472 JAMES CLERK MAXWELL. 



The result of each experiment was first expressed by an 

 equation in which the colours in the outer circle, with their 

 respective quantities as coefficients, appeared on the left of 

 the equation, and those in the inner circle on the right side. 

 Thus, employing V for vermilion, U for ultramarine, and EG- 

 for emerald green, SW for snow-white, and Bk for black, the 

 equation 



means that a sector of vermilion occupying 37 divisions of 

 the outer circle, combined with a sector of ultramarine 

 occupying 27 divisions, and a sector of emerald green 

 occupying 36 divisions, produced, on spinning, a colour which 

 matched that obtained by a sector of white paper occupying 

 28 divisons, and a sector of black occupying 72 divisions. 

 The sum of the coefficients on each side of the equation is 

 of course 1. 



The mode of conducting an experiment is best described 

 in Maxwell's own words : 



As an example of the method of experimenting, let us endea- 

 vour to form a neutral gray by a combination of vermilion, ultra- 

 marine, and emerald green. The most perfect results -are ob- 

 tained by two persons acting in concert, when 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 arrange- 

 ments must then be changed so as to render 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 observer 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 



