CHAP, ii.] SIGHT. 027 



colours can be taken in such proportions as with a proper addition, 

 if necessary, of white to produce the sensations of all other colours 1 . 

 That is to say, given three standard sensations, all the other 

 sensations may be gained by the proper mixture of these. 



It is obvious from the foregoing that our real colour sensations 

 are much fewer in number than those which we appear to have 

 when we look on the colours of the spectrum or of nature; that 

 rays of light awake in us certain simple sensations, which mixed in 

 various proportions reproduce all our sensations. And the question 

 arises, what is the nature or what are the characters of these simple 

 sensations ? 



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 

 recognise all other sensations as various mixtures of these. Thus 

 red and yellow are to us quite distinct: we do not recognise any 

 thing common to the two ; but orange is obviously a mixture of red 

 and yellow. The sensations caused by different kinds of light 

 which thus appear to us distinct, and which we may speak of as 

 '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 recognise a mixture 

 of two or more of these. 



This result of common experience suggests the idea that these 

 fundamental sensations are the primary or simple sensations, 

 spoken of above as those out of which all other sensations may be 

 supposed to be compounded. And a theory has been proposed to 

 reconcile the various facts of colour vision, with the supposition 

 that we possess these six fundamental sensations. This theory, 

 known as that of Hering, is somewhat as follows. The six sensa- 

 tions readily fall into three pairs, the members of each parr having 

 analogous relations to each other. White and black naturally go 

 together, the one being the antagonistic or correlative of the other. 

 There is a similar connection between red and green, the one being 

 the complementary of the other, and between yellow and blue 

 which are similarly complementary. We saw reason, a short time 

 back (p. 518), for believing that vision originates in the changes 

 taking place in certain visual substances (or a visual substance) in 

 the retina. And the theory of which we are speaking supposes that 

 there exist in the retina, or at least somewhere in the visual appa- 

 ratus, three distinct visual substances which are continually under- 

 going a double metabolism, one constructive, of assimilation or 

 building up, and the other destructive, of dissimilation or breaking 

 down. One of these substances is further of such a nature that 



1 A few highly saturated colours cannot be so reproduced, but a mixture of any 

 one of them with white can. We may perhaps therefore speak of these saturated 

 colours as being reproduced by a proper combination of the three arbitrarily selected 

 colours, with the subtraction of white. 



