70 



THE VERTEBRATE RETINA 



overall ocular threshold, reduce acuity; and in others the flat lens which 

 produces a broad image, spreading over enormous numbers of visual 

 cells, thereby increases the resolution but at the same time lowers the 

 brightness of the image and thus reduces the sensitivity of the eye as 

 a whole. 



By far the most important factor in endowing the rods with their 

 great sensitivity is the substance which is called Visual purple' or better, 

 rhodopsin. This is a deep red pigment which is formed slowly but con- 

 tinuously in the rod outer segment. The greater its concentration there, 

 the more light is absorbed and the more effective is that light as a stim- 

 ulus for vision. Since rhodopsin is destroyed by light, it builds up to 

 higher concentration in dim light or darkness than in bright light. Thus 



rods alone 



(log) Intensity (log) Intensity 



Fig. 27 — Evidence for the Duplicity Theory (see text). 



the sensitivity of the rods automatically increases just when it will do 

 the most good, due to the excess of rhodopsin-formation over destruc- 

 tion, and decreases when that in turn is desirable, due to the excess of 

 rhodopsin-destruction over formation, in bright light. Moreover, the em- 

 ployment of rhodopsin for increasing sensitivity does not entail any 

 sacrifice of resolving power by the rod-mechanism, and there are few 

 vertebrates whose rods get along without it. 



It is rhodopsin which is largely, perhaps entirely responsible for 'dark 

 adaptation', the familiar result of which is our ability to see quite well 

 around us in a theater after a few minutes in our seat, although we may 

 have had to feel to see whether the seat was empty, when we first came in. 



Rhodopsin is entirely absent from cones at all times; and there is per- 

 haps so little of it in rods when they are brightly illuminated that they 

 must then fall back upon the intrinsic outer-segment-volume factor and 



