78 DISSERTATION SECOMJ. {, A rt ii. 



of easy transmission and easy reflection, and forms one of the 

 most singular parts of his optical discoveries.^ It is so unlike 

 any thing which analogy teaches us to expect, that it has 

 often been viewed with a degree of incredulity, and regard- 

 ed as at best but a conjecture introduced to account for cer- 

 tain optical phenomena. This, however, is by no means a 

 just conclusion, for it is, in reality, a necessary inference 

 from appearances accurately observed, and is no less en- 

 titled to be considered as a fact than those appearances 

 themselves. The difficulty of assigning a cause for such 

 extraordinary alternations cannot be denied, but does not 

 entitle us to doubt the truth of a conclusion fairly deduced 

 from experiment. The principle has been confirmed by 

 phenomena that were unknown to Newton himself, and pos- 

 sesses this great and unequivocal character of philosophic 

 truth, that it has served to explain appearances which were 

 tiot observed till long after the time when it first became 

 known. 



We cannot follow the researches of Newton into what re- 

 gards the colours of thick plates, and of bodies in general. 

 \Ve must not, however, pass over his explanation of refrac- 

 tion, which is among the happiest to be met with in any part 

 of science, and has the merit of connecting the principles of 

 Optics with those of Dynamics. 



The theory from which the explanation we speak of is 

 deduced, is, that light is an emanation of particles, moving in 

 straight lines with incredible velocity, and attracted by the 

 particles of transparent bodies. . When, therefore, light falls 

 obliquely on the surface of such a body, its motion may be 

 resolved into two, one parallel to that surface, and the 

 other perpendicular to it. Of these, the first is not affected 

 by the attraction of the body, which is perpendicular to its 

 own surface ; and, therefore, it remains the same in the re- 

 fracted that it was in the incident ray. But the velocity 



