THE PERCEPTION OF LIGHT AND COLOR. 187 



tion of the different regions of the retina shows that the cones, which 

 are on the average 20 times less numerous than the rods, alone exist in 

 the yellow spot, but are more and more outnumbered by the rods as 

 one approaches the periphery. The cones of the " macula " or yellow 

 spot are, besides, more elongated and smaller than those of the rest of 

 the retina. The multipolar cells, referred to above, increase in number 

 and are only bipolar in the macuJa, but they disappear in the fovea 

 which lies at the center of the spot. 



Besides these details in regard to the constitution of the retina, those 

 concerning visual purple or erythropsine must be given. For many 

 years certain anatomists had called attention to a red pigment in the 

 retinas of certain animals, but the credit of emphasizing the importance 

 of this substance belongs to Boll. 1 In 1876 he discovered the existence 

 of a red coloring matter in the rods of the frog, which undergoes a change 

 if subjected to the influence of light. This material, which remains 

 unchanged in frogs kept in obscurity, becomes paler when the animals 

 are exposed to the light, and if they are kept in sunlight the retina 

 becomes colorless. Retinas removed in darkness may take several 

 minutes to decolorize in daylight. In mammals the decolorization is 

 much more rapid. Boll established, furthermore, that the color lost in 

 frogs exposed to sunlight is reproduced in darkness, but he was not able 

 to recognize the nature of this coloration, which he was led to attribute 

 to a lamellar structure of the rods and not to a coloring matter. Kiihne 

 succeeded, on the other hand, in isolating the coloring matter by means 

 of a solution of bile or of cholate of sodium. The solution of visual 

 purple thus obtained changes from red to yellow under the influence of 

 light, and Anally becomes colorless. This decoloration, according to 

 Kiihne, is, moreover, much more rapid in that region of the spectrum 

 comprised between the greenish-yellow and the indigo than elsewhere. 

 Bed is the least active color, even less active than the ultraviolet rays. 



The yellow material, produced by a partial decomposition of the vis- 

 ual red, follows a slightly different law of decoloration. The chemical 

 action of radiations being related to their absorption, the decolorizing 

 action, nearly absent in the red and yellow, is at a maximum in the 

 violet and is quite strongly developed in the ultraviolet. 



Without wishing to enter into a discussion of the details of these 

 experiments, we can hardly omit reference to veritable photographs, 

 known under the name of optograms, which Kiihne has been able to 

 produce on the retinas of rabbits and frogs. It might be x>ointed out 

 that, disregarding certain apparent exceptions, nocturnal animals have 

 a retina abundantly provided with visual purple, while the purple, as 

 well as the rods, is wanting in those animals which sleep at night — for 

 example, poultry. 



The visual purple has still another characteristic property. Previous 



1 An article by Weiss on the chemical theory of vision, in La Revue G6nerale, 

 March 30, 1895, is freely made use of in the resume" of this subject. 



