EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 129 



Thus, in the case of pale chrome, we have, from the same set of experiments, 

 34 PC + -55U + -12 EG='37 SW + '63 Bk (2). 



We may also make experiments in which the resulting tint is not a neutral 

 gray, but a decided colour. Thus we may combine ultramarine, pale chrome, and 

 black, so as to produce a tint identical with that of a compound of vermilion 

 and emerald-green. Experiments of this sort are more difficult, both from the 

 inability of the observer to express the difference which he detects in two tints 

 which have, perhaps, the same hue and intensity, but differ in purity ; and also 

 from the complementary colours which are produced in the eye after gazing too 

 long at the colours to be compared. 



The best method of arriving at a result in the case before us, is to render 

 the hue of the red and green combination something like that of the yellow, to 

 reduce the purity of the yellow by the admixture of blue, ^nd to diminish its 

 intensity by the addition of black. These operations must be repeated and 

 adjusted, till the two tints are not merely varieties of the same colour, but 

 absolutely the same. An experiment made 5th March gives 



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



That these experiments are really evidence relating to the constitution of the 

 eye, and not mere comparisons of two things which are in themselves identical, 

 may be shewn by observing these resultant tints through coloured glasses, or by 

 using gas-light instead of day-light. The tints which before appeared identical 

 will now be manifestly different, and will require alteration, to reduce them to 

 equality. 



Thus, in the case of carmine, we have by day-light, 



440 + '22 U+ -34 EG ='17 SW+'83Bk, 

 while by gas-light (Edinburgh) 



47 C + -08 U-K45 EG = '25 SW + 75 Bk, 



which shews that the yellowing effect of the gas-light tells more on the white 

 than on the combination of colours. If we examine the two resulting tints 

 which appeared identical in experiment (3), observing the whirling discs through 

 a blue glass, the combination of yellow, blue, and black, appears redder than the 

 other, while through a yellow glass, the red and green mixture appears redder. 

 So also a red glass makes the first side of the equation too dark, and a green 

 glass makes it too light. 



VOL. I. 17 



