432 HISTORY OF OPTICS. 



those of the fourteenth Section of the first Book 9 , in 

 which the law of the sines in refraction is proved 

 on the hypothesis that the particles of bodies act 

 on light only at very small distances ; and the pro- 

 position of the eighth Section of the second Book 10 ; 

 in which it is pretended to be demonstrated that 

 the motion propagated in a fluid must diverge 

 when it has passed through an aperture. The for- 

 mer proposition shows that the law of refraction, 

 an optical truth which mainly affected the choice 

 of a theory, (for about reflection there is no dif- 

 ficulty on any mechanical hypothesis,) follows from 

 the theory of emission : the latter proposition was 

 intended to prove the inadmissibility of the rival 

 hypothesis, that of undulations. As to the former 

 point, the hypothetical explanation of refraction, 

 on the assumptions there made, the conclusion is 

 quite satisfactory: but the reasoning in the latter 

 case, (respecting the propagation of undulations,) is 

 certainly inconclusive and vague; and something 

 better might the more reasonably have been ex- 

 pected, since Huyghens had at least endeavoured 

 to prove the opposite proposition. But supposing 

 we leave these properties, the rectilinear course, 

 the reflection, and the refraction of light, as pro- 

 blems in which neither theory has a decided ad- 

 vantage, what is the next material point ? The 

 colours of thin plates. Now, how does Newton's 

 theory explain these ? By a new and special sup- 



9 Principia, Prop. 94. et scq. I0 Ib. Prop. 42. 



