192 THE PERCEPTION OF LIGHT AND COLOR. 



cides invariably with the region of maximum absorption for visual 

 yellow, the region corresponding to a maximum intensity of the yellow 

 may occupy two different positions, according to the subject selected. 

 Hence this peculiarity would correspond exactly to the existence of two 

 different purples, the one red and the other violet, the absorption bands 

 corresponding to the two varieties of Daltonism. 



From these facts Ebbinghaus concludes that the visual purple is 

 involved in the perception of yellow, and the visual yellow in per- 

 ception of blue. In addition to these two substances, which are 

 derived from one another, he assumes the existence of a third, namely, 

 visual white, which by its transformation gives rise to the perception 

 of white and gray. In normal eyes he assumes a new substance which, 

 by transforming itself like visual purple, would give rise to the percep- 

 tions of red and green. It is quite evident what an important role 

 unverified hypotheses play in this theory as in that of Koenig. Never- 

 theless, it might be remarked that the study of subjects afflicted with 

 achromatopsia, more or less complete, is susceptible of furnishing many 

 interesting results. 



Von Kries and his pupils have for the most part been engaged in 

 observations of this nature. But if they believe that they have over- 

 thrown Hering's theory, others like Kirschmann think that they can 

 show the insufficiency of every theory admitting only three sources of 

 luminous sensations. On the basis of a study of color-blindness the 

 latter believes that he has established that the customary division of 

 cases of color-blindness into blindness for blue and yellow and blind- 

 ness for red and green is insufficient to explain the facts. 



With such an accumulation of data some are very likely to be in 

 contradiction to the others, and hence there rises the need of a synthetic 

 treatise criticising the different theories and combining them to form a 

 new one which can coordinate all psychological, physiological, patho- 

 logical, histological, and chemical data appertaining to the subject of 

 color vision. M. Victor Henri is of the opinion that a publication of 

 Prof. G. E. Muller, of Gottingen, on the "Psycho-physics of visual 

 sensation" meets this desideratum. A very interesting analysis of his 

 results is to be found in the fourth volume of L'Annee Psychologique, 

 but we shall have to limit ourselves to a statement of the conclusions 

 reached. All visual sensations are based on six chemical processes of 

 the retina, corresponding to the sensations of white, black, red, green, 

 yellow, blue; the author adopts, moreover, on the whole, the views of 

 Von Kries and Parinaud in regard to the role of the cones and rods. 

 He, however, contends that the visual substances are the same in both. 



In a separate publication on the visual sensations produced by the 

 galvanic current, Muller announces it as a general fact that when the 

 current passes through the eyes toward the back of the head the sub- 

 ject experiences a bright blue-red color sensation, while if the current 

 passes in the opposite direction the sensation produced is that of a 



