100 THE VISUAL PROCESS 



(usually ascribed to tinting of the vitreous by bilirubin — but E. Sachs 

 finds no such yellowing in icteric dogs; perhaps the retina is colored). 



Photochemistry of Color Vision — So much as to suggestions re- 

 garding what goes on in the higher reaches of the chromatic visual 

 mechanism. Now, what objective realities can we point to, in the way 

 of a physiological mechanism for analyzing and transmitting assort- 

 ments of wavelengths in and from the eye? Sadly, only one dubious 

 photochemical substance of ambiguous properties. 



In 1930, Gotthilft von Studnitz reported the first revelation of a 

 retinal photochemical since the discovery of Boll and Kiihne. Studnitz 

 has never given the material a real name — it is just the *Zapfensubstanz' 

 {i.e., cone-substance) . Several years later Wald in this country, without 

 reference to Studnitz's work, hypothesized a cone substance which he 

 named iodopsin (iodos = violet) on the assumption that if one could iso- 

 late and concentrate it, it would be found to be violet in color. 'Iodop- 

 sin', however, was based upon technical methods which Studnitz has ever 

 since insisted could not possibly have indicated his own zapfensubstanz, 

 but rather involved a serious error on Wald's part, Studnitz has con- 

 sequently refrained from applying Wald's appropriate name to the sub- 

 stance which he has claimed to be able to extract and study. For any 

 detailed discussion of the zapfensubstanz, the reader must go to the 

 work of Studnitz cited in the bibliography. No one outside of his group 

 has worked on the substance in all the years since its announcement. 

 Remarks on it here will be brief, 



Studnitz first identified this photosensitive substance by comparing 

 the capacity of a fresh retina for absorbing light, before and after being 

 exposed to strong light. After such exposure, the retina was found to 

 be more transparent than before, which could apparently only be the 

 result of the destruction of some photosensitive substance. The first 

 retinae employed were duplex; so, to eliminate rhodopsin from the pic- 

 ture, Studnitz repeated his experiments on some pure-cone retinae. Here 

 also he found the substance, which therefore must be in the cones. He 

 learned how to study it by itself in rhodopsin-bearing retinae, though not 

 how to isolate its effects very well from those of cone oil-droplet pigments, 

 which come out in the same solvents and are slightly photosensitive. 



By comparing the change, before and after the bleaching with strong 

 light, in the amount of various monochromatic lights absorbed, Studnitz 

 was enabled to plot a curve of the absorption spectrum of the zapfen- 



