ON THE THEORY OF COMPOUND COLOURS. 441 



On the Comparison of Colour-blind with ordinary Vision by means of Observations 



with Coloured Papers. 



In March 1859 I obtained a set of observations by Mr Simpson, of the 

 relations between six coloured papers as seen by him. The experiments were 

 made with the colour-top in the manner described in my paper in the Trans- 

 actions of the Royal Society of Edinburgh, Vol. xxi. pt. 2, p. 286 ; and the 

 colour-equations were arranged so as to be equated to zero, as in those given 

 in the Philosophical Magazine, July, 1857. The colours were Vermilion (V), 

 ultramarine (U), emerald-green (G), ivory-black (B), snow-white (W), and pale 

 chrome-yellow (Y). These six colours afford fifteen colour-blind equations, since 

 four colours enter into each equation. Fourteen of these were observed by 

 Mr Simpson, and from these I deduced three equations, giving the relation of 

 the three standards (V), (U), (G) to the other colours, according to his kind of 

 vision. From these three equations I then deduced fifteen equations, admitting 

 of comparison with the observed equations, and necessarily consistent in 

 themselves. 



The comparison of these equations furnishes a test of the truth of the theory 

 that the colour-blind see by means of two colour-sensations, and that therefore 

 every colour may be expressed in terms of two given colours, just as in ordinary 

 vision it may be expressed in terms of three given colours. The one set of 

 equations are each the result of a single observation ; the other set are deduced 

 from three equations in accordance with this theory, and the two sets agree to 

 within an average error = 2' 1. 



TABLE b. 



2. 

 3. 

 4. 

 5. 

 VOL. I. 56 



