CHRONOPHOTOGRAPHY ON FIXED PLATES 57 



measured. Such chronophotographic pictures contain 

 the two necessary elements for understanding a move- 

 ment, namely, a notion of space as well as that of 

 time ; nevertheless, as we shall see, it is often difficult 

 to harmonize two such incompatible notions without 

 having recourse to certain expedients. 



Influence of the Extent of Surface covered by the 

 Moving Object. — If the object under observation covers 

 only a small surface in the direction of movement, a 

 large number of images may be obtained without super- 

 position or confusion, as, indeed, we noticed in the case 



Fig. 37.— A man walking. Cbruiiophotograi>hy on a fixed plate. 



of the moving ball. As far, then, as time is concerned 

 we have a very complete picture, whereas that of space 

 is very restricted. 



Now, if we take a series of images of a man walking, 

 the question of space becomes a most complicated one. 

 Each image must be spread over a considerable surface 

 if it is to show the various positions assumed by the 

 head, arms and legs. Now, the larger the space covered 

 by the image, the smaller must be the number that 

 can be taken on one plate without superposition and 

 confusion. With a lar^e animal, a horse for instance, 



