344 THE SENSES 



in numberless illusions. We suspect that chemical processes take place 

 in the retina when light falls on it. What then exactly are these processes 

 in terms of modern chemistry? Why are there no rods in the fovea? 

 Why do different zones of the retina see particular colors only? Why 

 do we not experience a hole in our monocular landscape where the blind- 

 spot is ? Is color-blindness a defect of the retina or of the brain ? Ques- 

 tions of which these are only important examples beset us everywhere 

 while observing the immensely complex facts of vision. Their explana- 

 tion, however, in statements to which all must agree, are quite beyond 

 us as yet. 



THE PERCEPTION OF LIGHT depends in some way on the chemical 

 changes produced in some substance found in the rods or the cones or in 

 both by the vibrations of the "ether" entering the eye. This is probably 

 some chemical change, but just what is by no means certain as yet. It 

 appears to be in the nature of a bleaching of a pigment. In addition 

 the etheric vibrations cause the outer parts of the rods to increase in 

 diameter and the inner parts to thicken and shorten; on the other hand, 

 the lower parts of the cones contract. These movements, like those of 

 the pigment, depend much on the nervous system. The bleaching of 

 the visual purple is not at all so influenced. It is obvious how inadequate 

 such statements are. Such chemical reactions and other movements, 

 whatever they are, could do practically nothing toward explaining the 

 wonderful experience we call light, but they would have great interest 

 for the physiology of the nervous system. 



THEORIES OP COLOR-VISION. These have been numerous, those of 

 Young, Helmholtz, Hering, Wundt, and Mrs. Franklin being perhaps 

 the most conspicuous. Out of these theoretical complexities we need 

 discuss, even briefly, no more than the last-mentioned. 



Mrs. Franklin's theory of color-vision takes account of various facts 

 recently learned which give it much weight and promise, perhaps more 

 than any other. It is likely that the child for many weeks, perhaps 

 months, after birth lacks the color-sense. This theory supposes, then, 

 that the visual substance present at birth in the retina is affected at first 

 only by light ranging in tone from white to black through the grays, 

 but that the substance decomposes through this kind of photic influ- 

 ence and so stimulates the optic nerve-fibers. In the first or second years 

 of life some of this gray-perceiving retinal substance develops into two 

 other retinal pigments, one for blue-perception and one for yellow. 

 A little later, part of the yellow-seeing substance in turn differentiates into 

 a green-perceiving substance and a red-perceiving substance. All of 

 these pigments except the gray-perceiving, congenital one, develop in the 

 cones. This theory, then, supposes that it is the cones of the adult retina 

 that represent color, while the rods react only to light-waves of the black- 

 white sort. Each color-sensation corresponds to one combination of 

 ether wave-lengths striking the retina, every anabolic and katabolic 

 color-process being dependent for its stimulation on certain vibration- 

 numbers in the ether. There is no end to the number of combinations 



