AS PERCEIVED BY THE EYE. 281 



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

 65pc + -55U+12EG. 



To find it, we begin by determining /3 the centre of gravity of '55 U + -12 EG, 

 then, joining /3 w, 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) :: /3^v : wTp~c 



which determines the position of p c. 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. 1) has been laid down from observations made in day- 

 light, 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 pre- 

 vious 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 (1) and (2), we have 



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



Now, joining U with p c, and V with EG, the only common point is that at 

 which they cross, namely 7. 



Measuring the parts of the line VEG, we find them in the proportion of 



■58 V and -42 EG =1-00 7 

 Similarly, the line tjpc is divided in the proportion 



•78pcand -22 U = 1-00 7 

 But 78 pc must be divided by 1*97, to reduce it to PC, as was previously ex- 

 plained. The result of calculation is, therefore, 



•39 PC + -22 U + -39 Bk = -58 V + -42 EG 



the black being introduced simply to fill up the circle. 



This result differs very little from that of experiment (3), and it must be re- 

 collected that these are single experiments,made independently of theory, and 

 chosen at random. 



