352 LESLIE B. AREY 



conditions actually obtaining in the basin, the reader may decide 

 for himself. 



There is one further circumstance which on casual considera- 

 tion might be held responsible for the quantitative divergence 

 between my results and those of Garten and Weiss. They con- 

 tinued their experiments in most cases for five hours, while my 

 determinations lasted on the average perhaps three hours. It 

 seems plausible that the long-continued action of very weak 

 light might register an effect not manifest in shorter periods. 

 That this possibility is not operative in the cases under consid- 

 eration follows from certain other observations of Garten and 

 Weiss. They report maximal cone retraction in the fish after 

 the weakest grade of light employed had acted in one series for 

 three hours and in another crucial series for one and one-half 

 hours. Garten also records that in light too weak to distin- 

 guish colors by, a shortening of the cones occurred ('eintritt') 

 in one hour. 



The facts developed in this investigation may for conve- 

 nience be consolidated into the following statement : although re- 

 sponses of the visual cells and retinal pigment may be initiated 

 at lower intensities of light, an approach to a maximal response 

 is first elicited at an intensity which permits the reading of ordi- 

 nary print. This signifies that the threshold of stimulation of the 

 visual cells and retinal pigment is high; or, in other words, the 

 assumed great photic sensitivity of these elements is disproved. 

 Furthermore, since the responses in weak light are substantially 

 identical with those in darkness, the mechanical conditions are 

 present for a theoretically more efficient dim-light and bright- 

 light vision, as postulated (compare p. 345). 



SUMMARY 



The threshold of stimulation of the visual rods and cones and 

 of the retinal pigment, at which they exhibit their characteristic 

 photomechanical changes, is high. 



The alleged great sensitivity of these elements to light of ex- 

 tremely low intensity is consequently disproved. 



