250 



A. M. Banta 



The lamp container was sometimes placed quite near the heat 

 screen, which was always about 5 cm. from the end of the outer 

 tank. In such cases the interval between the lamp container and 

 the tank was not enough to permit the use of the half-cylinder, but 

 the space was<:arefully covered over with black cloth. When the 

 half-cylinder was used, the interval between the light-container and 

 the cylinder was likewise covered with black cloth [CS'), and in 

 a similar manner the interval between the cylinder and the tank 

 {CS"). At the end of the light-container nearer to the tank the 

 size of the opening in the opaque screen [S] was regulated so as to 

 .allow approximately only such rays to enter the half-cylinder as 

 would reach the tank directly, i.e., without being reflected from 

 the sides of the cylinder. A vertically sHding screen at the 

 near end of the tank was used to cut out all rays except those 

 entering below the surface of the water. 



>^ 



1 





^^/;//^/^^ 



Fig. I. 



Fig. I. — Diagram showing ground plan of apparatus used in experiments with horizontal illumina- 

 tion. CS', C5", cloth screens; B.C, half -cylinder; if 5, heat screen; IT, inner tank; L, source of illum- 

 ination; LC, lamp container; OT , OT', outer tank; .S, screen with adjustable opening; 5'5", opaque 

 screens. 



The Asellidae experimented with were ordinarily not very active, 

 but after being handled and placed in new quarters they kept 

 moving intermittently for some time. Hence they were generally 

 allowed considerable time to become adjusted to their new sur- 

 roundings before the experiments with light began. This was found 

 desirable because otherwise thigmotactic or other stimuli resulting 

 from the new conditions were for a time predominant, the light 

 stimulus being at first so ineffective that the animals wandered 

 about with apparent indifference to it. Sometimes the animals 



