Page:The story of the comets.djvu/235

XII. Copeland and Lohse followed the comet day by day till its perihelion passage on June 10, and had the gratification of seeing the bright sodium lines, D, develope in the most striking manner. On June 6, the D lines were seen beautifully double, and the continuous spectrum was so much the less important, the light of the comet was so nearly mono- chromatic, that with a wide slit Lohse was able to see "the perfect image of the comet, head and tail, . . . in the light of the D line, very bright and clear. . . . The sight was really magnificent; exactly like a prominence".

Other bright lines were observed, especially in the red, and there were faint traces of the ordinary carbon bands. But the sodium light was predominant, and, next to that, the general continuous spectrum.

The Great Comet of 1882 approached the Sun much nearer than Wells's Comet had done; and inferring that the development of sodium light in the spectrum of that body had been due to the heat it experienced as it approached the Sun, Copeland and Lohse naturally expected to find the D lines bright also in the spectrum of this new visitor. The inference was justified. "The expected bright sodium lines exhibited themselves to the eye of the observer with a brilliancy and neatness quite comparable to the C line in prominences, so well defined and clear did they stand out on the bright daylight spectrum." But besides the D lines a number of other bright lines were seen, notably the E lines and some other prominent iron lines, together with 5 lines in the red which had no counterparts in the dark lines of the solar spectrum.

A beautiful illustration was afforded by this comet of the effect of the motion of a body on the lines of its spectrum. The comet was receding from the Earth at this time, and the lines of its spectrum were therefore displaced a little towards the red, the bright D and E lines being seen therefore just on the redward side of their dark counterparts in the solar spectrum. This observation was not only made by Copeland and Lohse, at Dun Echt, but on the same day, Sept. 18, by Thollon and Gouy at Nice; and in both cases the