A translation of Dr. Vogel’s interesting paper will be found printed in extenso in Appendix E, and his lithographed drawings of the spectrum in the green and red portions of the Aurora respectively on Plate VI. figs. 2 and 3. The observations of April 9th by Dr. Vogel are probably, up to the present time, the most exact of any one Aurora, and I have therefore in most cases used them for comparison.

Mr. Backhouse’s catalogue of lines.

Mr. Backhouse, in a letter to ‘Nature,’ commenting upon my catalogue of lines, gave the following as the latest determinations from his own observations:—

No. 1.Wave-length6060
2.”5660
3.”5165
4.”5015
6.”4625
7.”4305

(6060 must be a mistake for 6260, and 5660 for 5560.—J. R. C.) Mr. Backhouse never saw a line at 5320 again. He found the continuous spectrum to reach from No. 2 to No. 7, being brightest from a little beyond No. 2 to No. 6. This part of the spectrum did not give him so much the idea of a true “continuous spectrum” as of a series of bright bands too close to be distinguished.

Subsequent full catalogue of Auroral lines.

I have subsequently, in another section of this Chapter, added a full catalogue of the Auroral lines, prepared by myself from the foregoing and other sources and observations; and I also append to it a Plate [Plate XII.], in which these lines are positioned and the wave-lengths and names of observers are given. The numbers of the lines on the Plate correspond with those in the catalogue. The solar spectrum and the spectrum of the blue base of a candle-flame are added for purposes of comparison. [The telluric bands in the solar spectrum are shown more distinctly than they actually appear, and do not profess to give details.]

Flickering of the Green Line.

Flickering of the green line. Herschel’s observation. J. R. Capron’s observation.

A. S. Herschel noticed this, April 9th (1871?). He says:—“A remarkable circumstance connected with the appearance of the single line observed on this occasion was the flickering and frequent changes with which it rose and fell in brightness; apparently even more rapidly than the swiftly travelling waves, or pulsations of light, that repeatedly passed over the streamers, near the northern horizon, towards which the spectroscope was directed.” In the spectrum of the Aurora of 20th October, 1870, I saw and noted the green line as “considerably flickering;” and in the Aurora of 4th February, 1872, I again saw and noted “the peculiar flickering” I had remarked in 1870. I have not seen the peculiarity noted by other observers.