186 THE PERCEPTION OF LIGHT AND COLOR. 



these experiments were made on eyes adapted to obscurity. Under these 

 circumstances it might possibly be questioned if, when the luminous 

 points observed become sufficiently bright, the adaptation of the cor- 

 responding points of the retina does not diminish. However, if this 

 were the case, it should result in increasing the smallest perceptible 

 difference for the more refrangible colors. It is cpiite evident that Oliar- 

 pentier's explanation only differs in form from that of Parinaud. As 

 a counterproof it would be interesting to repeat Charpentier's experi- 

 ments on eyes adapted to light, the eye being subjected to full daylight 

 between two consecutive observations. In passing I might call atten- 

 tion to a complex phenomenon, easily observable, in which the Pur- 

 kinje phenomenon is greatly exaggerated by the effect of contrast. 



In a poorly illuminated church just at daybreak the stained-glass 

 windows at a certain moment assume an aspect that is at first surpris- 

 ing; of all the colors, the blue alone appears with any brilliancy. 

 •It would be superfluous to emphasize the fact that the contrast of 

 the yellowish color of the interior light is added to the Purkinje 

 phenomenon. 



There is another fact to which I have not as yet alluded, but which 

 I have always observed in myself. When attempting, in a poor 

 light, to read fine print I instinctively make use of monocular vision. 

 This appears to result from the fact that under these circumstances one 

 naturally has recourse to indirect vision, which is alone benefited by 

 adaptation to obscurity and which is difficult to adjust to binocular 

 vision. It would be possible to continue the enumeration of phenomena 

 related to the fundamental observations of Oharpentier and Parinaud, 

 but it is better to turn now to the examination of the anatomical struc- 

 tures and the chemico-physiological phenomena which appear to be 

 related to the characteristic perceptions which we have been discussing. 



II. — PHYSIOLOGICAL PHENOMENA. 



It is well known that the optic nerve, after having entered the eye- 

 ball at the blind spot, designated as the papilla, spreads out in such a 

 manner as to form what is called the internal layer of the retina, but 

 which could as well be called its external layer, for while it is internal 

 from the geometric standpoint, as being nearer to the center of the 

 eyeball, it is external with respect to the tissues which envelop the eye. 

 The nerve fibers thus spread out curve back toward the exterior, and 

 finally terminate in the layer of rods and cones, or Jacob's membrane, 

 about 50 microns thick. This layer is covered with pigmentary cells, 

 in which the terminal organs of the optic nerve are more or less buried. 

 It should be pointed out, furthermore, that the nerve fibers, before 

 reaching the membrane, penetrate, one by one, large cells which are 

 provided with many prolongations toward the external layers. This 

 description is quite abbreviated, for not less than 10 different layers 

 are distinguishable within the thickness of the retina. An examina- 



