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Chapter IX DEVELOPMENT OF THE NEGATIVE
AFTER the picture has been taken, the cameraman delivers the film to the negative developing department, where it is developed and fixed in a manner very similar to that adopted in developing still pictures. Before proceeding with the development of the entire film, when the exposure and light conditions are unknown, a short piece is cut off and developed independently, so that the proper treatment may be determined without endangering the entire reel.
The exposed film is wrapped spirally around a light rectangular frame or rack, for convenience in handling, and is then dipped into a tank containing the developing solution. This arrangement enables the operator to agitate the film in the solution and examine it without danger of injury to the delicate sensitized surface. After the negative has been developed to the required density it is placed in the fixing bath of sodium hyposulphite where it remains until all the remaining active silver salts in the emulsion are dissolved out leaving an image of reduced metallic silver which can no longer be affected by the light.
Fixing having been completed, the film is thoroughly washed in clean water to remove the last traces of hypo. The film is next dried upon large revolving wooden drums, usually driven by power. The motion of the drums throws off any small drops of water that may adhere to the back of the film and keeps a constant stream of warm air moving over the emulsion side.
In some laboratories before drying, the film is given a final treatment in dilute solution of glycerine and water. A small percentage of the glycerine remains with the film even after it has dried and owing to the moisture absorbing properties of the glycerine enough moisture is retained to keep the film in a soft and pliable condition. When the glycerine has been lost after a considerable service, by evaporation or other cause, the film becomes brittle and must be given another treatment in the glycerine bath. This is a precaution that Is not needed so much today as modern film is much more pliable than that used a few years ago when the glycerine bath was a necessity.
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