448 ON THE THEORY OF THREE PRIMARY COLOURS. 



If we could excite one of these sets of nerves without acting on the 

 others, we should have the pure sensation corresponding to that set of nerves. 

 This would be truly a primary colour, whether the nerve were excited by pure 

 or by compound light, or even by the action of pressure or disease. 



If such experiments could be made, we should be able to see the primary 

 colours separately, and to describe their appearance by reference to the scale 

 of colours in the spectrum. 



But we have no direct consciousness of the contrivances of our own bodies, 

 and we never feel any sensation which is not infinitely complex, so that we 

 can never know directly how many sensations are combined when we see a 

 colour. Still less can we isolate one or more sensations by artificial means, so 

 that in general when a ray enters the eye, though it should be one of the 

 pure rays of the spectrum, it may excite more than one of the three sets of 

 nerves, and thus produce a compound sensation. 



The terms simple and compound, therefore, as applied to colour-sensation, 

 have by no means the same meaning as they have when applied to a ray of 

 light. 



The speaker then stated some of the consequences of Young's theory, and 

 described the tests to which he had subjected it: 



1st. There are three primary colours. 



2nd. Every colour is either a primary colour, or a mixture of primary 

 colours. 



3rd. Four colours may always be arranged in one of two ways. Either 

 one of them is a mixture of the other three, or a mixture of two of them 

 can be found, identical with a mixture of the other two. 



4th. These results may be stated in the form of colour-equations, giving 

 the numerical value of the amount of each colour entering into any mixture. 

 By means of the Colour Top*, such equations can be obtained for coloured 

 papers, and they may be obtained with a degree of accuracy shewing that the 

 colour-judgment of the eye may be rendered very perfect. 



The speaker had tested in this way more than 100 different pigments and 

 mixtures, and had found the results agree with the theory of three primaries 



* Described in the Trant. of the Royal Society of Edinburgh, Vol. XXL, and in the Phil. Mag. 



