454 FRANCIS B. SUMNER 



down) the fishes became decidedly pale within an hour. A second 

 inversion of the wall resulted in the fishes becoming much darker 

 (about medium shade) in 4| hours. It would seem, however, 

 that even in this position the white surface did have some influence, 

 since the fishes did not become very dark until the removal of 

 the wall (leaving the vertical surfaces entirely black), when they 

 became so within two hours. 



Two other specimens were, at the outset, subjected to the same 

 tests as the preceding ones, and with substantially the same results. 

 After several further changes of the visual stimulus, they were 

 again subjected to the influence of the black-walled, white-bot- 

 tomed tank, in which they now displayed a somewhat intermediate 

 shade. The black-and-white metal wall was next put in (now 

 one-quarter white), the white band being below. After twenty 

 minutes, one fish hadacquirednearly the maximum degree of pallor, 

 but the other had undergone no change. 



Two more fishes kept in this box for two days, remained dark, 

 one being of about maximum darkness, the other somewhat paler. 

 The black-and-white wall (one-quarter white) was now inserted, 

 the white band, as before, being below. After two days, both 

 fishes had become pale, though not of maximum pallor. After 

 removal of the wall, they remained in this condition for a day. 



Thus, it is interesting to note that when the bottom and the 

 adjacent zone upon the vertical walls were white, even though this 

 zone were no broader than 4| centimeters, the fishes reacted 

 much (though not quite) the same is in an all-white tank.- When, 

 on the other hand, the bottom was white, and the adjacent zone 

 upon the vertical walls was black, even the presence of an over- 

 lying band of white, 9 centimeters wide, was not sufficient to call 

 forth a truly pale condition in any of the four specimens thus 

 tested. 



It would hence seem, at first glance, that quantitative relations 

 alone could not determine which of these two components of the 

 visual stimulus should prove effective. In endeavoring to decide 

 this point, however, one must distinguish between the potential 

 and the actual visual fields. What the fish, from a given position, 



