﻿Notices respecting New Books. 335 



observed ratio of the distance between the components 

 of H a and EL. It seems also difficult to reconcile with the 

 observed unequal intensity of the components. Ifc may- 

 be mentioned here that there is perhaps another way of 

 explaining the observed doubling without introducing new 

 assumptions as to a complicated internal structure of the 

 hydrogen nucleus. For small velocities of the electron, 

 the calculation gives the same result whether the orbits 

 are assumed to be circular or not ; but taking the variation 

 of the mass into account, it can be shown that for higher 

 velocities the orbits will not be stationary unless they are 

 circular. In other cases the orbit will rotate round an axis 

 through the nucleus and perpendicular to the plane of the 

 orbit, in much the same way as if the atom were placed in a 

 magnetic field. It might therefore be supposed that we 

 would obtain a doubling of the lines if the orbits are not 

 circular. The frequency of this rotation of the orbit will 

 depend on the degree of excentricity. For very small 

 alterations from the circular orbit the ratio between the 

 frequency of rotation of the orbit and the frequency of 



277-V 



revolution of the electron is given by „ t> , 9 , which for 



ft J n 2 c 2 h 2 



n = 2 is of the same order of magnitude as the doubling 

 of the hydrogen lines observed. In view, however, of the 

 great number of new assumptions involved in such calcula- 

 tions, it seems to be of very little use to consider this 

 question in detail until more accurate measurements of 

 the distance between the components and especially of its 

 variation for the different hydrogen lines have been made. 



I hope in a later paper to deal more fully with some of 

 the problems briefly considered here, and to discuss in some 

 detail the main principles involved in the application of the 

 quantum theory to the problem of series spectra and the 

 structure of the atom. 



Yours faithfully, 

 University, Manchester, X. BOHR. 



January 12th, 1915. 



XXXVII. Notices respecting New JJooks. 



The Theory of Relativity. L. Silbersteijt, Ph.D. Pp. viii + 295. 

 Macmillan & Co. Ltd. : London, 1914. Price 10s. net, 



r PHIS book is founded on a course of lectures delivered at 

 -*- University College, London ; but the exposition has been 

 made more systematical and has been largely extended so as to 

 include all the most important aspects of the subject. The most 



