CHRONOPHOTOGRAPHY ON FIXED PLATES 55 



at regular intervals of time, an interrupted trajectory 

 will be obtained (lower curve, Fig. 36). This repre- 

 sents the successive positions assumed by the moving 

 object at each moment when light is admitted. This 

 is the chronophotographic trajectory In this method 

 the intervals of time separating two images are of 

 constant and known duration. 



To obtain the best possible results, the object must 

 be brightly illuminated and the background absolutely 

 dark. The duration of the exposure must be very 

 brief, in order that the object may not move an ap- 

 preciable distance during a single admission of light. 



Fig. 36. — Simple trajectory and chronophotographic trajectory of a bright ball 

 moving in front of a dark background. 



The original form of the chronophotographic appa- 

 ratus was very simple. It consisted of an ordinary 

 camera and lens. Within the body of the camera, in 

 front of the plate, a fenestrated diaphragm was fixed. 

 This rotated at a perfectly uniform rate by means of 

 a crank and regulator. The sensitized plate was held 

 in a frame and fixed in a position so that the object 

 was focussed accurately upon it. As each slit in the 

 diaphragm came into position, the plate received an 

 impression of the illuminated object, representing the 

 actual form and position of the object at that particular 

 moment. Now, as the object became displaced between 

 successive exposures, a series of impressions was ob- 

 tained exactly corresponding to the shape and position 



