BERING'S THEORY OF COLOR VISION. 529 



and destruction of the red-green substance are equal no 

 color sensation is aroused by it; and we get gray, due to 

 those simultaneously occurring changes in the black-white 

 substance which are always present, but were previously 

 more or less cloaked by the results of the changes in 

 the red-green substance. Eed and green in certain pro- 

 portions cause then a white or gray sensation, not because 

 they supplement one another, as on Young's theory, but 

 because they mutually cancel; and so for other comple- 

 mentary colors. 



Moreover, according to Hering, destruction of a visual 

 substance going on in one region of the retina promotes 

 construction and accumulation of that substance elsewhere, 

 but especially in the neighborhood of the excited spot. 

 Hence, wheu a white square on a black ground is looked 

 at, destruction of the black-white substance overbalances 

 construction in the place on which the image of the square 

 falls, but around this construction occurs in a high degree. 

 When the eyes are shut, this latter retinal region, with its 

 great accumulation of decomposable material, is highly 

 irritable and^ under the internal stimuli causing the idio- 

 retinal light, breaks down comparatively fast, causing the 

 corona, which may be intensely luminous; for with the 

 closed eye the total constructive and destructive processes 

 in the visual apparatus are small, and so the excess of de- 

 struction in the coronal region bears a large ratio to the 

 sum of the whole processes. The student must apply this 

 theory for himself to the other phenomena of contrasts and 

 negative images, as also to the gradual disappearance of 

 differences between light and dark objects when looked at 

 for a time with steady fixation; the general key being the 

 principle that anything leading to the accumulation of a 

 yisual substance increases its decompositions under stimu- 

 lation, and vice versa. The main value of Hering's theory 

 is that it attempts to account physiologically for phenomena 

 previously indefinitely explained psychologically by such 

 terms as "errors of judgment," which really leave the 

 whole matter where it was, since if (as we must believe) 

 mind is a function of brain, the errors of judgment have 



