DISCOVERY OF THE LAW OF REFRACTION. 379 



able to satisfy himself; and he at last 1 is obliged 

 to content himself with an approximate rule, which 

 makes the refraction partly proportional to the 

 angle of incidence, and partly, to the secant of that 

 angle. In this way he satisfies the observed re- 

 fractions within a difference of less than half a 

 degree each way. When we consider how simple 

 the law of refraction is, (that the ratio of the 

 sines of the angles of incidence and refraction is 

 constant for the same medium,) it appears strange 

 that a person attempting to discover it, and draw- 

 ing triangles for the purpose, should fail ; but this 

 lot of missing what afterwards seems to have been 

 obvious, is a common one in the pursuit of truth. 



The person who did discover the Law of the 

 Sines, was Willebrord Snell, about 1 621 ; but the 

 law was first published by Descartes, who had seen 

 Snell's papers-. Descartes does not acknowledge 

 this law to have been first detected by another ; 

 and after his manner, instead of establishing its 

 reality by reference to experiment, he pretends to 

 prove a priori that it must be true 3 , comparing, 

 for this purpose, the particles of light, to balls 

 striking a substance which accelerates them (FA). 



But though Descartes does not, in this instance, 

 produce any good claims to the character of an 

 inductive philosopher, he showed considerable skill 

 in tracing the consequences of the principle when 



1 L. U. K. Life of Kepler, p. 115. 



* Huyghens, Dioptrica, p. 2. s Diopt. p. 53. 



