184 The Aether as an Elastic Solid. 



This theory, in one modification or another, held its ground 

 until forty years later it was overthrown by the facts of 

 anomalous dispersion. 



The distinction between aether and ponderable matter was 

 more definitely drawn in memoirs which were published 

 independently in 1841-2 by F. E. Neumann* and Matthew 

 O'Brien.f These authors supposed the ponderable particles to 

 remain sensibly at rest while the aether surges round them, and 

 is acted on by them with forces which are proportional to its 

 displacement. ThusJ the equation of motion of the aether 

 becomes 



rP& 



p ~ 2 = - (k + ^n) grad div e - n curl curl e - Ce, 

 ot 



where C denotes a constant on which the phenomena of dis- 

 persion depend. For polarized plane waves propagated parallel 

 to the axis of x, this equation becomes 



9 2 e 9 2 e 



fg^5r*5 



and substituting 



e = e 



where r denotes the period and V the velocity of the light, we 

 have 



G T , 



772 - P 4^3 r ' 



an equation which expresses the dependence of the velocity on 

 the period. 



The attempt to represent the properties of the aether by 

 those of an elastic solid lost some of its interest after the 

 rise of the electromagnetic theory of light. But in 1867, 



* Berlin Abhandlungen aus dem Jahre 1841, Zweiter Teil, p. 1 : Berlin, 1843. 

 t Trans. Camb. Phil. Soc. vii (1842), p. 397. 

 J O'Brien, loc. cit, 15, 28. 



