and forty thousand passed over the 
 heavens, as seen from the place of observation, in three 
 hours. 
 
 Only in very rare instances do these fiery substances fall 
 upon the surface of the earth ; when they do, they are called 
 balls of fire ; and occasionally they reach the earth before 
 they are completely burnt out or evaporated ; they are then 
 termed meteoric stones, aerolites, or meteoric iron. They 
 are also divided into accidental meteors and meteoric 
 showers, according as to whether they traverse the heavens 
 in every direction at random, or appear in great numbers 
 following a common path, thus indicating that they are parts 
 of a great whole. 
 
 It is now generally received, and placed almost beyond 
 doubt by the recent observations of Schiaparelli, Le Verrier, 
 Weiss, and others, that these meteors, for the most part 
 small, but weighing occasionally many tons, are fragmentary 
 masses, revolving, like the planets, round the sun, which in 
 their course approach the earth, and, drawn by its attraction 
 into our atmosphere, are set on fire by the heat generated 
 through the resistance offered by the compressed air. 
 
 The chemical analysis of those meteors which have fallen 
 to the earth in a half-burnt condition in the form of meteoric 
 stones, proves that they are composed only of terrestrial ele- 
 
1 62 RECREATIONS IN POPULAR SCIENCE. [SCHELLEM. 
 
 merits, which present a form and combination commonly 
 met with in our planet. Their chief constituent is metallic 
 iron, mixed with various silicious compounds ; in combina- 
 tion with iron, nickel is always found, and sometimes also 
 cobalt, copper, tin, and chromium ; among the silicates, 
 olivine is especially worthy of remark as a mineral very 
 abundant in volcanic rocks, as also augite. There have also 
 been found in the meteoric stones hitherto examined, oxygen, 
 hydrogen, sulphur, phosphorus, carbon, aluminium, magne- 
 sium, calcium, sodium, potassium, manganese, titanium, lead, 
 lithium, and strontium. 
 
 The height at which meteors appear is very various, and 
 ranges chiefly between the limits of forty-six and ninety-two 
 miles. The mean may be taken at sixty-six miles. The 
 speed at which they travel is also various, generally about 
 half as fast again as that of the earth's motion round the 
 sun, or about twenty-six miles in a second : the maximum 
 and minimum differ greatly from this amount, the velocity 
 of some meteors being estimated at fourteen miles, and that 
 of others at one hundred and seven miles in a second. 
 
 When a dark meteorite of this kind, having a velocity of 
 one thousand six hundred and sixty miles per minute, en- 
 counters the earth, flying through space at a mean rate of 
 one thousand one hundred and forty miles per minute, and 
 when through the earth's attraction its velocity is further 
 increased two hundred and thirty miles per minute, this body 
 meets with such a degree of resistance, even in the highest 
 and most rarefied state of our atmosphere, that it is impeded 
 in its course, and loses in a very short time a considerable 
 part of its momentum. By this encounter there follows a 
 result common to all bodies which, while in motion, sud- 
 denly experience a check. When a wheel revolves very 
 rapidly, the axletree or the drag which is placed under the 
 wheel is made red hot by the friction. When a cannon ball 
 strikes suddenly with great velocity against a plate of iron, 
 which constantly happens at target practice, a spark is seen 
 
 20 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 163 
 
 to flash from the ball, even in daylight ; under similar cir- 
 cumstances, a lead bullet becomes partially melted. The 
 heat of a body consists in the vibratory motion of its small- 
 est particles ; an increase of this molecular motion is synon- 
 ymous with a higher temperature ; a lessening of this vibra- 
 tion is termed decreasing heat, or the process of cooling. 
 Now, if a body in motion as, for instance, a cannon ball 
 strike against an iron plate, or a meteorite against the 
 earth's atmosphere, in proportion as the motion of the body 
 diminishes, and the external action of the moving mass be- 
 comes annihilated by the pressure of the opposing medium 
 upon the foremost molecules, the vibration of these particles 
 increases ; this motion is immediately communicated to the 
 rest of the mass, and by the acceleration of this vibration 
 through all the particles, the temperature of the body is 
 raised. This phenomenon, which always takes place when 
 the motion of a body is interrupted, is designated by the 
 expression the conversion of the motion of the mass into 
 molecular action or heat ; it is a law without exception, that 
 where the external motion of the mass is diminished, an 
 inner action among its particles, or heat, is set up in its place 
 as an equivalent, and it may be easily supposed that even in 
 the highest and most rarefied strata of the earth's atmos- 
 phere, the velocity of the meteorite would be rapidly dimin- 
 ished by its opposing action, so that, shortly after entering 
 our atmosphere, the vibration of the inner particles would 
 become accelerated to such a degree as to raise them to a 
 white heat, when they would either become partially fused, 
 or, if the meteorite were sufficiently small, it would be dissi- 
 pated into vapor, and leave a luminous track behind it of 
 glowing gas. 
 
 Haidinger, in a theory embracing all the phenomena of 
 meteorites, explains the formation of a ball of fire round the 
 meteor by supposing that the meteorite, in consequence of 
 its rapid motion through the atmosphere, presses the air be- 
 fore it till it becomes luminous. The compressed air in 
 
 21 
 
164 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 which the solid particles of the surface of the meteorite 
 glow then rushes on all sides, but especially over the surface 
 of the meteor behind it, where it encloses a pear-shaped vac- 
 uum which has been left by the meteorite, and so appears to 
 the observer as a ball of fire. If several bodies enter the 
 earth's atmosphere in this way at the same time, the largest 
 among them precedes the others, because the air offers the 
 least resistance to its proportionately smallest surface ; the 
 rest follow in the track of the first meteor, which is the only 
 one surrounded by a ball of fire. When, by the resistance 
 of the air, the motion of the meteor is arrested, it remains 
 for a moment perfectly still ; the ball of fire is extinguished, 
 the surrounding air rushes suddenly into the vacuum behind 
 the meteor, which, left solely to the action of gravitation, 
 falls vertically to the earth. The loud, detonating noise 
 usually accompanying this phenomenon finds an easy ex- 
 planation in the violent concussion of the air behind the 
 meteor, while the generally received theory that the detonat- 
 ing noise is the result of an explosion or bursting of the 
 meteorite, does not meet with any confirmation. 
 
 The circumstance that most meteors are extinguished be- 
 fore reaching the earth seems to show that their mass is but 
 small ; but this is not always the case. If the distance of a 
 meteor from the earth be ascertained, as well as its apparent 
 brightness as compared with that of a planet, it is possible, 
 by comparing its luminosity with that of a known quantity 
 of ignited gas, to estimate the degree of heat evolved in the 
 meteor's combustion. As this heat originates from the mo- 
 tion of the meteor being impeded or interrupted by the 
 resistance of the air, and as this motion or momentum is 
 exclusively dependent on the speed of the meteor, as well as 
 upon its mass, it is possible, when the rate of motion has 
 been ascertained by direct observation, to determine the 
 mass. Professor Alexander Herschel has calculated, by this 
 means, that those meteors of the 9th and loth of August, 
 1863, which equalled the brilliancy of Venus and Jupiter, 
 
 22 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 165 
 
 must have possessed a mass of from five to eight pounds, 
 while those which were only as bright as stars of the second 
 or third magnitude would not be more than about ninety 
 grains in weight. As the greater number of meteors are 
 less bright than stars of the second magnitude, the faint 
 meteors must weigh only a few grains ; for according to Pro- 
 fessor Herschel's computation, the five meteors observed on 
 the 1 2th of November, 1865, some of which surpassed in 
 brilliancy stars of the first magnitude, had not an average 
 weight of more than five grains ; and Schiaparelli estimated 
 the weight of a meteor from other phenomena to be about 
 fifteen grains. The mass, however, of the meteoric stones 
 which fall to the earth is considerably greater, whether they 
 consist of one single piece, such as the celebrated iron-stone 
 discovered by Pallas in Siberia, which weighed about two 
 thousand pounds, and the meteorites of the Mexican desert,* 
 
 * THE METEORITES OF THE MEXICAN DESERT. With the ob- 
 ject of fixing with greater precision the geographical position of 
 the meteoric masses that have been from time to time met with on 
 the Bolson de Mapini, Dr. J. Lawrence Smith has communicated a 
 paper to the American Journal of Science for November, 1871, 335. 
 There were already known the Cohahuila meteorite of 1854 (0> tne 
 Cohahuila meteorite of 1868 (2), the Chihuahua iron of 1854 (3), 
 still at the Hacienda de Conception, and weighing about four thou- 
 sand pounds, and the Tucson iron (4), found in 1854 on the north 
 of the Rio Grande, and having the form of a ring. This mass 
 weighs fr6m two to three thousand pounds. A fifth mass (5) has 
 since been heard of on the western border of the Mexican desert, 
 that has received the name of the San Gregario meteoric iron. It 
 measures six feet six inches in length, is five feet six inches high, 
 and four feet thick at the base. On one part of its surface, 1821 has 
 been cut with a chisel. Its weight is calculated to be about five 
 tons. An examination of a fragment showed it to be one of the 
 softer meteoric irons. It has a specific gravity of 7.84, and consists 
 of ninety-five per cent, of iron, and five per cent, of nickel, inclu- 
 sive of a little cobalt. Still more recently, news has arrived of the 
 discovery, in the central portion of the desert, of a huge meteorite 
 (6), larger than any yet found in this locality. Dr. Lawrence 
 Smith's paper is illustrated with a little map, indicating their rel- 
 
 23 
 
166 RECREATIONS IN POPULAR SCIENCE. [SCHELLE*. 
 
 or of a cloud composed of many small bodies, which pen- 
 etrate the earth's atmosphere in parallel paths, and which, 
 from a simultaneous ignition and descent upon the earth, 
 present the appearance of a large meteor bursting into sev- 
 eral smaller pieces. Such a shower of stones, accompanied 
 by a bright light and loud explosion, occurred at L'Aigle, in 
 Normandy, on the 26th of April, 1803, when the number of 
 stones found in a space of fourteen square miles exceeded 
 two thousand. In the meteoric shower that fell at Kiiya- 
 hinga, in Hungary, on the 9th of June, 1866, the principal 
 stone weighed about eight hundred pounds, and was accom- 
 panied by about a thousand smaller stones, which were 
 strewed over an area of nine miles in length by three and 
 one quarter broad. 
 
 It must not be supposed, however, that the density of such 
 a cosmieal cloud is as great, when out of the reach of the 
 attraction of the sun and the earth, as when its constituents 
 fall upon the earth's surface. Schiaparelli calculates, from 
 the number of meteors observed yearly in the month of Au- 
 gust, that the distance between any two must amount, on the 
 average, to four hundred and sixty miles. As the cosmieal 
 clouds which produce the meteors approach the sun in their 
 wanderings from the far-off regions of space, they increase 
 in density some million times ; therefore the distance between 
 any two meteors, only a few grains in weight, before the 
 cloud begins to be condensed, may be upwards of forty 
 thousand miles. 
 
 The most striking example of such a cosmieal cloud, 
 composed of small bodies loosely hung together, and existing 
 
 ative positions. He believes they are the result of two falls. The 
 Tucson iron has characters that distinguish it from the remaining 
 five. The latter, he considers, fell at an epoch probably far remote, 
 moving from north-west to south-west during their descent. I and 
 2 fell first, 85 miles apart. The distances between the larger masses 
 are, from 2 to 6, 135 miles; from 6 to 5, 165 miles; and from 5 to 
 3, about 90 miles 
 
 24 
 
SCHELLEN.J SPECTRUM ANALYSIS DISCOVERIES. 167 
 
 with hardly any connection one with another, is exhibited in 
 the meteoric showers occurring periodically in August and 
 November. It is an ascertained fact that, on certain nights 
 in the year, the number of meteors is extraordinarily great, 
 and that at these times they shoot out from certain fixed 
 points in the heavens. The shower of meteors which hap- 
 pens every year on the night of the loth of August, pro- 
 ceeding from the constellation of Perseus, is mentioned in 
 many old writings. The shower of the I2th and I3th of 
 November occurs periodically every thirty-three years, for 
 three years in succession, with diminishing numbers ; it was 
 this shower that Alexander Von Humboldt and Bonpland 
 observed on the I2th of November, 1799, as a real rain of 
 fire. It recurred on the I2th of November, 1833, in such 
 force that Arago compared it to a fall of snow, and was lately 
 observed again in its customary splendor in North America, 
 on the 1 4th of November, 1867. Besides these two prin- 
 cipal showers, there are almost a hundred others, recurring 
 at regular intervals ; each of these is a cosmical cloud, com- 
 posed of small, dark bodies, very loosely held together, like 
 the particles of a sand cloud, which circulate round the sun 
 in one common orbit; The orbits of these meteor streams 
 are very diverse ; they do not lie approximately in one plane, 
 like those of the planets, but cross the plane of the earth's 
 orbit at widely different angles. The motion of the indi- 
 vidual meteors ensues in the same direction in one and the 
 same orbit ; but this direction is in some orbits in conformity 
 with that of the earth and planets, while in others it is in the 
 reverse order. 
 
 The earth, in its revolution round the sun, occupies every 
 day a different place in the universe; if, therefore, a mete- 
 oric shower pass through our atmosphere at regular inter- 
 vals, there must be at the place where the earth is at that 
 time an accumulation of these small cosmical bodies, which,, 
 attracted by the earth, penetrate its atmosphere, are ignited! 
 by the resistance of the air, and become visible as falling 
 
 25 
 
1 68 RECREATIONS IN POPULAR SCIENCE. 
 
 stars. A cosmical cloud, however, cannot remain at a fixed 
 spot in our solar system, but must circulate round the sun as 
 planets and comets do ; whence it follows that the path of a 
 periodic shower intersects the earth's orbit, and the earth 
 must either be passing through the cloud, or else very near 
 to it, when the meteors are visible to us. 
 
 The meteor shower of the loth of August, the radiant 
 point of which is situated in the constellation of Perseus, 
 takes place nearly every year, with varying splendor ; we 
 may therefore conclude that the small meteors composing 
 this group form a ring round the sun, and the earth evqry 
 loth of August is at the spot where this ring intersects our 
 orbit ; also that the ring of meteors is not equally dense in 
 all parts: here and there these small bodies must be very 
 thinly scattered, and in some places even altogether wanting. 
 
 Fig. XIX. shows a very small part of the elliptic orbit 
 which this meteoric mass describes round the sun S. The 
 earth encounters this orbit on the loth of August, and goes 
 straight through the ring of meteors. The dots along the 
 ring indicate the small, dark meteors which ignite in our 
 atmosphere, and are visible as shooting stars. The line m is 
 the .line of intersection of the earth's orbit and that of the 
 meteors.; the line P S shows the direction of the major axis 
 of .their orbit. This axis is fifty times 'greater than the mean 
 diameter of , the earth's orbit; the orbit of the meteors is in- 
 clined to that of the earth at an angle of 64 3', and their 
 motion is retrograde, or contrary to that of the earth. 
 
 The November shower is not observed to take place every 
 year on the I2th or I3th of that month, but it is found that 
 every thirty-three years an extraordinary shower occurs on 
 those days, proceeding from a point in the constellation of 
 Leo. The meteors composing this shower, unlike the Au- 
 gust one, are not distributed along the whole course of their 
 orbit, so as to form a ring entirely filled with meteoric par- 
 ticles, but constitute a dense cloud, of an elongated form, 
 which completes its revolution round the sun in thirty-three 
 
 26 
 
SCHBLLEN.] SPECTRUM ANALYSIS DISCOVERIES. 169 
 
 years, and crosses the earth's path at that point where the 
 earth is every I3th of November. 
 
 When the November shower reappears after the lapse of 
 
 FIG. XIX. 
 
 ORBIT OF THE METEOR SHOWER OF THE 10th OF AUGUST. 
 
 thirty-three years, the phenomenon is repeated during the 
 two following years on the I3th of that month, but with 
 diminished splendor ; the meteors, therefore, extend so far 
 
 27 
 
1 70 RECREATIONS IN POPULAR SCIENCE. [SCHELLKN. 
 
 along the orbit as to require three years before they have all 
 crossed the earth's path at the place of intersection ; they 
 are, besides, unequally distributed, the preceding part being 
 much the most dense. 
 
 A very small part of the elliptic orbit, and the distribution 
 of the meteors during the November shower, is represented 
 in Fig. XX. As shown in the drawing, this orbit intersects 
 that of the earth at the place where the earth is about the 
 I4th of November, and the motion of the meteors, which 
 occupy only a small part of their orbit, and are very un- 
 equally distributed, is retrograde, or contrary to that of the 
 earth. The inclination of this orbit to that of the earth is 
 only 17 44' ; its major axis is about ten and one third times 
 greater than the diameter of the earth's orbit, and the period 
 of revolution for the densest part of the meteorites round 
 the sun S is thirty-three years three months. 
 
 From all we have now learned concerning the nature and 
 constitution of comets, nebulas, cosmical clouds, and mete- 
 oric swarms, an unmistakable resemblance will be remarked 
 among these different forms in space. The affinity between 
 comets and meteors had been already recognized by Chladni, 
 but Schiaparelli, of Milan, was the first to take account of 
 all the phenomena exhibited by these mysterious heavenly 
 bodies, and with wonderful acuteness to treat successfully 
 the mass of observations and calculations which had been 
 contributed during the course of the last few years by Oppol- 
 zer, Peters, Bruhns, Heis, Le Verrier, and other observers. 
 He not only shows that the orbits of meteors are quite coin- 
 cident with those of comets, and that the same object may 
 appear to us at one time as a comet and at another as a 
 shower of meteors, but he proves also, by a highly elegant 
 mathematical calculation, that the scattered cosmical masses 
 known to us by the name of nebulae would, if in their 
 journey through the universe they were to come within the 
 powerful attraction of our sun, be formed into comets, and 
 these again into meteoric showers. 
 
 28 
 
SCHBLLBN.] SPECTRUM ANALYSIS DISCOVERIES. 171 
 
 The following is a short statement of Schiaparelli's theory. 
 Nebulae are composed of cosmical matter, in which, as yet, 
 there is no central point of concentration, and which has not 
 become sufficiently dense to form a celestial body,' in the 
 ordinary sense of the term. The diffuse substance of these 
 cosmical clouds is very loosely hung together; its particles 
 are widely separated, thus constituting masses of enormous 
 extent, some of which have taken a regular form, and some 
 not. As these nebulous clouds may be supposed to have, 
 like our sun, a motion in -space* it will sometimes happen 
 that such a cloud comes within reach of the power of attrac- 
 tion of our sun. The attraction acts more powerfully on 
 the preceding part of the nebula than on the further and fol- 
 lowing portion; and the nebula, while still at a great dis- 
 tance, begins to lose its original spherical form, and becomes 
 considerably elongated. Other portions of the nebulous 
 mass follow continuously the preceding part, until the sphere 
 is converted into a long cylinder^ the foremost part of which, 
 that towards the sun, is denser and more pointed than the 
 following part, which retains a portion of its original 
 breadth. As it nears the sun, this transformation of the 
 nebulous cloud becomes more complete : illuminated by the 
 sun, the preceding part appears to us as a dense nucleus, and 
 the following part, turned away from the sun, as a long tail, 
 curved in consequence of the lateral motion preserved by the 
 nebula during its progress. Out of the original spherical 
 nebula, quite unconnected with our solar system, a comet 
 has been formed, which, in its altered condition, will either 
 pass through our system, to wander again in space, or else 
 remain as a permanent member of our planetary system. 
 The form of the orbit in which it moves depends on the 
 original speed of the cloud, its distance from the sun, and 
 the direction of its motion, and thus its path may be ellipti- 
 cal, hyperbolical, or parabolical; in the last two cases, the 
 comet appears only once in our system, and then returns to 
 wander in the realms of space ; in the former case, it abides 
 
 29 
 
I7 2 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 with us, and accomplishes its course round the sun, like the 
 planets, in a certain fixed period of years. From this it is 
 evident that the orbits of comets may occur at every possible 
 angle to that of the earth, and that their motion will be 
 sometimes progressive and sometimes retrograde. 
 
 The history of the cosmical cloud does not, however, end 
 with its transformation into a comet. Schiaparelli shows 
 in a striking manner that, as a comet is not a solid mass, but 
 consists of particles each possessing an independent motion, 
 the head or nucleus nearer the sun must necessarily complete 
 its orbit in less time than the more distant portions of the 
 tail. The tail will therefore lag behind the nucleus in the 
 course of the comet's revolution, and the comet, becoming 
 more and more elongated, will at last be either partially or 
 entirely resolved into a ring of meteors. In this way, the 
 whole path of the comet becomes strewn with portions of 
 its mass, with those small, dark, meteoric bodies which, 
 when penetrating the earth's atmosphere, become luminous, 
 and appear as falling stars. Instead of the comet, there now 
 revolves round the sun a broad ring of meteoric stones, 
 which occasion the phenomena we every year observe as the 
 August meteors. Whether this ring be continuous, and the 
 meteoric masses strewn along the whole course of the path 
 of the original comet, or whether the individual meteors, as 
 in the November shower, have not filled up entirely the 
 whole orbit, but are still partially in the form of a comet, is 
 in the transformation of a cosmical cloud through the influ- 
 ence of the sun only a question of time ; in course of years, 
 the matter composing a comet which describes an orbit 
 round the sun must be dispersed over its whole path ; if the 
 original orbit be elliptical, an elliptic ring of meteors will 
 gradually be formed from the substance of the comet, of 
 the same size and form as the original orbit. 
 
 Schiaparelli has in fact discovered so close a resemblance 
 between the path of the August meteors and that of the 
 comet of 1862, No. III., that there cannot be any doubt as 
 
 30 
 
SCHKLLEN.] SPECTRUM ANALYSIS DISCOVERIES. 173 
 
 to their complete identity. The meteors to which we owe 
 the annual display of falling stars on the loth of August 
 are not distributed equally along the whole course of their 
 orbit ; it is still possible to distinguish the agglomeration of 
 meteoric particles which originally formed the cometary 
 nucleus from the other less dense parts of the comet ; thus 
 in the year 1862, the denser portion of this ring of meteors, 
 through which the earth passes annually on the loth of Au- 
 gust, and which causes the display of falling stars, was seen 
 in the form of a comet, with head and tail as the densest 
 parts, approached the sun and earth in the course of that 
 month. Oppolzer, of Vienna, calculated with great accu- 
 racy the orbit of this comet, which was visible to the naked 
 eye. Schiaparelli had previously calculated the orbit of the 
 meteoric ring, to which the shooting stars of the loth of 
 August belong before they are drawn into the earth's atmos- 
 phere. The almost perfect identity of the two orbits justi- 
 fies Schiaparelli in the bold assertion that the comet of '1862, 
 No. III., is no other than the remains of the comet out of 
 'which the meteoric ring of the ivth of August has been 
 formed in the course of time. The difference between the 
 comet's nucleus and its tail, that has now been formed into a 
 ring, consists in that while the denser meteoric mass forming 
 the head approaches so near the earth once in every hundred 
 and twenty years as to be visible in the reflected light of the 
 sun, the more widely scattered portion of the tail composing 
 the ring remains invisible, even though the earth passes 
 through it annually on the loth of August. Only fragments 
 of this ring, composed of dark meteoric particles, become 
 visible as shooting stars when they penetrate our atmosphere 
 by the attraction of the earth, and ignite by the compression 
 of the air. 
 
 A cloud of meteors of such a character can naturally only 
 be observed as a meteor shower when in the nodes of its 
 orbit, that is to say, in those points where it crosses the 
 earth's orbit, and then only when the earth is also there at 
 
174 RECREATIONS IN POPULAR SCIENCE. [SCHELLEN. 
 
 the same time, so that the meteors pass through our atmos- 
 phere. The nebula coming within the sphere of attraction 
 
 FIG. XX. 
 
 Orbits of the August and November Meteor Showers. 
 (Orbits of Comets III., 1862, and I., 1866.) 
 
 of our solar system, would, at its nearest approach to the 
 sun (perihelion), and in .the neighboring portions of its orbit, 
 
 32 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 175 
 
 appear as a comet, and when it grazed the earth's atmos- 
 phere would be seen as a shower of meteors. 
 
 Calculation shows that this ring of meteors is about ten 
 thousand nine hundred and forty-eight millions of miles in 
 its greatest diameter. As the meteoric shower of the loth 
 of August lasts about six hours, and the earth travels at the 
 rate of eighteen miles in a second, it follows that the breadth 
 of this ring, at the place where the earth crosses it, is four 
 million forty-three thousand five hundred and twenty miles. 
 In Fig. XX., A B represents a portion of the orbit of the 
 comet of 1862, No. III., which is identical with that (Fig. 
 XIX.) of the August shower. 
 
 The calculations of Schiaparelli, Oppolzer, Peters, and 
 Le Verrier have also discovered the comet producing the 
 meteors of the November shower, and have found it in the 
 small comet of 1866, No. I., first observed by Tempel, of 
 Marseilles. Its transformation into a ring of meteors has 
 not proceeded nearly so far as that of the comet of 1862, 
 No. III. Its existence is of a much more recent date ; and 
 therefore the dispersion of the meteoric particles along the 
 orbit, and the consequent formation of the ring, is but 
 slightly developed. 
 
 According to Le Verrier, a cosmical nebulous cloud en- 
 tered our system in January 126, and passed so near the 
 planet Uranus as to be brought by its attraction into an el- 
 liptic orbit round the sun. This orbit is the same as that of 
 the comet discovered by Tempel, and calculated by Oppol- 
 zer, and is identical with that in which the November group 
 of meteors make their revolution. 
 
 Since that time, this cosmical cloud, in the form of a 
 comet, has completed fifty-two revolutions round the sun, 
 without its existence being otherwise made known than by 
 the loss of an immense number of its components, in the 
 form of shooting stars, as it crossed the earth's path in each 
 revolution, or in the month of November in every thirty- 
 three years. It was only in its last revolution, in the year 
 
 33 
 
176 RECREATIONS IN POPULAR SCIENCE. [SCHELLW. 
 
 1866, that this meteoric cloud, now forming part of our solar 
 system, was first seen as a comet. 
 
 The orbit of this comet is much smaller than that of the 
 August meteors, extending at the aphelion as far as the orbit 
 of Uranus, while the perihelion is nearly as far from the sun 
 as our earth. The comet completes its revolution in about 
 thirty-three years and three months, and encounters the 
 earth's orbit as it is approaching the sun towards the end of 
 September. It is followed by a large group of small mete- 
 oric bodies, which form a very broad and long tail, througli 
 which the earth passes on the I3th of November. Those 
 particles which come in contact with the earth, or approach 
 so near as to be attracted into its atmosphere, become ignited, 
 and appear as falling stars. As the earth encounters the 
 comet's tail, or meteoric shower, for three successive years 
 at the same place, we must conclude the comet's track to 
 have the enormous length of seventeen hundred and seventy- 
 two millions of miles. In Fig. XX., C D represents a por- 
 tion of the orbit of this comet, which is identical with the 
 orbit of the November meteors. 
 
 By the side of these important conclusions, which the ob- 
 servation and acuteness of modern astronomers have been 
 able to make concerning the nature and mutual connection 
 of nebulae, comets, meteors, and balls of fire, the results of 
 spectrum analysis, as applied to meteors, will seem to be 
 exceedingly scant This is easy to understand when we 
 reflect how rapidly these fiery meteors rush through our at- 
 mosphere, and how difficult it is to lay hold of them with 
 the spectroscope during their instantaneous apparition. Be- 
 fore the instrument can be directed to a meteor or ball of 
 fire, and the focus adjusted, the object has disappeared from 
 view. The application, therefore, of spectrum analysis to 
 these fleeting visitors is left almost entirely to chance, and is 
 mainly confined to those nights in which yearly, or at certain 
 known periods, an extraordinary shower of falling stars is 
 expected to occur. 
 
 34 
 
SCHELLEN.] SPECTRUM ANALYSIS DISCOVERIES. 177 
 
 The principal result of the investigations thus far made is 
 confined, therefore, to the establishment of the fact that me- 
 teors consist of incandescent solid bodies, and that a differ- 
 ence is discernible in the chemical composition of the August 
 and November meteoric showers. 
 
 The November shower of 1868 was observed by Secchi. 
 Among the numerous meteors that left a train of light be- 
 hind them, was one, the track of which lasted fifteen min- 
 utes, and was at first sufficiently bright to allow of examina- 
 tion by a prism. Secchi found the spectrum to be discon- 
 tinuous, and the principal bright bands and lines were red, 
 yellow, green, and blue. Besides this observation, Secchi 
 was so fortunate as to see two meteors in the spectroscope : 
 the magnesium line appeared with great distinctness, besides 
 which some lines were also seen in the red. 
 
 On account of the great difficulty of observing meteors 
 with a narrow setting of the slit, ordinary spectroscopes are 
 not suited to this purpose. The only resource, therefore, is 
 to substitute a cylindrical lens for the slit. There can be no 
 doubt, however, that an apparatus will be invented which 
 will be employed in future with great success in the investi- 
 gation of meteors by means of spectrum analysis. 
 
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