VITAMIN A-DEFICIENCY AND THE RETINA 147 



Fig. 103. Curves showing final cone and rod thresholds of two subjects on 

 a diet nearly free from vitamin A. The diagonally shaded part represents the 

 normal diet, the clear part the diet free from vitamin A, and the black part the 

 normal diet plus 50,000 units of vitamin^ A daily. (From Hecht and Mandel- 

 baum, 1939, /. A. M. A., vol. 112.) 



in threshold, followed by a gradual decrease, and nearly two 

 months were required for the subjects to return to normal. 

 It became apparent from these observations that the return 

 to normal thresholds took a much longer time (even with 



