VITAMIN A-DEFICIENCY AND THE RETINA 145 



Having established the spread in the dark-adaptation of 

 a normal population, Hecht and Mandelbaum (op. cit.) 

 studied the dark-adaptation of four young men on a diet 

 with decreased vitamin A (150 units of vitamin A daily). 





S 5 



1 4 



c 



10 15 20 



Time in Dark - Minutes 



25 



30 



Fig. 101. Showing the course of dark-adaptation in 110 normal persons. 

 The points represent the single measurements made with two subjects yielding 

 the highest and lowest values of the final threshold. The dotted area contains 

 80 per cent of the population. The cone adaptation curve is indicated with 

 solid circles; the rod adaptation curve by unfilled circles. (From Hecht and 

 Mandelbaum, 1939, J. A. M. A., vol. 112.) 



The diet was maintained for about forty days after which 

 they were returned to normal food habits. Two were given 

 daily supplements of 50,000 units of vitamin A for six weeks 

 afterward. The dark-adaptation curves of one of these 

 subjects with vitamin A deprivation are shown in Figure 102 

 where it is seen that both cone and rod thresholds are 

 affected by the vitamin lack, and that rod vision is affected 

 more than cone vision, as evidenced by the more rapid and 

 extensive rise of the rod section of the curve during the diet. 



