Page:The story of the comets.djvu/229

XII. the first-fruits obtained give promise of a much richer harvest to follow.

In the case of perhaps no class of heavenly bodies has the spectroscope yielded information of so entirely an unexpected nature as in that of comets. The first comet observed spectroscopically was the Comet of 1864 (i.) by Donati, who found it to yield only 3 bright lines, showing the presence of a glowing gas. Huggins and Secchi, in 1866, found Tempel's Comet likewise gave 3 bright lines and a continuous spectrum in addition. No dark lines were perceived in the latter, the light being probably too faint. But in the continuous spectrum yielded by Coggia's Comet of 1874 some dark lines were seen, and therefore it is reasonable to conclude that the continuous spectrum when given by comets is due to reflected sunlight.

It was an important advance thus to learn that the light of the comet came from two sources; the one from the Sun, by reflection; the other from the comet itself. But the spectroscope speedily revealed a further and unsuspected fact: that intrinsic cometary light was due to glowing hydro-carbon vapour. For Huggins and Secchi in examining the head of Winnecke's Comet in 1868 saw 3 shaded bands, besides the continuous spectrum, and on comparing them with the spectrum of olefiant gas, found that they were exactly coincident with the 3 principal bands of the so-called hydro-carbon spectrum, agreeing with them not merely in position, but in general appearance and in the manner in which they faded away. Coggia's Comet yielded the same result. Since then various comets have been subjected to spectroscopic scrutiny with results which are, on the whole, in remarkable accordance. Three comets, however, stand out from the rest, Brorsen's Comet as observed by Huggins in 1868; Comet iii. of 1877 as observed by Copeland at Dun Echt; and Holmes's Comet as observed by Keeler in 1892. Excluding these, all the others have shown 3 bright bands coincident with one or other of the carbon spectra.