312 MOVEMENT 



as they appear in nature. Bat a combination of the 

 zootrope and chronophotography has further possibili- 

 ties, for it enables the observer to follow movements, 

 which would otherwise be impossible to examine, by 

 slowing down the motion to any desired rate. We 

 have already pointed out during a single stroke of the 

 wing, which lasts J of a second, a series of twelve 

 photographs can be taken at intervals of c } 6 of a second. 

 Now, these twelve photographs, which correspond to a 

 single stroke of the wing, can be made to pass before 

 the eye in one second. This succession is sufficiently 

 rapid to produce an impression of continuous motion. 

 Under these conditions, the rate of movement is reduced 

 to one-fifth of its actual velocity, and the eye can follow 

 it in all its phases, whereas, in a living bird, only a 

 confused flutter of the wings can be distinguished. 



In the same way, by slowing down the phases of a 

 horse's paces by means of the zootrope, they can be 

 more easily analyzed than by observations made directly 

 on the animal. 



It is not, however, only by reason of their rapidity 

 that some movements elude observation, sometimes 

 their very slowness renders them inaccessible to our 

 senses, take, for instance, the growth of animals and 

 plants. These movements may, however, become quite 

 visible if they are photographed at considerable inter- 

 vals of time, and the corresponding series of images 

 passed rapidly before the eyes by means of the 

 zootrope. 



Professor Mach, of Vienna, suggests a curious line 

 of research by means of this method. His idea is to 

 take a number of photographs of an individual at equal 

 intervals of time, from earliest infancy until extreme 

 old age, and then to arrange the series of images thus 

 obtained in Plateau's phenakistoscope. If this were 

 done, a series of changes, which had been brought 



