SIB G. STOKES, BAET., ON THE PERCEPTION OP COLOUR. 257 



on the manner in which our bodily organisms are affected. 

 I am not now speaking of physiology, but in the first 

 instance of pure observation and experiment. 



We know that by allowing two lights of different 

 colours to mix together, we get a sensation of some other 

 colour differing in general from the two. In certain cases it 

 may be we actually get the sensation of white, in which 

 case the colours are what is called complementary. The 

 study of the effect of mixing colours together was, at 

 first, somewhat impeded by confusing two different things, 

 the colour obtained by mixing together two coloured 

 pigments, and the colour resulting from mixing together 

 the colours which the two pigments exhibit when taken 

 separately. It is probable that even still some misappre- 

 hension arising from this source exists in the minds of those 

 who have not studied the subject. The difierence is now, 

 however, well known, and there are methods known whereby 

 coloured lights may be mixed together, and whereby we 

 may study the sensations which these mixtures produce, 

 which it would take too long now for me to go into. That 

 subject has been very well worked out by the late Professor 

 Maxwell, who has written some very elaborate papers, 

 giving the results of experiments on the effect of the super- 

 position of lights of different colours. It has long been 

 supposed that in light there is in some sense a kind of 

 triplicity, as if there were three kinds of light which give us 

 all the sensations of colour by mixing them together. This 

 triplicity might be either what I will call objective or 

 subjective. We know that lights of different retrangibility 

 are capable of being separated, forming a spectrum, and as 

 we observe the spectrum we have gradations of colour, red, 

 yellow, and so on, until we get up to the violet. Now, 

 some have speculated on the possibility of there being an 

 objective equality of light, ansAvering, I will say, to redness 

 or greenness or blueness or whatever trio of colours we 

 may take. Sir David Brewster imagined that there was 

 such an objective triplicity, and that really light from a 

 given part of the spectrum, though it cannot be decomposed 

 by the prism, was nevertheless compound, and that there 

 were three kinds of light there coexisting in different 

 proportions, the difference of the proportions varying 

 according to the part of the spectrum we are considering. 

 He supposed that though light of any particular refran- 

 gibility could no longer be separated by the prism, 



