CHAP, m.] SIGHT. 895 



say from X 525 to X 535 ; no rays but these are affecting the 

 retina at the time, and the result is the sensation which we 

 call spectral green. But we might easily so arrange matters 

 that a certain amount of white light, that is of light of all 

 wave-lengths of the visible spectrum, should fall on the area in 

 question at the same time 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 proportion 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 " saturated " 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 indi- 

 vidual 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 yel- 

 low 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." 



561. 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, 



