BERING'S THEORY. JJJ 



matter, this substance during the activity of the metabolic process undergoes decomposition or 

 "disassimilation" ; while during rest it must be again renewed, or "assimilate" new 

 material. Hering assumes that for the perception of white and black, two different qualities 

 of the chemical processes take place in the visual substance, so that the sensation of white 

 corresponds to the disassimilation (decomposition), and that of black to the assimilation 

 (restitution) of the visual substance. According to this view, the different degrees of distinct- 

 ness or intensity with which these two sensations appear, occur in the several transitions between 

 pure white and deep black ; or, the proportions in which they appear to be mixed (grey), 

 correspond to the intensity of these two psycho-physical processes. Thus, the consumption 

 and restitution of matter in the visual substance are the primary processes in the sensation of 

 white and black. In the production of the sensation of white, the consumption of the visual 

 substance is caused by the vibrating ethereal waves acting as the discharging force or stimulus, 

 while the degree of the sensation of whiteness is proportional to the quantity of the matter 

 consumed. The process of restitution discharges the sensation of black ; the more rapidly it 

 occurs, the stronger is the sensation of black. The consumption of the visual substance at one 

 place causes a greater restitution in the adjoining parts. Both processes influence each other 

 simultaneously and conjointly. [In the production of a visual sensation, it is important to 

 remember that the condition of one part of the retina influences contemporaneously the 

 condition of adjoining parts of the retina, i.e., "the sensation which arises through the 

 stimulation of any given point of the retina, is also a function of the state of other immediately 

 contiguous points."] This explains physiologically the phenomenon of contrast of which the 

 old view could give only a psychical interpretation (p. 782). 



Similarly, colour sensation is regarded as a sensation of decomposition (disassimilation) and 

 of restitution (assimilation) ; in addition to white, red and yellow are the expression of 

 decomposition ; while green and blue represent the sensation of restitution. Thus, the visual 

 substance is subject to three different ways of chemical change or metabolism. We may 

 explain in this way the coloured phenomena of contrast and the complementary after-images. 

 The sensation of black- white may occur simultaneously with all colours ; hence, every colour 

 sensation is accompanied by that of dark or bright, so that we cannot have an absolutely pure 

 colour. There are three different constituents of the visual substance ; that connected with 

 the sensation of black-white (colourless), that with blue-yellow, and that with red-green. 

 All the rays of the visible spectrum act in disassimilating the black-white substance, but the 

 different rays act in different degrees. The blue-yellow or the red-green substances, on the 

 other hand, are disassimilated only by certain rays, some rays causing assimilation, whilst 

 others are inactive. Mixed light appears colourless when it causes an equally strong dis- 

 assimilation and assimilation in the blue-yellow and in the red-green substance, so that the 

 two processes mutually antagonise each other, and the action on the black-white substance 

 appears pure. Two objective kinds of light, which together yield white, are not to be 

 regarded as complementary, but as antagonistic, kinds of light, as they do not supplement 

 each other to produce white, but only allow this to appear pure, because, being antagonistic, 

 they mutually prevent each other's action. 



The imperfection of the Young-Helmholtz theory of colour sensation is that it recognises 

 only one kind of excitability, excitement, and fatigue (corresponding to Hering's disassimilation), 

 and that it ignores the antagonistic relation of certain light rays to the eye. It does not regard 

 white as consisting of complementary light rays, which neutralise each other by their action on 

 the coloured visual substance, but as uniting to form white {Hering). 



[While it suffices to explain a great many of the phenomena of light and colour, e.g., the 

 mixing of colours and complementary colours, it does not satisfactorily explain contrast or 

 colour-blindness. Fick admits that it does not explain the following important fact : Every 

 ray of light, while exciting a colour sensation if it falls on a sufficient area of the posterior 

 polar part of the eyeball, provided it acts on an extremely limited part of the retina, even if 

 it be coloured light, produces a whitish impression. This is exactly the opposite of what we 

 should expect, viz., the smaller the area of retina acted on, the more readily should the parti- 

 cular nerve-ending be excited and a pure colour sensation result] 



In applying this theory to colour-blindness ( 397), we must assume that those 

 who are red-blind want the red-green visual substance ; there are but two partial 

 spectra in their solar spectrum, the black-white and the yellow-blue. The position 

 of green appears to such an one to be colourless ; the rays of the red part of the 

 spectrum are visible, so far as the sensation of yellow and white produced by these 

 rays is strong enough to excite the retina. Hering divides his spectrum into a 

 yellow and ablue half. A violet-blind person wants the yellow-blue visual substance ; 

 in his spectrum there are only two partial spectra, the black- white and the red-green. 

 In cases of complete colour-blindness, the yellow-blue and red-green substances 

 are absent. Hence, such a person has only the sensation of bright and dark. The 



