THE SENSATION OF SIGHT. 215 



colours of the spectrum, and to which we give the names red, 

 yellow, blue, violet, purple. Thus, with regard to tone, colours 

 form a series which returns upon itself ; a series which we com- 

 plete when we allow the terminal colours of the rainbow to pass 

 into one another through purple and crimson. It is in fact the 

 same which we describe as arranged around the circumference of 

 the colour disc. 



The fulness or saturation of colours is greatest in the pure 

 tints of the spectrum, and becomes less in proportion as they are 

 mixed with white light. This, at least, is true for colours pro- 

 duced by external light, but for our sensations it is possible to 

 increase still further the apparent saturation of colour, as we 

 shall presently see. Pink is a whitish-crimson, flesh-colour a 

 whitish-scarlet, and so pale green, straw-colour, light blue, Ac., 

 are all produced by diluting the corresponding colours with 

 white. All compound colours are, as a rule, less saturated than 

 the simple tints of the spectrum. 



Lastly, we have the difference of brightness, or strength of 

 light, which is not represented in the colour-disc. As long as we 

 observe coloured rays of light, difference in brightness appeal's 

 to be only one of quantity, not of quality. Black is only dark 

 ness that is, simple absence of light. But when we examine 

 the colours of external objects, black corresponds just as much to 

 a peculiarity of surface in reflection, as does white, and therefore 

 has as good a right to be called a colour. And as a matter of 

 fact, we find in common language a series of terms to express 

 colours with a small amount of light. We call them dark (or 

 rather in English, deep) when they have little light but are 'full ' 

 in tint, and grey when they are 'pale.' Thus dark blue conveys 

 the idea of depth in tint, of a full blue with a small amount of 

 light ; while grey- blue is a pale blue with a small amount of 

 light. In the same way, the colours known as maroon, brown, 

 and olive are dark, more or less saturated tints of red, yellow 

 and green respectively. 



In this way we may reduce all possible actual (objective) 

 differences in colour, so far as they are appreciated by the eye, to 

 three kinds ; difference of hue (tone), difference of fulness (satura- 



