REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 41 



stereoscopic binocular. This gives a magnification of 65 diameters, 

 with a working distance of 3 cm., and permits exact observation of 

 the movements of the individual Stentors. To one who has worked 

 only with the monocular microscope, the use of the stereoscopic binocu- 

 lar in studying the movements of small organisms will be a revelation. 



The vessel containing the Stentors is first covered with a dark screen 

 and the Stentors are allowed to become equally distributed throughout 

 the dish. The screen is then raised, allowing the light from the window 

 to fall upon the Stentors. Those which are swimming in any other 

 direction than away from the window now turn and in a short time 

 are swimming toward the side of the dish away from the window. 



With the Braus-Driiner the movements of individuals are observed 

 at the moment of removing the screen. Some turn at once, while most 

 continue for a few seconds in the direction in which they are swimming 

 and then turn. All turn in every case toward the right aboral side. 

 The turning is continued or repeated until the anterior end is directed 





FIG. 15.* 



away from the window ; then the direct course is continued, carrying the 

 Stentor to the side of the dish away from the window. The direction 

 of turning is thus determined by an internal factor the structure 

 of the body. 



The behavior of the Stentors may be controlled and studied more 

 exactly by a different order of experimentation. The animals are 

 placed in a shallow rectangular glass vessel on a dark background, in 

 a room that is entirely dark save for two incandescent electric lights 

 A and B (Fig. 15). These are clamped in position, one on each side 

 of the dish containing the Stentors, and about eight inches from it. 

 Both these lights can be turned on at once ; both can be extinguished 

 or one can be turned on while the other is turned off. When only one 

 is turned on the direction of the light can be instantly reversed by 

 simultaneously extinguishing this one and turning on the other. 



With both lights extinguished the Stentors in the vessel are allowed 

 to become equally distributed ; then B is illuminated. In a short time 



* FIG. 15. Method of testing the reaction of Stentor to light. A and B are 

 incandescent electric lights. 



