PHOTOCHEMISTRY OF COLOR VISION 103 



then integrated centrally where they set off the respective three com- 

 ponent processes of the synthetic mechanism. But, for any one wave- 

 length there is another on the other side of the peak of the absorption 

 spectrum of the zapfensubstanz, which at the same intensity would break 

 down the same amount of the substance into, presumably, the same end 

 products. How then could these two wavelengths possibly arouse differ- 

 ent sensations? It is impossible to imagine how any one substance could 

 serve as the analytical mechanism by which purple light is translated 

 into 'redness modulated' and Violet-ness modulated' impulses in a single 

 optic nerve fiber. For the cones to generate three qualitatively different 

 impulses, it would appear that they must contain a triplex photochemical 

 system. 



In truth, the working out of the photochemical system of the cone 

 may long continue to seem the most difficult branch of the physiology 

 of the eye. To absorb more light in one part of the visible spectrum than 

 another, a substance must be colored. In the present state of our knowl- 

 edge we must suppose that there are tiny amounts of three differently- 

 colored photosensitive substances in the cone's outer segment. With the 

 very sloppiest of technique, we can mount the fresh dark-adapted retina 

 of a frog or a goldfish on the microscope and still see the rich wine of 

 rhodopsin filling its rods. But with the most careful of methods, we can 

 succeed in seeing living cones only as completely colorless structures, 

 whose bland innocence conceals invisible traces of three important some- 

 things — to our utter exasperation. 



