AS PERCEIVED BY THE EYE. 279 



Investigation of the Law of the Perception of Colour. 



Before proceeding to the deduction of the elementary laws of the perception of 

 colour from the numerical results previously obtained, it will be desirable to point 

 out some general features of the experiments which indicate the form which these 

 laws must assume. 



Returning to experiment (1), in which a neutral gray was produced from red, 

 blue, and green, we may observe, that, while the adjustments were incomplete, 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, hav- 

 ing 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 in- 

 stance, redness, blueness, and greenness. This is confirmed 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 showing that colour depends on three things is, by consider- 

 ing 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 shown that these two methods of considering colour 

 may be deduced one from the other, and are capable of exact numerical com- 

 parison. 



On a Graphical 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 ope- 

 rations are confined to a plane is preferable to one requiring space of three di- 

 mensions, 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- 



VOL. XXI. PART II. 4 F 



