Part 2

Mezzotint apparently has never been used for the reproduction of scientific subjects. Indeed, in a sense, this process is much too artistic for the purpose. At their best, illustrations reproduced by this method have mystery and depth and give the imagination much employ; in a word, they are subjective rather than objective, qualities unsuited for our purpose.

PHOTOGRAVURE. Photogravure may next be considered, for although it is a photo-mechanical process, it corresponds to mezzotint. Excellent results may be obtained by its use provided the drawings--usually executed in monochrome such as sepia--be really good, otherwise they are hardly worth reproducing by this relatively expensive method.[A]

[Footnote A: This account refers only to plates made and printed entirely by hand, not to photogravure for printing on a rotary machine.]

Photogravure is particularly suitable for the reproduction of drawings showing a large amount of detail and made up of a variety of tones rather than lines or stipple.

The photographic part of the process is essentially the same as making a carbon print from a photographic negative. This consists in exposing under the negative the carbon tissue, which is a mixture of gelatine, in which is dissolved bichromate of potash, and a suitable pigment. Such a film of bichromate gelatine is, when dry, sensitive to light. If no light gains access to it, the gelatine is readily soluble in warm water; if light acts upon it, the gelatine becomes insoluble in proportion to the degree of its exposure. Obviously, the pigment will be retained in varying degrees according to the relative insolubility of the different parts of the gelatine.

The carbon tissue having been exposed, is rolled down on a wet sheet of paper covered with some adhesive and is dried under pressure. The paper is then soaked in warm water when the basis of the carbon tissue easily peels off; the picture is developed by laving in warm water, which will dissolve the gelatine in proportion to its exposure to the light. The print when dry is remarkably permanent and, from the picturesque point of view, is infinitely superior to the ordinary silver print.

The method of making the photogravure plate is, in outline, as follows:

The original drawing is photographed, and it is very important that the negative should be as perfect as possible.

From the negative, a positive is made upon transparency carbon tissue which is mounted upon a sheet of plate glass. The procedure is, in essentials, exactly the same as described above for the making of a carbon print. This positive when dry may be touched up; after which a negative, which also may be touched up, is made from it upon an ordinary sheet of carbon tissue.

The negative so obtained is transferred to a prepared plate of copper, developed with warm water and dried.

The copper plate is prepared as follows: after being well polished until quite free from all scratches, the surface is dusted over with finely powdered resin or, more usually, bitumen. The plate is then heated until the dust adheres.

After the carbon negative has been stuck on to the plate, developed and dried, the margins and back of the copper are protected with an acid-resisting varnish. When dry, the plate is placed in the etching bath of nitric acid or, more generally, of ferric chloride. The etching fluid will pass through the thinnest parts of the negative first, so that the surface of the copper will be etched to a degree corresponding to the thickness of the gelatine. The high lights on the negative obviously will be represented by thick coatings of gelatine, consequently such parts will be but slightly etched and vice versa.

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

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.

The art, which was discovered by Senefelder towards the end of the eighteenth century, depends on the fact that grease and water are immiscible: a drawing made with a greasy pigment upon a suitable surface adheres very strongly, whilst those parts free from it retain water, so that when damped and rolled up (_i.e._, inked), the ink used will stick only to the lines, etc., of the drawing, but not to the other parts.

Clearly the surface is all important, and this is provided by lithographic stone, a limestone occurring in Germany, France, England and Canada. The best stones occur at Solenhofen near Munich, those from other localities being inferior in quality. Incidentally it may be mentioned that zinc and aluminium plates are not infrequently used in place of stone.

Lithographic stones vary in hardness, colour and grain. For the best work the stone should be homogeneous and of a hardness suitable for the subject; the colour affords an indication of the hardness, the lighter-coloured stones, which are much the commoner, being softer than the darker.

There are two modes of procedure; the drawing may be made direct on the stone with lithographic ink or crayon--both being mixtures of tallow, wax, soap and shellac, with a sufficiency of pigment to render the drawing visible to the artist--or else the drawing may be made upon transfer paper.

The former method, although the more satisfactory and often used by artists, is seldom pursued in scientific drawings except when professional draughtsmen are employed. In such cases it may be necessary to reverse the drawing, which is conveniently done by viewing it in a mirror, and, of course, all lettering must be reversed.

The majority of amateur draughtsmen make their drawings in pencil or ink and these the lithographer traces upon lithographic transfer paper and transfers them to the stone; he, the lithographer, may merely trace the salient features and work the drawing up on the stone. The transfer papers are coated with gelatine, starch or gum, or mixtures of these substances, the idea being to interpose between the real paper and the pigment--in the form of lithographic crayon or ink--some substance soluble in water which will hold the pigment and prevent it soaking into the paper, so that a transfer has only to be damped through the back, pressed on to the stone and peeled off. The work, together with more or less of the film, will thus be transferred on to the stone and, of course, will be reversed, since the part uppermost on the stone will be the back of the original drawing.

The original drawings may be made upon the transfer paper direct, and in so doing mistakes in tracing will be obviated. Suitable papers are made for various purposes, e.g., smooth for ink work and variously granulated for crayon (see Plate 1, which was drawn by Mr. Harry Becker on transfer paper).

Another advantage in drawing directly upon the transfer paper is that the draughtsman can make corrections pretty easily for, if needs be, a bad piece of work can be entirely cut out and a fresh piece of paper inserted.

Assuming that the transfer method has been employed, the stone must be prepared according as the drawing is made with ink or with crayon.

The stone is first thoroughly ground, in order to rid it of all traces of previous work, and then polished for ink work or grained--_i.e._ roughened--for crayon work, the small points produced taking up the crayon in proportion to the amount present on the transfer and the pressure used.

The transfer is then damped with water, sometimes with a dilute solution of nitric acid, and placed in position on the stone, which is then passed two or three times through the lithographic press until dry. Then the back of the paper is damped and the sheet peeled off.

The stone is next proved, _i.e._, prepared for printing.

It is first carefully examined for broken lines and other blemishes, which are touched up with ink or crayon. The stone is then painted over with a solution of gum in water which is allowed to dry, it is then washed in water and rolled up with ink. The drawing will now be clearly visible, for if properly inked the clear parts of the stone will not take the pigment, so that any parts which require cleaning up may be deleted. This is accomplished by means of a pencil of snake stone, a piece of pumice stone, an acid stump--a rod of hard wood, the sharpened end of which is dipped into nitric acid--or with a scraper. The stone is again washed and rolled up strongly with ink and etched with a dilute solution of nitric acid which is applied with a sponge; then the surface is again gummed and the stone allowed to dry. It is sometimes necessary to re-etch the stone; if so, the damp stone is rolled up with thin ink and allowed to dry, it is then dusted over with finely powdered resin, the superfluous resin is removed by means of a wet sponge, and the surface is painted over with a solution of gum arabic mixed with dilute nitric acid. If the resin is well incorporated with the ink, the work will suffer no damage in the process. The acid gum is then dabbed off with a rag, the stone is cleaned up with turpentine, rolled up once more, gummed and finally set aside to dry.

All this appears complicated, but it is very necessary to get a good surface for printing. The action of the gum does not appear to be clearly understood, the nitric acid obviously will etch the stone, so that the gum will easily penetrate. It is sometimes supposed that the arabic acid of the gum enters into a chemical composition with the calcium carbonate, making a film which is the real ink-resisting surface. This film has not a long life, so that in printing it is necessary to renew it periodically by the application of gum solution.

If possible, the stone should be allowed to rest for a day or two after proving, in order that the ink may sink well in.

Before printing, the gum is washed off and the stone allowed to remain in the press-room until its temperature is the same as its surroundings. The stone is then thoroughly and evenly damped all over, placed in the press, and rolled up with lithographic ink; the paper is then laid on, and the whole passed through the lithographic press. After the first few pulls it will be seen whether all is well. The essentials of a good impression are these: the lines must be black and not grey, provided black ink is used; the lines must not be wider or blacker ("smutty") than those on the stone, nor must they be ragged or broken ("rotten").

In printing, the stone must be damped and inked before each impression is taken, and occasionally re-gumming is required. Good printing requires a considerable amount of ability, especially in the case of crayon drawings.

The paper used is a very important matter, the selection of which can be safely left to the lithographer, provided he be a good one, unless the author possesses the necessary technical knowledge. If a smooth paper is required, and the paper is not to be damped before printing, India paper is best and plate paper next best. All coarse or grained papers must be damped before printing.

As has already been remarked, lithography is a good process for scientific work; but, unfortunately, considering the number of lithographic plates published, really first-class examples are rare. This is largely due to the original draughtsman; it is unreasonable to expect a lithographer, in all probability ignorant of the subject of the plate, to turn out first-class reproductions of drawings which are obviously bad. On the other hand, lithographers vary greatly in their capabilities, and indifferent plates may be entirely due to their ability not being first rate.

As drawings have to be traced, mistakes are apt to occur; the proofs should, therefore, be carefully examined, for a certain amount of correction can be made on the stone.

The following works contain excellent lithographs, which should be studied by those interested in the subject.

Bornet et Thuret: _Notes Algologiques_. Paris, 1876-1880. This contains some of the best work, illustrative of science, known to the present author. The original drawings mostly were made by Bornet, and the lithography was carried out by Riocreux--one of the best if not the greatest of botanical artists--Arnoul, Picart and Pierre.

Davis and Thurnam: _Crania Britannica_. London, 1865.

Mirbel: _Sur le Cambium_, Paris, 1842. The plates provide excellent examples of ink lithography by Laplante.

Von Mohl: _Schriften botanischen Inhalts_. Tuebingen, 1845. Good examples by Federer.

The first volumes of the _Annales des Sciences Naturelles_ (Paris) may be referred to for lithographic work earlier than the above (1820).

For more modern examples the following may be consulted:

Blackman and Welsford: _Fertilisation in Lilium_, Annals of Botany, Vol. 27, 1913.

Gravis: _Recherches anatomiques sur les organes vegetatifs de l'Urtica Dioica_, Bruxelles, 1885. This memoir contains both good and indifferent plates.

Keibel: _Normentafeln zu Entwicklungsgeschichte der Wirbeltiere_, Jena, 1904.

Reed: _A Study of the Enzyme-secreting Cells in the Seedlings of Zea Mais and Ph[oe]nix dactylifera_. Annals of Botany, Vol. 18, 1904.

Semon: _Zoologische Forschungsreisen in Australien_, Jena, 1904.

Vaizey: _On the Morphology of the Sporophyte of Splachnum luteum_, Annals of Botany, Vol. 5, 1890.

Woodburn: _Spermatogenesis in Blasia pusilla_, Annals of Botany, Vol. 27, 1913.

Several memoirs in the _Fauna und Flora des Golfes von Neapel_ (Berlin) are illustrated by excellent lithographic plates. Many good examples of chromolithography also will be found there.

CHROMOLITHOGRAPHY. Lithography is much used for the reproduction of coloured pictures and illustrations, the process being termed chromolithography. The principles involved are the same as for ordinary work, but it is necessary to print from several stones, one for each colour. It is obvious that much skill is required, for the employment of different colours will give a large number of secondary and tertiary tints when printed one above the other in various combinations. Thus, by printing part of a design in yellow and the other part in blue, the finished product would show three colours--yellow, green and blue, and by the use of three primary colours a large number of different tints may be obtained.

As already mentioned, each colour is printed by a separate stone, there is thus no limit--excepting that of expense--to the number of different colours which can be obtained.

In practice it is usual to make an outline of the essential parts of the composition on a stone, known as the keystone, which is not necessarily used in printing the picture. An impression of this outline is taken upon a sheet of paper, which is used to transfer the design on to the stones, on each of which the artist will draw only those parts which he desires to be printed in one particular pigment.

Although the sequence of colours is generally blue, red and yellow, it is obvious that various changes in this order must be made according to the colours used and the exact tint required. For instance, a body colour such as cadmium yellow would precede a glaze such as madder-lake; again, two distinct tints may be obtained from red and blue, for example, according to the order of printing--red upon blue will give a mauve, whilst blue upon red will give a purple.

A knowledge of pigments is thus all important, and in printing, the superposition must be perfect.

Plate 2 is an example of a chromolithograph. Miss O. Johnston first drew the outline of the plant, which was phototransferred on to the stone. An impression was then pulled and tinted by the artist, and from this tinted impression the colour stones were made by the lithographer. It may be added that only three colours were used in printing the plate.

Examples:

Baur: _Einfuehrung in die experimentelle Vererbungslehre_ (Plate 1). Berlin, 1911.

Bruce and others: _A Note on the Occurrence of a Trypanosome in the African Elephant_. Proceedings of the Royal Society of London, B. Vol. 81, 1909.

Cropper: _The Development of a Parasite of Earthworms_. _Id._ Vol. 85, 1912.

Oliver: _On Sarcodes sanguinea_. Annals of Botany, Vol. 4, 1889-1891.

Rubbel: _UEber Perlen_ ... Zoologische Jahrbuecher, Vol. 32, 1911-12.

Biometrika, 1906-7, Vol. 5, Plate 23.

Mention has been made of the value of a knowledge of colours. The subject is much too extensive to be considered adequately on the present occasion even if it were desirable; its importance, however, warrants a few passing remarks.[A]

[Footnote A: See Ridgway: _Color Standards and Color Nomenclature_.]

No two people will describe in the same way the colour of, say, a rose petal; both will have a different conception of the colour "crimson." The majority have but a limited sense of colour, and even when this faculty is possessed, the personal equation looms large; further, the ordinary names of colours are quite inadequate for descriptive purposes. For these reasons the importance of a scientific system of colour nomenclature and colour standards is all important. By the use of such a scheme, the exact colour of an object can be found by comparison with an adequate chart, and the name there given will convey to others exactly what colour is described or desired. The plumage of a bird or the colour of a flower can thus be described correctly, and an author can indicate exactly the colour desired in certain parts of a chromolithograph or other reproduction in colour.

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 pass, 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 glass, 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 albumin, and placed in a warm oven to dry. If metal plates are used, such as zinc or copper, this preliminary coating is unnecessary; glass 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.