EVIDENCE FOR DUPLICITY OF VISION 73 



as well, with acuity rising more rapidly above the cone threshold than 

 below it in most animals (Fig. 28). If we knew very accurately this 

 relationship for pure-rod and pure-cone animals, we would expect to 

 find their curves of acuity-versus-intensity to be kinkless. 



As a final illustration of the difference in behavior of rods and cones, 

 we shall consider the rate of dark adaptation, or increase in sensitivity 

 in darkness following exposure to bright light. The graph of this in- 

 crease (Fig. 28) again shows a fairly well-defined kink owing to the fact 

 that the cones reach their maximum sensitivity at a rapid rate before the 

 sensitivity of the rods begins, slowly, to increase at all. In pure-rod, 

 duplex, and pure-cone eyes the expected differences in the slope of the 

 curve, and in the presence or absence of a kink, are indeed found when 

 such criteria of sensitivity as the behavior of the pupil or the electrical 

 discharges from the retina are recorded. 



We have surely seen enough evidence now to convince ourselves 

 of the duplicity of the visual process. The complexities of the above 

 evidence may seem rather appalling to the innocent reader; so, let us 

 try, in the next chapter, to make the process of vision seem fairly 

 simple after all! 



