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164 PRINCIPLES OF CINEMATOGRAPHY
element to transfer synchronising marks from one film to another, and also to thread the films correctly when such marks are supplied, was a potential source of error. With negatives which required the use of light-changing operations it was also necessary to depend on the operator not only to thread 'grading-cards' in true synchronism with the films but also, in the case of semi-automatic printers, to remember to pre-set the exposure selector lever before the corresponding notch had passed through the negative head.
Because of this the Bell & Howell Company introduced their fully automatic printing machine. This machine prints both sound and picture in one operation and also automatically adjusts the light-changing device. The machine was so designed that the sound and picture negatives, together with the rolls of matt film used to control the light changes, remained permanently threaded in the printer until all the required copies had been made. This was accomplished by arranging for prints to be made from either the start or the end of the negatives, that is to say, the first print was made by running the negative from the start to the end, the machine then automatically stopped with the negatives still threaded in the heads. The operator then loaded a new roll of unexposed positive film into the machine and, on re-starting, the negatives then travelled from the ends back to the start positions whilst printing this second copy.
The operation of this machine will be more clearly understood from Figure 68 which shows the general layout of the mechanism and the film paths. Three main operations are performed by this machine, firstly, the sound track is printed on to the positive film at the 'Sound Gate', indicated by 'SG\ secondly, a thin black line is printed on either side of the sound track by passing the positive film through the 'Line Gate', shown at 'LG', and, thirdly, the picture is printed by passing the positive and picture negative films continuously through the 'Picture Gate', shown at 'PG\ Both sound and picture heads print by contact between the negatives and positive film and are of rotary, or continuous type. All the sprocket wheels, by which the films are positively driven, are indicated by the letter 'A' in the figure. Spring-loaded tension rollers are indicated by the letter 'B\ and guide rollers rotating about fixed centres are indicated by the letter 'C or are unmarked. All the films are cleaned automatically both before and after passing the exposing points by passing them between a pair of nozzles, indicated at 'D\ containing 'IT shaped tubes having a series of fine holes drilled on the inner side to direct a jet of compressed air against both sides of the films. Compressed air is also