THE THEORY OF COLOURS IN RELATION TO COLOUR-BLINDNESS. 123 



By spinning the top, each colour is presented to the eye for a time pro- 

 portional to the angle of the sector exposed, and I have found by independent 

 experiments, that the colour produced by fast spinning is identical with that 

 produced by causing the light of the different colours to fall on the retina at 

 once. 



By properly arranging the discs, any given colour may be imitated and 

 afterwards registered by the graduation on the rim of the top. The principal 

 use of the top is to obtain colour-equations. These are got by producing, by 

 two different combinations of colours, the same mixed tint. For this purpose 

 there is another set of discs, half the diameter of the others, which lie above 

 them, and by which the second combination of colours is formed. 



The two combinations being close together, may be accurately compared, and 

 when they are made sensibly identical, the proportions of the different colours 

 in each is registered, and the results equated. 



These equations in the case of ordinary vision, are always between four 

 colours, not including black. 



From them, by a very simple rule, the different colours and compounds have 

 their places assigned on the triangle of colours. The rule for finding the position 

 is this : Assume any three points as the positions of your three standard colours, 

 whatever they are ; then form an equation between the three standard colours, 

 the given colour and black, by arranging these colours on the inner and outer 

 circles so as to produce an identity when spun. Bring the given colour to the 

 left-hand side of the equation, and the three standard colours to the right hand, 

 leaving out black, then the position of the given colour is the centre of gravity 

 of three masses, whose weights are as the number of degrees of each of the 

 standard colours, taken positive or negative, as the case may be. 



In this way the triangle of colours may be constructed by scale and compass 

 from experiments on ordinary vision. I now proceed to state the results of 

 experiments on Colour-Blind vision. 



If we find two combinations of colours which appear identical to a Colour- 

 Blind person, and mark their positions on the triangle of colours, then the 

 straight line passing through these points will pass through all points corre- 

 sponding to other colours, which, to such a person, appear identical with the first 

 two. 



We may in the same way find other lines passing through the series of 



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