EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 



153 



(In order to reduce these wave-lengths to their actual lengths in the eye, 

 each must be divided by the index of refraction for that kind of light in the 

 medium in which the physical effect of the vibrations is supposed to take place.) 



Although these experiments are not in themselves sufficient to give the com- 

 plete theory of the curve of homogeneous colours, they determine the most 

 important element of that theory in a way which seems very accurate, and I 

 cannot doubt that when a philosopher who has so fully pointed out the im- 

 portance of general theories in physics turns his attention to the theory of 

 sensation, he will at least establish the principle that the laws of sensation can 

 be successfully investigated only after the corresponding physical laws have been 

 ascertained, and that the connection of these two kinds of laws can be appre- 

 hended only when the distinction between them is fully recognised. 



NOTE IV. 

 Description of the Figures. Plate I. 



No. 1. is the colour-diagram already referred to, representing, on Newton's principle, the relations of 

 different coloured papers to the three standard colours vermilion, emerald-green, and ultra- 

 marine. The initials denoting the colours are explained ill the list at page 276, and the 

 numbers belonging to them are their coefficients of intensity, the use of which has been 

 explained. The initials H.R., H.B., and H.G., represent the red, blue and green papers 

 of Mr HAY, and serve to connect this diagram with No. (2), which takes these colours for 

 its standards. 

 VOL. I. 20 



