474 ADAPTATIONS TO PHOTIC QUALITY 



ing the negative side and with first Bauer, later Karl von Frisch, cham- 

 pioning the positive. The influence of Carl von Hess can hardly be ex- 

 aggerated, for he devised some ingenious procedures, and helped his 

 critics to improve their own work by his continual insistence that the 

 brightness factor was not properly controlled in previous and contemp- 

 orary work. He himself tended to avoid the use of techniques in which 

 any control of brightness was theoretically necessary. Some of his 

 assumptions and interpretations were so repugnant to others, however, 

 that his work served to stimulate an outpouring of research which might 

 not otherwise have been done even yet. Being a very great physiologist, 

 Hess made very great mistakes when he made any at all; and other 

 investigators were quick to point them out. 



Hess used two methods particularly : that of preference, and the study 

 of the pupil-contracting effects of the colors of lights. His argument was 

 as follows : The totally color-blind human eye and the dark-adapted nor- 

 mal eye (which is color-blind) see the green region of the spectrum as 

 brightest, whereas to the normal light-adapted eye the yellow region of 

 the solar spectrum is most luminous. This shift in the position of the 

 peak of maximal brightness is of course the Purkinje shift (Fig. 30, p. 

 87), and is accompanied by a relative decrease in the brightness of red 

 and a relative increase in the brightness of blue stimuli on passing from 

 full light-adaptation toward dark-adaptation, upon reducing the intensity 

 of illumination. If for fishes (or other vertebrates — or invertebrates!) the 

 brightest spectral region is the green when they are light-a-dapttd, and if 

 they show no Purkinje phenomenon, then they are color-blind. If their 

 pupils close further in response to green light than to other colored lights 

 of equal physical energy-content, then the green is brightest for them, 

 and they are therefore totally color-blind. Hess applied this argument 

 not only to fishes but to a host of other animals as well. 



The fallacies inherent in this argument are glaring ones, and the most 

 important of them have been repeatedly explained by others. There is 

 no justification whatever for assuming that the curves of spectral lum- 

 inosity, with or without color-vision, must be the same for any animal as 

 for the human. The writer would go even further, and insist that an 

 animal could have color vision and yet have no Purkinje phenomenon — 

 the latter exists at all, in man or animals, as a sheer fortuity : the peaks 

 of absorption of the rod and cone photosensitive substances are not iden- 

 tical in location in the spectrum. If they were identical (and they might 

 just as well be), there would be no Purkinje shift. The scotopic absorp- 



