364 A CENTURY OF SCIENCE 



the velocity of light, with a probable error of one part in 

 ten thousand. Later investigations by Newcomb and 

 Michelson (31, 62, 1886) gave substantially the same 

 result. So great has been the accuracy of these terres- 

 trial determinations that recent practice has been to cal- 

 culate from them and the angle of aberration the earth's 

 orbital velocity, and hence the distance of the earth from 

 the sun. This indirect method of measuring the astro- 

 nomical unit has a probable error no greater than the best 

 parallax methods of the astronomer. (J. Lovering, 36, 

 161,1863.) 



Aberration is a first order effect, i. e., it depends upon 

 the first power of the ratio of the velocity of the earth to 

 the velocity of light, and at first sight it seemed to prove 

 conclusively that the earth must be in motion relative to 

 the luminiferous 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 



