1889.] of the various Species of Heavenly Bodies. 223 



The proximity of the brightest part of one band (5586) to the aurora 

 line is notable, but considering that the aurora line is always sharp, 

 Schuster concludes that there is no connexion between the spectrum 

 of the aurora and that of the negative-pole glow of oxygen (quoted 

 by Capron, 'Aurorse,' p. 130). 



Comparison with the Spectrum of Hydrogen. 



Similarly, all attempts to identify the spectrum of the aurora with 

 that of hydrogen, another constituent of our atmosphere (in the form 

 of water vapour), have failed. On this point Capron remarks: 

 " No principal line, and one subsidiary line only,* actually coincide 

 with the aurora spectrum, this last being that to which Dr. Yogel 

 assigns an identical wave-length, viz., 5189 " (' Aurora,' p. 109). 



That this coincidence is of no importance is obvious when it is 

 remembered that there are a great number of such lines in the 

 spectrum of hydrogen, and that no experiments have been recorded 

 indicating that this line is more persistent than the others. 



Comparison with the Spectrum of Phosphoretted Hydrogen. 



Next in importance to comparisons of the aurora spectrum with 

 air spectra is the comparison with the flame of phosphoretted 

 hydrogen, in connexion with Angstrom's suggestion that the 

 characteristic green line may be due to phosphorescence or fluor- 

 escence. The spectrum of phosphoretted hydrogen consists of 

 several bands, the centres of the four brightest being at 526*3, 510*6, 

 560'5 and 599'4 (Lecoq de Boisbaudran, ' Spectres Lumineux,' p. 189). 

 These bands brighten when the flame is artificially cooled, especially 

 the less refrangible ones. 



On this subject, Capron says : " Having regard to the near 

 proximity of the phosphoretted hydrogen band to the bright aurora 

 line, to the circumstance of this band brightening by reduction of 

 temperature (a phenomenon probably connected with ozone), to the 

 peculiar brightening of one line in the green in the " aurora " and 

 " phosphorescent " tubes (the phosphorescent tubes probably con- 

 taining 0), and to the observed circumstance that the electric 

 discharge has a phosphorescent or fluorescent afterglow (isolated, I 

 believe, by Faraday), I feel there is strong evidence in favour of such 

 an origin to the principal aurora line, if not to the red line as well " 

 ('Auroree,'p. 126). 



But the mere fact of one of the phosphoretted hydrogen bands, 

 and that only the third in order of brightness, falling near the 

 characteristic aurora line cannot be supposed to be anything more 



* The subsidiary lines of hydrogen constitute what I described as the structure- 

 spectrum of hydrogen in my paper of November 17, 1887. 



Q 2 



