Dr BREWSTER on a New Analysis of Solar Light. 135 



the coloured portions could be separately submitted to the action 

 of absorbent media. 



In its practical applications, this new principle exceeded my 

 most sanguine expectations, and has furnished me with the means, 

 not only of analyzing the colours of natural bodies, but of deter- 

 mining the causes from which these colours originate. An ac- 

 count of these, and of other applications of it, will form the sub- 

 ject of separate communications, and I shall confine myself at 

 present to the single observation, that, in applying this method 

 of absorption to the decomposition of the solar rays, I have been 

 able to insulate white light, both in the orange and in the green 

 space, and thus obtain the most ample proof of the peculiar ana- 

 lysis of white light which it has been the object of this paper to 

 establish. 



By means of this analysis we are now able to explain the phe- 

 nomena observed by those who are insensible to particular colours. 

 The eyes of such persons are blind to red light ; and when we 

 abstract all the red rays from a spectrum constituted as above 

 described, there will be left two colours, blue and yellow, the only 

 colours which are recognised by those who have this defect 

 of vision. To such eyes light is always seen in the red space, but 

 this arises from the eye being sensible to the yellow and blue rays 

 which are mixed with the red light. 



Hence blue light will be seen in the place of the violet, and a 

 greenish-j/eJ/ottf will appear in the orange and red spaces, or 

 what is the same thing, the spectrum will consist only of the yel- 

 low and the blue spectra shewn in Figs. 2 and 3. The physiolo- 

 gical fact, and the optical principle, are therefore in perfect ac- 

 cordance ; and while the latter gives a precise explanation of 

 the former, the former yields to the latter a new and an unex- 

 pected support. 



