MICIIELSON's RF.(!ENT IJE.SEARCIIE.S OX LKHIT. 457 



the i)i'oximity of llie two iiiiri'ors which wouhl iiiort' lh;iii Icihuicc those 

 of the long distances used by Foucault and JVIiclielson. 



The greatest dillticulty wliich the nndulatory theory of li^lit has en- 

 countered is found in the attempted reconciliation between the re<inire- 

 nients of the refraction of light and the aberration of light. To explain 

 refraction, the density of the luniiniferous a'ther must be greater when 

 the ii\dex of refraction is greater. If a body moves, it must carry its 

 inclosed a'ther 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 a'ther in the telescope does not move with the telescope; that the 

 a'ther sifts through the telescope, the a'ther in front taking the i)lace of 

 the a'ther left behind; or, as Young expressed it, that the a'ther liows 

 through the air and solid earth as easily as the wind blows through the 

 trees of a forest. 



The difficulty can be eluded by snp[)osing that a refracting body car- 

 ries along with it as much of the aether as it possesses in excess of what 

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

 ways sifting through it, would maintain its a'ther at a constant density. 

 What this fraction is which must travel with the body was calculated 

 by Fresuel. But while the refracting power has been protected, how 

 is it with aberration '? That would be increased to a small extent. But 

 as the aberration is very small, only about 20^" at its maximum, the 

 required change in its value might be masked by ordinary errors of ob- 

 servation. Boscovich suggested to Lalande, in 17G(3, that a telescope 

 filled with water instead of air would test the theory; but he made no 

 experiment. Wdson, of Glasgow, also proposed a water telescope in 

 1782. In the couKse of time it appeared that not oiily was the effect of 

 the earth's motion 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 ? AVell, that depends altogether on wdiether 

 the tether is blowing along as w^ell as the air. If it is, then its motion nuist 

 help the light on a little; but if the a'ther is at rest, no motion of the 

 air, or of matter of any kind, can make any <lifference. According to 

 Fresnel, the free aether is at rest, the bound is in nu^tion. Therefore 

 the speed of light will be changed by the motion of the niedium; but 

 only by a fraction, depending on its index of refraction, — infinitesimal 

 for air, but sensible for water. 



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

 velocity of light would produce on aberration and refraction. JJe saw 

 that a change of 5 per cent-, in the velocity of light would alter the aber- 

 ration by only one second, whereas the refraction in a prism of 45^ would 

 be aflfecte<l to the extent of two minutes. lie observed the zenith dis- 

 tances of stars with and without the prism; and also the deviation of 

 stars which passed the meridian at A. m. and 6 p. m. Tlie observa- 



