I20 LIGHT AND THE BEHAVIOR OF ORGANISMS 



movable glower in an intensity of 150 ca. m. After they 

 had oriented the glower was suddenly pulled toward the 

 aquarium until it could be clearly seen that many of the 

 specimens responded with the avoiding reaction. By re- 

 peating this many times it was found that it required an 

 increase from 150 ca. m. to 444 ca. m. (or 294 ca. m.) to 

 throw them out of orientation. This may then be called 

 the threshold with the posterior end illuminated. 



Frequently during the progress of the preceding experi- 

 ments the ray direction was changed and the light intensity 

 increased simultaneously. It was found that when the 

 intensity was thus increased from 150 ca. m. to 321 ca. m. 

 (or 171 ca. m.), nearly all responded at once regardless of 

 the surface turned toward the source of light after the 

 change was made; and under these conditions they could 

 be seen to turn toward the light as w^ell as from it. But 

 when the intensity was increased from 226 ca. m. to 321 

 ca. m. (or 95 ca. m.), only those responded in which the 

 oral side faced the light after the direction of the rays was 

 changed, and these also responded when the ray direction 

 was changed without an increase in light intensity, as repre- 

 sented in Fig. 14. 



Judging from these results a change in the position of a 

 Stentor from one in which the posterior end faces the source 

 of light to one in which the oral side faces it, is equivalent to 

 increasing the intensity nearly threefold ; and a change from 

 a position in which the aboral side is illuminated to one in 

 which the oral side faces the light is equivalent to doubling 

 the intensity. These considerations show clearly how a 

 stimulation in a field of uniform and constant light intensity 

 can be produced by change of intensity. The fact that 

 Stentors are so much more sensitive when the oral surface 

 is illuminated than when the aboral surface or the posterior 

 end is exposed, points toward the presence of a highly sensi- 

 tive region in the neighborhood of the oral opening in Sten- 

 tors. The precise location of this region is a subject for 

 future investigation. The following experiments however 



