CH. XI] MOVING PICTURE SHUTTER 421 



that a water-cell be used ( 848) in cases where an automatic fire 

 shutter is not provided; where a motor is used to drive the 

 machine; for all experimental work and for every person running 

 a moving picture machine who has not had abundant experience in 

 operating. It is so easy to let the film stop for a second, or to have 

 the film break leaving a tag end of film in the aperture plate, and 

 wonder afterward what started the fire. 



588. Setting or "timing" the shutter. The shutter should 

 be mounted on the spindle used to turn it in such a way that it will 

 cut off the light from the screen during the time when the film is in 

 motion. If the shutter is not set exactly right in the beginning it 

 is often a rather tedious job to correct its position, but by going at 

 the matter systematically the difficulty is greatly lessened. 



Shutters of the one-wing type can, of course, be set in only one 

 way but shutters of the two- or three-wing types may have wings of 

 different widths. In this case the widest wing is the one which 

 should intercept the light while the film moves. 



The easiest way to set the shutter would, of course, be to run 

 the machine very slowly and watch the picture on the screen. If 

 no shutter were used the picture would seem to jump up, and be 

 replaced by a picture which comes up from below. When the 

 shutter is in place, if the picture seems to jump up just before the 

 light is out, the shutter is said to be too "late" and it must be 

 loosened on its shaft and turned slightly forwards, that is, in the 

 direction in which it is turning. The shutter is then fastened 

 securely in position. If, the picture jumps into place from below 

 just after the light comes on, the shutter is said to be too "early" 

 and it must be turned slightly backwards. That the shutter may 

 be correctly set when it is turning rapidly as well as when it is 

 moving slowly, it is well to hold the outside of the shutter or the 

 shaft on which it turns with the finger so as to take up lost motion. 

 When in rapid rotation all the lost motion is taken up on account 

 of air friction. 



587a. With a two-lens condenser the water-cell can be put between the 

 condenser and the aperture plate (fig. 206). 



