A RETINAL MECHANISM OF EFFICIENT VISION 349 



upper temperature limits alone. From these statements it fol- 

 lows that to obtain significant results from experimentation 

 upon frogs a temperature of about 15°C. must be maintained, 

 while with fishes ordinary summer temperature of 22°C., or 

 higher, is favorable. 



Experiments were conducted in a large, windowless room into 

 which weak daylight of a non-directive nature could be introduced 

 from a second room; the latter, in turn, received its light directly 

 from windows located at the end far from the single communi- 

 cating door. Animals were tested under five- different conditions 

 of illumination: 1) total darkness; 2) light in which the presence 

 of objects could just be determined; 3) light of an intensity 

 which allows the certain identification of bright colors ; 4) light 

 which just permits the reading of ordinary journal print; 5) 

 bright, diffuse daylight. Exposures lasted two or three hours or 

 more. 



As experience proved, these seemingly rough criteria of light 

 intensity are sufficiently accurate for the purposes required; 

 with a little practice such grades can be kept fairly uniform. 

 Permanent preparations of Perenyi-fixed, paraffin-infiltrated sec- 

 tions served as a basis for study. 



To further brevity and clearness, the results obtained from 

 experimentation will be condensed to mere summaries. 



EXPERIMENTATION 



A. Retinal pigment 



1. Frog. In total darkness, and in light of just sufficient 

 strength to allow objects or colors to be discerned, the pigment 

 lies in a narrow stratum near the chorioid. When the illumi- 

 nation is increased just sufficiently to allow the reading of ordi- 

 nary print, the pigment, for the most part, becomes expanded, 

 in some cases completely, in others in a zone only three-fourths 

 the maximal breadth. 



2 For Ameiurus another intensity— one which enables ordinary print to be 

 easily read — was also utilized. 



