226 Mr. J. N. Lockyer. On the Classification [Jan. 10, 



quently what experiments have now conclusively proved the presence 

 of iron. 



Mr. Gaprons Conclusions. 



In reviewing the above theory to explain the origin of aurora 

 up to 1879, Mr. Band Capron makes the following statement : " As 

 the general result of spectrum work on the aurora up to the present 

 time, we seem to have quite failed in finding any spectrum which, 

 as to position, intensity, and general character of lines, well coincides 

 with that of the aurora. Indeed, we may say we do not find any 

 spectrum so nearly allied to portions even of the aurora spectrum as 

 to lead us to conclude that we have discovered the true nature of one 

 spectrum of the aurora (supposing it to comprise, as some consider, 

 two or more). The whole subject may be characterised as still a 

 scientific mystery." (' Auroras,' p. 171.) 



II. Lemstroms Observations. 



The next contribution to our knowledge of aurora spectra of any 

 importance is that of Lemstrom's (' L'Aurore Boreale,' 1886). All 

 previous observers who attempted to identify the spectrum of the 

 aurora with that of atmospheric air failed to do so, but Lemstrom 

 asserts (p. 158) that the twelve lines which have been recorded in 

 aurora spectra are nearly all seen in the spectrum of a Geissler tube 

 containing the same gases as those constituting our atmosphere. The 

 differences in the relative intensities he believes to be due to condi- 

 tions of temperature and pressure.* Although the auroral line 

 (wave-length 557) does not agree perfectly with the line at 558 seen 

 in the spectrum given by his appareil de Vaurore boreale (air vacuum- 

 tubes illuminated by sparks from a Holtz machine), he regards the 

 atmospheric origin of the aurora spectrum as completely demon- 

 strated. He states (p. 138) that the characteristic line of the aurora 

 spectrum is always seen in the light produced by the discharge of an 

 electric current (by means of his appareil $ ecoulement) from the top 

 of a mountain. He gives a table of auroral lines compared with the 

 lines in the spectra of rarefied air, as observed by himself, and by 

 Vogel and Sundell under other conditions. The air lines recorded by 

 Yogel nearly all coincide with lines recorded as oxygen lines by 

 Schuster (' Phil. Trans.,' 1879) ; but it is important to note that 

 some of the strongest lines mapped by Schuster are absent from 

 Vogel's list (see fig. 16). So that, even if we allow that some of the 

 aurora lines fall near lines of oxygen, the absence of the brightest 

 oxygen lines from the spectrum is sufficient evidence for us to conclude 



* " Si 1'on se demande pourquoi on ne yoit point, dans 1'aurore polaire toutes les 

 raies existant dans ces gaz, 1' experience repond que les raies des gaz changent 

 selon la temperature et la pression de ces gaz." ('L'Aurore Borgale,' p. 158.) 



