COLOUR VISION. 271 



of a musical chord. A colour, therefore, must be regarded as a single thing, 

 the quality of which is capable of variation. 



To bring a quality within the grasp of exact science, we must conceive it 

 as depending on the values of one or more variable quantities, and the first 

 step in our scientific progress is to determine the number of these variables 

 which are necessary and sufficient to determine the quality of a colour. We 

 do not require any elaborate experiments to prove that the quality of colour 

 can vary in three and only in three independent ways. 



One way of expressing this is by saying, with the painters, that colour 

 may vary in hue, tint, and shade. 



The finest example of a series of colours, varying in hue, is the spectrum 

 itself. A difference in hue may be illustrated by the difference between adjoining 

 colours in the spectrum. The series of hues in the spectrum is not complete ; 

 for, in order to get purple hues, we must blend the red and the blue. 



Tint may be defined as the degree of purity of a colour. Thus, bright 

 yellow, buff, and cream-colour, form a series of colours of nearly the same hue, 

 but varying in tint. The tints, corresponding to any given hue, form a series, 

 beginning with the most pronounced colour, and ending with a perfectly neutral 

 tint. 



Shade may be defined as the greater or less defect of illumination. If 

 we begin with any tint of any hue, we can form a gradation from that colour 

 to black, and this gradation is a series of shades of that colour. Thus we 

 may say that brown is a dark shade of orange. 



The quality of a colour may vary in three different and independent ways. 

 We cannot conceive of any others. In fact, if we adjust one colour to another, 

 so as to agree in hue, in tint, and in shade, the two colours are absolutely 

 indistinguishable. There are therefore three, and only three, ways in which a 

 colour can vary. 



I have purposely avoided introducing at this stage of our inquiry anything 

 which may be called a scientific experiment, in order to shew that we may 

 determine the number of quantities upon which the variation of colour depends 

 by means of our ordinary experience alone. 



Here is a point in this room : if I wish to specify its position, I may do 

 so by giving the measurements of three distances namely, the height above the 

 floor, the distance from the wall behind me, and the distance from the wall 

 at my left hand. 



