362 ADAPTATIONS TO SPACE AND MOTION 



But there is another difference between the two percepts which cannot 

 be explained away and cannot be reconciled with the idea of a com- 

 pletely identical basis for the two. The movingnesses seen in both cases 

 may seem much alike, but they have utterly different sources. In real 

 motion, the movingness-blur is of physiological origin, and resides in the 

 retina. The overlapped photochemical images which produce it are chron- 

 ologically older than the foremost, newest image of the moving object. 

 But in the ^-phenomenon it is obvious that the generation of the mov- 

 ingness cannot possibly commence in the brain until after the retina has 

 been hit by the second light — otherwise, what would determine the direc- 

 tion the movingness was to take? In some way however, the impression 

 of motion reaches consciousness before the impression of the final posi- 

 tion of the movement — the second light — gets there. The phsysiological 

 sequence of events is: id) reception of the first light; (b) reception of 

 the second light; (c) instigation of the fiUing-in process, the percept of 

 movingness. But the perceptual sequence is: (a) light in initial position; 

 (b) movingness; (c) light in its final position. 



Real and stroboscopic movements are thus deceptively similar subjec- 

 tively; but the only things they share in common objectively are their 

 dependence upon similar conditions of brightness and distance, and the 

 role played in each by the persistence time. This is probably not a coin- 

 cidence, for the close imitation of real movement by apparent movement 

 under the same spatial, temporal, and intensity-conditions presumably 

 has some biological value. What it is, we cannot say. The writer would 

 suggest — most gingerly! — that perhaps when a primitive, stupid verte- 

 brate saw a moving object pass behind an obstacle and emerge again, he 

 could not be trusted to know that it was all one object, and not two 

 different ones, unless he had an automatic means of maintaining the one- 

 ness of the object during the moment when it was hidden from him. 

 Whatever the incentive may have been for the evolution of the 'fiUing-in' 

 process in the ^-phenomenon, it is difficult to see what good its retention 

 has done us — unless one belongs to the growing number who regard the 

 movies as an absolute necessity. 



Stroboscopic Vision in Animals — It is fairly certain that the lower 

 animals in general do have perception of stroboscopic apparent move- 

 ment. At least, it is well established experimentally for fishes and can be 

 inferred from such phenomena as the dog's interest in, and obvious 

 deception by, motion pictures, coupled with his complete indifference 

 toward still pictures. If apparent-movement perception exists for the 



