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166 PRINCIPLES OF CINEMATOGRAPHY
roller *C, over a driving sprocket 'A', under a tensioning roller *B' and, via guide roller 'C, is fed through the sound gate in contact with the positive film and underneath this film. A similar series of rollers and sprockets guide the film downwards to the take-up spool 'SN2' and, here again, before winding up, the film passes through a second cleaning gate 'D\ The picture negative follows a path similar to that of the sound negative and, in this case, is seen to travel from TN^ towards TN2\
The exposure of the sound track is controlled by a roll of matt film having a variable width clear centre section, bounded by opaque marginal areas. This film is lead over the sound matt gate 'SMG' which surrounds a housing containing a prism, by which the printing light is conducted from the source at the rear of the machine upwards to the sound gate, and through the matt film. To conserve film the matt is moved intermittently through the gate at 'SMG' at a speed one-quarter that of the negative and positive films. The matt is therefore lead from supply spool 'SM', through the air cleaning gate 'D', around a constant speed sprocket 'A' and is then formed into a free loop to enable the intermittent motion at the gate 'SMG' to operate satisfactorily. A similar path conducts the matt film to take-up spool 'SM2'. In a similar matter a second matt film is used to control the exposure given at the picture gate, and travels from a supply spool TM/, through the picture matt gate ?MG', to the take-up spool ?M2'. This film also passes through air cleaning gates before and after passing through the exposing gate.
In the illustration the positive film will be completely exposed when it is fully wound up on spool 'P2'. When this occurs the sound negative will have automatically come to rest with the last few feet of trailer still threaded in the machine, but with the bulk of the film wound up on spool 'SN2'. Similarly the sound matt film will still be threaded through the machine, but with the bulk of film on spool 'SM2\ Both picture negative and picture matt films will also automatically stop when still threaded in the machine, but with the bulk of film on spools 'PN2' and TM2* respectively. The machine operator will then remove the exposed positive film and replace a roll of unexposed film on the same spool, that is, on spool 'P2\ When this film is completely threaded the machine is set in reverse, and the positive film travels towards 'P^ whilst the negatives and matts travel back to their starting positions.
Obviously, the layout of the negatives and matts to ensure that adequate and correctly designed leaders and trailers are included, together with the automatic stopping devices, is a very intricate