484 SLONAKER. [Vol. XIII. 



tions to the rods and cones, I shall not refer to them further. 

 Mrs. Franklin (39) bases her theory on carefully conducted 

 experiments testing the sensitiveness of different regions of 

 the retina to various colors and intensities of light. She 

 assumes the presence of two kinds of molecules in the photo- 

 chemical substance of the retina: (i) gray molecules which 

 give rise to the sensation of gray; (2) color molecules which 

 have been differentiated from the gray, and whose atom.s of 

 the external layer are arranged in three directions at right 

 angles to each other. These give the sensation of color. She 

 would thus attribute to the rods the perception of uncolored 

 light, for she says (40, a) : " In the very eccentric part of the 

 retina the differentiation of the color molecule out of the gray 

 molecule has not taken place; these parts of the retina are 

 chiefly useful to us in warning us of danger from moving 

 insects and other enemies, and for this the power to detect dif- 

 ferences of brightness is sufificient." Again (41): " Only the 

 cones are sensitive to variations of color; they must be extremely 

 sensitive to variations of intensity in white light as well, — 

 otherwise the fovea would not be the place with which we make 

 out the minutest variations of line and shade in an intricate 

 drawing. If the cones only give color, they do not give color 

 only!' Her experiments, as well as those of Konig (44), 

 further show that the fovea is blind to blue, and is not able to 

 perceive other colors when the illumination is faint, seeing them 

 only as "different intensities of gray" (40, b). In color-blind 

 people she finds that they are blind in the center of the fovea, 

 but have come to use a small spot on the edge of the fovea as 

 the point of acute vision (42). The maximum sensitiveness 

 of the retina to faint impression is found to be about 25° from 

 the fovea where it is four times as sensitive, and at 50° it is 

 still twice as sensitive as the fovea. These gray and color 

 molecules are, of course, only theoretical and cannot be demon- 

 strated. The gray molecules, without doubt, correspond to 

 the visual purple of other writers, which is found only in the 

 rods. The results of the various experiments on the sensitive- 

 ness of the retina to different colors correspond closely with 

 the arrangement of the rods and cones. In the center of the 



