506 DARK ADAPTATION OF EYE 



of decomposed material necessary for the initiation of a visual effect 

 at a given moment? This really amounts to a demand for the 

 objective basis of variations in the irritabihty of the retinal mechan- 

 ism. We must know why at any given moment a certain number of 

 units of decomposed photosensitive material is necessary for the pro- 

 duction of a visual response, before the strikingly regular variations 

 of these quantities can attain any basic signiificance. Here we meet 

 with the second obstacle in the way of an interpretation, because the 

 question cannot be cleared up in terms of the existing data of retinal 

 physiology. Indeed it is difficult to conceive of experiments on the 

 vertebrate retina so designed as to give an objective answer to these 

 two questions. 



It must be remembered that the important point of the data of 

 retinal adaptation is not merely the fact of adaptation, but the con- 

 sistently regular sequence in the course of adaptation. Given the 

 means of answering the two questions relating to visual reception, 

 this orderly progress of dark adaptation might be attacked with 

 profit. Lacking them, it is small wonder that the data are meaning- 

 less in themselves, and that they have failed to add to a possible 

 hypothesis for the basis of visual reception. 



rv. 



Although retinal physiology has not been able to surmount the 

 difficulties previously enumerated, there are some experiments re- 

 cently made with invertebrates that may help in this connection. 

 The work on the light sensibility of Mya and Ciona (Hecht, a, b, c, d) 

 has demonstrated two aspects of the sensory process which are inti- 

 mately connected with the problem of retinal adaptation. The first 

 of these is concerned with the relation between the intensity of the 

 stimulating light and its photochemical effect in photoreception. 

 The second presents an objective basis for the meaning of variations 

 in irritability. 



According to the hypothesis suggested for it, the photosensory 

 mechanism in Mya is composed of two processes, one following the 

 other in point of time. The initial process is a photochemical reac- 

 tion; the subsequent one is an ordinary chemical reaction which is 



