346 ADAPTATIONS TO SPACE AND MOTION 



unless he can easily outrun that enemy. We may be sure that any animal 

 can see, as clearly as if it were motionless, any object moving as fast or 

 somewhat faster than the animal himself can go. It will be recalled that, 

 other things being equal, the size of an animal's eyes is related to his 

 speed of locomotion (Leuckart's ratio). Bearing in mind that visual 

 acuity tends to rise with eye size (see p. 171), we shall shortly see why 

 this should be. 



Some objects move too slowly or too fast for any motion to be seen. 

 Suppose we consider only perceptible movements, and separate them into 

 slow, medium, and fast. The ranges of absolute speeds embraced by 

 these terms will vary from species to species and, of course, with the dis- 

 tance of the moving object from the observer. Let us call 'slow' all move- 

 ments during which the character and details of the object are as clearly 

 seen as when the object is still. There will be even slower movements 

 which will not be seen at all. When we watch the minute-hand of a clock, 

 for example, we are aware from time to time that it has taken a new posi- 

 tion; but we cannot honestly say that we see it move. Let us coin a term 

 and say that the movement of the clock hand is for us, psychologically, 

 infra-perceptible. 



The fastest movements we can detect are those in which we are unable 

 to detect direction. An object can flit so rapidly across the whole visual 

 field that we are unable to say whether it went from right to left or from 

 left to right. Here, we do perceive motion, but not a movement at a cer- 

 tain rate over a certain distance. Still faster objective movements may be 

 supra-perceptible, where the speed is so high that nothing is seen at all. 



By elimination, 'medium' movements are those in which not only a 

 change of the position of an object can be detected, but also the changing 

 of position. The nature of the moving object can be made out more or 

 less well. It is with the perception of medium movements that we are 

 most concerned. The whole percept of such a movement may be 

 described as a comet, whose head is the object and whose tail is a blur 

 which we interpret as 'movingness'. When a cartoonist suggests motion 

 by putting a series of partial outlines behind an object, he has wrought, 

 better than he knows, a realistic diagram of movingness as a train of 

 overlapped after-images. 



So, objective movements may be : 



A. Infra-perceptible — so slow that only a change of position is noted 

 from time to time. 



