EPOCH OF YOUXG- AND FBESNEL. 89 



endeavored to prove tlie opposite proposition. But supposing we leave 

 these properties, the rectilinear course, the reflection, and the refraction 

 of light, as problems in which neither theory has a decided advantage, 

 what is the next material point ? The colors of thin plates. Now, 

 how does Newton's theory explain these ? By a new and special sup- 

 position ; that of fits of easy transmission and reflection : a supposi- 

 tion which, though it truly expresses these facts, is not borne out by 

 any other phenomena. But, passing over this, when we come to the 

 peculiar laws of polarization in Iceland spar, how does Newton's meet 

 this ? Again by a special and new supposition ; that the rays of light 

 have sides. Thus we find no fresh evidence in favor of the emission 

 hypothesis springing out of the fresh demands made upon it. It may 

 be urged, in reply, that the same is true of the undulatory theory ; 

 and it must be allowed that, at the time of which we now speak, its 

 superiority in this respect was not manifested ; though Hooke, as we 

 have seen, had caught a glimpse of the explanation, which this theory 

 supplies, of the colors of thin plates. 



At a later period, Newton certainly seems to have been strongly 

 disinclined to believe light to consist in undulations merely. " Are 

 not," he says, in Question twenty-eight of the Opticks, "all hypo- 

 theses erroneous, in which light is supposed to consist in prcssion or 

 motion propagated through a fluid medium ?" The arguments which 

 most weighed with him to produce this conviction, appear to have 

 been the one already mentioned, that, on the undulatory hypothesis, 

 undulations passing through an aperture would be diffused ; and again, 

 his conviction, that the properties of light, developed in various 

 optical phenomena, " depend not upon new modifications, but upon 

 the original and unchangeable properties of the rays." (Question 

 twenty-seven.) 



But yet, even in this state of his views, he was very far from aban- 

 doning the machinery of vibrations altogether. He is disposed to use 

 such machinery to produce his "fits of easy transmission." In his 

 seventeenth Query, he says, 11 " when a ray of light falls upon the sur- 

 face of any pellucid body, and is there refracted or reflected ; may not 

 waves of vibrations or tremors be thereby excited in the refracting or 

 reflecting medium at the point of incidence ? . . . . and do not these 

 vibrations overtake the rays of light, and by overtaking them succes- 

 sively do they not put them into the fits of easy reflecticn and easy 



11 Opticks, p. 322. 



