54 GERTRUDE M. WHITE 



x 4 cm. at the end where the water entered. Opposite this open- 

 ing was placed a Nernst lamp having an intensity of about 

 ten candlemeters in the center of the trough. 



Trout about one week old were placed in the center of the 

 trough, four at a time in most instances, since the trough was 

 found to be large enough to hold that many without inter- 

 ference. Their positions were noted just before the water was 

 turned on and again five minutes later. Twenty-five Brook 

 Trout were used. Of these twenty-three moved away from 

 the light and two fish did not change their positions. 



This seems to show that Brook Trout become negative to 

 current when a light is placed at the head of the stream. This 

 probably means that in natural conditions, when they have to 

 choose between shelter and cool water, they seek shelter. 



8. Carbon Dioxide and Light 



Since an excess of Carbon Dioxide is a condition which a 

 growing fish is likely to meet, the way in which a Brook Trout 

 responds to it and the effect it has upon its light reactions is 

 very important in determining whether or not the trout is to 

 escape from the unfavorable conditions, and swim into purer 

 water, where there is more oxygen. 



For the purpose of discovering the effect of an excess of carbon 

 dioxide upon the response to light, fifty individuals were tested 

 separately in the apparatus used for the light experiments in 

 a five per cent solution of carbonated water. 1 The strength 

 of the light for one-half the fish was two candlemeters, for the 

 other half about eight candlemeters. The results of the two 

 were similar. 



Almost immediately after a trout was placed in the carbo- 

 nated water, the fins began to move more rapidly and the mouth 

 to open and close with a gulping motion. Shelf ord ('14) de- 

 scribes this same condition in the fish that he tested in an excess 

 of carbon dioxide. In the present experiments, the trout were 

 stimulated to great activity, swimming continually from one end 

 of the dish to the other with no apparent reference to the light. 

 They were as likely to stop at the end of the dish toward the 

 light as at that away from it. Excess of carbon dioxide appar- 

 ently causes the Brook Trout to be indifferent to light. 



1 The solution of carbonated water was made by adding carbonated water from 

 a siphon to the ordinary city water, which is taken from Lake Mendota. 



