716 



LIGHT AND LIFE 



o 



+ 25% B/eoch 

 • 35% bleach 

 O Total Bleoch 



(0 



/5 



20 25 



Minutes 

 Fig. 4. Measurements of rhodopsin regeneration 15° from fovea in man. Curves 

 for 35% and 25% bleach slid to the right so that zero time comes to points 

 indicated. Points when rods first appear in dark adaptation curve shown by 

 vertical arrows. 



i 



result after a total bleach, black circles after 35% and crosses after 

 25% bleaching. The latter two sets of points have been slid to the 

 right so that their zero times come at the points indicated, and 

 demonstrate that the chemical course of regeneration is much the same 

 from any level onward no matter what happened before this point. 



When the pigment was seen to have regenerated to the 85% level, 

 Fuortes shifted to a dark-adaptation apparatus and determined his 

 threshold, using test flashes of yellow and of green (alternating each 

 minute) . These two lights had been matched so that they were iden- 

 tical by rod vision, but the yellow was nearly 1 log unit brighter 

 by cone vision. 



When 85% of rhodopsin had regenerated the rods had not yet 

 become more sensitive than the cones, for the yellow test light was 

 found to be much the brighter. A few minutes saw the transition to 

 rod vision, with equal efficacy for yellow and green. The moments 

 when the green test reached this rod threshold curve are shown in 

 Fig. 4. They correspond to about 90% rhodopsin regeneration in 

 all cases. The whole question, then, of the relation of rod threshold 

 to rhodopsin regeneration is restricted to the final 10 per cent of 

 the process, because earlier than this the rods are less sensitive than 

 the cones. 



