ADJUSTMENT OF FLATFISHES 427 



The remaining backgrounds to be considered consisted of hard, 

 smooth surfaces, differing thus from all of the preceding ones, 

 which consisted of more or less finely divided materials. Thus 

 we have: 



9. The white marble bottom of a large aquarium, and strips of 

 white opaque glass, used in the bottom of smaller jars. Under 

 the influence of such backgrounds, all normal specimens became 

 considerably paler in the course of a few days at the most. A num- 

 ber of specimens which were kept in the large aquarium for a 

 period of 14 of 15 days became extremely pale, some of them 

 matching verj^ well the marble bottom, which at the end of this 

 period was considerably discolored by a deposit of diatoms or other 

 microscopic plants. A pure white, or even a very close approach 

 to this, was not, however, assumed by any of my specimens, even 

 upon bottoms of opaque white glass. -'^ The closest approximation 

 to this was a very pale yellow or straw color, not verj^ different 

 from the '128 D' of the 'Code des Couleurs.'" 



10. Nearly black bottoms, made of slate or glass, painted with 

 asphalt varnish. Upon such backgrounds several of the fishes 

 assumed an appearance which was as dark or darker than that dis- 

 played on the dark mixed sand. In no case, however, was there 

 in such cases a very close approach to black, while one of the fishes, 

 although apparently normal, remained of a pale brown hue for 

 several days. 



11. A tank, much larger than the rectangular jars used in most 

 of these experiments, having glass walls and a glass .bottom, 

 through which light was reflected from below by a mirror, in- 

 clined at an angle of about 45°. The lateral walls of the tank 

 were covered with black cloth, and the top by a sheet of galvan- 

 ized iron, painted black within. In looking into this tank from 

 above, the observer of course perceived a well-lighted field, prac- 

 tically coextensive with the entire bottom. Upon first thought, it 



-^ No. 11 was kept for thirty-three days uninterruptedly upon marble and 

 white glass. 



" This comparison was made in the case of one specimen (No. 12), which, 

 however, probably did not become quite as pale as no. 11 and possibly some 

 others. 



