The first point to be cleared up was that of chemical composition. Leisurely measurements verified the presence above the sun's surface of hydrogen in prodigious volumes, but showed that sodium had nothing to do with the orange-yellow ray identified with it in the haste of the eclipse. From its vicinity to the D-pair (than which it is slightly more refrangible), the prominence-line was, however, designated D_3, and the unknown substance emitting it was named by Lockyer "helium." Its terrestrial discovery ensued after twenty-six years. In March, 1895, Professor Ramsay obtained from the rare mineral clevite a volatile gas, the spectrum of which was found to include the yellow prominence-ray. Helium was actually at hand, and available for examination. The identification cleared up many obscurities in chromospheric chemistry. Several bright lines, persistently seen at the edge of the sun, and early suspected by Young[596] to emanate from the same source as D_3, were now derived from helium in the laboratory; and all the complex emissions of that exotic substance ranged themselves into six sets or series, the members of which are mutually connected by numerical relations of a definite and simple kind. Helium is of rather more than twice the density of hydrogen, and has no chemical affinities. In almost evanescent quantities it lurks in the earth's crust, and is diffused through the earth's atmosphere.
The importance of the part played in the prominence-spectrum by[Pg 195] the violet line of calcium was noticed by Professor Young in 1872, but since H and K lie near the limit of the visible spectrum, photography was needed for a thorough investigation of their appearances. Aided by its resources, Professor George E. Hale, then at the beginning of his career, detected in 1889 their unfailing and conspicuous presence.[597] The substance emitting them not only constitutes a fundamental ingredient of the chromosphere, but rises, in the fantastic jets thence issuing, to greater heights than hydrogen itself. The isolation of H and K in solar prominences from any other of the lines usually distinctive of calcium was experimentally proved by Sir William and Lady Huggins in 1897 to be due to the extreme tenuity of the emitting vapour.[598]
Hydrogen, helium, and calcium form, then, the chief and unvarying materials of the solar sierra and its peaks; but a number of metallic elements make their appearance spasmodically under the influence of disturbances in the layers beneath. In September, 1871, Young[599] drew up at Dartmouth College a list of 103 lines significant of injections into the chromosphere of iron, titanium, chromium, magnesium, and many other substances. During two months' observation in the pure air of Mount Sherman (8,335 feet high) in the summer of 1872, these tell-tale lines mounted up to 273;[600] and he believes their number might still be doubled by steady watching. Indeed, both Young and Lockyer have more than once seen the whole field of the spectroscope momentarily inundated with bright rays, as if the "reversing layer" had been suddenly thrust upwards into the chromosphere, and as quickly allowed to drop back again. The opinion would thus appear to be well-grounded that the two form one continuous region, of which the lower parts are habitually occupied by the heaviest vapours, but where orderly arrangement is continually overturned by violent eruptive disturbances.
The study of the forms of prominences practically began with Huggins's observation of one through an "open slit" February 13, 1869.[601] At first it had been thought possible to examine them only in sections—that is, by admitting mere narrow strips or "lines" of their various kinds of light; while the actual shape of the objects emitting those lines had been arrived at by such imperfect devices as that of giving to the slit of the spectroscope a vibratory moment rapid enough to enable the eye to retain the[Pg 196] impression of one part while others were successively presented to it. It was an immense gain to find that their rays had strength to bear so much of dilution with ordinary light as was involved in opening the spectroscopic shutter wide enough to exhibit the tree-like, or horn-like, or flame-shaped bodies rising over the sun's rim in their undivided proportions. Several diversely-coloured images of them are formed in the spectroscope; each may be seen under a crimson, a yellow, a green, and a deep blue aspect. The crimson, however (built up out of the C-line of hydrogen), is the most intense, and is commonly used for purposes of observation and illustration.
Friedrich Zöllner was, by a few days, beforehand with Huggins in describing the open-slit method, but was somewhat less prompt in applying it. His first survey of a complete prominence, pictured in, and not simply intersected by, the slit of his spectroscope, was obtained July 1, 1869.[602] Shortly afterwards the plan was successfully adopted by the whole band of investigators.
A difference in kind was very soon perceived to separate these objects into two well-marked classes. Its natural and obvious character was shown by its having struck several observers independently. The distinction of "cloud-prominences" from "flame-prominences" was announced by Lockyer, April 27; by Zöllner, June 2; and by Respighi, December 4, 1870.
The first description are tranquil and relatively permanent, sometimes enduring without striking change for many days. Certain of the included species mimic terrestrial cloud-scenery—now appearing like fleecy cirrus transpenetrated with the red glow of sunset—now like prodigious masses of cumulo-stratus hanging heavily above the horizon. The solar clouds, however, have the peculiarity of possessing stems. Slender columns can ordinarily be seen to connect the surface of the chromosphere with its outlying portions. Hence the fantastic likeness to forest scenery presented by the long ranges of fiery trunks and foliage occasionally seeming to fringe the sun's limb. But while this mode of structure suggests an actual outpouring of incandescent material, certain facts require a different interpretation. At a distance, and quite apart from the chromosphere, prominences have been perceived, both by Secchi and Young, to form, just as clouds form in a clear sky, condensation being replaced by ignition. Filaments were then thrown out downward towards the chromosphere, and finally the usual appearance of a "stemmed prominence" was assumed. Still more remarkable was an observation made by Trouvelot at Harvard College Observatory, June 26, 1874.[603] A gigantic comma-shaped prominence, 82,000 miles high, vanished[Pg 197] from before his eyes by a withdrawal of light as sudden as the passage of a flash of lightning. The same observer has frequently witnessed a gradual illumination or gradual extinction of such objects, testifying to changes in the thermal or electrical condition of matter already in situ.
The first photograph of a prominence, as shown by the spectroscope in daylight, was taken by Professor Young in 1870.[604] But neither his method, nor that described by Dr. Braun in 1872,[605] had any practical success. This was reserved to reward the efforts towards the same end of Professor Hale. Begun at Harvard College in 1889,[606] they were prosecuted soon afterwards at the Kenwood Observatory, Chicago. The great difficulty was to extricate the coloured image of the gaseous structure, spectroscopically visible at the sun's limb, from the encompassing glare, a very little of which goes a long way in fogging sensitive plates. To counteract its mischievous effects, a second slit,[607] besides the usual narrow one in front of the collimator, was placed on guard, as it were, behind the dispersing apparatus, so as to shut out from the sensitised surface all light save that of the required quality. The sun's image being then allowed to drift across the outer slit, while the plate holder was kept moving at the same rate, the successive sectional impressions thus rapidly obtained finally "built up" a complete picture of the prominence. Another expedient was soon afterwards contrived.[608] The H and K rays of calcium are always, as we have seen, bright in the spectrum of prominences. They are besides fine and sharp, while the corresponding absorption-lines in the ordinary solar spectrum are wide and diffuse. Hence, prominences formed by the spectroscope out of these particular qualities of violet light, can be photographed entire and at once, for the simple reason that they are projected upon a naturally darkened background. Atmospheric glare is abolished by local absorption. This beautiful method was first realised by Professor Hale in June, 1891.
A "spectroheliograph," consisting of a spectroscopic and a photographic apparatus of special type, attached to the eye-end of an equatoreal twelve inches in aperture, was erected at Kenwood in March, 1891; and with its aid, Professor Hale entered upon original researches of high promise for the advancement of solar physics. Noteworthy above all is his achievement of photographing both prominences and faculæ on the very face of the sun. The latter[Pg 198] had, until then, been very imperfectly observed. They were only visible, in fact, when relieved by their brilliancy against the dusky edge of the solar disc. Their convenient emission of calcium light, however, makes it possible to photograph them in all positions, and emphasises their close relationship to prominences. The simultaneous picturing, moreover, of the entire chromospheric ring, with whatever trees or fountains of fire chance to be at the moment issuing from it, has been accomplished by a very simple device. The disc of the sun itself having been screened with a circular metallic diaphragm, it is only necessary to cause the slit to traverse the virtually eclipsed luminary, in order to get an impression of the whole round of its fringing appendages. And the record can be extended to the disc by removing the screen, and carrying the slit back at a quicker rate, when an "image of the sun's surface, with the faculæ and spots, is formed on the plate exactly within the image of the chromosphere formed during the first exposure. The whole operation," Professor Hale continues, "is completed in less than a minute, and the resulting photographs give the first true pictures of the sun, showing all of the various phenomena at its surface."[609] Most of these novel researches were, by a remarkable coincidence, pursued independently and contemporaneously by M. Deslandres, of the Paris Observatory.[610]
The ultra-violet prominence spectrum was photographed for the first time from an uneclipsed sun, in June, 1891, at Chicago. Besides H and K, four members of the Huggins-series of hydrogen-lines imprinted themselves on the plate.[611] Meanwhile M. Deslandres was enabled, by fitting quartz lenses to his spectroscope, and substituting a reflecting for a refracting telescope, to get rid of the obstructive action of glass upon the shorter light-waves, and thus to widen the scope of his inquiry into the peculiarities of those derived from prominences.[612] As the result, not only all the nine white-star lines were photographed from a brilliant sun-flame, but five additional ones were found to continue the series upward. The wave-lengths of these last had, moreover, been calculated beforehand with singular exactness, from a simple formula known as "Balmer's Law."[613] The new lines, accordingly, filled places in a manner already prepared for them, and were thus unmistakably associated with the hydrogen-spectrum. This is now known to be represented in prominences by twenty-seven lines,[614] forming a kind of harmonic progression, only
Photographs of the Solar Chromosphere and Prominences.
Taken with the Spectroheliograph of the Kenwood Observatory, Chicago, by Professor George E. Hale.
four of which are visibly darkened in the Fraunhofer spectrum of the sun.
The chemistry of "cloud-prominences" is simple. Hydrogen, helium, and calcium are their chief constituents. "Flame-prominences," on the other hand, show, in addition, the characteristic rays of a number of metals, among which iron, titanium, barium, strontium, sodium, and magnesium are conspicuous. They are intensely brilliant; sharply defined in their varying forms of jets, spikes, fountains, waterspouts; of rapid formation and speedy dissolution, seldom attaining to the vast dimensions of the more tranquil kind. Eruptive or explosive by origin, they occur in close connection with spots; whether causally, the materials ejected as "flames" cooling and settling down as dark, depressed patches of increased absorption;[615] or consequentially, as a reactive effect of falls of solidified substances from great heights in the solar atmosphere.[616] The two classes of phenomena, at any rate, stand in a most intimate relation; they obey the same law of periodicity, and are confined to the same portions of the sun's surface, while quiescent prominences may be found right up to the poles and close to the equator.
The general distribution of prominences, including both genera, follows that of faculæ much more closely than that of spots. From Father Secchi's and Professor Respighi's observations, 1869-71, were derived the first clear ideas on the subject, which have been supplemented and modified by the later researches of Professors Tacchini and Riccò at Rome and Palermo. The results are somewhat complicated, but may be stated broadly as follows. The district of greatest prominence-frequency covers and overlaps by several degrees that of the greatest spot-frequency. That is to say, it extends to about 40° north and south of the equator.[617] There is a visible tendency to a second pair of maxima nearer the poles. The poles themselves, as well as the equator, are regions of minimum occurrence. Distribution in time is governed by the spot-cycle, but the maximum lasts longer for prominences than for spots.
The structure of the chromosphere was investigated in 1869 and subsequent years by Professor Respighi, director of the Capitoline Observatory, as well as by Spörer, and Brédikhine of the Moscow Observatory. They found this supposed solar envelope to be of the same eruptive nature as the vast protrusions from it, and to be made up of a congeries of minute flames[618] set close together like[Pg 200] blades of grass. "The appearance," Professor Young writes,[619] "which probably indicates a fact, is as if countless jets of heated gas were issuing through vents and spiracles over the whole surface, thus clothing it with flame which heaves and tosses like the blaze of a conflagration."
The summits of these filaments of fire are commonly inclined, as if by a wind sweeping over them, when the sun's activity is near its height, but erect during his phase of tranquillity. Spörer, in 1871, inferred the influence of permanent polar currents,[620] but Tacchini showed in 1876 that the deflections upon which this inference was based ceased to be visible as the spot-minimum drew near.[621]
Another peculiarity of the chromosphere, denoting the remoteness of its character from that of a true atmosphere,[622] is the irregularity of its distribution over the sun's surface. There are no signs of its bulging out at the equator, as the laws of fluid equilibrium in a rotating mass would require; but there are some that the fluctuations in its depth are connected with the phases of solar agitation. At times of minimum it seems to accumulate and concentrate its activity at the poles; while maxima probably bring a more equable general distribution, with local depressions at the base of great prominences and above spots.
A low-lying stratum of carbon-vapour was, in 1897, detected in the chromosphere by Professor Hale with a grating-spectroscope attached to the 40-inch Yerkes refractor.[623] The eclipse-photographs of 1893 disclosed to Hartley's examination the presence there of gallium;[624] and those taken by Evershed in 1898 were found by Jewell[625] to be crowded with ultra-violet lines of the equally rare metal scandium. The general rule had been laid down by Sir Norman Lockyer that the metallic radiations from the chromosphere are those "enhanced" in the electric spark.[626] Hence, the comparative study of conditions prevalent in the arc and the spark has acquired great importance in solar physics.
The reality of the appearance of violent disturbance presented by the "flaming" kind of prominence can be tested in a very remarkable manner. Christian Doppler,[627] professor of mathematics at Prague, enounced in 1842 the theorm that the colour of a luminous body, like the pitch of a sonorous body, must be changed by movements[Pg 201] of approach or recession. The reason is this. Both colour and pitch are physiological effects, depending, not upon absolute wave-length, but upon the number of waves entering the eye or ear in a given interval of time. And this number, it is easy to see, must be increased if the source of light or sound is diminishing its distance, and diminished if it is decreasing it. In the one case, the vibrating body pursues and crowds together the waves emanating from it; in the other, it retreats from them, and so lengthens out the space covered by an identical number. The principle may be thus illustrated. Suppose shots to be fired at a target at fixed intervals of time. If the marksman advances, say twenty paces between each discharge of his rifle, it is evident that the shots will fall faster on the target than if he stood still; if, on the contrary, he retires by the same amount, they will strike at correspondingly longer intervals. The result will of course be the same whether the target or the marksman be in movement.
So far Doppler was altogether right. As regards sound, anyone can convince himself that the effect he predicted is a real one, by listening to the alternate shrilling and sinking of the steam-whistle when an express train rushes through a station. But in applying this principle to the colours of stars he went widely astray; for he omitted from consideration the double range of invisible vibrations which partake of, and to the eye exactly compensate, changes of refrangibility in the visible rays. There is, then, no possibility of finding a criterion of velocity in the hue of bodies shining, like the sun and stars, with continuous light. The entire spectrum is slightly shifted up or down in the scale of refrangibility; certain rays normally visible become exalted or degraded (as the case may be) into invisibility, and certain other rays at the opposite end undergo the converse process; but the sum total of impressions on the retina continues the same.
We are not, however, without the means of measuring this sub-sensible transportation of the light-gamut. Once more the wonderful Fraunhofer lines came to the rescue. They were called by the earlier physicists "fixed lines;" but it is just because they are not fixed that, in this instance, we find them useful. They share, and in sharing betray, the general shift of the spectrum. This aspect of Doppler's principle was adverted to by Fizeau in 1848,[628] and the first tangible results in the estimation of movements of approach and recession between the earth and the stars, were communicated by Sir William Huggins to the Royal Society, April 23,[Pg 202] 1868. Eighteen months later, Zöllner devised his "reversion-spectroscope"[629] for doubling the measurable effects of line-displacements; aided by which ingenious instrument, and following a suggestion of its inventor, Professor H. C. Vogel succeeded at Bothkamp, June 9, 1871,[630] in detecting effects of that nature due to the solar rotation. This application constitutes at once the test and the triumph of the method.[631]
The eastern edge of the sun is continually moving towards us with an equatorial speed of about a mile and a quarter per second, the western edge retreating at the same rate. The displacements—towards the violet on the east, towards the red on the west—corresponding to this velocity are very small; so small that it seems hardly credible that they should have been laid bare to perception. They amount to but 1/150th part of the interval between the two constituents of the D-line of sodium; and the D-line of sodium itself can be separated into a pair only by a powerful spectroscope. Nevertheless, Professor Young[632] was able to show quite satisfactorily, in 1876, not only deviations in the solar lines from their proper places indicating a velocity of rotation (1·42 miles per second) slightly in excess of that given by observations of spots, but the exemption of terrestrial lines (those produced by absorption in the earth's atmosphere) from the general push upwards or downwards. Shortly afterwards, Professor Langley, then director of the Allegheny Observatory, having devised a means of comparing with great accuracy light from different portions of the sun's disc, found that while the obscure rays in two juxtaposed spectra derived from the solar poles were absolutely continuous, no sooner was the instrument rotated through 90°, so as to bring its luminous supplies from opposite extremities of the equator, than the same rays became perceptibly "notched." The telluric lines, meanwhile, remained unaffected, so as to be "virtually mapped" by the process.[633] This rapid and unfailing mode of distinction was used by Cornu with perfect ease during his investigation of atmospheric absorption near Loiret in August and September, 1883.[634]
A beautiful experiment of the same kind was performed by M. Thollon, of M. Bischoffsheim's observatory at Nice, in the summer of 1880.[635] He confined his attention to one delicately defined group of four lines in the orange, of which the inner pair are solar (iron) and the outer terrestrial. At the centre of[Pg 203] the sun the intervals separating them were sensibly equal; but when the light was taken alternately from the right and left limbs, a relative shift in alternate directions of the solar, towards and from the stationary telluric rays became apparent. A parallel observation was made at Dunecht, December 14, 1883, when it was noticed that a strong iron-line in the yellow part of the solar spectrum is permanently double on the sun's eastern, but single on his western limb;[636] opposite motion-displacements bringing about this curious effect of coincidence with, and separation from, an adjacent stationary line of our own atmosphere's production, according as the spectrum is derived from the retreating or advancing margin of the solar globe. Statements of fact so precise and authoritative amount to a demonstration that results of this kind are worthy of confidence; and they already occupy an important place among astronomical data.
The subtle method of which they served to assure the validity was employed in 1887-9 by M. Dunér to test and extend Carrington's and Spörer's conclusions as to the anomalous nature of the sun's axial movement.[637] His observations for the purpose, made with a fine diffraction-spectroscope, just then mounted at the observatory of Upsala, were published in 1891.[638] Their upshot was to confirm and widen the law of retardation with increasing latitude derived from the progressive motions of spots. Determinations made within 15° of the pole, consequently far beyond the region of spots, gave a rotation-period of 38-1/2, that of the equatorial belt being of 25-1/2 days. Spots near the equator indeed complete their rounds in a period shorter by at least half a day; and proportionate differences were found to exist elsewhere in corresponding latitudes; but Dunér's observations, it must be remembered, apply to a distinct part of the complex solar machine from the disturbed photospheric surface. It is amply possible that the absorptive strata producing the Fraunhofer lines, significant, by their varying displacements at either limb, of the inferred varying rates of rotation, may gyrate more slowly than the spot-generating level. Moreover, faculæ appear to move at a quicker pace than either;[639] so that we have, for three solar formations, three different periods of average rotation, the shortest of which belongs to the faculæ, one of intermediate length to the spots, and the most protracted to the reversing layer. All, however, agree in lengthening progressively from the equator towards the poles. Professor Holden aptly compared the sun to "a vast whirlpool where the[Pg 204] velocities of rotation depend not only on the situation of the rotating masses as to latitude, but also as to depth beneath the exterior surface."[640]
Sir Norman Lockyer[641] promptly perceived the applicability of the surprising discovery of line-shiftings through end-on motion to the study of prominences, the discontinuous light of which affords precisely the same means of detecting movement without seeming change of place, as do lines of absorption in a continuous spectrum. Indeed, his observations at the sun's edge almost compelled recourse to an explanation made available just when the need of it began to be felt. He saw bright lines, not merely pushed aside from their normal places by a barely perceptible amount, but bent, torn, broken, as if by the stress of some tremendous violence. These remarkable appearances were quite simply interpreted as the effects of movements varying in amount and direction in the different parts of the extensive mass of incandescent vapours falling within a single field of view. Very commonly they are of a cyclonic character. The opposite distortions of the same coloured rays betray the fury of "counter-gales" rushing along at the rate of 120 miles a second; while their undisturbed sections prove the persistence of a "heart of peace" in the midst of that unimaginable fiery whirlwind. Velocities up to 250 miles a second, or 15,000 times that of an express train at the top of its speed, were thus observed by Young during his trip to Mount Sherman, August 2, 1872; and these were actually doubled in an extraordinary outburst observed by Father Jules Fényi, on June 17, 1891, at the Haynald Observatory in Hungary, as well as by M. Trouvelot at Meudon.[642]
Motions ascertainable in this way near the limb are, of course, horizontal as regards the sun's surface; the analogies they present might, accordingly, be styled meteorological rather than volcanic. But vertical displacements on a scale no less stupendous can also be shown to exist. Observations of the spectra of spots centrally situated (where motions in the line of sight are vertical) disclose the progress of violent uprushes and downrushes of ignited gases, for the most part in the penumbral or outlying districts. They appear to be occasioned by fitful and irregular disturbances, and have none of the systematic quality which would be required for the elucidation of sun-spot theories. Indeed, they almost certainly take place at a great height above the actual openings in the photosphere.
As to vertical motions above the limb, on the other hand, we have direct visual evidence of a truly amazing kind. The projected[Pg 205] glowing matter has, by the aid of the spectroscope, been watched in its ascent. On September 7, 1871, Young examined at noon a vast hydrogen cloud 100,000 miles long, as it showed to the eye, and 54,000 high. It floated tranquilly above the chromosphere at an elevation of some 15,000 miles, and was connected with it by three or four upright columns, presenting the not uncommon aspect compared by Lockyer to that of a grove of banyans. Called away for a few minutes at 12.30, on returning at 12.55 the observer found—
"That in the meantime the whole thing had been literally blown to shreds by some inconceivable uprush from beneath. In place of the quiet cloud I had left, the air, if I may use the expression, was filled with flying débris—a mass of detached, vertical, fusiform filaments, each from 10′ to 30′ long by 2′ or 3′ wide,[643] brighter and closer together where the pillars had formerly stood, and rapidly ascending. They rose, with a velocity estimated at 166 miles a second, to fully 200,000 miles above the sun's surface, then gradually faded away like a dissolving cloud, and at 1.15 only a few filmy wisps, with some brighter streamers low down near the photosphere, remained to mark the place."[644]
A velocity of projection of at least 500 miles per second was, by Proctor's[645] calculation, required to account for this extraordinary display, to which the earth immediately responded by a magnetic disturbance, and a fine aurora. It has proved by no means an isolated occurrence. Young saw its main features repeated, October 7, 1881,[646] on a still vaster scale; for the exploded prominence attained, this time, an altitude of 350,000 miles—the highest yet chronicled. Lockyer, moreover, has seen a prominence 40,000 miles high shattered in ten minutes; while uprushes have been witnessed by Respighi, of which the initial velocities were judged by him to be 400 or 500 miles a second. When it is remembered that a body starting from the sun's surface at the rate of 383 miles a second would, if it encountered no resistance, escape for ever from his control, it is obvious that we have, in the enormous forces of eruption or repulsion manifested in the outbursts just described, the means of accounting for the vast diffusion of matter in the solar neighbourhood. Nor is it possible to explain them away, as Cornu,[647] Faye,[648] and others have sought to do, by substituting for the rush of matter in motion, progressive illumination[Pg 206] through electric discharges, chemical processes,[649] or even through the mere reheating of gases cooled by expansion.[650] All the appearances are against such evasions of the difficulty presented by velocities stigmatised as "fabulous" and "improbable," but which, there is the strongest reason to believe, really exist.
On the 12th of December, 1878, Sir Norman Lockyer formally expounded before the Royal Society his hypothesis of the compound nature of the "chemical elements."[651] An hypothesis, it is true, over and over again propounded from the simply terrestrial point of view. What was novel was the supra-terrestrial evidence adduced in its support; and even this had been, in a general and speculative way, anticipated by Professor F. W. Clarke of Washington.[652] Lockyer had been led to his conclusion along several converging lines of research. In a letter to M. Dumas, dated December 3, 1873, he had sketched out the successive stages of "celestial dissociation" which he conceived to be represented in the sun and stars. The absence from the solar spectrum of metalloidal absorption he explained by the separation, in the fierce solar furnace, of such substances as oxygen, nitrogen, sulphur, and chlorine, into simpler constituents possessing unknown spectra; while metals were at that time still admitted to be capable of existing there in a state of integrity. Three years later he shifted his position onward. He announced, as the result of a comparative study of the Fraunhofer and electric-arc spectra of calcium, that the "molecular grouping" of that metal, which at low temperatures gives a spectrum with its chief line in the blue, is nearly broken up in the sun into another or others with lines in the violet.[653] This came to be regarded by him as "a truly typical case."[654]
During four years (1875-78 inclusive) this diligent observer was engaged in mapping a section of the more refrangible part of the solar spectrum (wave-lengths 3,800-4,000) on a scale of magnitude such that, if completed down to the infra-red, its length would have been about half a furlong. The attendant laborious investigation, by the aid of photography, of metallic spectra, seemed to indicate the existence of what he called "basic lines." These held their ground persistently in the spectra of two or more metals after all[Pg 207] possible "impurities" had been eliminated, and were therefore held to attest the presence of a common substratum of matter in a simpler state of aggregation than any with which we are ordinarily acquainted.
Later inquiries have shown, however, that between the spectral lines of different substances there are probably no absolute coincidences. "Basic" lines are really formed of doublets or triplets merged together by insufficient dispersion. Of Thalèn's original list of seventy rays common to several spectra,[655] very few resisted Thollon's and Young's powerful spectroscopes; and the process of resolution was completed by Rowland. Thus the argument from community of lines to community of substance has virtually collapsed. It was replaced by one founded on certain periodical changes on the spectra of sun-spots. They emerged from a series of observations begun at South Kensington under Sir Norman Lockyer's direction in 1879, and continued for fifteen years.[656]
The principle of the method employed is this. The whole range of Fraunhofer lines is visible when the light from a spot is examined with the spectroscope; but relatively few are widened. Now these widened lines alone constitute (presumably) the true spot-spectrum; they, and they alone, tell what kinds of vapour are thrust down into the strange dusky pit of the nucleus, the unaffected lines taking their accustomed origin from the over-lying strata of the normal solar atmosphere. Here then we have the criterion that was wanted—the means of distinguishing, spectroscopically and chemically, between the cavity and the absorbing layers piled up above it. By its persistent employment some marked peculiarities have been brought out, such as the unfamiliar character of numerous lines in spot-spectra, especially at epochs of disturbance; and the strange individuality in the behaviour of every one of these darkened and distended rays. Each seems to act on its own account; it comports itself as if it were the sole representative of the substance emitting it; its appearance is unconditioned by that of any of its terrestrial companions in the same spectrum.
The most curious fact, however, elicited by these inquiries was that of the attendance of chemical vicissitudes upon the advance of the sun-spot period. As the maximum approached, unknown replaced known components of the spot-spectra in a most pronounced and unmistakable way.[657] It seemed as if the vapours emitting lines of iron, titanium, nickel, etc., had ceased to exist as such, and their[Pg 208] room been taken by others, total strangers in terrestrial laboratories. These were held by Lockyer to be simply the finer constituents of their predecessors, dissociation having been effected by the higher temperature ensuing upon increased solar activity. But Father Cortie's supplementary investigations at Stonyhurst[658] modified, while they in the main substantiated, the South Kensington results. They showed that the substitution of unknown for known lines characterizes disturbed spots, at all stages of the solar cycle, so that no systematic course of chemical change can be said to affect the sun as a whole. They showed further[659]—from evidence independent of that obtained by Young in 1892[660]—the remarkable conspicuousness in spot-spectra of vanadium lines excessively faint in the Fraunhofer spectrum. Lockyer's "unknown lines" may probably thus be accounted for. They represent absorption, not by new, but by scarce elements, especially, Father Cortie thinks, those with atomic weights of about 50. The circumstance of their development in solar commotions, largely to the exclusion of iron, is none the less curious; but it cannot be explained by any process of dissociation.
The theory has, however, to be considered under still another aspect. It frequently happens that the contortions or displacements due to motion are seen to affect a single line belonging to a particular substance, while the other lines of that same substance remain imperturbable. Now, how is this most singular fact, which seems at first sight to imply that a body may be at rest and in motion at one and the same instant, to be accounted for? It is accounted for, on the present hypothesis, easily enough, by supposing that the rays thus discrepant in their testimony, do not belong to one kind of matter, but to several, combined at ordinary temperatures to form a body in appearance "elementary." Of these different vapours, one or more may of course be rushing rapidly towards or from the observer, while the others remain still; and since the line of sight across the average prominence-region penetrates, at the sun's edge, a depth of about 300,000 miles,[661] all the incandescent materials separately occurring along which line are projected into a single "flame" or "cloud," it will be perceived that there is ample room for diversities of behaviour.
The alternative mode of escape from the perplexity consists in assuming that the vapour in motion is rendered luminous under[Pg 209] conditions which reduce its spectrum to a few rays, the unaffected lines being derived from a totally distinct mass of the same substance shining with its ordinary emissions.[662] Thus, calcium can be rendered virtually monochromatic by attenuation, and analogous cases are not rare.
Sir Norman Lockyer only asks us to believe that effects which follow certain causes on the earth are carried a stage further in the sun, where the same causes must be vastly intensified. We find that the bodies we call "compound" split asunder at fixed degrees of heat within the range of our resources. Why should we hesitate to admit that the bodies we call "simple" do likewise at degrees of heat without the range of our resources? The term "element" simply expresses terrestrial incapability of reduction. That, in celestial laboratories, the means and their effect here absent should be present, would be an inference challenging, in itself, no expression of incredulity.
There are indeed theoretical objections to it which, though probably not insuperable, are unquestionably grave. Our seventy chemical "elements," for instance, are placed by the law of specific heats on a separate footing from their known compounds. We are not, it is true, compelled by it to believe their atoms to be really and absolutely such—to contain, that is, the "irreducible minimum" of material substance; but we do certainly gather from it that they are composed on a different principle from the salts and oxides made and unmade at pleasure by chemists. Then the multiplication of the species of matter with which Lockyer's results menace us, is at first sight startling. They may lead, we are told, to eventual unification, but the prospect appears remote. Their only obvious outcome is the disruption into several constituents of each terrestrial "element." The components of iron alone should be counted by the dozen. And there are other metals, such as cerium, which, giving a still more complex spectrum, would doubtless be still more numerously resolved. Sir Norman Lockyer interprets the observed phenomena as indicating the successive combinations, in varying proportions, of a very few original ingredients;[663] but no definite sign of their existence is perceptible; "protyle" seems likely long to evade recognition; and the only intelligible underlying principle for the reasonings employed—that of "one line, one element"—implies a throng beyond counting of formative material units.
Thus, added complexity is substituted for that fundamental unity of matter which has long formed the dream of speculators. And it[Pg 210] is extremely remarkable that Sir William Crookes, working along totally different lines, has been led to analogous conclusions. To take only one example. As the outcome of extremely delicate operations of sifting and testing carried on for years, he finds that the metal yttrium splits up into five, if not eight constituents.[664] Evidently, old notions are doomed, nor are any preconceived ones likely to take their place. It would seem, on the contrary, as if their complete reconstruction were at hand. Subversive facts are steadily accumulating; the revolutionary ideas springing from them tend, if we interpret them aright, towards the substitution of electrical for chemical theories of matter. Dissociation by the brute force of heat is already nearly superseded, in the thoughts of physicists, by the more delicate process of "ionisation." Precisely what this implies and involves we do not know; but the symptoms of its occurrence are probably altogether different from those gathered by Sir Norman Lockyer from the collation of celestial spectra.
A. J. Ångström of Upsala takes rank after Kirchhoff as a subordinate founder, so to speak, of solar spectroscopy. His great map of the "normal" solar spectrum[665] was published in 1868, two years before he died. Robert Thalèn was his coadjutor in its execution, and the immense labour which it cost was amply repaid by its eminent and lasting usefulness. For more than a score of years it held its ground as the universal standard of reference in all spectroscopic inquiries within the range of the visible emanations. Those that are invisible by reason of the quickness of their vibrations were mapped by Dr. Henry Draper, of New York, in 1873, and with superior accuracy by M. Cornu in 1881. The infra-red part of the spectrum, investigated by Langley, Abney, and Knut Ångström, reaches perhaps no definite end. The radiations oscillating too slowly to affect the eye as light may pass by insensible gradations into the long Hertzian waves of electricity.[666]
Professor Rowland's photographic map of the solar spectrum, published in 1886, and in a second enlarged edition in 1889, opened fresh possibilities for its study, from far down in the red to high up in the ultra-violet, and the accompanying scale of absolute wave-lengths[667] has been, with trifling modifications, universally adopted.[Pg 211] His new table of standard solar lines was published in 1893.[668] Through his work, indeed, knowledge of the solar spectrum so far outstripped knowledge of terrestrial spectra, that the recognition of their common constituents was hampered by intolerable uncertainties. Thousands of the solar lines charted with minute precision remained unidentified for want of a corresponding precision in the registration of metallic lines. Rowland himself, however, undertook to provide a remedy. Aided by Lewis E. Jewell, he redetermined, at the Johns Hopkins University, the wave-lengths of about 16,000 solar lines,[669] photographing for comparison with them the spectra of all the known chemical elements except gallium, of which he could procure no specimen. The labour of collation was well advanced when he died at the age of fifty-two, April 16, 1901. Investigations of metallic arc-spectra have also been carried out with signal success by Hasselberg,[670] Kayser and Runge, O. Lohse,[671] and others.
Another condition sine quâ non of progress in this department is the separation of true solar lines from those produced by absorption in our own atmosphere. And here little remains to be done. Thollon's great Atlas[672] was designed for this purpose of discrimination. Each of its thirty-three maps exhibits in quadruplicate a subdivision of the solar spectrum under varied conditions of weather and zenith-distance. Telluric effects are thus made easily legible, and they account wholly for 866, partly for 246, out of a total of 3,200 lines. But the death of the artist, April 8, 1887, unfortunately interrupted the half-finished task of the last seven years of his life. A most satisfactory record, meanwhile, of selective atmospheric action has been supplied by the experiments and determinations of Janssen, Cornu and Egoroff, by Dr. Becker's drawings,[673] and Mr. McClean's photographs of the analysed light of the sun at high, low, and medium altitudes; and the autographic pictures obtained by Mr. George Higgs, of Liverpool, of certain rhythmical groups in the red, emerging with surprising strength near sunset, excite general and well-deserved admiration.[674] The main interest, however, of all these documents resides in the information afforded by them regarding the chemistry of the sun.
The discovery that hydrogen exists in the atmosphere of the sun was made by Ångström in 1862. His list of solar elements[Pg 212] published in that year,[675] the result of an investigation separate from, though conducted on the same principle as Kirchhoff's, included the substance which we now know to be predominant among them. Dr. Plücker of Bonn had identified in 1859 the Fraunhofer line F with the green ray of hydrogen, but drew no inference from his observation. The agreement was verified by Ångström; two further coincidences were established; and in 1866 a fourth hydrogen line in the extreme violet (named h) was detected in the solar spectrum. With Thalèn, he besides added manganese, titanium, and cobalt to the constituents of the sun enumerated by Kirchhoff, and raised the number of identical rays in the solar and terrestrial spectra of iron to no less than 460.[676]
Thus, when Sir Norman Lockyer entered on that branch of inquiry in 1872, fourteen substances were recognised as common to the earth and sun. Early in 1878 he was able to increase the list provisionally to thirty-three,[677] all except hydrogen metals. This rapid success was due to his adoption of the test of length in lieu of that of strength in the comparison of lines. He measured their relative significance, in other words, rather by their persistence through a wide range of temperature, than by their brilliancy at any one temperature. The distinction was easily drawn. Photographs of the electric arc, in which any given metal had been volatilised, showed some of the rays emitted by it stretching across the axis of the light to a considerable distance on either side, while many others clung more or less closely to its central hottest core. The former "long lines," regarded as certainly representative, were those primarily sought in the solar spectrum; while the attendant "short lines," often, in point of fact, due to foreign admixtures, were set aside as likely to be misleading.[678] The criterion is a valuable one, and its employment has greatly helped to quicken the progress of solar chemistry.
Carbon was the first non-metallic element discovered in the sun. Messrs. Trowbridge and Hutchins of Harvard College concluded in 1887,[679] on the ground of certain spectral coincidences, that this protean substance is vaporised in the solar atmosphere at a temperature approximately that of the voltaic arc. Partial evidence to the same effect had earlier been alleged by Lockyer, as well as by Liveing and Dewar; and the case was rendered[Pg 213] tolerably complete by photographs taken by Kayser and Runge in 1889.[680] It was by Professor Rowland shown to be irresistible. Two hundred carbon-lines were, through his comparisons, sifted out from sunlight, and it contains others significant of the presence of silicon—a related substance, and one as important to rock-building on the earth, as carbon is to the maintenance of life. The general result of Rowland's labours was the establishment among solar materials, not only of these two out of the fourteen metalloids, or non-metallic substances, but of thirty-three metals, including silver and tin. Gold, mercury, bismuth, antimony, and arsenic were discarded from the catalogue; platinum and uranium, with six other metals, remained doubtful; while iron was recorded as crowding the spectrum with over two thousand obscure rays.[681] Gallium-absorption was detected in it by Hartley and Ramage in 1889.[682]
Dr. Henry Draper[683] announced, in 1877, his imagined discovery, in the solar spectrum, of eighteen especially brilliant spaces corresponding to oxygen-emissions. But the agreement proved, when put to the test of very high dispersion, to be wholly illusory.[684] Nor has it yet been found possible to identify, in analysed sunlight, any significant bright beams.[685]
The book of solar chemistry must be read in characters exclusively of absorption. Nevertheless, the whole truth is unlikely to be written there. That a substance displays none of its distinctive beams in the spectrum of the sun or of a star, affords scarcely a presumption against its presence. For it may be situated below the level where absorption occurs, or under a pressure such as to efface lines by widening and weakening them; it may be at a temperature so high that it gives out more light than it takes up, and yet its incandescence may be masked by the absorption of other bodies; finally, it may just balance absorption by emission, with the result of complete spectral neutrality. An instructive example is that of the chromospheric element helium. Father Secchi remarked in 1868[686] that there is no dark line in the solar spectrum matching its light; and his observation has been fully confirmed.[687] Helium-absorption[Pg 214] is, however, occasionally noticed in the penumbræ of spots.[688]
Our terrestrial vital element might then easily subsist unrecognisably in the sun. The inner organisation of the oxygen molecule is a considerably plastic one. It is readily modified by heat, and these modifications are reflected in its varying modes of radiating light. Dr. Schuster enumerated in 1879[689] four distinct oxygen spectra, corresponding to various stages of temperature, or phases of electrical excitement; and a fifth has been added by M. Egoroff's discovery in 1883[690] that certain well-known groups of dark lines in the red end of the solar spectrum (Fraunhofer's A and B) are due to absorption by the cool oxygen of our air. These persist down to the lowest temperatures, and even survive a change of state. They are produced essentially the same by liquid, as by aërial oxygen.[691]
It seemed, however, possible to M. Janssen that these bands owned a joint solar and terrestrial origin. Oxygen in a fit condition to produce them might, he considered, exist in the outer atmosphere of the sun; and he resolved to decide the point. No one could bring more skill and experience to bear upon it than he.[692] By observations on the summit of the Faulhorn, as well as by direct experiment, he demonstrated, nearly thirty years ago, the leading part played by water-vapour in generating the atmospheric spectrum; and he had recourse to similar means for appraising the share in it assignable to oxygen. An electric beam, transmitted from the Eiffel Tower to Meudon in the summer of 1888, having passed through a weight of oxygen about equal to that piled above the surface of the earth, showed the groups A and B just as they appear in the high-sun spectrum.[693] Atmospheric action is then adequate to produce them. But M. Janssen desired to prove, in addition, that they diminish proportionately to its amount. His ascent of Mont Blanc[694] in 1890 was undertaken with this object. It was perfectly successful. In the solar spectrum, examined from that eminence, oxygen-absorption was so much enfeebled as to leave no possible doubt of its purely telluric origin. Under another form, nevertheless, it has been detected as indubitably solar. A triplet of dark lines low down in the red, photographed from the sun by Higgs and[Pg 215] McClean, was clearly identified by Runge and Paschen in 1896[695] with the fundamental group of an oxygen series, first seen by Piazzi Smyth in the spectrum of a vacuum-tube in 1883.[696] The pabulum vitæ of our earth is then to some slight extent effective in arresting transmitted sunlight, and oxygen must be classed as a solar element.
The rays of the sun, besides being stopped selectively in our atmosphere, suffer also a marked general absorption. This tells chiefly upon the shortest wave-lengths; the ultra-violet spectrum is in fact closed, as if by the interposition of an opaque screen. Nor does the screen appear very sensibly less opaque from an elevation of 10,000 feet. Dr. Simony's spectral photographs, taken on the Peak of Teneriffe,[697] extended but slightly further up than M. Cornu's, taken in the valley of the Loire. Could the veil be withdrawn, some indications as to the originating temperature of the solar spectrum might be gathered from its range, since the proportion of quick vibrations given out by a glowing body grows with the intensity of its incandescence. And this brings us to the subject of our next Chapter.
FOOTNOTES:
[596] Phil. Mag., vol. xlii., p. 380, 1871.
[597] Astr. Nach., No. 3,053, Amer. Jour., vol. xlii., p. 162; Deslandres, Comptes Rendus, t. cxiii., p. 307.
[598] Proc. Roy. Society, vol. lxi., p. 433.
[599] Phil. Mag., vol. xlii., p. 377.
[600] Frost-Scheiner, Astr. Spectroscopy, pp. 184, 423.
[601] Proc. Roy. Soc., vol. xvii., p. 302.
[602] Astr. Nach., No. 1,769.
[603] Am. Jour. of Science, vol. xv., p. 85.
[604] Journ. Franklin Institute, vol. xl., p. 232a.
[605] Pogg. Annalen, Bd. cxlvi., p. 475; Astr. Nach., No. 3,014.
[606] Astr. Nach., Nos. 3,006, 3,037.
[607] This device was suggested by Janssen in 1869.
[608] Astr. and Astrophysics, vol. xi., pp. 70, 407.
[609] Astr. and Astrophysics, vol. xi., p. 604.
[610] Comptes Rendus, t. cxiii., p. 307.
[611] Astr. and Astrophysics, vol. xi., p. 50.
[612] Ibid., pp. 60, 314.
[613] Wiedemann's Annalen der Physik, Bd. xxv., p. 80.
[614] Evershed, Knowledge, vol. xxi., p. 133.
[615] Secchi, Le Soleil, t. ii., p. 294.
[616] Lockyer, Chemistry of the Sun, p. 418.
[617] L'Astronomie, August, 1884, p. 292 (Riccò); see also Evershed, Jour. British Astr. Ass., vol. ii., p. 174.
[618] Averaging about 100 miles across and 300 high. Le Soleil, t. ii., p. 35.
[619] The Sun, p. 192.
[620] Astr. Nach., No. 1,854.
[621] Mem. degli Spettroscopisti Italiani, t. v., p. 4; Secchi, ibid., t. vi., p. 56.
[622] Its non-atmospheric character was early defined by Proctor, Month. Not., vol. xxxi., p. 196.
[623] Astroph. Jour., vol. vi., p. 412.
[624] Ibid., vol. xi., p. 165.
[625] Ibid., p. 243.
[626] Sun's Place in Nature, pp. 111, 288.
[627] Abh. d. Kön. Böhm Ges. d. Wiss., Bd. ii., 1841-42, p. 467.
[628] In a paper read before the Société Philomathique de Paris, December 23, 1848, and first published in extenso in Ann. de Chim. et de Phys., t. xix., p. 211 (1870). Hippolyte Fizeau died in September, 1896.
[629] Astr. Nach., No. 1,772.
[630] Ibid., No. 1,864.
[631] A. Cornu, Sur la Méthode Doppler-Fizeau, p. D. 23.
[632] Am. Jour. of Sc., vol. xii., p. 321.
[633] Ibid., vol. xiv., p. 140.
[634] Bull. Astronom., February, 1884, p. 77.
[635] Comptes Rendus, t. xci., p. 368.
[636] Month. Not., vol. xliv., p. 170.
[637] See ante, p. 147.
[638] Recherches sur la Rotation du Soleil, Upsal, 1891.
[639] Harzer, Astr. Nach., No. 3,026; Stratonoff, Ibid., No. 3,344.
[640] Publ. Astr. Pacific Soc., vol. ii., p. 193.
[641] Proc. Roy. Society, vols. xvii., p. 415; xviii., p. 120.
[642] Comptes Rendus, t. cxii., p. 1421; t. cxiii., p. 310.
[643] At the sun's distance, one second of arc represents about 450 miles.
[644] Amer. Jour. of Sc., vol. ii., p. 468, 1871.
[645] Month. Not., vol. xxxii., p. 51.
[646] Nature, vol. xxiii., p. 281.
[647] Comptes Rendus, t. lxxxvii., p. 532.
[648] Ibid., t. xcvi., p. 359.
[649] A. Brester, Théorie du Soleil, p. 66.
[650] Such prominences as have been seen to grow by the spread of incandescence are of the quiescent kind, and present no deceptive appearance of violent motion.
[651] Proc. Roy. Soc., vol. xxviii., p. 157.
[652] "Evolution and the Spectroscope," Pop. Science Monthly, January, 1873.
[653] Proc. Roy. Soc., vol. xxiv., p. 353. These are the H and K of prominences. H. W. Vogel discovered in 1879 a hydrogen-line nearly coincident with H (Monatsb. Preuss. Ak., February, 1879, p. 118).
[654] Proc. Roy. Soc., vol. xxviii., p. 444.
[655] Many of these were referred by Lockyer himself, who first sifted the matter, to traces of the metals concerned.
[656] Chemistry of the Sun, p. 312; Proc. Roy. Society, vol. lvii., p. 199.
[657] Lockyer's Chemistry of the Sun, p. 324.
[658] Month. Not., vol. li., p. 76.
[659] Ibid., vol. lviii., p. 370.
[660] Astr. and Astrophysics, vol. xi., p. 615.
[661] Thollon's estimate (Comptes Rendus, t. xcvii., p. 902) of 300,000 kilometres, seems considerably too low. Limiting the "average prominence region" to a shell 54,000 miles deep (2′ of arc as seen from the earth), the visual line will, at mid-height (27,000 miles from the sun's surface), travel through (in round numbers) 320,000 miles of that region.
[662] Liveing and Dewar, Phil. Mag., vol. xvi. (5th ser.), p. 407.
[663] Chemistry of the Sun, p. 260.
[664] Nature, October 14, 1886.
[665] The normal spectrum is that depending exclusively upon wave-length—the fundamental constant given by nature as regards light. It is obtained by the interference of rays, in the manner first exemplified by Fraunhofer, and affords the only unvarying standard for measurement. In the refraction spectrum (upon which Kirchhoff's map was founded), the relative positions of the lines vary with the material of the prisms.
[666] Scheiner, Die Spectralanalyse der Gestirne, p. 168.
[667] Phil. Mag., vol. xxvii., p. 479.
[668] Astr. and Astrophysics, vol. xii., p. 321; Frost-Scheiner, Astr. Spectr., p. 363.
[669] Published in Astroph. Jour., vols. i. to vi.
[670] Astr. and Astrophysics, vol. xi., p. 793.
[671] Astroph. Jour., vol. vi., p. 95.
[672] Annales de l'Observatoire de Nice, t. iii., 1890.
[673] Trans. Royal Society of Edinburgh, vol. xxxvi., p. 99.
[674] Rev. A. L. Cortie, Astr. and Astrophysics, vol. xi., p. 401. Specimens of his photographs were given by Ranyard in Knowledge, vol. xiii., p. 212.
[675] Ann. d. Phys., Bd. cxvii., p. 296.
[676] Comptes Rendus, t. lxiii., p. 647.
[677] Ibid., t. lxxxvi., p. 317. Some half dozen of these identifications have proved fallacious.
[678] Chemistry of the Sun, p. 143.
[679] Amer. Jour. of Science, vol. xxxiv., p. 348.
[680] Berlin Abhandlungen, 1889.
[681] Amer. Jour. of Science, vol. xli., p. 243. See Appendix, Table II.
[682] Astrophy. Jour., vol. ix., p. 219; Fowler, Knowledge, vol. xxiii., p. 11.
[683] Amer. Jour, of Science, vol. xiv., p. 89; Nature, vol. xvi., p. 364; Month. Not., vol. xxxix., p. 440.
[684] Month. Not., vol. xxxviii., p. 473; Trowbridge and Hutchins, Amer. Jour. of Science, vol. xxxiv., p. 263.
[685] Scheiner, Die Spectralanalyse, p. 180.
[686] Comptes Rendus, t. lxvii., p. 1123.
[687] Rev. A. L. Cortie, Month. Not., vol. li., p. 18.
[688] Young, The Sun, p. 135; Hale, Astr. and Astrophysics, vol. xi., p. 312 Buss, Jour. Brit. Astr. Ass., vol. ix., p. 253.
[689] Phil. Trans., vol. clxx., p. 46.
[690] Comptes Rendus, t. xcvii., p. 555; t. ci., p. 1145.
[691] Liveing and Dewar, Astr. and Astrophysics, vol. xi., p. 705.
[692] Comptes Rendus, t. lx., p. 213; t. lxiii., p. 289.
[693] Ibid., t. cviii., p. 1035.
[694] Ibid., t. cxi., p. 431.
[695] Astroph. Jour., vols. iv., p. 317; vi., p. 426.
[696] Trans. Roy. Soc. Edin., vol. xxxii., p. 452.
[697] Comptes Rendus, t. cxi., p. 941; Huggins, Proc. Roy. Soc., vol. xlvi., p. 168.
article by Agnes Mary Clerke
from The Project Gutenberg eBook of A Popular History of Astronomy During the Nineteenth Century
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