THE SENSE OF VISION. 783 



bered that observations on the augruentor and inhibitory cardiac nerves have 

 shown us that nerve-stimulation may produce very contrary effects. There 

 seems to be, therefore, no serious theoretical difficulty in supposing that light 

 rays of different wave-lengths may produce opposite metabolic effects upon the 

 substances in which changes are associated with visual sensations. 



A more serious objection lies in the difficulty of distinguishing between the 

 sensation of blackness, which, on Hering's hypothesis, must correspond to active 

 anabolism of the white-black substance, and the sensation of darkness (such as 

 we experience when the eyes have been withdrawn for some time from the 

 influence of light), which must correspond to a condition of equilibrium of 

 the white-black substance in which neither anabolism nor katabolism is 

 occurring. 



Another objection to the Hering theory is to be found in the results of 

 experiments in comparing grays or whites produced by mixing different colored 

 rays under varying intensities of light. The explanation given by Hering of 

 the production of white through the mixture of blue and yellow or of red and 

 green is that when either of these pairs of complementary colors is mixed 

 the anabolic and the katabolic processes balance each other, leaving the corre- 

 sponding visual substance in a condition of equilibrium. Hence, the white- 

 black substance being alone stimulated, the result will be a sensation of white 

 corresponding to the intensity of the katabolic process caused by the mixed 

 rays. Now, it is found that when blue and yellow are mixed in certain pro- 

 portions on a revolving disk a white can be produced which will, with a certain 

 intensity of illumination, be undistinguishable from a white produced by mix- 

 ing red and green. If, however, the intensity of the illumination is changed, 

 it will be found necessary to add a certain amount of white to one of the mix- 

 tures in order to bring them to equality. On the theory that complementary 

 colors produce antagonistic processes in the retina it is difficult to understand 

 why this should be the case. 



A color theory which is in some respects more in harmony with recent 

 observations in the physiology of vision has been proposed by Mrs. C. L. 

 Franklin. In this theory it is supposed that, in its earlier periods of de- 

 velopment, the eye is sensitive only to luminosity and not to color i. e. it 

 possesses only a white-black or (to use a single word) a gra^-perceiving sub- 

 stance which is affected by all visible light rays, but most powerfully by those 

 lying near the middle of the spectrum. The sensation of gray is supposed to 

 be dependent upon the chemical stimulation of the optic nerve-terminations by 

 some product of decomposition of this substance. 



In the course of development a portion of this gray visual substance becomes 

 differentiated into three different substances, each of which is affected by rays 

 of light corresponding to one of the three fundamental colors of the spectrum 

 viz. red, green, and blue. When a ray of light intermediate between two 

 of the fundamental colors falls upon the retina, the visual substances corre- 

 sponding to these two colors will be affected to a degree proportionate to the 

 proximity of these two colors to that of the incident ray. Since this effect is 



