128 EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 



the operator arranges the colours and spins the top, leaving the eye of the 

 observer free from the distracting effect of the bright colours of the papers when 

 at rest. 



After placing discs of these three colours on the circular plate of the top, 

 and smaller discs of white and black above them, the operator must spin the 

 top, and demand the opinion of the observer respecting the relation of the 

 outer ring to the inner circle. He will be told that the outer circle is too 

 red, too blue, or too green, as the case may be, and that the inner one is too 

 light or too dark, as compared with the outer. The arrangement must then be 

 changed, so as to render the resultant tint of the outer and inner circles more 

 nearly alike. Sometimes the observer will see the inner circle tinted with the 

 complementary colour of the outer one. In this case the operator must interpret 

 the observation with respect to the outer circle, as the inner circle contains only 

 black and white. 



By a little experience the operator will learn how to put his questions, and 

 how to interpret their answers. The observer should not look at the coloured 

 papers, nor be told the proportions of the colours during the experiments. 

 When these adjustments have been properly made, the resultant tints of the 

 outer and inner circles ought to be perfectly indistinguishable, when the top 

 has a sufficient velocity of rotation. The number of divisions occupied by the 

 different colours must then be read off on the edge of the plate, and registered 

 in the form of an equation. Thus, in the preceding experiment we have ver- 

 milion, ultramarine, and emerald green outside, and black and white inside. The 

 numbers, as given by an experiment on the Gth March 1855, in daylight without 

 sun, are 



37 V + -27 U + -36 EG = '28 SW + 72 Bk (1). 



The method of treating these equations will be given when we come to the 

 theoretical view of the subject. 



In this way we have formed a neutral gray by the combination of the 

 three standard colours. We may also form neutral grays of different intensities 

 by the combination of vermilion and ultramarine with the other greens, and thus 

 obtain the quantities of each necessary to neutralize a given quantity of the 

 proposed green. By substituting for each standard colour in succession one of the 

 colours which stand under it, we may obtain equations, each of which contains 

 two standard colours, and one of the remaining colours. 



