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. "A true half tone will be best obtained by not relying entirely on the mechanical means, but assisting them with some hand work, either in the shape of re-etching or engraving, or both." The application of hand engraving to photo-mechanical work has been chiefly due to American process workers, who applied the technique of the wood engraver's art to the amplification of their half tone blocks. =Tri-chromatography.=--The "Three Colour Process" is more or less an application of half tone engraving to chromo-typography. The colours, each in their relative value, are produced by purely photo-mechanical methods--the colours of the original copy being dissected by means of specially prepared colour screens. Half tone blocks are made from each of the three negatives, and superimposed in accurate register in the subsequent printing, when, of course, the primary colours, red, blue, and yellow, are used. The process possesses brilliant and effective illustrative power, offers ample scope for the ingenuity and manipulative skill of artist, engraver, and printer, and promises well-nigh unlimited possibilities as a medium of pictorial expression. (_c_) =Photogravure.=--Photogravure may be very briefly described. It is a photo-mechanical process, in which rich, soft tones of surpassing delicacy and undeniably artistic effect are striking peculiarities. Unlike "line" and "half tone" engraving, it is an intaglio process, in which the printer as well as the etcher must possess a profound artistic perception. [Illustration: Reproduction by R. J. EVERETT & SONS' "INK-PHOTO" Process. Plate ENGRAVING FOR ILLUSTRATION WITHIN A MILE OF EDINBURGH TOWN.]