ADJUSTMENT OF FLATFISHES 453 



the specimens took on a somewhat paler shade, looking as if a 

 very thin 'wash' of white had been spread over their bodies.^' 



Pale fishes placed in this box either became nearly as dark as 

 when kept in an all-black box (three specimens) , or at least of a 

 medium shade (one specimen). One of these specimens acquired 

 the mottled appearance referred to above. 



5. The movable walls, when painted uniformly, were used in 

 order to change the outlook of the fish without otherwise disturb- 

 ing it. The results obtained from their use were the same as those 

 following the transfer of the animals from one tank to another. 

 They need not detain us here. Highly interesting results were 

 obtained, however, with a wall painted partly black and partly 

 white, the line of division between the two areas being horizontal. 

 In the first experiments the white and black portions were of equal 

 extent, that is to say, the wall, which was 18 centimeters high, was 

 divided into a white and a black half. Later, it was painted so 

 that the white band occupied only a fourth of the height of the 

 wall. 



The movable wall, thus painted, was used only in the white- 

 bottomed, black-walled box. It served, therefore, to add a cer- 

 tain amount of white to the vertical surfaces of the box. Some of 

 the results from its use seem worth recording in detail. 



Two fishes (dealt with together) which had remained dark in this 

 box, were found to become pale when an all-white movable wall 

 was introduced, returning to the dark condition, however, when 

 this wa^ removed. The half-white, half-black wall was then in- 

 serted, the white half being uppermost. No change occurred, 

 even after 8 hours. Upon the reversal of the wall (white half now 



''^ Here, and in all similar cases, it was absolutely necessary to place fishes together 

 in the same box before comparing them. The white walls, in the present case, 

 reflected enough light upon the surface of the animal to give it a paler appearance 

 than when in an all-black box. Accordingly, specimens from one of the latter were 

 transferred to the present box, and a comparison made with the fishes which had 

 been kept in this for some time. The reverse change was likewise made, both lots 

 being compared together in all-black tank. A mirror was also used in examining 

 specimens kept in dark boxes, the fishes being illuminated by reflected light. 

 Without such precautions, one may easily be led into error in judging of the less 

 pronounced changes of shade. 



