WRITTEN FOR THE MENTOR BY PAUL L. ANDERSON ILLUSTRATION FOR THE MENTOR. VOL. 6, No. 12, SERIAL No. 160 COPYRIGHT, 1918, BY THE MENTOR ASSOCIATION, INC.
THE MENTOR ·· AUGUST 1, 1918 DEPARTMENT OF FINE ARTS
_Artist Photographer, Author of “Pictorial Photography”_
Entered as second-class matter March 10, 1913, at the postoffice at New York, N. Y., under the act of March 3, 1879. Copyright, 1918, by The Mentor Association, Inc.
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exposure was made before the plate dried; but these plates were inconvenient to handle and have been almost entirely superseded by the gelatine dry plate.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.