132 LESLIE B. AREY 



Diffused tlaylight 



30 minutes, rods two-thirds extended 



45 minutes, maximal light adaption. (Cones also light-adapted) 

 Darkness 



15 to 20 minutes, rods, in most cases, almost completely shortened 



30 minutes, maximal dark adaption. (Cones still in position of light 

 adaption) 



The quicker response of the rods in darkness than in Hght is 

 noteworthy. The rod shortens in the dark, the cone in the hght; 

 since in both cases the process of shortening is more vigorous 

 than the lengthening, it would appear that the contractility of 

 either type of cell is the responsible factor and the relative effici- 

 ency of light and darkness is not primarily involved. In the last 

 analysis, however, the situation may not be reducible to such 

 simple terms. If these responses are merely the expression of the 

 action of hght and darkness on the protoplasmic myoids, why 

 should the direction of movement of the two elements be opposed? 

 A discussion of this phase of the problem will be found in another 

 place. 



I attempted no experimentation upon the cones of the frog. 

 Angelucci ('90) stated that after an exposure to candle-light 

 for 5 minutes, the cones were strongly retracted, although other 

 experiments of his do not seem to support this conclusion. Her- 

 zog ('05) found that at medium light intensity complete light- 

 adaption occurred in 2^ minutes. 



The most surprising discovery in this series of determina- 

 tions, taken as a whole, was the length of time required to com- 

 plete the adaption of the rod and cone cells in comparison with 

 the retinal pigment. From the results of earlier workers, I 

 had expected the positional changes of these cells to be com- 

 pleted in a few minutes, hence the actual values obtained were 

 wholly unlooked for, and were only accepted after many repe- 

 titions of individual experiments. 



