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



strong had this tendency become by the middle of the 

 nineteenth century that the English school of physicists 

 were attributing rigidity, density and nearly all the prop- 

 erties of material media to the ether. In fact most 

 physicists seemed to have forgotten that no experiment 

 had ever given direct evidence of the existence of such a 

 medium. Not until the first decade of the twentieth cen- 

 tury was it realized that the experimental evidence actu- 

 ally pointed in quite the opposite direction, and that a 

 new point of view was needed in dealing with those phe- 

 nomena of light and electromagnetism which had been 

 previously described in terms of a universal medium. 

 Some account of the development of the ether theory 

 and of the origin and growth of the point of view which 

 has its principal exemplification in the principle of rela- 

 tivity is essential for an understanding of present ten- 

 dencies in formulating a philosophic basis for scientific 

 thought. 



In the time of Newton and for a century after there was 

 much controversy between the adherents of two irrecon- 

 cilable theories of light. Hooke had suggested that 

 light is a wave motion traveling through a homogeneous 

 medium which fills all space, and Huygens had shown 

 that the law of refraction can be deduced at once from 

 this hypothesis if it is assumed that the velocity of light 

 in a transparent body is less than that in free ether. 

 However, Newton, impressed by the fact that a ray 

 obtained by double refraction in Iceland spar differs from 

 a ray of ordinary light just as a rod of rectangular cross 

 section differs from one of circular cross section, and 

 seeing no way of explaining this dissymmetry in terms 

 of a wave motion analogous to longitudinal sound waves, 

 adhered to the view that light consists of infinitesimal 

 particles shot out from the luminous body with enormous 

 velocities. So great was his reputation on account of his 

 discoveries in other fields that this theory of light' held 

 sway among his contemporaries and successors until the 

 labors of Young and Fresnel at the beginning of the 

 nineteenth century definitely established the undulatory 

 theory. However, in spite of the fact that a corpuscular 

 theory of light made the assumption of an ether unneces- 

 sary in so far as the simpler of the observed phenomena 

 are concerned, even Newton postulated the existence of 

 such a medium, partly in order to explain the more com- 



