RETINAL PHOTOPIGMENTS 135 



which is converted to retinene and protein when treated 

 with acids and heavy metal chlorides (Wald, 193oa, 19356). 

 The droplets and retinene, however, not only have a 

 common (rod) origin and lipoidal structure, but both are 

 know^n to occur under similar conditions, viz., in retinae 

 dark-adapted at room temperature and treated with certain 

 acids and heavy metal chlorides.' Wald (op. cit.) has shown 

 that retinene is liberated in the isolated fresh retina (or in 

 solution), through the break-down of visual purple on brief 

 exposure to light at room temperature. It is present in 

 light-adapted fresh retinae at 0°C, at which low temperature 

 the final bleaching of retinene to vitamin A and protein is 

 inhibited. It is demonstrable also in large quantities in 

 either isolated dark-adapted retinae (or in visual purple 

 solution) when they have been treated with acids, heavy 

 metal chlorides, or chloroform. 



Since Wald (19356, 1936, 1938) then demonstrated that 

 the conversion of retinene to vitamin A or its reconversion 

 to visual purple is a thermal reaction which is inhibited at 

 low temperatures, w^e (Johnson and Detwiler, op. cit.) 

 endeavored to test further the validity of our theory that 

 the droplets may be retinene by determining Avhether they 

 (the droplets) exhibit a behavior similar to that of retinene 

 under like conditions of temperature and illumination. This 

 involved experiments to determine 1) w^hether the droplets 

 remain demonstrable in the retinae of animals light -adapted 

 at 0°C, after having been dark-adapted at room temperature, 

 and 2) whether the droplets are either incapable of demon- 

 stration or sparse in the retinae of animals dark-adapted at 

 0°C (following light-adaptation at room temperature). 



Using frogs (Rana pipiens) a series of nine experiments 

 was performed in which the animals were subjected to 

 various conditions of temperature and illumination (Table 4, 

 columns 1 and 2). In each case it was known from Wald's 

 (op. cit.) studies on the isolated fresh retina what materials 

 predominate in both fresh and fixed retinae under given 

 conditions (Table 4, columns 3 and 4). It w^as know^n from 



