GENERAL INTRODUCTION 3 



lating the organisms, others appear to be equally positive 

 that all rays are active in this process, and still others say 

 that the stimulating efficiency of different rays varies in 

 different organisms and in the same organism under different 

 conditions. 



Many investigators have apparently not thoroughly 

 analyzed the problems concerning reactions: To them the 

 question regarding orientation, e.g., has been merely: Is it 

 ray-direction or intensity difference that regulates this? 

 And with regard to this question they have failed to see 

 that there may be a vast difference in effect between 

 direction of rays in the field and direction through the 

 organism; between diversity in light intensity in \he field 

 and variation on different parts of the surface of the or- 

 ganism. Moreover they have failed to appreciate the 

 importance of difference in sensitiveness of different parts 

 of the reacting organism, and the consequent effect of 

 change in position on stirnulatlon. 



An illustration will serve to emphasize the Importance 

 of distinguishing these characteristics. Suppose we have 

 an elongated opaque organism the anterior end of which 

 is more sensitive than the posterior, and suppose that 

 this organism is in a field of direct sunlight without any 

 other obstruction. Now It is evident that under such 

 conditions the Intensity In the field is uniform, but the 

 intensity on the Illuminated side of the organism may be 

 almost infinitely higher than that on its shaded side, since 

 no light can get through the organism, and If the organism 

 changes Its axial relation with reference to the ray direction, 

 the intensity on the surface may change just as much as it 

 would if the organism moved about in a field in which the 

 intensity was not uniform. Moreover if the organism takes 

 a position in which the sensitive anterior end is shaded by 

 the rest of the body it is of course in a lower effective inten- 

 sity of light than it would be If this end were illuminated. 

 Here again we see that a change in axial position In a field 

 uniformly illuminated may produce the same effect as 



