EVIDENCE FOR DUPLICITY OF VISION 71 



the extrinsic summation-difference to retain any lead over the cones in 

 the matter of sensitivity. But when the rods are working to best advan- 

 tage, at intensities below the cone threshold, the intrinsic factor of their 

 rhodopsin content far outweighs the combined effect of the other two. 

 So important is rhodopsin in this regard, and so deeply involved in the 

 fundamental chemical events of the visual process itself, that a large part 

 of the first section of the next chapter will be devoted to this magic 

 chemical whose effect is: "Now you don't see anything; now you do!" 



Evidence for Duplicity of Vision — Essentially, then, the Duplicity 

 Theory states that the retina contains a sensitivity mechanism and an 

 acuity mechanism, and identifies these with the rods and cones respec- 

 tively. If both of these mechanisms are in operation only through a 



(log) Intensity Time In Darkness 



Fig. 28 — ^Further evidence for the Duplicity Theory (see text). 



certain transitional range of intensities, and only one or the other of 

 them can operate effectively below and above this range, we might ex- 

 pect that many phases of visual physiology would exhibit differences in 

 accordance with whether one, both, or the other mechanism were 

 in action. This is indeed the case. When graphs of various visual physi- 

 ological processes are plotted, a characteristic 'kink' is always to be seen 

 in the curve, marking the change-over from predominantly rod- to pre- 

 dominantly cone-control of the process in question. Moreover, when 

 such curves are plotted for stimuli restricted to the pure-cone (foveal) 

 portion of the human retina, or are plotted for animals with cone-sim- 

 plex retinas, there is no kink— the whole curve resembles the cone portion 

 of the graph of a rod-and-cone, duplex, retina. And of course pure-rod 

 retinae yield curves which lack kinks and simulate the below-the-kink 

 portion, or rod portion, of a duplex retina's graph. 



