in the Seventeenth Century. 19 



course ; and differences of density of the aether between one 

 material medium and another account on these principles for 

 the reflexion and refraction of light. The condensation or 

 rarefaction of the aether due to a material body extends to 

 some little distance from the surface of the body, so that the 

 inflexion due to it is really continuous, and not abrupt; and 

 this further explains diffraction, which Newton took to be 

 " only a new kind of refraction, caused, perhaps, by the 

 external aethers beginning to grow rarer a little before it 

 came at the opake body, than it was in free spaces." 



Although the regular vibrations of Newton's aether were not 

 supposed to constitute light, its irregular turbulence seems to 

 have represented fairly closely his conception of heat. He 

 supposed that when light is absorbed by a material body, 

 vibrations are set up in the aether, and are recognizable as 

 the heat which is always generated in such cases. The 

 conduction of heat from hot bodies to contiguous cold ones he 

 conceived to be effected by vibrations of the aether propagated 

 between them ; and he supposed that it is the violent agitation 

 of aethereal motions which excites incandescent substances to 

 emit light. 



Assuming with Newton that light is not actually con- 

 stituted by the vibrations of an aether, even though such 

 vibrations may exist in close connexion with it, the most 

 definite and easily conceived supposition is that rays of light 

 are streams of corpuscles emitted by luminous bodies. Although 

 this was not the hypothesis of Descartes himself, it was so 

 thoroughly akin to his general scheme that the scientific men 

 of Newton's generation, who were for the most part deeply 

 imbued with the Cartesian philosophy, instinctively selected 

 it from the wide choice of hypotheses which Newton had offered 

 them ; and by later writers it was generally associated with 

 Newton's name. A curious argument in its favour was drawn 

 from a phenomenon which had then been known for nearly half 

 a century : Vincenzo Cascariolo, a shoemaker of Bologna, had 

 discovered, about 1630, that a substance, which afterwards 



C 2 



