EXPERIMENTS ON THE PERCEPTION OF COLOUR. 267 



brilliancy of the colour, and black is liable to the greatest mistakes. I would 

 recommend this method of examining a series of experiments as a means of 

 detecting partial colour-blindness, by the different accuracy in observing differ- 

 ent colours. The next operation is to combine all the equations according to 

 their values. Each was first multiplied by a coefficient proportional to its ac.- 

 curacy, and to the coefficient of white in that equation. The result of adding 

 all the equations so found is given in equation (W). 



Equation (Y) is the result of similar operations with reference to the 

 yellow on each equation. 



We have now two equations from which to deduce six new equations, by 

 eliminating each of the six colours in succession. We must first combine the 

 equations, so as to get rid of one of the colours, and then we must divide by 

 the sum of the positive or negative coefficients, so as to reduce the equations 

 to the form of the observed equations. The results of these operations are given 

 in Table V., along with the means of each group of six observations. It will 

 be seen that the differences between the results of calculation from two equations 

 and the six independent observed equations are very small. The errors in red 

 and green are here again somewhat less than in blue and yellow, so that there 

 is certainly no tendency to mistake red and green more than other colours. 

 The average difference between the observed mean value of a colour and the 

 calculated value is 77 of a degree. The average error of an observation in any 

 group from the mean of that group was '92. No observation was attempted 

 to be registered nearer than one degree of the top, or ^fa of a circle ; so that 

 this set of observations agrees with the theory of three primary colours quite 

 as far as the observations can warrant us in our calculations ; and I think that 

 the human eye has seldom been subjected to so severe a test of its power of 

 distinguishing colours. My eyes are by no means so accurate in this respect as 

 many eyes I have examined, but a little practice produces great improvement 

 even in inaccurate observers. 



I have laid down, according to Newton's method, the relative positions of 

 the five positive colours with which I worked. It will be seen that W lies 

 within the triangle VUG, and Y outside that triangle. 



The first combination, Equation I., consisted of blue, yellow, and green, 

 taken in such proportions that their centre of gravity falls at W. 



342 



