Discoveries and Inventions of the Nineteenth Century

sheet of paper is placed over the plate, and a moderate pressure is applied, when the excess of ink is squeezed out and escapes. That which remains in the hollows of the plate, becoming set by cooling, adheres to and is removed with the paper, giving in each part a force of tint proportional to its quantity, that is, according to the depth of the hollow in the plate. The paper is laid aside to dry, and although the picture has at first a certain relief, yet the gelatine ink dries down, the picture becoming so flat that no difference of the surface is perceptible. It will be observed that this mode of printing rests upon a distinctly new principle—namely, the production of shades and gradations of tints by the varying quantity of the ink laid upon the different parts of the paper. The method is in this respect identical with that by which the water-colour painter produces his gradations; for the colour is applied in transparent layers, and the depth of the tint produced depends upon the mass of the pigment laid on, and is greater or less according as the white of the paper is more or less visible through the film of colouring matter. The gradations of tint in wood and steel engraving and in lithographs are dependent upon quite another principle—namely, the varying distribution of spots, patches, or lines in black ink of uniform intensity. The Woodbury print has all the detail and clearness of the photograph, together with a 643certain softness, produced by the transparency of the colouring matter, not found in the ordinary photographic print. The method admits of any desired tint being given to the prints, and these are perfectly unchangeable by light. Thus the result is a print which secures every good quality of a photograph without any of the unpleasant ones, such as hardness, harsh tints, opacity, fugaciousness. The prints may be taken on plates of glass, and they then form beautiful transparencies. Such prints constitute most admirable slides for the magic lantern, since the semi-transparent colouring matter, and the soft gradations, produce charming effects. Another ingenious invention of Mr. Woodbury’s provides a means of making the sunbeam engrave a mezzotint copper plate from a photograph. The action of light on bichromated gelatine is here again taken advantage of. A film is prepared similar to that used in the above-described Woodbury process proper, but the gelatine is mixed with some powdered or granular material, so that it may give rise to a granulated texture in the resulting plate. This film is treated exactly in the same way as before with regard to exposing, washing with warm water, drying, &c. The product is a very thin sheet, having a mezzotint-like surface, with more or less grain according to the action of the light. The white parts are perfectly freed from the granular matter by the solution of the gelatine, while in the darkest parts there is the greatest accumulation. The dry film in this condition is pressed into soft metal, and by a double process of electrotyping and subsequent facing with steel, a plate is obtained fit for printing at the copper-plate press. The firm of Messrs. Goupil and Co., of Paris, extensively employ this process for the preparation of the illustrations in that elegant publication, “The Portfolio.” Another method of photographic engraving lately projected by Mr. Woodbury is the following: a plate of steel is covered with a layer of gelatine, mixed with a certain proportion of gum and glucose, and dried in a dark room. This is exposed to the action of light under a transparent photograph on glass. When afterwards this gelatine layer is breathed upon, the moisture attaches itself to the portions which have not been acted on by the light, and these become more or less sticky. Sand or emery sifted to three different degrees of fineness is then sprinkled over the plate, beginning with the coarsest, which attaches itself to the most sticky parts. The less sticky parts are incapable of retaining these larger particles; while the finest sand, which is sprinkled on last, is held by parts of the plate that are even very slightly sticky; but the places where the light has been intense are dry, and none of the sand adheres. The gelatine layer is then completely dried, and the plate, being covered with another of soft metal, is placed in a press, by which a granular impression is produced on the soft metal, and this may then be copied in copper by the electrotype process. The larger particles of sand produce deeper depressions in the plate, and thus a gradation of tint is obtained. Amongst other applications of the gelatine relief devised by Mr. Woodbury is that of producing a watermark in paper. A very delicate relief is firmly attached to a plate of steel or zinc, and when paper is rolled in contact with these plates, it receives an impression of the design, all the delicate half-tints being represented in the slight opacity of the paper. Mr. Woodbury is at present engaged in perfecting a method for wedding his own process to that of chromo-lithography, by first printing the different tints on the paper, and then transferring the Woodbury prints to the top of the colours. The transparency of the gelatine and ink is such that the most brilliant effects are attainable in this way. 644Bichromated gelatine is also the agent employed in photolithography, the image of a negative photograph being thus rendered insoluble in a layer of gelatine spread on the stone, which is acted on by acids, &c., in the usual way, after the soluble portions have been removed by water. As there are also methods of using the lithographic process with plates of zinc instead of stones, so there are processes of impressing the image photographically upon the zinc. Of the general nature of the processes of zincography, photolithography, and photozincography the reader will now probably be able to form some idea, but the details need not here be described. The last two, and some other processes for printing photographic effects mechanically, all labour under the defect of imperfectly rendering the half-tints of a picture. This remark does not apply to the Woodbury process. The photo-lithographic process gives marvellous results in cases where no gradations are required. Thus a whole page of the Times newspaper may be lithographed in a space not exceeding half of this page, and although the characters may be indistinguishable to the naked eye, a lens will show them perfectly. Similarly, we may obtain within the compass of an octavo page a photo-lithograph of one of Hogarth’s large engravings, which will show every touch of the original artist’s burin. There is reason to hope that the time is not far distant when all our tedious mechanical methods of reproducing drawings by wood or steel engravings will be superseded by processes which will give us absolute facsimiles of every touch of the artist’s pencil; and when some process, giving all the delicacy and truthfulness of Mr. Woodbury’s prints, will supply us with faithful transcripts of nature for book illustration at a cost not exceeding that of the ordinary methods. So far as relates to one style of drawing, these requirements appear to be nearly realized in the process termed the graphotype, which reproduces mechanically, in the form of a metal plate with all the lines in relief, a design which the artist has etched on a flat surface. This is effected in the following manner: Chalk is powdered very finely, and sifted through wire gauze having very narrow meshes. A quantity of this is spread upon a smooth plate of metal, and subjected to an intense pressure by means of an hydraulic press. The particles of the chalk cohere into a mass, having sufficient firmness to admit of its surface being drawn upon in the same manner as a block of boxwood. The drawing is effected with an ink composed of lampblack and glue, a finely-pointed camel’s-hair brush being employed; but the shades must be produced by lines and strokes as in wood engraving. When the ink is quite dry, the surface is rubbed with a fitch brush or with velvet; and by this brushing the particles of chalk not protected by the inked strokes are loosened and carried off. In a short time the chalk between the strokes becomes quite hollowed out; and when a depth of about one-eighth of an inch has been attained, every line remains standing in relief exactly as in an engraved wood block. A strong solution of silicate of potash is then poured upon the chalk, which its chemical action converts into a kind of stone without in any way altering the forms. Although this artificial stone is quite hard, so that impressions may at once be taken from it, yet it is incapable of enduring the wear and tear of the printing-press. Accordingly a mould is taken from it, and this is made, by some of the processes of casting or electrotyping already described, to furnish a metal stereotype plate. 645 THE LINOTYPE MACHINE. Fig. 313a.—The Linotype Machine. Among recent inventions in connection with printing, the linotype machine calls for special mention. In this machine a great number of actions are combined and co-ordinated with the utmost ingenuity, but such mechanism does not lend itself to popular description, and we must confine ourselves to a statement of what it effects, recommending the reader to avail himself of some opportunity of seeing the apparatus at work. It will not then be needful to give details of every 646one of the very numerous parts, which present in the ensemble a great appearance of complication, the more so that much ingenuity has been exerted to make the machine compact, which is a practical point of great importance. The disposition of parts is not, therefore, that which is calculated to show each movement clearly to the spectator, but that by which the least space is occupied. The machine is driven by belting from a main shaft, turned by a steam-engine, gas-engine, electro-motor, or other regular source of power, and rotated at such a rate that the main pulley of the machine itself (14½ inches in diameter) shall make about 60 revolutions per minute. Fig. 313a shows the general aspect of the machine and seat for the one operator required, but as we are not undertaking a detailed and complete description of the whole mechanism, no letters of reference are given; but the reader will be able, from the following diagrams, to identify the more important parts, and form a general idea of their action and purpose. In this machine great use is made of the contrivances called cams, several of which may be observed in the sketch towards the side of the machine on the left, being fixed on and turning with its main shaft. They consist of plates, or open rims of various forms, which move levers, etc., in any required way, and at any required period of the revolution. Fig. 313b.—A Matrix. Fig. 313c.—Diagram of Movements. Fig. 313d.—A Line of Matrices. The linotype is not a type-setting and type-distributing machine, but one in which the form is stereotyped line by line; hence its name of linotype. The mould, or matrix, is made up of a number of brass matrices, each of which consists of a flat plate having on its edge a letter incised. One of these is represented on Fig. 313b wherein a is the hollow letter. At the upper end the plate is cut into a number of notches like the teeth of a saw, only that some of the teeth have their points cut off, leaving steps, as it were, with faces parallel to the longer edges of the matrix. There may be seen one of these at b, and on the opposite side of the V-shape, three may be observed. The number and arrangement of the cut-away notches is different for the