STROBOSCOPIC VS. REAL MOVEMENT 357 



ary. One experiences this illusion after watching a waterfall or a stream 

 for a time, and then turning one's attention to objects on the bank. 



The third, most important and most interesting kind of apparent 

 movement is that called variously stroboscopic or cinematoscopic move- 

 ment, or the '^-phenomenon'. It is obtained either when identical or 

 slightly differing images fall in succession upon neighboring retinal 

 areas, or when slightly different images fall successively on the same 

 retinal spot. 



The stroboscope (meaning 'whirling looker') was invented almost 

 simultaneously by Plateau and Stampfer more than a century ago. In 

 any of its many forms, it is a device for making moving objects appear 

 to be stationary, where the object — usually a rotating one — has a regular 

 and serially-repeated pattern. The reader can make a simple stroboscope 

 (see Fig. 125a) in the following way: Take a disc of cardboard about 



Fig. 125 — Simple stroboscopes (see text). 



eight inches in diameter and puncture its center with a pencil which can 

 then serve as an axle. Draw a few radial pencil lines on the disc, evenly 

 spaced. Punch small holes through or between these radii, equal in 

 number to the latter, equally spaced apart, and equidistant from the 

 center. Now place the disc with the pencilled radii facing a mirror, and, 

 with the eye looking into the mirror through one of the small holes, 

 spin the disc. 



One sees the pattern of pencil lines 'standing still', like the spokes of 

 a motionless wheel, no matter what the speed of the disc. The eye sees 

 the group of lines reflected in the mirror every time a hole comes along, 

 but the small hole permits such a brief glimpse that no motion of the 

 lines is perceptible. Since, through each hole, lines are always to be seen 

 pointing in the same set of directions, only this single changeless pattern 

 can be seen. 



