[ 33 ] 



IV. On a possible Cause of the Bright Line observed by M. 

 Angstrom in the Spectrum of the Aurora Borealis. By A. S. 

 Davis, B.A., Mathematical Master, Leeds Grammar School. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IN a letter upon "A Theory of Nebulae and Comets," published 

 in your Magazine for last month, I endeavoured to show 

 that there are grounds for believing that the solid or liquid 

 bodies composing a meteoric band are surrounded by very rare 

 and extensive atmospheres. I assumed that the meteoric bodies 

 are so numerous and the atmosphere of each so extensive, that 

 in the neighbourhood of the sun, and as far from it as the tails 

 of comets are formed, the atmospheres of the different bodies 

 encroach upon one another, and so form a continuous envelope 

 of gaseous matter about the sun. As the tails of comets are 

 known to extend to a much greater distance from the sun than 

 the distance of the earth, we must admit that the earth is moving 

 through this gaseous envelope. Though the matter which forms 

 it is exceedingly rare, yet it must be much condensed in the 

 neighbourhood of a large attracting body like the earth. The 

 question then arises, is there any evidence of the existence of 

 such gaseous matter in our atmosphere ? 



I think that the spectrum of the aurora borealis indicates the 

 existence of this matter in the higher regions of the atmosphere. 



The spectrum of an aurora observed by M. Angstrom* con- 

 sists mainly of one bright line not belonging to any known ter- 

 restrial substance, besides several very faint atmospheric lines 

 and some faint bands of light. This proves that there exists in 

 the upper regions of the atmosphere a kind of matter not known 

 to exist in an appreciable quantity in the lower strata. More- 

 over, M. Angstrom has found the same bright line in the spec- 

 trum of the zodiacal light, which shows that this matter is of 

 the same kind as that which exists in the sun's envelope. 



From the great superiority in the brightness of this line in 

 the auroral spectrum, compared with the atmospheric lines, we 

 might be led to suppose that the matter to which it is due either 

 exists in larger quantities than the elements of the atmosphere 

 in those regions from which the light comes, or else that the 

 electrical currents render it more luminous than the other matter 

 present. 



This, however, is not necessarily the case ; for the light due 



* Poggendorff's Annalen, May 1869, and Phil. Mag. September 1869. 

 Phil. May. S. 4. Vol. 40. No. 264. July 1870. D 



