680 



HANDBOOK OF PHOTOGRAPHY 



vides a high-gamma duplicating positive (No. 204), shown in Fig. 21. Duplicating 

 positives are usually distinguished by a lavender base. 



Cameras and Their Operation. — A motion-picture camera is essentiallj^ a mecha- 

 nism for driving film intermittently and exposing successive frames while the film is at 

 rest. Its basic elements are shown schematically in Fig. 22. An unexposed roll of 

 negative is carried on a freeh^ revolving core on one side (left) of a film magazine. 



Fig. 24 J.. — Detail of camera parts (shutter, sprocket, gear train). 



Fig. 24:B. — Detail of camera parts (film movement mechanism). 



The usual amount of film in such a feed roll is 1000 ft., although 400-ft. magazines are 

 also furnished. The film is pulled down through a light trap by a sprocket wheel and 

 its associated guide rollers, whence, in the form of a loop, it is carried to and held flat 

 in a gate behind the lens. Returning to the other side of the sprocket wheel, it is 

 finally wound on the take-up roll (right), the core of which is tension driven, so that 

 as the diameter of the roll increases the rotational speed is proportionate!}" diminished. 



