50 EEPOKT ON PHYSICAL OPTICS. 



ceived to consist of a certain quantity of white light, and of an 

 excess of the light of two of the simple colours ; and if this excess 

 be absorbed, a white light will be the result, which will be inde- 

 composable by the prism. This result of his hypothesis has been 

 experimentally confirmed by Sir David Brewster.* 



These views, if finally established, sever the connexion be- 

 tween the colour of a ray and its refrangibility, laid down by 

 Newton ; and the former must be supposed to depend not on the 

 length of the wave but on some other element of the vibratory 

 movement. 



III. Diffraction. 



It has been already stated that Newton considered the undu- 

 lations of an ethereral medium to be a necessary part of his theory, 

 and that that theory, as maintained by its author, differed from 

 the theory of Huygens and of Hooke only by the addition of a 

 new hypothesis. The necessity of something extraneous to the 

 undulations of the ether seems to have been admitted by Newton, 

 mainly to account for the right-lined propagation of the rays of 

 light ; and a careful consideration of his optical writings leaves 

 the impression, that had the wave-theory alone appeared to ex- 

 plain this fact, Newton would not have hesitated to embrace it. 

 This explanation has been spoken of in another place, and it has 

 been shown to follow from that theory, that the light which en- 

 counters an obstacle must diminish rapidly in intensity within the 

 edge of the geometric shadow. It now remains to consider the 

 other phenomena which arise under these circumstances ; and it 

 will be found that the same theory affords the most complete 

 account, not only of their general characters, but even of their 

 numerical details. 



In order to understand the theory of shadows, it is necessary to 

 investigate their laws in the simple case in which the magnitude 

 of the luminous body is reduced to a point. The effects thus pre- 

 sented were first observed by Grimaldi, and they have been since 

 studied as a separate branch of optical science, under the title of 

 diffraction or inflexion. Grimaldi found that when a small opaque 

 body was placed in the cone of light, admitted into a dark cham- 



* " On a New Analysis of Solar Light." Edin. Trans., 1831. 



