CHARACTERISTICS OF PHOTOGRAPHIC MATERIALS
A photograph consists of a deposit of black silver known as the image, distributed in such a manner as to represent the tones of the subject. To the eye the image appears more or less continuous, as if it were made by applying a solution of a black dye. Examination at high magnification, however, reveals that it is made up of a lot of separate particles, which are so small and so close together that the eye normally cannot separate them. The photograph owes its particular discrete structure to the fact that the silver of the image is derived from small separate crystals of compounds of silver which make films, plates, and papers respond to light (Figure 6). Materials used for making negatives in the camera consist of silver bromide, usually containing a small percentage of silver iodide. The sensitive substance in printing papers is either silver bromide or chloride. The crystals range in size up to about 1/10,000 in., depending on the kind of material. They are larger on the average in fast plates and films than they are in the slow materials.
The sensitive silver compounds are applied to the support or carrier in gelatin, in the form of an "emulsion." They actually are formed in the gelatin during the manufacture of the emulsion, by being precipitated from solutions of potassium bromide or chloride and silver nitrate. The emulsion is coated in a thin layer on film, glass, or paper and allowed to dry to a layer which is less than 1/1,000 in. thick. The gelatin serves as a means of holding the silver compounds on the support and keeping the crystals separate from one another. It also plays a very important part in determining the sensitivity of the material to light and in influencing the course of development. The art of making emulsions involves the control of the crystals so that they are of a desired range of sizes, and the carrying out of the