A SIGHT REFLEX SHOWN BY STICKLEBACKS. 83 



This reflex is, on the whole, most striking in its simplicity, but 

 it is none the less puzzling for it might seem from the above ex- 

 periments that the sticklebacks, and probably some other kinds 

 of fish, form an exception to the quite general rule among ani- 

 mals, that by a "compensatory motion " of either the eyes, head, 

 or whole body, the visual field is kept relatively constant. 1 



To test the validity of this view several kinds of fish were ex- 

 perimented with in order to determine their reactions to changes 

 in the visual field. 



1. Sticklebacks among other fish were placed in a cylindrical 

 glass vessel which was suspended from above. Beneath and fit- 

 ting quite closely about this aquarium was placed a cylindrical 

 galvanized iron dish resting on a turn-table. The interior of this 

 iron dish was striped alternately black and white. When this 

 visual field was rotated the fish were very definitely oriented and 

 swam with it, although the aquarium and water remained station- 

 ary. The fish made the compensatory effort to keep the visual 

 field constant. 



2. The animals were placed in the same cylindrical aquarium 

 and the water set in motion by a gentle circular motion of the 

 hand the idea being to simulate the classic turn-table experi- 

 ments. As a result the animals headed up stream (rheotropism). 

 Their efforts were materially increased by rotating the visual field 

 opposite to the moving water, or they could be brought to rest so 

 that they floated with the stream by rotating the field at about 

 the speed of the moving water and in the same direction. Dr. 

 Lyon was at the same time conducting experiments on rheotro- 

 pism and by several most ingenious methods demonstrated an 

 orientation where no current existed and where the reaction was 

 produced only by a changing visual field. 2 Most of the fish he 

 tested showed the same tendency to keep a constant visual field. 



The results of these experiments seem at variance with those 

 set forth in the first part of this paper, but there is this difference 

 to be noted. The reflex depends upon a stationary visual field 

 with the stimulating object moving before this background, a fact 

 which offers a possible explanation of the peculiar reflex. If 



1 E. P. Lyon, American Join nal of Physiology, 1899, III. 



2 E. P. Lyon, American Journal of Physiology, 1904, XII., p. 153- 



