Part 3

The sensitising solution is made up of gelatine and bichromate of potash dissolved in water; before use it is filtered, freed from air bubbles and heated to not more than 120 deg. F. The plate is now placed on a stand, which is provided with levelling screws, in the oven, and, when the temperature has reached 120 deg. F., an amount of the bichromate gelatine solution sufficient to make a thickness of film proper for the mode of printing to be employed is poured upon the plate.

The oven is kept at a constant temperature, 120 deg. F., until the gelatine is dry, when it is allowed to cool gradually.

Whilst the gelatine is setting, precautions against vibration must be taken else the plate will be spoilt.

When dry, the collotype plate is sensitive to light and moisture; its surface shows a more or less regular series of convolutions which resemble those of the outer surface of the human brain, although, of course, very much smaller. The character of the grain is very important, for if it be too fine it will not take up a sufficiency of ink, and, on the other hand, if too coarse it will yield coarse impressions.

A reversed negative, of a quality beyond reproach, must be made of the original; if the subject is dark or has heavy shadows the negative is frequently slightly over exposed so as to soften them.

The collotype plate is then exposed under the negative and washed in cold water until the yellow bichromate no longer comes away. It is then dried.

In printing, the plate is damped and rolled up with ink as in lithographic printing; the amount of ink adhering to the film depends on the extent to which the different parts have been acted on by the light, as has already been mentioned. The moistening of the plate--mis-termed etching--is best done with dilute glycerine containing 75 per cent. of water, which when first applied should be allowed to remain on for about half-an-hour. The excess of moisture is taken up with a sponge or a ball of rag, and then the plate is inked and printed in a lithographic or a collotype press. The picture is usually masked with tin foil in order that its edges may be quite clean.

Of the faults which may occur, the following may be alluded to. A mottled appearance may obtain in the high lights; this is due to the coating of gelatine being too thick. More commonly, the reproductions may appear flat owing to the degradation of the high lights; this is a sign that the sensitive film has been acted upon by moisture during its critical existence between the drying and the washing out of the potassium bichromate, or that the temperature has been too low.

The following contain good examples of collotype.

Karsten and Schenck: _Vegetationsbilder_, Jena.

Oliver: _Notes on Trigonocarpus and Polylophospermum_. New Phytologist, Vol. 3, 1904.

Semon: _Zoologische Forschungsreisen in Australien_. Jena. 1912.

Thompson: _The Anatomy and Relationships of the Gnetales_. Annals of Botany, Vol. 26, 1912.

See also Plates 3, 4, 5 and 9 in the present work.

THE PREPARATION OF ILLUSTRATED PAGES. Of the processes dealt with, photogravure lithography and collotype are those most generally used at the present day for the printing of plates or insets. Half-tone also is employed, a process which will be considered later since it is essentially relief printing. This, therefore, is a convenient opportunity to make a few general observations on plates.

Plates should only be employed for the reproduction of subjects of such complexity that cannot be reproduced satisfactorily by figures in the text.

A plate or page made up of several illustrations should look well as a whole; in other words, it should not outrage all the canons of composition, it should have some pictorial effect. At the same time, for facility of reference, the individual figures should run in a convenient sequence. This latter point is so important that a plate composed really well is distinctly rare, for a compromise nearly always has to be made. At the same time there are, apparently, comparatively few authors who pay much attention to plate design.

Although it is not desired to write of the laws of pictorial composition, attention may be drawn to a few points which are amongst those generally neglected.

The figures should not be crowded together; a reasonable amount of margin should be left around each. They should be arranged, as far as possible, in such a way that a sense of balance is maintained. As to how this is to be accomplished will depend upon the nature of the illustrations; if they are all about equal in tone, the largest ones should preponderate towards the base of the plate, and not _vice versa_. The difference which this makes will be obvious if the two accompanying illustrations (Figs. 1 and 2, Plate 3) be compared. The first is a reduced copy of the plate as it was published: it will be noticed that it has a top-heavy appearance, which is corrected in the second figure by the simple device of turning it upside down.

If, on the other hand, the figures are some lighter and others darker, the latter should form the base, since low tones give the idea of solidity; this is so marked that in cases where the figures vary much in size and tone, the darker ones may nearly always be situated at the base or at any rate low down on the plate unless they are very much smaller than the lighter toned ones.[A]

[Footnote A: If, of course, the reader understands chiaroscuro, he will take no notice of this paragraph, but arrange his plates in accordance with his ability.]

An examination of the figures on Plates 4 and 5 will roughly illustrate these points. The upper figure of Plate 4 is well designed, and no improvement could be made, bearing in mind the compromise alluded to above. The lower figure is, however, not so good, it was obviously a difficult one to arrange; it would have been improved if Figs. 23, 26 and 28 could have been placed in the top tier, but this would have seriously disturbed the sequence. The first illustration on Plate 5 is well designed; it would, however, have been improved by interchanging 8A and 11B.

We may now pass on to the individual figures; these should shew the essential features, together with some surrounding and comparatively extraneous matter; often there is included too much of unimportance and its retention means a waste of valuable space. The first thing to do, therefore, is to trim, if needs be, the figures; their shape is more or less immaterial, provided that in cases where there are a large number of illustrations on one plate, they are not all alike. The American fashion of circular figures is particularly displeasing, at any rate to the author.

Having trimmed the figures, the next point to decide is whether any require reduction; if they do, cut out pieces of paper (referred to as patterns below) of the size which the figures will ultimately appear: on the whole, it is better to avoid reduction of the originals, for without a good deal of experience it is very difficult to judge exactly what the result will be; a good idea, however, may be gained by the use of a diminishing glass.

The size of the available surface of the plate should now be ruled on a white sheet of fairly thick cardboard, and the figures, or their patterns, arranged so as to be easy of reference, to compose as well as may be, and spaced in such a way that, in the case of a quarto plate to be folded vertically, no figure is placed so that the fold will pass through its centre. Nothing is more irritating than having an illustration spoilt in this way.

All this may be done by arranging in different ways until a satisfactory result is obtained, a process which may take an hour or two. The figures should then be pasted down, covered with several sheets of blotting paper and placed in a press. A press is seldom available; when such is the case, a number of heavy books serve equally well.

The lettering must next be attended to. The individual figures are usually designated by numbers; this is a bad method, since it involves referring to the description of the plate. The best way is to use a number, and after it to add the name of the plant or animal, and, if needs be, a description as short as may be. If the author can "print" or write reasonably well, well and good; if not, it is better to attach a slip to the plate with full directions relating to lettering, and to write in pencil on the plate the titles, etc., required in the proper places for the guidance of the craftsman. The typewriter is sometimes employed for this purpose by authors; it is purely a matter of taste, but some readers feel a slight shock when this method is resorted to.

In some cases a key to the plate printed on tough translucent tissue paper and having the necessary information, guide lines, etc., is inserted with the plate.[A]

[Footnote A: See Kerner and Oliver: _Natural History of Plants_ (First Edition) London, 1894.]

In the case of glossy chromolithographs this practice is best avoided, for the key is apt to stick to the plate if too much pressure is used when the book is bound.

With regard to the "catch letters" used to indicate different parts: these should be as obvious as possible, and the guide lines should be either in black or in white ink, according to the general tone of the illustration. These lines should be conspicuous without being heavy. Not infrequently they, together with the lettering, are printed on the plate by a second impression in red ink.

The foregoing is primarily the business of the author; with regard to editors and publishers, all plates should be mounted in a manner to facilitate reference and should be printed on suitable paper; the former is seldom or never done. All plates which must be constantly referred to in reading the text should have a selvedge as broad as the book, so that when unfolded the whole plate is visible, no matter what page is being read. This would, no doubt, prove an additional expense, but this should not militate against the suggestion here made, not by any means an innovation, for in many cases it would save the expense of mounting on guards, and, further, the additional expenditure could be saved several times over in other ways.

With regard to paper, this generally is satisfactory; unfortunately, highly glazed paper, mis-termed art paper, with an enamelled or chromo surface, and consisting chiefly of china clay and size, is generally used for printing the best half-tone reproductions. For this purpose a paper with a suitable surface, obtained by means other than those mentioned and not too costly, is highly desirable, since art paper has the reputation of being not at all permanent, owing to the deleterious action of moisture, and is somewhat brittle. When used, art paper, if folded, should have a proper paper hinge along the fold.

Half-tones are occasionally printed on a kind of vegetable parchment, a paper which should be more extensively used since it will sometimes, but not always, give as good a reproduction as art paper, and the final result is more pleasing from the artistic point of view.

RELIEF PRINTING

RELIEF PRINTING

In order that illustrations may be incorporated in the text, the blocks used must be in relief the same as the type; a mixture of intaglio and relief is impossible, for the whole surface must be level in order to be inked by the rollers, which deposit the pigment evenly, so that only one tone of colour--that of the ink--is possible.

Up to quite recent times wood cuts and engravings were the only means available for text-illustrations, so that this method may next be considered.[A]

[Footnote A: See Treviranus, C.L.: _Die Anwendung des Holtzschnitts zur bildlichen Darstellung von Pflanzen_. Leipzig, 1855.]

WOOD CUTS AND ENGRAVINGS. The invention of illustrating by means of wood blocks followed closely on the heels of the use of moveable types for printing. The Chinese were the first, as far as is known, to use these methods of printing and illustration; in the western world the first wood blocks date from the beginning of the fifteenth century.

All the earlier cuts were made, commonly on pear wood, on the longitudinal face of the wood, in technical language "on the plank," and seemingly, in many instances, were made from drawings in ink. By cutting on the plank, the craftsmen were enabled to make large blocks, but were prohibited from doing anything more than relatively simple and straightforward work. Such blocks are known as wood cuts; wood engravings were not made until the possibilities of a hard wood like box carved upon the transverse section were discovered at a much later date. This is, strictly speaking, wood engraving, an art which almost entirely, if not quite, superseded the older craft, on account of its great possibilities; indeed, wood engravers imitated metal engraving so closely as to deceive many. But such work was enormously laborious; for instance, in the case of a fishing net, if the string were to be printed black, the engraver would have to cut out hundreds of small diamond-shaped pieces of wood in order that the string of the net should be in relief. But few artists would do this of their own free will, and generally such laborious work will only be found in wood-engravings which were intended for the reproduction of ink drawings or other kinds of pictures where the lines, shading, etc., had to be faithfully copied. This point may be illustrated by the accompanying cut (Fig. 2), which was made by my friend Mr. Geoffrey Oliver, who at the time was totally uninstructed in the art and knew nothing of its literature. It will be seen that he, quite unconsciously, treated his wood in the same way as an engraver would his metal; the result, of course, is just the opposite to metal engraving since the printing of the wood block is the reverse to intaglio.

In fact, the cut illustrates the three fundamentals of wood engraving; the white line made by cutting out the wood so that no impression will be obtained when printed; the white space which is similarly obtained; and the black space, which is made by leaving the wood untouched. It was, however, necessary to employ the black line, otherwise the tape with which the two men--the artist and his father--are measuring the trunk of the tree would be invisible where it crosses the sky. In a word, the little picture illustrates very nicely the legitimate use of wood in the graphic arts.

As already remarked, the majority of the earlier wood cuts and engravings are reproductions of line drawings, so that although we may admire and often marvel at the technical ability of the engraver, the credit for what artistic merit such illustrations may possess must, in the majority of cases, go to the draughtsman.

The work of the earlier wood engravers may be conveniently studied in _A Lyttel Booke of Nonsense_, by R. D., London, 1912. (See also the relevant works cited under Literature, p. 94).

Bewick, of course, is an outstanding example of an artist who used wood engraving for illustrating natural history; the methods he pursued may be studied in the tailpiece on p. 11, which was printed from an electrotype of the original block.

Wood engraving, up to quite recent times, was the method of reproducing text figures; not only for scientific books and periodicals, but also for general literature and journals.

Much of this work is of outstanding excellence; for scientific work the following may be studied:

Duchartre: _Elements de Botanique_. Paris, 1867. The drawings were made by Riocreux and engraved by Leblanc.

Baillon: _Histoire des Plantes_, Paris, 1887. This work contains some beautiful wood engravings, reproductions of drawings by Faguet.

Bentham: _Handbook of the British Flora_, London, 1865. The engravings are from drawings by W. H. Fitch.

Deschanel: _Natural Philosophy_, London, 1890. The engravings, many of which are of excellent quality, are by Laplante, Rapine and others. In many cases, notably in the representation of the rays of light passing through lenses and also in the illustrations of snow crystals, the use of the white line is admirably demonstrated.

Kerner: _Pflanzenleben_, Leipzig, 1888. This contains some excellent engravings by Winkler and others.

Le Maout et Decaisne; _Traite general de Botanique_, Paris, 1876. This work contains splendid examples by Riocreux and Steinheil (see Fig. 8).

Oliver: _First Book of Indian Botany_, London, 1869. This contains some characteristic work of W. H. Fitch.

It does not appear to be generally known that excellent reproductions in colour may be obtained from wood blocks by superposed printing in a manner comparable to that followed in chromolithography although, of course, in the present instance, the blocks are in relief (Fig. 1).

From the foregoing account it is obvious that the engraving even of a small illustration, except it be in mere outline, involves a considerable amount of labour; in fact, if the subject were large it was usual to cut it up into areas and distribute between several engravers, the finished blocks finally being joined together to make the block of the whole picture. Hence it is not surprising to find that when the photo-mechanical processes were perfected, the older methods of reproduction were ousted by the newer, more especially since they are much less expensive; these, therefore, may next be considered.

THE HALF TONE PROCESS.--For the making of a relief block by photo-mechanical means, the main difficulty is the proper rendition of the tones intermediate between black and white; this has been solved, at any rate in part, by the discovery of the half-tone process.

If an ordinary photographic negative be highly magnified, it will be seen that the high lights, the low lights, and the intermediate tones are made by the varying density of the reduced silver. In the lighter parts the small black particles are surrounded by colourless areas, whilst in the dark regions small colourless patches are surrounded by black areas owing to the closeness of the particles of silver (Plate 5, Fig. 2).

What is required, therefore, is a relief block which will print a number of dots of equal density but of unequal size. Vervasser illustrates the point in an ingenious way: a plate, covered with a number of cones, is supposed to be acted upon by light in such a way that the cones are truncated in varying degrees according to intensity of the light falling upon them. The section of such a plate would therefore shew a curve (Fig. 3); now if the truncated cones be brought down to one level and a print taken from them, the high lights would be represented by black dots surrounded by white areas and so on.

This illustrates the principle which obtains in the making of half-tones in which the image is made up of a large number of dots varying in size but all equally dense, so that when viewed from a suitable distance the dots are individually invisible but compose to give gradations of light and shade. In other words, the structure obtaining in a photographic negative is, in a sense, realised by optical chemical means, although the dots in a half-tone block are much coarser than those in a negative (Plate 5, Fig. 3).

This result is obtained by interposing between the diaphragm of the camera and the negative--for the half-tone process is a photo-mechanical one--a glass screen covered with intersecting engraved lines (Fig. 4). As a matter of fact, each screen consists of two plates of glass similarly ruled and cemented face to face so that the lines intersect.

It may at first be thought that the effect of such a screen placed in front of the negative would be to produce merely a cross hatching on the reproduction; this, however, is not the case; if the screen be placed in a proper position relative to the negative and the size of the diaphragm of the camera, the picture will be reproduced in a series of dots of varying size.

The optical and other reasons for this phenomenon must be sought elsewhere,[A] but the following brief consideration will serve to illustrate what happens. The rays of light which ultimately reach the sensitive plate are acted upon by two lenses, that of the camera and the meshes of the screen, each one of which acts as a lens on the principle of the pin-hole camera. Each mesh, therefore, brings the image of the diaphragm to a focus on the negative, but the lens of the camera focusses the picture as a whole, thus the amount of light falling on the different pin-holes will vary in intensity, and hence the dots produced will vary in size, for it is assumed, with good reason, that each dot is built up from its centre and radially expands according to the amount of light acting upon it.

[Footnote A: See Verfasser, _loc. cit._, p. 94.]

It is obvious that the quality of the resulting picture will depend, other things being equal, upon the coarseness of the screen employed. Screens are ruled with lines varying from 50 to 400 to the inch: the lower rulings give very coarse reproductions, and are only used for posters, whilst the higher rulings yield very fine impressions and are employed only for the best work. It is hardly necessary to remark that the finer the screen the better must be the skill of the printer. To illustrate the difference in the results obtained by the use of different screens, the two figures on Plate 6 have been prepared; both were made from the same negative, but for the upper figure a 100-line screen was used, and for the lower a 200-line screen. It will be observed that there is more contrast in the former, and more detail in the latter. Authors should therefore mention when sending in their original pictures the qualities they require in the reproduction; it must, however, be remembered that the blocks made from the finer ruled screens will not print satisfactorily except on more or less highly glazed paper, to the use of the "art" varieties of which there are objections on aesthetic and other grounds.

Before passing on it may be mentioned that screens with patterns other than that represented in Fig. 4 are sometimes employed; for instance, the wavy-line screen gives the impression of coarse collotype.

The preparation of the blocks may now be briefly dealt with.

A negative of the picture, using a screen suitable for the purpose, is taken on a special dry gelatine plate ("process" plates) or on some other form of negative, _e.g._, wet collodion which is most commonly employed. This negative requires very careful development in order to get the dots right.

From the negative a positive is made upon a copper or zinc plate, suitably coated with a sensitive film. The usual practice is to coat the polished metal plate with a mixture of water, albumen, fish glue, ammonium bichromate, chromic acid and ammonia; the plate is then dried and, when cooled, exposed under the negative. The action of the light on such a film, the essentials of which are the albumen, the glue or gelatine and a chromate, has already been described. The mixture becomes more or less insoluble in water, according to the intensity of the light falling upon it.