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PRINCIPLES OF MOVIES 7 time ago, before 16 mm, 8 mm or 17.5 mm film, another size was introduced—28 mm—but this did not become popular and very soon was forgotten. Photography depends on chemistry for its operation. This is not intended as a primer on photography, but a very brief explanation of what goes on inside the camera will help the student appreciate some of the problems he will face in the film and television industry. Photography has come a long way since the crude experiments in the eighteenth century, but it still depends on the same thing—the chemical effect of light on a film coated with certain chemicals. Despite all the progress that has been made, silver still plays a very important part in the operation. The base is made by combining various ingredients, which de- pend on the type of film being made, and pouring the resulting thick liquid onto a slowly rotating drum perhaps 5-6 feet wide. The drum speed is designed so that all the volatile solvents have evapo- rated in a little less than one revolution. The resulting sheet is pulled off continuously and rolled for convenience; it may be as long as 2000 feet and is of uniform thickness except for the edges which are discarded. Since the characteristics of every batch of base mixture are not constant, but vary even from roll to roll, the advice to ob- tain all film for a particular production with the same batch num- ber is readily understood in its application to base as well as emulsion. The coating is really much more important, for it is this which takes and records the product of the producers, cameramen, actors, and the whole film company. Therefore, it is essential that it be uniform in as many respects as possible. Gelatin is used to support the particles of silver salts which constitute the active agents in the emulsion. It also acts in the same way as manganese dioxide in a dry cell: that is, it is a depolarizer and removes halogen produced by the action of light on the silver salts. Emulsion is made by dissolving gelatin and heating it. Silver bromide is added and an excessive quantity of potassium bromide, the latter being added as a sort of catalyst. Silver nitrate is then added, and minute crystals of silver halide are precipitated; these