The Reactions of Plananans to Light 141 



second form of variation in intensity is directly due to the fact 

 that the organism has a solid form and is not homogeneously photo- 

 receptive. 



The direction of light with reference to the organism, presenting 

 as the latter does a structurally diversified form, is influential only 

 as regards the position of the source of light, just as in the case of 

 a fixed point. 



Any change in the position of the source results, then, in a 

 redistribution of the intensities falling upon the organism, so that 

 again the intensity received varies in accordance with the position 

 of the light. 



It is this factor of position in light that has been termed the direc- 

 tive influence of light and it is seen to be due to variations in 

 the intensity of light with reference to the organism, and not to 

 any peculiar property of light iself. By "non-directive light," 

 on the other hand, is understood those conditions which secure 

 for the organism equalized or symmetrical intensity with respect 

 to the parts stimulated. If this interpretation is correct there can 

 be no response, strictly speaking, to the direction of light exclu- 

 sive of intensity although the factor of intensity may be continually 

 modified by that of direction in the light relations of organisms. 



b The Modifying Influence of Direction 



It is undeniable that the planarians experimented upon exhib- 

 ited without exception a definite characteristic phototaxis, that 

 is to say, they habitually go either toward or away from the source 

 of light according as they are respectively positive or negative. 

 In analyzing this phototaxis it seemed desirable to eliminate so 

 far as possible the factor of intensity, but the attempt to do this 

 was only partially successful owing to physical limitations. A 

 step was made, however, toward subjecting worms to directive 

 light without at the same time exposing them to a variation in 

 intensity by inserting a biconvex lens between the source of the 

 illumination and the aquarium, thus making the diverging rays 

 of light parallel throughout their course in the aquarium. By this 

 means was formed a field equal in its amount of illumination at 



