STROBOSCOPIC VS. REAL MOVEMENT 361 



across the field of the camera. When this is done, the finger is seen to 

 jerk across the screen, taking twenty new positions per second, and does 

 not appear to be in motion at all. 



Before we leave the movies, it is worth-while to point out that the 0-phe- 

 nomenon can be seen binocularly. A real movement appears just as con- 

 tinuous if we blink our eyes alternately while observing it. So also, the 

 ^-phenomenon occurs if each eye sees only one of the stimuli. A movie 

 will still move even if shutters placed before the two eyes are opened and 

 closed alternately in synchrony with the alternate frames of the film — 

 it is only necessary to take and project the picture at twice-normal speed, 

 to prevent flicker. This phenomenon is the basis of some methods of 

 making stereoscopic motion pictures. 



A common illustration of the 0-phenomenon, often suggested in psy- 

 chological text books because it requires no apparatus and is therefore 

 'simple', is actually of the binocular type — with special complications: 

 if a finger is held still before the eyes, and the eyes are blinked alter- 

 nately, the finger is seen to move from side to side. Actually, the single 

 finger cannot represent our pair of 0-phenomenon lights — it appears in 

 two positions, to begin with, only if we are accommodating beyond it. 

 The two diplopic images of the finger then have different apparent posi- 

 tions because of their different parallaxes with background objects. If 

 one accommodates and converges steadily upon the finger, it will not 

 'move' at all when the eyes are alternated. Blinking the eyes makes it 

 difficult to maintain the convergence for the finger. Try propping up a 

 pencil instead, and occluding the eyes alternately with your hands, held 

 before them, while watching the pencil. The pencil will not 'move', un- 

 less you fixate something beyond it. 



Now, can the machinery with which we see real movements be, actu- 

 ally, our machinery for stroboscopic perception? When the distance and 

 duration of real and apparent movements is objectively the same, they 

 appear equally moving; but the real movement may seem slower and 

 smoother and the apparent movement a bit jerky. This jerkiness we can 

 attribute to the fact that the impact of the second stimulus upon its 

 retinal spot is sudden. The relative retardation of the real movement is 

 perhaps due to the circumstance that intermediate retinal areas are actu- 

 ally receiving stimulation; for, as is well-known, we see an occupied space 

 as longer than an unoccupied one, and if a movement traverses the two 

 in the same time, it will seem to traverse the occupied space more slowly. 



