SYNTHESIS OF MOVEMENT 305 



they depend on the physiological property of the retina 

 of retaining for a brief moment the impression of an 

 image after the object which has produced it has 

 disappeared. The duration of this retinal picture is 

 estimated at -^ part of a second. So that if an image 

 is placed before our eyes ten times in a second the idea 

 of discontinuity is lost, and the images appear to 

 be in continual evidence. 



If the images shown to us are represented in the 

 successive positions assumed by the object in motion, 

 the impression conveyed to the eye is that of a con- 

 tinuous movement with no intermission. Now, we have 

 seen that not only 10, but even 20, 40, or 60 images 

 can be produced by chronophotography per second. 

 If the 60 photographs taken during one step of a 

 galloping horse could be passed before the eyes at the 

 rate of 10 per second, the duration of the whole step 

 would be spread over a period of 6 seconds, and hence 

 we should have a considerable time in which to observe 

 the motion of the limbs, so hard to follow under normal 

 conditions. 



In the same way a flying bird could be represented 

 with slower wing movement, and so too with other 

 phenomena which escape notice on account of their 

 extreme rapidity. 



Inversely, when a movement is so slow as to escape 

 observation, photographs could be taken of it at long 

 intervals and presented to the eyes in sufficiently rapid 

 succession to allow of the changes being clearly per- 

 ceptible. In other cases, if the photographs are pre- 

 sented to the eye at the same intervals as separate 

 the successive exposures, the movement will appear as 

 it actually took place. Such is the use of the strobo- 

 scope. We will now show the successive developments 

 of this method. 



Plateau's Phenakistoscope. — Everybody knows the 



