REPORT ON PHYSICAL OPTICS. 32S 



David Brewster. According to this philosopher, white light 

 consists of but three simple colours, — red, yellow, and blue ; 

 and the solar spectruni is composed of three overlapping spectra 

 of these colours, the intensity of each of which is greatest at 

 the point where that colour is strongest in the compound spec- 

 trum. According to this view, then, all the colours in the solar 

 spectrum are compound, and consist of red, yellow, and blue 

 light, in different proportions. These compound colours can- 

 not be analysed by the prism, in as much as the rays of which 

 they consist at any point of the spectrum have the same refran- 

 gibility ; and it is only by the different action of absorbing 

 media on their constituent elements that their compound nature 

 can be detected. Each of them may be conceived 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 indecomposable 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 vi- 

 bratory movement. 



III. Diffraction. 



It has been already stated that Newton considered the undu- 

 lations of an ethereal medium to be a necessary part of his the- 

 ory, 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 explain 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 encounters 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 shadoius, it is necessary 



* " On a New Analysis of Solar Light," Ed'm. Trans. 1831. 

 Y 2 



