:66 



NATURE 



[February i 6, 1893 



z to violet, Hering's coordinates u, v, iv will have the 

 values 



_ .r - 2/ + ^ 



v/6 



V3 ' N^2 



u denoting the white element, and being measured along 

 the axis of the pyramid ; w denoting the red-green ele- 

 ment, and being measured at right angles to the axis of 

 white, in the plane containing the green edge of the 

 pyramid ; %' denoting the yellow-blue element, and being 

 measured at right angles to the plane of it, w. 



Positive values of w correspond to purple red, and 

 negative values to green. Positive values of 7/ correspond 

 to yellow, and negative values to blue. 



I give these equations in this definite shape for the 

 purpose of showing, by a definite system of representa- 

 tion, that the arbitrariness which attends the choice of 

 three colours, in terms of which the rest are to be 

 specified, affords sufficient latitude to admit of the em- 

 ployment of three such different specifying elements as 

 are adopted by Hering. 



If only positive values of x,y, z are to be admissible, 

 the expression for ?e shows that every kind of light must 

 excite the white sensation positively, and consequently 

 that no kind of objective light can produce a pure sensa- 

 tion either of the red-green or of the yellow-blue kind. 

 Hence the pure unmixed "opposed colour-sensations" 

 are such as we never have had or can have, and are 

 separated from all colour-sensations that we have ever 

 had by a much wider gap than the pure sensations which 

 Young's theory supposes, although these latter extend 

 somewhat beyond the range of objective colours. By 

 subjecting portions of the retina to special influences 

 (as we shall explain in treating of after-images) we can 

 at least approximate to Young's elementary sensations; 

 while these same methods, when we attempt to approxi- 

 mate to Hering's pure sensations, give results opposite 

 to what his theory would lead us to expect. 



Hering assumes, in accordance with the brief expres- 

 sion of his theory in the above equations, that white light 

 excites only the white-black visual substance and excites 

 it always positively ; that yellow light, besides doing 

 this, excites the blue-yellow visual substance, as does 

 also blue light, but in opposite sense. On the other 

 hand, when blue and yellow lights are in exact equili- 

 brium, they have no action on the blue-yellow visual 

 substance.^ Similar remarks apply to the excitements of 

 the red-green visual substance by red and green light. 



The sensation of luminosity is identified by Hering 

 with the sensation of white. He accordingly maintains 

 that the pure sensation of blue or of yellow involves no 

 sensation of luminosity. I must confess that personally 

 I can form no conception of a colour which has no degree 

 of less or greater luminosity, and therefore think such an 

 abstraction not tolerable in a system which, on other 

 points, makes its appeal to the immediate testimony of 

 inner consciousness, and claims by this means to establish 

 its superiority to other systems. 



Differences of intensity must, however, occur in the 

 opposed colour-sensations if they involve no difference 

 of brightness. In comparing saturated blue with equally 

 luminous pale blue, Hering would regard the white sen- 

 sation as equally intense in both, but the blue sensation 

 as stronger in the saturated blue. 



As the physiological basis of the "opposed colour-sensa- 

 tions " Hering takes the two opposite processes of organic 

 change, namely, the decomposition of the organic mass 

 by activity, and its restoration under the influence of the 

 circulation of the blood, which carries oxygen stored up 

 in it and feebly united with it. The former process is 



I This was a point which Hering left doubtful in the earlier statements of 

 his system, so that it was not clear whether he assumed three or six inde- 

 pendent variables. According to his more recent explanations the state- 

 ment given in the text may fairly be said to represent his view. 



NO. I 2 16, VOL. 47] 



called dissimilation, and the latter assimilation. Which 

 of the two opposed sensations corresponds to dissimila- 

 tion and which to assimilation is left undecided, both in 

 the case of blue-yellow and of red-green. The physio- 

 logical improbabilities of this assumption have in part 

 been pointed out already, and we shall return to the 

 subject in treating of after-images. 



This assumption of double nerve-working was originally 

 applied by Hering to the white-black visual substance also. 

 At the present time he adheres to the hitherto-received 

 doctrines of nerve-physiology to the extent of holding 

 that, in the case of this substance, all light excites only 

 dissimilation and the sensation of white ; and on the 

 other hand want of light produces only assimilation and 

 restoration of excitability. That during this latter pro- 

 cess a sensation of darkness is experienced, all are 

 agreed. The difference is purely theoretical. According 

 to the older view, which I have defended, we must, in 

 order to perceive that there is luminosity in a particular 

 part of the field of view at a given time, be able to dis- 

 tinguish at another time that this perception is wanting. 

 This perception that a sensation which might be there is 

 not there contains in itself a testimony as to the condition 

 of the organ at the time, which is different from all 

 sensations of incident light ; and in this sense we call it 

 also a sensation — the sensation of darkness. 



Hering, on the contrary, maintains that the sensation 

 of black must have its own special physiological basis of 

 excitation, and seeks it in assimilation, going on in the 

 white-black visual substance. 



From the foregoing account the reader will gather that 

 Hering's theory, if we overlook its physiological views, is 

 able to explain all hitherto established facts of colour 

 mixture as well as, but not better than. Young's theory. 

 It diff"ers only in its special choice of elementary excita- 

 tions ; and this choice, if we admit negative values of 

 them, suffices for expressing the facts, just as any axes 

 of co-ordinates suffice for a problem of solid geometry. 



Hering's objections to Young's theory reduce them- 

 selves, in his latest statement, to the following: — 



"In the Young- Helmholtz theory, the assumption of 

 the three elementary colour-sensations is ii priori repul- 

 sive, because these sensations are not presentable : and 

 notoriously, according to necessity, now one set and now 

 another set of elementary colour-sensations are assumed." 

 As to this, I have already remarked that the funda- 

 mental sensations of Young's theory, in so far as they 

 differ from objective colours, can be approximated to, by 

 the method of partial fatigue of the retina, much more 

 closely than Hering's pure opposed-colour-sensations. If 

 different upholders of Young's theory have made different 

 assumptions as to the three primary colours, and have 

 assigned different weights to various facts which bear on 

 the distinction, this affords no justification whatever for 

 the imputation that they have changed their assumptions 

 according to necessity. It is always better to acknow- 

 ledge existing doubt than to dogmatise. 



Hering goes on, " If the excitations belonging to the 

 three elements have correspondingly distinct physiolo- 

 gical causes, one would expect that these sensations 

 would have something special about them." 



This they have, in my opinion, in the prominent glow 

 of colour-saturation ; for which, again, the theory of 

 opposed-colours furnishes no basis of explanation. 



He continues, "Yellow gives, for example, much more 

 the impression of a simple or elementary sensation than 

 violet, and yet we are told that the latter is an elementary 

 sensation and the former a mixture of simultaneous sen- 

 sations of red and green, or at least, in some way, the 

 product of the simultaneous existence of the principal 

 excit3,tions corresponding to these two elementary 

 sensations." 



What a deceitful test apparent inner consciousness is 

 in such matters, we can see from the examples of two 



