﻿the Rays producing Aurora Borealis. 233 



For a-rays from radium emanation we find a> = 16°'5. This 



result is a most important one when taken in connexion with the 

 position of the auroral zone. According to the chart given 

 by Fritz, the auroral zone for equal frequency on the northern 

 and southern sky is situated at an angular distance from the 

 magnetic axis of about 17° or 18°, consequently a-rai/s of the 

 ordinary kind have just the proper stiffness to give the right 

 position of the auroral zone. 



The relative position of the bands corresponding to a-rays 

 from the radium family is given in the following table : — 



Table IV. 



Substance. 



Distance from 



Distance between 



Apparent angle 



Ra band. 



successive bands. 



between succes. bands. 



Ka 



km. 











24 km. 



11°5 



Pol 



24 „ 



15 „ 



7°-2 





KaEni. ... 



39 „ 











25 „ 



ll°-9 



EaA 



64 „ 











69 „ 



33° 



i EaC 



1 



133 „ 







The bands are supposed to be seen near the zenith at a 

 height of 120 km. 



The material at my disposal gives no means of any exact 

 comparison with observations. Such comparison, however, 

 may be possible by the development of the photographic 

 method. But this much can already be said, that the values 

 found are of the ri°ht order, or, differences of stiffness as those 

 of a-rays from ordinary radioactive substances are competent 

 to explain the distances between successive bands actually 

 observed. 



The Light of Aurora. 



The various lines in the auroral spectrum have not yet 

 been interpreted with any certainty by means of spectra 

 physically known. The greater number of lines have pro- 

 bably rightly been identified with lines of the spectrum of 

 air and some of the rare gases ; but the origin of the most 

 conspicuous line with the wave-length X = 5570 is still 

 unknown. 



As early as 1874 Angstrom * came to the conclusion 



o 



* Angstrom, Pogg. Ann. JubcUnl. p. 424 (1874.). 

 Phil. Mag. S. 6. Vol. 23. No. 134. Feb, 1912. U 



