Talking pictures : how they are made, how to appreciate them (c. 1937)

Developing the Film tive film as it is placed in the camera has on a celluloid base an emulsion coating of several chemicals, the principal of which is nitrate of silver. The famous quality of nitrate of silver is this : when it is exposed to light, its white crystals turn black. Put this exposed film in a bath of a special chemical formula and it is "developed"; that is to say, the unexposed nitrate of silver is removed, leaving light portions on the film. The black portions on which the light had acted chemically remain untouched. But this developed film is quite the opposite in light values from the exhibition print which, when strong light is shown through it, projects enlarged and moving pictures, lifelike and natural, on theatre screens. Developed negative film portions which the camera saw black or dark, are in reality light because little illumination came through the lens. On the contrary, black portions mean that a great deal of light was focused on the nitrate of silver emulsion. To bring these light relations back to normal a different type of film called positive is exposed to light coming through the negative. This is called printing from a negative, and the result is a positive print. Positive prints in celluloid form are the counterpart of the paper prints made from still-life negatives exposed in a hand camera. It requires about two hundred and fifty prints of each picture to supply the American theatres alone. These two hundred and fifty prints are divided between thirtyfive or forty regional offices called exchanges. Since motion pictures are perishable products, the value of which diminishes with each day of their age, profits in [229]