In treating of Photo-lithography we do not intend to describe the various general photographic processes, as there are many cheap and good treatises on the subject already published. Most of these photographic manuals, however, address themselves to portrait and landscape work, and we shall briefly point out the specialities of certain instruments which are best adapted to the technical purpose in view. Before doing so, however, we must give our readers an idea of the complete process of photography, so as to enable them to understand the details afterwards, to be presented.

179. Photo-lithography is a method of producing a copy of a print or drawing in line of the same or altered dimensions. This print is of such a nature that it can be multiplied from stone or zinc at the ordinary lithographic press or machine. It is necessary that the subject to be copied should consist of visible lines or dots to insure distinctness in the reproduction. The copying is done by photography upon glass. What is technically called a “negative” is first of all produced. If this negative is viewed by transmitted light, it will be seen that those parts which correspond to the white ground of the original drawing or engraving are more or less dark and opaque, while the copy of the drawing itself consists of transparent lines.

This negative is put into a photographic printing or pressure frame, and a piece of chemically prepared paper is placed face downwards upon it, in contact with that side of the glass upon which the picture has been produced. The back is then secured in its place, and the glass side exposed to the light.