GEORGE WALD 



745 



5.5 



3.0-- 



z.s- 



o 



OS-- 



0-- 



• log Thresholal 

 O Rhodopsin 



dO 



60 90 



Time in Dark {mm.) 



o 



JO 



- zo 



ao 



-SO 



^ -JOO 



120 



Fig. 7. Dark adaptation in tlic normal rat. Following a high light adaptation, 

 which raised the ERG threshold about 3.5 log imits (about 3000 times), the animals 

 were left in darkness. Periodically the visual (ERG) threshold was measured in 

 one of them, or one was sacrificed to determine the rhodopsin content of the 

 retina. During dark adaptation the log threshold (left) falls parallel with the 

 rise of rhodopsin concentration (right). Both changes are completed in about 

 100 minutes (12, 14). (From Dowling). 



Fig. 7 sho-^s's these results also. It is apparent that during dark 

 adaptation, the logarithm of the threshold falls — i.e., the log sensi- 

 tivity rises — in strict parallelism with the rise of rhodopsin concen- 

 tration. 



Fig. 8 shows the resultant, approximately linear relationship be- 

 tween the ihodopsin concentration and log threshold. It does so 

 also for another kind of situation. When rats are deprived of vitamin 

 A, the first deficiency symptom that appears is night-blindness, caused 

 by the inability to form normal amounts of rhodopsin. In this con- 

 dition the visual threshold rises, just as it does in light adaptation; 

 and one can ask again what relationship obtains between the rise 

 of threshold and the rhodopsin concentration. Fig. 8 shows that here 

 again the relationship is approximately linear, and is indeed identi- 

 cal with that observed during dark adaptation in a normal animal. 



Within the limitations of these measurements, which cover the 

 range of rhodopsin concentrations from the maximum found in the 

 normal dark-adapted animal (here 100) to about 5 per cent of this 



