THE HERON HOUSE.
Half-tone from photogram. (Original 14 x 11 inches.)
Half-Tone Process.
As has been already said, it is of course necessary to produce from the original a printing surface of such a kind as shall take ink and print an image therefrom. Now it will of course be obvious that with ink and white paper we can only produce two things—black and white—and that therefore all the intermediate shades must be produced by a greater or less number of black dots. The process under consideration, in common with some others, is based upon the fact that gelatine or albumen sensitised with bichromate of ammonium or potassium, becomes insoluble after being acted upon by light. A solution of bitumen in benzole also forms a light sensitive coating which is frequently used in these processes. If we were to expose such a sensitive film to light under (for example) a photographic negative of a figure taken against a light background and then washed it in a suitable solvent, those parts which had been protected from light by the opaque portions of the negative, such as the background, the face, hands, and white portions of the dress, would dissolve away, leaving the insoluble or light affected portions standing.
LINE REPRODUCTION FROM PEN AND INK.
(Original 14 x 11 inches.)
If we were now to ink these portions, we could print a black image which would represent the general form of the portrait like the old-fashioned silhouettes, or shadow pictures; but the "half-tone" process provides for the breaking up of the solid black image, substituting for it black dots closer and more numerous where the darker greys are, and less so where the shading is lighter. A very considerable variation in apparent tint may thus be effected.
The actual method is briefly thus:—What is known as a "screen" is first made by taking a photographic negative of a print from a copper-plate, on which has been ruled about 120 fine parallel lines to the inch. This is done with extreme care and exactness, so that the negative gives a clear transparent copy of the ruled lines of microscopic sharpness. In many cases the ruled copy is moved round a quarter of a circle during the exposure, thus resulting in the effect of double ruling, the lines intersecting each other at right angles. Or two printings may be made from the copper-plate to produce the same result and the negative then taken. We shall thus have a negative having the appearance of a very fine lattice of transparent glass on an opaque ground.