Motion picture projection : an elementary text book (1928)

MOTION PICTURE PROJECTION 567 trap, will show that much thought has been given to the designing and finishing of this surface, which must of necessity be perfectly smooth as well as accurately squared and lined up. It will be noted also that the section of film operating between the intermittent sprocket and upper loop is the section which intermittently stops and starts while the balance of the active film moves constantly. With this intermittent sprocket action pulling the film into aperture position with a sudden start, and stopping it just as quickly, there naturally is a tendency to create either a slight vibration or a buckling of the film, which in reality is similar to holding a loose loop of string between the two hands and suddenly jerking it taut, when it will be noticed that the string vibrates considerably. It must be realized also that any movement on the part of the film at the aperture position, being magnified so greatly upon the screen, does much to prevent a perfectly steady picture. In order that the film will be at perfect right angle with the lens or optical train, and so that the tension or film pads will function at their best, it is necessary that the replacing of the film trap unit be followed out with extreme care as to its proper alignment. To replace : — The really important fact to bear in mind, is the lining-up process which is followed after the two film trap screws S-133-C, P.4, have been entered into place. These screws together with several locating pins hold the film trap in position so that the alignment is made possible at this time. Now by turning to Plate No. 2 we see the mechanism as it appears with the film