144 VERTEBRATE PHOTORECEPTORS 



With the estabhshment of vitamin A as an important 

 component in both rod and cone visual cycles, it became 

 apparent that visual tests might well be employed for the 

 estimation of vitamin A in the body and for its detection 

 in early stages of vitamin A deprivation, before any definite 

 clinical symptoms of deficiency become recognizable. Such 

 tests were made by Edmund and Clemmesen (1936) with 

 visual intensity discrimination as an index, and by other 

 investigators using dark-adaptation as a criterion (Jeans 

 and Zentmire, 1934; Jeghers, 1937; Palmer and Blumberg, 

 1937; Isaacs, Jung, and Ivy, 1938; Gridgeman and Wilkinson, 

 1938; Hecht, 19386; Hecht and Mandelbaum, 1939, 1940; 

 Wald, Jeghers and Arminio, 1938; and others). 



Hecht and Mandelbaum (1939) measured the dark-adapta- 

 tion of one hundred and ten 'normal' persons and obtained 

 the highest and lowest values of final threshold. The data 

 are presented in Figure 101. Normal dark-adaptation pro- 

 ceeds in two steps. The first is rapid and over in a few 

 minutes, whereas the second is late in starting and goes on 

 for a half hour or more. The rapid first adaptation repre- 

 sents the behavior of the cones (Figure 101, solid circles) 

 while the delayed and slower adaptation (open circles) 

 represents the rod function. These authors pointed out 

 that dark-adaptation measurements in order to be numeri- 

 cally precise and quantitatively valid depend upon a number 

 of conditions as follows: 1) the intensity and 2) duration of 

 the light present before dark-adaptation begins, 3) the area, 

 4) the retinal location, 5) the color, and 6) the duration of 

 the hght used for measuring the course of dark-adaptation. 

 These six conditions were found to influence profoundly 

 the relative extent, the duration, and the form of the two 

 limbs of the dark-adaptation curve, as well as the time of 

 appearance of the cone-rod transition. 



Hecht and Shlaer (1938) designed an adaptometer for 

 controlling intensities, times, positions, and areas, so that 

 the dark-adaptation measurements were made, in all in- 

 stances, under controlled conditions. 



