Incidentally it may be mentioned that machine etching, by which a fine spray of the etching fluid is distributed over the plate, has recently come into vogue, for it is claimed that the results print better and are in other ways an improvement upon the older method.
The plate may now pass through the hands of an engraver, who removes any blemishes, as far as is possible, improves the high lights, and so on; in fact, a skilful engraver can improve the plate considerably.
After the plate is trimmed, and the superfluous metal cut out by means of a routing machine, it is firmly tacked to a wooden mount, usually of oak, but sometimes of mahogany, especially if the plate is large. In order to obtain the best results, the printing, in a typographical machine, should be done on highly calendered paper--so-called "art" paper; in fact, this is absolutely essential if a fine screen has been used; it is only the blocks made with the coarser screens that will give fair prints on ordinary paper. For this reason reproductions made by the half-tone process are very generally treated as plates unless the glazed paper is used throughout the book. The process is used principally for the reproduction of photographs, and for pencil or wash drawings.
With regard to photographs, it has already been mentioned that authors should send the negative or two or three prints differently toned, in order that the operator can choose the one most likely to give the best result.
It is sometimes difficult in a photograph of a landscape to obtain a negative in which the particular feature it is desired to represent--_e.g._, in photographs of vegetation--stands out with the requisite contrast. This is due to the position of the sun at the time of exposure, or to the use of ordinary plates. The remedy for the first is to take the photograph when the proper light obtains; with regard to the second, the use of colour correct plates, together with a colour screen in front of the lens, will obviate the defect. Since for scientific purposes the correct interpretation of the various tones of the vegetation, for example, may not be essential, variously coloured screens may be used in order to emphasise a particular feature. For instance, it will be noticed how well the bushes in Plate 7 stand out. This effect was obtained by the use of a panchromatic plate in conjunction with a red colour screen.