1889.] principal Line in the Spectrum of the Aurora. 431 



diameter. The definition is very good. Though the automatic 

 arrangement works well, I always take the precaution to measure 

 only small differences of position of the line to be determined from 

 lines near it, the wave-lengths of which are known. 



The observations were made on February 4, 1874. There was a 

 brilliant Aurora, showing a whitish light ; a direct- vision spectro- 

 scope resolved this light into a brilliant line in the yellow and a 

 faint continuous spectrum. 



The " Grubb " spectroscope was directed from the window of the 

 observatory upon the brightest part of the Aurora. In the first 

 instance, an estimation by eye was made of the position of the bright 

 line by comparing it in the instrument with the spectrum of a spirit 

 lamp. The bright line was seen to fall on the more refrangible side 

 of the line for which Watts gives the wave-length 5582,* Angstrom 

 and Thalen 5583,t by from one-fifth to one-fourth of the distance of 

 this line from the beginning of the band. If we take one-fourth, we 

 have X5569-6; one-fifth gives \5572'3. The mean of these values 

 gives for the 



Aurora line X 5570'9 (1). 



The cross-wires of the spectroscope were then brought upon the 

 line, and the reading 3476 showed the line to fall about midway 

 between two strong lines in the spectrum of tin, X 5564 and X 5587 

 respectively, according to my measures. J The position of the cross 

 was then compared directly with those lines in the spectrum of an 

 induction spark taken between electrodes of tin. The further details 

 of this comparison are not given in my note-book, but the result 

 only, which placed the 



Aurora line at X 5571 (2). 



Consulting my map of the chemical elements, I found that there 

 was a line of tellurium very near this place, namely, at X 5575, I 

 therefore brought the spark from tellurium before the slit, when the 

 cross appeared on the more refrangible side of the tellurium line. 

 The measure of the distance of the cross from this line came out 

 equal to X 0003. The place given in my paper for this line of tellurium 

 is 5575. Thalen gives for the same line 5574'1. if we take the 

 mean of these values and deduct 0003, we get for 



The line of the Aurora X 5571'5 (3). 



There are strong lines of iron very near this position in the 



* ' Phil. Mag.,' vol. 41, 1871, p. 14. 



f " Spectres des Metalloides," 'Nov. Act. Soc. Sci. Upsal.,' vol. 9, 1875 (p. 29). 



J "Spectra of the Chemical Elements," ' Phil. Trans.,' 1864, p. 139. 



' Brit. Assoc. Kep.,' 1885, p. 292. 



