CHAP, in.] SIGHT. 1223 



of white and black, sometimes it is used to comprise these two 

 sensations as well. 



When we examine the spectrum we are able to perceive a very 

 large number of different colours, we experience a multitude of 

 sensations, no two of which are exactly alike. There are certain 

 broad differences which we express by common names, such as red, 

 orange, yellow and the like. But we can go much further than 

 this. If we take any part of the spectrum, the green for instance, 

 we find that a very slight change in the wave-length produces a 

 change in the character of the sensation. For convenience sake 

 we call a whole group of sensations green ; but we are obliged to 

 admit that there are several kinds of green, several distinct kinds 

 of sensations, though we do not possess names for all of them ; a 

 trained eye will recognize that within the green of the spectrum, 

 the sensation produced by one part is a different sensation from 

 that produced by an adjoining part differing in wave-length from 

 the former by an exceedingly small amount. The same is the 

 case with other parts of the spectrum. And in general we may 

 say that any change in the wave-length will produce a change in 

 the sensation, so that we might speak of almost each wave-length 

 as producing a separate sensation. 



On the other hand we also easily recognize that the sensations 

 produced by the spectrum are not all wholly unlike, that some 

 are allied to others, and that in some cases one sensation is 

 intermediate between two other sensations and partakes of the 

 nature of both. We recognize the sensation produced by the 

 part of the spectrum lying between the green and the yellow as 

 partaking on the one hand of the nature of the sensation of green 

 and on the other hand of yellow, and call it yellowish green 

 or greenish yellow ; we similarly recognize a greenish blue or a 

 blueish green, and so on. This suggests that our colour sensa- 

 tions are in reality mixed sensations, that the multitude of 

 different sensations to which the spectrum gives rise are brought 

 about not by each wave-length giving rise to a separate and 

 independent sensation, but by means of a certain smaller number 

 of primary sensations excited in different degrees by different 

 wave-lengths and mixed in various proportions. 



756. This view is confirmed when we study in a systematic 

 manner the results of mixing or fusing together colour sensations. 



The best method of fusing colour sensations is that of allowing 

 two different parts of the spectrum to fall on the same part of the 

 retina at the same time. We may make use of surfaces coloured 

 with pigments, but in doing so we must bear in mind the nature 

 of the colour of pigments. A pigment possesses colour because 

 when white light falls upon it some of the rays are absorbed 

 while others are reflected. Thus gamboge absorbs the blue rays 

 very largely as well as to a slight extent the red rays, but reflects 

 the yellow rays and with these many of the green rays; indigo 



