338 THE ZOOLOGIST. 



be detected without difficulty ; but it seems quite probable that 

 if they had been in a larger aquarium they might have been 

 entirely overlooked in a cursory glance. It would have been 

 interesting to observe whether the fishes would have changed to 

 their original colour if they had been taken from aquarium 2 

 and put into aquarium 1, but unfortunately this aquarium had 

 been cleaned out and the pebbles removed in the meantime. 



2. In aquarium 2 the fishes remained until April 6th, i. e. 

 for five and a half weeks. During this time the whitish pebbles 

 and rocks of this aquarium had become somewhat darker in 

 colour owing to the effects of light and the growth of green and 

 brownish vegetable matter. I believe, though it is very difficult to 

 be quite certain in such a case, that the fishes also had become 

 a little darker in colour, their green tinge being more pronounced. 



3. I now resolved to observe the effects of placing the fishes 

 in a third aquarium whose bottom and three sides were of dark 

 blue slate, the top of the aquarium being almost wholly covered 

 with a wooden board to exclude light, and the glass front turned 

 away from the window. In aquarium 3 the two fishes were 

 placed at 10.10 a.m. on April 6th. At 10.50 I looked at them 

 and saw at once that they were unmistakably darker in colour. 

 At 12.5 I examined them again, removing them rapidly from 

 the dark aquarium 3 to a small glass tank and carrying them 

 to the window in order to do so. Both fishes were now dark 

 green, perhaps of a darker green than they had been when I 

 took them from aquarium 1, their irises being coppery red and 

 their fins greenish, but the beautiful bronze tint of the body 

 was now almost absent. 



4. I allowed the two Tench to remain in aquarium 3 until 

 the morning of the 8th. At 12.35 I removed one of them to a 

 white enamelled dish, containing about a gallon of water, placed 

 before a window in a fairly light spot. A distinct change could 

 be observed in this fish at 1.30, and by 2.30 the change back to 

 the sickly pale yellow colour had been accomplished. The 

 individual left in the dark aquarium 3 was, of course, unaltered. 

 When, at about 4.15, the two fishes were put together in the 

 little glass tank and carried to the window, the contrast between 

 the dark green individual and the pale yellow one was very 

 striking. 



