Photographs are reproduced tor printing by what is known as the half-tone process. Line bloeks giving merely the outlines are produced in the same manner, except that no screen is used. Considerable plant is required to do the work thoroughly. A negative showing strong contrasts is taken on a photo-mechanical or wet collodion plate, a screen of sheet glass, ruled with a network of line lines, being intei'posed between the lens and the plate. The screen usually contains about 120 crossed Hues to the inch, but for work on tine surface paper 240 lines to the inch can be used. This screen breaks up the shadows into flue dots of varying size. A sheet of zinc coated with gelatine or fish glue, and sensitised with bichromate of potash, is then placed in contact with the negative. The parts exposed to light become insoluble in hot water, as in the carbon process. The unexposed parts are washed away, leaving the zinc bare between the dots. The plate is then immersed in nitric acid, which etches it or cuts into it.

It is then mounted on a block of wood or metal to bring it level with the type.