COLOR VISION 887 



green and yellow-blue. To these is then added a white-black substance, so that 

 we have in reality three pairs of recipient elements. Actual sensations of color are 

 derived from these groups of substances by processes of assimilation and dis- 

 similation, as follows : 



Dissimilation Assimilation 



Red-green red green 



Yellow-blue yellow blue 



White-black white black 



Like all other constituents of our body, these substances are first broken down and 

 then again reformed. They undergo catabolism and anabolism. Thus, if white 

 light falls upon the retina, the white-black pigment is reduced, this process 

 giving rise in consciousness to the sensation of white. As soon as this stimulation 

 ceases, the white-black substance is reformed, setting up in consciousness the sen- 

 sation of black. But, this recipient is also affected by rays of different wave-lengths 

 so that the sensations of white and black frequently occur together with those of 

 the other colors. The yellow-blue and red-green recipient elements, however, are 

 affected exclusively by rays of their specific wave-lengths. 



The Ladd-Franklin theory of color vision ^ assumes that the colorless sensations 

 of white, gray and black are produced by the excitation of a photo-chemical 

 substance, designated as gray. While this recipient element is present in the rods 

 as well as in the cones, only the latter contain it in a form to give rise t6 sensations 

 of different colors. On exposure to fight this substance is dissociated, the result 

 being different shades of gray. This is the only reaction possible in the rods, and 

 hence, these elements give rise exclusively to this particular sensation. In the 

 coneS; on the other hand, this substance is present in a differentiated form, allow- 

 ing the development of more complex reactions. The molecules of gray substance 

 here assume a multiple form so that only certain portions of them are dissociated 

 by the light. The molecular substance is divided into two parts, one of which is 

 sensitive to the rays of slow vibration, and the other to those of rapid vibration. 

 The excitation of the first yields yellow and that of the second blue. The yellow 

 recipient is again divided into two parts, one of which receives the longest visible 

 rays (red) and the other the rays giving rise to the spectral green. Thus, the com- 

 plete dissociation of the red, green and blue recipients produces gray, while the 

 simultaneous dissociation of the red and green evokes the same sensation as the 

 dissociation of the entire yellow recipient. 



Since this theory necessitates certain new chemical conceptions pertaining to the 

 differentiation of the molecule, and its complete and partial dissociation, it cannot 

 be regarded as anything more than a provisional explanation until definite experi- 

 mental proof has been furnished for these contentions. It is true, however, that 

 it accounts for certain facts pertaining to color-blindness in a more accurate man- 

 ner than the two theories mentioned previously. In addition, it furnishes an 

 explanation for the variations in the visual sensations mediated by the peripheral 

 zone of the retina. 



Color-blindness. — The terms of amblyopia and amaurosis are 

 employed to indicate an obscurity and loss of sight. In this category 

 are also placed certain congenital defects of the sense of color which 

 are present in about 3 per cent, of the eyes examined, but are relatively 

 rare in woman. In most cases, both eyes are affected and a hereditary 

 tendency is unmistakable. It also seems that this disorder is more 

 common among the poorly educated classes.^ 



1 Psychological Review, 1894, 1896 and 1899. 



2 Holmgren, Color Blindness in its Relations to Accidents by Rail and Sea, 

 Smithsonian Institution Reports, 1878. 



