Numerous investigators, Daguerre, Niépce, Fox Talbot, and others, have been credited with the discovery of photography, but the fact seems to be that these, and many more, merely contributed, each in his turn, some portion of the total that goes to make up the art as it now stands. Photography means, literally, “light-writing,” the name being derived from two Greek words, phos, light, and graphein, to write. The practice of photography depends primarily on the fact that certain chemical compounds are changed into other compounds by the action of light. Another fact is closely allied with this, namely, that a suitably constructed lens of glass or other transparent material, or a fine needle-hole used instead of a lens, will project the image of objects placed in front of it. A camera, then, consists of a light-tight box having at one end an arrangement for holding a lens or a card with a needle-hole in it, and at the other end a device for holding some light-sensitive chemical to receive the image projected by the lens. In modern practice this light-sensitive chemical is almost always bromide of silver or a mixture of the bromide with other silver compounds, these chemicals being held in an emulsion of gelatine. When the gelatine emulsion is coated in a thin film on a sheet of glass the result is known as a dry-plate, or, simply, a plate. When it is coated on a strip of celluloid wound on rollers so that successive portions may be exposed to light, it is called a roll film, and when it is coated on separate sheets of celluloid, arranged like a pad, to be exposed successively, it is called a film pack. A similar emulsion coated on paper gives bromide or gas-light paper, which, as will be seen later, is used for making prints. At one time wet collodion plates were generally used, a sheet of glass being coated with collodion and sensitized by bathing it with iodide of silver. The exposure was made before the plate dried; but these plates were inconvenient to handle and have been almost entirely superseded by the gelatine dry plate. The prepared plate, of whatever type it may be, is placed in the camera and exposed for a longer or shorter time, depending on circumstances, to the light projected by the lens, but no image is visible after exposure, (unless, indeed, the exposure has been tremendously excessive,) and the plate must be developed.
From a platinum print by Gertrude Käsebier
“BLESSED ART THOU AMONG WOMEN”
Portrait Photograph
From a photograph by Paul L. Anderson
A COUNTRY STATION MASTER
Portrait Photograph
There are about fifty different reducing agents on the market; most of them are derived from coal-tar, though some are made from nut-galls, lichens, and other substances. The developer consists of a solution in water of one or more of these reducing agents, with other chemicals to control the action, the exposed plate being bathed in this solution, either in the dark or in a light to which the plate is not sensitive. Wherever light has acted on the silver salts the developer causes metallic silver to be deposited in proportion to the amount of light-action, so that on holding the developed plate up to the light a dense deposit is seen in those parts representing the brightest portions of the subject, while the shadows of the original are represented by thin areas, and the half-tones by deposits of intermediate density. For this reason the developed plate is called a negative. The plate is then bathed in a solution of sodium thiosulphate (generally called “hypo”), which dissolves the unaffected silver salt but does not affect the metallic silver—or at least does so only very slowly. Next, the plate is washed in water to remove all unnecessary chemicals, and is dried. The ordinary plate is sensitive only to ultra-violet (invisible) and violet light, so it cannot render truthfully any subject having color, but by the addition of certain aniline dyes to the emulsion it may be rendered sensitive to green in addition to violet and ultra-violet; it is then described as orthochromatic (“right-colored”) or isochromatic (“equally-colored”). Still other dyes extend the sensitiveness to include not only ultra-violet but also the entire visible spectrum; such a plate is called pan-chromatic (“all-colored”).