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465 MOTION PICTURE HANDBOOK
become much worn. It is absolutely essential to good results on the screen that the film be held absolutely flat over the aperture during the time the picture is being projected, and this is not likely to be done if (a) the aperture plate tracks be appreciably worn; (b) the shoes or springs do not set squarely on the tracks, but one or both of them is over to one side. Worn aperture plate tracks are likely to produce a buckling of the film, with consequent in and out of focus effect in the center of the picture. This is particularly true of the type of mechanism which employs a limber tension spring, instead of a stiff tension shoe. By this I do not wish to be understood as saying that in and out of focus effect is always due to the above causes. It may also be due to an old, dry, shrunken film, or ta too much pressure by the tension springs.
General Instruction No. 12. — It is of the utmost importance that the sprocket idlers be kept in line with the sprocket, so that each side of the idler is equidistant from the face of the sprocket, and that the distance of the idler from the face of the sprocket be two thicknesses of a film or a trifle less. If the sprocket idlers be not so set there is likely to be trouble, particularly at the lower sprocket. Losing the lower loop through the film climbing the sprocket teeth is very often directly due to the improper setting of the idler. It is either out of line with the sprocket or too close to or too far away from the sprocket. Many do not realize the importance of a close adjustment of their sprocket idlers. Never allow your sprocket idler to "ride the film" — that is to say, to bear on it with pressure. This is especially bad if the pressure is greater on one side than on the other, and will most likely cause the film to climb the sprocket at the first bad patch. This does not apply to the Edison machines. Their idler rollers ride directly on the film, which is held in place by deep flanges at either end of all sprockets. See to it that your sprocket idlers turn; if they do not they will soon develop a flat spot, and sooner or later this means trouble.
General Instruction No. 13. — It is highly important that the intermittent sprocket shaft and the cam or fly-wheel shaft be kept in exact alignment with each other. The position of the cam or fly-wheel shaft is fixed and cannot be changed. It will readily be seen that if the intermittent sprocket shaft be out of line with the cam or fly-wheel shaft— that is to say, if one end of the intermittent sprocket shaft be 'high or low with relation to the other end — it will bring one end of the intermittent sprocket lower than the other end, and the