SirW-HUGGINS and 
 SpectroscopicAstronomy 
 
 E-WMAUNDER r FR.A.S " 
 
 I 
 
 THE PEOPLE'S -BOOKS 
 
SIR WILLIAM HUGGINS 
 
 AND SPECTROSCOPIC 
 
 ASTRONOMY 
 
 BY E. W. MAUNDER, F.R.A.S. 
 
 OF THE ROYAL OBSERVATORY, GREENWICH 
 AUTHOR OF "ASTRONOMY WITHOUT A TELESCOPE" 
 
 "THE ASTRONOMY OF THE BIBLE" 
 "ARE THE PLANETS INHABITED?" ETC. 
 
 LONDON: T. C. & E. C. JACK 
 67 LONG ACRE, W.C., AND EDINBURGH 
 NEW YORK: DODGE PUBLISHING CO. 
 
CONTENTS 
 
 CHAP PAB 
 
 I. THE HERSCHEL OF THE SPECTROSCOPE . 7 
 
 II. THE SOURCE OF THE RAINBOW ... 12 
 
 in. THE SECRET OF THE RAINBOW . . . 17 
 
 IV. THE STORY OF SODIUM .... 23 
 
 V. THE STORY OF HYDROGEN .... 31 
 
 VI. THE STORY OF CARBON .... 39 
 
 VII. THE STORY OF IRON 43 
 
 VHI. THE STORIES OF MAGNESIUM AND TITANIUM 53 
 
 IX. THE STORY OF CALCIUM . ... 56 
 
 X. THE SECOND STORY OF HYDROGEN . . 63 
 
 XI. THE STORY OF HELIUM .... 73 
 
 XH. THE STORY OF NEBULIUM .... 79 
 
 xm. SUMMARY 89 
 
 INDEX 93 
 
 272820 
 
SIR WILLIAM MUGGINS AND 
 SPEGTROSCOPIG ASTRONOMY 
 
 CHAPTER I 
 
 THE HERSCHEL OF THE SPECTROSCOPE 
 
 SEVERAL of the volumes of this series are devoted to the 
 biographies of eminent men, and in such a case the 
 personality of the man himself, and the struggles through 
 which he had to pass furnish the chief points dwelt 
 upon. Thus Vol. XXIII gives a sketch of the life of 
 Thomas Huxley, and, as was inevitable with such a 
 subject, the character of the man himself as a man among 
 men, a born fighter and controversialist, quick, reso- 
 lute, fearless, is placed in vivid portraiture before the 
 reader. 
 
 WiLLiAMHuGGiNS, fifteen months the senior of Huxley, 
 was a man of an entirely different frame and disposition ; 
 not less devoted to the study of Nature, and endowed 
 with gifts not inferior to those of his great contemporary, 
 the path along which his inclination drew him was that 
 of quiet persistent research, and public controversy 
 was pre-eminently distasteful to him. For the one 
 man, the joy of conflict furnished the crowning zest 
 in every enterprise, without which it lacked something 
 of the true flavour of life ; to the other, dispute or 
 contention spoiled any inquiry however attractive in 
 itself. The greatest of all men that Science has pro- 
 duced Newton complained " Philosophy is such an 
 
8 SIR WILLIAM HUGGINS 
 
 ^ntly ti-Mgious ]ady that a man had as good be 
 engaged in lawsuits as have to do with her," and he 
 shrank so much from such discussions as even to con- 
 template giving up scientific research. " I see I have 
 made myself a slave to philosophy, but if I get free of 
 this present business, I will resolutely bid adieu to it 
 eternally, except what I do for my private satisfaction 
 or leave to come out after me ; for I see a man must 
 either resolve to put out nothing new, or to become a 
 slave to defend it." And with his great predecessor, 
 Huggins would have been in full agreement. The 
 research itself, the meeting and overcoming the diffi- 
 culties it presented, the patient, cautious, infinitely 
 careful working up to stable conclusions, these were 
 what attracted him, not the plaudits of the crowd that 
 watch the efforts of men contending in an arena. And 
 it must be remembered that this quiet, withdrawn, and 
 patient work is the true Science, the thing of real value ; 
 even in the case of those, like Huxley, whose circum- 
 stances and gifts lead them into the dust and heat of 
 controversy. Though Huggins was little more than 
 a mere name to most of his countrymen, his work is the 
 very type and example of scientific research ; patient 
 and resourceful, cautious and bold, far-seeing and, above 
 all, conscientious and exact. 
 
 WILLIAM HUGGINS was born on February 7, 1824, in 
 London, and in 1856 he built himself an observatory at 
 Tulse Hill, then quite out of London. Here for four 
 or five years he made many observations of the sur- 
 faces of the sun and planets, especially of Jupiter. 
 These early observations have been overshadowed by 
 the great achievements of later years, but they gave 
 evidence of many of the qualities that were to be 
 brilliantly displayed later on. 
 
 But none of the then recognised departments of 
 astronomical observation appealed to Huggins as offer- 
 ing him the sphere of activity he sought. As he himself 
 has recorded in a retrospect in the Nineteenth Century 
 Review for June, 1897 : 
 
HERSCHEL OF THE SPECTROSCOPE 9 
 
 " I soon became a little dissatisfied with the routine character 
 of ordinary astronomical work, and in a vague way sought about 
 in my mind for the possibility of research upon the heavens in 
 a new direction or by new methods. It was just at this time, 
 when a vague longing after newer methods of observation for 
 attacking many of the problems of the heavenly bodies filled 
 my mind, that the news reached me of KirchhofPs great discovery 
 of the true nature and the chemical constitution of the sun from 
 his interpretation of the Fraunhofer lines. 
 
 " This news was to me like the coming upon a spring of water 
 in a dry and thirsty land. Here at last presented itself the 
 very order of work for which in an indefinite way I Mas looking 
 namely, to extend his novel methods . of research upon the 
 sun to the other heavenly bodies. A feeling as of inspiration 
 seized me : I felt as if I had it now in my power to lift a veil 
 which had never before been lifted ; as if a key had been put 
 into my hands which would unlock a door which had been 
 regarded as for ever closed to man the veil and door behind 
 which lay the unknown mystery of the true nature of the 
 heavenly bodies. This was especially work for which I was 
 to a great extent prepared, from being already familiar with the 
 chief methods of chemical and physical research." 
 
 To extend by means of the spectroscope the work 
 that Kirchhoff had begun, from the sun to every class 
 of luminary that the heavens present this was the idea 
 that filled the mind of Huggins, the enterprise to which 
 he forthwith devoted himself for nearly half a century. 
 The fundamental principle of the new research was 
 not his, but it was peculiarly hig to see in how many 
 new directions it might be applied, and to meet and 
 overcome the enormous practical difficulties before 
 him. 
 
 For Kirchhoff had worked upon the sun, and this 
 supplies to us ten thousand million times the light from 
 Sirius, the brightest of all stars ; yet in some respects 
 even sunlight was none too bright for the new method 
 of analysis. It might well have daunted even the most 
 sanguine to apply the spectroscope, not to the sun only, 
 but to stars, to planets, and to fainter objects still, 
 such as comets and nebulae. 
 
 Yet Huggins obtained the most signal success. He 
 
10 SIR WILLIAM HUGGINS 
 
 identified the presence of hydrogen, sodium, magnesium, 
 iron, and other elements in a number of the brighter 
 stars ; he determined the character of the spectra of 
 comets and nebulae ; he took prompt advantage of the 
 invention of the gelatine dry plate and applied it to 
 the new research. Prior to the invention of the spectro- 
 scope, the study of the movements of the heavenly 
 bodies had been astronomy in the strict sense ; and 
 Bessel, the greatest astronomer of his time, had some- 
 what scornfully refused to recognise any other celestial 
 research as worthy of the name ; but Huggins showed 
 how it was possible to observe the movements of 
 heavenly bodies, not as they appear projected on the 
 vault of the sky, but in the direction at right angles to 
 that projection along the radial lines which have the 
 earth as their centre. 
 
 In almost every department of the physical study of 
 the heavenly bodies, Huggins either led the way, or 
 was one of the earliest and most resourceful workers, 
 so that R. A. Proctor was justified in applying to him 
 the title of the " Herschel of the Spectroscope." For, 
 just as Sir William Herschel, though he neither in- 
 vented the telescope nor was the first to apply it 
 to astronomical observation, yet used it as none had 
 used it before to " break through the barriers of the 
 heavens," to ascertain the structure of the sidereal 
 universe, so in his turn Huggins, who neither in- 
 vented the spectroscope nor was the first to use it 
 in astronomy, yet stamped his name and impress on 
 almost every department of research to which it was 
 directed. 
 
 In the following chapters, no attempt is made to set 
 forth the life of Huggins in narrative form ; he lived 
 for his science ; his life was his work. To the world in 
 general, Huggins was little known, except as a name, 
 held in vague regard. By his fellow-workers in Science, 
 whatever their nationality and department of study, he 
 was esteemed one of the few who are in the first rank. 
 The highest position in Science in this country is the 
 
HERSCHEL OF THE SPECTROSCOPE 11 
 
 Presidentship of the Royal Society, and that he held 
 for five years. In 1902, when the Order of Merit was 
 instituted by King Edward, he was chosen to be one 
 of the twelve who were the original members of the 
 Order. The principal national learned societies abroad, 
 both in Europe and America, enrolled him as honorary 
 or foreign member, and his colleagues in Science were 
 continually referring to him for his counsel and judgment 
 in scientific questions. 
 
CHAPTER II 
 
 THE SOURCE OF THE RAINBOW 
 
 Bur what is the Spectroscope, of which Huggins was 
 thus so great a master ? What is the Spectrum, and 
 what is meant by Spectrum Analysis ? 
 
 Light reveals to us two properties of natural objects 
 their form and their colour. The form does not concern 
 us here, but colour is of the very essence of our present 
 inquiry. 
 
 We are apt to think of the colour of an object as 
 something added to it, just as a painter adds his pig- 
 ments to his canvas. But a little thought reveals the 
 contrary to be the case. If we enter a photographer's 
 dark room and close the red window, we have evidently 
 not increased the light in the room, but diminished it ; 
 the white light was more than the red. So, too, if we 
 watch a richly-coloured landscape during the coming on 
 of an eclipse of the Sun, we see that as the light diminishes 
 the objects of most vivid hue disappear, while white 
 objects still remain visible. The colour white is more, 
 not less, than the colour red, or the colour yellow. 
 Something is lost to us when we fill a window, not with 
 clear glass, but with red or green ; something is lost 
 if we colour a white wall with red or green paint. 
 
 SIR ISAAC NEWTON was the first to prove that this is 
 the case, and that coloured light is white light from 
 which some element has been removed ; that white 
 light is the sum of light of all colours. He was experi- 
 menting on the properties of a prism; that is, of a 
 piece of glass, triangular in section, and he found what 
 had indeed been known before that a beam of white 
 
 12 
 
THE SOURCE OF THE RAINBOW 13 
 
 light on passing through a prism suffers two changes, 
 one of direction and one of character or colour. If the 
 Sun shines through a small hole in the window shutter 
 of a darkened room, a bright spot of light is seen to fall 
 on the wall or floor ; a spot exactly in a straight line 
 with the opening and with the Sun ; but if a prism 
 be placed in the path of the ray, then the light is turned 
 aside at the prism, and bent out of its former course. 
 This is what is called the refraction of light : the 
 bending it from a straight course. 
 
 But this bending is not the only change. If the 
 original beam were received on a white screen, say a 
 sheet of cardboard, and this screen were placed at right 
 angles to the beam, then the spot of light would be, like 
 its source, perfectly round. But the refracted beam 
 makes a spot neither round nor white, but elongated 
 and coloured ; it is a strip, not a circle of light ; it is 
 red at one end, shading into yellow, green, blue, and 
 indigo in the middle, and violet at the other end ; it looks 
 like, and indeed is, a slice cut out of the arch of a 
 rainbow. This second change, the spreading out of the 
 white light into a coloured streak, is known as dis- 
 persion, and Newton gave the name of the Spectrum 
 to the many-coloured image thus produced. 
 
 From what source do these colours spring ? The light 
 entering the room is white ; whence has it derived the 
 colours ? It was this question, unasked by others, that 
 Newton set himself to solve. First, he found that light 
 travels in a straight line after passing through a prism, 
 as it did before entering, for the refraction, the bending, 
 takes place at the two surfaces of the prism and there 
 alone ; at the surface where the light enters and at that 
 where it passes out. Next, the spreading out is regular. 
 If the screen is put close to the prism, the coloured 
 image is short ; if it is a long way off, it is lengthened 
 out, but in the exact proportion of the greater distance 
 of the screen ; the angular amount of the spreading is 
 the same in each case. 
 
 But if an aperture be made in the screen, and a 
 
14 SIR WILLIAM HUGGINS 
 
 beam from the many-coloured band of light, say from 
 the yellow region or the green, be allowed to travel 
 further on and to pass through a second prism, this yellow 
 or green beam will thereby suffer refraction a second 
 time, but no further dispersion. Light from the green 
 region will remain green, from the yellow will remain 
 yellow, and the second prism will not cause it to take 
 on any tint of red or violet. 
 
 From such simple experiments, Newton proved that 
 white light is made up of light of many different colours, 
 all travelling along the same path, and so, to our sight, 
 indistinguishable the one from the other. But on en- 
 countering a prism, these rays of different colours are 
 differently refracted, bent from their course in different 
 degrees, and hence are partially separated from each 
 other ; their dispersion is due to the differences of their 
 refraction. 
 
 The red light that comes through a photographer's 
 window is not white light that has had red light added 
 to it, but white light from which the blue, green and 
 yellow elements have been stopped out The red glass 
 is opaque to them. In like manner the red petals of a 
 geranium add nothing to the light falling on them ; on 
 the contrary they absorb the blue, green and yellow, 
 and reject, i.e., reflect, only the red. 
 
 It is easy to reconstruct white light from the many- 
 coloured band of the " Spectrum " by allowing it to fall 
 on a second prism, like the first in shape and material, 
 but placed in the reverse position. Or if the spectrum be 
 allowed to fall upon a series of very small mirrors these 
 can be tilted so as to reflect the particular part of the 
 spectrum falling on each, to meet at the same point of 
 a suitable screen. In this way it can be shown that 
 white light can be produced by the mixture of rays from 
 three, or even from only two, different parts of the spec- 
 trum if these are properly selected. 
 
 The Rainbow is the Spectrum seen in Nature when 
 the Sun shines out after a shower of rain, and its light 
 falls on a layer of the atmosphere still laden with minute 
 
THE SOURCE OF THE RAINBOW 15 
 
 particles of moisture, the light is partly reflected from 
 the internal surfaces of each rain-drop and partly re- 
 fracted, and because refracted it also suffers dispersion ; 
 the amount of refraction is different for the different 
 colours. Hence the observer sees the sevenfold band 
 of colour spread out before him on part of the circum- 
 ference of a circle of which he is himself the centre. 
 This being so, the old fable that a crock of fairy gold 
 waits him who will dig in the ground from which the foot 
 of the Rainbow springs, is easily explained ; for the 
 man who follows the Rainbow finds it always reced- 
 ing from him ; he is always at its centre, never 
 where it meets the earth ; it is the pursuit of the 
 unattainable. 
 
 But the old fable has had a wonderful fulfilment in 
 the fairyland of Science ; for those who have dug to find 
 the root of the Rainbow, the source from which it springs, 
 have found an inexhaustible treasure ; a wonderful 
 store of information about 1he stars above us, and the 
 earth under our feet ; about the innermost structure of 
 the ultimate particles of matter. 
 
 Newton distinguished seven colours in the Rainbow 
 or spectrum : red, orange, yellow, green, blue, indigo 
 and violet, but each of the seven melts into the next on 
 either side without any definite limit. PIAZZI SMYTH 
 raised the number to ten by placing deep-red before 
 red, and dividing green into citron or yellowish-green, 
 emerald or full green, and glaucous or bluish-green, 
 and the fuller division has some advantages. But even 
 assuming that there are but seven different colours in 
 the spectrum, it is clear that in Newton's experiment 
 they would overlap, for a round hole to admit the light 
 would mean a series of round images and his spectrum 
 was not seven times as long as it was broad. Newton 
 appears to have recognised this, and actually tried ad- 
 mitting the light through a narrow slit, so as to have a 
 series of narrow coloured images side by side, over- 
 lapping each other as little as possible. But for some 
 reason, perhaps simply because his chief object in these 
 
16 SIR WILLIAM HUGGINS 
 
 experiments was the improvement of the telescope, he 
 failed to notice a remarkable feature that the spectrum 
 produced in this manner is able to show. He had dis- 
 covered the way in which the Rainbow was formed, but 
 he had not discovered its Secret. 
 
CHAPTER III 
 
 THE SECRET OF THE RAINBOW 
 
 ONE hundred and thirty years passed after Newton's 
 first experiments on the spectrum before another added 
 to his work. But in 1802, WOLLASTON, looking casually 
 through a prism at a narrow opening in a window-blind, 
 perceived that the rainbow-tinted image of the slit was 
 crossed by several dark lines parallel to the line of opening. 
 These he supposed to be the boundaries of the different 
 colours, and being satisfied with this explanation, he did 
 not push the inquiry any further. Twelve years later 
 a German optician, FRATJNHOFER, turned his attention 
 to the subject, and devised a more compact instrument 
 for examining the spectrum than the dark room with a 
 hole in the window-shutter. With him, as with Wol- 
 laston, the light was admitted to the prism through a 
 narrow slit, but after passing through the prism, it was 
 not thrown on a screen, but examined directly by means 
 of a small telescope. Under these circumstances, 
 Fraunhofer obtained a spectrum of great purity ; the 
 images of the slit given by rays of different refrangibility, 
 that is refracted in different degrees, overlapped as 
 little as possible, and from end to end of the spectrum 
 it was crossed by hundreds of fine dark lines, each an 
 image in negative of the slit. 
 
 Thus the solar spectrum was seen at last ; seen as 
 composed of an infinite number of colours gently graded 
 the one to the other from a dull deep brownish-red 
 through fiery red and glowing yellow, vivid green and 
 restful blue, to a violet that died away into a sombre 
 tint that may best be described as a lavender-grey. But 
 
 17 B 
 
18 SIR WILLIAM HUGGINS 
 
 the whole succession of living colours were crossed and 
 interrupted by an immense number of very thin dark 
 lines, differing in breadth and intensity of darkness, and 
 arranged in a fashion apparently irregular. 
 
 Fraunhofer perceived that he had opened the way 
 to an inquiry of great importance. What caused these 
 lines, what was their explanation, where did they take 
 their rise ? These were questions that pressed for 
 answers, and so far as his short life and many occupations 
 permitted him, he succeeded in obtaining them. 
 
 These dark lines were not characteristic of the spec- 
 trum of every bright body ; a terrestrial substance heated 
 up to incandescence gives all the colours of the rainbow, 
 uninterrupted by any dark lines. Thus if a piece of iron 
 be heated in a fire, it first becomes what we call " red- 
 hot," and when looked at through a prism only the red 
 end of the spectrum is seen. As the iron is heated still 
 more strongly, it shines more brightly, but with a whiter 
 light, and as seen through the prism, its spectrum 
 lengthens out ; the colours extend into the orange and 
 yellow. And as the process of heating it is continued, 
 the iron becomes at length white-hot ; it shines bril- 
 liantly, and as looked at through the prism, gives the 
 entire succession of colours from red to violet ; but, 
 unlike the Sun, its spectrum shows no interruption ; 
 there are no dark lines. And what is true of iron is true 
 of practically all other incandescent solids ; they yield 
 continuous and uninterrupted spectra. 
 
 Fraunhofer's first work in tracing out the origin of 
 these dark lines, called since his time the Fraunhofer 
 lines, was to make a careful map of them, and he 
 counted 574 between the red and the violet, of which 
 number he represented 354 in his map. The principal 
 lines he distinguished by letters of the alphabet, and 
 determined their refrangibility. With the dispersion 
 that he used these chief lines appear as follows : A is 
 a thick dark line at the extreme red end of the spectrum ; 
 B is also a broad line ; between the two is a cluster of 
 several lines, called a ; C is dark and fine ; all four of 
 
THE SECRET OF THE RAINBOW 19 
 
 these are in the red. D is a very close pair of dark lines 
 in the orange-yellow ; E is the middle and darkest of 
 a group in the yellowish -green ; b a group of four dark 
 lines where the green is more of an emerald tint ; F seems 
 to be about the boundary between green and blue ; 
 G is in a crowded cluster in the indigo ; and H is a pair 
 of bands near the limit of vision in the extreme violet. 
 
 Then, turning his attention to other sources of light, 
 Fraunhofer found that the Moon, some of the planets, 
 the clouds, the general sky, all of which are bright by 
 reflecting the light of the Sun, show exactly the same 
 lines in their spectra. The stars, with their feeble light, 
 were more difficult of examination, but as each star was 
 but a point in the teles ope, there was no need to pass 
 the light through a slit ; all that was required was to 
 place a prism before the object-glass of the telescope. 
 Observing in this way, Fraunhofer found that they did 
 not all give the same dark lines. Some, like Arcturus, 
 resembled the Sun very closely in the spectra that they 
 yielded ; others, like Sirius and Antares, differed from 
 it widely, and not less widely from each other. 
 
 This was important, for it showed that the cause of the 
 Fraunhofer lines was neither in the earth's atmosphere 
 nor in interstellar space, but lay in the Sun or star itself, 
 or in its immediate surroundings. 
 
 Yet this was not the case with all the dark lines and 
 bands. Some of them were soon proved to be, not 
 formed in or round the Sun or star, but in the atmosphere 
 of the earth ; they were telluric lines. For when the 
 Sun was low down in the sky, as at rising or setting, and 
 was therefore seen through a great thickness of atmos- 
 phere, such lines were numerous, broad and dark ; as 
 it rose higher and was looked at through a thinner 
 stratum of atmosphere, these lines became thinner and 
 fainter or disappeared altogether. And though the 
 spectra of Sirius or Vega when high in the heavens were 
 quite unlike that of the Sun, the lines that came into 
 them when they approached the horizon were just the 
 same as those that under the same condition invaded 
 
20 SIR WILLIAM HUGGINS 
 
 the solar spectrum. After Fraunhofer's death in 1826 
 at the age of thirty -nine, BBEWSTER and GLADSTONE 
 made a careful map of these telluric lines. 
 
 Some of the lines in the solar spectrum had been ex- 
 plained, but the great majority, all those indeed that 
 truly belonged to the Sun, still remained without any 
 clue to their meaning ; they were still the Secret of 
 the Rainbow. 
 
 Fraunhofer's work was far greater than this brief 
 outline would suggest. For one thing he measured the 
 wave-lengths of many of the chief spectral lines. Light 
 consists of wave-movements, very short and rapid ; so 
 short that 50,000 waves of green light or 40,000 of 
 orange light are comprised in a single inch. It is indeed 
 because white light is made up of vibrations differing 
 in length that on passing through a prism it is decom- 
 posed into lights of different colours, i.e. of different 
 wave-lengths. For if abeam of white light passes from 
 a rare medium to one that is denser, it suffers a retarda- 
 tion ; if from a dense medium to a rarer, the effect is 
 the reverse. If it encounters a plate of the dense 
 medium, and the beam of light strikes the two surfaces 
 at right angles, no change is effected ; the retardation 
 upon entering the plate is balanced by the acceleration 
 upon leaving it. But if the denser medium is triangular 
 in section, and the beam strikes the two surfaces ob- 
 liquely, the beam is turned from its course, and the 
 shorter the wave-length, the more waves to an inch, the 
 greater is the amount of the turning. Violet light being 
 that of shortest wave-length, is refracted, or bent out of 
 its course, the most strongly ; deep-red, being that of 
 longest wave-length, is least deviated. The different 
 vibrations, which entered the prism en masse indis- 
 tinguishable from each other, leave it therefore by dif- 
 ferent paths and are seen apart. 
 
 But the lights of different wave-lengths may be 
 separated from each other, not only by passing them 
 through a prism, but also by using a grating. This 
 may roughly be described as a sieve for the vibrations 
 
THE SECRET OF THE RAINBOW 21 
 
 of light, the divisions of the grating not being very much 
 further apart than the wave-lengths of a single vibration. 
 The first grating made by Fraunhofer was ingenious : 
 two long screws of very delicate pitch were screwed 
 through two pieces of wood, so as to make a square 
 frame, and fine wire was wound over and over the two 
 screws between their threads ; the wires were then 
 soldered to the screws, and lastly the screws with the 
 w^ooden supports, were sawn through lengthways, so 
 that two frames, each carrying a fine wire grating, were 
 obtained. Later Fraunhofer made a grating by ruling 
 a piece of glass, by a fine diamond point, with a vast 
 number of fine parallel lines at equal distances apart. 
 Modern gratings are ruled either on glass or on speculum 
 metal, and these latter may be either plane or concave ; 
 the number of lines to the inch being usually about 
 14,400, but occasionally of 10,000 or 20,000 lines. 
 
 One property of a grating is that the amount of de- 
 viation produced in any ray is a function cf its wave- 
 length, and it was from this property that Fraunhofer 
 was able to measure the wave-lengths of the chief lines ; 
 such measures showing that the dark lines are permanent 
 features of the solar spectrum, fixed as to their places 
 in it ; fixed, that is to say, as to their refrangibilities and 
 the lengths of the vibrations corresponding to them. 
 
 For the edge of the great A band in the deep-red there 
 are 33,447 waves to the inch ; for the similar edge of B, 
 36,985 ; for C in the bright red, 38,701 ; the close pair 
 of lines in the orange-yellow, D x and D 2 , give 43,078 and 
 43,122 respectively; E in the citron-green, 48,195; 
 F in the glaucous-green, 52,246 ; G in the indigo, 58,959 ; 
 and H where the violet is fading into grey, 64,001. 
 Wave-lengths are not usually expressed in this manner, 
 though perhaps it gives the clearest idea of the minuteness 
 of these vibrations. Usually they are expressed as being 
 so many tenth-metres in length ; a tenth-metre being 
 a metre divided by 10 10 ; it is the ten-millionth part of 
 a millimetre. This scale was adopted by ANGSTROM, 
 who made the first great map of the diffraction spectrum, 
 
22 SIR WILLIAM HUGGINS 
 
 that is, the spectrum as given by a grating, and hence 
 a o tenth-metre is usually known as an Angstrom Unit 
 (A. U.). A table of 23,000 lines has been prepared by 
 ROWLAND in which the wave-length of each is given to 
 one-thousandth part of an Angstrom Unit ; those for 
 the chief Fraunhof er lines standing as under : 
 
 A, 7594-059 B, 6867'461 C, 6563-054 D lf 5896*154 D 2 , 5890'182 
 E, 5270-106 F, 4861-496 G, 4307'988 H, 3968-620 
 
 The sun, then, sends us light of every conceivable 
 wave-length from those of deep-red to lavender-grey, 
 except for the special wave-lengths corresponding to 
 those of the dark Fraunhof er lines. But what gives rise 
 to these lines themselves ; why is the bright spectrum 
 of the sun broken at these points ; why does the sun 
 fail to send us light of these particular wave-lengths ? 
 
CHAPTER IV 
 
 THE STORY OF SODIUM 
 
 A FAMILIAR tale will often bear retelling if it be told 
 from a new standpoint, and that which was well known 
 may gather freshness from the new setting. So I am 
 fain to hope that the story of the work of the spectro- 
 scope in the field of astronomy, familiar as it is to most, 
 and difficult as it is to some, may, if presented in a new 
 manner, be acceptable to the former and perhaps helpful 
 to the latter. 
 
 The leading principle of the arrangement which I 
 propose to adopt in the remaining chapters of this book 
 is this : The various stages of the progress in spectro- 
 scopy have been associated with different elements. 
 One element has furnished the key to one problem, 
 another to another ; the simplicity of one has proved 
 of value in one difficulty ; the complexity of another 
 has been not less useful in other conditions. 
 
 To write the history of spectroscopy neither from the 
 point of strict chronological order, which is the method 
 of the annalist ; nor by following in succession the 
 labours of the great men who have developed the science, 
 the method of the biographer ; nor by passing in review, 
 one by one, the various celestial bodies to which the 
 spectroscope has been applied, the method of the as- 
 tronomer, might well seem at first sight to close the way 
 to any logical treatment. And yet I think that to call 
 up one by one the various chemical elements which have 
 taught us the most, and to interrogate them as to the 
 parts which they have severally played in the working 
 out of the science, may well have a force and simplicity 
 
 23 
 
24 SIR WILLIAM HUGG1NS 
 
 of its own, and lend a quasi-dramatic touch to the 
 narrative. 
 
 It is hard to realise how essentially modern the whole 
 science of spectroscopy really is. One hundred years 
 ago this method of analysis had not been dreamed of, 
 even in its simplest form. Yet some of its simpler 
 phenomena must surely have been noticed. It was at 
 one time much the custom to ornament chandeliers and 
 candlesticks with long triangular pieces of glass which 
 were called " lustres." The bright rainbow-coloured 
 images which these produced when the light of the Sun 
 or of the candles fell upon them were much admired, 
 yet no one seems to have had any idea that these bright 
 streaks with their regular gradations of colours red, 
 orange, yellow, green, blue, violet had any significance 
 than that of pleasing the eye with an unwonted effect. 
 No doubt it often happened that in looking through a 
 " lustre " at a candle-flame, it had been noticed that 
 most bright objects when viewed in this way entirely 
 lost their shape, every portion of their form being spread 
 out into a rainbow-coloured streak, yet the candle-flame 
 would show itself in perfect outline right amongst the 
 broad band of colours. So too, when looking at the 
 firelight, while some of the flames as seen through the 
 lustre would lose their shape entirely in a broad con- 
 fusion of bright colours, here and there a particular 
 flame would stand out by itself, apparently quite un- 
 changed by the effect of the prism ; its shape perfectly 
 recognisable and unaltered by any fringe of red or blue 
 light on either edge. 
 
 If anyone at any time made this observation, then, 
 all unknown to himself, he was watching the spectrum 
 of one of the elements, the spectrum of Sodium. 
 
 The meaning of this appearance will be better under- 
 stood if we repeat the experiment and slightly vary its 
 conditions. For example, take three sources of light : 
 an incandescent gas mantle, an ordinary candle flame, 
 and a spirit lamp to which some colour and luminosity 
 have been given by sprinkling the wick with carbonate 
 
THE STORY OF SODIUM 25 
 
 of soda or common salt. In all three cases one pre- 
 caution must be observed ; the prism must not be held 
 between the eye and the flame directly, but consider- 
 ably to one side of the latter, the point of the prism being 
 away from the flame. And with all three the same 
 effect is seen ; the coloured image does not appear to 
 lie in the direction of the flame, but in the direction of 
 the prism. But the image is of quite a different char- 
 acter in the three cases. The incandescent mantle 
 gives a coloured glow in which no resemblance to the 
 shape of the mantle can be traced. The candle flame 
 likewise shows all the prismatic colours in a broad stripe, 
 unlike the candle flame in outline ; but on the coloured 
 stripe may be seen, between the red and green, a faint 
 yellow image of the flame, an image which becomes 
 bright and distinct if a few grains of salt are added to 
 the candle wick. The spirit lamp treated with carbonate 
 of soda shows itself practically unchanged when viewed 
 through the prism ; every detail of the flame is just as 
 we see it when looked at direct. Apparently the only 
 effect which the prism has had upon it is that of a seem- 
 ing change in its location. 
 
 The cause of the difference in effect of the prism on 
 these three sources of light is easily understood. The 
 spirit lamp shines by light of only one colour ; its image 
 is seen through the prism in that same one colour and 
 unaltered in shape. The incandescent mantle shines 
 by white light every colour is represented in its light, 
 no one preponderates. The candle flame shines by light 
 of every colour, but one particular colour preponderates ; 
 each colour produces its own image of the candle flame, 
 but these, by overlapping, are confused together, and 
 the only image that can be recognised is the one due to 
 the excess of yellow light. In the gas-mantle, as no one 
 colour preponderates, no image due to any one colour 
 can be isolated from the rest, and instead of a picture 
 of the gas mantle, there is only a broad streak of the 
 colours of the rainbow. 
 
 If the experiment is varied so that the three sources 
 
26 SIR WILLIAM HUGGINS 
 
 of light, enclosed in a metal box, shine out only through 
 a small opening in it, in like manner we should see the 
 shape of that opening a round hole, a narrow straight 
 slit, or other shape perfectly reproduced in yellow 
 light from the spirit lamp ; in the candle flame it would 
 be faintly seen on a broad background of prismatic 
 colour ; whilst the gas-mantle would show these 
 colours alone without any clear sharp image of the open- 
 ing through which the light is shining. But in the case 
 of the candle flame, the yellow image of a narrow slit 
 would be more easily recognised than that of a round 
 hole ; for with the round, the red on one side and the green 
 on the other would overlap the yellow image more 
 seriously than in the straight and narrow slit. To use 
 a technical expression, the slit gives a " pure " spectrum. 
 
 The one essential portion of a spectroscope is a prism 
 or some other means for making light of different colours 
 follow different paths. A narrow slit is also required 
 in almost every instance, but the practical efficiency of 
 a spectroscope is immensely increased by placing a lens 
 between the slit and the prism, and having the slit in 
 its focus. This lens, technically called the collimator, 
 renders the rays that diverge from the slit parallel as 
 they fall on the prism. If a small telescope be now used 
 to view the spectrum formed by the prism, the instru- 
 ment may be said to be complete. Several prisms or a 
 grating may be substituted for the single prism, and so 
 the separating power of the spectroscope may be in- 
 creased, and many accessories and refinements may be 
 added, but no essential alteration is made in the first 
 principle of the spectroscope. 
 
 When Fraunhof er, the first to make and use a spectro- 
 scope, observed by its means a candle flame, he noticed 
 that it gave, not one yellow image of the slit, but two 
 exceedingly close together. The yellow light, therefore, 
 which preponderates in the flame is not of one kind 
 merely, but of two, differing from each other only very 
 slightly. This shows the importance of having a 
 complete spectroscope with which to work, for a prism 
 
THE STORY OF SODIUM 27 
 
 by itself would not enable the eye to distinguish be- 
 tween the two images of the flame, which, though really 
 present, are so similar and so close together, that with- 
 out an extremely narrow slit and a telescope to view 
 the spectrum, they could not be separated the one from 
 the other. 
 
 If we repeat Fraunhofer's observation with the salt- 
 fed spirit lamp, we see at first when the slit is wide open, 
 only one broad yellow bright line, whose breadth roughly 
 corresponds to that of the slit opening. As the slit is 
 gradually closed, this bright line narrows likewise, until 
 at length it has a distinct triple appearance, the centre 
 portion being twice as bright as the two exterior portions. 
 As the slit is still further narrowed, this central brighter 
 portion becomes rapidly thinner, until at length it 
 vanishes, and a thin hair-line of blackness separates the 
 two bright lines. These two bright lines will each narrow 
 further as the slit is further closed, the dark space 
 separating them widening, since the centres of the two 
 bright lines remain at the same distance apart through- 
 out. It is clear that the triple appearance when the 
 slit is wider is due to the overlapping of the two images. 
 
 That all spectra consist of series of variously coloured 
 images of the source of light, becomes yet more apparent 
 when a flame of richer colour than that of the salt-fed 
 lamp is examined. If the spectroscope be turned to- 
 wards the " red fire " used in theatres a great number 
 of bright lines are seen in the red, each widening with a 
 widening slit, and narrowing as it is closed ; and chang- 
 ing in form if the form of slit be changed. Most of the 
 lines seen from the " red fire " are recognised as those 
 given by a spirit lamp fed with chloride of strontium ; 
 and as early as 1827, SIR JOHN HERSCHEL found one 
 particular grouping of red lines so characteristic of the 
 chloride of strontium, and other groupings as character- 
 istic of other substances, that, as he expressed it, " the 
 colours thus contributed by different objects to flame 
 afford in many cases a ready and neat way of detecting 
 extremely minute quantities of them." It afforded also 
 
28 SIR WILLIAM HUGGINS 
 
 the means of distinguishing between flames that to the 
 unassisted eye appeared practically identical. Thus 
 chloride of lithium gives a flame very like that from 
 chloride of strontium, but in the spectroscope the lithium 
 flame gives but one red line or image of the slit, and that 
 of strontium gives several red lines, besides a blue line 
 whose presence would not have been suggested by the 
 mere appearance of the flame itself. A new method of 
 analysing substances had been originated that of 
 Spectrum Analysis. 
 
 None of these spectra, however, are so universal as 
 that of the close pair of yellow lines seen in the candle 
 flame or salt-fed spirit lamp ; for no matter what sub- 
 stance is volatilised, this unmistakable pair is sure to 
 put in an appearance more or less strongly, and it is 
 most conspicuous when common salt or carbonate of 
 soda, or some other compound of sodium is added to 
 the flame, though it can be detected when, to all appear- 
 ance, sodium has been carefully excluded. So much 
 is this the case that FOX-TALBOT was driven to conclude 
 that the ubiquitous yellow pair were due to the pre- 
 sence of water. More careful methods ended, however, 
 in showing that they are really due to sodium and that 
 their constant presence is explained, partly by the fact 
 that sodium is more widely distributed than any had 
 supposed, and partly by the extraordinary delicacy of 
 the spectroscopic method of analysis ; for it has been 
 calculated that the one-two hundredth part of the 
 millionth of a grain of sodium will reveal its presence 
 by this unmistakable pair of lines. 
 
 As mentioned in the preceding chapter, Fraunhofer 
 had noted in the solar spectrum a very close pair of dark 
 lines in the orange-yellow to which he gave the letter D. 
 He further noted that these two dark lines fell in the 
 same part of the spectrum, and were the same distance 
 apart as the two bright yellow lines which he had al- 
 ready detected in the spectrum of the candle flame, 
 which many years later were found to be characteristic 
 of sodium. 
 
THE STORY OF SODIUM 29 
 
 So the matter stood for many years. But in 1859, 
 Kirchhofl: of Heidelberg resolved to test if this alleged 
 correspondence were perfect, and therefore he observed 
 daylight through a Bunsen flame strongly coloured by 
 sodium. He saw the dark D lines change into bright 
 ones ; the Bunsen flame supplying light of precisely the 
 quality which was lacking in daylight. He then allowed 
 direct sunlight to shine through the flame, but to his 
 astonishment found that now the dark lines D were not 
 changed to bright by the sodium flame, but were actu- 
 ally deepened in their darkness. 
 
 He pushed the experiment further, and instead of sun- 
 light, employed the limelight in which no spectroscope, 
 however powerful, had ever detected any dark lines. 
 " When this light was allowed to pass through a suitable 
 flame coloured with sodium, dark lines were seen in the 
 spectrum in the position of the sodium lines." Instead 
 therefore of the limelight with the sodium flame in front 
 of it, giving a spectrum of all the colours plies two bright 
 lines in the yellow, the place of the two bright lines 
 which the sodium flame, had it been by itself, would 
 have given, was taken by two dark lines. So far as the 
 two D lines were concerned, he had made an artificial 
 solar spectrum. 
 
 This experiment illustrates the three chief kinds of 
 spectra. The limelight gave a perfectly continuous 
 spectrum, generally characteristic of incandescent solid 
 substances. The sodium flame gave a bright-line spec- 
 trum, generally characteristic of a glowing gas. The 
 sodium flame, when the limelight was seen through it, 
 gave an absorption spectrum. 
 
 And thus Sodium, by the extreme simplicity of its 
 spectrum, by its almost aggressive individuality, had 
 unlocked the mystery of the dark lines which Fraun- 
 hofer was the first to map, which for a whole generation 
 had perplexed inquirers. We must suppose that the 
 surface of the Sun is glowing with white light like the 
 limelight giving a continuous and unbroken spectrum, 
 that is to say light of every conceivable colour and 
 
30 SIR WILLIAM HUGGINS 
 
 between us and that glowing surface there is associated 
 with the Sun an atmosphere of glowing sodium vapour. 
 This, could we see it by itself, would give us for its 
 spectrum the two bright yellow lines of the salt-fed spirit 
 lamp, but when we look at the white light of the Sun 
 through it, it stops out or absorbs light of the same quality 
 as itself it has the power to emit, so causing the con- 
 tinuous solar spectrum to be broken by the presence of 
 the two dark absorption-lines D. Therefore Sodium, 
 the terrestrial element with which we are so familiar 
 here, exists likewise in the Sun, exists in the form of a 
 glowing gas. 
 
 Since one terrestrial element has been recognised in 
 the Sun in this manner, it has needed but to apply the 
 same principle further in order to recognise many more. 
 
 The spectrum of Sodium, therefore, furnished the 
 key to the significance of the dark lines of the solar 
 spectrum, and a new science, the science of celestial 
 chemistry, was brought into existence. 
 
CHAPTER V 
 
 THE STORY OF HYDROGEN 
 
 THE spectrum of Sodium is of extreme simplicity and 
 of striking individuality ; a pair of bright lines, twins 
 of golden light, so like, so close together, they are un- 
 mistakable wherever seen. The spectrum of Hydrogen 
 is also simple, but not to the same degree. 
 
 If a vacuum-tube be filled with pure hydrogen gas 
 and then exhausted until only a trace of the gas is left, 
 and an electric current passed through it, the gas will 
 glow with a rosy light. (The middle part of the tube 
 is drawn out into a capillary, the bore of the tube being 
 there reduced to a mere thread in breadth, and the chief 
 light proceeds from this narrow section ; the light from 
 the broader ends of the tube does not concern us here.) 
 This rosy or crimson light resolves itself in the spectro- 
 scope into four bright lines ; the first a bright full red, 
 the second greenish -blue, the third indigo or deep blue, 
 the fourth violet. And corresponding to these four 
 bright lines of hydrogen, known as Hydrogen Alpha, 
 Beta, Gamma and Delta (a, /3, y, 6), are four of the chief 
 dark lines in the solar spectrum. Of these two are the 
 lines called by Fraunhofer C and F ; a third is " near G " 
 and is sometimes loosely called by that letter ; the fourth 
 was lettered by Fraunhofer as h. 
 
 Hydrogen, therefore, is one of the gases existing in a 
 glowing condition above the solar photosphere. Emit- 
 ting light of these four special wave-lengths itself, it has 
 the power of absorbing light of the same quality. Hence 
 it is opaque to these four special radiations from the 
 solar photosphere, and looking down on the Sun through 
 
 31 
 
32 SIR WILLIAM HUGGINS 
 
 a shell of this luminous hydrogen, light of these four 
 qualities is intercepted and the solar spectrum yields 
 us these four dark lines. Hydrogen as well as sodium 
 is a solar constituent. 
 
 So far Kirchhoff had gone. It was Huggins' work to 
 carry the same research from the Sun to the stars. 
 
 As already pointed out, the light of the Sun is im- 
 measurably greater than that of even the brightest star. 
 But the star has one advantage over the Sun ; the 
 former presents all its light as concentrated in the tele- 
 scope to a single point of extreme minuteness while the 
 Sun's light is spread over a broad disc, of which only a 
 small part can be covered by the slit of the spectroscope 
 at any one time. But this gain brings with it a serious 
 drawback ; the spectrum of a star is not a broad rainbow- 
 coloured band, but an exceedingly narrow rainbow- 
 coloured line, finer than a thread, too narrow for trans- 
 verse lines interrupting it to be measured successfully 
 and compared in their positions with the bright lines 
 obtained from terrestrial elements. 
 
 Huggins overcame these difficulties by broadening 
 the image of the star in the direction of the length of 
 the slit, while preserving it unaltered in the transverse 
 direction, by the use of a cylindrical lens ; but the details 
 of the arrangements by which he measured the exact 
 positions of the stellar lines and caused the spectra of 
 various terrestrial elements to be directly compared 
 with that of the star, cannot be described in the limited 
 area at our disposal. Suffice it to say that in conjunction 
 with MILLER, he made a detailed study in the laboratory 
 of the spectra of nearly all the terrestrial elements, 
 carefully computing the wave-lengths of all the lines, 
 and then compared these with the spectra of a 
 number of the brighter stars, line by line. He thus 
 detected the presence in various stars of hydrogen, 
 sodium, magnesium, calcium, and iron. In all, the 
 spectra of about 100 of the brightest stars were thus 
 examined. 
 
 But though the lines of hydrogen were seen in the 
 
THE STORY OF HYDROGEN 33 
 
 spectra of most stars, they were not seen in all, nor did 
 they offer the same characteristics in all spectra in which 
 they appeared. RUTHERFURD early pointed out that, 
 judged by the test of the hydrogen lines, there are three 
 chief types of star : those that show these four lines 
 broad, diffused and dark, the only important lines in the 
 spectrum ; those where the four lines are dark but 
 narrow, interspersed amongst a congregation of lines 
 of other substances ; and those which show no hydrogen 
 lines at all. SECCHI, a little later, added a fourth type, 
 and noted that a few stars do not come exactly under 
 any of the four heads, for there are several stars in the 
 constellation Orion with the four hydrogen lines among 
 their chief characteristics, and yet those lines are narrow 
 and not very dark, and there are one or two stars that 
 show the hydrogen lines bright on the bright background 
 of the spectrum. There are thus six chief types of 
 stellar spectra, and in a general way the colour of the 
 star itself appears to be connected with the character of 
 the hydrogen line. 
 
 Secchi's results may be summarised as under : 
 
 Type I. White or bluish stars. Examples : Sirius, Vega, 
 
 Castor. Hydrogen lines predominant and broad, 
 
 dark and diffused. 
 Type II. Yellow stars. Examples : the Sun, Arcturus, 
 
 Capella, Pollux. Hydrogen lines dark and distinct, 
 
 but not broad. Many other lines. 
 Type III. Orange stars. Examples : Alpha Orionis, 
 
 Antares, Alpha Herculis. Hydrogen lines feeble. 
 
 Spectrum crossed by fluted bands, sharp towards 
 
 the violet and fading off towards the red. 
 Type IV. Red stars. Examples : only a few faint stars. 
 
 Hydrogen lines wanting. Spectrum crossed by 
 
 fluted bands, sharp towards the red and fading 
 
 off towards the violet end. 
 Type I, Variety a. Example : Rigel. Hydrogen lines 
 
 predominant, but narrow and not very dark. 
 
 Colour of stars slightly tending to a greenish tint. 
 
 See p. 76. 
 
 o 
 
34 SIR WILLIAM HUGGINS 
 
 Type I, Variety 6. Examples : Gamma Cassiopeiae and 
 Beta Lyrae. Hydrogen lines bright. 
 
 A striking example of this sixth class was witnessed 
 on May 12, 1866, when a faint 9th magnitude star in 
 the Northern Crown, since generally known as T Coronae, 
 suddenly attained a brightness surpassing that of a 
 2nd magnitude star, and then rapidly faded away till 
 it again became of the 9th magnitude by the end of the 
 month. On the 16th of May, when it was of about the 
 4th magnitude, Huggins examined its spectrum and 
 found that in addition to a spectrum apparently of 
 Secchi's Third Type that of the orange stars it showed 
 four bright lines, two of which were the well-known red 
 and greenish-blue lines of hydrogen, its a and (3 lines 
 corresponding to the dark Fraunhofer lines C and F. 
 Their great brightness showed that the luminous gas 
 was hotter than the stellar surface. Many such New 
 Stars, or Novae, have been observed since, and in every 
 case hydrogen has shown its characteristic lines, bright 
 and glowing on the background of the star's continuous 
 spectrum. 
 
 In the case of T Coronae it is necessary, however, to 
 differentiate. The continuous spectrum of the star 
 resembled that of Secchi's Third Type, that of the 
 orange stars ; and many of these are strongly variable, 
 and show the hydrogen lines bright at maximum. This 
 is the case with the star Mira Ceti, which goes through, 
 its changes in eleven months, and is often 1000 times as 
 bright at maximum as at minimum. The change in 
 T Coronae was twice as great as this ; but no greater 
 than some other variables show, and it may perhaps be 
 that it should be classed with these rather than with 
 Novae. But in any case a change of brightness so great 
 is evidence that both Mira Ceti and T Coronae are bodies 
 of a very different order from our Sun. 
 
 The four bright lines of hydrogen, in the red, greenish- 
 blue, indigo and violet, soon showed themselves again 
 in another connection ; viz. in the immediate surround- 
 
THE STORY OF HYDROGEN 35 
 
 ings of the Sun. Total eclipses had revealed that when 
 the Sun itself is hidden by the dark body of the Moon, a 
 narrow rim of crimson light surrounds the Moon, and 
 weirdly shaped prominences of the same tint arise from it 
 here and there. It needed a number of eclipses to estab- 
 lish the point that these bright crimson " flames " be- 
 long to the Sun itself. Huggins had pointed out that 
 if the prominences were composed of glowing gas, they 
 would give a spectrum of bright lines, and not a contin- 
 uous spectrum, and it would be possible to see them in 
 the spectroscope even when the Sun was not eclipsed. 
 JANSSEN, who observed the eclipse of 1868, remarked 
 then how intense the bright lines of the prominences 
 were, looked for them after the eclipse was over, and 
 found that he could readily see them, and LOCKYER, ob- 
 serving at home in England a few weeks later, indepen- 
 dently succeeded in finding a prominence by its bright 
 lines. Once the method was hit upon, it was easy to 
 do that which could not be well done in the few moments 
 of an eclipse, to determine the exact positions of the 
 bright lines of the prominence spectrum, and they were 
 seen to correspond to the four dark Fraunhofer lines, 
 C, F, " near G," and h ; that is to the four lines of 
 hydrogen, in the red, greenish-blue, indigo and violet ; 
 together with a beautiful golden-hued line near the dark 
 D lines of Sodium and hence known as D 3 . The chromo- 
 sphere or shallow layer from which the prominences 
 rise, gave the same bright lines, so that the Sun was seen 
 to be surrounded by a shell of highly heated hydrogen, 
 mingled with at least one other element. 
 
 A new department of observation, that of the solar 
 prominences and chromosphere, was thus opened, and 
 Huggins showed that a narrow slit was not necessary 
 in their case, as the differently coloured images of a 
 prominence would not overlap or interfere with one 
 another. If a spectroscope of sufficient power to weaken 
 the background of diffused sunlight from the sky was 
 used, the slit could be opened wide, wide enough to see 
 the entire shape of the prominence, or, if the instrument 
 
36 SIR WILLIAM HUGGINS 
 
 were so arranged, to take in a wide range of the solar 
 circumference. 
 
 Of the four familiar lines of hydrogen, its a, /3, y, and 
 B lines, the one in the full red, corresponding to the 
 Fraunhofer line C, proved to be the most suitable for 
 use in prominence work, where light was abundant, but 
 in a faint object the easiest line to recognise was /3, the 
 one in the greenish-blue, corresponding to the Fraun- 
 hofer line F. Thus it was the first recognised in the 
 spectra of " bright-line " stars like Gamma Cassiopeiae, 
 of Novae like T Coronse, or variable stars like Mira Ceti ; 
 though usually when instrumental power was increased, 
 a and y could be detected later, and even 8 as well. So 
 to the early workers with the spectroscope, the " F line," 
 greenish-blue hydrogen, was the first to be looked for 
 in a new field, and when Huggins examined the spectrum 
 of Jupiter, this F line of hydrogen showed up as broader 
 and darker than the same line in the solar spectrum. 
 Not only so, but beside the Fraunhofer lines there was 
 in the red a strong dark band unknown to the Sun. 
 VOGEL and afterwards MAUNDER examined also Saturn, 
 and the far more difficult objects, Uranus and Neptune, 
 and though the two latter were too faint to show the 
 dark lines of the Fraunhofer spectrum, all yielded 
 several dark bands of which that in the red recognised in 
 Jupiter was one, and another was a broad dark band in 
 the position of F. It results from this that it is highly 
 probable that the four outer planets still emit some 
 light of their own, for it is patent that the light which 
 they send to us bears a very considerably higher propor- 
 tion to the amount which they receive than is the case 
 with the Moon or Mars, and yet the absorption exercised 
 by the planet's atmosphere is manifestly much greater 
 also. 
 
 The same F line, the greenish-blue line of hydrogen, 
 in the spectra of the stars was put by Huggins to a most 
 notable use. It has been said above in Chapter III, 
 that the place of a line in the spectrum is invariable, its 
 refrangibility, its wave-length undergoes no change. 
 
THE STORY OF HYDROGEN 37 
 
 This is the broad fact of the case, but the statement 
 requires some qualification and correction. The wave- 
 length of a particular radiation can be changed in effect, 
 if not in reality, by the rapid app oach of the sources of 
 light and the observer, whether this be due to the motion 
 of one or both combined. For in this case, so far as the 
 observer is concerned, the waves of light are practically 
 shortened by his movement to meet them ; and the 
 line will have its wave-length diminished, that is its 
 refrangibility will be increased ; as compared with the 
 same radiation, obtained from a source of light at rest 
 with relation to the observer, it will be displaced towards 
 the violet end of the spectrum. Conversely a movement 
 apart of the source of light and the observer will mean 
 a displacement towards the red. 
 
 It was Huggins who first worked out this principle 
 and put it into practical application in order to deter- 
 mine the movements of the stars either towards or away 
 from the earth ; that is radially, in the line of sight. 
 Motion at right angles to this direction, motion as seen 
 projected on the vault of heaven, could be observed and 
 measured in the telescope ; but hitherto it had seemed 
 Impossible that any information as to radial motion 
 could ever be obtained. For this work a very consider- 
 able dispersion is required, and hence in order that the 
 spectrum may not be too much weakened, a telescope 
 with a very large object-glass is needed to collect suffi- 
 cient light. In Secchi's work on star-types a slit might 
 be dispensed with, as the image of the star formed by 
 the cylindrical lens is almost a mathematical line ; 
 but in this a narrow slit is of primary importance, for 
 otherwise the star might shift its position, and therefore 
 the whole spectrum, including the particular line under 
 examination, would likewise move in one direction or 
 the other. A similar fictitious appearance of displace- 
 ment will be caused if the light from the terrestrial sub- 
 stance which gives the comparison spectrum does not 
 enter the slit perfectly at right angles to it. A further 
 difficulty arose from the fact that a very strong absorp- 
 
38 SIR WILLIAM HUGGINS 
 
 tion line in the star spectrum had necessarily to be chosen 
 for measurement, and this is apt to be broad and ill- 
 defined at the edges. This is particularly the case with 
 the lines of hydrogen in stars of Secchi's First Type, 
 which offered the greatest number of subjects bright 
 enough for the observation. 
 
 The work proved too difficult for the resources then 
 at Huggins' command, and also for those at Greenwich, 
 where the same research was long carried on. But the 
 introduction of rapid dry plates rendered it possible to 
 substitute photographic observation for direct visual 
 work at the spectroscope, and this improvement, effected 
 by Vogel, as will be narrated later, soon raised the method 
 to a high degree of effectiveness. 
 
 The spectrum of Hydrogen furnished the means of 
 classifying the stars, enabled the solar chromosphere 
 and prominences to be studied without an eclipse, and 
 supplied a method by which the movements of stars 
 could be measured in the line of sight. 
 
CHAPTER VI 
 
 THE STORY OF CARBON 
 
 A CANDLE flame, or better a gas flame fed with carbonate 
 of soda or with common salt, gives as we have seen the 
 simplest of spectra, the twin bright golden lines of sodium. 
 The radiations from the bluish base of either of these 
 flames, or better from the inner cone of a " roaring " 
 Bunsen burner, yield in the spectroscope a vision of 
 extreme beauty, an exquisite poem in greenish light. 
 Nor is it merely a metaphor to speak of it as a poem, for 
 that which renders it so charming is the faultless rhythm 
 that is displayed, the perfectly metrical character of 
 the succession of the bright images. The same spectrum, 
 brighter and therefore more perfectly seen, is given 
 (with other lines) by the carbons of the ordinary electric 
 arc. 
 
 Three bright bands reveal themselves ; the first of 
 a delicate citron tint, the second full emerald green, 
 and the third of a rich blue, each band beginning boldly 
 and sharply on the side towards the red, and fading away 
 gradually on the side towards the violet. More light, 
 more dispersion, and a narrower slit break up each of 
 the three bands into a succession of flutings that is to 
 say narrower bands, bright and sharp on the redward 
 side, fading towards the violet ; and reveal also two 
 similar bands of flutings beyond the three chief bands, 
 one in the orange-red, the other in the violet. And these 
 flutings again can be broken up into a succession of 
 bright lines with a most rhythmical arrangement of 
 brightness and position. 
 
 This beautiful spectrum was first described and 
 
 39 
 
40 SIR WILLIAM HUGGINS 
 
 measured by SWAN, and it is often, for the sake of dis- 
 tinctness, called after him the " Swan " spectrum. For 
 there has been long controversy as to its true origin ; 
 whether it is due to carbon itself or to some compound 
 of carbon, and if the latter, with what other element the 
 carbon is combined. Carbon itself gives a line spectrum, 
 and the idea that the " Swan " spectrum is due to simple 
 carbon is now abandoned by many, but there are still 
 two views respecting it, one that it is the spectrum of 
 a hydrocarbon acetylene, the other that it is due to 
 carbon monoxide. There is another spectrum, likewise 
 of fluted bands, as beautiful as this " Swan " spectrum 
 and very similar to it, that is evidently given by a com- 
 pound of carbon and oxygen, either the monoxide or 
 the dioxide. BALY, SMITHELLS, and others who regard 
 the " Swan spectrum " as belonging to the monoxide, 
 naturally ascribe the second to the dioxide. 
 
 But it is the " Swan " spectrum, considered by many 
 astronomers as that of the hydrocarbons, that is of chief 
 interest in astronomy. It is found in two classes of 
 objects vastly different from each other ; in the one it is 
 seen in positive as bright bands, in the other, in nega- 
 tive as dark absorption flu tings. 
 
 The former class is that of Comets. As early as 1864, 
 DONATE was able to examine the spectrum of a comet 
 and to ascertain that it showed three bright bands, 
 thus proving that it shone, not by mere reflection of 
 sunlight, but by radiance of its own. The identification 
 of the bands fell to Huggins, who showed that they 
 correspond to the three brightest of the beautiful 
 " Swan " spectrum. 
 
 Not that comets never yield a continuous spectrum 
 significant of reflected sunlight. There has been much 
 variety in this respect, and while some comets have 
 yielded hardly anything but the " Swan " bright bands, 
 others have given spectra almost wholly continuous. 
 More striking still, the two comets of 1882, both of which 
 approached the Sun very nearly, both shone almost en- 
 tirely by the light of sodium vapour when near peri- 
 
THE STORY OF CARBON 41 
 
 helion, and this spectrum in the comet that came second in 
 the year and made the nearer approach of the two, finally 
 gave way to that of iron. Under the intense heat to 
 which this comet was exposed, first sodium and later iron 
 were volatilised. See p. 52. 
 
 The second class of objects is that of the Red Stars ; 
 the members of Secchi's Fourth Type. Their broad dark 
 bands, sharply defined on the redward side and fading 
 off towards the violet, were shown by Secchi, Vogel and 
 DUNER to be due to the reversal of the " Swan " spec- 
 trum. Huggins would seem to have compared these 
 stars, not with the " Swan " spectrum but with that 
 ascribed to carbon dioxide, and hence failed to 
 detect an identity between the two ; but the agree- 
 ment of the Red Star bands with those of the " Swan " 
 spectrum has long been fully established. The Red Stars 
 therefore give the typical cometary spectrum reversed ; 
 so far as these bands are concerned, comets yield us what 
 we should perceive if we could see a Red Star under 
 conditions such as give us the spectrum of the Sun's 
 chromosphere in an eclipse. 
 
 But the compound of carbon giving rise to the " Swan " 
 spectrum is not the only combination of carbon shown by 
 comets. The first comet bright enough to give a photo- 
 graph of its spectrum was seen in 1881, and Huggins 
 secured a very important picture of it. The spectrum 
 showed a continuous background, crossed by a number 
 of the most conspicuous Fraunhofer lines, such as G, h, H 
 and K. The comet, therefore, shone partly by reflecting 
 the light of the Sun. But it also yielded a spectrum of 
 bright lines, of which the strongest group was beyond 
 H and K at the beginning of the ultra-violet region, and 
 a fainter lay between h and G. From the measures of 
 their positions which Huggins secured, LIVEING and 
 DEWAE, proved that " these two groups indicate the 
 presence of cyanogen, and are not to be seen in hydro- 
 carbons unless nitrogen is also present." Huggins 
 pointed out that both meteoric iron and stony meteorites 
 yield hydrogen, carbonic oxide, and nitrogen ; thus the 
 
42 SIR WILLIAM HUGGINS 
 
 spectroscopic examination of comets fully confirmed 
 their association with meteors, which had been estab- 
 lished by an altogether different class of evidence. 
 FOWLER has further indemnified bands seen in the tail 
 of a comet with a peculiar spectrum yielded by carbonic 
 oxide at very low pressure. 
 
 The " Swan " carbon spectrum occurs the more fre- 
 quently in astronomical physics, but the second banded 
 carbon spectrum, that of a compound with oxygen, is 
 historically of even greater importance. For Piazzi 
 Smyth, who was in the habit of drawing his spectra 
 and expressing the positions of his lines on a scale of 
 " wave -numbers to an inch," that is on the inverse 
 scale of that of wave-lengths, made a special study and 
 a fine map of the great green band of this beautiful 
 spectrum, which he considered as that of carbonic 
 oxide. This map he submitted in November 1883 to 
 the inspection of ALEXANDER HERSCHEL, who pointed 
 out " that the ' wave -numbers ' of these lines formed 
 two perfectly similar arithmetical series, so placed with 
 respect to each other that the fifth line of the first series 
 was coincident with the first of the second. Further, 
 the ninth line of the first series showed as a close doublet, 
 and the tenth and following lines as doublets, each 
 slightly wider than the preceding one." It was therefore 
 clear that the component lines of this particular band, 
 at least, formed an harmonic series, and the discovery 
 stimulated the search for a similar underlying law in 
 other spectra, apparently heterogeneous and without 
 order in their arrangement. 
 
 In the spectra of the compounds of Carbon, therefore, 
 the " Swan "spectrum revealed itself bright in Comets, 
 and reversed, that is dark, in Red Stars ; that of cyanogen 
 linked comets with meteors, while the spectrum of 
 carbonic oxide yielded the first case in which an harmonic 
 arrangement of lines was definitely recognised in the 
 spectrum of a terrestrial substance. 
 
CHAPTER VII 
 
 THE STORY OF IRON 
 
 AFTER Kirehhoff had made the all-important comparison 
 of the pair of dark Fraunhofer lines, D, in the solar spec- 
 trum, with the pair of bright yellow lines given by 
 Sodium, and had grasped the significance of the relation 
 between them, he went on to compare the spectra of 
 other elements with the solar dark lines. In so doing 
 he passed from a spectrum, like that of Sodium, of the 
 utmost simplicity to one of greater complexity, for he 
 chose iron, that yields more than 2000 lines, irregularly 
 distributed through all the colours from deep-red to 
 lavender-grey. But iron is a much more refractory 
 substance than Sodium, and the method that was suitable 
 for its spectrum, i.e. feeding a spirit lamp or a Bunsen 
 flame with a solution of some salt of the element to be 
 examined, is no longer available. A much higher tem- 
 perature can be obtained by the use of the oxy-hydrogen 
 blow-pipe, the jet of the mixed gases being directed upon 
 the substance which is to give the spectrum. 
 
 But the electric arc or the electric spark are in most 
 cases more effective and more convenient. In the use 
 of the electric arc the carbon of the positive pole may 
 have a hole bored through it lengthways, the hole being 
 filled with the substance under scrutiny ; or the positive 
 carbon top may be shaped like a cup and the substance 
 inserted little by little as required. Or, and in the case 
 of iron in particular the method is very suitable, the 
 poles may be made of the metal itself. 
 
 But for many purposes, spark spectra are preferred, 
 the sparks from an induction coil being caused to pass 
 
 43 
 
44 SIR WILLIAM HUGGINS 
 
 between points of the substance under scrutiny, or it 
 may be between a platinum point and a solution of a 
 salt, one or more Ley den jars usually being employed 
 in the secondary circuit. For the illumination of gases 
 of which the spectra are desired, it is usual to fill a 
 " vacuum tube " with the gas. The tube is then 
 exhausted until only a trace of the gas remains, neces- 
 sarily at a low pressure, and this is submitted to the 
 action of the discharge from a coil. 
 
 Of these three methods of procuring the bright line 
 spectra of terrestrial elements the flame, the arc and 
 the spark Kirchhoff used the last in order to volatilise 
 iron, and directing his spectroscope towards the Sun, 
 exposed the lower half of the slit to its light, and by 
 means of a reflecting prism placed in front of the upper 
 half, brought the image of the spark between the iron 
 poles into juxtaposition with it in the field of view. In 
 the eye-piece of the spectroscope, the solar spectrum, 
 the long bright band of the rainbow colours, interrupted 
 by many thousands of fine dark lines, was seen in the 
 upper half of the field (as the telescope gave an inverted 
 image), and in the lower half hundreds of narrow bright 
 lines due to iron, ranged irregularly through the same 
 succession of colours. And broadly speaking, each 
 bright line of iron was represented in place and intensity 
 by a dark line in the solar spectrum ; the single lines 
 by single lines, the double lines by double, broad or 
 intense lines by lines that were broad or intense, lines 
 that were narrow or feeble by lines that had the same 
 characteristics. As the iron spectrum in Kirchhoffs 
 comparison with the Fraunhofer lines gave more than 
 60 such coincidences, the evidence of the presence 
 of iron in the Sun is strong indeed. But this number 
 has been raised by later workers and more powerful 
 instruments to more than 2000, so that more careful 
 comparisons have immensely increased the force of 
 Kirchhoffs original conclusion. 
 
 Of these 2000 lines of iron among the most prominent 
 are a close pair of noble dark lines in the full green ; they 
 
THE STORY OF IRON 45 
 
 occur in a crowded part of the spectrum and were 
 distinguished by the letter E by Fraunhofer, whose 
 attention they attracted in his pioneer work. 
 
 Beside Sodium and Iron, Kirchhotf was able to identify 
 the presence in the solar spectrum of the lines of six 
 metals : magnesium, calcium, nickel, barium, copper 
 and zinc. He suspected also that cobalt was there, 
 but failed to recognise evidence of gold, mercury, anti- 
 mony, arsenic, and lithium. He also failed to recognise 
 silver, lead, silicon, strontium, aluminium, cadmium and 
 tin, but these, as well as cobalt, have been detected 
 since. 
 
 It was the examination of the bright line spectra of 
 all the principal terrestrial elements and the determina- 
 tion of their wave-lengths that occupied Huggins in the 
 first years of his devotion to this new field of research, 
 and from that he proceeded to the difficult task of con- 
 fronting the dark lines in the feeble spectra of the 
 brighter stars with these bright metallic lines, with the 
 result of establishing that the stars also are built up of 
 the same elements as those that form the structure of 
 our Earth and of its Sun. 
 
 The spectra of the Sun and of the stars are therefore 
 built up in that same way. In each we have a contin- 
 uous spectrum interrupted by numerous dark lines or 
 bands ; in each the continuous spectrum proceeds 
 from a brilliant shell that is known as the Photosphere, 
 giving white light, light that is to say, of all degrees of 
 refrangibility that the eye can perceive, while the dark 
 lines are caused by the absorption of certain gases lying 
 immediately above the photosphere ; gases capable of 
 emitting the very same radiations that they absorb. If 
 the photosphere could be seen alone it would yield a 
 spectrum like that of the limelight ; all the colours 
 from deep -red to violet and lavender-grey would be seen 
 in their proper order and succession without any break 
 or interruption. If the gases could be seen alone they 
 would yield a spectrum of bright lines, lines identical in 
 their position in the spectrum, that is in their wave-length, 
 
46 SIR WILLIAM HUGGINS 
 
 their refrangibility, with the dark lines actually observed. 
 For the solar spectrum as seen is the photospheric spec- 
 trum viewed through these gases, and therefore inter- 
 rupted by dark lines due to their absorption. 
 
 And though the photosphere cannot be seen free from 
 the absorption of the luminous gases that overlie it, 
 yet on occasion these gases can be seen apart from the 
 photosphere ; viz. in a total eclipse of the Sun. In 
 such eclipses, prominences and chromosphere had been 
 repeatedly seen before the spectroscope had been applied 
 to solar research ; and after it had become a weapon of 
 astronomical observation, it was in the eclipse of 1868 
 that the spectra of the prominences, bright lines due 
 largely to hydrogen, had first been seen. So the thin 
 stratum in the lowest part of the chromosphere, where 
 these bright-line gases chiefly lie, reveals its spectrum 
 for a second or two at the beginning and end of totality. 
 Ordinarily the sky immediately round the Sun is filled 
 with a glare due to the scattering of sunlight in the atmo- 
 sphere, but as the Moon passes before the Sun this glare 
 is cut off and the shallow shell of gases can then make 
 its presence apparent. " The moment the Sun is hidden, 
 through the whole length of the spectrum, in the red, 
 the green, the violet, the bright lines flash out by hun- 
 dreds and thousands, almost startlingly ; as suddenly 
 as stars from a bursting rocket-head, and as evanescent, 
 for the whole thing is over within two or three seconds." l 
 
 This beautiful spectacle of the " Flash," as from its 
 evanescence it has been appropriately termed, is the 
 " sudden reversal into brightness and colour of the 
 countless dark lines of the spectrum." The spectra of 
 the gases that by their absorption give rise to the dark 
 lines of the Fraunhofer spectrum are here seen in their 
 true aspect, apart from the photosphere, as bright lines ; 
 and the stratum is therefore known as the Reversing 
 Layer. 
 
 Among these lines, those of iron are very numerous 
 and range from a height of about If seconds of arc to 
 1 The Sun, by C. A. Young, p. 82. 
 
THE STORY OF IRON 47 
 
 rather more than twice that amount. Many other 
 elements are also represented ; some of them, such as 
 hydrogen and calcium, rising to a height of 10" or more 
 and forming the chromosphere. And just as hydrogen 
 often rises in irregular forms to immense heights from the 
 chromosphere, forming the prominences which can be 
 observed by means of the spectroscope without an 
 eclipse, so from time to time the reversing layer is dis- 
 turbed by violent commotions, and metallic elements 
 like iron are carried up sufficiently high to show their 
 bright lines without the intervention of an eclipse, and 
 on occasion something that almost suggested a general 
 reversal of the Fraunhofer spectrum has been thus 
 observed at the base of a prominence, even hundreds 
 of very short bright lines being seen. Generally, how- 
 ever, only a few bright lines of metals are seen in the 
 chromosphere, but certain lines of sodium, magnesium, 
 strontium, titanium and iron show themselves pretty 
 readily. Metallic eruptions, as these outbreaks rich in 
 metallic lines are called for the sake of distinctness, 
 are usually violent and short-lived in their movements 
 and changes ; many of the hydr gen prominences, on 
 the contrary, though of much greater size, are com- 
 paratively slow-moving and stable. 
 
 But these metals thus invading the chromosphere 
 do not muster all their lines in full on such occasions ; 
 many fail to answer to the roll-call. At one time in the 
 early history of spectroscopy, it was supposed that each 
 element had only a single spectrum, absolutely invari- 
 able in the number of its lines, in their positions and 
 relative brightness. Now it is known that spectra are 
 alterable in a number of particulars according to the 
 conditions under which they are produced ; the number 
 of lines shown may vary, their actual brightness may 
 suffer change, and their relative brightness, their width 
 and even to some extent their wave-length or position 
 in the spectrum may be altered. Sir Norman Lockyer 
 has devoted many years' work to the study in the labora- 
 tory of these changes in metallic spectra, chiefly in two 
 
48 SIR WILLIAM HUGGINS 
 
 directions. Of these the first is concerned with the 
 distinction between the long and the short lines given 
 by a particular substance. If, say, the arc is used with 
 the poles horizontal, and a spectroscope having its slit 
 vertical is turned upon it, the light comes generally 
 from all parts of the arc. But if a lens is interposed 
 between the arc and the slit so as to form an image of 
 the arc on the slit-plate, only the light from the part 
 of the image of the arc that actually falls on the slit will 
 enter the spectroscope ; that is to say, only the light 
 from a narrow section of the arc at right angles to the 
 poles. If the core of the arc gives one set of lines and 
 the outer stratum another, the lines belonging to the 
 core will be short, those belonging to the outer shell will 
 be long, and this whether they are also given by the core 
 or not. The spark may also be used in the same way. 
 Or the arc or spark may be arranged parallel to the slit, 
 and an image of them formed by a lens as before. In 
 this case, if the points between which the spark is being 
 taken are somewhat far apart the lines due to the metal 
 may be mere points close to the electrodes, while the 
 lines of air or of the particular gas in which the spark 
 is being taken extend from one electrode to the other. 
 
 The vapour of the metal under examination will 
 naturally be densest and most highly heated close to 
 the poles, coolest and rarest at the greatest distance 
 from them. The long lines are therefore those given 
 most easily by the substance under examination ; they 
 are the lines developed when the substance is present 
 only in small quantities or when the temperature is not 
 relatively high. There need be no surprise then, that 
 the Sun does not show all the lines of every element ; 
 thus aluminium is chiefly represented in the solar spec- 
 trum only by two strong lines in the violet, but these 
 are the two " longest " lines of its spark spectrum. 
 
 Again, when the spectrum of a metal as given by the 
 arc is compared with that given by the spark, many 
 lines are seen to be common to the two as would be ex- 
 pected, but the relative brightness of the lines has under- 
 
THE STORY OF IRON 49 
 
 gone a change, some being distinctly stronger in the 
 spark. Thus Lockyer writes : " Neglecting, then, all 
 changes at the lowest temperatures, but including the 
 flame spectrum, four distinct temperature stages are 
 indicated by the varying spectra of the metals ; for 
 simplicity I limit myself to iron as an example. These 
 are 
 
 1. The flame spectrum, consisting of a few lines and 
 flutings only, including several well-marked lines, some 
 of them arranged in triplets. 
 
 2. The arc spectrum, consisting, according to Rowland, 
 of 2000 lines or more. 
 
 3. The spark spectrum, differing from the arc spectrum 
 in the enhancement of some of the short lines and the 
 reduced relative brightness of others. 
 
 4. A spectrum consisting of a relatively very small 
 number of lines which are intensified in the spark." J 
 
 In passing from the arc spectrum of iron to that of 
 the spark, there is a marked change in the relative in- 
 tensities of the lines ; some lines disappearing, many 
 becoming feebler, others brightening, and new lines 
 appearing. If the spark be increased in intensity, this 
 type of change is increased likewise, so that the lines of 
 increasing strength, the " enhanced lines " to use 
 Lockyer's term, might conceivably in the sequel be the 
 sole survivors if we could view the hottest part of the 
 spark in complete isolation. 
 
 The spectrum of an element therefore is not invariable 
 always and under all conditions the same ; and in strict 
 analogy the solar spectrum suffers change in accordance 
 with the region from which it is obtained. The spectrum 
 of the general photosphere itself is indeed practically 
 constant ; but when that photosphere is torn apart in a 
 sunspot or raised into the bright ridges of a facula, then 
 other regions offer themselves for scrutiny, and with 
 changed conditions a modification of the spectrum is 
 observed. 
 
 1 Inorganic Evolution, by Sir Norman Lockyer, p. 32. 
 
 D 
 
50 SIR WILLIAM HUGGINS 
 
 The spectrum of a sunspot shows first of all a duller 
 background than that of the general solar surface ; the 
 continuous spectrum is weaker, and this is more marked 
 in the umbra than in the penumbra, just as the umbra 
 is the least luminous part of the spot. There is appar- 
 ently a general absorption of light over a spot. 
 
 But there is also a selective absorption. Many dark 
 lines are evident, and not a few dark lines or bands come 
 into evidence ; lines that are either entirely absent 
 from the spectrum of the general surface, or are at least 
 exceedingly difficult to detect there. On the other 
 hand, some dark lines are weakened ; others, amongst 
 them those of hydrogen, are often reversed; i.e. they 
 are seen as bright lines. 
 
 The spectrum of the chromosphere, as we have al- 
 ready seen, differs widely from that of the general sur- 
 face, and yet more widely from the spectrum of a sun- 
 spot. " The long lines seen in laboratory experiments 
 are suppressed, and the feeble lines exalted in the 
 spectrum of the chromosphere. In the Fraunhofer 
 spectrum, the relative intensities of the lines are quite 
 different from those of coincident lines in the chromo- 
 sphere." 
 
 " In the visible region of the spectrum, iron is repre- 
 sented by nearly a thousand Fraunhofer lines ; in the 
 chromosphere it has only two representatives." 
 
 " In sunspots we deal with one set of iron lines, in 
 the chromosphere with another." 1 
 
 Extending the same inquiry to stellar spectra, 
 Lockyer has been able to show that the stars can be 
 grouped according to the " enhanced lines " of iron, 
 and of other elements that they show. Thus the most 
 important lines seen bright in the solar chromosphere 
 correspond to the chief " enhanced lines," and many 
 of these again are strongly marked in the dark lines of 
 the star Alpha Cygni. 
 
 Lockyer considers that this progressive change in 
 the spectrum of iron from flame to arc, and arc to spark, 
 1 Inorganic Evolution, p. 34. 
 
THE STORY OF IRON 51 
 
 is due to dissociation, i.e. to the break up of the metal 
 into simpler constituents through the great increase of 
 temperature in the changed conditions. Baly questions 
 if the temperature of the spark should be considered 
 so enormously hotter than that of the arc. He admits 
 " that the simpler spectrum of the spark may be due to 
 a simpler form of matter, as assumed by Lockyer, is 
 < r ery probable, but it by no means follows that tempera- 
 ture is the factor which aids this dissociation." * Others 
 question the validity of the proof that iron is thus dis- 
 sociated, but, however the progression in the form of 
 the spectrum be explained, the beauty and importance 
 of Lockyer's working out of this progression, and of his 
 application of it to the comparative study of stellar 
 spectra, remain unaltered. For the present, it may be 
 adopted as at least a convenient hypothesis that the 
 spark spectrum of iron represents a higher temperature 
 than that of the arc, and that the presence of strongly 
 marked " enhanced lines " in the spectrum of a star 
 indicates a more fervid condition than where they are 
 absent or relatively feeble. 
 
 The spectrum of iron figures importantly in two other 
 departments of the " new astronomy." As soon as the 
 introduction of rapid dry photographic plates rendered 
 it possible to photograph stellar spectra with a sufficient 
 dispersion, Vogel devoted himself to perfecting an in- 
 strument in which he could apply photography to the 
 problem Huggins had attacked the determination of 
 the motions of stars in the line of sight by measuring 
 the displacement of their lines. When this had been 
 achieved, iron, by its richness in the number of its lines, 
 presented itself as an ideal element for a comparison 
 spectrum, and a single photograph gave, not one case 
 only where a stellar line was confronted with its terres- 
 trial equivalent, but scores or even hundreds, and the 
 method, which, so long as it was confined to direct visual 
 observation, failed to obtain success, became fertile in 
 rich results. It has now become possible to determine 
 1 Spectroscopy, by E. C. C. Baly. 
 
52 SIR WILLIAM HUGGINS 
 
 the speed with which the Earth is travelling in its orbit, 
 and so to ascertain the dimensions of the orbit and the 
 Sun's distance, by observing the change in the appar- 
 ent motion of a star from the time when we are moving 
 most directly towards it, to the time when, six months 
 later, we are moving most directly away. Further, 
 from observations of a large number of stars, the motion 
 in space of the solar system itself has been inferred, and 
 CAMPBELL, from observations of 285 stars, deduces that 
 this motion is at the rate of thirteen miles a second, and 
 is directed towards a point in the constellation Hercules. 
 
 On one occasion, the lines of iron flashed out in the 
 spectrum of an object very unlike Sun or stars. On 
 September 18, 1882, COPELAND, Astronomer Royal for 
 Scotland, watching the great comet of that year as it 
 reached its perihelion, saw not only the bright yellow 
 pair of sodium lines blaze out, but also six brilliant lines 
 in the yellow and green, which proved to be coincident 
 with six chief lines of iron. At this time the centre of 
 the comet was only 300,000 miles from the surface of 
 the Sun, and the temperature to which it was exposed 
 was amply sufficient to volatilise iron. The lines were 
 not long visible, and as the comet receded from the Sun, 
 the lines of sodium faded out likewise. But the obser- 
 vation, short as it, was, was sufficient to show that the 
 comet, however filmy and ghostlike its outer structure, 
 yet possessed an inner core rich in the metal so char- 
 acteristic of one class of meteorites iron, 
 
 The spectrum of Iron has illustrated the changes 
 brought about in the appearance of a spectrum by 
 increase of temperature, and has thus provided a means 
 for grouping the stars in order of temperature. It has 
 also furnished a most useful comparison spectrum for 
 the determination of the rate of motion of stars in the 
 line of sight. 
 
CHAPTER Vin 
 
 THE STORIES OF MAGNESIUM AND TITANIUM 
 
 THE spectrum of sodium, as used by Kirchhoff , was on 
 of great simplicity ; a pair of strong lines in the orange- 
 yellow, quite close together. The entire spectrum of 
 sodium, as we know it now, is much richer than this, 
 but the D lines still remain the typical lines of sodium. 
 
 Similarly the typical lines of magnesium are a strong 
 triplet in the emerald -green, with wave-lengths at 5184, 
 5173, and 5168. It will be seen that the three com- 
 ponents of the triplet, known among the dark Fraun- 
 hofer lines of the solar spectrum as b l9 6 2 , and 6 4 , are not 
 at equal intervals apart, but the space separating 6 X 
 from 6 2 is about double that separating 6 2 from 6 4 . So, 
 too, the three lines are not of equal intensity ; 6 X is the 
 strongest, 6 2 somewhat less pronounced, 6 4 much the 
 weakest of the three. The letter 6 3 is given in the solar 
 spectrum to a line due to iron that intervenes between 
 6 2 and 6 4 . 
 
 " This pretty and striking arrangement of three strong 
 green lines renders the spectrum of magnesium quite 
 unmistakable whenever encountered, whether in the 
 spectra of the Sun or stars ; and the presence and strength 
 of the 6 lines of magnesium, together with those of the 
 D lines of sodium, form a ready index to the corre- 
 spondence in type between the spe trum of a given star 
 and that of the Sun ; and as early as 1863, Huggins 
 had confronted Aldebaran (Alpha Tauri) and Betelgeux 
 (Alpha Orionis) with both sodium and magnesium, and 
 found that both stars yielded the typical lines of these 
 two metals, but that Aldebaran showed them the more 
 
 53 
 
54 SIR WILLIAM HUGGINS 
 
 clearly. Betelgeux gave a spectrum of Secchi's Third 
 Type, with dark shaded flutings as well as dark lines, 
 and the b lines fell on one of these flutings and the D 
 lines on another, but the origin of the flutings them- 
 selves was then unknown ; they could be ascribed to 
 neither element. 
 
 In the solar chromosphere the b lines of magnesium, 
 together with the D lines of sodium, readily flash out ; 
 they are observed as regular constituents of its lower 
 strata when these can be observed in a total eclipse of the 
 Sun, and they are characteristic lines of those small bright 
 violent prominences known as " metallic eruptions." 
 
 As the simple spectrum of the double lines of sodium 
 was put into contrast with the complexity of that of iron 
 with more than 2000 lines, so the triplet of magnesium 
 may well be contrasted with the hundreds of lines that 
 make up the spectrum of another metal titanium. 
 Titanium also, like sodium, magnesium, and iron, shows 
 itself readily not only by dark absorption lines in the 
 spectra of the Sun, and of stars of the solar (or 
 Second) Type, but also in the solar chromosphere. 
 
 But these two metals have, like others, not only their 
 elemental spectra, but their spectra as compounds. 
 Under ordinary conditions we do not expect to see the 
 spectra of compounds in the Sun, for its temperature 
 is so high that compounds are dissociated there, the 
 component elements being driven apart. But in sun- 
 spots we have regions of local differences of temperature 
 and pressure, and there we also find local differences in 
 the solar spectrum. 
 
 In 1880, Maunder discovered a series of broad dark 
 lines in the spectrum of a sunspot, close to the b lines 011 
 the side nearer the red. These lines followed each other 
 closely, like the lines of a fluting. But while spectra 
 of separate lines are in general characteristic of elements, 
 spectra of flutings are in general characteristic of com- 
 pounds. Thus the two fluted spectra of carbon noted 
 in Chapter VI are regarded by many as the spectra of 
 its compounds with hydrogen and oxygen respectively. 
 
MAGNESIUM AND TITANIUM 55 
 
 In 1907, Fowler succeeded in proving that Maunder's 
 series of broad lines or " bands " in the spectra of sun- 
 spots, was due to a compound of magnesium with 
 hydrogen magnesium hydride a discovery of the 
 utmost importance in the light that it throws on the 
 temperature of sunspots, as it is clear that the tempera- 
 ture where that compound can be abundantly formed 
 must be considerably lower than that of the solar surface 
 in general. 
 
 Now fluted spectra are characteristic of two types of 
 stars : of Secchi's Third Type, which may loosely be 
 called the orange stars, in which the dark flutings are 
 dark and sharp on the side nearer the violet, and fade 
 gradually away in the red direction, and of Secchi's 
 Fourth Type, the red stars, in which the dark flutings 
 are dark and sharp on the side nearer the red, and fade 
 gradually away in the violet direction. The latter were 
 early seen to be the reverse of the spectrum shown by 
 comets the reversal, that is to say, of the " Swan " 
 spectrum, the spectrum of a carbon compound. And 
 in 1904, Fowler \vas able to show that the bands of 
 the Third Type, the bands seen in Betelgeux, Mira Ceti, 
 and many other stars, were due to the reversal of the 
 spectrum of titanium oxide. Some of the flutings of the 
 same compound have also been found in the extreme red 
 of the spectra of sunspots. Thus these Third Type stars 
 have been brought into continuity with the Second or 
 Solar stars ; they give evidence of the presence of the 
 same elements as in the Sun, but at a lower temperature. 
 
 The spectra of Magnesium and Titanium have thus 
 explained the origin of the flutings in the spectra of 
 sunspots and of the Third Type stars, and have furnished 
 an index of their relative temperature. 
 
CHAPTER IX 
 
 THE STORY OF CALCIUM 
 
 WHEN Fraunhofer made his first map of the solar spec- 
 trum, he noticed, far down in the extreme violet, a pair 
 of dark lines, or rather, a pair of broad shaded dark 
 bands, standing like two giant pillars at the gateway 
 of the darkness. For just beyond these bands the 
 spectrum ceases to impress ordinary human sight, 
 though the photographic plate is sensitive much further 
 in this direction. These two bands are now known as 
 the H and K lines, and are attributed to the element 
 Calcium. 
 
 These are not the only lines of calcium in this part of 
 the spectrum. Farther in the obscure region beyond 
 K the ultra-violet, as it is termed there is another 
 strongly-marked pair of lines, with the same intervals 
 between the members of the pair as between H and K. 
 On the other side of H, there is a strong single line in 
 the indigo-blue, with a wave-length of 4227, H and K 
 having wave-lengths of 3968 and 3934 ; while the pair 
 of lines beyond K stand at 3737 and 3706. But as early 
 as the year 1872, C. A. YOUNG, working on the spectrum 
 of the chromosphere and prominences, was able to point 
 out that " the selection of lines," seen bright in the 
 chromosphere, " seems most capricious ; one is taken 
 and another is left, though belonging to the same ele- 
 ment, of equal intensity, and close beside the first." 
 He especially noted that while H and K are almost 
 always observable in the chromosphere, the strong blue 
 line, 4227, as well as the other lines of the metal, are 
 very seldom seen. Thus while the H line was seen on 
 
 66 
 
THE STORY OF CALCIUM 57 
 
 75 occasions out of 100, and the frequency of K natur- 
 ally less often seen, since it is nearer the limit of our 
 vision was 50, the blue line at 4227 had a frequency 
 of only 3. 
 
 This apparently capricious selection in the spectrum 
 of the chromosphere of a very few lines out of the many 
 characteristic of an element, was ascribed by Lockyer 
 to the effect of the extremely high temperature of the 
 chromosphere. He considered that " iron, calcium, and 
 magnesium, have probably a definite spectrum consist- 
 ing of a few lines, which can only be completely pro- 
 duced at a temperature higher than any which is at 
 present available in laboratory experiment." But, as 
 the observations of Lockyer himself would suggest, 
 there is another explanation which would account for 
 this selection, and in 1897 Huggins gave the question 
 a thorough examination. Using a spark of very small 
 intensity, he found all the five above-mentioned lines of 
 calcium, very intense ; H and K being broad, and all 
 the five lines " winged." i.e. with diffused fringes on 
 both sides. This was when both electrodes were of 
 metallic calcium. The substitution of platinum for 
 calcium in one electrode diminished the quantity of 
 calcium vapour present, and was accompanied by a 
 narrowing and sharpening of the lines, particularly 
 of 3737 and 3706. Then both electrodes were taken of 
 platinum, and a trace of calcium was supplied by moist- 
 ening them with a strong solution of calcium chloride ; 
 this made the blue line faint, while H and K lost their 
 diffused edges, and became fairly sharp though still 
 pretty broad. The electrodes were then repeatedly 
 washed with pure water to remove even these slight 
 traces of calcium, with the result that the blue line 
 disappeared, the other four lines became narrowed and 
 sharper, and lastly the lines at 3737 and 3706 vanished, 
 leaving H and K quite fine sharp lines. As the spark 
 was not altered in intensity throughout the series of 
 experiments, it would appear that these progressive 
 changes were due, not to increase of temperature, but 
 
58 SIR WILLIAM HUGGINS 
 
 to a diminution in the quantity that is to say, in the 
 density of the calcium vapour present. There could 
 be no question of the gradual break-up of the element, 
 and of its dissociation into simpler substances. 
 
 The lines H and K rise higher in the solar chromosphere 
 even than the lines of hydrogen, and their appearance 
 at various distances from the Sun's limb reproduce 
 pretty closely the changes obtained by Huggins in these 
 experiments so far as the breadth and diffuseness of 
 the two lines are concerned. 
 
 Fathers SIDGREAVES and COBTIE at Stonyhurst have 
 shown that the H and K lines of the general light of 
 the Sun show at times a " reversal " that is, a bright line 
 is shown in the middle of the dark band. The Sun 
 therefore to this extent approximates to that class of 
 stars showing bright lines as well as dark in their spectra ; 
 it is to a small extent a "bright-line star." But when 
 an image of the Sun is formed by a telescope on the 
 slit of the spectroscope, so that different parts of the 
 disc can be studied in detail, such a " reversal " is very 
 often seen. The bright line in the centre of the dark 
 band, which is itself fairly broad, also frequently shows 
 a further reversal, a narrow dark line being detected in 
 its centre. 
 
 These three successive forms, say of the K line, 
 correspond in breadth therefore to the K line as seen 
 beyond the limb at different distances from it that 
 is to say, at different heights above the Sun's sur- 
 faceand recall the different breadths obtained by 
 Huggins in the experiments just cited. The narrower 
 the line and the higher above the photosphere, the rarer 
 we may suppose the gas to be ; so that the three divi- 
 sions of the line may be taken as corresponding to three 
 different levels ; the broad dark band corresponding to 
 the reversing layer, the bright line to the chromosphere, 
 and the narrow dark central line to its highest region. 
 
 The slit of a spectroscope allows only a narrow strip 
 of the image of the Sun to be examined at a time ; but 
 if we remove the spectroscope while we retain the slit 
 
THE STORY OF CALCIUM 59 
 
 and place an eye-piece behind it, we can see the whole 
 of the Sun piecemeal, by allowing its image to drift 
 across the slit. It would indeed be difficult to sketch 
 the details of the Sun's surface in this way, but it would 
 be possible to photograph them by the simple device 
 of moving the photographic plate at precisely the same 
 speed as the solar image was moving ; or, both the 
 image of the Sun and the sensitive plate might be fixed, 
 and the slit might be moved at a uniform rate across 
 both. This is, indeed, the method actually employed 
 in the ordinary photography of the Sun. The Sun's 
 light is so intense that even very slow plates are almost 
 instantly fogged when they are exposed to it in a tele- 
 scope ; it is not possible to uncover and cover the ob- 
 ject-glass with sufficient rapidity to get an unfogged 
 image. A brass plate in which there is a very narrow 
 slit is therefore placed in the focus of the telescope, 
 cutting off all light from the sensitive plate except what 
 comes through the slit, and this slit is drawn very rapidly 
 across the solar image by a powerful spring. The 
 different parts of the photographic plate are thereby 
 successively exposed for a very small fraction of a second 
 to different parts of the Sun, and a complete image of 
 the whole Sun is built up on the plate. 
 
 Now by replacing the spectroscope behind the slit, 
 and fixing a second slit in the plane of the spectrum so 
 as to allow the light corresponding to one ray only to 
 pass through, we get substantially the same arrange- 
 ment, except that we confine the light used to that single 
 radiation. As the lines of the solar spectrum are images 
 of the first slit, if we exactly fit the second slit to one of 
 these lines, we shut out afl light except that one par- 
 ticular ray, and see the Sun in light of that one refrangi- 
 bility or colour only ; we should seem to be looking out 
 through a single narrow slit at a Sun painted in pure 
 monochrome, red, green, blue, or violet, according to the 
 line chosen. 
 
 And we could photograph the entire Sun piecemeal, 
 either by keeping the spectroscope fixed and moving 
 
60 SIR WILLIAM HUGGINS 
 
 both the solar image and the photographic plate in exact 
 accord, or by keeping the image and the plate both fixed 
 and moving the entire spectroscope, with both its slits 
 in a single piece across both. In both methods a com- 
 plete image of the entire Sun will be built up on the 
 plate. 
 
 This device of a spectroscope with two slits occurred 
 to several astronomers at the same time, but G. E. HALE 
 was the first to overcome the immense difficulties in- 
 volved in its construction and employment and to make 
 a " spectroheliograph," as an instrument of this kind is 
 called, a practical success. DESLANDBES has introduced 
 a modification of the device in order to register the rate 
 of motion of the gas-streams over the solar surface. 
 
 The first lines selected for application to the spectro- 
 heliograph were the two giant lines of calcium, H and K ; 
 their breadth and darkness rendering them the easiest 
 to work with ; indeed Hale soon found that he could 
 work with small portions of either line. He could set the 
 second slit somewhere on the general broad band of K, 
 which he distinguished as K b or on the bright line in its 
 centre, K 2 , or even on the very narrow dark line, K 3 , 
 which was so often seen in the centre of K 2 . Each of 
 these could in turn be isolated to give a monochromatic 
 picture, and thus images of the Sun corresponding to 
 three widely different levels could be obtained. 
 
 The spectroheliograph has thus not only enabled the 
 prominences and chromosphere beyond the Sun's limb 
 to be registered with a fulness of detail that the old 
 laborious method of sketching each separate prominence 
 in turn could never give, but the whole disc of the Sun 
 has been shown to be covered by a network or mottling 
 of bright markings to which Hale has given the name of 
 " flocculi." These flocculi have a sort of general corre- 
 spondence to the bright ridges known as " faculse," seen 
 directly in the telescopic image of the Sun, or on ordinary 
 photographs of it ; but the spectroheliograph enables 
 them to be photographed in all regions of the disc, even 
 at the centre, where faculae as ordinarily seen cannot be 
 
THE STORY OF CALCIUM 61 
 
 traced. These flocculi are particularly bright and 
 closely aggregated over or around sunspots ; so that 
 where the usual methods of observation will show a great 
 group of dark sunspots, the spectroheliograph will reveal 
 a very bright cloud of calcium vapour, the extent and 
 brightness of which will vary according to the position 
 on the K line, where the second slit has been placed ; 
 or we may say, in the light of Huggins' experiments on 
 the density of calcium vapour, to the level above the 
 solar surface to which that position of the slit will cor- 
 respond. 
 
 Not all the flocculi are bright ; sometimes in a com- 
 plete spectroheliogram of the Sun taken in K-light, a 
 long dark rift will show itself, and these seem closely 
 associated with long ridges of prominences. 
 
 The invention of this particular modification of the 
 spectroscope the spectroheliograph has been hardty 
 less important than that of the spectroscope itself. It 
 has opened up to study an entirely new field in solar 
 physics, the study of the strata and currents of the solar 
 atmosphere, not merely beyond the limb of the Sun, but 
 also above the face of the disc. 
 
 The difference in the appearance of the H and K lines 
 of calcium according as the gas is rare or condensed 
 furnished Huggins with a means for a far more detailed 
 classification of stellar spectra than that due to Secchi. 
 In the spectrum of the Sun, the H and K lines form a 
 pair, in breadth and distinctness, but in many stars the 
 two have lost their likeness. H still remains dark, 
 broad, and diffused while K is faint, narrow, and sharp, 
 and it becomes an easy task to arrange those stellar 
 spectra that agree as to the prominence of H, according 
 to the distinctness which they give to K. Thus Regulus, 
 Gamma LJTSB, Gamma Andromedee, Vega, Sirius, Castor 
 (the fainter member of the pair), Altair, Procyon, and 
 Gamma Cygni, are all members of Secchi 's First Type, 
 showing the lines of hydrogen broad and dark. But 
 the charts of their spectra given by Huggins in his 
 great Atlas of Representative Spectra show that K is all 
 
62 SIR WILLIAM HUGGINS 
 
 but invisible in Regulus, faint in Gamma Lyrae, a little 
 more distinct in Vega and Sirius, dark, but narrow and 
 sharp, in the fainter component of Castor, darker and 
 broader in Altair, and a full twin to H in Procyon and 
 Gamma Cygni. These two last-named stars approxi- 
 mate to Secchi's Second Type stars, with spectra like 
 that of the Sun, wherein H and K stand out twin giants 
 at the boundary between the violet and the invisible 
 ultra-violet beyond. Thus within the limits of this one 
 type of spectrum, according to Secchi's classification, 
 there are six or seven well-marked subdivisions, capable 
 of being arranged in unbroken orderly succession, 
 according to the degree of distinctness with which the 
 K line of calcium reveals itself. 
 
 The spectrum of Calcium therefore has enabled a more 
 minute and orderly classification to be made of stellar 
 spectra. But, above all, it has led to the invention of 
 the spectroheliograph, and thus to the registration by 
 photography of the distribution of the regions of varying 
 luminosity at different heights above the solar surface. 
 
CHAPTER X 
 
 THE SECOND STORY OF HYDROGEN 
 
 THE introduction of gelatine as a vehicle for the silver 
 salts in photography and the supersession of the old 
 " wet " collodion plates, by " dry plates " of far higher 
 sensitiveness offered Huggins the means, of which he at 
 once availed himself, for the extension of his spectro- 
 scopic researches, and he led the way in making use of the 
 possibilities of the new process. But this involved a 
 great alteration in the form of the instrument to be em- 
 ployed. If photography offered the prospect of much 
 help, it likewise introduced many fresh difficulties, which 
 Huggins encountered and overcame with limitless fore- 
 thought, ingenuity, and patience. 
 
 Thus the eye regards one small portion of the spectrum 
 at a time, and it can be brought to its appropriate focus 
 while actually under observation ; the photographic 
 plate will record the spectrum as a whole, and that not 
 merely in the visual portion, but far within the invisible 
 ultra-violet. All this has to be brought to focus to- 
 gether on the plate, and the focussing must be made 
 before the exposure begins, and must remain unchanged 
 throughout its whole duration. Then in direct eye 
 observation, if the star, through imperfect clock-driving 
 of the telescope, wander off the narrow slit of the spectro- 
 scope, the observer at once sees what has happened, 
 and brings it back by the slow-motion rod ; in photo- 
 graphic work, such wandering would simply mean that 
 the star would leave no image of its spectrum on the 
 plate. This difficulty was overcome partly by the 
 neat device of silvering the face of the slit plate and view- 
 
 63 
 
64 SIR WILLIAM HUGGINS 
 
 ing it by a little telescope. The images of the star and 
 of the slit were therefore seen together, and could be 
 kept in their proper relative positions. The devotion 
 of Lady Huggins, the unwearied assistant of her husband 
 in all his work, did the rest, as she watched these images 
 and guided the telescope through all the long hours of 
 the protracted exposures required by these photographs. 
 
 In this department of stellar photography the two 
 chief pioneers were Huggins in England and HENRY 
 DRAPER in the United States, and the latter was the first 
 to secure a successful photograph of the spectrum of 
 Vega. 
 
 Huggins had indeed obtained an impression of that 
 of Sirius nine years earlier, but this had not showed 
 any absorption lines ; in 1876, however, Vega yielded 
 him a most instructive negative. The visual spectrum 
 of Vega and other stars of Secchi's First Type is crossed 
 by four broad dark lines or bands, corresponding to the 
 four lines of hydrogen in the red, blue-green, indigo, and 
 violet. Huggins' photograph was not sufficiently sen- 
 sitive to the red and green to bring out the first two 
 bands, but the indigo and violet bands were strongly 
 marked, followed by five similar bands in the ultra- 
 violet. The entire spectrum of Vega thus displayed 
 in all nine bands that tended to come closer and closer 
 together the farther that they were followed into the 
 ultra-violet, and in 1880 Huggins was able to increase 
 the number to twelve by the addition of three more, 
 still farther in the ultra-violet. The manifestly syste- 
 matic way in which the series of lines tends to converge 
 led Huggins to suggest that the lines must be " intimately 
 connected with one another and present the spectrum 
 of one substance " ; and G. JOHNSTONE STONE Y decided 
 that there could remain very little doubt that these 
 lines were all due to hydrogen. " The evidence of their 
 all being members of one physical system is made very 
 plain when their positions are plotted down, for it then 
 becomes conspicuous that they lie on, or very near, a 
 definite curve which could not happen by chance.'* 
 
THE SECOND STORY OF HYDROGEN 65 
 
 Johnstone Stoney proceeded to show that the wave- 
 lengths of several of the lines bore a simple numerical 
 relationship to each other, but that it would be neces- 
 sary to assume that some members of the series were 
 missing. In 1885, BALMER discovered that the wave- 
 lengths of the four hydrogen lines in the visual part of 
 the spectrum bear a striking relation to the number 
 3645-6. Thus we have 
 
 Colour Line Wave-length 
 
 Red C 6562-10 9/5 
 
 Bluish-green F 4860*74 16/12 
 
 Indigo "nearG" 4340-1 25/21 
 
 Violet h 4101-2 36/32 
 
 and it became evident that the fractions could be written 
 as : 
 
 3 a 4 2 5 2 6 2 
 
 32_4 ; 4 2 -4' 5 2 -4' 6 2 -4 
 
 or generally as f , where ra is given by the in- 
 
 tegers in regular succession. As this formula, when 
 carried on to higher values of ra, was found to yield the 
 lines observed in the ultra-violet in Vega, no doubt could 
 be felt that they also were due to hydrogen ; nor was 
 it long before AMES was able to produce them in the 
 laboratory. Later, photographs of stellar spectra by 
 Iluggins and others, and of the chromospheric lines in 
 total eclipses by EVERSHED, DYSON and MITCHELL have 
 enabled the series to be traced yet further still, and in 
 all no fewer than twenty -nine lines have been observed, 
 the fifth of which agrees very nearly in place with the 
 H line of calcium. This explains the apparent dissimi- 
 larity between the H and K lines in certain stellar 
 spectra ; in such cases H is really a line of hydrogen, 
 not one of the Great Twin Brethren of calcium. 
 
 This striking discovery by which all the twenty-nine 
 lines of hydrogen were shown to be the working out of 
 one simple principle, soon led the way to other develop- 
 ments. RUNGE, KAYSER and PASCHEN found the alka- 
 
 E 
 
66 SIR WILLIAM HUGGINS 
 
 line metals, sodium, lithium, potassium, &c., give not one 
 series of lines but three ; the Principal series containing 
 the lines strongest in the spectrum, and most easily 
 reversed, while the two Subordinate series are distin- 
 guished from each other by the fact that one gives 
 faint, sharp lines, the other, lines somewhat stronger 
 but diffuse. Thus the three series may be distinguished 
 as Principal, Diffuse and Sharp, the last two being 
 often made up of doublets or triplets according to the 
 element in question. 
 
 RYDBERG has carried the inquiry yet further and has 
 shown that a definite mathematical relationship exists 
 between the three series, so that it is possible to calcu- 
 late the wave-lengths of the lines in the Principal series 
 when those in the Sharp series are known. In 1896, 
 E. C. PICKERING discovered a new series of lines in the 
 spectrum of Zeta Puppis, a star that shows the lines of 
 hydrogen very distinctly. But the new series bore an 
 evident relation to the familiar series of hydrogen ; 
 indeed a slight change in Balmer's formula would give 
 the well-known hydrogen lines when even values of m 
 were used ; i.e. when ra=6, 8, 10, &c., but the new 
 Zeta Puppis lines when the odd values, w=ll, 13, 
 15, &c., were employed. Rydberg showed that this 
 was a case of the two Subordinate series of one element, 
 the familiar series belonging to the Diffuse, the new 
 lines to the Sharp series ; and from the known values 
 of these he calculated the wave-lengths of the Principal 
 series. This had one line in the blue at 4688, and the rest 
 so deep down in the ultra-violet that our atmosphere 
 prevents our tracing any astronomical spectrum so far. 
 
 In 1867, WOLF and RAYET chanced upon three small 
 stars in Cygnus with a very remarkable spectrum, 
 consisting both of bright and dark lines on a continuous 
 background. Six stars more of similar character were 
 detected in 1883 by Copeland, and since then further 
 discoveries of the same kind have been numerous. 
 These stars are noticeable not only for their spectra, but 
 also for their distribution ; they are found chiefly along 
 
THE SECOND STORY OF HYDROGEN 67 
 
 the very spine of the Milky Way or in the Great Magel- 
 lanic Cloud, which appears to be a detached cluster of 
 an allied nature. 
 
 The continuous spectrum of the Wolf-Rayet stars is 
 traceable in fair strength into the ultra-violet. The 
 dark absorption lines and bands which are superposed 
 upon it are mostly of an unknown origin, the metallic 
 elements not being represented. Hydrogen gives some 
 of its lines as dark and some bright, and this in both 
 series, whether the familiar Diffuse series or the Sharp 
 series discovered by Pickering. But in the blue stands 
 a broad bright band that is now recognised as the wanted 
 chief line of the Principal series of hydrogen ; and the 
 same line has been photographed by MAX WOLF from the 
 central " dark space " of the King nebula in Lyra. 
 Fowler also photographed it as a high level line in the 
 solar chromosphere during the eclipse of 1898. The 
 same observer also obtained the line in the laboratory 
 as at wave-length 4686 (Rydberg's calculated wave- 
 length not being quite exact), together with its three 
 predicted companions in the ultra-violet, and at the 
 same time detected three members of another Principal 
 series of hydrogen lines. 
 
 RITZ, who has carried Rydberg's inquiries as to the 
 mathematical relationships of the wave-frequencies of 
 the lines of elements a stage further, has computed the 
 wave-lengths of a series of hydrogen lines occurring in the 
 invisible portion of the spectrum beyond its less re- 
 frangible end, the " infra-red," as it is termed, and two 
 of these lines have been photographed by Paschen. 
 Thus instead of one series of lines due to hydrogen, five 
 are now known, all closely related to each other by their 
 mathematical formula though utterly unlike in arrange- 
 ment, position in the spectrum, and appearance. 
 
 The presence in the Wolf-Rayet stars of lines charac- 
 teristic of the same element, but some bright and some 
 dark, would seem to be a question of relative temperature. 
 A solid body, as everyone knows, as its temperature is 
 increased, glows first with a dull red, which becomes 
 
68 SIR WILLIAM HUGGINS 
 
 more vivid as the heating process is continued until at 
 length it is white hot. If the process is watched in the 
 spectroscope, a faint continuous spectrum is first seen 
 far down in the deep-red which gradually gets brighter 
 and extends further and further towards the orange, 
 then into the yellow and green, and so on, until the 
 object is fully white hot, and the spectrum reaches to 
 the extreme violet. So with a glowing gas ; the lines in 
 the red show themselves strongest at the lowest tem- 
 peratures, those further towards the violet become more 
 conspicuous as the temperature is increased. In the 
 same way the general continuous spectrum of a star will 
 extend further into the ultra-violet and the position of 
 its brightest region will be more in that direction the 
 higher the temperature. 
 
 The same is observed with regard to the Sun. The 
 red C line of hydrogen will be observed to a greater 
 height in a prominence than the bluish-green line F, and 
 this again will appear taller than the line " near G " 
 in the indigo, or the h line in the violet. Just then, as in 
 a spectroheliogram taken on K 3 , the narrow dark line in 
 the centre of the " reversal " of the K line is regarded 
 as giving a view of calcium at the highest level while 
 the edge of the broad dark K band, K 1? gives the lowest 
 level ; so a spectroheliogram on C is regarded as represent- 
 ing substantially a higher hydrogen level than F, and 
 F again as higher than " near G." It is, therefore, not 
 unnatural to find stars with " chromospheres " so hot 
 as to give the first line of hydrogen, or even the first two 
 as bright, while those further towards the violet re- 
 main dark. 
 
 The Wolf-Rayet stars, for all that they show this 
 peculiarity of some lines bright and others dark from 
 the same element, are stable in the amount of the light 
 they radiate. But many bright-line stars are very vari- 
 able ; in particular the stars of Secchi's Third Type, 
 the orange stars, with their spectra crossed by a series 
 of dark flutings, each with a sharp dark edge on the side 
 nearer the violet, but fading away towards the red. In 
 
THE SECOND STORY OF HYDROGEN 69 
 
 " Mira " Ceta, which may be taken as the type of these 
 " long period variables," the star yields 1000 times more 
 light at maximum that at minimum, the complete cycle 
 of variation running its course in eleven months, and 
 at maximum the lines of hydrogen gleam bright, but 
 their shining appears to be confined to the lines of the 
 violet and ultra-violet ; the C and F lines do not share 
 in the quickening of their comrades. And one of the 
 violet lines also remains dark, the line H ; but in this 
 case Miss CLERKE suggests, since the H line of hydrogen 
 and the H line of calcium are so close to each other in 
 position, that where either is at all broad the two overlap, 
 that the atmosphere of Mira may contain a stratum of 
 cooler calcium vapour, higher up than the stratum giving 
 the bright hydrogen. On this assumption the dark H 
 line of calcium of the upper stratum would conceal the 
 bright H line of hydrogen from the lower. 
 
 Henry Draper has already been mentioned as a pioneer 
 in photographing stellar spectra. His early death in 
 1882 put an end to the work he was carrying out with so 
 much skill and success, but his widow, resolving to erect 
 a suitable monument to her husband's memory, placed 
 his telescopes at the disposal of E. C. Pickering and 
 supplied him with ample funds to carry on, at the 
 Harvard College Observatory, the researches in this 
 field that her husband had commenced. In this way 
 a great photographic survey of stellar spectra was under- 
 taken for the northern heavens, and as far as S. Dec. 25 ; 
 10351 stars down to the 8th magnitude being included 
 in the " Draper Catalogue." These photographs were 
 obtained with the simplest possible form of spectroscope, 
 that used by Fraunhofer in his examination of stellar 
 spectra. A prism is placed before the object glass of the 
 photographic telescope. That is all ; there is no slit, 
 110 collimator, no image formed on the slit-plate by a 
 condensing telescope ; the only two parts are the prism 
 and the telescope for forming an image of the spectrum 
 on the photographic plate. 
 
 Miss MAUBY, who examined the photographs and 
 
70 SIR WILLIAM HUGGINS 
 
 classified the spectra, detected that the lines in the spec- 
 trum of Zeta Ursse Majoris were sometimes single, some- 
 times double. This was true of all the lines in the 
 spectrum, but the hydrogen lines being the strongest 
 the fact was best exhibited by them. The cause of this 
 change in appearance was found to be that this star, 
 usually known as Mizar, was really a close pair, the com- 
 ponents of which move round their common centre of 
 gravity in a little under 21 days. At one part of then" 
 orbit therefore both stars are moving across the line of 
 sight, and the lines are all single ; at another, one star 
 is approaching us and the other receding from us. In 
 this case the lines of the first star are shifted, by the 
 effect of that motion, towards the violet, the lines of 
 the other star towards the red, and hence in the common 
 spectrum of the two stars all the absorption lines are 
 seen as doubled. In 1889 Miss Maury found that Beta 
 Aurigse, Menkalinan, showed the same effect but in a 
 much shorter time ; the swing to and fro of the lines 
 takes place in two days, involving a revolution in four 
 days. 
 
 Some stars, as for instance Capella, have a spectrum 
 showing a combination of two different types ; in other 
 words, the single star that we see is really two close to- 
 gether but of unlike spectra. In such cases, as some 
 lines belong entirely to one star, some entirely to the 
 other, not all the lines are doubled, but they move re- 
 latively to each other in the compound spectrum due to 
 the aggregate light of the two stars. In the case of 
 Spica (Alpha Virginis), Vogel found only one spectrum 
 visible, but from careful comparison with hydrogen, 
 inferred that the star was sometimes receding from 
 us and sometimes approaching us ; and therefore that 
 it must have an orbital motion round an invisible com- 
 panion. So many instances of this kind have now been 
 discovered that the Director of the Lick Observatory, 
 on Mount Hamilton, W. W. Campbell, estimates that 
 one star out of every five is probably a " spectroscopic 
 binary," i.e. a double star with only one member visible, 
 
THE SECOND STORY OF HYDROGEN 71 
 
 but the orbital motion of which is shown by these changes 
 in its radial motion relatively to the Earth. 
 
 Sometimes a spectroscopic binary reveals its nature 
 by the circumstance that one of the two stars is dark, 
 and the dark member of the pair passes at times between 
 the Earth and the bright member, thus producing a 
 partial eclipse of its primary. The model star of this 
 class is Algol (Beta Persei), a variable star to the naked 
 eye, dropping every 69 hours from mag. 2*3 to mag. 3*4 ; 
 thus losing three -sevenths of its light. The period of 
 decline and recovery lasts about 9 h. 20 m. ; for the 
 remaining 60 hours the star retains its full brightness. 
 
 Goodricke, the young deaf-mute who, in 1782, re- 
 discovered the variability of Algol, conjectured that its 
 changes were due to an eclipsing satellite ; Vogel, 
 rather more than a century later, proved this to be the 
 case, by showing that the bright star is moving in an 
 orbit with a speed of more than 26 miles a second ; 
 17 hours before minimum it recedes from us at 24 miles, 
 17 hours after minimum it approaches us at 28*7 miles 
 a second ; at the time of minimum, that is to say at 
 mid-eclipse, and midway between one eclipse and the 
 next, its motion of approach is 2*3 miles a second. 
 From these data, the size and mass of Algol, and of its 
 unseen companion, and their distance apart, have been 
 computed. The diameter of Algol is more than a, million 
 miles, that of the dark companion about 830,000 miles ; 
 the distance between their centres is a little more than 
 three million miles ; and the combined masses of the 
 two bodies amount to two-thirds that of the Sun. 
 
 Algol-type stars, so far as yet discovered, all give 
 spectra in which metallic lines are relatively weak and 
 inconspicuous ; so that our knowledge of their move- 
 ments rests chiefly on the changing displacement of the 
 dark absorption lines due to hydrogen. 
 
 Another short-period variable, Beta Lyrse, is dis- 
 tinguished by the appearance of the lines of hydrogen 
 in brightness, the C line glowing out like a red danger 
 signal. Beta Lyrae is, like Algol, an eclipse variable, 
 
72 SIR WILLIAM HUGGINS 
 
 but both members of the pair are bright, and as they 
 alternately eclipse each other, there are two minima, 
 one more marked than the other, in each period of 
 nearly thirteen days. But the changes in the spectrum 
 of the combined stars are most intricate and perplexing. 
 BELOPOLSKY, however, succeeded in proving that a strong 
 dark line in the blue at wave -length 4482, due to mag- 
 nesium, belongs to one member of the system, and the 
 bright line F of hydrogen to the other ; and that the 
 changes in the double spectra of the system go forward 
 in a general correspondence with the changes in its 
 aggregate brightness that follow on the alternating 
 eclipses. 
 
 The study by photography of the spectrum of Hydrogen 
 as given us first by the heavenly bodies, and next in the 
 laboratory, has led to the solution of the great problem 
 of the relations of the various lines in a given spectrum. 
 They form definite series, calculated by formulae of one 
 general type, the chief constant in which is the same for 
 all elements. Hydrogen, too, revealed the existence of 
 a new class of binary systems the spectroscopic 
 doubles. 
 
CHAPTER XI 
 
 THE STORY OF HELIUM 
 
 HELIUM, the solar element, as its name implies, was 
 first discovered in the spectrum of the Sun ; it was later, 
 by more than half a century, that it was " run to earth." 
 The first vague glimpse of its spectrum would seem to 
 have been caught by MAGRINI at Milan, during the 
 total solar eclipse of 1842, July 8. In those early days 
 of spectrum analysis, a momentary glance through a 
 flint glass prism at the eclipsed Sun was all that was 
 attempted, but that glance revealed its immediate sur- 
 roundings in three vivid colours red, yellow and blue. 
 It was not until twenty-six years later, at another solar 
 eclipse, that of August 18, 1868, in India, that the sig- 
 nificance of this observation was grasped. CAPT. JOHN 
 HERSCHEL, when setting the slit of his spectroscope on 
 a brilliant prominence, needed but to give " a single 
 glance and the problem was solved. Three vivid 
 lines red, orange, blue ; no others, and no trace of a 
 continuous spectrum." The observation showed that 
 the prominences are wholly gaseous, and are parts 
 of the Sun. A second problem still remained as to 
 the nature of the gas, and it was not until later, when 
 Janssen observed the uneclipsed chromosphere, that it 
 was found that the blue and red lines belonged to 
 hydrogen, but that the bright yellow line was not the 
 double D-line of sodium, but lay further towards the blue 
 than D 2 , by more than twice the distance that separated 
 the latter from its twin brother D r 
 
 At first this new yellow line, D 3 , was by some supposed 
 to come from an allotropic form of hydrogen, but the 
 
 73 
 
74 SIR WILLIAM HUGGINS 
 
 difference in its behaviour from the undoubted hydrogen 
 lines, C and F, finally caused it to be attributed to a gas, 
 as yet unknown, and given the name of " helium." 
 For another quarter of a century, helium could only be 
 examined in the Sun or in certain of the stars, until 
 1895 when RAMSAY obtained it from the mineral cleveite. 
 Even then its identity with solar helium was in dispute, 
 for in the cleveite gas Runge and Paschen found that 
 D 3 was a double not a single line, but the doubt was no 
 sooner raised than it was set at rest by the observation, 
 both by Huggins and by Hale, that the solar D 3 is also 
 double. 
 
 No sooner was this settled than another question was 
 raised for solution ; whether helium was a single ele- 
 ment, or the mixture of two elements helium and 
 " parhelium." At that time only one series of lines 
 had been recognised in the spectrum of hydrogen. In 
 the spectra of the alkalies, Kayser and Runge had found 
 that in each case there were three distinct series a 
 Principal and two Subordinate series. But the gas 
 from cleveite gave, not one series like hydrogen, nor 
 three like lithium, but six, and of these, some were series 
 of single lines, some of doublets. Kayser and Runge 
 wrote : " We have accordingly here to do with six 
 series, among which it twice happens that two series 
 converge towards the same place. . . . Both the sys- 
 tems prove to have a similar appearance and all the 
 lines of the first system are stronger than the corres- 
 ponding lines of the second system. Further, it appears 
 that both these spectra are very similar to the spectra 
 of the alkalies. . . . We can distinguish in each of them 
 two subordinate series diverging from the same situation, 
 of which the brighter is the more closely spaced. More- 
 over, each system contains a Principal series whose lines 
 are stronger than those of the Subordinate series, and 
 which extends to shorter wave-lengths." l In Nature 
 for September 26, 1895, they summed up the argument 
 
 1 Proc. Berlin Alcad., June 20, 1895; Phil. Mag., Sept. 1895, 
 p. 298. 
 
THE STORY OF HELIUM 75 
 
 for a mixture of two elements thus : " There is no instance 
 of an element whose spectrum contains two pairs of 
 series ending at the same place. This suggested to us 
 the idea that the two pairs of series belonged to different 
 elements. . . . We therefore believe the gas in cleveite 
 to consist of two, and not more than two constituents. 
 We propose to call only one of the constituents helium 
 the one to which the bright yellow double line belongs, 
 whose spectrum altogether is the stronger one while the 
 other constituent ought to receive a new name." In fact 
 each set of three series was analogous to the complete 
 spectrum of a distinct element. Nevertheless, not only 
 was " parhelium " never separated from helium, but the 
 very argument for its separate existence was turned into 
 disproof, when it was found that oxygen also claimed six 
 series. Hydrogen, too, has recently been shown to have 
 two Principal series, and moreover these two are related to 
 each other in the same manner as the Diffuse and Sharp 
 series. 
 
 As mentioned in Chapter IX, in his report on the total 
 solar eclipse of 1898, January 22, Fowler pointed out 
 that there was a high level chromospheric line about 
 wave-length 4686, which was possibly identical with 
 . the first Principal line of hydrogen, and a line of unknown 
 origin in about the same position was shown in a photo- 
 graph of the spectrum of a helium tube at the Solar 
 Physics Observatory, South Kensington. A careful 
 watch was kept for this line in all helium laboratory 
 experiments, but without success until very recently 
 it was recognised in a tube of helium prepared from 
 cleveite in which hydrogen was the chief impurity. 
 So far it has hot been found possible to obtain the line 
 in question from hydrogen alone under apparently 
 identical conditions, though there is not the slightest 
 difficulty in obtaining it when helium is also present, 
 and it needs only a very small proportion of hydrogen 
 to cause 4686 to show up brightly. At present it can 
 only be said that it seems to require both hydrogen and 
 helium to be present for this line to show up. 
 
76 SIR WILLIAM HUGGINS 
 
 On Earth, helium was first derived only from cleveite 
 and other rare mineral earths. But the study of the 
 radio-active elements, so far as it has gone within the 
 last few years that these substances have been recog- 
 nised, has revealed one fact which is of great impor- 
 tance. Radium, the best known of these elements, 
 is a metal the compounds of which resemble those 
 of barium. But its distinctive property is that it 
 undergoes spontaneous disintegration and decay, giving 
 off particles of helium and a gaseous emanation which 
 is ultimately resolved into helium and nothing else. 
 As radium yields helium, the first term of the series of 
 chemically inactive elements, TILDEN suggests that it 
 seems legitimate to suppose a similar origin for neon, 
 argon, krypton and xenon, the rare and inert gases that 
 Ramsay discovered in our atmosphere. None of these 
 last four elements have been observed in the spectrum 
 of the Sun or any of the stars, but helium is, as its name 
 implies, an important constituent of the Sun's atmos- 
 phere. But helium does not usually exhibit any ab- 
 sorption lines in the photospheric spectrum, though its 
 dark lines have been seen over sunspots ; it usually 
 only manifests its presence by bright lines seen beyond 
 the limb. Were our Sun so distant that we could 
 examine spectroscopically neither the chromosphere nor 
 the spots, we should not know that helium was on the 
 Sun, though it surrounds it to a depth of 5000 miles. 
 
 Helium is, further, one of the chief constituents 
 of gaseous nebulae ; it shows its characteristic absorp- 
 tion lines in stars of Secchi's First Type, Variety a, 
 which are immersed in nebulae or closely connected with 
 them. Helium stars are not scattered indiscriminately 
 over the heavens, but tend to congregate towards the 
 plane of the Milky Way ; indeed it is conjectured that 
 Milky Way clusterings are largely composed of helium 
 stars, large and small. This may not hold good in actu- 
 ality but only in appearance, since the great intrinsic 
 brightness of helium stars (which are intensely white) 
 may leave them visible when other redder stars have 
 
THE STORY OF HELIUM 77 
 
 vanished through distance. For helium stars are very 
 distant only Regulus and Beta Centauri are within 
 measurable distances from us. 
 
 The brightest helium star is Rigel (Beta Orionis) ; 
 it is so distant that all attempts to measure the space 
 between it and our Sun have failed, but if its proper 
 motion be apparent only, and due to the Sun's motion 
 in space, then its actual brightness will come out as 
 8000 times as great as that of Alpha Centauri, our nearest 
 neighbour and a twin to our Sun. This great intrinsic 
 brightness of helium stars is counted as a sign of youth 
 and a sign of high temperature ; hydrogen lines are 
 present in helium stars, but as these " age," if we may 
 so speak of stars, the helium lines lose their predomi- 
 nance as compared with those of hydrogen. And a 
 significant characteristic of helium stars is the presence 
 in them of the lines of the non-metals oxygen, silicon, 
 carbon and the absence of metallic lines ; as the 
 metals appear the helium spectrum vanishes. Variable 
 stars sometimes show either bright or dark helium lines 
 and Gamma Cassiopeiae and P Cygni show both together. 
 All'" temporary stars " or Novae at one stage of their 
 evolution exhibit both helium emission and absorption. 
 
 The Orion stars were classed by Secchi with wonderful 
 intuition, as a distinct subdivision of his First Type. This 
 was before helium had been recognised in the Sun's 
 chromosphere, or in stars, and long before it had been 
 isolated in the laboratory. But his prescience was 
 amply justified, and in Huggins' magnificent photo- 
 graphic Alias of Representative Stellar Spectra al- 
 ready alluded to, the gradation of spectra is admirably 
 brought out. Beta Lyrse, with the lines of helium bright, 
 comes first ; then follows Bellatrix (Gamma Orionis), 
 with the helium lines dark and distinct and hydrogen 
 narrow, and Rigel, with weaker helium and stronger 
 hydrogen, followed by Alpha Cygni with still feebler 
 helium absorption and hydrogen lines still narrow, and 
 by Regulus (Alpha Leonis), where the hydrogen lines 
 are broad and dark. 
 
78 SIR WILLIAM HUGGINS 
 
 Then succeed a number of spectra of Secchi's First 
 Type proper, with the full series of hydrogen lines domi- 
 nating them all, yet with a gradual increase in the 
 strength of the lines of the metals. The K line of calcium 
 is seen at first as a narrow line not very strongly marked, 
 while the -H line, here a member of the hydrogen series, 
 is broad and dark. Further spectra give instances of 
 the gradual weakening of the hydrogen series and the 
 strengthening of the metallic lines until H and K appear 
 as the twin giants of the spectra of Gapella, the Sun, 
 and Arcturus, the chief representatives of Secchi's 
 Third Type. After these comes Betelgeux (Alpha 
 Orionis), with the calcium H and K lines less pronounced, 
 hydrogen practically gone, and the dark flutings of 
 titanium oxide in evidence. 
 
 The great collections of stellar spectra made at Har- 
 vard College under the classifications of Miss Maury and 
 Miss CANNOX, yield practically the same result. All 
 the thousands of spectra examined may, with only few 
 exceptions, be arranged in one continuous series, from 
 the brilliantly white stars, strong in helium, a class to 
 which the spectroscopic binaries belong, to those like 
 Vega and Sirius where the broad lines of hydrogen pre- 
 dominate, then to stars where, as in the Sun, metallic 
 lines are in chief force, and so to stars where the flutings 
 of metallic compounds are the paramount feature. 
 
 The spectrum of Helium furnishes the first example 
 of an element noted in an astronomical body and later 
 recognised in the laboratory. Like hydrogen, its lines 
 supply an apt means for the classification of stellar 
 spectra ; but unlike hydrogen it is chemically very 
 inert. Lastly, it appears to furnish an example of the 
 formation of one element from the disintegration of 
 another. 
 
CHAPTER XII 
 
 THE STORY OF NEBULITJM 
 
 " TOWARDS the end of the last century the elder Herschel, 
 from his observations at Slough, came very near suggest- 
 ing what is doubtless the true nature, and place in the 
 Cosmos, of the nebulae. I will let him speak in his own 
 words : 
 
 " A shining fluid of a nature unknown to us. 
 
 " What a field of novelty is here opened to our conceptions ! 
 . . . "We may now explain that very extensive nebulosity, ex- 
 panded over more than sixty degrees of the heavens, about 
 the constellation of Orion ; a luminous matter accounting much 
 better for it than clustering stars at a distance . . . 
 
 " If this matter is self-luminous, it seems more fit to produce a 
 star by its condensation, than to depend on the star for its exis- 
 tence. 
 
 " This view of the nebulae as parts of a fiery mist out of 
 which the heavens had been slowly fashioned, began, 
 a little before the middle of the present century, at least 
 in many minds, to give way before the revelations of the 
 giant telescopes which had come into use, and especially 
 of the telescope, six feet in diameter, constructed by the 
 late Earl of Rosse at a cost of not less than 12,000. 
 
 "Nebula after nebula yielded, being resolved appa- 
 rently into innumerable stars, as the optical power was 
 increased ; and so the opinion began to gain ground that 
 all nebulae may be capable of resolution into stars. 
 According to this view, nebulae would have to be regarded 
 not as early stages of an evolutional progress, but rather 
 as stellar galaxies already formed, external to our 
 system cosmical ' sandheaps ' too remote to be sepa- 
 
 79 
 
80 SIR WILLIAM HUGGINS 
 
 rated into their component stars. Lord Rosse himself 
 was careful to point out that it would be unsafe from 
 his observations to conclude that all nebulosity is but 
 the glare of stars too remote to be resolved by our in- 
 struments. In 1858 Herbert Spencer showed clearly 
 that, notwithstanding the Parsonstown revelations, 
 the evidence from the observation of nebulae up to that 
 time was really in favour of their being early stages of 
 an evolutional progression. 
 
 " On the evening of August 29, 1864, I directed the 
 telescope for the first time to a planetary nebula in 
 Draco. The reader may now be able to picture to him- 
 self to some extent the feeling of excited suspense, 
 mingled with a degree of awe, with which, after a few 
 moments of hesitation, I put my eye to the spectroscope. 
 Was I not about to look into a secret place of creation ? 
 
 " I looked into the spectroscope. No spectrum such 
 as I expected. A single bright line only ! At first I 
 suspected some displacement of the prism, and that I 
 was looking at a reflection of the illuminated slit from 
 one of its faces. This thought was scarcely more than 
 momentary ; then the true interpretation flashed upon 
 me. The light of the nebula was monochromatic, and 
 so, unlike any other light I had as yet subjected to 
 prismatic examination, could not be extended out to 
 form a complete spectrum. After passing through the 
 two prisms it remained concentrated into a single bright 
 line, having a width corresponding to the width of the 
 slit, and occupying in the instrument a position at that 
 part of the spectrum to which its light belongs in re- 
 frangibility. A little closer looking showed two other 
 bright lines on the side towards the blue, all the three 
 lines being separated by intervals relatively dark. 
 
 " The riddle of the nebulae was solved. The answer, 
 which had come to us in the light itself, read : Not an 
 aggregation of stars, but a luminous gas. Stars after 
 the order of our own sun, and of the brighter stars, 
 would give a different spectrum ; the light of this nebula 
 had clearly been emitted by a luminous gas. With an 
 
THE STORY OF NEBULIUM 81 
 
 excess of caution, at the moment I did not venture to 
 go further than to point out that we had here to do with 
 bodies of an order quite different from that of the stars. 
 Further observations soon convinced me that, though the 
 short span of human life is far too minute relatively 
 to cosmical events for us to expect to see in succession 
 any distinct steps in so august a process, the probability 
 is indeed overwhelming in favour of an evolution in 
 the past and still going on, of the heavenly hosts. A 
 time surely existed when the matter now condensed 
 into the sun and planets filled the whole space occupied 
 by the solar system, in the condition of gas, which then 
 appeared as a glowing nebula, after the order, it may be, 
 of some now existing in the heavens. There remained 
 no room for doubt that the nebulae, which our telescopes 
 reveal to us, are the early stages of long processions of 
 cosmical events, which correspond broadly to those 
 required by the nebular hypothesis in one or other of 
 its forms." l 
 
 Such is the account given by the Master himself of 
 the most epoch-making observation which fell even to 
 his good fortune and skill. 
 
 In the next four years, Huggins examined spectro- 
 scopically about seventy nebulae, and of about one- 
 third of these he could assert that they do not yield 
 light of many different refrangibilities and therefore 
 do not form a continuous spectrum. These nebulse 
 show in the telescope a greenish tint, indeed in the case 
 of some like the great Nebula in Orion, the green is 
 vivid, due to its light being so largely concentrated in 
 the line in the green at wave-length 6007, that first 
 caught Huggins' attention in the nebula in Draco, near 
 the pole of the ecliptic. This line has been therefore 
 known since as the Chief Nebular Line. In later years 
 W. H. Wright has extended the list of nebular lines to 
 19, extending from wave-length 5007 in the green to 
 3726 in the violet ; some of the lines being due to hydro- 
 gen, one or two to helium, and one at 3729 in the violet 
 1 Nineteenth Century Review, June 1897. 
 
 P 
 
82 SIR WILLIAM HUGGINS 
 
 to an unnamed substance as yet undetected on the 
 Earth. 
 
 Not all nebulae are of this class. The " white " 
 nebulae, of which the Great Nebula in Andromeda is a 
 type, gave a very different image when a few days after 
 this first observation, Huggins turned his spectroscope 
 upon it. He writes : " Its light was distributed 
 throughout the spectrum and consequently extremely 
 faint. The brighter middle part only could be seen, 
 though I have since proved, as I at first suggested might 
 be the case, that the blue and the red ends are really not 
 absent, but are not seen on account of their feebler 
 effect upon the eye. Though continuous, the spectrum 
 did not look uniform in brightness, but its extreme 
 feebleness made it uncertain whether the irregularities 
 were due to certain parts being enhanced by bright lines, 
 or the other parts enfeebled by dark lines. ' ' l 
 
 But whatever the " white " nebulae like those in 
 Andromeda or Canes Venatici might be, the " green " 
 nebulae are certainly not groups of stars, ill-defined and 
 irresolvable by reason of their vast distance from us ; 
 some of them are structures of a different order ; they 
 are masses of glowing gas without being condensed like 
 our Sun, or covered by a photospheric envelope. 
 
 The Chief Nebular Line has not, as yet, been found 
 in the spectrum of any terrestrial substance, but in the 
 hope of such ultimate discovery the element producing 
 it has been called Nebulium. Nor does the Sun yield 
 any trace of it. But there is one class of star in which 
 at one epoch of its existence the Chief Nebular Line 
 is found. In 1876, a New Star of about the 3rd mag. 
 appeared in the constellation Cygnus, and showed the 
 usual bright lines of hydrogen and helium. It gradu- 
 ally faded and was lost to sight in the twilight in March 
 1877, but when it was looked for again in the following 
 September, Copeland found that its spectrum was re- 
 duced to the bright green Chief Nebular Line. To all 
 appearance it had been changed into a planetary nebula. 
 1 Nineteenth Century Review, June 1897. 
 
THE STORY OF NEBULIUM 83 
 
 The next instance of a new or " temporary " star 
 occurred in 1885, the Nova appearing in the Great Nebula 
 in Andromeda. Spectroscopically this object yielded 
 little information as it was only about the seventh 
 magnitude. But in 1892 a Nova appeared in the con- 
 stellation Auriga and yielded a most instructive spectrum. 
 The lines of hydrogen, helium and calcium were bright 
 as in the solar prominences, and each bright line was 
 accompanied by a dark absorption line on the side 
 nearer the violet. Huggins, in particular, was most 
 successful in his observations of the star, whether made 
 directly or by photography. On February 22, he ob- 
 tained a photograph showing the entire hydrogen series 
 in the violet and ultra-violet as bright lines, each with 
 a dark absorption line beyond it. Many other bright 
 lines were detected much further in the ultra-violet, 
 and several of the bright hydrogen lines were seen to be 
 double or even multiple in structure. 
 
 The star gradually faded from February 1, when it 
 was discovered by DR. ANDERSON, down to April 26, 
 when it was only of the 16th mag. But in the following 
 August it was re-discovered, being then of the 10th mag. 
 Examined in the spectroscope, the experience of Nova 
 Cygni was repeated ; the spectrum was that of a plane- 
 tary nebula, the Chief Nebular Line was the dominant 
 feature, and the other bright lines were also nebular ; 
 indeed Campbell recognised nearly all the eighteen 
 that he had measured as characteristic of planetary 
 nebulae. 
 
 In the following years several faint Novse recorded 
 themselves on the spectroscopic survey plates taken at 
 Harvard College and were detected by MRS. FLEMING. 
 All showed the bright prominence spectrum of Nova 
 Aurigse, coupled with the corresponding dark absorption 
 lines on the side nearer the violet. But on the morning 
 of February 22, 1901, Dr. Anderson caught a bright 
 Nova hi Perseus while it was still on the upgrade ; all 
 previous Novse having been discovered after the moment 
 of greatest brilliance and while declining. Photographs 
 
84 SIR WILLIAM HUGGINS 
 
 of the neighbourhood showed that 28 hours earlier it 
 must have been below the 12th mag. ; now it was of 
 the 2nd, and for the next night it was brighter than 
 
 was continuous at first with narrow 
 dark fflKs due to helium and hydrogen, then the bright 
 lines of both elements broke out, flanked, according to 
 the precedent of Nova Aurigse, by dark lines on the side 
 towards the violet. Then as the star declined in bright- 
 ness, the absorption faies faded, and the bright bands 
 in the blue charac^^^c of the Wolf-Rayet stars ap- 
 
 the 
 
 peared. Finally ^>jHHr ^ ne Nova had taken on the 
 nebular spectrum. 
 
 On August 22, Max Wolf obtained a direct photograph 
 of some nebulous extensions to the Nova, which BJTCHEY 
 on September found to form a great spiral nebula en- 
 circling the star, and later photographs showed the 
 nebula expanding in all directions at a speed that sug- 
 gested that it had taken its origin at the star itself 
 shortly before the first sudden increase in its light. 
 Some astronomers have considered that the star was 
 originally involved in a nebula, previously unseen, and 
 that what had been observed was simply the successive 
 illumination of its different parts by the light radiated 
 from the Nova at its outburst. BARNARD is, however, 
 clear that this theory is irreconcilable with the photo- 
 graphs, which testify to an actual movement of the 
 nebulous matter. 
 
 Of three later Novse, one discovered in Lacerta by 
 ESPIN on December 30, 1910, and two others in Gemini, 
 one detected in 1903, and the second in 1912, only the 
 last-named has added to our spectroscopic information, 
 and it has confirmed the testimony of its two great pre- 
 decessors of Auriga and Perseus. All the stars of the 
 class show a brilliant chromospheric spectrum in their 
 first stage of decline after the outburst ; that is to say 
 the lines of hydrogen and helium, and possibly other 
 well-known lines of the solar chromosphere, are bright. 
 They are, however, flanked on the more refrangible side 
 
THE STORY OF NEBULIUM 85 
 
 by corresponding dark absorption lines. Many com- 
 plex changes occur in the positions and characters of the 
 lines of this double order, but eventually they fade out 
 and give place to the bright lines typical of a 
 nebula. At this stage they can be detecte 
 scope by the nearly monochromatic green 
 give ; at another they yield a sharp image in two quite 
 different foci. And this is characteristic of all Novae ; at 
 one stage, if they are viewed directly by the telescope 
 they will give a sharp image in I/fro quite different foci. 
 One of these images is a brigh|^tiinson, and Earned 
 has pointed out that new 8 *IIP an ^ e detected as 
 such by this crimson image forTBra^ile. But only for 
 a while. The spectroscope show that this crimson 
 colour is due to the red C line of \a stupendous output 
 of hydrogen, and as the Nova fades, the hydrogen 
 becomes less distinct and the nebular lines make their 
 appearance. 
 
 The nature and origin of Novae still remain unsolved. 
 The collision of two stars, the near approach of two 
 stars, causing a disruptive tidal wave in both, the rush 
 of a star through a nebula have all been suggested in 
 turn ; none of the three hypotheses agree fully with all 
 the known facts. Huggins concluded in 1893 from his 
 observations of Nova Aurigae, that in the outburst of 
 these " temporary stars," " We have not to do mainly 
 with cold matter raised suddenly to a high temperature 
 by a collision of any form, but rather, for the most part, 
 as was suggested by Dr. Miller and myself in 1866 in the 
 case of the first temporary star examined with the 
 spectroscope, with an outburst of existing hot matter 
 from the interior of the star or stars ; indeed, with 
 phenomena broadly similar to, but on an immensely 
 grander scale than, those with which we are familiar 
 in the periodic greater and lesser disturbances of the 
 Sun's surface. 
 
 " Such grand eruptions may well be expected to take 
 place as stars cool, and if in two or more dull and com- 
 paratively cool stars such a state of things were im- 
 
86 SIR WILLIAM HUGGINS 
 
 ininent, then the tidal action due to their near approach 
 might be amply adequate to determine, as by a trigger 
 action, such eruptions. 
 
 " Under such conditions, fluctuations of brightness 
 and subsequent partial renewals of the eruptive dis- 
 turbances might well take place." 1 
 
 This conclusion announced twenty years ago is that 
 to which astronomers appear to be brought to-day. 
 
 But what of the gas characteristic of nebulae, and of 
 this final stage of Novse ? In the eighties, Lockyer 
 made an attempt to identify the Chief Nebular Line 
 with a " fluting " of magnesium, but Huggins was able 
 to show that the nebular ray is slightly more refrangible 
 than the magnesium fluting-edge, and it is sharp and 
 fine instead of showing a one-sided haze like the fluting. 
 We have not yet found it on the Earth, so that it cannot 
 be analysed and weighed in the laboratory ; must we 
 relegate it to the utterly unknown ? 
 
 No, for within the last two years, in a series of 
 masterly papers communicated to the Royal Astrono- 
 mical Society, J. W. NICHOLSON has, so to speak, 
 outflanked the problem, and attacked it from the 
 theoretical side. Assuming the existence of elements 
 with the simplest possible type of atom, simpler than 
 any yet discovered on the earth, the main conception 
 involved in the structure of their atoms is that of the 
 nature of positive electricity, and this is supposed to 
 exist in small spherical volume distributions of uniform 
 density, whose radius is small in comparison even with 
 that of an electron, but whose mass is relatively very 
 large. This single positive unit, with a single ring 
 of electrons rotating in a plane round it, constitutes 
 a " simple " atom, and he premises that an atom of 
 nebulium, when it is electrically neutral, has a ring of 
 four electrons, rotating uniformly in a plane circle at 
 equal distances round a positive nucleus whose electric 
 charge is four times that of any one electron. If the 
 atom loses one or more electrons, the remaining electrons 
 1 Proceedings of the Royal Society, vol. liv. p. 30, 1893. 
 
THE STORY OF NEBULIUM 87 
 
 take up equidistant positions and the atom becomes 
 positively charged. Similarly if the atom takes up one 
 or more additional electrons, it becomes negatively 
 charged. These electrons form a system which rotates 
 uniformly round the positive unit. The various periods 
 of vibration given for the atom of nebulium when 
 this is neutral and when it is positively and negatively 
 charged to various degrees, are shown by Nicholson 
 to account for 10 out of Wright's 19 nebular lines, 
 and included in these are one at 3836 which had been 
 hitherto attributed to hydrogen, and another at 4027, 
 attributed to helium, while the Chief Nebular Line at 
 5007, is by the same means shown definitely to be due 
 to nebulium. He was moreover able to predict the 
 presence of a line which was afterwards found in a 
 photographic spectrum by Wright. Dr. Nicholson has 
 also shown that the radius of the atom of nebulium 
 is 3*453 tenth -metres ; a tenth -metre being the ten 
 thousand millionth part of a metre. 
 
 Nebulium is not the only " unknown " gas in the 
 universe. There is one part of the Sun's surroundings 
 that we only see when the Sun is eclipsed the Corona 
 a pearly-tinted glory that stretches out from the sun 
 in irregular rays and petal -like forms to distances that 
 are thousands or millions of miles beyond the chromo- 
 sphere. The element or elements that constitute the 
 corona, have not yet been identified on the Earth, but 
 it gives a series of bright lines, the chief being one of 
 wave-length 5303 in the green. Nicholson has been 
 investigating this also, and though he has not yet pub- 
 lished the nature of the atom whose vibrations give rise 
 to this chief " coronium " line he has traced many other 
 of the coronal lines to the atom of " proto-fluorine," 
 that being the name he has given to the theoretical 
 element whose atoms when neutral have five negatively 
 charged electrons rotating in a plane circle round a posi- 
 tive unit whose charge is five times that of any one of 
 the rotating electrons. This atom is rather larger 
 than that of nebulium, having a radius of 5 *802 tenth- 
 
88 SIR WILLIAM HUGGINS 
 
 metres ; but the velocity of its electrons round the posi- 
 tive unit is 2*1 times greater than of the electrons of 
 nebulium. 
 
 The spectrum of Nebulium, though the element itself 
 is still unknown in terrestrial laboratories, first demon- 
 strated the existence of true nebulse in the depths of space. 
 
CHAPTER XIII 
 
 SUMMARY 
 
 IN the preceding chapters an attempt has been made 
 to give a sketch of the chief results obtained by the appli- 
 cation of the spectroscope to astronomy during the fifty 
 years that have elapsed since Huggins entered the 
 field. Few nowadays realise how small was the instru- 
 ment with which he began his great work. His tele- 
 scope indeed was a good one, having an object glass 
 of 8 inches aperture made by Alvan Clark, but his 
 first star spectroscope, every detail of which had to be 
 thought out and designed by himself, had a collimator 
 of only three-fifths of an inch aperture, and of 4| inches 
 focal length. Yet after he had been at work only four 
 years with this modest equipment, he was able in a 
 lecture delivered before the British Association at Not- 
 tingham in 1866, to summarise the advances already 
 made in the following terms. 
 
 1. All the brighter stars, at least, have a structure analogous 
 to that of the sun. 
 
 2. The stars contain material elements common to the sun and 
 earth. 
 
 3. The colours of the stars have their origin in the chemical 
 constitution of the atmospheres which surround them. 
 
 4. The changes in brightness of some of the variable stars are 
 attended with changes in the lines of absorption of their spectra. 
 
 5. The phenomena of the star in Corona appear to show that 
 in this object at least great physical changes are in operation. 
 
 6. There exist in the heavens true nebula. These objects 
 consist of luminous gas. 
 
 The Royal Society came to his assistance in 1870, 
 and put at his disposal a fine equatorial carrying two 
 
 on 
 
90 SIR WILLIAM HUGGINS 
 
 telescopes, one a refractor of 15 inches aperture, the other 
 a Cassegrain reflector of 18 inches, and with these all 
 his subsequent work was effected ; the little spectro- 
 scope that he used at first being finally superseded by 
 a fine 4-inch Rowland grating for ordinary work and a 
 spectroscope with quartz lenses and iceland spar prism 
 for taking photographs in the ultra-violet. 
 
 Dr. Huggins commenced his spectroscopic studies 
 in collaboration with Dr. W. Allen Miller, but this associ- 
 ation did not last long, as other duties claimed Dr. 
 Miller's attention. For thirteen years Huggins laboured 
 alone, but in 1875 he married Miss Margaret Lindsay 
 Murray, who at once entered enthusiastically into her 
 husband's studies and proved herself a most able and 
 devoted assistant. 
 
 In 1891 he became President of the British Associa- 
 tion, and his Presidential address at its Cardiff Meeting 
 was devoted to carrying on the history of the develop- 
 ment of prismatic astronomy during the quarter of a 
 century which had elapsed since his former lecture. 
 Six years later he contributed an historical note on the 
 work carried out by his wife and himself to the Nineteenth 
 Century Review, concluding : " We found the nw 
 astronomy newly born in a laboratory at Heidelberg ; 
 to astronomers she was 
 
 ... a stranger 
 Born out of their dominions. 
 
 We take leave of her in the full beauty of a vigorous 
 youth, receiving homage in nearly all the observatories 
 of the world." In 1899 Sir William Huggins, as he had 
 now become, brought out in conjunction with Lady 
 Huggins, a magnificent Atlas of Representative Stellar 
 /Spectra with explanatory text, mainly devoted to the dis- 
 cussion of the " evolutional order of the stars and the 
 interpretation of their spectra." In 1909 Sir William 
 and Lady Huggins followed the Atlas by a second volume 
 of the Publications of the Tulse Hill Observatory. 
 This last was a collection of his scientific papers, and 
 
SUMMARY 91 
 
 marked the completion of his life-work. "These 
 original observations," the Authors write in the Preface, 
 " notwithstanding the serious drawbacks and limitations 
 which must be present in pioneering work, have in their 
 collected form all the interest of a nascent science, 
 since they led directly to, and indeed themselves formed 
 a not inconsiderable part of the foundations of, a new 
 branch of Astronomy, which extends the chemistry 
 and physics of the Earth to the heavenly bodies, and 
 under the new name of Astrophysics, is to-day zealously 
 cultivated in all the principal observatories of the world." 
 He still continued to take an active interest in all 
 scientific progress. " Only a week before his death he 
 took part in a meeting of a joint committee of the Royal 
 and Royal Astronomical Societies for making arrange- 
 ments for the publication of a collected edition of Sir 
 William Herschel's papers, an undertaking due largely 
 to his initiative. . . . His death, at the age of eighty- 
 six, took place in a nursing home in London on May 
 12, 1910, unexpectedly, after one day's illness." * 
 
 In this little volume only the chief features of Huggins' 
 researches, only the leading discoveries effected in as- 
 tronomy by the spectroscope have been noticed ; a 
 few spectra only of those of the many elements have been 
 described. More than this could not be accomplished 
 in the space. Yet the author is fain to hope that some 
 indication has been given of the chief result of this half- 
 century of triumphant research, namely the testimony 
 which the spectroscope has given to the essential unity 
 of creation. 
 
 A given substance has its physical qualities. It may 
 be hard, tough and heavy, or soft and light ; dark in 
 appearance or bright. These are its outward qualities. 
 Next come its chemical affinities ; it behaves in this or 
 that manner when acted on by various reagents. But 
 it has qualities more intimate still, more completely 
 
 1 Obituary Notice, Proceedings of the Royal Society, Series A, 
 Vol. 86, p. xix. 
 
92 SIR WILLIAM HUGGINS 
 
 characteristic of it, because they are the qualities of its 
 ultimate molecules and it is these qualities which give 
 to each substance its appropriate spectrum. 
 
 For the lines of a spectrum are the evidence of the 
 vibrations which the ultimate molecules of that element 
 take up, to which they respond, and the wonderful in- 
 tricacy which many of these spectra display has already 
 yielded to analysis and been found to conform to a 
 formula of a comparatively simple type, with its chief 
 constant the same for all elements. 
 
 Turning from the ultimate molecule, an entity so 
 small that it necessarily escapes our most powerful 
 means of magnification to render it directly visible, 
 to the enormous suns transcending this Earth of ours 
 many millionfold in size, and the same testimony to 
 unity is not less apparent. They reveal to the spectro- 
 scope the presence of the same elements ; they are built 
 on the same plan ; their differences are but differences of 
 size or age or development ; they exhibit every possible 
 gradation in one and the same series ; the unity of the 
 whole is manifest. 
 
 Astronomy is the oldest of the sciences ; Astro- 
 physics, that is to say, Spectroscopic Astronomy, is the 
 youngest. Yet they concur in their Testimony. The 
 Universe is One. 
 
INDEX 
 
 ABSORPTION, general, 50 
 
 y Cassiopeia}, 34, 36, 77 
 
 Fraunhofer, 17, 18, 21, 26, 
 
 Absorption, selective, 50 
 
 Castor, 33, 61, 62 
 
 27, 29, 56 
 
 Absorption lines, 30 
 Aldebaran, 53 
 
 a Centauri, 77 
 /3 Centauri, 77 
 
 Fraunhofer lines, 9, 18, 21, 
 22, 34, 46, 47 
 
 Algol, 71 
 
 Chromosphere, 35, 46, 47, 
 
 
 Altair, 61 
 
 50, 56, 57, 58, 68, 73, 76 
 
 GEMINI, new stars in, 84 
 
 Aluminium, 45, 48 
 
 Clark, Alvan, 89 
 
 Gladstone, 20 
 
 Ames, 65 
 Anderson, Dr., 83 
 
 Clerke, Miss, 69 
 Cleveite, 74, 76 
 
 Gold, 45 
 Goodricke, 71 
 
 7 Andromedse, 61 
 
 Clouds, spectrum of, 19 
 
 Grating, 20, 21 
 
 Andromeda, great nebula 
 
 Cobalt, 45 
 
 
 in, 82, 83 
 
 Collimator, 26 
 
 HALE, 60, 74 
 
 Angstrom, 21 
 
 Comets, 40, 41, 42, 52, 55 
 
 Helium, 73-78, 81, 83, 84 
 
 Angstrom unit, 22 
 
 Copeland, 52, 66, 82 
 
 Hercules, 52 
 
 Autares, spectrum of, 19, 
 
 Copper, 45 
 
 a Herculis, 33 
 
 83 
 
 Corona, 87 
 
 Herschel, Alexander, 42 
 
 Antimony, 4 
 
 T Corona;, 34 
 
 Herschel, Capt., 73 
 
 Arcturus, spectrum of, 19, 
 
 Coronium, 87 
 
 Herschel, Sir John 27 
 
 35, 78 
 
 Cortie, 58 
 
 Herschel, Sir W., 79, 91 
 
 Argon, 76 
 
 Cyanogen, 41, 42 
 
 Huggins, Lady, 64, 90 
 
 Arsenic, 45 
 
 a Cygni, 50, 77 
 
 Huggins, Sir W., 7, 8, 32, 
 
 Atlas of Representative 
 
 y Cygni, 61, 62 
 
 34, 35, 36, 37, 38, 40, 41, 
 
 Spectra, 61, 77, 90 
 
 P Cygni, 77 
 
 45, 51, 53, 67, 58, 61, 63, 
 
 /S Aurigse, 70 
 Auriga, new star in, 83, 84, 
 
 Cygnus, new star in, 82, 83 
 Cylindrical lens, 32 
 
 64, 65, 74, 77, 81, 82, 85, 
 86, 89, 90 
 
 85 
 
 
 Huxley, 7, 8 
 
 
 DESLANDRES 60 
 
 Hydrogen, 31-38, 41, 46, 
 
 BALMER, 65, 66 
 
 Dewar, 41 
 
 47, 63-72, 75, 77, 78, 81, 
 
 Baly, 40, 51 
 Barium. 45, 76 
 Barnard, 84 
 
 Diffraction spectrum, 21 
 Dispersion, 13 
 
 83,84 
 INFRA-RED, 67 
 
 Bellatrix (Gamma Orionis), 
 
 Donati, 40 
 
 Iron, 41, 43-52, 53, 54, 57 
 
 77 
 Belopolsky, 72 
 
 Draco, planetary nebula 
 in, 80 
 
 JANSSEN, 35, 73 
 
 Bessel, 10 
 Brewster, 20 
 Bunsen flame, 29, 39, 43 
 
 Draper Catalogue, 69 
 Draper, Henry, 64, 67 
 Duner 41 
 
 Jupiter, 36 
 KAYSER, 65 
 
 CADMIUM, 45 
 
 Dyson, 65 
 
 Kirchhoff, 9, 29, 32, 43, 44, 
 53 
 
 Calcium, 45, 47, 56-62, 78, 
 
 ESPIN 84 
 
 Krypton, 76 
 
 Campbell, 52, 70, 83 
 
 Eversted, 60 
 
 LACERTA, new star in, 84 
 
 Canes Venatici, nebula in, 
 
 
 Lead, 45 
 
 82 
 
 FACULA, 49 
 
 Line of sight, 37, 51 
 
 Cannon, Misg, 78 
 
 Flash spectrum, 41, 46 
 
 Lines, enhanced, 49, 50, 51 
 
 Capella, 33, 78, 84 
 
 Fleming, Mrs., 83 
 
 Lines, long, 48 
 
 Capillary, 31, 70 
 
 Flocculi,;60, 61 
 
 Lines, reversed, 50, 58 
 
 Carbon, 39-42, 54, 77 
 
 Fluting, 39, 86 
 
 Lines, short, 48 
 
 Carbon dioxide, 40 
 
 Fowler, 55, 67, 75 
 
 Lithium, 28, 45, 66, 74 
 
 Carbon monoxide, 41, 42 ! Fox-Talbot, 28 
 
 Liveing, 41 
 
 93 
 
94 
 
 INDEX 
 
 Lockye:, 3D, 17, 4.-, 19, 50, 1 Planets, ?9 
 
 Spectrum, arc, 43, 48, 49, 
 
 51, 57, 86' 
 
 Platinum, 7 
 
 50, 51 
 
 /3 Lyrae, 34, 71, 77 
 
 Pleiades, 76 
 
 Spectrum, bright-line, 29 
 
 y Lyrae, 61, 62 
 
 Pollux, 33 
 
 Spectrum, continuous, 29 
 
 Lyra, ring nebula in, 67 
 
 Potassium, 66 
 
 Spectrum, fluted, 54 
 
 
 Prism, 12 
 
 Spectrum, spark. 43, 48, 
 
 MAGELLANIC CLouD.great, 
 
 Procyon, 61, 62 
 
 49, 50, 51 
 
 67 
 
 Prominences, 35, 56, 73, 76 
 
 Spencer, Herbert, 80 
 
 Magnesium, 45, 47, 53-55, 
 
 Proto-fluorine, 87 
 
 Spica, 70 
 
 57,86 
 
 Puppis, 66 
 
 Stars, Wolf -Rayet, 67, 68, 
 
 Magrini, 73 
 
 
 84 
 
 Maunder, 36, 54, 55 
 
 RADIUM, 76 
 
 Stellar spectra, 19 
 
 Maury, Miss, 69, 70, 78 
 
 Rainbow, 12, 14, 15 
 
 Stellar spectra, types of, 
 
 Menkalinan, 70 
 
 Ramsay, 74, 76 
 
 33 
 
 Mercury, 45 
 
 Rayet, 66 
 
 Stoney, G.iJohnstone,64,65 
 
 Metallic eruptions, 47, 54 
 
 Red stars, 33, 41, 42 
 
 Strontium, chloride of, 27, 
 
 Meteorites, 41, 42, 52 
 
 Refraction, 13 
 
 28, 45, 47 
 
 Milky Way, 67, 76 
 
 Regulus (a Leonis), 61, 62, 
 
 Sun, 9, 78 
 
 Miller, 32, 85, 90 
 Mira Ceti, 34, 36, 55, 69 
 
 77 
 Reversal of lines, 50. 58, 
 
 Sun, spectrum of, 17, 18, 19 
 Sunspots, 49, 50, 55 
 
 Mitchell, 65 
 
 68 
 
 Swan, 40 
 
 Mizar, 70 
 
 Reversing layer, 46, 47 
 
 "Swan " spectrum, 40, 41, 
 
 Moon, 19 
 
 Rigel (8 Orionis), 33, 77 
 Rjtchey, 84 
 
 42, 55 
 
 NEBULA, green, 82 
 
 Ritz, 67 
 
 TELLURIC LINES, 19 
 
 Nebula, white, 82 
 
 Rosse, Earl of, 79, 80 
 
 Tenth-metres, 21 
 
 Nebular Line, Chief, 81, 
 
 Rowland, 22 
 
 Tilden, 76 
 
 82, 83, 86, 87 
 
 Runge, 65, 74 
 
 Tin, 45 
 
 Nebulium, 79-88 
 
 Rutherfurd, 33 
 
 Titanium, 47, 53-55 
 
 Neon, 76 
 
 Rydberg, 66, 67 
 
 
 Neptune, 36 
 
 
 ULTRA-VIOLET, 56, 67 
 
 New stars, 34, 36, 77, 82- 
 86 
 
 SATURN, 36 
 Secchi, 33, 37, 41 
 
 Uranus, 36 
 Ursa>Majoris, 70 
 
 Newton, 7, 12, 15, 17 
 
 Series of lines, 66, 74, 75 
 
 
 Nicholson, J. W., 86, 87 
 
 Sidgreaves, 58 
 
 VACUUM TUBE, 31 
 
 Nickel, 45 
 
 Silicon, 45, 77 
 
 Variables, eclipse, 71 
 
 Nitrogen, 41 
 Nova3. See New Stars 
 
 Silver, 45 
 Sirius, 9, 19, 33, 61, 62, 64, 
 
 Variables, long period, 69 
 Variables, short period, 71 
 
 
 78 
 
 Vega, 19, 33, 61, 62, 64, 6r, 
 
 a ORIONIS, 33, 53, 54, 55, 
 
 Smithells, 40 
 
 557 
 
 77, 78 
 
 Smyth, Piazzi, 15, 42 
 
 Vogel, 36, 38, 41, 51, 70, 71 
 
 Orion, nebula in, 81 
 
 Sodium, 23-30, 35, 39, 41, 
 
 
 Oxygen, 75, 77 
 Oxy-hydrogen flame, 43, 49 
 
 43, 45, 47, 52 
 Spectroheliogram, 61 
 
 WAVE-LENGTHS, 20 
 Wolf, 66 
 
 
 Spectroheliograph, 60, 61 
 
 Wolf, Max, 67, 84 
 
 PARHELIUM, 74 
 
 Spectroscope, 12, 26 
 
 Wollaston, 17 
 
 Paschen, 65, 67, 74 
 
 Spectroscopic binary, 70 
 
 Wright, W. H., 81, 87 
 
 Perseus, new star in, 83, 84 
 
 Spectrum, the, 12, 13, 14, 
 
 
 Photosphere, 45, 46 
 
 26 
 
 YOUNG, 56 
 
 Pickering, E. C., 69 
 Pickering, W. H., 66, 67 
 
 Spectrum, absorption, 29 
 Spectrum analysis, 12, 28 
 
 ZINC, 45 
 
 8/13 
 
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