32 STUDIES IN GENERAL PHYSIOLOGY 



3. The dependence of the orientation on the intensity of 

 the rays of light. It is a peculiarity of all animal as well as 

 plant structures that only external stimuli of a certain inten- 

 sity can call forth reactions. It can easily be shown that at 

 the approach of twilight there comes a time when the rays of 

 diffuse daylight coming through a window no longer attract 

 caterpillars of Porthesia chrysorrhoea. 



If the animals are between two sources of light of differ- 

 ent intensities, that having the greater intensity is the more 

 effective. This can easily be shown by bringing the animals 

 into a room into which light enters from opposite directions. 

 Other conditions being the same, the animals move to the 

 window nearest them. A maximum limit for the intensity 

 of the light cannot be established, as direct sunlight is in 

 itself effective. Artificial sources of light above a certain 

 intensity and containing the more refrangible rays affect the 

 animals in the same manner as the natural sources of light. 

 In a dark room caterpillars are attracted by a kerosene flame 

 as markedly as moths; the caterpillars, however, are not 

 burned, because they move so slowly that they have time to 

 turn back before the zone of fatal temperature is reached. 

 Such animals as are attracted by direct sunlight may also be 

 attracted by the candle flame, exactly as is the case in posi- 

 tively heliotropic plants. 



4. At a constant intensity light acts as a continuous 

 source of stimulation. If the test-tube which is placed with 

 its longitudinal axis perpendicular to the window is left 

 undisturbed, the animals remain permanently on the side 

 nearest the window. Under these conditions we can also 

 safely open the room side of the vessel without a single 

 animal changing its position or escaping from its cage. It is 

 remarkable, however, that when the test-tube has been left 

 undisturbed all day, the animals keep their position during 

 the night. In this way I have kept animals for several days 



