Chapter 4 



THE VISUAL PROCESS 



(A) ScoTOPic Vision 



Any attempt to depict the events which intervene between the impact 

 of Ught upon the retina and the registration, in consciousness, of the 

 quahtative and quantitative aspects of vision, must necessarily be largely 

 guess-work, and can be lucid and connected only if it is dogmatic. The 

 following treatment is such an attempt, made for the sake of the reader 

 rather than for the sake of the subject. The literature of the field of 

 visual physiology is vast and unorganized, and largely unreadable with- 

 out a considerable background of mathematics. Paraphrased sans mathe- 

 matics, it is bound to seem largely a series of unfounded generalizations 

 to any astute physiologist who may read it; but, these latter gentry have 

 yet to promulgate an inclusive theory of vision in which a sophomore 

 cannot pick great holes. In the present state of knowledge, one descrip- 

 tion of what goes on in vision is almost as good as another, and may be 

 the best one for the beginning reader if, at least, he is able to follow it 

 without miring down in equations. 



Rhodopsin — Perhaps the greatest advance which has ever been made 

 in this field was the discovery of the photosensitivity of the rod pigment, 

 rhodopsin, by Boll in 1876, and the elucidation of most of its properties 

 by Kiihne in the years immediately following. But rhodopsin was at first 

 used to explain too much, and during its history many of its original 

 attributes have had to be taken away from it. Physiologists have relin- 

 quished their beliefs about rhodopsin most reluctantly, since the less one 

 can credit to it, the farther away seem the solutions of some of the 

 fundamental problems of vision. However, in very recent years some 

 progress has been made in the study of other photosensitive substances 

 in the retina, which may be found to do some of the things formerly 

 credited to rhodopsin itself. 



Rhodopsin was once supposed to be the sine qua non of all of verte- 

 brate photoreception, and owing to the attention it commanded, photo- 

 chemical theories of vision rapidly came to be the only ones seriously 

 considered. But it was soon seen that if vision does have a strictly photo- 



