A camera is a light-tight box containing a movable convex lens at one end and a screen at the opposite end. Light from the object to be photographed passes through the lens, falls upon the screen, and forms an image there. If we substitute for the ordinary screen a plate or film coated with silver chloride or any other silver salt, the light which falls upon the sensitive plate and forms an image there will change the silver chloride and produce a hidden image. If the plate is then removed from the camera in the dark, and is treated as described in the preceding Section, the image becomes visible and permanent. In practice some gelatin is mixed with the silver salt, and the mixture is then poured over the plate or film in such a way that a thin, even coating is made. It is the presence of the gelatin that gives plates a yellowish hue. The sensitive plates are left to dry in dark rooms, and when the coating has become absolutely firm and dry, the plates are packed in boxes and sent forth for sale.
Glass plates are heavy and inconvenient to carry, so that celluloid films have almost entirely taken their place, at least for outdoor work.
FIG. - A camera.