The Aether as an Elastic Solid. 187 



where w denotes the velocity of the ponderable body. If the 

 body is an ordinary isotropic one, and if we consider light 

 propagated parallel to the axis of z, in a medium moving in 

 that direction, the light- vector being parallel to the axis of x, 

 the equation reduces to 



d'e x d'e x id 9V 



O 7b ' Q]A. I -f- IV I 6 r i 



' O/2 ^W- \ ^/ ^i/v / 



C7t (j6 \(7 (72'/ 



substituting 



,-/(*- FO, 



where V denotes the velocity of propagation of light in the 

 medium estimated with reference to the fixed aether, we obtain 



for V the value 



/ n \k o\A 



\p + pt p + 



The absolute velocity of light is therefore increased by the 

 amount piAw/(p + piA) owing to the motion of the medium ; 

 and this may be written (/** - 1) wjfjc, where ju denotes the 

 refractive index ; so that Boussinesq's theory leads to the same 

 formula as had been given half a century previously by Fresnel.* 

 It is Boussinesq's merit to have clearly asserted that all 

 space, both within and without ponderable bodies, is occupied 

 by one identical aether, the same everywhere both in inertia 

 and elasticity; and that all aethereal processes are to be re- 

 presented by two kinds of equations, of which one kind expresses 

 the invariable equations of motion of the aether, while the other 

 kind expresses the interaction between aether and matter. 

 Many years afterwards these ideas were revived in connexion 

 with the electromagnetic theory, in the modern forms of which 

 they are indeed of fundamental importance. 



* Cf. p. 115 sqq. 



