746 



LIGHT AND LIFE 



0.5" 



10 



1.5 



Loc^ 



Z.O 2.5 5. a 



ThreshoioC 



d.5 



Fig. 8. Relation between the rhodopsin content of the rat retina and the 

 logarithm of the visual (ERG) threshold, in animals night-blind owing to vitamin 

 A deficiency, and in normal animals dining dark adaptation. The relationship is 

 virtually identical in both cases, the log threshold or log sensitivitv varying linearly 

 with the retinal content of rhodopsin (14, 1.5, 12). 



amount, the relationship between visual threshold and rhodopsin 

 concentration is described reasonably well by the simple equation 

 (15): 



^ ^' = 0.28 log ^' 



R. 



lo 



in which Ig and Rg are respectively the threshold and rhodopsin con- 

 centration in normal, dark-adapted animals, and I^ and jR^ are respec- 

 tively the thresholds and rhodopsin concentrations in vitamin A-defi- 

 cient or partly dark-adapted animals. 



This is satisfactory so far as it goes, yet clearly not the end of the 

 story. So, for example, as the rhodopsin concentration approaches 

 zero, one might expect the visual threshold to rise toward infinity. 

 According to this equation, however, it rises only toward a limiting 

 value 3.6 log units above the normal dark-adapted threshold (i.e., 

 log /„ -|- 3.6) . Bowling's measurements of rhodopsin concentration 

 in these retinas never reached below about 5 per cent of the maximal 

 values. It seems probable that at still lower concentrations, what we 

 drew as a straight line shoidd become convex to the origin, so as to 

 approach infinite threshold at zero rhodopsin concentration. 



