THE PERCEPTION OF LIGHT AND COLOR. 189 



engagement of electricity or heat, while several experimenters have 

 verified that it is otherwise in the case at hand. Since 1874 — that is to 

 say, before the discovery of visual purple — Dewar has recognized that 

 the action of light on the retina is accompanied by the development of 

 an electro-motive force, measurable by means of a galvanometer. 

 Having confirmed this observation, Johannes Chatin established the 

 effect of obscuration on the intensity of the current and the unequal 

 action of the different radiations; in addition, he verified that the 

 greatest electro-motive force is found in those species in which the 

 purple predominates, as in the lobster. All these circumstances tend 

 to show that the disengagement of electricity is principally caused by 

 physico-chemical action that has its seat in the visual purple. 



The conceptions of Parinaud, which may indeed be reduced .to the 

 perception of colors by the cones and of colorless light by the rods 

 under the action of the purple, have been readopted by Von Kries 

 since 1894, and, indeed, M. Victor Henri, in referring to them, constantly 

 calls it the theory of Kries. Parinaud, who claims very properly the 

 rights of priority, moreover adds that Schultze, as early as 1866, 

 pointed out the probable difference in the role of the cones and that of 

 the rods, a difference which he based on their unequal distribution in 

 the retina and on the diminution in the intensity of color vision at its 

 periphery, where the cones are rare. Schultze's opinion, however, 

 remained unnoticed, but fortunately it has now been completed by the 

 discovery of the purple and all the investigations tliat have been made 

 upon it. Therefore Schultze appears to deserve an eminent place in 

 the development of the subject near Parinaud. The investigations of 

 Koenig and of Kiihne appear to be much more original than those of 

 Kries; for by comparing the absorption curves of visual purple and 

 yellow, in different parts of the spectrum, with those of the luminous 

 impression and its variation for the different spectral colors, they 

 have truly completed the results of Parinaud. In the article of Weiss, 

 already referred to, I found some very curious curves, showing the 

 proportionality between the luminous impression and the absorption 

 of the rays by the visual purple. We shall have occasion to return to 

 these investigations in Part III. 



III. — THEORIES OF COLOR PERCEPTION. 



In all that precedes we have seen that certain anatomical elements 

 appear to be involved in color perception and that the different radia- 

 tions exercise an influence more or less great on the luminous sen- 

 sation, but nothing has so far beeu said concerning the mechanism of 

 differentiation in color perception. Here it must be clearly recognized 

 that we are on unsafe ground; on account of the lack of sufficient 

 experimental evidence, the hypotheses become more audacious and are 

 often too far removed from the possibility of experimental control. 



