266 EXPERIMENTS ON THE PERCEPTION OP COLOUR. 



Blue, green, and yellow must therefore be combined on the large discs, and 

 stand on one side of the equation, and black and white, on the small discs, must 

 stand on the other side. In order to facilitate calculations, the colours are 

 always put down in the same order; but those belonging to the small discs 

 are marked negative. Thus, instead of writing 



we write + 54U + 14G - 32W - 68B + 32Y = 0. 



The sum of all the positive terms of such an equation is 100, being the 

 whole number of divisions in the circle. The sum of the negative terms is 

 also 100. 



The second equation consists of all the colours except blue ; and in this 

 way we obtain six different combinations of five colours. 



Each of these combinations was formed by the unassisted judgment of my 

 eye, on six different occasions, so that there are thirty-six independent observa- 

 tions of equations between five colours. 



Table I. gives the actual observations, with their dates. 



Table II. gives the result of summing together each group of six equations. 



Each equation in Table II. has the sums of its positive and negative co- 

 efficients each equal to 600. 



Having obtained a number of observations of each combination of colours, 

 we have next to test the consistency of these results, since theoretically two 

 equations are sufficient to determine all the relations among six colours. We 

 must therefore, in the first place, determine the comparative accuracy of the 

 different sets of observations. Table III. gives the averages of the errors of 

 each of the six groups of observations. It appears that the combination IV. is 

 the least accurately observed, and that VI. is the best. 



Table IV. gives the averages of the errors in the observation of each colour 

 in the whole series of experiments. This Table was computed in order to detect 

 any tendency to colour-blindness in my own eyes, which might be less accurate 

 in discriminating red and green, than in detecting variations of other colours. 

 It appears, however, that my observations of red and green were more accurate 

 than those of blue or yellow. White is the most easily observed, from the 



