120 MOVEMENT 



A crank placed behind the chronophotographic 

 apparatus turns all the wheels of the instrument, as 

 well as the circular diaphragms. A movement, so 

 rapid as this must necessarily be, is bound to be 

 continuous, for it would be impossible, as in the case 

 of the photographic gun, to remit or continue the 

 movement of such heavy bodies. The film itself 

 comes to rest at the moment of exposure, arrested by 

 a special mechanism which allows it to continue its 

 movement as soon as the image has been taken. 



Necessity for arresting the Progress of the Fi^m at the 

 Moment of Exposure. — Some people have thought that, 

 by using such a complicated apparatus as that which 

 we have employed for arresting the movement of the 

 film, we have given ourselves unnecessary trouble, and 

 it has been said that for very short exposures the move- 

 ment of the film might be neglected. It would be 

 easy to prove by calculation that, during the period of 

 the exposure, say j^qq part of a second, the film 

 would move enough to deprive the photographs of 

 that clearness upon which their value depends. But 

 it is simpler and perhaps more convincing to show by 

 an experiment that without these periods of arrest 

 good images are not to be obtained. 



By alternately suppressing and inducing an arrest 

 of the film at the moment of exposure, we obtained 

 a series of images which were alternately blurred and 

 distinct. In Fig. 85 two such consecutive images are 

 shown. The different degrees of definition is so 

 obvious that it is useless to further insist on the 

 necessity of arresting the film during the period of 

 exposure. 



The Moment to choose for taking the Photograph. — 

 When the chronophotographic apparatus is pointed at 

 the object the movements of which are to be studied, 

 the wheels are put in motion by turning a crank, 



