36 SCIENTIFIC THOUGHT. 



out, which in the course of the eighteenth century com- 

 bined to establish what I termed the astronomical view 

 of nature. The undulatory theory of light, established 

 by Young and Fresnel during the first quarter of the 

 nineteenth century, was a breaking away from what 

 then seemed to many Continental philosophers a prom- 

 ising line of thought, a unifying principle in natural 

 philosophy. As long as light was thought to consist of 

 particles, however minute, which were projected from 

 luminous centres, the mechanical laws of impact, of at- 

 traction and repulsion, could be applied ; and they went 

 a considerable way in apparently explaining the ordinary 

 phenomena of light, such as motion in straight lines, re- 

 flexion, and refraction. They failed indeed in the case 

 of diffraction or inflection, and still more in those pheno- 

 mena which were misleadingly grouped under the term 

 polarisation. The new theory seemed specially adapted 

 to these more recently discovered phenomena, but it had 

 to be admitted that the explanation of reflexion and 

 refraction of light at the surface of polished, transparent, 

 20. or opaque bodies met with considerable difficulties. The 



Problems 



as to the new theory had introduced the conception of an all- 

 nature of 



the ether, pervading, apparently imponderable substance, the ether. 

 The reintroduction of this conception into physical 

 science was repugnant to many thinkers of the then 

 prevailing school, 1 and it became more so when it had 



measured the speed of light in 

 various media. He proved that 

 light moves faster in air than in 



1 One of the crucial tests for 

 deciding between the corpuscular 

 and the wave theory of light was 

 the relative speed with which light 

 travels in air and in water, i.e., in 

 a refracting substance. Foucault, 

 in 1850, by a very ingenious method, 

 improved since by Mitchelson, 



water, whereas on the corpuscular 

 theory the speed of light in water 

 must be to its speed in air as 4 to 3 

 approximately. " This finally dis- 

 posed of the corpuscular theory " 



