SENSATION. 19 



ing purely on physiological action of the terminal apparatus of 

 vision. Hering is the most prominent supporter of this view. 

 By great originality in devising experiments and by insist- 

 ing on rigid care in conducting them, he has been able to 

 detect the faults in the psychological theory and to practi- 

 cally establish the validity of his own. Every visual sensa- 

 tion, he maintains, is correlated to a physical process in the 

 nervous apparatus. Contrast is occasioned, not by a false 

 idea resulting from unconscious conclusions, but by the 

 fact that the excitation of any portion of the retina — and 

 the consequent sensation — depends not only on its own 

 illumination, but on that of the rest of the retina as well. 



" If this psycho-physical process is aroused, as usually happens, by 

 light-rays impinging on the retina, its nature depends not only on the 

 nature of these rays, but also on the constitution of the entire nervous 

 apparatus which is connected with the organ of vision, and on the state 

 in which it finds itself." * 



When a limited portion of the retina is aroused by ex- 

 ternal stimuli, the rest of the retina, and especially the 

 immediately contiguous parts, tends to react also, and in 

 such a way as to produce therefrom the sensation of the 

 opposite degree of brightness and the complementary color 

 to that of the directly-excited portion. When a gray spot 

 is seen alone, and again when it appears colored through 

 contrast, the objective light from the spot is in both cases 

 the same. Helmholtz maintains that the neural process 

 and the coYresponding sensation also remain unchanged, but 

 are differently interpreted ; Hering, that the neural process 

 and the sensation are themselves changed, and that the 

 ' interpretation ' is the direct conscious correlate of the 

 altered retinal conditions. According to the one, the con- 

 trast is psychological in its origin ; according to the other, 

 it is purely physiological. In the cases cited above where 

 the contrast-color is no longer apparent — on a ground with 

 many distinguishable features, on a field whose borders are 

 traced with black lines, etc., — the psychological theory, as 

 we have seen, attributes this to the fact that under these 

 circumstances we judge the smaller patch of color to be an 



* E. Heriug, iu Hermaim's Haiidbucli d Physiologie, in. 1, p. 565. 



