yo Herbert Eiigeue Walter 



areas of light were respectively 33.16 and 68.18 cm. the ratio was 

 practically the same as when the two intensities were 16.3 and 

 33.16 cm., yet the percentage of responses in the two cases is 

 decidedly different, being 10.5 per cent in the former, and 56 per 

 cent in the latter case When the lesser of the two lights was 33.16 

 cm. there were invariably fewer responses than when the lesser 

 light was 16.3 cm. The latter intensity is undoubtedly nearer 

 the planarian's optimum intensity, and the apparently inhibitive 

 action of the higher intensities agrees perfectly with certain facts 

 already detailed, as, for instance, that the activities of Planaria 

 gonocephala were less pronounced at 431 cm. than in lesser inten- 

 sities; and, again, that all planarians show more responses on 

 going into a lesser than when going into a greater intensity. 



Attempts were made in some other ways to subject planarians 

 to areas of contrasted intensities and, although the results were 

 less satisfactory in general than those obtained by the method of 

 using two overhead lights of different intensities just described, 

 yet certain facts were brought out that may be worth recording. 



In the first of these attempts two concentric rings of white 

 paper, each about two centimeters wide and having between them 

 a space of a couple of centimeters, were fastened to the under sur- 

 face of the aquarium floor. The white paper thus arranged 

 reflected the light upward and made areas of gradually increased 

 intensity as compared with the remainder of the aquarium floor 

 through which the light passed without reflection. Worms placed 

 in the center of these circles would consequently be obliged to 

 pass from one intensity of light directly to another, whatever the 

 direction of the radius they might be taking. When worms were 

 actually tested, it was found that they exhibited considerable 

 modification in their movements, particularly when approaching 

 the edge of the paper backgrounds. 



Owing to the considerable thickness of the plate-glass floor of 

 the aquarium as well as to the fact that white paper is a surface 

 which scatters the light falling upon it, it was found that there was 

 formed, not a sharp line of demarkation between two intensities, 

 but rather a penumbra-like margin of intermediate light. This 

 apparatus was therefore abandoned as unsatisfactory. 



