646 PHYSIOLOGY 



The green rays excite the green percipient element strongly, the 

 other two slightly, with green sensation as a result. Blue rays excite 

 green and violet percipient elements to a moderate extent, and the 

 red rays to a somewhat less extent ; blue is therefore a mixed sensa- 

 tion composed chiefly of green and violet. Simultaneous excitation 

 of all three elements evokes a sensation of white or grey, according 

 to the intensity. 



The cases of defective colour-vision which occur the so-called 

 colour-blindness are regarded under this theory as due to the 

 absence of one or other of the elements which determine the primary 

 colour-sensations. The normal eye is spoken of as trichromatic 

 since it has three primary colour-sensations. Two kinds of dichromatic 

 eyes are described those in which the red sensation is lacking, and 

 those in which the green sensation is lacking. In either of these 

 cases the subject confuses red and green. The ripe cherry on a tree 

 they may distinguish from the leaves, not by their colour, but by 

 form or difference in their luminosity. It is only when they are 

 tested by means of the spectrum that we find that whereas in the 

 first case (red blindness) there is insensibility to the red end of the 

 spectrum, in the second case the red end of the spectrum is seen as 

 well as by the normal person, so that the defect must be located 

 towards the middle of the spectrum. Theoretically, 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 Hering 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 considers that there is a special 

 substance in the retina, dissimilation or catabolism 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 

 catabolism of the white-black substance. This breaking down excites 

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



