HISTORY OF OPTICS. 



Grimaldi's experiments on fringes, of winch we have spoken a little 

 while ago. And the great authority of the period, Newton, adopted 

 the opposite hypothesis, that of emission, and gave it a currency 

 among his followers which kept down the sounder theory for above a 

 century. 



Newton's first disposition appears to have been by no means averse 

 to the assumption of an ether as the vehicle of luininiferous undula- 

 tions. When Ilooke brought against his prismatic analysis of light 

 some objections, founded on his own hypothetical notions, Newton, in 

 his reply, said, 8 " The hypothesis has a much greater affinity with his 

 own hypothesis than he seems to be aware of; the vibrations of the 

 ether being as useful and necessary in this as in his." This was in 

 1672 ; and we might produce, from Newton's writing, passages of the 

 same kind, of a much later date. Indeed it would seem that, to the 

 last, Newton considered the assumption of an ether as highly probable, 

 and its vibrations important parts of the phenomena of light ; but he 

 also introduced into his system the hypothesis of emission, and having 

 followed this hypothesis into mathematical detail, while he has left all 

 that concerns the ether in the form of queries and conjectures, the 

 emission theory has naturally been treated as the leading part of his 

 optical doctrines. 



The principal propositions of the Principia which bear upon the 

 question of optical theory are those of the fourteenth Section of the 

 first Book, 8 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 proposition 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 former proposition shows that the law of refraction, 

 an optical truth which mainly affected the choice of a theory, (for 

 about reflection there is no difficulty 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 undula- 

 tions,) is certainly inconclusive and vague ; and something better might 

 the more reasonably have been expected, since Huyghens had at least 



6 Phil. Trans, vii. 50S7. Principia, Prop. 94, et scg. 10 Ib. Prop. 42. 



