A Popular History of Astronomy During the Nineteenth Century Part 21

[Footnote 487: _Mitth._, No. lii., p. 58 (1881).]

[Footnote 488: _Ibid._, No. xii., p. 192. Baxendell, of Manchester, reached independently a similar conclusion. See _Month. Not._, vol.

xxi., p. 141.]

[Footnote 489: Wolf, _Mitth._, No. xv., p. 107, etc. Olmsted, following Hansteen, had already, in 1856, sought to establish an auroral period of sixty-five years. _Smithsonian Contributions_, vol. viii., p. 37.]

[Footnote 490: Hahn, _Ueber die Reziehungen der Sonnenfleckenperiode zu meteorologischen Erscheinungen_, p. 99 (1877).]

[Footnote 491: _Report Brit. a.s.s._, 1881, p. 518; 1883, p. 418.]

[Footnote 492: The Rev. A. Cortie (_Month. Not._, vol. lx., p. 538) detects the influence of a short subsidiary cycle, Dr. W. J. S. Lockyer that of a thirty-five year period (_Nature_, June 20, 1901). Professor Newcomb (_Astroph. Jour._, vol. xiii., p. 11) considers that solar activity oscillates uniformly in 11.13 years, with superposed periodic variations.]

[Footnote 493: _Opere_, t. iii., p. 412.]

[Footnote 494: _Mitth._, Nos. vii. and xviii.]

[Footnote 495: _Observations at Redhill_, p. 248.]

[Footnote 496: _Comptes Rendus_, t. xcv., p. 1249.]

[Footnote 497: _Ibid._, t. xciii., p. 827; t. xcvi., p. 1418.]

[Footnote 498: _Ibid._, t. c, p. 593.]

[Footnote 499: Ellis, _Proc. Roy. Society_, vol. lxiii., p. 70.]

[Footnote 500: Schultz, _Astr. Nach._, Nos. 2,817-18, 2,847-8; Wilsing, _Ibid._, No. 3,039; Belopolsky, _Ibid._, No. 2,722.]

[Footnote 501: _Report Brit. a.s.s._, 1892, p. 635.]

[Footnote 502: A. W. Augur, _Astroph. Jour._, vol. xiii., p. 346.]

[Footnote 503: _Report Brit. a.s.s._, 1862, p. 16 (pt. ii.).]

[Footnote 504: _Mem. R. A. S._, vol. xxi., p. 161.]

[Footnote 505: _Month. Not._, vol. xxiv., p. 162.]

[Footnote 506: _Am. Jour. of Science_, vol. vii., 1874, p. 92.]

[Footnote 507: Young, _The Sun_, p. 103.]

[Footnote 508: _Ann. Bur. Long._, 1879, p. 679.]

[Footnote 509: _Ibid._, 1878, p. 689.]

[Footnote 510: _Himmelsphotographie_, p. 273.]

[Footnote 511: Ranyard, _Knowledge_, vols. xiv., p. 14, xvi., p. 189; see also the accompanying photographs.]

CHAPTER III

_RECENT SOLAR ECLIPSES_

By observations made during a series of five remarkable eclipses, comprised within a period of eleven years, knowledge of the solar surroundings was advanced nearly to its present stage. Each of these events brought with it a fresh disclosure of a definite and unmistakable character. We will now briefly review this orderly sequence of discovery.

Photography was first systematically applied to solve the problems presented by the eclipsed sun, July 18, 1860. It is true that a daguerreotype,[512] taken by Berkowski with the Konigsberg heliometer during the eclipse of 1851, is still valuable as a record of the corona of that year; and some subsequent attempts were made to register partial phases of solar occultation, notably by Professor Bartlett at West Point in 1854;[513] but the ground remained practically unbroken until 1860.

In that year the track of totality crossed Spain, and thither, accordingly, Warren de la Rue transported his photo-heliograph, and Father Secchi his six-inch Cauchoix refractor. The question then primarily at issue was that relating to the nature of the red protuberances. Although, as already stated, the evidence collected in 1851 gave a reasonable certainty of their connection with the sun, objectors were not silenced; and when the side of incredulity was supported by so considerable an authority as M. Faye, it was impossible to treat it with contempt. Two crucial tests were available. If it could be shown that the fantastic shapes suspended above the edge of the dark moon were seen under an identical aspect from two distant stations, that fact alone would annihilate the theory of optical illusion or "mirage"; while the certainty that they were progressively concealed by the advancing moon on one side, and uncovered on the other, would effectually detach them from dependence on our satellite, and establish them as solar appendages.

Now both these tests were eminently capable of being applied by photography. But the difficulty arose that nothing was known as to the chemical power of the rosy prominence-light, while everything depended on its right estimation. A shot had to be fired, as it were, in the dark. It was a matter of some surprise, and of no small congratulation, that, in both cases, the shot took effect.

De la Rue occupied a station at Rivabellosa, in the Upper Ebro valley; Secchi set up his instrument at Desierto de las Palmas, about 250 miles to the south-east, overlooking the Mediterranean. From the totally eclipsed sun, with its strange garland of flames, each observer derived several perfectly successful impressions, which were found, on comparison, to agree in the most minute details. This at once settled the fundamental question as to the substantial reality of these objects; while their solar character was demonstrated by the pa.s.sage of the moon _in front_ of them, indisputably attested by pictures taken at successive stages of the eclipse. That forms seeming to defy all laws of equilibrium were, nevertheless, not wholly evanescent, appeared from their ident.i.ty at an interval of seven minutes, during which the lunar shadow was in transit from one station to the other; and the singular energy of their actinic rays was shown by the record on the sensitive plates of some prominences invisible in the telescope. Moreover, photographic evidence strongly confirmed the inference--previously drawn by Grant and others, and now with fuller a.s.surance by Secchi--that an uninterrupted stratum of prominence-matter encompa.s.ses the sun on all sides, forming a reservoir from which gigantic jets issue, and into which they subside.

Thus, first-fruits of accurate knowledge regarding the solar surroundings were gathered, while the value of the brief moments of eclipse gained indefinite increase, by supplementing transient visual impressions with the faithful and lasting records of the camera.

In the year 1868 the history of eclipse spectroscopy virtually began, as that of eclipse photography in 1860; that is to say, the respective methods then first gave definite results. On the 18th of August, 1868, the Indian and Malayan peninsulas were traversed by a lunar shadow producing total obscuration during five minutes and thirty-eight seconds. Two English and two French expeditions were despatched to the distant regions favoured by an event so propitious to the advance of knowledge, chiefly to obtain the verdict of the prism as to the composition of prominences. Nor were they despatched in vain. An identical discovery was made by nearly all the observers. At Jamkandi, in the Western Ghauts, where Lieutenant (now Colonel) Herschel was posted, unremitting bad weather threatened to baffle his eager expectations; but during the lapse of the critical five and a half minutes the clouds broke, and across the driving wrack a "long, finger-like projection" jutted out over the margin of the dark lunar globe. In another moment the spectroscope was pointed towards it; three bright lines--red, orange, and blue--flashed out, and the problem was solved.[514] The problem was solved in this general sense, that the composition out of glowing vapours of the objects infelicitously termed "protuberances" or "prominences" was no longer doubtful; although further inquiry was needed for the determination of the particular species to which those vapours belonged.

Similar, but more complete observations were made, with less atmospheric hindrance, by Tennant and Janssen at Guntoor, by Pogson at Masulipatam, and by Rayet at Wha-Tonne, on the coast of the Malay peninsula, the last observer counting as many as nine bright lines.[515] Among them it was not difficult to recognise the characteristic light of hydrogen; and it was generally, though over-hastily, a.s.sumed that the orange ray matched the luminous emissions of sodium. But fuller opportunities were at hand.

The eclipse of 1868 is chiefly memorable for having taught astronomers to do without eclipses, so far, at least, as one particular branch of solar inquiry is concerned. Inspired by the beauty and brilliancy of the variously tinted prominence-lines revealed to him by the spectroscope, Janssen exclaimed to those about him, "Je verrai ces lignes-la en dehors des eclipses!" On the following morning he carried into execution the plan which formed itself in his brain while the phenomenon which suggested it was still before his eyes. It rests upon an easily intelligible principle.

The glare of our own atmosphere alone hides the appendages of the sun from our daily view. To a spectator on an airless planet, the central globe would appear attended by all its splendid retinue of crimson prominences, silvery corona, and far-spreading zodiacal light projected on the star-spangled black background of an absolutely unilluminated sky. Now the spectroscope offers the means of indefinitely weakening atmospheric glare by diffusing a constant amount of it over an area widened _ad libitum_. But monochromatic or "bright-line" light is, by its nature, incapable of being so diffused. It can, of course, be _deviated_ by refraction to any extent desired; but it always remains equally concentrated, in whatever direction it may be thrown. Hence, when it is mixed up with continuous light--as in the case of the solar flames s.h.i.+ning through our atmosphere--it derives a _relative_ gain in intensity from every addition to the dispersive power of the spectroscope with which the heterogeneous ma.s.s of beams is a.n.a.lysed.

Employ prisms enough, and eventually the undiminished rays of persistent colour will stand out from the continually fading rainbow-tinted band, by which they were at first effectually veiled.

This Janssen saw by a flash of intuition while the eclipse was in progress; and this he realised at 10 A.M. next morning, August 19, 1868--the date of the beginning of spectroscopic work at the margin of the un.o.bscured sun. During the whole of that day and many subsequent ones, he enjoyed, as he said, the advantage of a prolonged eclipse. The intense interest with which he surveyed the region suddenly laid bare to his scrutiny was heightened by evidences of rapid and violent change. On the 18th of August, during the eclipse, a vast spiral structure, _at least_ 89,000 miles high, was perceived, planted in surprising splendour on the rim of the interposed moon. If was formed as General Tennant judged from its appearance in his photographs, by the encounter of two mounting torrents of flame, and was distinguished as the "Great Horn."

Next day it was in ruins; hardly a trace remained to show where it had been.[516] Janssen's spectroscope furnished him besides with the strongest confirmation of what had already been reported by the telescope and the camera as to the continuous nature of the scarlet "sierra" lying at the base of the prominences. Everywhere at the sun's edge the same bright lines appeared.

It was not until the 19th of September that Janssen thought fit to send news of his discovery to Europe. It seemed little likely to be antic.i.p.ated; yet a few minutes before his despatch was handed to the Secretary of the Paris Academy of Sciences, a communication similar in purport had been received from Sir Norman Lockyer. There is no need to discuss the narrow and wearisome question of priority; each of the compet.i.tors deserves, and has obtained, full credit for his invention.

With noteworthy and confident prescience, Lockyer, in 1866, before anything was yet known regarding the const.i.tution of the "red flames,"

ordered a strongly dispersive spectroscope for the express purpose of viewing, apart from eclipses, the bright-line spectrum which he expected them to give. Various delays, however, supervened, and the instrument was not in his hands until October 16, 1868. On the 20th he picked up the vivid rays, of which the presence and (approximately) the positions had in the interim become known. But there is little doubt that, even without that previous knowledge, they would have been found; and that the eclipse of August 18 only accelerated a discovery already a.s.sured.

Sir William Huggins, meanwhile, had been tending towards the same goal during two and a half years in his observatory at Tulse Hill. The principle of the spectroscopic visibility of prominence-lines at the edge of an uneclipsed sun was quite explicitly stated by him in February, 1868,[517] and he devised various apparatus for bringing them into actual view; but not until he knew where to look did he succeed in seeing them.

Astronomers, thus liberated, by the acquisition of power to survey them at any time, from the necessity of studying prominences during eclipses, were able to concentrate the whole of their attention on the corona. The first thing to be done was to ascertain the character of its spectrum.

This was seen in 1868 only as a faintly continuous one; for Rayet, who seems to have perceived its distinctive bright line far above the summits of the flames, connected it, nevertheless, with those objects.

On the other hand, Lieutenant Campbell ascertained on the same occasion the polarisation of the coronal light in planes pa.s.sing through the sun's centre,[518] thereby showing that light to be, in whole or in part, reflected suns.h.i.+ne. But if reflected suns.h.i.+ne, it was objected, the chief at least of the dark Fraunhofer lines should be visible in it, as they are visible in moonbeams, sky illumination, and all other sun-derived light. The objection was well founded, but was prematurely urged, as we shall see.

On the 7th of August, 1869, a track of total eclipse crossed the continent of North America diagonally, entering at Behring's Straits, and issuing on the coast of North Carolina. It was beset with observers; but the most effective work was done in Iowa. At Des Moines, Professor Harkness of the Naval Observatory, Was.h.i.+ngton, obtained from the corona an "absolutely continuous spectrum," slightly less bright than that of the full moon, but traversed by a single green ray.[519] The same green ray was seen at Burlington and its position measured by Professor Young of Dartmouth College.[520] It appeared to coincide with that of a dark line of iron in the solar spectrum, numbered 1,474 on Kirchhoff's scale.

But in 1876 Young was able, by the use of greatly increased dispersion, to resolve the Fraunhofer line "1474" into a pair, the more refrangible member of which he considered to be the reversal of the green coronal ray.[521] Scarcely called in question for over twenty years, the identification nevertheless broke down through the testimony of the eclipse-photographs of 1898. Sir Norman Lockyer derived from them a position for the line in question notably higher up in the spectrum than that previously a.s.signed to it. Instead of 5,317, its true wave-length proved to be 5,303 ten millionths of a millimetre;[522] nor does it make any show by absorption in dispersed sunlight. The originating substance, designated "coronium," of which nothing is known to terrestrial chemistry, continues luminous[523] at least 300,000 miles above the sun's surface, and is hence presumably much lighter even than hydrogen.

A further trophy was carried off by American skill[524] sixteen months after the determination due to it of the distinctive spectrum of the corona. The eclipse of December 22, 1870, though lasting only two minutes and ten seconds, drew observers from the New, as well as from the Old World to the sh.o.r.es of the Mediterranean. Janssen issued from beleaguered Paris in a balloon, carrying with him the _vital parts_ of a reflector specially constructed to collect evidence about the corona.

But he reached Oran only to find himself shut behind a cloud-curtain more impervious than the Prussian lines. Everywhere the sky was more or less overcast. Lockyer's journey from England to Sicily, and s.h.i.+pwreck in the _Psyche_, were recompensed with a glimpse of the solar aureola during _one second and a half_! Three parties stationed at various heights on Mount Etna saw absolutely nothing. Nevertheless important information was s.n.a.t.c.hed in despite of the elements.

The prominent event was Young's discovery of the "reversing layer." As the surviving solar crescent narrowed before the encroaching moon, "the dark lines of the spectrum," he tells us, "and the spectrum itself, gradually faded away, until all at once, as suddenly as a bursting rocket shoots out its stars, the whole field of view was filled with bright lines more numerous than one could count. The phenomenon was so sudden, so unexpected, and so wonderfully beautiful, as to force an involuntary exclamation."[525] Its duration was about two seconds, and the impression produced was that of a complete reversal of the Fraunhofer spectrum--that is, the subst.i.tution of a bright for every dark line.