FOOTNOTES:

The idea that solar maculation depends in some way upon theposition of the planets occurred to Galileo in 1612.[493]It has been industriously sifted by a whole bevy of modern solar physicists. Wolf in 1859[494]found reason to believe that the eleven-year curve is determined by the action of Jupiter, modified by that of Saturn, and diversified by influences proceeding from the earth and Venus. Its tempting approach to agreement with Jupiter's period of revolution round the sun, indeed, irresistibly suggested a causal connection; yet it does not seem that the most skilful "coaxing" of figures can bring about a fundamental harmony. Carrington pointed out in 1863, that while, duringeight successive periods, from 1770 downwards, there were approximate coincidences between Jupiter's aphelion passages and sun-spot maxima, the relation had been almost exactly reversed in the two periods preceding that date;[495]and Wolf himself finally concluded that the Jovian origin must be abandoned.[496]M. Duponchel's[497]prediction, nevertheless, of an abnormal retardation of the maximum due in 1881 through certain peculiarities in the positions of Uranus and Neptune about the time it fell due, was partially verified by the event, since, after an abortive phase of agitation in April, 1882, the final outburst was postponed to January, 1894. The interval was thus 13.5 instead of 11.1 years; and it is noticeable that the delay affected chiefly the southern hemisphere. Alternations of activity in the solar hemispheres were indeed a marked feature of the maximum of 1884, which, in M. Faye's view,[498]derived thence its indecisive character, while sharp, strong crises arise with the simultaneous advance of agitation north and south of the solar equator. The curve of magnetic disturbance followed with its usual strict fidelity the anomalous fluctuations of the sun-spot curve. The ensuing minimum occurred early in 1889, and was succeeded in 1894 by a maximum slightly less feeble than its predecessor.[499]

It cannot be said that much progress has been made towards the disclosure of the cause, or causes, of the sun-spot cycle. No external influence adequate to the effect has, at any rate, yet been pointed out. Most thinkers on this difficult subject provide a quasi-explanation of the periodicity in question through certain assumed vicissitudes affecting internal processes;[500]Sir Norman Lockyer and E. von Oppolzer reach the same end by establishing self-compensatory fluctuations in the solar atmospheric circulation;Dr. Schuster resorts to changes in the electrical conductivity of space near the sun.[501]In all these theories, however, the course of transition is arbitrarily arranged to suit a period, which imposes itself as a fact peremptorily claiming admittance, while obstinately defying explanation.

The question so much discussed, as to the influence of sun-spots on weather, does not admit of a satisfactory answer. The facts of meteorology are too complex for easy or certain classification. Effects owning dependence on one cause often wear the livery of another; the meaning of observed particulars may be inverted by situation; and yet it is only by the collection and collocation of particulars that we can hope to reach any general law. There is, however, a good deal of evidence to support the opinion—the grounds for which were primarily derived from the labours of Dr. Meldrum at Mauritius—that increased rainfall and atmospheric agitation attend spot-maxima; while Herschel's conjecture of a more copious emission of light and heat about the same epochs has recently obtained some countenance from Savélieff's measures showing a gain in the strength of the sun's radiationpari passuwith increase in the number of spots visible on his surface.[502]

The examination of what we may call thetextureof the sun's surface derived new interest from a remarkable announcement made by Mr. James Nasmyth in 1862.[503]He had made (as he supposed) the discovery that the entire luminous stratum of the sun is composed of a multitude of elongated shining objects on a darker background, shaped much like willow-leaves, of vast size, crossing each other in all possible directions, and possessed of unceasing relative motions. A lively controversy ensued. In England and abroad the most powerful telescopes were directed to a scrutiny encompassed with varied difficulties. Mr. Dawes was especially emphatic in declaring that Nasmyth's "willow-leaves" were nothing more than the "nodules" of Sir William Herschel seen under a misleading aspect of uniformity; and there is little doubt that he was right. It is, nevertheless, admitted that something of the kind may be seen in the penumbræ and "bridges" of spots, presenting an appearance compared by Dawes himself in 1852 to that of a piece of coarse straw-thatching left untrimmed at the edges.[504]

The term "granulated," suggested by Dawes in 1864,[505]best describes the mottled aspect of the solar disc as shown by modern telescopes and cameras. The grains, or rather the "floccules,"with which it is thickly strewn, have been resolved by Langley, under exceptionally favourable conditions, into "granules" not above 100 miles in diameter; and from these relatively minute elements, composing, jointly, about one-fifth of the visible photosphere,[506]he estimates that three-quarters of the entire light of the sun are derived.[507]Janssen agrees, so far as to say that if the whole surface were as bright as its brightest parts, its luminous emission would be ten to twenty times greater than it actually is.[508]

The rapid changes in the forms of these solar cloud-summits are beautifully shown in the marvellous photographs taken by Janssen at Meudon, with exposures reduced at times to 1/100000 of a second! By their means, also, the curious phenomenon known as theréseau photosphériquehas been made evident.[509]This consists in the diffusion over the entire disc of fleeting blurred patches, separated by a reticulation of sharply-outlined and regularly-arranged granules. The imperfect definition in the smudged areas may be due to agitations in the solar or terrestrial atmosphere, unless it be—as Dr. Schemer thinks possible[510]—merely a photographic effect. M. Janssen considers that the photospheric cloudlets change their shape and character with the progress of the sun-spot period;[511]but this is as yet uncertain.

The "grains," or more brilliant parts of the photosphere, are now generally held to represent the upper termination of ascending and condensing currents, while the darker interstices (Herschel's "pores") mark the positions of descending cooler ones. In the penumbræ of spots, the glowing streams rushing up from the tremendous sub-solar furnace are bent sideways by the powerful indraught, so as to change their vertical for a nearly horizontal motion, and are thus taken, as it were, in flank by the eye, instead of being seen end-on in mamelon-form. This gives a plausible explanation of the channelled structure of penumbræ which suggested the comparison to a rude thatch. Accepting this theory as in the main correct, we perceive that the very same circulatory process which, in its spasms of activity, gives rise to spots, produces in its regular course the singular "marbled" appearance, for the recording of which we are no longer at the mercy of the fugitive or delusive impressions of the human retina. And precisely this circulatory process it is which gives to our great luminary its permance as asun, or warming and illuminating body.

FOOTNOTES:[405]Mem. R. A. S., vol. xxi., p. 157.[406]Ibid., p. 160.[407]Month. Not., vol. xxi., p. 144.[408]Le Soleil, t. i., pp. 87-90 (2nd ed., 1871).[409]Seeante, p. 58.[410]Observations at Redhill (1863), Introduction.[411]Month. Not., vol. xxxvi., p. 142.[412]Cape Observations, p. 435,note.[413]Month. Not., vol. x., p. 158.[414]Rosa Ursina, lib. iii., p. 348.[415]Observations at Redhill, p. 8.[416]Op., t. iii., p. 402.[417]Rosa Ursina, lib. iv., p. 601. Both Galileo and Scheiner spoke of theapparentor "synodical" period, which is about one and a third days longer than thetrueor "sidereal" one. The difference is caused by the revolution of the earth in its orbit in the same direction with the sun's rotation on its axis.[418]Rosa Ursina, lib. iii., p. 260.[419]Faye,Comptes Rendus, t. lx., p. 818.[420]Ibid., t. xii., p. 648.[421]Proc. Am. Ass. Adv. of Science, 1885, p. 85.[422]Observations at Redhill, p. 221.[423]Am. Jour. of Science, vol. xi., p. 169.[424]Month. Not., vol. xix., p. 1.[425]Vierteljahrsschrift der Naturfors. Gesellschaft(Zürich), 1859, p. 252.[426]Lockyer,Chemistry of the Sun, p. 428.[427]Maunder,Knowledge, vol. xv., p. 130.[428]Month. Mon., vol. l., p. 251.[429]Maunder,Knowledge, vol. xvii., p. 173.[430]Astr. Nach., No. 1,315.[431]As late as 1866 an elaborate treatise in its support was written by F. Coyteux, entitledQu'est-ce que le Soleil? Peut-il être habité?and answering the question in the affirmative.[432]The subsequent researches of Plücker, Frankland, Wüllner, and others, showed that gases strongly compressed give an absolutely unbroken spectrum.[433]Comptes Rendus, t. lx., pp. 89, 138.[434]Ibid., t. c., p. 595.[435]Bull. Meteor. dell Osservatorio dell Coll. Rom., Jan. 1, 1864, p. 4.[436]Quart. Jour. of Science, vol. i., p. 222.[437]Ann. de Chim. et de Phys., t. xxii., p. 127.[438]Phil. Trans., vol. clix., p. 575.[439]Les Mondes, Dec. 22, 1864, p. 707.[440]Comptes Rendus, t. lx., p. 147.[441]Proc. Roy. Society, vol. xvi., p. 29.[442]Recherches sur le Spectre Solaire, p. 38.[443]Am. Jour. of Science, 1881, vol. xxi., p. 41. Hastings stipulated only for some member of the triad, carbon, silicon, and boron.[444]Ranyard,Knowledge, vol. xvi., p. 190.[445]Young,The Sun, p. 337, ed. 1897.[446]H. Draper,Quart. Journ. of Sc., vol. i., p. 381; alsoPhil. Mag., vol. xvii., 1840, p. 222.[447]Reproduced in Arago'sPopular Astronomy, plate xii., vol. 1.[448]Report Brit. Ass., 1859, p. 148.[449]Phil. Trans., vol. clii., p. 407.[450]Researches in Solar Physics, part i., p. 20.[451]Both the phrase and the method were suggested by Faye, who estimated the average depth of the luminous sheath of spots at 2,160 miles.Comptes Rendus, t. lxi., p. 1082; t. xcvi., p. 356.[452]Month. Not., vol. lv., p. 74.[453]Sidgreaves,Ibid., p. 282; Cortie,Ibid., vol. lviii., p. 91.[454]Explained by East as refraction-effects.Jour. Brit. Astr. Ass., vol. viii., p. 187.[455]Proc. Roy. Soc., vol. xiv., p. 39.[456]Potsdam Publicationen, No. 18;Astr. Nach., Nos. 3,000, 3,287.[457]Faye,Comptes Rendus, t. cxi., p. 77; Bélopolsky,Astr. Nach., No. 2,991.[458]Ibid., Nos. 3,275, 3,344.[459]Lockyer,Contributions to Solar Physics, p. 70.[460]Le Soleil, p. 87.[461]Proc. Roy. Soc., vol. xv., p. 256.[462]Phil. Mag., vol. xvi., p. 460.[463]Recherches sur la Rotation du Soleil, p. 12.[464]Hale,Astr. and Astrophysics, vol. xi., p. 814.[465]Jour. Brit. Astr. Ass., vol. i., p. 177.[466]Comptes Rendus, t. lxxv., p. 1664;Revue Scientifique, t. v., p. 359 (1883). Mr. Herbert Spencer had already (inThe Reader, Feb. 25, 1865) put forward an opinion that spots were of the nature of "cyclonic clouds."[467]The Sun, p. 174. For Faye's answer to the objection, seeComptes Rendus, t. xcv., p. 1310.[468]A revised edition appeared in 1897.[469]Astr. and Astrophysics, vol. xii., p. 832.[470]Proc. Roy. Soc., No. 244.[471]Astr. Nach., No. 3,146;Astr. and Astrophysics, vol. xii., pp. 419, 736.[472]Sirius, Sept., 1893;ibid., vol. xxiii., p. 97;Astrophy. Jour., vol. i., p. 112 (Wilczynski), p. 178 (Keeler); vol. ii., p. 73 (Hale).[473]Month. Not., vol. xx., p. 13.[474]Ibid., p. 15.[475]Am. Jour., vol. xxix. (2nd series), pp. 94, 95.[476]The magnetic disturbance took place at 11.15A.M., three minutes before the solar blaze compelled the attention of Carrington.[477]Phil. Trans., vol. cli., p. 428.[478]Maunder,Journal Brit. Astr. Ass., vol. ii., p. 386; Miss E. Brown,Ibid., p. 210; Month. Not., vol. lii., p. 354.[479]Observatory, vol. xxi., p. 387; Maunder,Knowledge, vol. xxi., p. 228; Fényi,Astroph. Jour., vol. x., p. 333.[480]Ibid., p. 336; W. Anderson, Observatory, vol. xxii., p. 196.[481]Proc. Roy. Society, vol. lii., p. 307; Rev. W. Sidgreaves,Mem. R. A. S., vol. liv., p. 85.[482]Report on Solar and Terrestrial Magnetism, Washington, 1898, p. 27.[483]Astr. and Astrophysics, vol. xi., p. 611.[484]Ibid., p. 819 (Sidgreaves).[485]See J. Rand Capron,Phil. Mag., vol. xv., p. 318.[486]Mittheilungen über die Sonnenflecken, No. ix.,Vierteljahrsschrift der Naturforschenden Gesellschaft in Zürich, Jahrgang 4.[487]Mitth., No. lii., p. 58 (1881).[488]Ibid., No. xii., p. 192. Baxendell, of Manchester, reached independently a similar conclusion. SeeMonth. Not., vol. xxi., p. 141.[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.[490]Hahn,Ueber die Reziehungen der Sonnenfleckenperiode zu meteorologischen Erscheinungen, p. 99 (1877).[491]Report Brit. Ass., 1881, p. 518; 1883, p. 418.[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.[493]Opere, t. iii., p. 412.[494]Mitth., Nos. vii. and xviii.[495]Observations at Redhill, p. 248.[496]Comptes Rendus, t. xcv., p. 1249.[497]Ibid., t. xciii., p. 827; t. xcvi., p. 1418.[498]Ibid., t. c, p. 593.[499]Ellis,Proc. Roy. Society, vol. lxiii., p. 70.[500]Schultz,Astr. Nach., Nos. 2,817-18, 2,847-8; Wilsing,Ibid., No. 3,039; Bélopolsky,Ibid., No. 2,722.[501]Report Brit. Ass., 1892, p. 635.[502]A. W. Augur,Astroph. Jour., vol. xiii., p. 346.[503]Report Brit. Ass., 1862, p. 16 (pt. ii.).[504]Mem. R. A. S., vol. xxi., p. 161.[505]Month. Not., vol. xxiv., p. 162.[506]Am. Jour. of Science, vol. vii., 1874, p. 92.[507]Young,The Sun, p. 103.[508]Ann. Bur. Long., 1879, p. 679.[509]Ibid., 1878, p. 689.[510]Himmelsphotographie, p. 273.[511]Ranyard,Knowledge, vols. xiv., p. 14, xvi., p. 189; see also the accompanying photographs.

[405]Mem. R. A. S., vol. xxi., p. 157.

[406]Ibid., p. 160.

[407]Month. Not., vol. xxi., p. 144.

[408]Le Soleil, t. i., pp. 87-90 (2nd ed., 1871).

[409]Seeante, p. 58.

[410]Observations at Redhill (1863), Introduction.

[411]Month. Not., vol. xxxvi., p. 142.

[412]Cape Observations, p. 435,note.

[413]Month. Not., vol. x., p. 158.

[414]Rosa Ursina, lib. iii., p. 348.

[415]Observations at Redhill, p. 8.

[416]Op., t. iii., p. 402.

[417]Rosa Ursina, lib. iv., p. 601. Both Galileo and Scheiner spoke of theapparentor "synodical" period, which is about one and a third days longer than thetrueor "sidereal" one. The difference is caused by the revolution of the earth in its orbit in the same direction with the sun's rotation on its axis.

[418]Rosa Ursina, lib. iii., p. 260.

[419]Faye,Comptes Rendus, t. lx., p. 818.

[420]Ibid., t. xii., p. 648.

[421]Proc. Am. Ass. Adv. of Science, 1885, p. 85.

[422]Observations at Redhill, p. 221.

[423]Am. Jour. of Science, vol. xi., p. 169.

[424]Month. Not., vol. xix., p. 1.

[425]Vierteljahrsschrift der Naturfors. Gesellschaft(Zürich), 1859, p. 252.

[426]Lockyer,Chemistry of the Sun, p. 428.

[427]Maunder,Knowledge, vol. xv., p. 130.

[428]Month. Mon., vol. l., p. 251.

[429]Maunder,Knowledge, vol. xvii., p. 173.

[430]Astr. Nach., No. 1,315.

[431]As late as 1866 an elaborate treatise in its support was written by F. Coyteux, entitledQu'est-ce que le Soleil? Peut-il être habité?and answering the question in the affirmative.

[432]The subsequent researches of Plücker, Frankland, Wüllner, and others, showed that gases strongly compressed give an absolutely unbroken spectrum.

[433]Comptes Rendus, t. lx., pp. 89, 138.

[434]Ibid., t. c., p. 595.

[435]Bull. Meteor. dell Osservatorio dell Coll. Rom., Jan. 1, 1864, p. 4.

[436]Quart. Jour. of Science, vol. i., p. 222.

[437]Ann. de Chim. et de Phys., t. xxii., p. 127.

[438]Phil. Trans., vol. clix., p. 575.

[439]Les Mondes, Dec. 22, 1864, p. 707.

[440]Comptes Rendus, t. lx., p. 147.

[441]Proc. Roy. Society, vol. xvi., p. 29.

[442]Recherches sur le Spectre Solaire, p. 38.

[443]Am. Jour. of Science, 1881, vol. xxi., p. 41. Hastings stipulated only for some member of the triad, carbon, silicon, and boron.

[444]Ranyard,Knowledge, vol. xvi., p. 190.

[445]Young,The Sun, p. 337, ed. 1897.

[446]H. Draper,Quart. Journ. of Sc., vol. i., p. 381; alsoPhil. Mag., vol. xvii., 1840, p. 222.

[447]Reproduced in Arago'sPopular Astronomy, plate xii., vol. 1.

[448]Report Brit. Ass., 1859, p. 148.

[449]Phil. Trans., vol. clii., p. 407.

[450]Researches in Solar Physics, part i., p. 20.

[451]Both the phrase and the method were suggested by Faye, who estimated the average depth of the luminous sheath of spots at 2,160 miles.Comptes Rendus, t. lxi., p. 1082; t. xcvi., p. 356.

[452]Month. Not., vol. lv., p. 74.

[453]Sidgreaves,Ibid., p. 282; Cortie,Ibid., vol. lviii., p. 91.

[454]Explained by East as refraction-effects.Jour. Brit. Astr. Ass., vol. viii., p. 187.

[455]Proc. Roy. Soc., vol. xiv., p. 39.

[456]Potsdam Publicationen, No. 18;Astr. Nach., Nos. 3,000, 3,287.

[457]Faye,Comptes Rendus, t. cxi., p. 77; Bélopolsky,Astr. Nach., No. 2,991.

[458]Ibid., Nos. 3,275, 3,344.

[459]Lockyer,Contributions to Solar Physics, p. 70.

[460]Le Soleil, p. 87.

[461]Proc. Roy. Soc., vol. xv., p. 256.

[462]Phil. Mag., vol. xvi., p. 460.

[463]Recherches sur la Rotation du Soleil, p. 12.

[464]Hale,Astr. and Astrophysics, vol. xi., p. 814.

[465]Jour. Brit. Astr. Ass., vol. i., p. 177.

[466]Comptes Rendus, t. lxxv., p. 1664;Revue Scientifique, t. v., p. 359 (1883). Mr. Herbert Spencer had already (inThe Reader, Feb. 25, 1865) put forward an opinion that spots were of the nature of "cyclonic clouds."

[467]The Sun, p. 174. For Faye's answer to the objection, seeComptes Rendus, t. xcv., p. 1310.

[468]A revised edition appeared in 1897.

[469]Astr. and Astrophysics, vol. xii., p. 832.

[470]Proc. Roy. Soc., No. 244.

[471]Astr. Nach., No. 3,146;Astr. and Astrophysics, vol. xii., pp. 419, 736.

[472]Sirius, Sept., 1893;ibid., vol. xxiii., p. 97;Astrophy. Jour., vol. i., p. 112 (Wilczynski), p. 178 (Keeler); vol. ii., p. 73 (Hale).

[473]Month. Not., vol. xx., p. 13.

[474]Ibid., p. 15.

[475]Am. Jour., vol. xxix. (2nd series), pp. 94, 95.

[476]The magnetic disturbance took place at 11.15A.M., three minutes before the solar blaze compelled the attention of Carrington.

[477]Phil. Trans., vol. cli., p. 428.

[478]Maunder,Journal Brit. Astr. Ass., vol. ii., p. 386; Miss E. Brown,Ibid., p. 210; Month. Not., vol. lii., p. 354.

[479]Observatory, vol. xxi., p. 387; Maunder,Knowledge, vol. xxi., p. 228; Fényi,Astroph. Jour., vol. x., p. 333.

[480]Ibid., p. 336; W. Anderson, Observatory, vol. xxii., p. 196.

[481]Proc. Roy. Society, vol. lii., p. 307; Rev. W. Sidgreaves,Mem. R. A. S., vol. liv., p. 85.

[482]Report on Solar and Terrestrial Magnetism, Washington, 1898, p. 27.

[483]Astr. and Astrophysics, vol. xi., p. 611.

[484]Ibid., p. 819 (Sidgreaves).

[485]See J. Rand Capron,Phil. Mag., vol. xv., p. 318.

[486]Mittheilungen über die Sonnenflecken, No. ix.,Vierteljahrsschrift der Naturforschenden Gesellschaft in Zürich, Jahrgang 4.

[487]Mitth., No. lii., p. 58 (1881).

[488]Ibid., No. xii., p. 192. Baxendell, of Manchester, reached independently a similar conclusion. SeeMonth. Not., vol. xxi., p. 141.

[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.

[490]Hahn,Ueber die Reziehungen der Sonnenfleckenperiode zu meteorologischen Erscheinungen, p. 99 (1877).

[491]Report Brit. Ass., 1881, p. 518; 1883, p. 418.

[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.

[493]Opere, t. iii., p. 412.

[494]Mitth., Nos. vii. and xviii.

[495]Observations at Redhill, p. 248.

[496]Comptes Rendus, t. xcv., p. 1249.

[497]Ibid., t. xciii., p. 827; t. xcvi., p. 1418.

[498]Ibid., t. c, p. 593.

[499]Ellis,Proc. Roy. Society, vol. lxiii., p. 70.

[500]Schultz,Astr. Nach., Nos. 2,817-18, 2,847-8; Wilsing,Ibid., No. 3,039; Bélopolsky,Ibid., No. 2,722.

[501]Report Brit. Ass., 1892, p. 635.

[502]A. W. Augur,Astroph. Jour., vol. xiii., p. 346.

[503]Report Brit. Ass., 1862, p. 16 (pt. ii.).

[504]Mem. R. A. S., vol. xxi., p. 161.

[505]Month. Not., vol. xxiv., p. 162.

[506]Am. Jour. of Science, vol. vii., 1874, p. 92.

[507]Young,The Sun, p. 103.

[508]Ann. Bur. Long., 1879, p. 679.

[509]Ibid., 1878, p. 689.

[510]Himmelsphotographie, p. 273.

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

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 Königsberg 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 themfrom 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 passage of the moonin frontof 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 identity 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 assurance by Secchi—that an uninterrupted stratum of prominence-matter encompasses 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, assumed 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-là en dehors des éclipses!" 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 widenedad libitum. But monochromatic or "bright-line" light is, by its nature, incapable of being so diffused. It can, of course, bedeviatedby 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 shining through our atmosphere—it derives arelativegain in intensity fromevery addition to the dispersive power of the spectroscope with which the heterogeneous mass of beams is analysed. Employ prisms enough, and eventually theundiminishedrays 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 10A.M.next morning, August 19, 1868—the date of the beginning of spectroscopic work at the margin of the unobscured 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 least89,000 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 anticipated; 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 competitors deserves, and has obtained, full credit for his invention. With noteworthy and confident prescience, Lockyer, in 1866, before anything was yet known regarding the constitution 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 assured.

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 passing through the sun's centre,[518]thereby showing that light to be, in whole or in part, reflected sunshine. But if reflected sunshine, 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, Washington, 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 aposition for the line in question notably higher up in the spectrum than that previously assigned 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 shores of the Mediterranean. Janssen issued from beleaguered Paris in a balloon, carrying with him thevital partsof 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 shipwreck in thePsyche, were recompensed with a glimpse of the solar aureola duringone second and a half! Three parties stationed at various heights on Mount Etna saw absolutely nothing. Nevertheless important information was snatched 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 substitution of a bright for every dark line.

Now something of the kind was theoretically necessary to account for the dusky rays in sunlight which have taught us so much, and have yet much more to teach us; so that, although surprising from its transitory splendour, the appearance could not strictly be called "unexpected." Moreover, its premonitorysymptom in the fading out of these rays had been actually described by Secchi in 1868,[526]and looked for by Young as the moon covered the sun in August 1869. But with the slit of his spectroscope placednormallyto the sun's limb, the bright lines gave a flash too thin to catch the eye. In 1870 the position of the slit wastangential—it ran along the shallow bed of incandescent vapours, instead of cutting across it: hence his success.

The same observation was made at Xerez de la Frontera by Mr. Pye, a member of Young's party; and, although an exceedingly delicate one, has since frequently been repeated. The whole Fraunhofer series appeared bright (omitting other instances) to Maclear, Herschel, and Fyers in 1871, at the beginning or end of totality; to Pogson, at the break-up of an annual eclipse, June 6, 1872; to Stone at Klipfontein, April 16, 1874, when he saw "the field full of bright lines."[527]But between the picture presented by the "véritable pluie de lignes brilliantes,"[528]which descended into M. Trépied's spectroscope for three seconds after the disappearance of the sun, May 17, 1882, and thefamiliar one of the dark-line solar spectrum, certain differences were perceiving, showing their relation to be not simply that of a positive to a negative impression.

A "reversing layer," or stratum of mixed vapours, glowing, but at a lower temperature than that of the actual solar surface, was an integral part of Kirchhoff's theory of the production of the Fraunhofer lines. Here it was assumed that the missing rays were stopped, and here also it was assumed that the missing rays would be seen bright, could they be isolated from the overpowering splendour of their background. This isolation is effected by eclipses, with the result—beautifully confirmatory of theory—ofreversing, or turning from dark to bright, the Fraunhofer spectrum. The completeness and precision of the reversal, however, could not be visually attested; and a quarter of a century elapsed before a successful "snap-shot" provided photographic evidence on the subject. It was taken at Novaya Zemlya by Mr. Shackleton, a member of the late Sir George Baden-Powell's expedition to observe the eclipse of August 9, 1896;[529]and similar records in abundance were secured during the Indian eclipse of January 22, 1898,[530]and the Spanish-American eclipse of May 28, 1900.[531]The result of their leisurely examination has been to verify the existence of a "reversing-layer," in the literal sense of the term. It is true that no single "flash" photograph is an inverted transcript of theFraunhofer spectrum. The lines are, indeed, in each case—speaking broadly—the same; but their relative intensities are widely different. Yet this need occasion no surprise when we remember that the Fraunhofer spectrum integrates the absorption of multitudinous strata, various in density and composition, while only the upper section of the formation comes within view of the sensitive plates exposed at totalities, the low-lying vaporous beds being necessarily covered by the moon. The total depth of this glowing envelope may be estimated at 500 to 600 miles, and its normal state seems to be one of profound tranquillity, judging from the imperturbable aspect of the array of dark lines due to its sifting action upon light.

The last of the five eclipses which we have grouped together for separate consideration was visible in Southern India and Australia, December 12, 1871. Some splendid photographs were secured by the English parties on the Malabar coast, showing, for the first time, the remarkable branching forms of the coronal emanations; but the most conspicuous result was Janssen's detection of some of the dark Fraunhofer lines, long vainly sought in the continuous spectrum of the corona. Chief among these was the D-line of sodium, the original index, it might be said, to solar chemistry. No proof could be afforded more decisive that this faintechoing backof the distinctive notes of the Fraunhofer spectrum, that the polariscope had spoken the truth in asserting a large part of the coronal radiance to be reflected sunlight. But it is usually so drenched in original luminosity, that its special features are almost obliterated. Janssen's success in seizing them was due in part to the extreme purity of the air at Sholoor, in the Neilgherries, where he was stationed; in part to the use of an instrument adapted by its large aperture and short focus to give an image of the utmost brilliancy. His observation, repeated during the Caroline Island eclipse of 1883, was photographically verified ten years later by M. de la Baume Pluvinel in Senegal.[532]

An instrument of great value for particular purposes was introduced into eclipse-work in 1871. The "slitless spectroscope" consists simply of a prism placed outside the object-glass of a telescope or the lens of a camera, whereby the radiance encompassing the eclipsed sun is separated into as many differently tinted rings as it contains different kinds of light. These tinted rings were simultaneously viewed by Respighi at Poodacottah, and by Lockyer at Baikul. Theirphotographic registration by the latter in 1875 initiated the transformation of the slitless spectroscope into the prismatic camera.[533]Meanwhile, the use of an ordinaryspectroscope by Herschel and Tennant at Dodabetta showed the green ray of coronium to be just as bright in a rift as in the adjacent streamer. The visible structure of the corona was thus seen to be independent of the distribution of the gases which enter into its composition.

By means, then, of the five great eclipses of 1860-71 it was ascertained: first, that the prominences, and at least the lower part of the corona, are genuine solar appurtenances; secondly, that the prominences are composed of hydrogen and other gases in a state of incandescence, and rise, as irregular outliers, from a continuous envelope of the same materials, some thousands of miles in thickness; thirdly, that the corona is of a highly complex constitution, being made up in part of glowing vapours, in part of matter capable of reflecting sunlight. We may now proceed to consider the results of subsequent eclipses.

These have raised, and have helped to solve, some very curious questions. Indeed, every carefully watched total eclipse of the sun stimulates as well as appeases curiosity, and leaves a legacy of outstanding doubt, continually, as time and inquiry go on, removed, but continually replaced. It cannot be denied that the corona is a perplexing phenomenon, and that it does not become less perplexing as we know more about it. It presented itself under quite a new and strange aspect on the occasion of the eclipse which visited the Western States of North America, July 29, 1878. The conditions of observation were peculiarly favourable. The weather was superb; above the Rocky Mountains the sky was of such purity as to permit the detection of Jupiter's satellites with the naked eye on several successive nights. The opportunity for advancing knowledge was made the most of. Nearly a hundred astronomers, including several Englishmen, occupied twelve separate posts, and prepared for an attack in force.

The question had often suggested itself, and was a natural one to ask, whether the corona sympathises with the general condition of the sun? whether, either in shape or brilliancy, it varies with the progress of the sun-spot period? A more propitious moment for getting this question answered could hardly have been chosen than that at which the eclipse occurred. Solar disturbance was just then at its lowest ebb. The development of spots for the month of July, 1878, was represented on Wolf's system of "relative numbers" by the fraction 0·1, as against 135·4 for December, 1870, an epoch of maximum activity. The "chromosphere"[534]was, for the most part, shallow and quiescent;its depth, above the spot zones, had sunk from about 6,000 to 2,000 miles;[535]prominences were few and faint. Obviously, if a type of corona corresponding to a minimum of sun-spots existed, it should be seen then or never. Itwasseen; but while, in some respects, it agreed with anticipation, in others it completely set it at naught.

The corona of 1878, as compared with those of 1869, 1870, and 1871, was generally admitted to be shrunken in its main outlines and much reduced in brilliancy. Lockyer pronounced it ten times fainter than in 1871; Harkness estimated its light at less than one-seventh that derived from the mist-blotted aureola of 1870.[536]In shape, too, it was markedly different. When sun-spots are numerous, the corona appears to be most fully developed above the spot-zones, thus offering to our eyes a rudely quadrilateral contour. The four great luminous sheaves forming the corners of the square are made up of rays curving together from each side into "synclinal" or ogival groups, each of which may be compared to the petal of a flower. To Janssen, in 1871, the eclipsing moon seemed like the dark heart of a gigantic dahlia, painted in light on the sky; and the similitude to the ornament on a compass-card, used by Airy in 1851, well conveys the decorative effect of the beamy, radiated kind of aureola, never, it would appear, absent when solar activity is at a tolerably high pitch. In his splendid volume on eclipses,[537]with which the systematic study of coronal structure may be said to have begun, Mr. Ranyard first generalised the synclinal peculiarity by a comparison of records; but the symmetry of the arrangement, though frequently striking, is liable to be confused by secondary formations. He further pointed out, with the help of careful drawings from the photographs of 1871 made by Mr. Wesley, the curved and branching shapes assumed by the component filaments of massive bundles of rays. Nothing of all this, however, was visible in 1878. Instead, there was seen, as the groundwork of the corona, a ring of pearly light, nebulous to the eye, but shown by telescopes and in photographs to have a fibrous texture, as if made up of tufts of fine hairs. North and south, a series of short, vivid, electrical-looking flame-brushes diverged with conspicuous regularity from each of the solar poles. Their direction was not towards the centre of the sun, but towards each summit of his axis, so that the farther rays on either side started almost tangentially to the surface.

But the leading, and a truly amazing, characteristic of thephenomenon was formed by two vast, faintly-luminouswingsof light, expanded on either side of the sun in the direction of the ecliptic. These were missed by very few careful onlookers; but the extent assigned to them varied with skill in, and facilities for seeing. By far the most striking observations were made by Newcomb at Separation (Wyoming), by Cleveland Abbe from the shoulder of Pike's Peak, and by Langley at its summit, an elevation of 14,100 feet above the sea. Never before had an eclipse been viewed from anything approaching that altitude, or under so translucent a sky. A proof of the great reduction in atmospheric glare was afforded by the perceptibility of the corona four minutes after totality was over. For the 165 seconds of its duration, the remarkable streamers above alluded to continued "persistently visible," stretching away right and left of the sun to a distance of at least ten million miles! One branch was traced over an apparent extent of fully twelve lunar diameters, without sign of a definite termination having been reached; and there were no grounds for supposing the other more restricted.

The resemblance to the zodiacal light was striking; and a community of origin between that enigmatical member of our system and the corona was irresistibly suggested. We should, indeed, expect to see, under such exceptionally favourable atmospheric conditions as Professor Langley enjoyed on Pike's Peak, therootsof the zodiacal light presenting near the sun just such an appearance as he witnessed; but we can imagine no reason why their visibility should be associated with a low state of solar activity. Nevertheless this seems to be the case with the streamers which astonished astronomers in 1878. For in August, 1867, when similar equatorial emanations, accompanied by similar symptoms of polar excitement, were described and depicted by Grosch[538]of the Santiago Observatory, sun-spots were at a minimum; while the corona of 1715, which appears from the record of it by Roger Cotes[539]to have been of the same type, preceded by three years the ensuing maximum. The eclipsed sun was seen by him at Cambridge, May 2, 1715, encompassed with a ring of light about one-sixth of the moon's diameter in breadth, upon which was superposed a luminous cross formed of long bright branches lying very nearly in the plane of the ecliptic, and shorter polar arms so faint as to be only intermittently visible. The resemblance between his sketch and Cleveland Abbe's drawing of the corona of 1878 is extremely striking. It should, nevertheless, be noted that some conspicuous spots were visible on the sun's discat the time of Cotes's eclipse, and that the preceding minimum (according to Wolf) occurred in 1712. Thus, the coincidence of epochs is imperfect.

Professor Cleveland Abbe was fully persuaded that the long rays carefully observed by him from Pike's Peak were nothing else than streams of meteorites rushing towards or from perihelion; and it is quite certain that the solar neighbourhood must be crowded with such bodies. But the peculiar structure at the base of the streamers displayed in the photographs, the curved rays meeting in pointed arches like Gothic windows, the visible upspringing tendency, the filamentous texture,[540]speak unmistakably of the action of forces proceedingfromthe sun, not of extraneous matter circling round him.

A further proof of sympathetic change in the corona is afforded by the analysis of its light. In 1878 the bright line so conspicuous in the coronal spectrum in 1870 and 1871 had faded to the very limit of visibility. Several skilled observers failed to see it at all; but Young and Eastman succeeded in tracing the green "coronium" ray all round the sun, to a height estimated at 340,000 miles. The substance emitting it was thus present, though in a low state of incandescence. The continuous spectrum was relatively strong; faint traces of the Fraunhofer lines attested for it an origin, in part by reflection; and polarisation was undoubted, increasing towards the limb, whereas in 1870 it reached a maximum at a considerable distance from it. Experiments with Edison's tasimeter seemed to show that the corona radiates a sensible amount of heat.

The next promising eclipse occurred May 17, 1882. The concourse of astronomers which has become usual on such occasions assembled this time at Sohag, in Upper Egypt. Rarely have seventy-four seconds been turned to such account. To each observer a special task was assigned, and the advantages of a strict division of labour were visible in the variety and amount of the information gained.

The year 1882 was one of numerous sun-spots. On the eve of the eclipse twenty-three separate maculæ were counted. If there were any truth in the theory which connected coronal forms with fluctuations in solar activity, it might be anticipated that the vast equatorial expansions and polar "brushes" of 1878 would be found replaced by the star-like structure of 1871. This expectation was literally fulfilled. No lateral streamers were to be seen. The universal failure to perceive them, after express search in a sky of the most transparent purity, justifies the emphatic assertion thatthey were not there. Instead, the type of corona observed in Indiaeleven years earlier, was reproduced with its shining aigrettes, complex texture and brilliant radiated aspect.

Concordant testimony was given by the spectroscope. The reflected light derived from the corona was weaker than in 1878, while its original emissions were proportionately intensified. Nevertheless, most of the bright lines recorded as coronal[541]were really due, there can be no doubt, to diffused chromospheric light. On this occasion, the first successful attempt was made to photograph the coronal spectrum procured in the ordinary way with a slit and prisms, while the prismatic camera was also profitably employed. It served to bring out at least one important fact—that of the uncommon strength in chromospheric regions of the twin violet beams of calcium, designated "H" and "K"; and prominence-photography signalised its improvement by the registration, in the spectrum of one such object, of twenty-nine rays, including many of the ultra-violet hydrogen series discovered by Sir William Huggins in the emission of white stars.[542]

Dr. Schuster's photographs of the corona itself were the most extensive, as well as the most detailed, of any yet secured. One rift imprinted itself on the plates to a distance of nearly a diameter and a half from the limb; and the transparency of the streamers was shown by the delineation through them of the delicate tracery beyond. The singular and picturesque feature was added of a bright comet, self-depicted in all the exquisite grace of swift movement betrayed by the fine curve of its tail, hurrying away from one of its rare visits to our sun, and rendered momentarily visible by the withdrawal of the splendour in which it had been, and was again quickly veiled.

From a careful study of these valuable records Sir William Huggins derived the idea of a possible mode of photographing the coronawithout an eclipse.[543]As already stated, its ordinary invisibility is entirely due to the "glare" or reflected light diffused through our atmosphere. But Huggins found, on examining Schuster's negatives, that a large proportion of the light in the coronal spectrum, both continuous and interrupted, is collected in the violet region between the Fraunhofer lines G and H. There, then, he hoped that, all other rays being excluded, it might prove strong enough to vanquish inimical glare, and stamp on prepared plates, throughlocalsuperiority in illuminative power, the forms of the appendage by which it is emitted.

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