﻿230 Mr. L. Vegard on the Properties of 



^) = 180° + « and 180° — a may equally well correspond to a 

 theoretical maximum ; and thus, if we consider the aurorse 

 regardless of their peculiar forms, the theory gives no reason 

 for supposing orbits corresponding to 4>=180° — a more 

 probable than orbits corresponding to <E>=180 o + a. 



So far the theory of the diurnal distribution leaves the 

 question with regard to the sign of the charge undecided. 



If, however, we take into account the theoretical conditions 

 for the formation of various forms, e. g. the draperies, we 

 may get a step further. 



Stormer, from his mathematical investigations, has suc- 

 ceeded in giving a fairly simple explanation of the formation 

 of the thin and long drapery bands. Regarding details I 

 must refer to his papers *. It may here be sufficient to 

 notice that it is essential for his explanation that has a 

 great value, at least greater than 180°. If the rays correspond 

 to the interval (I,) <£ must be in the interval 180°«I>^360 o , 

 and if they follow some infinite orbit of the interval (I2), 

 <£ might be even greater. 



Thus, according to Stormer, the draperies, so far from 

 being produced by the simplest orbits, are rather caused by 

 rays following the more complicated ones, and thus the reason 

 for supposing that a negative radiation produces the draperies 

 loses its foundation. 



In all cases investigated most drapery forms are found 

 near the evening maximum : if so a positive radiation 

 is the more likely ; for, in the case of a positive radiation, 

 the production of an evening maximum of drapery forms 

 would only require <£ >■ 180°, while in the case of a negative 

 radiation it would require <I> :> 360 . 



Explanation of Equidistant Bands. 



8. The explanation of a thin drapery band given by Stormer 

 requires a radiation which is strictly homogeneous. 



But, whatever the explanation may be with regard to details 

 if we at all stick to the hypothesis that aurorgeare formed by 

 electric radiation from the sun, a homogeneous radiation 

 will be necessary to account for the very sharp boundaries 

 of extremely thin drapery bands ; for a heterogeneous radia- 

 tion composed of rays whose stiffness varies fairly continuously 

 between wide limits will produce a broad and diffuse band. 

 This follows from the law of motion of electric rays in the 

 earth's magnetic field, because a variation of stiffness will 

 correspond to a motion of the point where the ray strikes the 



* Arch, des Sciences, &e. p. 336 (1907). 



