FACTORS IN MOVEMENT-DETECTION 351 



'refractory period', because no identical stimulus presented within this 

 period can be perceived as separate from the preceding one. The period 

 has been called by von Uexkiill the 'biological moment' — the shortest 

 discriminable unit of time for the animal. This name for it has justifica- 

 tion only if it be found that in all sensory modalities the duration of the 

 'moment' is about the same. And, Uexkiill did find very good agree- 

 ment. Thus for example, a snail fuses visual impressions coming at four 

 or five per second, and cannot distinguish mechanical taps on its foot, 

 at this same frequency, from a steady pressure. We fuse movie frames 

 at 16 per second, and the lowest frequency of auditory impulses which 

 we fuse into a steady tone is also 16 per second. But, with each of the 

 senses, the duration of the 'moment' is profoundly influenced both by the 

 intensity of the stimulus and the adaptation-condition of the sense-organ. 



We can now understand how the persistence time affects the percep- 

 tion of movements at different speeds. As an object moves slowly across 

 the field we see it with the same clarity, at each instant, that we would 

 if its image were motionless upon the part of the retina which it strikes 

 at that instant. The after-images of the object are being given adequate 

 time in which to fade. With increasing speed, the image of the object in 

 a given position is overlapped by, and blurred by, the after-images of the 

 object in its just-previous positions. We see this blur as movingness and, 

 if the visual acuity of the particular retinal area is extremely low, the 

 movingness may seem disembodied. If, now, the object traverses the en- 

 tire field within the period of the persistence time, we will obviously see 

 nothing but blur, and perhaps cannot decide the direction of the move- 

 ment. And, if the object is of such brightness, and moves at such speed, 

 that its image endures for too short a time on any one spot of retina to 

 arouse any sensation, the flight of the object — a bullet, for instance — 

 becomes supra-perceptible. But by enlarging or brightening the object 

 (as with a howitzer shell, or a tracer bullet) we may restore visibility of 

 its flight even at terrific speed. 



A few years ago, quite a furore was created by the scientific announce- 

 ment that a deer-fly can travel at 800 miles per hour. Skepticism took 

 various forms. A biochemist computed that the fly would consume its 

 own weight in food every hundred yards or so at such a level of muscular 

 activity. Langmuir, of the General Electric Company, noting that the 

 deer-fly at its speediest is still visible, swung a fly-sized lead pellet at the 

 end of a wire at known speeds. When the linear speed of the pellet was 

 13 miles per hour, the pellet was blurred. At 26 miles per hour it was 



