THE SPECTRUM: HUE, LUMINOSITY, SATURATION 29 



If now the pure spectral colours be successively mixed with gradually 

 increasing quantities of white light they become paler until eventually 

 no colour can be distinguished. They are said to become less saturated. 



A given colour may therefore be defined by its hue, its luminosity, 

 and its degree of saturation. In regard to the hue or tone the matter 

 is relatively simple, so long as we adhere to the term hue or tone in 

 this definite sense. The terms tint, nuance, shade and so on should 

 be avoided. 



With regard to luminosity and saturation the matter is by no means 

 so simple. An unsaturated colour is also an impure colour in the 

 physical sense of the word, for it no longer consists solely of rays from 

 a single small region of the spectrum. But we are confronted with 

 another fact, less easy of explanation, viz. that great increase of in- 

 tensity of the light not only alters the hue, but also alters the saturation, 

 so that eventually it produces only the sensation of white light. It 

 would seem therefore that luminosity is in some recondite sense an 

 inherent " whiteness " in the colour itself, differing in degree in different 

 spectral colours and varying with the intensity of those colours. Clearly 

 we are here, at the outset, face to face with a physiological fact of immense 

 importance, and much of the difficulty of colour vision is concerned 

 with this fact. 



The terms hue or tone {Farbenton), brightness or luminosity {Hellig- 

 keit), and saturation or purity {Sdttigung) are now generally used in the 

 well-defined senses given above. One has to be careful, however, in 

 reading the older and some modern works. Thus Aubert^ uses Farben- 

 ton for Clerk-Maxwell's " hue " (" one may be more blue or more red 

 than the other, that is, they may differ in hue ") ; Farbenniiance for 

 Helmholtz's " Sdttigungsgrad " and Grassmann's Intensitdt des bei- 

 gemischten Weiss and Clerk-Maxwell's " tint " (" 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 dift'er in tint") ; Farbenmtensitdt for Helmholtz's LicktstdrJce 

 and Clerk-Maxwell's " shade " (" one may be lighter or darker than the 

 other ; that is, the tints may differ in shade "). Edridge-Green uses 

 the terms hue, luminosity and purity. Hue is employed in the usual 

 sense. Of luminosity, however, he says : " No coloured object can 

 have the luminosity of a white object reflecting practically the whole 

 of the light impinging upon it. Therefore if we take absolute reflection 

 as 100, a fraction of 100 will give the relative luminosity of any body." 



1 Physiologic der Netzhaut, Breslau, 1865. 



