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370 THE ADVANCE OF PHOTOGRAPHY
aperture //6'5 and shutter slot lj inches, fifteen exposures should be made per second.
For very rapidly moving objects, such as cyclists racing, trains in motion, etc., use an aperture //4, with shutter opening one inch, and make twenty to twenty-five exposures per second.
It is absolutely essential that the films used should be accurately perforated, and fit the sprocket wheels both in the camera and projecting machines, otherwise it would be impossible to obtain steady projections.
For projecting the standard-sized films a lantern with some form of cinematograph attachment is required. The object of the attachment is of course to move the film across the field in such a manner that an observer sees upon the screen a reproduction of the movements represented upon the film. This is sometimes done by a series of sprocket wheels which move intermittently and so cause the film to be dragged across the field of view in a somewhat unpleasant, jerky manner. Other models provide an ingenious arrangement of cog-wheels, and a piston rod and plunger, which at every revolution gently pushes the film forward over the extent of one of the small photographs.
It is usually arranged that the picture remains before the eye some four or five times as long as the period of change, so that the tendency for any " nicker " may be reduced to a minimum. Some form of shutter which usually revolves on a steel spindle is supplied, because in some films, particularly those in which light objects occur on a dark background, the rapid movement of the film produces a sort of rain effect. This can be eliminated by a properly arranged shutter, and the entire picture thus made more sharply defined.
The critical frequency at which nicker appears has been shown by Porter to be proportional to the log. of intensity of illumination. Lehmann's experiments and