EPOCH OF YOUNG AND FRESNEL. 441 



the colours of thin plates has converted that pre- 

 possession which I before entertained for the undu- 

 latory theory of light, into a very strong conviction 

 of its truth and efficiency ; a conviction which has 

 since been most strikingly confirmed by an analysis 

 of the colours of striated surfaces" He here states 

 the general principle of interferences in the form 

 of a proposition. (Prop, viii.) " When two undu- 

 lations from different origins coincide either per- 

 ^ctly or very nearly in direction, their joint effect 

 is a combination of the motions belonging to them." 

 He explains, by the help of this proposition, the 

 colours which were observed in Coventry's micro- 

 meters, in which instrument lines were drawn on 

 glass at a distance of 1 -500th of an inch. The inter- 

 ference of the undulations of the rays reflected from 

 the two sides of these fine lines, produced periodical 

 colours. In the same manner, he accounts for the 

 colours of thin plates, by the interference of the 

 light partially reflected from the two surfaces of 

 the plates. We have already seen that Hooke had 

 long before suggested the same explanation; and 

 Young says at the end of his paper, "It was not 

 till I had satisfied myself respecting all these phe- 

 nomena, that I found in Hooke's Micrographia 

 a passage which might have led me earlier to a 

 similar opinion." He also quotes from Newton 

 many passages which assume the existence of an 

 ether; of which, as we have already seen, Newton 

 suggests the necessity in these very phenomena, 



