638 



NATURE 



[Oct. 30, 1879 



although we can follow the ether vibrations to the 

 retina, it has not been possible to trace what happens 

 beyond. We can, indeed, compare the physical influences 

 with the sensations produced, but we are obliged, in doing 

 so, to skip over the intervening physiological steps where 

 the chief interest lies. Hence the backward study of the 

 processes, as inferred from their results, affords the best 

 chance of success. 



As to the general nature of the action of the visual 

 substance, we have a choice between the idea of mecha- 

 nical vibrations and that of chemical changes. Modern 

 physiology points to the latter, for the general physiology 

 of the nerves has sufficiently shown that all movement 

 and all activity of the nervous substance produces chemi- 

 cal changes in it, and all our representations of changes 

 of sensitiveness, fatigue, and restoration after activity, 

 are founded on the assumption of such chemical changes. 

 And however varied may be the views as to the details of 

 this action, so far is certain that the continual presence of 

 chemical processes in every vital and sensitive substance 

 is a fact, and that material change is the most universal 

 known property of every living thing. 



It is therefore taken for granted that light produces 

 chemical changes in the nervous apparatus of the visual 

 organs ; and what we term fatigue and change of sensi- 

 tiveness depend, by general consent, on chemical changes 

 of the sensitive substance. Hitherto, however, it has 

 been customary only to consider this (so far as black 

 and white are concerned) as an effect of white light ; 

 the element of black being neglected altogether, as 

 already explained. The author proposes to correct this 

 error, and he formulates his extended theory as follows : — 

 The two kinds of sensation which we call white {or 

 li^ht) and black {or aark) correspond to two distinct 

 kinds of chemical action in the visual substances and 

 the various proportions in which these appear in the 

 rnixed sensation of gray, correspond to the same propor- 

 tions of intensity of these tv/o psycho-physical processes. 



This is the simplest explanation conceivable, and it 

 fulfils every condition demanded by general nervous 

 physiology. We must assume a sensitive substance in 

 the visual apparatus, which suffers a change by the action 

 of light, and this change is generally believed to be a 

 chemical one ; and when the stimulating action is removed 

 there must be a corresponding change in the other direc- 

 tion, giving a return to the normal condition. If the 

 former change is assumed to be a partial consumption of 

 matter, then the opposite change must be a restitution ; 

 if the former change is an analytical or disintegrating one, 

 the latter must be a synthetical or reintegrating one, and 

 so on. 



Now the latter process, by which ' the living organic 

 substance replaces the quantity lost by stimulation or 

 activity, is usually called assimilation, and the author 

 retains this name. The previous or contrary process, 

 where the loss is caused by stimulation or activity, he 

 calls dissitnilation. Having to use these terms very often 

 he denotes them by the letters A and D respectively. 



These two processes result from the knowledge of 

 physiology in general, and if they are correct, there is no 

 reason why, as heretofore, only one of them, D, should 

 be admitted to a theory of visual perception, and the other, 

 A, excluded from it. The author's theory of the black- 

 white sensation, therefore, embodies the proposition that 

 the scnsatioji of white corresponds to dissimilation, and 

 that of black to assimilation of the visual substance, so 

 that our visual sensations furnish a psychical expression 

 of the correlation of the changes in the matter of this 

 substance. 



The following propositions are easily deducible from 

 this principle. 



The degree of lightness or darkness of a colourless 

 visual sensation corresponds with the proportion between 

 the intensities or magnitudes of the D and A actions 



respectively. For the medium gray these actions arc 

 equal, (_=_.= i ), so that the state of the visual 



substance remains constant. For a lighter gray D is 

 greater than A, while for a darker gray the reverse is the 

 case. 



If we call all stimulating actions which favour dissi- 

 milation, D stimuli, and if we borrow from general 

 physiology the proposition that the magnitude of the 

 reaction with which an organ answers to its stimulus 

 depends also on the mass of the excitable substance it 

 offers to be acted on, we get the principle that the 

 magnitude of the dissimilation caused by a Y>-stimulus 

 depends not only on the force of this stimulus, but also on 

 the quantity of the excitable substance present. 



But the ability of an excitable substance to be set by a 

 stimulus, in a state of excitation, is called its excitability 

 {Enegbarkeit), and the previous proposition may be thus 

 expressed : — 



Every increase of the excitable substance necessitates a 

 raising, every decrease a lowering, of the Vi-excitability of 

 the visual organ. Hence the sensation of medium gray 

 implies a uniformity, every brighter sensation a decrease, 

 and every darker sensation an increase of the D-excita- 

 bility. And it follows that if, at the same time, images 

 of different brightness fall on two places of equal D- 

 excitability, the place of the brighter sensation will have 

 its excitability lowered, and vice versa. 



The author further explains the law, that in any 

 compound sensation, the prominence of any particular 

 single one is expressed by the ratio which the magnitude 

 or " weight " of that sensation bears to the sum of all the 

 sensations present. For example, in a gray, the pro- 

 minence of white = „, , , . If the sensation of yellow 

 W -|- B 



is also present in an amount = Y, it will be = . 

 and the prominence of yellow will be 



■ W + B + Y' 

 Y 

 ■ W + B -f Y' 



These being the chief features of the author's theory 

 he goes on to show how it is applied to explain the various 

 phenomena already mentioned, particularly those of sub- 

 jective vision. 



The first point necessary to be explained is what may 

 be called the normal state of the visual organs, i.e., the 

 sensation experienced when the eyes have long been 

 closed and darkened, as on awaking in a perfectly dark 

 room. It has already been explained that this sensation, 

 although dark, is far removed from what we know as 

 black. It follows from the theory that in this state the U 

 and the A actions should be in equilibrium, i.e., about 



equally great or — = i, according to which the sensation 

 A 



should correspond to the medium gray. 



The author remarks on the fact that, comparing the 

 actual sensation with the brightest sun-light on the one 

 hand and the blackest known velvet on the other, it would 

 seem to be far nearer the black than the white ; but we 

 have no reason to believe that the darkest sensation we 

 can get at all approaches absolute blackness. He thinks 

 it possible that if we could get rays as near the black end 

 of the scale as sun-light is near the white end, we should 

 find their effect as powerful. But such rays do not exist 

 in nature, and in our ignorance of them we cannot define 

 accurately what may or may not be the true medium 

 gray. 



Next is given an explanation oi Simultaneous Contrasts 

 The before-mentioned experiments have shown that when 

 one part of the visual organ is stimulated by light, the 

 effect is to darken the sensation in the parts around. The 

 theory admits of the explanation of this in several ways :, 

 but the author prefers the following ; — In a partial stimu- 



