CHAP. in.J SIGHT. 93 



the three sensations. Thus the sensation of orange (0 in the 

 figure) is brought about by a mixture of a great deal of the 

 primary red with much less of the primary green, and hardly any 

 of the primary blue ; the orange sensation is converted into a 

 yellow sensation by diminishing the primary -red and largely 

 increasing the primary green, the primary blue undergoing also 

 some slight increase. And similarly with all the other sensations. 

 When all the three primary sensations are together excited, each 

 to its whole extent, as when ordinary light falls on the retina, the 

 result is a sensation of white. According to this theory, black is 

 simply the absence of sensation from the visual apparatus. 



It will be understood that the pure primary red sensation 

 need not necessarily appear in any of our actual sensations of red ; 

 we may suppose that it is always more or less mixed with the 

 primary green and even with the primary blue. So also we may 

 suppose that we never actually experience the primary sensations 

 of green or of blue ; to this point we shall return. 



In the view, as originally put forward by Young, the three 

 primary sensations were supposed to be represented by three sets 

 of fibres, each set of fibres being differently affected by different 

 rays of light, and the impulses passing to the brain along each set 

 awakening a distinct sensation. No such distinction of fibres can 

 be found in the retina ; but an anatomical basis of this kind is not 

 necessary for the theory ; we can easily conceive of the same 

 fibre transmitting three distinct kinds of impulses ; and indeed, as 

 we shall see later on, there are more ways than one by which we 

 can imagine the sensations to be differentiated. 



762. Another theory, that of Hering, starts from the 

 observation that when we examine our own sensations of light we 

 find that certain of these seem to be quite distinct in nature from 

 each other, so that each is something sui generis, whereas we 

 easily recognize all other colour sensations as various mixtures of 

 these. Thus red and yellow are to us quite distinct : we do not 

 recognize any thing common to the two , but orange is obviously 

 a mixture of red and yellow. Green and blue are equally distinct 

 from each other and from red and yellow, but in violet and purple 

 we recognize a mixture of red and blue. White again is quite 

 distinct from all the colours in the narrower sense of that word, 

 and black, which we must accept as a sensation, as an affection of 

 consciousness, even if we regard it as the absence of sensation 

 from the field of vision, is again distinct from everything else. 

 Hence the sensations, caused by different kinds of light or by the 

 absence of light, which thus appear to us distinct, and which we 

 may speak of as ' native ' or ' fundamental ' sensations, are white, 

 black, red, yellow, green, blue. Each of these seems to us to have 

 nothing in common with any of the others, whereas in all other 

 colours we can recognize a mixture of two or more of these. 



This result of common experience suggests the idea that these 



