122 VERTEBRATE PHOTORECEPTORS 



by the pigment epithelium, thereby favoring the idea that 

 the pigment cell is responsible for its generation. Although 

 Klihne (op. cit.) reported that visual purple in solution will 

 regenerate some of its color in the dark, this observation 

 has apparently never been confirmed until recently by 

 Hecht (1936) and his co-workers. The properties of rho- 

 dopsin in solution have been examined recently by Wald 

 (1938) in marine fishes, frogs, and mammals. He showed 

 that the bleaching of neutral rhodopsin in solution includes 

 a photic and at least three thermal (^'dark") processes. 

 Thermal reactions account for approximately half of the 

 total fall in extinction at 500 m^u. He showed also that the 

 spectrum of pure rhodopsin corresponds in form and position 

 with the spectral sensitivity of human rod vision, computed 

 at the retinal surface. 



Boll (op. cit.) observed that, in frogs, the pigment epithe- 

 lium firmly adheres to the retina when exposed to light — 

 thus implying some functional relation between this tissue 

 and the visual purple. The carotenoid nature of visual 

 purple has been studied extensively by Wald (19356, 1936), 

 who has shown also that the pigment epithelium contains 

 large stores of vitamin A and xanthophyll esters. Since the 

 presence of vitamin A is now well known to be essential to 

 the production of visual purple, it becomes apparent that 

 the storage of this substance in the pigment epithelium 

 firmly establishes an important relation between this cellular 

 layer and the retina proper. Light liberates from the visual 

 purple a carotenoid (retinene) which by a reaction independ- 

 ent of light (thermal) is converted to vitamin A. Vitamin A 

 and retinene are also the precursors of visual purple. The 

 visual processes, therefore, constitute a cycle (p. 125). 



It is interesting in this connection to cite the early formu- 

 lated photochemical theory of Hecht (1920, 1927), who 

 studied extensively the photochemistry of light responses 

 in invertebrates and in the human retina. He was led to 

 the conclusion that the photosensitive mechanism is com- 

 posed of two processes. The initial one has the character- 



