From an artistic point of view, photo-engraving possesses equally important features. It translates the artist's work with extraordinary facility and accuracy, retaining a satisfactory proportion of its expressive feeling, and reproducing subtleties of drawing and texture which it would be difficult, if not quite impossible, to obtain by any other process. Of the many photo-mechanical engraving processes, all of which are more or less associated with pictorial illustration, three at least merit further consideration. [Illustration: Fig. 5.--Cross Section of Cyanide Furnace. The "Line Process."] (_a_) =The "Line" Process.=--The "line" process is applicable only to the reproduction of line drawings or prints, in which the design is represented in simple black and white, with only such gradations of tone as may be suggested by lines or dots. For the reproduction of pen-and-ink drawings, it has found considerable favour with illustrators, and many even of the more conservative artists are compelled to appreciate its merits and acknowledge its value. An interesting account of the compulsory acceptance of process engraving by the famous illustrator "Du Maurier" is suggestive of at least one valuable peculiarity of this method of reproduction. Owing to failing sight, Du Maurier found it increasingly difficult to introduce into his drawings on the wood block that amount of detail which he considered necessary for the adequate expression of his ideas. Eventually he was compelled to make pen-and-ink drawings on a much larger scale than was his wont, and to have them reproduced as photo-line-blocks, the reduction being made as required. (_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. [Illustration: Fig. 6.--Process Engraving. _Block by the Arc Engraving Co. Ltd., London._] 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.