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420 TREVOR FAULKNER [j. s. M. P. E.
The straining or tearing of perforations is, in most cases, due to the condition of the film itself, and can not always be averted by competent projectionists even when the projectors are in good operating condition.
This damage usually occurs during the first four or five passages of the film through the projector, and results from the depositing of emulsion on the shoes of the aperture plate. These deposits rarely accumulate to such an extent that the pressure between the aperture plate and the tension on the opposite side of the film is increased sufficiently to cause the teeth of the intermittent sprocket to tear the perforations by forcing the film down in front of the aperture; but the smallest deposit of emulsion on either of these shoes may, by baking, become so hard that its action on the emulsion of the film then passing over it is similar to that of passing a diamond over a pane of glass. When the diamond does not scratch the surface of the glass, it drags very little; but once it "bites" into the glass, the drag or traction may become considerable. This kind of effect occurs with film. Before the deposit of emulsion has acquired a "cutting edge," the film is permitted to slide over the deposit rather easily, and there is little probability of damaging the film. But once this smooth surface is changed, by further accretion of emulsion, into a cutting edge, it begins to cut into the emulsion, creating a resistance to the free passage of the film so that the teeth of the intermittent sprocket may tear the perforations in forcing the image past the aperture.
When such deposits of emulsion accumulate and interfere with the free passage of the film over the aperture plate, the projectionist is warned of the situation by the excessive noise made by the intermittent movement in protest against its overload. The projectionist can do only one of two things : either place a lubricant of some kind on the film before it arrives at the aperture plate, or stop the projector and remove the deposit that is causing the trouble. Neither of these procedures is followed by the average projectionist except in emergencies.
Scratches in the emulsion, in the picture area of the film, are nearly always caused by carelessness in handling the film, either in the projection room or in the exchange inspection room, or by improperly adjusting projectors or allowing dirt to accumulate in them.
Long runs of film, made under the best conditions of operation,