(b) Half Tone.—"Half tone" process engraving, as distinguished from the "line" process, is the reproduction of a design or copy which has in its composition gradations of tone in the form of flat tints. Wash drawings and photographs present characteristic examples of such copies.
The true relative value of these medium or half tones can only be retained in the half tone engraving by breaking up the picture into most minute sections, and thereby producing a grain or series of dots of varying size and contiguity according to the requirements of the drawing. This grain or "screen" effect is produced by the interposition of
a network of finely ruled lines in the form of a screen between the lens and the sensitive plate when photographing. The optical principle involved is beyond the sphere of this work, but the effect produced is a matter of vital importance, and requires careful consideration.
The coarser the ruling of a screen, consistent of course with the class of work for which it is required, the more vigorous and consequently more effective the reproduction will appear. The variety of tones will be greater, and the textures will appear richer. Small prints are naturally subjected to a close inspection; the screen effect, therefore, should be less obtrusive than in larger ones. It may also be useful to know that a finely ruled screen will reproduce the minute details of a copy.
Artistic Restoration.—It is somewhat doubtful if the half tone engraving, pure and simple, would ever have any real artistic value for pictorial illustration but for some method of restoring those qualities which are so considerably reduced when copying a picture through the line screen. The pure half tone consists of a grain of varying gradations over the whole design. There are, therefore, no pure whites even in the highest lights. The use of the roulette and graver for accentuating light and shade is therefore not only permissible but decidedly advantageous, for the monotony of a mechanical grain is thereby relieved, and the print produced will be an effective and accurate translation of the artistic sketch.