241 RECENT PROGRESS OF THE THEORY OF VISION. 



which returns upon itself; a series which we complete 

 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 described 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 produced 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, &c., 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 appears to be only one of quantity, not of quality. 

 Black is only darkness that is", simple absence of light. 

 But when \ve examine the colours of external objects^ black 

 x^t5offesponds 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 

 6 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 re- 

 spectively. 



