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330 PRINCIPLES OF CINEMATOGRAPHY
difficult to manufacture a split sprocket wheel, having a gap formed around the circumference at the position of the sound track, it is generally found that the 35-mm sprocket consists of one half only. This is indicated at 'B' and it is usual to provide a stationary curved gate through which the sprocket teeth project and over which the film may ride. A small stationary aperture may then be cut in the gate in the position of the sound track and centred with the optical axis of the lens system. Since the hub of the 35-mm sprocket projects some distance beyond the sprocket flange it is not possible to illuminate the 35-mm gate by a straight optical system.
Light from a heavy filament low voltage lamp 'L' is used to illuminate the track, lens '(V forming an image of the filament at the plane of lens 'Og' whilst lens '02' forms an image of the uniformly illuminated lens '(V at the plane of the 35-mm track. Since, in this arrangement, the lamphouse must be located to one side of the driving motor, it is necessary to turn the light beam through a right-angle by employing a prism 'P^ between the two lenses 'CV and '02'. After passing through lens '02' the light beam is displaced inwards towards the rotating sprocket by a small double prism ab shown at T2\ The required image reduction is then obtained by the use of two cylindrical lenses '03' and '05' and one high quality spherical lens *04\ Lenses '03' and '05' are only operative in the lateral plane and serve to partially neutralise the power of the spherical lens. In the longitudinal plane objective lens '04' is alone controlling the image formation since lenses '03' and '05' have no curvature in this plane. The large prism 'P3' known as a roof prism not only directs the fight from the 35-mm side of the machine around to the 16-mm side but also inverts the image in the longitudinal direction so that the image formed on the 16-mm film travels in the same direction as the film itself.
Design of Printing Gates
With machines of the type just described it is obviously necessary to use stationary gates at the printing point and to cause the film to slide over these surfaces by engagement with sprocket teeth arranged to project through a slot in the gates. The gates must therefore be formed in a curved path and mounted in close proximity to the sprocket wheels as seen in Figure 146. This figure shows a side elevation of the layout of one film path it may be either the 16-mm or 35-mm side of the machine since both are identical in arrangement although somewhat different in actual