2i6 SCIENCE AND METHOD. 



aberration itself, it would be explained by a kind of 

 refraction produced at the surface of separation of the 

 ether in repose in the interstellar spaces and the ether 

 carried along by the Earth's movement. It is upon 

 this hypothesis (the total translation of the ether) that 

 Hertz's theory of the Electro-dynamics of bodies in 

 motion is founded. 



2. Fresnel, on the contrary, supposes that the ether 

 is in absolute repose in space, and almost in absolute 

 repose in the air, whatever be the velocity of that air, 

 and that it is partially displaced by refringent mediums. 

 Lorentz has given this theory a more satisfactory form. 

 In his view the ether is in repose and the electrons 

 alone are in motion. In space, where the ether alone 

 comes into play, and in the air, where it comes almost 

 alone into play, the displacement is nil or almost nil. 

 In refringent mediums, where the perturbation is pro- 

 duced both by the vibrations of the ether and by 

 those of the electrons set in motion by the agitation of 

 the ether, the undulations 2.xq partially carried along. 



To help us to decide between these two hypotheses, 

 we have the experiment of Fizeau, who compared, by 

 measurements of fringes of interference, the velocity of 

 light in the air in repose and in motion as well as in 

 water in repose and in motion. These experiments 

 have confirmed Fresnel's hypothesis of partial dis- 

 placement, and they have been repeated with the 

 same result by Michelson. HertrSs theory, therefore, 

 must be rejected. 



