EXPERIMENTS ON THE PERCEPTION OF COLOUR. 265 



combinations must be equivalent to two equations only, if the theory of three 

 primaries be true. 



The method which I have found most convenient for registering the result 

 of an experiment, after an identity of tint has been obtained in the inner and 

 outer circles, is the following: 



Write down the names or symbols of the coloured discs each at the top of 

 a column, and underneath write the number of degrees of that colour observed, 

 calling it + when the colour is in the outer circle, and when it is in the inner 

 circle; then equate the whole to zero. In this way the account of each colour 

 is kept in a separate column, and the equations obtained are easily combined and 

 reduced, without danger of confounding the colours of which the quantities have 

 been measured. The following experiments were made between the 3rd and llth 

 of September, 1856, about noon of each day, in a room fronting the north, 

 without curtains or any bright- coloured object near the window. The same 

 combination was never made twice in one day, and no thought was bestowed 

 upon the experiments except at the time of observation. Of course the gradua- 

 tion was never consulted, nor former experiments referred to, till each combi- 

 nation of colours had been fixed by the eye alone ; and no reduction was 

 attempted till all the experiments were concluded. 



The coloured discs were cut from paper painted of the following colours : 

 Vermilion, Ultramarine, Emerald-green, Snow-white, Ivory-black, and Pale 

 Chrome-yellow. They are denoted by the letters V, U, G, W, B, Y respectively. 

 These colours were chosen, because each is well distinguished from the rest, so 

 that a small change of its intensity in any combination can be observed. Two 

 discs of each colour were prepared, so that in each combination the colours might 

 occasionally be transposed from the outer circle to the inner. 



The first equation was formed by leaving out vermilion. The remaining 

 colours are Ultramarine-blue, Emerald-green, White, Black, and Yellow. We 

 might suppose, that by mixing the blue and yellow in proper proportions, we 

 should get a green of the same hue as the emerald-green, but not so intense, 

 so that in order to match it we should have to mix the green with white to 

 dilute it, and with black to make it darker. But it is not in this way that we 

 have to arrange the colours, for our blue and yellow produce a pinkish tint, and 

 never a green, so that we must add green to the combination of blue and yellow, 

 to produce a neutral tint, identical with a mixture of white and black. 



VOL. I. 34 



