OF ARTS AND SCIENCES. 389 



more than balance those of the long distances used by Foueault and 

 Michelson. 



The greatest difficulty which the undulatory theory of light has 

 encountered is found in the attempted reconciliation between the 

 requirements of the refraction of light and the aberration of light. 

 To explain refraction, the density of the lumiuiferous ether must be 

 greater when the index of refraction is greater. If a body moves, it 

 must carry its enclosed ether with it, as its refractive power does not 

 change. On the other hand, to explain the aberration of light, it must 

 be supposed that the ether in the telescope does not move with the 

 telescope ; that the ether sifts through the telescope, the ether in front 

 taking the place of the ether left behind ; or, as Young expressed it, 

 that the ether flows through the air and solid earth as easily as the 

 wind blows through the trees of a forest. 



The difficulty can be eluded by supposing that a refracting body 

 carries along with it as much of the ether as it possesses in excess of 

 what would exist in a vacuum of the same bulk. This, added to what 

 is always sifting through it, would maintain its ether at a constant 

 density. What this fraction is which must travel with the body was 

 calculated by Fresnel. But while the refracting power has been pro- 

 tected, how is it with aberration ? That would be increased to a small 

 extent. But as the aberration is very small, only about 20 J" at its 

 maximum, the required change in its value might be masked by ordi- 

 nary errors of observation. Boscovich suggested to Lalande, in 1766, 

 that a telescope filled with water instead of air would test the theory ; 

 but he made no experiment. Wilson of Glasgow also proposed a 

 water telescope in 1782. In the course of time it appeared that not 

 only was the effect of the earth's motiou on refraction and aberration 

 under trial, but also the solar parallax, the motion of the solar system, 

 and that of other stars. 



The case is clearly stated by Lodge in this way. Sound travels 

 quicker with the wind than against it. Is it the same with light? 

 Does light travel quicker with the wind? Well, that depends alto- 

 gether on whether the ether is blowing along as well as the air. If it 

 is, then its motion must help the light on a little ; but if the ether is 

 at rest, no motion of the air, or of matter of any kind, can make any 

 difference. According to Fresnel, the free is at rest, the bound is in 

 motion. Therefore the speed of light will be changed by the motion 

 of the medium ; but only by a fraction, depending on its index of re- 

 fraction, — infinitesimal for air, but sensible for water. 



At an early day Arago investigated the effect which a change in the 



