Com pens ntorv Motions 461 



On comparing the arrows in the four columns it will be seen that 

 w^hereas all in II seem to be related to the direction of the head 

 turning, none in I are so except 5 and 6 (retinal and spin impres- 

 sions). 



In experiments on the turntable (I) the factor 5 could be elim- 

 inated in a variety of ways: By surrounding the vessel containing 

 the frog with some opake material, or placing it in a tall opake 

 cylinder; by covering the eyes with the opake non-irritant mix- 

 ture already referred to, or with a pad of absorbent cotton mixed 

 with vaseline and lampblack; by placing the source of light on the 

 turntable with the animal. In all cases the turning of the head 

 in response to rotation was the same as in the usual rotation as to 

 direction; but frequently it was less in degree. In other words, 

 while the displacement of the retinal image can and does set up the 

 compensatory response, the eye is not the sole sense organ through 

 which such movements can be initiated. 



This leaves the spin as the only other factor to be further con- 

 sidered. According to the results indicated in columns A and 

 B of the table, the spin is the only factor (of those considered) 

 in addition to vision that can constantly set up the head turning. 

 In the experiments on the eccentric (columns C and D) where the 

 spin is already eliminated, the further elimination of sight results 

 in a total loss of the response. 



The slightest amount of spin is sufficient to set up a perceptible 

 amount of head turning; considerable displacement of the retinal 

 image is required to bring about the same amount of response. 

 It is possible to move the frog in a right line without the animal 

 giving any response whatever; but if the movement is not smooth, 

 that is, if there is vibration, or very slight turning in a horizontal 

 plane, the head responds at once. That the response to the spin 

 is quicker and greater in amount is also certain; the two factors 

 may be caused to operate in opposite directions in the following 

 manner: 



A dish holding the frog on a horizontal plane and facing the 

 observer, is swung about slowly by the observer at arm's length. 

 The head will be seen to turn in the same direction as the move- 

 ment of translation; that is, in a direction opposite to what we 



