432 Dr. W. Huggins. On the Wave-length of the [Mar. 7, 



spectrum, and I made use of these also for a further determination of 

 the place of the Aurora line. The cross, after having been placed 

 upon the line of the Aurora, was confronted with these lines in the 

 spectrum of iron. 



The condensed account in my note-book does not give further 

 particulars of this comparison, but states only that the place of the 



Aurora line came out X 5571*5 (4). 



Summing up these determinations we have 



(1) Eye-estimation X 5570*9 



(2) From tin 5571/0 



(3) From tellurium 5571'5 



(4) From iron 5571'5 



From these values I think that we are justified in taking for the 

 Aurora line, as a position very near the truth, 



X 5571 0-5 (5). 



Among the numerous determinations of other observers, those of 

 Professor H. C. Vogel in 1872* seem to me to have great weight. A 

 direct-vision spectroscope with a set of five prisms was used. The 

 reduction of the readings of the micrometer into wave-lengths was 

 based upon the repeated measures of 100 lines of the solar spectrum. 



The screw had been thoroughly examined. After each observation 

 of the Aurora line, readings were taken of the lines of sodium or of 

 hydrogen. The observations extended over four nights. On three 

 nights four separate readings were obtained ; on the fourth night 

 two only. Vogel gives as the mean result of the fourteen obser- 

 vations, 



Aurora line X 5571'3 0'92 (6). 



Perhaps I should state that I find, from a remark in my note-book, 

 that at the time of my observations, in 1874 I was not aware of 

 Vogel's results, and I could not, therefore, have been biassed in any 

 way by them. 



The recent observations on the spectrum of the Aurora by Gyllen- 

 skiold, at Cap Thordsen, in 1882, deserve special mention.f With 

 a Hoffmann spectroscope, furnished with a scale, he obtained at Cap 

 Thordsen in 1882 a mean result of A, 5568 1'6; later, in 1884, at 

 Upsala, with a Wrede spectroscope furnished with a micrometer 

 screw, a mean value for the Aurora line, X 5569 + 6'2. J Gyllenskiold 



* ' Leipzig Math. Phys. Berichte,' vol. 22, p. 285. 



f ' Observations faites au Cap Thordsen, Spitzberg, par 1' Expedition Suedoise,' 

 vol. 2, I : Aurores Boreales, par Carlheim-G-yllenskib'ld. Stockholm, 1886. 

 J Ibid., p. 166. 



