282 A. M. Banta 



the lamp-container and the hght end of the tank. The only light 

 which could reach the dark region came through the water from 

 the illuminated half of the tank. A 6-glower Nernst lamp sus- 

 pended within the lamp-container was the one most used. The 

 partition was set at such an angle within the lamp-container that 

 the middle of its edge was immediately below the Nernst lamp 

 This secured rays of Hght at right angles to the long axis of the 

 tank at the plane of division between the dark and the light re- 

 gions. 



In these experiments a sharp plane of separation between a 

 strongly illuminated and an entirely dark region was desired. To 

 the eye this plane seemed very distinct, though the dark region was 

 somewhat illuminated, perhaps to one-fiftieth the intensity of the 

 light region, due to the diffusion of light through the water and 

 also to reflection from the ground glass bottom of the inner tank. 

 The sides and end of the illuminated half of the tank were non- 

 reflecting, since they were lined with sheets of slate painted dead 

 black. 



Usually the observations were made from a position {X) near 

 the dark end of the tanks, the eye being placed slightly below the 

 level of the water in the tank. By looking through the glass ends 

 of the two tanks and the water within, one was able to observe 

 the whole surface of the inner tank and the animals upon it with- 

 out changing his position or disturbing the light hood. High 

 intensities of illumination were generally used and little diflficulty 

 was experienced in counting the animals in either the light or dark 

 regions. If the illumination was not sufficient to enable one to see 

 readily the animals in the darkened region, their outlines could be 

 observed by bringing the eye into such a position that the ani- 

 mals would appear silhouetted against the relatively intensely 

 illuminated space beyond. This method of observation was seldom 

 necessary, however. 



The 6-glower Nernst lamp was used at a distance of about 30 

 cm. from the floor of the tank. It produced considerable heat at 

 the surface of the water, but the heat thus produced was not 

 apparent at the bottom of the tank, for it was not enough to affect 

 a thermometer bulb placed there. According to Melloni ( cf. Mast 



