[Sidenote: Lines nine in number.] The lines have been classified and arranged by Lemström and others as nine in number, but I believe not more than seven have ever been seen simultaneously. The author of the article “Aurora Polaris,” in the ‘Encyclopædia Britannica,’ classes the lines as nine, and gives a table with the following results (to these I have added Herr Vogel’s lines, for the purpose of identification and comparison):— [Sidenote: Table from Encyc. Brit.] +------+-------------+------+--------+-------+ |No. of| Number of | Mean |Probable|Vogel’s| |line. |observations.| W.L. | error. |lines. | +------+-------------+------+--------+-------+ | 1. | 5 | 6303 | ± 8·1 | 6297 | | 2. | 10 | 5569 | ± 2·9 | 5569 | | 3. | 4 | 5342 | ±16 | 5390 | | 4. | 6 | 5214 | ± 5·4 | 5233 | | 5. | 4 | 5161 | ± 9·7 | 5189 | | 6. | 6 | 4984 | ±11 | 5004 | | 7. | 4 | 4823 | ± 9·3 | | | 8. | 8 | 4667 | ± 9·8 | 4663 | | 9. | 8 | 4299 | ± 9·3 | | +------+-------------+------+--------+-------+ The probable errors are large, and it is a question whether any thing is gained by thus endeavouring to average the lines. [Sidenote: Ångström’s line. Zöllner’s line in the red. Other Lines of the spectrum.] The principal and brightest line, in the yellow-green, is generally called “Ångström’s,” and his (probably the first) measurement of its position at 5567 adopted. This was in the winter of 1867-68, and he saw in addition, by widening the slit, traces of three very feeble bands situated near to F. Zöllner is credited with the first observation of the line in the red. These two lines are generally described as with similar characteristics, and in about the same respective positions, by all observers, and have never been remarked to spread into bands. The other lines in the spectrum are difficult to position, owing to the many discordant observations of them. They seem also variable in intensity as well as in number (sometimes even in the same Aurora), and are not unfrequently observed to have their places supplied by bands. [Sidenote: Second German expedition observations. Austro-Hungarian.] The spectroscope was used in the second German expedition, but only the one brightest line seen—Dr. Börgen stating he had never seen a trace of the weak lines in the blue and red, which were observed so distinctly with the same spectroscope on 25th October, 1870, after the return of the expedition. Lieutenant Weyprecht used a small spectroscope during the Austro-Hungarian Expedition, and saw only the well-known yellow-green line. [Sidenote: Swedish expedition, 1868. Lemström’s observations.]