Of the Recent Advances in Our Knowledge of the Photographic Process

While many materials are sensitive to light and can be used for making photographs, the present art of photography is based upon one specific process : The light-sensitive material is silver bromide in the form of extremely small crystals held in a layer of gelatin. These crystals are very sensitive to light when the reaction to light is made manifest by the process of development, that is, by the treatment of the exposed material with a solution of a suitable reducing agent. After exposure to light, the crystals are very much more easily reduced to metallic silver than when they have not been exposed, and as a result of this difference, the effect of exposure becomes visible as an image composed of metallic silver. Significant advances have been made in recent years in the study of the structure of the light-sensitive materials and of the factors which produce in the silver bromide crystals sensitivity so great that very small amounts of light are sufficient to make them developable ; in the study of the action of light itself and of the change which occurs in the exposed crystals ; and in the study of the development reaction by which the exposed crystals are reduced to metallic silver. The nature of the silver halide crystals and the origin of their sensitivity was elucidated between 1920 and 1930. An adequate theory of the nature of the reaction of light has only become available in the last three or four years, and our views about the nature of the development reaction have changed very radically in the last year or two.. A photomicrograph of the silver halide crystals of the lightsensitive emulsion is shown in Plate XII. These crystals are composed of silver bromide containing a small amount of silver iodide, and they may be dyed to sensitize them to the longer