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.

a.s.suming 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 pa.s.sed 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 sc.r.a.per.

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 pa.s.sed 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 ("s.m.u.tty") 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 coa.r.s.e 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-cla.s.s 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-cla.s.s 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, ill.u.s.trative 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_. Tubingen, 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 ill.u.s.trated 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 ill.u.s.trations, 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: _Einfuhrung 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 pa.s.sing 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.

[Ill.u.s.tration: PLATE 2.--GERANIUM COLUMBINUM.

A chromolithographic reproduction of a drawing by Miss O. Johnston]