422 MOVING PICTURE SHUTTER [Cn. XI 



Running the machine slowly with a film in the machine is entirely 

 practical provided the arc current is not extremely heavy, and 

 provided a water-cell is used (See 596, 779-782). 



When no water-cell is at hand the machine must be run rapidly. 

 In this case the rule for changing the position of the shutter is 

 exactly the same but the motion of each individual picture cannot 

 be seen. If one has a film which is nearly opaque, but has a few 

 spots in it, as a period on a title for example, there is an effect 

 known as "travel ghost" which is seen if no shutter is used or if 

 the shutter is incorrectly timed. This is caused by the persistence 

 of vision. As the white spot moves upward, it appears to be a 

 streak instead of a spot. If, now, the shutter is too late, the light 

 is not cut off until the spot starts to move upwards and a streak is 

 seen above the spot. If the shutter is too early, the light is turned 

 on while the spot is still moving upward and before it comes to 

 rest; the streak is then seen below the spot. 



If the shutter is too narrow the motion of the spot, both before 

 and after the light is cut off and the streak will be seen both above 

 and below the spot of light. 



589. Rule for setting or timing the shutter. If the streak or 

 travel ghost appears above the letters of the title, the shutter is too 

 late, turn it slightly forward on the shaft. 



If the streak or travel ghost appears below the letters of the title 

 the shutter is too early, turn it slightly backwards on the shaft. 



If the streak or travel ghost appears both- above and below the 

 letters of the title, the shutter blade is too narrow.. Use a shutter 

 with a wider blade. 



590. The best position of the shutter and the speed to prevent 

 flicker. The shutter may be placed in either of two positions ; it 

 may be just beyond the film and between it and the objective 

 (inside shutter) or it may be placed beyond the objective (outside 

 shutter). There is a difference in the effect produced depending 

 on which of these positions is chosen (fig. 225-226). 



With the inside shutter, when the machine is turned slowly the 

 image of the shutter can be seen somewhat out of focus traveling 

 from one side of the picture to the other. 



