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 Could the veil be withdrawn, some indications as to the originating temperature of the solar spectrum might be gathered from its range, since the proportion of quick vibrations given out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.
[Footnote 596: _Phil. Mag._, vol. xlii., p. 380, 1871.]
[Footnote 597: _Astr. Nach._, No. 3,053, _Amer. Jour._, vol. xlii., p. 162; Deslandres, _Comptes Rendus_, t. cxiii., p. 307.]
[Footnote 598: _Proc. Roy. Society_, vol. lxi., p. 433.]
[Footnote 599: _Phil. Mag._, vol. xlii., p. 377.]
[Footnote 600: Frost-Scheiner, _Astr. Spectroscopy_, pp. 184, 423.]
[Footnote 601: _Proc. Roy. Soc._, vol. xvii., p. 302.]
[Footnote 602: _Astr. Nach._, No. 1,769.]
[Footnote 603: _Am. Jour. of Science_, vol. xv., p. 85.]
[Footnote 604: _Journ. Franklin Institute_, vol. xl., p. 232_a_.]
[Footnote 605: _Pogg. Annalen_, Bd. cxlvi., p. 475; _Astr. Nach._, No. 3,014.]
[Footnote 606: _Astr. Nach._, Nos. 3,006, 3,037.]
[Footnote 607: This device was suggested by Janssen in 1869.]
[Footnote 608: _Astr. and Astrophysics_, vol. xi., pp. 70, 407.]
[Footnote 609: _Astr. and Astrophysics_, vol. xi., p. 604.]
[Footnote 610: _Comptes Rendus_, t. cxiii., p. 307.]
[Footnote 611: _Astr. and Astrophysics_, vol. xi., p. 50.]
[Footnote 613: Wiedemann's _Annalen der Physik_, Bd. xxv., p. 80.]
[Footnote 614: Evershed, _Knowledge_, vol. xxi., p. 133.]
[Footnote 615: Secchi, _Le Soleil_, t. ii., p. 294.]
[Footnote 616: Lockyer, _Chemistry of the Sun_, p. 418.]
[Footnote 617: _L'Astronomie_, August, 1884, p. 292 (Riccò); see also Evershed, _Jour. British Astr. Ass._, vol. ii., p. 174.]
[Footnote 618: Averaging about 100 miles across and 300 high. _Le Soleil_, t. ii., p. 35.]