A CENTURY'S PEOGEESS IN PHYSICS 361 



any possible solution of which seems irreconcilable with 

 the most basic truths of the science in question. The 

 greater the paradox grows, the more certain the advent 

 of a new point of view which will bring one step nearer 

 the comprehensive picture of nature which is the goal of 

 natural philosophy. 



The Ether. From the earliest times philosophers have 

 been attracted by the possibility of explaining physical 

 phenomena in terms of an all-pervading medium. So 

 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 



