EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 133 



Equation (2) then becomes 



65 pc + '55 U + -12 EG= 1-32 w. 

 Hence pc must be situated at a point such that w is the centre of gravity 



of - 



To find it, we begin by determining ft the centre of gravity of -55 U + '12EG, 

 then, joining fiw, the point we are seeking must lie at a certain distance on 

 the other side of w from c. This distance may be found from the proportion, 



65 : (-55 + -12) :: /3w : w pc, 



which determines the position of pc. The proper coefficient, by which the ob- 

 served values of PC must be corrected, is ||, or 1'97. 



We have thus determined the position and coefficient of a colour by a single 

 experiment, in which it was made to produce a neutral tint along with two of 

 the standard colours. As this may be done with every possible colour, the 

 method is applicable wherever we can obtain a disc of the proposed colour. In 

 this way the diagram (No. l) has been laid down from observations made in 

 daylight, by a good eye of the ordinary type. 



It has been observed that experiments, in which the resultant tint is neutral, 

 are more accurate than those in which the resulting tint has a decided colour, 

 as in experiment (3), owing to the effects of accidental colours produced in the 

 eye in the latter case. These experiments, however, may be repeated till a 

 very good mean result has been obtained. 



But since the elements of every colour have been already fixed by our 

 previous observations and calculations, the agreement of these results with those 

 calculated from the diagram forms a test of the correctness of our method. 



By experiment (No. 3), made at the same time with (l) and (2), we have 



'39 PC + '21 U + '40Bk = '59 V + '41 EG (3). 



Now, joining U with pc, and V with EG, the only common point is that 

 at which they cross, namely y. 



Measuring the parts of the line V EG, we find them in the proportion of 



58V and '42 EG = 1 '00 y. 



Similarly, the line U pc is divided in the proportion 



78 pc and -22U=l - OOy. 



