RELATIVITY OF MOVEMENT-PERCEPTION 347 



B. Perceptible as : 



1. Slow — where the percept is, so to say, all object and no blur. 



2. Medium — where the percept is comet-like, the object being 

 seen with a tail of blur, or 'movingness'. 



3. Fast — where the percept is all blur and no object, with direc- 

 tion difficult or impossible to decide. 



C. Supra-perceptible — so fast that nothing is seen at all. 



All of these definitions, it is understood, concern movements of objects 

 which are not being followed or 'pursued' by the eyes. Where voluntary 

 pursuit eye movements occur, all rules are off with regard to the changes 

 of the appearance of the moving object with changes in its speed. The 

 object may be seen as clearly as if motionless, if the pursuit movements 

 are precise enough to hold its image on the fovea. But even though this 

 image does not move over the retina, the images of background objects 

 do so move, and their apparent speed of movement helps us to gauge 

 the speed of the moving object. 



The Relativity of Movement-Perception — Relative movement of 

 the object and its background is essential for any accurate perception of 

 slow motions. In a darkroom, a single spot of light may be motionless 

 and yet appear to be moving, or moving and appear to be motionless; 

 for, eye movements of which we are unaware are then taking place and 

 the shift of the image of the spot over the retina is misinterpreted. This 

 is the explanation of the 'autokinetic movement' of a stationary spot of 

 light which we attempt to fixate, and think we are fixating, but which 

 seems to wander here and there over a considerable range. If two lights 

 are presented and only one is moved, we may see both as moving if they 

 are alike; but if one is larger or brighter than the other it tends to take 

 on the attributes of a 'ground' and we see the other light as moving even 

 though it may be the one which is actually stationary. For us to be sure 

 that an object is moving, it is ordinarily necessary that we be able to see 

 some other object which we know or believe to be stationary. In fact the 

 more other, motionless objects we can see, the better for our accuracy in 

 detecting the direction and extent of a motion. The minimal angular 

 velocity for our perception of motion is only one or two minutes of arc 

 per second of time when there are stationary objects in the field; but 

 when there are no such objects to serve as landmarks, the velocity of the 

 moving object must be made ten to twenty times as great. The local 

 signs of direction, and of change of direction, in our retinae work well 



