258 SIR G. STOKES, BART., ON TH K PERCEPTION OF COLOUR, 



nevertheless three kmds of light, capable separately of 

 exciting three different sensations of colonr, there co-existed, 

 and were capable of having their proportions altered by- 

 passing through suitable absorbing media, so that the 

 filtered light might affect the eye with a different sensation 

 of colour from the original. 



It has been shown, however, that this was a mistake, and 

 he appears to have been led into the error by being 

 deceived by tlie illusion due to contrast. So far as "we know, 

 there is absolutely nothing objectively in light answermg to 

 redness as such, or yellowness as such, or blueness as such, 

 although of course at one part of the spectrum there is 

 redness predominant, and so with the other parts. 



Dr. Young believed that there were, so to speak, three 

 primary sensations of colour, and that those sensations were 

 called up simultaneously when any colour was presented to 

 us — simultaneously, but in different proportions according to 

 the nature of that colour. Even the colours of the spectrum, 

 which are the purest colours that one can get, are, on his 

 theory, supposed to give rise to three primary sensations of 

 colour which are co-existent, but in different proportions- 

 according to the place in the spectrum. 



The sensations which Dr. Young supposed to be the 

 primary ones were red, green, and violet. Perhaps it ma_T 

 not be quite right to speak of those as the primary sensations, 

 but you may take them at any rate as three standards of 

 colour, and perhaps they are the best to take as standards, 

 because by their mixtures we can the most nearly reproduce 

 all other colours, of which we have an infinite number of 

 hues. That view was taken up and extended or rendered 

 in some respects more precise by Helmholtz, and so the 

 theory is now known as the Young-Helmholtz theory. 



Now, as 1 said, some very elaborate experiments were 

 made by the late Professor Maxwell in regard to our 

 sensations on the mixing together of different kinds of 

 light, and the result of his experiments showed that not 

 merely qualitatively, but quantitatively, one ma}' express 

 any colour in terms of three colours taken as standards. 



Suppose we take three colours as standards, and call 

 them X, Y, z, each supposed to be referred to a unit of its 

 own kind, and suppose that «, h, and c are numerical 

 co-efficients, which may be positive or negative. Then 

 any colour whatsoever (C) as regards its effect upon our 

 eyes — not by any means as regards its physical properties. 



