ON THE THEORY OF COMPOUND COLOURS. 245 



the negative values transferred to the other side of the equation. They were 

 all made by means of the colour-top, and were verified by repetition at different 

 times. It may be necessary to remark, in conclusion, with reference to the mode 

 of registering visible colours in terms of three arbitrary standard colours, that it 

 proceeds upon that theory of three primary elements in the sensation of colour, 

 which treats the investigation of the laws of visible colour as a branch of human 

 physiology, incapable of being deduced from the laws of light itself, as set forth 

 in physical optics. It takes advantage of the methods of optics to study vision 

 itself; and its appeal is not to physical principles, but to our consciousness of 

 our own sensations. 



