144 Prof. 0. W. Richardson on the 



say ^' = 0. Then the formulae (XVII.) are considerably 

 simplified, giving for the #-point of view : 



-11=11' + - 2 (v/"v) 



1 I 



- 2 e$ = u'v + f'v Y . . . (XVII.a) 



7 



\ef=f - + %*(? 

 Y •' J € c 2 v 



; 



Remembering that - v = v, we may write also, somewhat 

 more conveniently, 



Sp = 7 2JV V -f-i/'v], 



and similarly for the third of the above formulae. But as a 

 matter of fact the operator e, of quite simple structure, 

 makes no difficulty, wherever it stands. Moreover, its 

 appearance in our formulae has nothing artificial about it, 

 since it corresponds, in fact, to the very nature of the 

 transformation which is fundamental for the whole theory 

 of relativity. 



November, 1912. 



XV. The Asymmetric Emission of Secondary Rays. By 

 0. W. Richardson, Professor of Physics, Princeton 

 University *. 



IN 1908 Bragg f showed that when y rays passed normally 

 through a thin plate of matter the ionization produced 

 by the ft secondary rays thus stimulated was greater on the 

 emergent than on the incident side : C. D. Cooksey J showed 

 that similar effects were caused by the electronic emission 

 from thin plates illuminated by Rontgen rays. It was first 

 shown by 0. Stuhlmann§, and about the same time, inde- 

 pendently, by R. D. Kleeman ||, that when thin films of 

 metals are traversed normally by ultra-violet light, the 

 number of electrons emitted on the emergent is greater than 



* Communicated by the Author. 



t Nature, vol. lxxvii. p. 270, Jan. 23, 1908; Phil. Mag-, vol. xvi. 

 p. 918 (1908). 



X Nature, vol. lxxvii. p. 509 (1908). 



§ Nature, May 12, 1910; Phil. Mag. vol. xx. p. 331 (1910), vol. xxii. 

 p. 854 (1911). 



|| Nature, May 19, 1910; Proc. Roy. Soc. A. vol. lxxxiv. (1910). 



