1889.] of the various Species of Heavenly Bodies. 221 



nitrogen at wave-length 5224, of another with an oxygen line at 5189, 

 and of another with the strong nitrogen line 5004. The red line in 

 the spectrum he regards as having the same origin as the group of 

 lines in the spectrum of nitrogen which extends from 6218 to 6620, 

 and brightens towards the violet end, the change in appearance being 

 due to the faintness of the aurora. This, however, is not likely to be 

 the case, as the red line has been seen both bright and sharp (R. H. 

 Proctor, "Aurora," 'Encycl. Brit.,' 9th edit.). 



In the same paper, Yogel shows the close coincidences between the 

 aurora lines and lines in the spectrum of iron, but considers it more 

 in accordance with probability to regard the aurora spectrum as a 

 modification of the spectrum of atmospheric air. 



Angstrom's further Observations and Conclusions. 



In a later paper ('Nature,' vol. 10, p. 210), Angstrom arrives at 

 conclusions which may be thus briefly stated : 



(1.) That the aurora has two different spectra, one consisting of 

 the characteristic line, and the other consisting of the fainter lines. 



(2.) That the coincidences of the bright green line with a faint 

 line in the spectrum of air, as determined by Dr. Vogel, is purely 

 accidental, and also that there is no coincidence of any importance 

 with any member of the hydrocarbon group in which it falls. 



(3.) That the bright line is probably due to fluorescence or phos- 

 phorescence. 



(4.) That Vogel's theory of unknown conditions of temperature 

 and pressure being competent to produce the change from the ordi- 

 nary experimental spectrum of air to that given by the aurora, is 

 inadmissible. (Angstrom regarded the spectrum of a gas as invari- 

 able.) 



(5.) That moisture may be neglected in considering the nature of 

 the aurora spectrum. 



He describes an experiment on a glow equivalent to the glow of the 

 negative pole of an air vacuum-tube, in which the spectrum obtained 

 showed close coincidences with three faint lines in the aurora spectrum. 

 A layer of phosphoric anhydride is spread over the bottom of a flask 

 fitted with platinum wires ; after exhaustion with an air-pump, the 

 current from an induction coil is passed between the two platinums. 

 The flask then becomes filled with a violet light like that which, 

 under ordinary conditions, only appears at the negative pole. The 

 spectrum of this light shows the following close coincidences with 

 that of the aurora : 



VOL. XLV. 



