62 MOVEMENT 



successive positions occupied by the line which en- 

 gendered them. Now, if the intervals between the 

 exposures are precisely equal, we have an example of 

 stereoscopic chronophotography, and consequently a 

 complete expression of the movement. This method 

 is applicable in a great many cases in which we want 

 to know whether the moving object moves in one 

 plane only or in three. 



Method of multiplying the Number of Images without 

 producing Confusion. — The applications of chrono- 

 photography are, as we have seen, limited by inter- 

 ference from superposition and consequent confusion. 

 Now, the larger the space covered by the object, and 

 the slower the movement, the sooner does superposition 

 occur. Thus, if the space is large and the movement 

 slow, recourse must be had to certain measures, if we 

 want to obtain a photograph of the various positions 

 occupied in space. 



One method consists in taking alternating images, 

 another in separating the images on the plate by 

 making them fall on different parts. 



Alternating Images. — To obtain these, a stereoscopic 

 apparatus with two lenses is employed, both of which 

 are controlled by the same diaphragm. Such a 

 diaphragm should be circular, and contain only one 

 slit, which, as it rotates, alternately admits the light 

 first by the right and then by the left lens. Two 

 series of images will be thus obtained, which lie in two 

 parallel lines. The upper series corresponds to the 

 odd numbers, and the lower to the even numbers, as 

 is shown below — 



13 5 7 9 

 2 4 6 8 10 



In this way Fig. 43 was obtained, which shows the 

 various positions of the wings as assumed by a seagull 

 during flight. 



