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represented. Maxwell's object was to submit this theory to the 

 test of measurement. For this purpose he employed an apparatus 

 now well known as his Colour Top, in which a system of coloured 

 disks could be arranged round the axis of the top, so that a sector of 

 any required angular magnitude of each colour might be exposed. 

 When this system was spun rapidly it assumed the appearance of a 

 single tint. Young had himself employed an apparatus of a similar 

 character, consisting of circles painted with different colours. The 

 merit of Maxwell's top is in the ease with which the proportions of 

 the colours can be changed and definite measures taken of them. His 

 plan of experimenting was to obtain matches between various mixtures 

 of colours from the coloured disks, and these matches were expressed 

 by colour equations. Having selected three colours as standards of 

 reference, the choice being guided by the variety of combinations 

 producible from them, he was enabled, from the great number of 

 colour equations he obtained, to construct a colour diagram on the 

 principle suggested by Newton. The standard colours being at the 

 angular points of an equilateral triangle, any other colour, with a 

 proper coefficient attached to it, could be represented by a point in the 

 diagram, so that from a knowledge of this coefficient and the position 

 of the point, it is possible to say how the colour in question may be 

 obtained from the standards or combined with one or two of them to 

 form a match with the remainder. Any colour which can be produced 

 by a mixture of two of the standards will be represented by a point on 

 the side of the triangle joining them, and any colour from a mixture of 

 the three by a point inside the triangle. Had the standards, instead 

 of being arbitrary, been the three primary colours, any other colour 

 would be obtainable from them by their mixture. Maxwell shows how 

 one of the primary colours may be obtained. Colour-blindness is due 

 to the absence of one of the three primary sensations, and therefore 

 from the colour equations of the colour-blind it is possible to localise 

 on the diagram the tint corresponding to the absent sensation. From 

 experiments on two colour-blind subjects he found the tint was a red 

 approaching to crimson. 



The Colour Top was afterwards abandoned for another apparatus of 

 an ingenious character, known as the Colour Box, by means of which 

 the proportions of three colours forming white light of constant inten- 

 sity could be accurately measured. The main object of the measure- 

 ments was to determine the positions of the colours of the spectrum on 

 the colour diagram. The standard colours in this case were red, green, 

 and blue of ascertained wave-lengths. In all, sixteen definite points 

 of the spectrum were examined, these points being determined by a 

 contrivance in the apparatus for obtaining measures by which the 

 wave-lengths of all the colours admitted could be found. The results 

 of these elaborate experiments showed that from the red to the green 



