RELATIVE STIMULATING EFFICIENCY OF LIGHT 151 



the illumination in one beam was three times that in the other 

 the average angle was +14.4 degrees. This conclusion is sup- 

 ported by the data presented in table 1, as is shown in the pre- 

 ceding section. This seems to show that, in spite of the enormous 

 variation described, conclusions based upon averages of numerous 

 tests are trustworthy. We have consequently in the work de- 

 scribed in the following pages continuously dealt with such 

 averages. 



The cause of the variations mentioned is in itself an exceed- 

 ingly interesting problem. Why, for example, does a butterfly 

 under given conditions deflect zero degrees in one test and +36 

 degrees in the following test? We assume that this change is 

 due to internal changes of some sort or other. What these 

 changes are is not known, but we shall demonstrate later that 

 they are at times, at least, associated with the past experiences 

 of the individuals involved. 



V. RELATIVE STIMULATING EFFICIENCY OF INTERMITTENT AND 



CONTINUOUS LIGHT 



In the following experiments it was found that at certain 

 flash-frequencies the stimulating effect of intermittent light is 

 greater than that of continuous light of equal illumination; at 

 other flash-frequencies it is less than that of continuous light; 

 and at still others it is equal to that of continuous light. 



In these experiments the butterflies were exposed at the inter- 

 section of two horizontal beams of light which crossed at right 

 angles, as in the preceding experiments. The paths of the insects 

 were recorded by allowing them to walk over sheets of paper 

 covered with charcoal dust, as described previously. Each one 

 of 10 animals was given the following tests. An individual was 

 first given 5 trials in two beams of continuous light of equal il- 

 lumination. Before each trial the insect was placed so that it 

 faced a point about halfway between the two sources. As soon 

 as it had walked across the field of light it was gently picked up 

 in the fingers and placed again at the starting point preparatory 

 for another trial. Care was taken not to pinch the animal or to 



