106 PHYSICS OF ETHER 



of density within the medium proportional to the square of its index 

 and being convected in proportion to this excess of density, which 

 would give an apparent velocity to the ether of (1 f jc ~ 2 } v , instead of 

 the velocity of the earth. Stokes suggested, as a simpler idea, that we 

 suppose the ether is not convected but passes freely through the 

 earth, being condensed as it passes into a body in the ratio of 1 to /* 2 , 

 so that its velocity within the refracting medium becomes (1 f j -~' 2 ) v , 

 from the law of continuity. 1 



Babinet in the second-century period attempted to test Fresnel's 

 theory by examining the interference of two rays traversing a piece 

 of glass, the one in the direction of the earth's motion and the other 

 in the opposite direction. Stokes showed that a negative result was 

 not contrary to the theory of aberration, since the retardation would 

 be the same as if the earth were at rest. 



He showed further, what Fresnel had not proven to be true in 

 general, that on Fresnel's theory the laws of reflection and refraction 

 for single refracting media are uninfluenced by the motion of the 

 earth. In fact, Rayleigh has shown that, in using terrestrial sources, 

 no optical effect can be produced by any system of reflecting or 

 refracting optical surfaces moving as a rigidly connected system 

 relatively to the ether, if we take into account the Doppler "effect," 

 and neglect quantities of the second order of the aberration. Since, 

 as Stokes says, the theory of a quiescent ether may be dispensed with, 

 and as there is no good evidence that the ether moves quite freely 

 through the solid mass of the earth, he proposes to explain the 

 phenomenon of aberration on the undulation theory of light, upon 

 the supposition that the earth and the planets carry a portion of the 

 ether along with them, so that the ether close to their surfaces is 

 at rest relatively to those surfaces and diminishes in velocity till at 

 no great distance in space there is no motion. Cauchy had previ- 

 ously discussed the theory of a mobile ether, and had proposed to 

 explain aberration by a shearing of the wave-fronts due to the trans- 

 latory motion of the medium, but he did not develop his method 

 sufficiently to explain how much the aberration would be. 



On the other hand Stokes has specifically indicated his assumptions 

 and formulated his conclusions. He examines the displacements of 

 a wave-front in its passage from the ether at rest, across the region 

 of transition to the ether in the neighborhood of the observer, which 

 is at rest relatively to him. Adopting the same method which is 

 used in the case of an ether at rest in determining the wave-front at 

 any future time from that of a given one at any instant, he shows, 



1 If x is the velocity of the ether relative to the moving matter, and the dens- 

 ity of ether within it is M 2 , the density of free ether being unity, we have from 

 the law of continuity v (v z),u 2 and hence, 



