L. Page — A Century's Progress in Physics. 331 



conclusively that the earth must be in motion relative to 

 the luminif erous medium. Other questions had to be set- 

 tled, however, and one of these was whether or not light 

 coming from a star would be refracted differently when 

 passing through optical instruments from light which 

 had a terrestial origin. Arago subjected the matter to 

 experiment, and concluded that in every respect the light 

 from a star behaved as if the earth were at rest and the 

 star actually occupied the position which it appears to 

 occupy on account of aberration. Finally optical exper- 

 iments with terrestrial sources seemed to be in no way 

 affected by the motion of the earth through the ether. 



In order to account for these facts Fresnel advanced 

 the following theory. To explain the refraction that 

 takes place when light enters a transparent body, it is 

 necessary to assume that light waves travel more slowly 

 through matter than in free ether. Now the velocity of 

 sound is known to vary inversely with the square root of 

 the density of the material medium through which it 

 passes. Hence it is natural to assume that ether is con- 

 densed inside material objects to such an extent that 

 this same relation connects its density with the velocity 

 of light traveling through it. But when a lens or prism 

 is set in motion, Fresnel supposed it to carry along only 

 the excess ether which it contains, ether of the normal 

 density remaining behind. This assumption suffices to 

 explain Arago 's results, and yet fits in with the phenom- 

 enon of aberration. It gives for light traveling in the 

 direction of motion through a moving material medium 

 of index of refraction n an absolute velocity greater than 

 that when the medium is at rest by an amount 



-i)*' 



which is only a fraction of the velocity v which would 

 have to be added if convected matter carried along all 

 the ether which resides within it. This expression was 

 tested -directly, first by Fizeau in 1851, and later by 

 Michelson and Morley (31, 377, 1886) in this country. 

 The experiment consists in bifurcating a beam of light, 

 passing one half in one direction and the other in the 

 opposite direction through a stream of running water. 

 On reuniting the two rays the usual interference fringes 

 are produced. Reversing the direction of motion of the 



