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1949 AIR COOLING OF FILM 659
normal projection. The data for Fig. 11 were obtained during the projection of a roll of fresh film at normal intensities, where, if left to its own devices, without air impingement, the film would have taken an equilibrium position at approximately —0.026 inch. With a series of air flows directed both from the emulsion side and the base side of the film, it will be seen that this equilibrium position can be displaced to a maximum negative displacement of —0.035 inch and a maximum positive displacement of greater than +0.060 inch!
With no air on the base side and air impinging only from the emulsion side, it will be seen that near the mid-point of the air-flow range, the film was restrained from its normal negative drift to the extent that it was held nearly flat in the aperture, and that further air on the emulsion side displaced the film toward the projection lens. The action of air on the emulsion side only in pushing film toward the projection lens can be counterbalanced by air directed on the base side. It can be seen also from Fig. 11 that the highvelocity flow directed at the base side exerted enough force on the film so that, in combination with the normal forces producing negative drift, it prevented the air on the emulsion side from blowing the film toward the lens; actually, the resultant forces produced a greater amount of negative drift than would have been normal in this film. It must be realized that Fig. 11 represents only the effects of air forces on film which, without air, would have positioned around —0.026 inch; it does not give a definite answer about the effects of air forces ori film which might be at a different stage in its projection life, and might tend of its own accord to focus nearer zero or on the positive side of the gate.
In the discussion of in-and-out of focus, it was pointed out that this phenomenon takes place when the best compromise film focus is near zero, permitting some frames to drift negatively and others to drift positively. Actually, in-and-out of focus has been produced in film under radiation conditions that normally do not result in in-and-out, by displacing the film with an air jet to bring its focal position near zero. Thus, it becomes apparent that, for in-and-out of focus, air directed only at the emulsion side has two conflicting contributions: (1) it cools the film and by lowering the film temperature delays the onset of in-and-out of focus, and (2) it forces the film more nearly toward zero and therefore hastens the beginnings of in-and-out of focus. In order to obtain the full benefit of the cooling action, it is necessary, therefore, to counterbalance the mechanical force of the air jet on the emulsion surface (where most of the film cooling must be