34 On the Blight Line in the Spectrum of the Aurora Borealis. 



to the luminosity of the atmospheric elements will, in its pas- 

 sage through the non-luminous lower strata of the atmosphere, 

 be very greatly diminished, owing to absorption; whilst the 

 light due to any substance not present in the lower strata will 

 be almost wholly transmitted to the eye. Thus the relative in- 

 tensity of the rays due to the different kinds of matter will, 

 when they reach the eye, be very different from their relative in- 

 tensity before their passage through the atmosphere*. 



The absence of a corresponding dark line in the solar spec- 

 trum shows, I think, that the quantity of the gaseous matter is 

 very small; and this, taken in conjunction with the brightness 

 of the line in the auroral spectrum, shows that it is confined to 

 the higher parts of the atmosphere. 



Auroras are known to occur in the lower parts of the atmo- 

 sphere, in the region of the clouds ; and some have been observed 

 so low as to appear against a mountain as a background. It 

 would be interesting to know whether such auroras would exhibit 



o 



the same spectrum as that observed by M. Angstrom. 



o 



M. Angstrom asserts, as a deduction from his observations, 

 that the aurora is not due to electricity. The only other way 

 in which it seems possible to account for luminous matter in the 

 atmosphere is by supposing chemical action to be taking place, 

 and, as all chemical action between the constituents of the atmo- 

 sphere must have ceased long ago, if ever there was any chemi- 

 cal affinity between them ; we must suppose that this chemical 

 action is taking place between the elements of the atmosphere 

 and the newly introduced gaseous matter. I do not, however, 



6 



see that M. Angstrom's observation affords any ground for be- 

 lieving that the aurora is not the light due to electrical discharges. 



{ A. S. Davis. 



Roundhay Vicarage. 

 June 10, 1870. 



* I wish here to point out that the light from a nebula may be due to 

 only one of the different gases composing it ; for the light due to the other 

 gas may be wholly absorbed in passing outwards through its non-lumi- 

 nous portions. The presence of lines in the spectrum of a nebula due 

 only to one kind of matter, is therefore no proof that it is not composed 

 of more than one kind of matter. 



