TIME 15 



measure the exposure of a photographic shutter ; and 

 secondly, to measure the intervals of time which 

 separate consecutive exposures. Both of these methods 

 must be described, since it is necessary to be familiar 

 with them, so as to understand the analysis of move- 

 ment by means of photography. 



Measurement of the D oration of Exposure produced by 

 a Photographic Shutter. — Shutters are generally called 

 instantaneous, when they show in the photograph a 

 representation of a moving object as clearly as would 

 have been the case had the object been at rest. This 

 definition is not, however, strictly true, since shutters, 

 capable of giving a clear picture of passengers in the 

 street, may be incapable of doing so in the case of 

 the hoofs of a trotting horse. A still shorter time is 

 required to catch the various positions of the wing of 

 a flying bird, and still more so in the case of insects. 

 It must be possible then to 

 measure the exposure of a 

 shutter, and express the time 

 in fractions of a second. 



We shall see how photo- 

 graphy allows this measure- 

 ment to be made. Let a 

 bright needle rotate on a dial 

 (Fig. 11) covered with black 

 velvet, and graduated by white f.-.. n.-Needie spinning round the 



!• mi . n. .i chr. nomotr c dial, and measurin-' 



lines. lhe movement 01 the the duration of exposure. 



needle must be absolutely 



uniform. For. this purpose clockwork machinery 

 with a Foucault's regulator is employed. This 

 mechanism is hidden behind the dial, and the needle 

 makes one complete revolution in a second and a half, 

 say 90'". The circumference of the dial is divided 

 into eighteen equal parts, and consequently the con- 

 tained angle of each space corresponds to 5"\ 



