HELIOTROPISM OF 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 
[4 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 
the flies; for when I put 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 L 
whole rotation as though stunned, and fell 
upon the table. I was able to repeat this 4 =a, 
experiment twice. The fly, which was appar- Se 
ently sound, escaped while I was giving my 
attention to something else. FIG. 6 
FIG. 5 
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." 
1R&dl 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. [1903] 
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