﻿236 Properties of the Rays producing Aurora Borealis. 



fairly broad band, whicb. in the green spectrum may cover 

 as much as 500 A. 



The definite sharp lines observed in the auroral spectrum 

 are therefore no doubt due to light produced in the atmo- 

 sphere, and do not originate from the radiation itself. 



Concluding Remarks. 



From the study of the structure of the radiant forms of 



aurora, we found that the rays producing them had a number 



of characteristic properties in common with a-rays. 



The investigation with regard to diurnal distribution 

 to & 



showed that, so far as our present knowledge goes, the 

 assumption of a positive charge is not only a legitimate one, 

 but is even the most favourable for the explanation of the 

 occurrence of drapery forms. 



From the point of view of the radiation hypothesis the 

 extremely thin drapery bands have to be produced by a 

 strictly homogeneous radiation, and the parallel bands were 

 explained in a simple way by assuming groups of homogeneous 

 rays to emanate from the same source. This explanation 

 gave a strong argument in favour of the view that the solar 

 rays are not merely of the a-ray type, but are indeed a-rays 

 of some sort given out from radioactive substances. 



With regard to the position of aurora?, we found that the 

 stiffness of ordinary a-rays gave the right distance from the 

 magnetic axis, and their penetrating power was sufficient to 

 account for the altitude of a great part of the aurora? 

 observed. 



Thus, as far as our investigations have been carried, the 

 ordinary a-rays are found to possess the right charge, scatter- 

 ing, stiffness, and penetrating power, which are necessary to 

 explain most characteristic properties of a great number of 

 aurora?. 



But still we ought to be aware of the possibility that not 

 all auroral forms are necessarily produced in the same way 

 and by the same radiation. It is quite possible that aurora? 

 in certain cases may partly be secondary effects of precipi- 

 tations of solar rays, and that certain diffuse forms may be 

 produced by corpuscular rays. And even in the case of the 

 radiant forms with straight-lined structure, it is still a matter 

 for further investigation to determine in each case the atoms 

 or molecules which form the carriers of the electric rays. 



Provided relation (2) holds good for all possible velocities, 

 we found that an ordinary a-particle, even with the velocity 

 of light, could not reach within a distance of 27 km. from 



