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 dlue 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 ¢inzt. 
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. Butas 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 Newron’s Circle of Colours and Mayrr and Youne’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 IL. 4F 
