If the plate had not been laid with resin, the surface after etching would show more or less extensive depressions and elevations; but the grains of resin protect the copper immediately beneath them from the action of the acid, which consequently can only dissolve the exposed parts of the metal between the resinous particles. The result is, therefore, that the plate is covered over with numberless fine pits of varying depths. The deepest ones will, on printing, give the darkest tones, since they will hold more ink, the shallower ones will give the lighter tones, whilst the shallowest and those parts unetched will give the high lights. The plate is usually etched three or four times successively in varying strengths of fluid, after which the etching ground and gelatine is cleaned off. A strong copper-plate ink is then well rubbed in by means of a dabber, after which the ink from the surface is removed, first with a coarse piece of muslin and finally, with fine muslin. The ink must not be removed from the pits. The first pull is then taken as in a line engraving with a copper-plate press, and its appearance shows what corrections are necessary. The plate nearly always requires a certain amount of engraving; the high lights may be improved by means of a burnisher, the shadows by means of a rocker or a roulette--a small steel wheel the rim of which is beset with fine teeth--and so on. Finally, if a large edition is required, the plate is steel faced. Although much used for the reproduction of pictures, photogravure is too rarely employed for scientific purposes; this is to be regretted, for the process is admirably suited to the reproduction of photographs and drawings with delicate tones. As compared with the usual half-tone, the cost is high, and this no doubt militates against its use. Examples of outstanding excellence will be found in the _New Phytologist_, Vol. xi, 1912, plates 5 and 6. These are absolute facsimiles of the original drawings by Mr. McLean, both as regards colour and reproduction of tones. Plate 8 may also be examined and compared with plates 9 and 10 which are reproductions of the same subject in collotype and half-tone respectively. PLANE SURFACE PRINTING [Illustration: PLATE 1.--An original lithograph by Mr. Harry Becker.] PLANE SURFACE PRINTING LITHOGRAPHY. Of these methods of printing, lithography is the outstanding example: it is a method of reproduction possessed of great possibilities, for by its employment a facsimile of any drawing can be obtained. As a means of artistic expression it ranks high amongst the graphic arts, and, for the reproduction of drawings of a scientific nature, it is very popular, since it meets most requirements and is comparatively inexpensive.