ii2o VISION. 



explains the shortening in the case recorded hy Vintschgau and himself, partly 

 by a co-existing weakness of the red-green sense, and partly by a quantitative 

 anomaly. He found evidence that the colourless region at the violet end had a 

 red value, for when violet light was added to a red which was below the chro- 

 matic threshold, the mixture became red, while addition of violet light to a green 

 above the chromatic threshold produced a mixture which appeared colourless. 



Hering's theory and the retina.— It might be supposed that, 

 according to Hering's theory, the katabolic colours should produce an 

 electrical change in the retina in an opposite direction to the anabolic 

 colours. Red and yellow should cause a variation of one kind, and 

 green and blue of the opposite kind. This is not the case. This 

 supposition, however, neglects the black-white substance which is acted 

 on by all kinds of light, and the variation due to this would mask any 

 difference in the behaviour of the chromatic substances. In order to 

 support the theory, it would only be necessary to show that a green 

 of a definite white value gives a smaller variation than a red of equal 

 white value. Another factor to be taken into account is the influence of 

 spatial induction, which, by causing the opposite kind of change in the non- 

 stimulated area, would still further obscure the direct results of stimulation. 



The recent views considered on pp. 1101-1105 must, if correct, modify 

 the theory to some extent, at any rate in the form in which it is usually 

 understood. According to these views, the spectrum of low intensity of 

 total colour-blindness is the spectrum of visual purple, while the effect 

 of dark-adaptation depends on accumulation of this substance. This 

 suggests that Hering's black-white substance and visual purple may be 

 one and the same thing; against this is the undoubted fact that the 

 production of colourless sensation by colour mixture holds for the fovea ; 

 the colourless sensation cannot here be due to the combined white values 

 of the coloured lights. It must be remembered, however, in answer to 

 this and other difficulties, that Hering has never regarded his theory as 

 an expression of retinal processes. In one place 1 he expressly states that 

 his theory has nothing to do with the first immediate changes in the 

 peripheral organ. If Hering's substances are regarded as central, these 

 difficulties to a great extent disappear. Under ordinary conditions, the 

 central black-white action could be excited by two kinds of peripheral 

 activity, i.e. changes in visual purple, or changes in the chromatic 

 apparatus. As already mentioned, the existence of binocular colour 

 contrast suggests the central origin of contrast in general, and, if spatial 

 induction is central, temporal induction has probably the same seat. 



Hering has not only left the actual seat of the psycho-physical 

 processes undecided, but, further, has only adopted the conception of 

 three special substances, in order to assist the exposition of his views. 

 He would prefer 2 to speak of only one psycho-physical substance, capable 

 of manifold qualitatively different kinds of anabolism and katabolism, 

 each kind being capable of analysis into three components, corresponding 

 to three chief varieties of anabolism and katabolism. 



G. E. Midler 3 has lately suggested certain modifications of Hering's theory, 

 by means of which he thinks certain difficulties may be removed. He supposes 

 that visual purple acts as a sensitizer to the visual substances proper, and that, in 



1 Arch./, d. ges. Physiol., Bonn, 1887, Bd. xli. S. 39. 



2 Sitzungsb. d. k. Akad. d. TFissensch., Wien, 1889, Bd. xcviii. Abth. 3, S. 73. 



3 Ztschr. f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1896, Bd. x. S. 1 

 and 321; and 1897, Bd. xiv. S. 1 and 161. 



