116 The L uminiferous Medium, 



star as freed from aberration, or to its apparent direction as 

 affected by aberration ? The question whether rays coming 

 from the stars are refracted differently from rays origi- 

 nating in terrestrial sources had been raised originally by 

 Michell* ; and Kobison and Wilsonf had asserted that the focal 

 length of an achromatic telescope should be increased when it 

 is directed to a star towards which the earth is moving, owing 

 to the change in the relative velocity of light. AragoJ sub- 

 mitted the matter to the test of experiment, and concluded that 

 the light coming from any star behaves in all cases of reflexion 

 and refraction precisely as it would if the star were situated in 

 the place which it appears to occupy in consequence of aber- 

 ration, and the earth were at rest ; so that the apparent 

 refraction in a moving prism is equal to the absolute refraction 

 in a fixed prism. 



Fresnel now set out to provide a theory capable of explaining 

 Arago's result. To this end he adopted Young's suggestion, 

 that the refractive powers of transparent bodies depend on the 

 concentration of aether within them ; and made it more precise 

 by assuming that the aethereal density in any body is pro- 

 portional to the square of the refractive index. Thus, if c 

 denote the velocity of light in vacuo, and if c, denote its 

 velocity in a given material body at rest, so that /u = c/o { is the 

 refractive index, then the densities p and p l of the aether in 

 interplanetary space and in the body respectively will be 

 connected by the relation 



pi = n*P- 



Fresnel further assumed that, when a body is in motion, part 

 of the aether within it is carried along namely, that part which 

 constitutes tne excess of its density over the density of aether 

 in vacuo ; while the rest of the aether within the space occupied 

 by the body is stationary. Thus the density of aether carried 



* Phil. Trans., 1784, p. 35. 



t Trans. E. S. Edin., i, Hist., p. 30. 



J Biot, Astron. Phys., 3rd ed., v, p. 364. The accuracy of Arago's 

 experiment can scarcely have been such as to demonstrate absolutely his 

 result. 



ft, 



