THE THEORY OF COLOURS IN RELATION TO COLOUR-BLINDNESS. 



121 



3. Now, let us take the case of nature. We find that colours differ not 

 only in intensity and hue, but also in tint ; that is, they are more or less pure. 

 We might arrange the varieties of each colour along a line, which should begin 

 with the homogeneous colour as seen in the spectrum, and pass through all 

 gradations of tint, so as to become continually purer, and terminate in white. 



We have, therefore, three elements in our sensation of colour, each of which 

 may vary independently. For distinctness sake I have spoken of intensity, hue, 

 and tint ; but if any other three independent qualities had been chosen, the 

 one set might have been expressed in terms of the other, and the results identified. 



The theory which I adopt assumes the existence of three elementary sen- 

 sations, by the combination of which all the actual sensations of colour are 

 produced. It will be shewn that it is not necessary to specify any given colours 

 as typical of these sensations. Young has called them red, green, and violet ; but 

 any other three colours might have been chosen, provided ih&t white resulted 

 from their combination in proper proportions. 



Before going farther I would observe, that the important part of the theory 

 is not that three elements enter into our sensation of colour, but that there are 

 only three. Optically, there are as many elements in the composition of a ray 

 of light as there are different kinds of light in its spectrum ; and, therefore, 

 strictly speaking, its nature depends on an infinite number of independent 

 variables. 



I now go on to the geometrical form into which the theory may be thrown. 

 Let it be granted that the three pure sensations corre- 

 spond to the colours red, green, and violet, and that we 

 can estimate the intensity of each of these sensations 

 numerically. 



Let v, r, g be the angular points of a triangle, and 

 conceive the three sensations as having their positions at 

 these points. If we find the numerical measure of the 

 red, green, and violet parts of the sensation of a given 

 colour, and then place weights proportional to these parts 



at r, g, and v, and find the centre of gravity of the three weights by the 

 ordinary process, that point will be the position of the given colour, and the 

 numerical measure of its intensity will be the sum of the three primitive 

 sensations. 



In this way, every possible colour may have its position and intensity 



VOL. I. 16 



