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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.