102 



THE VISUAL PROCESS 



material, show three peaks instead of one. He thinks the precursor of 

 the substance is the carotenoid pigment of the cones' oil-droplets (for 

 this there is no evidence whatever) and points out that the multiplicity 

 of such pigments in turtles and birds suggests that several different 

 photochemicals, a la the multi-component color-vision theories, are formed 

 from them. How this works out in the lizard, which sees all colors and 

 yet has only yellow pigment in its oil-droplets — or in man, who has no 

 oil-droplets at all (see Chapter 8, section D), Studnitz does not tell us. 

 So far, then, we are told of but the one substance. Its very existence 

 is most dubious, for leading authorities are very skeptical of Studnitz's 



/ ,.' man, a ., 

 /^-photopic \\ \ 

 scotopic— \\ 



owl, scotopic- 



400 500 600 700 



Wavelength (mp) 



Fig. 35 — The Purkinje shift as shown 

 by the relative brightnesses of mono- 

 chromatic lights to the photopic and 

 scotopic human eye. Also, the relative 

 pupil -closing effectiveness of mono- 

 chromatic lights upon the scotopic 

 eye of an owl, Asia wilsonianus. 

 Redrawn from Hecht and Pirenne. 



<08 



I04 



fish-.;' 



'6g 



t 



12 m 

 h 



Q 

 08-j- 



600 550 500 450 



Wavelength (mp) 



Fig. 36 — Formation of acid (phosphoric?) in 

 retinje under monochromatic light — supposedly 

 owing to breakdown of the cones' photosensitive 

 material, and showing similarity to graphs of 

 photopic brightnesses. Redrawn from von Stud- 



claims and critical of his methods. Granting that Studnitz has really 

 found a cone-substance — it may really be three, but if so we know not 

 how to separate them. Its precursor is quite unknown; but its end- 

 product upon breakdown under light is supposed to be phosphoric acid 

 (Fig. 36) . When we try to understand the retinal part of the physiology 

 of color vision, a single zapfensubstanz seems more of a hindrance than 

 a help. And if we choose rather to believe in the solitary 'iodopsin' of 

 Wald and Qiase, we are no better off. Different wavelengths would 

 break down different amounts of the whole concentration of the sub- 

 stance, and we can easily imagine that corresponding kinds of optic 

 nerve impulses — differing in modulation or whatnot — are produced and 



