[Footnote 367: _Ibid._, vols. cxliii., p. 558, cxlvi., p. 505.]

[Footnote 368: _Observations on Light and Colours_, p. 35.]

[Footnote 369: _Phil. Trans._, vol. lxxv., p. 190.]

[Footnote 370: _Denkschriften_ (Munich. Ac. of Sc.), 1814, 1815, Bd. v., p. 197.]

[Footnote 371: _Edinburgh Journal of Science_, vol. v., p. 77. See also _Phil. Mag._, Feb., 1834, vol. iv., p. 112.]

[Footnote 372: _Ed. Phil. Trans.,_ vol. xxi., p. 411.]

[Footnote 373: _On the Absorption of Light by Coloured Media, Ed. Phil.

Trans._, vol. ix., p. 445 (1823).]

[Footnote 374: _Phil. Mag._, vol. xxvii, (ser. iii.), p. 81.]

[Footnote 375: _Report Brit. a.s.s._, 1835, p. 11 (pt. ii.). _Electrodes_ are the terminals from one to the other of which the electric spark pa.s.ses, volatilising and rendering incandescent in its transit some particles of their substance, the characteristic light of which accordingly flashes out in the spectrum.]

[Footnote 376: _Phil. Mag._, vol. xx., p. 93.]

[Footnote 377: _Annalen der Physik_, Bd. cxiii., p. 357.]

[Footnote 378: _Phil. Trans._, vol. xcii., p. 378.]

[Footnote 379: _Denkschriften_, Bd. v., p. 202.]

[Footnote 380: _Ibid._, p. 220; _Edin. Jour. of Science_, vol. viii., p.

9.]

[Footnote 381: _Denkschriften_, Bd. v., p. 222.]

[Footnote 382: _Arch. des Sciences_, 1849, p. 43.]

[Footnote 383: _Phil. Trans._, vol. cl., p. 159, _note_.]

[Footnote 384: _Ed. Phil. Trans._, vol. xii., p. 528.]

[Footnote 385: _Phil. Trans._, vol. cxxvi., p. 453. "I conceive," he says, "that this result proves decisively that the sun's atmosphere has nothing to do with the production of this singular phenomenon" (p. 455).

And Brewster's well-founded opinion that it had much to do with it was thereby, in fact, overthrown.]

[Footnote 386: _Monatsberichte_, Berlin, 1859, p. 664.]

[Footnote 387: _Abhandlungen_, Berlin, 1861, pp. 80, 81.]

[Footnote 388: _Ibid._, 1861, p. 77; _Annalen der Physik_, Bd. cxix., p.

275. A similar conclusion, reached by Balfour Stewart in 1858, for heat-rays (_Ed. Phil. Trans._, vol. xxii., p. 13), was, in 1860, without previous knowledge of Kirchhoff's work, extended to light (_Phil. Mag._, vol. xx., p. 534); but his experiments wanted the precision of those executed at Heidelburg.]

[Footnote 389: _Miscellaneous Works_, vol. i., p. 189.]

[Footnote 390: _Ed. Phil. Trans._, vol. ix., p. 458.]

[Footnote 391: _Ibid._, vol. xii., p. 519.]

[Footnote 392: _Quart. Jour. Chem. Soc._, vol. x. p. 79.]

[Footnote 393: A facsimile accompanied Sir H. Roscoe's translation of Kirchhoff's "Researches on the Solar Spectrum" (London, 1862-63).]

[Footnote 394: Estimated by Kirchhoff's at a _trillion to one_.

_Abhandl._, 1861, p. 79.]

[Footnote 395: _Phil. Mag._, vol. xxvii. (3rd series), p. 90.]

[Footnote 396: _L'Inst.i.tut_, Feb. 7, 1849, p. 45; _Phil. Mag._, vol.

xix. (4th series), p. 193.]

[Footnote 397: _Ann. d. Phys._, vol. cxviii., p. 110.]

[Footnote 398: _Phil. Mag._, vol. ix. (4th series), p. 327.]

[Footnote 399: Spectra may be produced by _diffraction_ as well as by _refraction_; but we are here only concerned with the subject in its simplest aspect.]

[Footnote 400: _Astrologia Gallica_ (1661), p. 189.]

[Footnote 401: _Pos. Phil._, vol. i., pp. 114, 115 (Martineau's trans.).]

[Footnote 402: _Proem Astronomiae Pars Optica_ (1640), _Op._, t. ii.]

[Footnote 403: _Pop. Vorl._, pp. 14, 19, 408.]

[Footnote 404: _Pos. Phil._, p. 115.]

CHAPTER II

_SOLAR OBSERVATIONS AND THEORIES_

The zeal with which solar studies have been pursued during the last half century has already gone far to redeem the neglect of the two preceding ones. Since Schwabe's discovery was published in 1851, observers have multiplied, new facts have been rapidly acc.u.mulated, and the previous comparative quiescence of thought on the great subject of the const.i.tution of the sun, has been replaced by a bewildering variety of speculations, conjectures, and more or less justifiable inferences. It is satisfactory to find this novel impulse not only shared, but to a large extent guided, by our countrymen.

William Rutter Dawes, one of many clergymen eminent in astronomy, observed, in 1852, with the help of a solar eye-piece of his own devising, some curious details of spot-structure.[405] The umbra--heretofore taken for the darkest part of the spot--was seen to be suffused with a mottled, nebulous illumination, in marked contrast with the striated appearance of the penumbra; while through this "cloudy stratum" a "black opening" permitted the eye to divine farther unfathomable depths beyond. The _hole_ thus disclosed--evidently the true nucleus--was found to be present in all considerable, as well as in many small maculae.

Again, the whirling motions of some of these objects were noticed by him. The remarkable form of one sketched at Wateringbury, in Kent, January 17, 1852, gave him the means of detecting and measuring a rotatory movement of the whole spot round the black nucleus at the rate of 100 degrees in six days. "It appeared," he said, "as if some prodigious ascending force of a whirlwind character, in bursting through the cloudy stratum and the two higher and luminous strata, had given to the whole a movement resembling its own."[406] An interpretation founded, as is easily seen, on the Herschelian theory, then still in full credit.

An instance of the same kind was observed by Mr. W. R. Birt in 1860,[407] and cyclonic movements are now a recognised feature of sun-spots. They are, however, as Father Secchi[408] concluded from his long experience, but temporary and casual. Scarcely three per cent. of all spots visible exhibit the spiral structure which should invariably result if a conflict of opposing, or the friction of unequal, currents were essential, and not merely incidental to their origin. A whirlpool phase not unfrequently accompanies their formation, and may be renewed at periods of recrudescence or dissolution; but it is both partial and inconstant, sometimes affecting only one side of a spot, sometimes slackening gradually its movement in one direction, to resume it, after a brief pause, in the opposite. Persistent and uniform notions, such as the a.n.a.logy of terrestrial storms would absolutely require, are not to be found. So that the "cyclonic theory" of sun-spots, suggested by Herschel in 1847,[409] and urged, from a different point of view, by Faye in 1872, may be said to have completely broken down.

The drift of spots over the sun's surface was first systematically investigated by Carrington, a self-const.i.tuted astronomer, gifted with the courage and the instinct of thoughtful labour.