EPOCH OF YOUNG AND FRESNEL 433 



position ; that of fits of easy transmission and 

 reflection : a supposition which, though it truly ex- 

 presses 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 these? Again by 

 a special and new supposition; that the rays of 

 light have sides. Thus we find no fresh evidence 

 in favour 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 undu- 

 latory 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 colours 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 pression or motion propagated through 

 a fluid medium?" The arguments which most 

 weighed with him to produce this conviction, ap- 

 pear 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 



VOL. II. F F 



