GENERAL PRINCIPLES 



11 



6. Photography has resolved in a most striking and beautiful 

 way the problems of kinetics. The instantaneous photograph 

 has already made it possible to take and fix on the sensitive 

 film the positions of a moving point or body from one moment 

 to another. Even this is too slow, however ; Muybridge and 

 Marey had recourse therefore to chronophotography ; its actual 

 principle being that the sensitive . film unrolls itself at a known 

 speed and by an arrangement of shutters, which mask and un- 

 mask the object rapidly, a series of exposures at very close in- 

 tervals can be taken. The instantaneous shutters only allow, 

 therefore, a photo-chemical action of a very short duration, for 

 the sensitiveness of gelatine-bromide is sueh that an exposure of 

 Tuzfcro of a second suffices in sunlight. 



In practice y^ of a second exposure is not exceeded. The 

 film advances at each closing of the shutter, and thus we get a 

 large number of images in series at equal intervals. 



Marey, with his chronophotograph, obtained pictures 9 centi- 

 metres square. To-day 200 photographs can be taken in a second. 

 If a moving body is to be photographed, the initial position of it 

 is fixed in relation to a datum line, and we place on the same 

 diagram copies of the various photo- 

 graphs joined together to form a 

 series. The various positions of the 

 same part of a body in relation to 

 time can then be followed. 



Instead of the moving film, Marey, 

 following Janssen, employed the fixed 

 plate ; in those circumstances, he 

 could only take a few pictures, and 

 the body could not have a large 

 surface in order to avoid super- 

 position,. When the object is large 

 (a galloping horse or a man jumping) 

 certain points or lines are emphasised 

 by making them brilliant, the 

 subject being black or draped in 

 black velvet and moving in front of 

 a black background. 



For examples white sticks or 

 stripes (fig. 17) will indicate the 

 spine, the line that joins the shoulders 

 and that which joins the hips. In 

 the Prince's Park, where Marey in- 

 stalled a laboratory, the subjects 

 passed, on a track of blackened 

 stones, before the opening of a shed 

 of which the walls and floor were 



