ON THE THEORY OF COMPOUND COLOURS. 413 



tions correspond to red, green, and violet. A blue ray, for example, though 

 homogeneous in itself, he conceives capable of exciting both the green and the 

 violet sensation, and therefore he would call blue a compound colour, though 

 the colour of a simple kind of light. The quality of any colour depends, 

 according to this theory, on the ratios of the intensities of the three sensations 

 which it excites, and its brightness depends on the sum of these three intensities. 

 Sir David Brewster, in his paper entitled " On a New Analysis of Solar 

 Light, indicating three Primary Colours, forming Coincident Spectra of equal 

 length*," regards the actual colours of the spectrum as arising from the inter- 

 mixture, in various proportions, of three primary kinds of light, red, yellow, 

 and blue, each of which is variable in intensity, but uniform in colour, from 

 one end of the spectrum to the other ; so that every colour in the spectrum 

 is really compound, and might be shewn to be so if we had the means of 

 separating its elements. 



Sir David Brewster, in his researches, employed coloured media, which, 

 according to him, absorb the three elements of a single prismatic colour in 

 different degrees, and change their proportions, so as to alter the colour of the 

 light, without altering its refrangibility. 



In this paper I shall not enter into the very important questions affecting 

 the physical theory of light, which can only be settled by a careful inquiry 

 into the phenomena of absorption. The physiological facts, that we have a 

 threefold sensation of colour, and that the three elements of this sensation are 

 affected in different proportions by light of different refrangibilities, are equally 

 true, whether we adopt the physical theory that there are three kinds of light 

 corresponding to these three colour-sensations, or whether we regard light of 

 definite refrangibility as an undulation of known length, and therefore variable 

 only in intensity, but capable of producing different chemical actions on different 

 substances, of being absorbed in different degrees by different media, and of 

 exciting in different degrees the three different colour-sensations of the human 

 eye. 



Sir David Brewster has given a diagram of three curves, in which the 

 base-line represents the length of the spectrum, and the ordinates of the curves 

 represent, by estimation, the intensities of the three kinds of light at each point 

 of the spectrum. I have employed a diagram of the same kind to express the 



* Transactions of the Royal Society of Edinburgh, Vol. xn. p. 1 23. 



