1226 CHARACTERS OF COLOURS. [BOOK in. 



that the green is falling upon it; the result would be a mixed 

 sensation, a sensation of spectral green mixed with the sensation 

 of white, and we should recognize this sensation as different from 

 the sensation of spectral green. Further by varying the pro- 

 portion of white to green falling on the area in question at 

 the same time we should have a whole series of different 

 sensations from a green in which there was hardly any white 

 to a white in which there was hardly any green. In such a 

 series of colour sensations we recognize a hue supplied by the 

 spectral colour, and we use the phrase more or less "satu- 

 rated " to express the proportion of white light ; when very 

 little white is present, we speak of the colour as being highly 

 saturated. It need hardly be said that not only individual 

 spectral colours, but all mixtures of these also, may be thus 

 " mixed with white." 



Again, taking a given area of the retina we may, on the one 

 hand, throw on to the area a small amount of a spectral colour in 

 such a way that all the elements of the retina in the area are 

 excited, to a slight degree, giving rise to a feeble sensation of that 

 colour ; but we may, on the other hand, so scatter a few rays over 

 the area that while some elements are excited others remain at 

 rest and yet in such way that the excitation of the whole area 

 still gives rise to one sensation only. We may speak of each of 

 these sensations as one in which the sensation of the spectral 

 colour is mixed or fused with the sensation which we call black ; 

 or we may distinguish the former as merely a feeble sensation 

 and the latter as more strictly mixed with black. Many of the 

 colours of the external world are of this nature ; thus the colours 

 which we call " browns " are mixtures of yellow or of red or of 

 both (and possibly of other spectral colours also) with more or 

 less black. In a similar way we may mix, not a spectral colour, 

 but white with black, various mixtures forming various " greys." 



758. Putting aside these more or less peculiar cases of 

 mixture with black, we may say that the character of a colour 

 depends (1) on the wave-lengths of the particular rays which, 

 either alone or in excess of other rays, are falling on a given area 

 of the retina ; (2) on the amount of this coloured light falling on 

 that area in a given time ; and (3) on the amount of white light 

 falling on that area at the same time. The first determines 

 what we call the hue, the second the intensity, and the third 

 the amount of saturation. Our common phrases do not distinguish 

 with sufficient accuracy these three conditions, which obviously 

 may exist under various combinations. On the one hand we 

 frequently use wholly unlike names for colours which differ only 

 in degree of saturation, such as carmine and pink ; on the other 

 hand we often use the same adjectives for quite different con- 

 ditions. It is desirable to employ the word ' pale/ to mean little 

 saturated, largely mixed with white, and the word 'deep' or 'rich ' 



