HELIOTROPISM or ANIMALS 69 



Positive heliotropism can be demonstrated in flies by the 

 same experiments as in plant lice ; only it must be noticed 

 that flies are provided with several more kinds of irrita- 

 bility than plant lice, and that in consequence heliotropism 

 may be obscured when other stimuli besides light come into 

 play. In one experiment, for example, I observed 

 that light, gravity, and heat were without effect 

 on the flies, because the animals always remained 

 on the cork stopper in the test-tube. Some sub- 

 stance was probably on the stopper that attracted 

 FIG. 5 f.j ie flj es . f or wnen I p u t the animals into a flask 

 with a clean glass stopper, they reacted to light. 



I am indebted to Professor Ernst Mach for a beautiful 

 observation on the influence of light on the orientation of 

 the house fly : 



Several years ago I accidentally made an observation which I 

 have never been able to follow further. While adjusting my rotat- 

 ing polarization apparatus in a dark room, by the help of sunlight, 

 whereby a bright quadratic picture a some 16 cm. across (Fig. 5) 

 was rotated three or four times per second in a circle of a radius of 

 30 cm., a fly F (Fig. 6) happened to enter 

 the bundle of rays LL, went through the 

 whole rotation as though stunned, and fell 

 upon the table. I was able to repeat this i 

 experiment twice. The fly, which was appar- 

 ently sound, escaped while I was giving my 

 attention to something else. FIG. 6 



In this case, then, the same effect was produced by rotat- 

 ing the rays of light as by revolving the fly on a centrifugal 

 machine. 1 



l Radl has recently come to the conclusion that the reactions of insects on a 

 centrifugal machine are indeed caused by the light. If this is correct, they are 

 identical with Mach's observation. [1903J 



