EXPERIMENTS ON COLOUK, AS PERCEIVED BY THE EYE. 131 



plete, the difference of the tints could be detected only by one circle appearing 

 more red, more green, or more blue than the other, or by being lighter or 

 darker, that is, having an excess or defect of all the three colours together. 

 Hence it appears that the nature of a colour may be considered as dependent 

 on three things, as, for instance, redness, blueness, and greenness. This is con- 

 firmed by the fact that any tint may be imitated by mixing red, blue, and 

 green alone, provided that tint does not exceed a certain brilliancy. 



Another way of shewing that colour depends on three things is by con- 

 sidering how two tints, say two lilacs, may differ. In the first place, one may 

 be lighter or darker than the other, that is, the tints may differ in shade. 

 Secondly, one may be more blue or more red than the other, that is, they may 

 differ in hue. Thirdly, one may be more or less decided in its colour ; it may vary 

 from purity on the one hand, to neutrality on the other. This is sometimes 

 expressed by saying that they may differ in tint. 



Thus, in shade, hue, and tint, we have another mode of reducing the 

 elements of colour to three. It will be shewn that these two methods of con- 

 sidering colour may be deduced one from the other, and are capable of exact 

 numerical comparison. 



On a Geographical Method of Exhibiting the Relations of Colours. 



The method which exhibits to the eye most clearly the results of this theory 

 of the three elements of colour, is that which supposes each colour to be repre- 

 sented by a point in space, whose distances from three co-ordinate planes are 

 proportional to the three elements of colour. But as any method by which the 

 operations are confined to a plane is preferable to one requiring space of three 

 dimensions, we shall only consider for the present that which has been adopted 

 for convenience, founded on Newton's Circle of colours and Mayer and Young's 

 Triangle. 



Vermilion, ultramarine, and emerald-green, being taken (for convenience) as- 

 standard colours, are conceived to be represented by three points, taken (for con- 

 venience) at the angles of an equilateral triangle. Any colour compounded of 

 these three is to be represented by a point found by conceiving masses propor- 

 tional to the several components of the colour placed at their respective angular 

 points, and taking the centre of gravity of the three masses. In this way, each 



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