THE EYE AS A SENSORY APPARATUS. 551 



previously more or less cloaked by the results of the changes 

 in the red-green substance. Ked 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 be- 

 cause they mutually cancel; and so for other complementary 

 colors. 



Moreover, according to Hering, destruction of a visual sub- 

 stance going on in one region of the retina promotes con- 

 struction and accumulation of that substance elsewhere, but 

 especially in the neighborhood of the excited spot. Hence, 

 when a white square on a black ground is looked at. destruc- 

 tion 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 accumu- 

 lation 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 destruction 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 visual substance increases its decomposi- 

 tions under given stimulation, and vice versa. The main 

 value of Bering's theory is that it attempts to account 

 physiologically for phenomena previously indefinitely ex- 

 plained psychologically by such terms as "errors of judg- 

 ment," 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 still to be accounted for on physio- ' 

 logical grounds, as due to conditions of the nervous system. 



The three visual substances, the anabolisms and katabol- 

 isms of which according to Hering give rise to color sensa- 

 tions, need not necessarily be in the retina itself: they may 

 be in the central nerve portions of the visual apparatus, 

 being excited through different nerve fibres excited by dif- 

 ferent lights falling on the retina. 



