148 VERTEBRATE PHOTORECEPTORS 



supplementary vitamin A) than the time required for the 

 maximal rise in threshold. Whereas, in some instances, the 

 rise in visual thresholds follows almost immediately upon 

 vitamin deprivation, there is also evidence of great variability 

 in different individuals. Wald, Jeghers, and Arminio (1938) 

 and Wald and Steven (1939) observed an immediate rise. 

 Other investigators, however, did not encounter such imme- 

 diate responses. Booher, Callison, and Hewston (1939) 

 worked on five subjects, which required 16, 27, 29, 39, and 

 124 days respectively on a vitamin A-deficient diet before 

 showing any recognizable rise in thresholds. Steffens, Bair, 

 and Sheard (1939) found only slight and temporary rises 

 in threshold for three subjects on a deficient diet for approxi- 

 mately six months. 



Hecht and Mandelbaum (1940) worked with a group of 

 seventeen young men placed on diets which were restricted 

 to foods calculated to yield about 150 International Units 

 of vitamin A daily. These were divided into three groups. 

 The first group of four subjects received no supplements 

 of other vitamins during the A-deficient dietary period. The 

 second group of eleven subjects received daily supplements 

 of 125 units each of vitamins Bi and G in the form of brewer's 

 yeast, and 1800 units of vitamin D as irradiated ergosterol. 

 In addition, each subject took 300 c.c. of skimmed milk 

 daily, and was encouraged to eat grapefruit for vitamin C. 

 The third group of two subjects received daily supplements 

 of 200 units of Bi and G, 2000 units of D, and 50 mgm. of 

 ascorbic acid, as well as 300 c.c. of skimmed milk. These 

 authors found an unmistakable immediate threshold rise 

 in fourteen of the seventeen subjects. Two individuals 

 showed hardly any rise for 60 days, and then the threshold 

 suddenly began to rise sharply. Another subject showed 

 an initial rise, then a drop, and fluctuated thus for 3 months 

 before there was an unmistakable typical threshold rise. 

 The supplementation of the A-deficient diet with other 

 vitamins apparently had no effect upon the behavior of the 

 thresholds. Subjects showing the most immediate rise fell 



