The Printed Book
by Harry G. Aldis, M.A. Cambridge at the University Press 1916
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Book illustrations Part 8
The picture is thus left in relief on the face of the plate which can then be printed from in the same manner as a woodcut. It is obvious that in printing from a relief block those portions of the face of the block which come in contact with the paper will produce corresponding solid marks on the paper, whether they be lines, dots, or larger portions of the surface. Since the marks thus made must be solid black, or one tone of colour if a colored ink is used, tints and shades cannot be rendered by this method; though the effect of them is approximatly attained in wood engraving by the use of lines graduated in thickness and distance from each other so as to produce the simulation of tint.
The problem of reproducing pictures composed of tints and light-and-shade (and not of lines) was solved by the invention of the Half-tone process, sometimes called Meisenbach process, now in general use for illustrations in books and magazines. This process is similar to that of making Line etchings, but, in taking the photograph for transfer to the metal plate, a glass screen, closely ruled with fine lines at right angles to each other, is interposed between the negative and the picture. The result of this is that the image on the negative is broken up into small dots which vary in size and density according to the amount of light reflected through the ruled screen by the different parts of the picture. The face of the plate and the resultant reproduction thus consist of a mass of minute dots which are not separately visible to the eye but by their varying texture give the effect of the tones of the original. This construction may readily be seen by examining one of these illustrations with a magnifying lens. In the lighter parts of the picture it will be noticed that the dots are small and distinctly separated by the white ground between them. In the middle tones the black and white are more nearly even, while in the shadows the texture becomes white dots on a black ground. In some of the coarser work used in newspaper illustration this effect may be detected without the aid of a lens.
In Photogravure and other intaglio processes it is the design itself that is exposed to the action of acid, and so is bitten into the surface of the plate which is then printed from after the manner of an etching. Collotype and Heliotype are printed from a flat surface and are akin to the process of lithography, the prints being made from a prepared surface of gelatine instead of from a stone.
From the earliest days of book illustration the attraction of colored pictures has found its votaries. Copies of books in which the illustrations have been colored are of common occurrence in all periods, and generally this addition is contemporary work. Sometimes this was done by or for the owner, but in many cases books were issued by the publisher with the illustrations either plain or colored. Initial letters printed in colour occur as early as Fust and SchoetIer's Psalter of 1457; and in the Book of Bt Albans, printed at St Albans in 1486, the heraldic shields are printed in colours. But until the eighteenth century little attempt was made to print illustrations in colors, and most of the colouring was done by hand.