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XVII. On Fresnel's Theory of the Aberration of Light. By 

 G. G. Stokes, M.A. t Fellow of Pembroke College, Cam- 

 bridge*. 



^^HE theory of the aberration of light, and of the absence 

 of any influence of the motion of the earth on the laws 

 of refraction, &c, given by Fresnel in the ninth volume of the 

 Annales de Chimie, p. 57, is really very remarkable. If we 

 suppose the diminished velocity of propagation of light within 

 refracting media to arise solely from the greater density of the 

 aether within them, the elastic force being the same as without, 

 the density which it is necessary to suppose the aether within 

 a medium of refractive index jx. to have is ju. 2 , the density in 

 vacuum being taken for unity. Fresnel supposes that the 

 earth passes through the aether without disturbing it, the 

 aether penetrating the earth quite freely. He supposes that a 

 refracting medium moving- with the earth carries with it a 

 quantity of aether, of density pr — 1-, which constitutes the ex- 

 cess of density of the aether within it over the density of the 

 aether in vacuum. He supposes that light is propagated 

 through this aether, of which part is moving with the earth, 

 and part is at rest in space, as it would be if the whole were 

 moving with the velocity of the centre of gravity of any por- 

 tion of it, that is, with a velocity ( 1 % )v, v being the velo- 

 city of^he earth. It may be observed however that the result 

 would be the same if we supposed the whole of the aether 

 within the earth to move together, the aether entering the 

 earth in front, and being immediately condensed, and issuing 

 from it behind, where it is immediately rarefied, undergoing 

 likewise sudden condensation or rarefaction in passing from 

 one refracting medium to another. On this supposition, the 

 evident condition that a mass v of the aether must pass in 

 a unit of time across a plane of area unity, drawn anywhere 

 within the earth in a direction perpendicular to that of the 



earth's motion, gives ( 1 ^ ) v f° r l ' ie velocity of the aether 



within a refracting medium. As this idea is rather simpler 

 than Fresnel's, I shall adopt it in considering his theory. 

 Also, instead of considering the earth as in motion and the 

 aether outside it as at rest, it will be simpler to conceive a ve- 

 locity equal and opposite to that of the earth impressed both 

 on the earth and on the aether. On this supposition the earth 

 will be at rest; the aether outside it will be moving with a ve- 

 locity v, and the aether in a refracting medium with a velocity 



* Communicated by the Author. 



