6 The Theory of the Aether 



on periodic time is a curious foreshadowing of one of the 

 great discoveries of Newton. 



The general explanation of light on these principles was 

 amplified by a more particular discussion of reflexion and 

 refraction. The law of reflexion that the angles of incidence 

 and refraction are equal had been known to the Greeks ; but 

 the law of refraction that the sines of the angles of incidence 

 and refraction are to each other in a ratio depending on the 

 media was now published for the first time.* Descartes gave 

 it as his own ; but he seems to have been under considerable 

 obligations to Willebrord Snell (b. 1591, d. 1626), Professor of 

 Mathematics at Leyden, who had discovered it experimentally 

 (though not in the form in which Descartes gave it) about 

 1621. Snell did not publish his result, but communicated it in 

 manuscript to several persons, and Huygens affirms that this 

 manuscript had been seen by Descartes. 



Descartes presents the law as a deduction from theory. 

 This, however, he is able to do only by the aid of analogy ;. 

 when rays meet ponderable bodies, " they are liable to be 

 deflected or stopped in the same way as the motion of a ball or 

 a stone impinging 011 a body " ; for " it is easy to believe that 

 the action or inclination to move, which I have said must be 

 taken for light, ought to follow in this the same laws as 

 motion."f Thus he replaces light, whose velocity of propagation 

 he believes to be always infinite, by a projectile whose velocity 

 varies from one medium to another. The law of refraction is 

 then proved as follows J : 



Let a ball thrown from A meet at B a cloth CBE, so weak 

 that the ball is able to break through it and pass beyond, but 

 with its resultant velocity reduced in some definite proportion,, 

 say 1 : k. 



Then if BI be a length measured on the refracted ray 

 equal to AB, the projectile will take k times as long to 

 describe BI as it took to describe AB. But the component 



* Dioptrique, Discount second. t Jbid., Discows premier. 



% Ibid., Discotirs second. 



