PHOTOLITHOGRAPHIC PROCESSES.--Of these methods of reproduction there are several, their value lying in the fact that the originals can be reduced or enlarged with the greatest of ease. The general principles are as follows.

A photographic negative is taken of the original drawing and a positive made on a film of bichromate gelatine. Wherever light reaches the film, the gelatine is rendered more or less insoluble according to the intensity of the light acting upon it; through the dark parts of the negative but little light will pa.s.s, so that the gelatine will remain soluble.

The exposure of the positive having been made, the film, which may be mounted on paper, is inked with lithographic ink in the dark room and then washed. The pigment will adhere to those parts acted on by light, but will wash away from those regions unacted upon; obviously the half-tones will retain ink in direct proportion to their density.

The developed positive is then transferred to a stone or zinc plate and impressions taken as in pure lithography for the dark parts are resistant to water and will take the ink, whilst the high lights will retain water and so will not be inked. The intermediate tones will take the pigment according to their density.

In distinction to the previous methods, corrections cannot be made except in so far as the negative can be touched up.

COLLOTYPE.--Of the various photolithographic methods which have from time to time been employed, collotype is the one in most general use at the present time, especially for the reproduction of photographs.

Collotype is a simple process which does not require so extensive a technical knowledge and ability as some of those previously described.

But notwithstanding this, the results are sometimes unsatisfactory and unequal; faults due to indifferent originals and to unsatisfactory conditions obtaining in the work rooms. The great drawbacks to good collotype are cold and dampness, and it is for these reasons that continental firms, blessed with a more stable climate, often produce much the best work. Provided the workshops are properly heated, the collotypers of this country ought to be able to turn out good work at all times of the year; indeed, the best firms do.

For this and for other processes in which photographs form the originals to be reproduced, authors should send the negative to the collotyper; if this be impossible, positives of the best possible quality, printed on ordinary P.O.P. paper, toned to various shades of purple, and also on smooth bromide paper, in ordinary black tones, should be provided in order that the collotyper can choose the print he most prefers to work with. Also, it is usual to glaze the prints.

The method is as follows. A piece of british plate gla.s.s, about half-an-inch in thickness, is ground on one side with fine emery powder, and then thoroughly washed and dried. The plate is covered with a filtered mixture of the colloids sodium silicate and dextrine or alb.u.min, and placed in a warm oven to dry. If metal plates are used, such as zinc or copper, this preliminary coating is unnecessary; gla.s.s plates, however, must have the substratum in order that the sensitised gelatine--which is next put on--may stick.

When the plate is dry, it is thoroughly washed with water in order to remove any free silicate; it is then dried and put away until required for use.

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 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 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 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 coa.r.s.e it will yield coa.r.s.e 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 pota.s.sium 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 ILl.u.s.tRATED 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 ill.u.s.trations 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 ill.u.s.trations; 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 ill.u.s.trations (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.

[Ill.u.s.tration: PLATE 3.]

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 ill.u.s.trate 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 ill.u.s.tration on Plate 5 is well designed; it would, however, have been improved by interchanging 8A and 11B.

[Ill.u.s.tration: PLATE 4.]

[Ill.u.s.tration: PLATE 5.]

We may now pa.s.s 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 s.p.a.ce. 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 ill.u.s.trations 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 gla.s.s.

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 s.p.a.ced 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 pa.s.s through its centre. Nothing is more irritating than having an ill.u.s.tration 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 t.i.tles, 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 ill.u.s.tration. 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.