EHEOTEOPISM 131 



When a human being has been rotated passively to the 

 right for some time, at the interruption of the passive 

 motion the eyes move slowly to the right and return 

 rapidly to the left. Only the slow motions give rise to 

 the sensation of an apparent motion of the objects and 

 hence after the sudden stopping of a passive rotation 

 to the right the objects seem to such a person to move 

 to the left. The geotropic after effect, after passive rota- 

 tion to the right, consists in inducing passive compensa- 

 tory motions to the right, i.e., in the opposite sense of the 

 orientation caused by the apparent motion of the visual 

 objects. Hence in the after effect the orienting effect 

 of the retina image and the centrifugal effect weaken 

 each other. 



Lyon 321 . 322 326 has shown that the phenomena which 

 were formerly described as rheotropism in fish are due 

 to the orienting effect of moving retina images. The 

 reader is familiar with the fact that many fish when 

 in a lively current have a tendency to swim against the 

 current. This phenomenon was believed to be due to 

 the friction of the water. Lyon showed that fish orient 

 themselves just as, well when they are put into a closed 

 glass bottle, which is dragged through the water, although 

 in this case they are not under the influence of any fric- 

 tion from the current. When the bottle is not moved 

 the fish swim in any direction inside the bottle. It is 

 obviously the motion of the retina images of the objects 

 on the bank of the brook which causes the "rheotropic" 

 orientation of fish. When driven backward by the current 

 or when dragged backward in a bottle through the water, 

 the objects on the bank of the river seem to move in the 

 opposite direction. The animal being compelled to keep 



