RECORDS. 197 



Every pursuit movement of the eyes is a definite muscular 

 reaction to retinal stimulation. As such it is evidently condi- 

 tioned both in direction and in velocity by some definite charac- 

 teristics of the stimulus which occasions it. Since its accuracy 

 can never transcend the accuracy of the data on which it occurs, 

 it follows that the kinesthetic factor from a reactive pursuit 

 movement could never correct nor materially augment the data 

 furnished by the stimulus. 



Moreover, the reaction of the eye involves a long reaction 

 interval, about 160—170. This suggests both the relative im- 

 portance of the actual motor response and a considerable elab- 

 oration of the sensory data in what seems like a simple reaction. 

 But any reaction interval at all renders it impossible for the 

 actual eye movement to parallel the movement of the object of 

 interest either in velocity or in amplitude. 



Experimental verification of the above takes two forms : 

 Whenever all other sensory data for the perception of motion 

 are suppressed, except the hypothetical kinesthetic factor, there, 

 is no immediate perception of motion. And whenever the 

 former are distorted by eye movements, the appearance of 

 motion is respectively decreased or increased, entirely without 

 correction by kinesthetic data. 



A horopter, said Mr. Stevens, will be formed when the two 

 eyes are so adjusted as to enable the image of the point fixed 

 to be located exactly at the maculas of the two retinas. It 

 follows that horopters succeed each other in endless variety and 

 with amazing rapidity. With every glance a new horopter is 

 developed. Two tenets constitute the essential foundation for 

 the doctrine of the horopter, the theory of actually horizontal 

 and actually vertical meridians of the retinas and a doctrine of 

 corresponding points. 



Corresponding points of the two retinas are those which an- 

 swer to proportional degrees of rotation of the eyes about the 

 center of rotation, and which, from given individual points in the 

 plane of fixation, each receive incident rays which must pass 

 through the nodal points. They represent, therefore, the rela- 

 tion between the muscular and the retinal senses. 



