Coiiipnisatory Motiofis 455 



cm. apart. Another portion of the cyHnder was of black paper 

 in which had been cut numerous holes of various shapes and 

 sizes. When in use the cylinder was always part striped and part 

 fenestrated, or part open and part one or the other of the described 

 surfaces. (The proportions were varied, but the character of 

 the surface did not seem to make a constant difference.) 



Frogs placed in the middle of this "circus" arrangement could 

 be made to turn their heads and to give the nystagmus or back- 

 jerk by revolving the cylinder, the same as when the animals them- 

 selves were rotated on the turntable. The response was not, how- 

 ever, equally marked in all cases, nor was it in any case as quick 

 as m the actual rotation ot the animal. When the revolving of 

 the cylinder was very rapid or very slow, there was no response 

 at all; but when the optimum rate was found, the responses were 

 well marked and continuous. These experiments with the mov- 

 ing environment would indicate that the visual impressions do, or 

 may, play an important role m setting up compensatory move- 

 ments; and in the case of the animal rotated on the turntable one 

 might conclude that it is the displacement of the retinal image 

 that is the constant, and therefore the determinant factor. But 

 such a conclusion would be false, and for the following reasons: 



If a frog is placed on the turntable, in every possible position 

 with relation to the pivot, and the table is turned to the right, 

 (that is, clockwise) the frog's head will always turn to the (ani- 

 mal's) left, and the animal will seek, humanly speaking, to keep 

 the same view before his eyes. But if now the vessel containing 

 the animal is completely surrounded by some opake material, the 

 frog will respond in precisely the same way. If the frog is taken 

 into a room almost dark — one barely light enough to permit the 

 observer to discern the outlines of the animal against a white 

 background — the animal will respond in the same way. If the 

 animal is placed upon the turntable together with the source of 

 illumination, and completely cut off from the sight of external 

 objects, rotation will result in the same reactions. If, finally, the 

 animal's eyes are covered with a mixture of vaseline and lamp- 

 black (which will entirely exclude vision without in the least irri- 

 tating the frog) the responses to rotation are still the same. 



