900 THEORIES OF COLOUR VISION. [BOOK nr. 



excited to a certain extent by the rays at the extreme red end, 

 is most powerfully affected by the rays at a little distance from 

 that end, the rays from this point onwards towards the blue end 

 producing less and less effect. The curve of the green primary 

 sensation begins later and reaches its maximum in the green of 

 the spectrum, while the violet or blue primary sensation is still 

 later and only reaches its maximum towards the blue end of the 

 spectrum. Each ray calls forth each primary sensation though 

 to a different degree, and the total result of each ray, or of each 

 group of rays, is determined by the proportionate amount of 

 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 violet; the orange sensation is converted 

 into a yellow sensation by diminishing the primary red and 

 largely increasing the primary green, the primary violet under- 

 going 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 sensa- 

 tion from the visual apparatus. 



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. 



565. 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 mix- 

 tures 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 nar- 

 rower 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 



