MATTER, SPACE, AND TIME 221 



intellectual supremacy, a stumbling block to his 

 weaker brethren, and an impediment to the cause 

 he had most at heart. 



In recent years the discovery of radio-activity 

 has revealed to us particles very like those that 

 Newton used to explain ordinary light. The /3 

 rays from radium are projected particles moving 

 with velocities nearly approaching that of light 

 itself. Newton's inscrutable insight, amounting 

 almost to an instinctive knowledge of Nature, has 

 again been demonstrated. His corpuscles cannot, 

 indeed, explain the phenomena of ordinary light ; 

 but similar corpuscles we find do exist, and their 

 properties as set forth by Newton are not so 

 unlike those actually occurring in the working of 

 Nature as men have assumed throughout the years 

 which separate the establishment of the undulatory 

 theory of light from the discovery of radio-activity. 



The corpuscular theory of light was replaced 

 by a theory of waves in a medium which already 

 had been recognised by Newton as a necessary 

 addition to his idea of corpuscles. Newton's 

 difficulty, which caused him to reject the undula- 

 tory hypothesis, namely, the rectilinear propaga- 

 tion of light, and the consequent possibility of 

 sharp shadows, was finally overcome by Fresnel 

 and Young, who showed that shadows were the 

 result of the minuteness of the wave-lengths of 

 light as compared with the dimensions of ordinary 

 obstacles. This cleared the way for the wave 

 theory as already formulated by Huygens, and 

 there arose a definite physical need for the exact 

 specification of an aether or luminiferous medium, 

 pervading all space, and the interstices, if not the 

 substance, of material objects. Such a medium. 



