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May, 1930] CAMERA MECHANISM 541
opinion which I have formed after many years of consideration. The next point is the gate. In a great many cameras, the pressure of the gate has been the factor in bringing the film to rest, because in most cases the driving pins have not exactly fitted the perforations . Lately we have been getting perforations to which we can make pins exactly fit ; but pins so made are liable to rapid wear. The method of moving the film is, however, the most important item. Individually I have a considerable objection to eccentrics or cams, and also to slots or slides of any kind. Pin joints in my opinion are the only rational means of converting rotation into reciprocation, and when we can design a movement having only pin joints in its construction, we shall have the ideal camera movement as far as the driving item is concerned.
It may be well to state here the reason for my objections to cams and slides. In the case of a cam which rotates against a more or less flat surface, to insure complete and continuous lubrication is somewhat difficult. The same objection applies in a lesser degree to slides. Dirt, dust, and grit, like the poor, we always have with us, and it is extremely difficult to guard against contamination of the sliding surfaces. As a cam rolls round it continually meets new parts of the surface against which it works and in many cases particles of matter become wedged and form a more or less efficient ball clutch. The same objection applies in a lesser degree to slides. When a cam becomes in the slightest degree worn, its action is necessarily characterized by back-lash and consequent noise. A slide does not suffer in exactly the same way, but as it travels twice over the middle positions, for each time it reaches either end position, it is liable to wear loose at the middle part and, if the wear be taken up, to wedge at the ends. This may be overcome almost completely by making it overrun its fitting position at each end of the stroke. Many camera movements depend on slides and these work very well if properly designed and fitted.
Sprocket trouble is not so prevalent as it used to be; in fact, with present day film, sprocket trouble seldom occurs. When the gauge varied, I got over sprocket trouble by a method which could be well followed now. I made the diameter of the sprocket somewhat smaller than normal, but with rather long teeth. I employed a shield instead of rollers, and placed this shield at such a distance from the sprocket as to allow the film considerable play. The effect was that when the film was short in gauge, it automatically set itself near to