124 VERTEBRATE PHOTORECEPTORS 



In fact even rod vision has been shown to be mediated by 

 sHghtly different photopigments. 



Fridericia and Holm (1925) and Tansley (1931) showed 

 that animals deprived of vitamin A become abnormally 

 insensitive to dim light (night blind), due to the failure to 

 synthesize visual purple. In recent years the problem of 

 vitamin A deficiency and raised visual thresholds has become 

 one upon which many investigations have been launched. 



The condition of night blindness has been known for ages 

 and its relation to poor nutrition recognized. It was seen 

 in men returning from long sea voyages, in people living in 

 prisons, and among Orthodox Russians during Lenten fasts. 

 The use of liver in its treatment has been widespread. 

 According to Tansley (1931) the earliest reference appears 

 in Eber's Papyrus dated about 1500 b.c. Liver treatment 

 was apparently recommended by Hippocrates. 



Some of the most extensive experimental work dealing 

 with the relation of vitamin A to vision has been done by 

 Wald, who has shown that vitamin A is the precursor of 

 visual purple as well as the product of its decomposition. 

 He has worked extensively upon frogs and fishes. 



Rhodopsin bleaches in light and is resynthesized in the 

 dark. In aqueous solution its absorption spectrum consists 

 of a broad band, maximal at 500 mju. It has the properties 

 of a protein, but it is a conjugated protein, the special prop- 

 erties of which involve principally a colored prosthetic group. 

 This is derived from the widely distributed class of yellow 

 to red, highly saturated, lipoidal pigments known as the 

 carotenoids (Wald, 19356). Dark-adapted retinas, owing 

 to their content of rhodopsin, are rose-colored. On irradia- 

 tion they bleach to an orange color. Wald (1938) has shown 

 that this reaction in solution consists of a succession of light 

 and ' dark ' (thermal) processes, the latter accounting for 

 at least half the total change in spectrum. The orange-colored 

 substance in neutral fat solvents yields a yellow lipoid pig- 

 ment which he has called retinene and which is a carotenoid. 

 In aqueous solution, retinene with a protein are the final 



