in the Seventeenth Century. 13 



rated originally at a point, and affirms that it is a sphere, 

 whose, centre is the point in question, and whose radii are 

 the rays of light issuing from the point. 



Hooke's next effort was to produce a mechanical theory of 

 refraction, to replace that given by Descartes. " Because," he 

 says, "all transparent mediums are not Homogeneous to one 

 another, therefore we will next examine how this pulse or motion 

 will be propagated through differingly transparent mediums. 

 And here, according to the most acute and excellent Philosopher 

 Des Cartes, I suppose the sine of the angle of inclination in the 

 first medium to be to the sine of refraction in the second, as the 

 density of the first to the density of the second. By density, I 

 mean not the density in respect of gravity (with which the 

 refractions or transparency of mediums hold no proportion), but 

 in respect only to the trajeetion of the Kays of light, in which 

 respect they only differ in this, that the one propagates the 

 pulse more easily and weakly, the other more slowly, but 

 more strongly. But as for the pulses themselves, they will 

 by the refraction acquire another property, which we shall now 

 endeavour to explicate. 



"We will suppose, therefore, in the first Figure, ACFD to be 



a physical Kay, or ABC and DEFto be two mathematical Kays r 

 trajected from a very remote point of a luminous body through 



