VISUAL SENSATIONS 579 



the defect must be located towards the middle of the spectrum. Theoreti- 

 cally, of course, violet blindness ought also to exist, but cases of this nature 

 are so rare that their very existence is doubted. Cases have also been recorded 

 with total colour-blindness (so-called monochromatic vision). Such cases 

 have been supposed to be endowed only with vision similar to that found in 

 the dark-adapted eye, i.e. with sensations only of white and black rather than 

 vision determined only by the existence of the violet-perceiving constituent 

 of the cones. Indeed the phenomena we have studied under the heading of 

 dark adaptation would tend to show that, if we accept the Young-Helmholtz 

 theory, we should add to the primary visual sensations those of light and 

 dark, which have their seat in the -rods. 



(6) THE HERING THEORY. According to Bering there are four, and 

 not three, primary colour-sensations, viz. red, yellow, green, and blue. In 

 this theory the sensations of white and black are also regarded as primary 

 visual sensations. These sensations are placed in three groups red and 

 green, yellow and blue, white and black. For each pair of sensations he con- 

 siders that there is a special substance in the retina, dissimilation or cata- 

 bolism of which gives rise to one colour-sensation ; anabolism or assimilation 

 to the other. Thus if white light falls on the retina, it causes a breaking 

 down or cataboiism of the white-black substance. This breaking down 

 excites certain fibres of the optic nerve, and produces in consciousness a 

 sensation of white. If the light be now removed, this breaking down gives 

 place to anabolism or building up of the white-black substance, which excites 

 the same nerve fibrils in a different way, giving rise to a sensation of black. 

 The white-black substance is affected not only by white light but also by the 

 colours red, green, yeUow, blue, and their mixtures. The other two visual 

 substances are affected only by red and green or by yellow and blue respec- 

 tively. Hence even the spectral colours do not give rise to pure sensations, 

 there being always some mixture of a sensation of white with the proper 

 colour-sensation. 



Most of the phenomena of colour- vision that we have mentioned above 

 can be equally well explained on either theory. Thus the fact that blue and 

 yellow together give rise to a sensation of white may be explained on the 

 Young-Helmholtz theory by saying that the stimulation of all three sets of 

 fibrils is equal, as will be seen by adding together the ordinates of each curve 

 in Fig. 296 at yellow and at blue. 



Adopting Bering's hypothesis, we may say that, anabolism and catabo- 

 iism being equally excited in the yellow- blue substance, no change in it takes 

 place, and the sole sensation is that produced by the stimulation of the white- 

 black substance. The pairs of colours that we have distinguished are there- 

 fore, according to this theory, not in the strict sense of the word comple- 

 mentary, but antagonistic. The fact that white light appears to us as a simple 

 sensation and gives us no suspicion of the coloured rays of which it may be 

 composed is in favour of Hering's theory. Cases of colour-blindness would 

 be reduced by Hering to two classes, viz. those in which the red-green sub- 

 stance is lacking and those in which the yellow-blue substance is lacking. 



