358 ADAPTATIONS TO SPACE AND MOTION 



The higher the speed, the smaller the holes must be to 'stop' the 

 motion, until impracticably small holes are required. In stroboscopy in 

 industry, for seeing the distortions of rapidly rotating parts, intermittent 

 illumination is more feasible. If a motor armature is whirling at ten 

 thousand revolutions per minute, and is illuminated by ten thousand 

 light-flashes per minute, each of very short duration, the armature is 

 seen as if standing still; but any distortions produced by its rotation are 

 visible and can be studied deliberately. 



Now, if our cardboard disc should have one or two more evenly- 

 spaced radii than the number of evenly-spaced holes (Fig. 125b), the 

 pencilled pattern will appear to rotate slowly forward. If the number of 

 holes is in excess (Fig. 125c), the spokes will seem to turn slowly back- 

 ward. These apparent movements of a pattern which (seen through any 

 one small hole) is always actually motionless, are stroboscopic apparent 

 movements. 



We can duplicate the essentials of this illusion in a darkroom with a 

 pair of small lights. If the two lights are a given distance apart and of 

 a given intensity, and are flashed in succession, there will be found a 

 time-interval of flashing at which one sees the first light apparently slide 

 over into the position of the second. Between the two end positions of 

 this 'single' light, a distinct blur of movingness is seen. 



If the time interval between the two flashes is too long, however, one 

 sees two lights flashing in sequence with no illusion of movement. This 

 is called the 'successive phase' of the illusion. Shortening the time 

 interval now brings back the optimal phase, in which the movement is 

 perceived. With very short time intervals, the 'simultaneous phase' is 

 reached, in which the two lights appear to flash together. 



This illusion is called the ^-phenomenon. The movement seen need 

 not be in a straight line — it always follows a course which it might be 

 'expected' to do. Thus, if the two stimulus-spots are both tangent to a 

 visible curved line, the first spot seems actually to roll along the curve 

 to reach the position of the second stimulus. If the two stimuli are lines 

 which, if presented simultaneously, would form a right angle, the appear- 

 ance seen in the optimal phase is of a single line pivoted at one end and 

 swinging through a right angle (Fig. 126). 



The ^-phenomenon sounds academic when thus described in terms of 

 unfamiliar apparatus, but we experience it about 200,000 times whenever 

 we sit through a movie show. It is the (^-phenomenon which makes the 

 movies move, for the motion picture camera is like our stroboscope with 



