128 TEOPISMS 



location of the illuminated element in the retina. This 

 fact induced the writer to suggest in his first publication 

 on tropisms that the act of focussing in our vision was 

 simply a phenomenon of heliotropism. "The general 

 principle of orientation of organisms to light is also mani- 

 fested in our act of binocular vision which results auto- 

 matically in such an orientation of the two retinae that 

 the image of the luminous point falls upon the two foveae 

 centrales of the retinae" (which are symmetrical ele- 

 ments). In other words, when an object causes us to turn 

 our eyes to it we are dealing with a phenomenon of forced 

 (heliotropic) movement. In order to prove this it is neces- 

 sary to show that a moving retina image can produce 

 forced movements determined by the direction of motion 

 of the luminous object. The difficulties inherent in the 

 proof for such a statement lie in the general prejudice 

 that the motions of an animal are directed to a purpose 

 and it is, therefore, necessary to devise experiments which 

 exclude the assumption of an interest on the part of the 

 animal in the motion. 



The writer observed years ago that when a fly is put 

 on a rotating disk it rotates in the opposite direction 

 from the disk. When the motion of the turntable ceases 

 these compensatory motions of the fly stop also and none 

 of the after effects mentioned at the end of the previous 

 chapter are noticed. 280 This suggested that the so-called 

 compensatory motions of insects on the turntable have a 

 different origin from that of vertebrates. The phenom- 

 enon was explained by Kadi, who proved that the com- 

 pensatory motions of insects on the turntable are pro- 

 duced in the eye and that they are due to the fact that the 

 eye tries automatically to fix the same object. 447 This 

 agrees with the observation of Lyon who had already 



