THE PERCEPTION OF LIGHT AND COLOR. 193 



dark greenish-yellow. Here is a confirmation of the theory of reversed 

 chemical reactions on the retina. It would seem to us that this would 

 be rather an argument in favor of the three substances of Hering thau 

 of the six substances of Muller, but possibly we have not entirely 

 comprehended his theory, a disadvantage often unavoidable when one 

 works with a mere resume, however well made. We are not confronted 

 by the same difficulty in reviewing the theories of MM. Nicati and 

 Charpentier, that of the former having been published in L'Annales 

 d'Ophtalmologie, January, 1895. After having pointed out that lumi- 

 nous sensations of very small dimensions, although corresponding to a 

 retinal field smaller than the rod itself, are recognized in their proper 

 colors, and having concluded therefrom that the same rod must be 

 capable of transmitting different colors, the author assumes that there 

 should correspond to the different chemical actions, produced by the 

 different radiations, variations in the quantity and tension of the 

 electric currents engendered by them. The short radiations having 

 an intense and rapid action, should develop currents of maximum 

 quantity and minimum tension. These can only flow through nerve 

 filaments of small resistance — that is to say, the short and thick ones — 

 while the currents of high tension are able to follow the loug and thin 

 filaments, which offer a greater resistance. These different currents 

 are discharged upon the optoblasts, which are differentiated by the 

 influence of habit and heredity, and hence result the differences in color 

 sensations. 



Much more subtle and more complex in its details is the theory of 

 Charpentier. As we have already pointed out, he distinguishes, together 

 with Parinaud, three functions of the retina — luminous sensibility, 

 chromatic sensibility, and visual sensibility. Up to this time we have 

 not dwelt much on the latter, but it is to play a preponderating role 

 in the theory we are about to discuss, and it is therefore necessary to 

 dwell on this function somewhat before taking up the consideration of 

 the theory. 



If, while in darkness, we determine the minimum illumination neces- 

 sary for the perception of a luminous surface of considerable extent, 

 we find there is no sensible difference in the aspect of that surface, 

 whether it be brightly or dimly illuminated; but if the surface be suf- 

 ficiently small it is seen under a minimum illumination as a diffused 

 spot with indistinct outlines always much larger than it really is. A 

 sharp perception of its form and dimensions requires a stronger 

 illumination. 



The enlargement of a dimly illuminated image is explained by the 

 diffusion of the luminous impression on the retina or in the nerve centers, 

 a fact which has been thoroughly verified, but distinct vision evidently 

 requires an additional stimulus or the excitation of a new physiological 

 element. 



By means of very small holes, a millimeter apart, in an opaque screen, 

 SM98 13 



