OF NATURAL PHILOSOPHY. 251 



success to the explanation of the planetary motions, 

 not merely a plausible, but a perfectly reasonable 

 and fair explanation of all the usual phenomena of 

 light known in his time. His own beautiful dis- 

 coveries, too, of the different refrangibilities of 

 the differently coloured rays, were perfectly well 

 represented in this theory, by simply admitting a 

 difference of velocity in the particles, which produce 

 in the eye the sensations of different colours. And had 

 the properties of light remained confined to these, 

 there would have been no occasion to have resorted 

 to any other mode of conceiving it. . 



(276.) A very different hypothesis had, however, 

 been suggested about the same period by Huyghens, 

 who supposed light to be produced in the same 

 manner with sound, by the communication of a 

 vibratory motion from the luminous body to a 

 highly elastic fluid, which he imagined as filling all 

 space, and as being less condensed within the limits 

 of space occupied by matter, and that to a greater or 

 less extent, according to the nature of the occupying 

 substance. Thus, in place of any thing actually 

 thrown off, he substituted waves, or vibrations, pro- 

 pagated in all directions from luminous bodies, 

 through this medium, or ether, as he called it. 

 Huyghens, being himself a consummate mathema- 

 tician, was enabled to trace many of the conse- 

 quences of this hypothesis, and to show that the 

 ordinary laws of reflection and refraction were repre- 

 sented or accounted for by it, as well as by Newton's. 

 But the hypothesis of Huyghens has not been fully 

 successful in accounting for what may be consi- 

 dered the chief of all optical facts, the production of 



