892 MIXING OF COLOURS. [BOOK in. 



When we examine the spectrum we are able to perceive a 

 very large number of different colours, we experience a multi- 

 tude 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 spec- 

 trum, 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 sen- 

 sations 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 sensa- 

 tion, so that we might speak of almost each wave-length as pro- 

 ducing a separate sensation. 



On the other hand we also easily recognize that the sensa- 

 tions 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 bluish green, and so on. This suggests 

 that our colour sensations 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 dif- 

 ferent degrees by different wave-lengths and mixed in various 

 proportions. 



559. This view is confirmed when we study in a syste- 

 matic 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 also 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 



