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Rh colours, which have been ground on a slab. The mixture is filtered through fine muslin. In making the tissue in large quantities the two ends of a piece of roll-paper are pasted together and the paper hung on two rollers; one of wood about 5 in. in diameter is fixed near the top of the room and the other over a trough containing the gelatin solution, the paper being brought into contact with the surface of the gelatin by being made to revolve on the rollers. The thickness of the coating is proportional to the rate at which the paper is drawn over the gelatin: the slower the movement, the thicker the coating. The paper is taken off the rollers, cut through, and hung up to dry on wooden laths. If it be required to make the tissue sensitive at once, 120 grains of potassium bichromate should be mixed with the ingredients in the above formula. The carbon-tissue when prepared should be floated on a sensitizing bath consisting of one part of potassium bichromate in 40 parts of water. This is effected by turning up about 1 in. from the end of the sheet of tissue (cut to the proper size), making a roll of it, and letting it unroll along the surface of the sensitizing solution, where it is allowed to remain till the gelatin film feels soft. It is then taken off and hung up to dry in a dark room through which a current of dry warm air is passing Tissue dried quickly, though not so sensitive, is more manageable to work than if more slowly dried. As the tissue is coloured, it is not possible to ascertain by inspection whether the printing operation is sufficiently carried out, and in order to ascertain this it is usual to place a piece of ordinary silvered paper in an actinometer, or photometer, alongside the carbon-tissue to ascertain the amount of light that has acted on it. There are several devices for ascertaining this amount, the simplest being an arrangement of a varying number of thicknesses of gold-beater’s skin. The value of 1, 2, 3, &c., thicknesses of the skin as a screen to the light is ascertained by experiment. Supposing it is judged that a sheet of tissue under some one negative ought to be exposed to light corresponding to a given number of thicknesses, chloride of silver paper is placed alongside the negative beneath the actinometer and allowed to remain there until it takes a visible tint beneath a number of thicknesses equivalent to the strength of the negative. After the tissue is remove printing-frame—supposing a double transfer is to be made—it is placed in a dish of cold water, face downwards, along with a piece of Sawyer’s flexible support. When the edges of the tissue begin to curl up, its surface and that of the flexible support are brought together and placed flat. The water is pressed out with an india-rubber squeezer or “squeegee” and the two surfaces adhere. About a couple of minutes later they are placed in warm water of about 90° to 100° F., and the paper of the tissue, loosened by the gelatin solution next it becoming soluble, can be stripped off, leaving the image (reversed as regards right and left) on the flexible support. An application of warm water removes the rest of the soluble gelatin and pigment. When dried the image is transferred to its permanent support. This usually consists of white paper coated with gelatin and made insoluble with chrome alum, though it may be mixed with barium sulphate or other similar pigments. This transfer-paper is made to receive the image by being soaked in hot water till it becomes slimy to the touch; and the surface of the damped print is brought into contact with the surface of the re transfer-paper, in the same manner as was done with the flexible support and the carbon-tissue. When dry the re transfer-paper bearing the gelatin image can be stripped off the flexible support, which may be used again as a temporary support for other pictures. If a reversed negative be used the image may be transferred at once to its final support instead of to the temporary flexible support, which is a point of practical value, since single-transfer are better than double-transfer prints.

Printing with Salts of Iron.—Sir John Herschel and Robert Hunt entered into various methods of printing with salts of iron. At the present time to or three are practised, being used in draughtsmen’s offices for copying tracings (see ).

Photo-mechanical Printing Processes.—Poitevin claimed to have discovered that a film of gelatin impregnated with potassium bichromate, after being acted upon by light and damping, would receive greasy ink on those parts which had been affected by light. But Paul Oreloth seems to have made the discovery previous to 1854, for in his patent of that year he states that his designs were inked with printing ink before being transferred to stone or zinc. C. M. Tessie de Motay (in 1865) and C. R. Marechal of Metz, however, seem to have been the first to produce half-tones from gelatin films by means of greasy ink. Their general procedure consisted in coating metallic plates with gelatin impregnated with potassium on ammonium bichromate or tri-chromate and mercuric chloride, then treating with silver oleate, exposing to light through a negative, washing, inking with a lithographic roller, and printing from the plates as for an ordinary lithograph. The half-tints by this process were very good, and illustrations executed by it are to be found in several existing works. The method of producing the plates, however, was most laborious, and it was simplified by A. Albert of Munich. He had been experimenting for many ears, endeavouring to make the gelatin Films more durable than those of Tessie de Motay. He added gum-resins, alum, tannin and other such matters, which had the property of hardening gelatin; but the difficulty of adding sufficient to the mass in its liquid state before

the whole became coagulated rendered these unmanageable. It at last occurred to him that if the hardening action of light were utilized by exposing the surface next the plate to light after or before exposing the front surface to the Film and the image, the necessary hardness might be given to the gelatin without adding any chemical hardeners to it. In Tessie de Motay’s process the hardening was almost absent, and the plates were consequently not durable. It is evident that to effect this one of two things had to be done: either the metallic plate used by Tessie de Motay must be abandoned, or else the film must be stripped off the plate and exposed in that manner. Albert adopted the transparent plate, and his success was assured, since instead of less than a hundred impressions being pulled from one plate he was able to take over a thousand. This occurred about 1867, but the formula was not published for two or three years afterwards, when it was divulged by Ohm and Grossman, one of whom had been employed by Albert of Munich, and had endeavoured to introduce a process which resembled Albert’s earlier efforts. The name of “Lichtdruck” was given about this time to these surface-printing processes, and Albert may be considered, if not the inventor, at all events the perfecter of the method Another modification of “Lichtdruck” was patented in England by Ernest Edwards under the name of “heliotype”

Woodbury Type.—This process was invented by W. Woodbury about the year 1864, though we believe that J. W. Swan had been working independently in the same direction about the same time. In October 1864 a description of the invention was given in the Photographic News. Marc Antoine A. Gaudin claimed the principle of the process, insisting that it was old, and basing his pretensions on the fact that he had printed with translucent ink from intaglio blocks engraved by hand; but at the same time he remarked that the application of the principle might lead to important results. It was just these results which Woodbury obtained, and for which he was entitled to the fullest credit. Woodbury subsequently introduced certain modifications, the outcome being what is known as the “stannotype process,” of which in 1880 he read a description before the French Photographic Society (see ).

Photo-lithography.—Reference has been made to the effect of light on gelatin impregnated with potassium bichromate, whereby the gelatin becomes insoluble, and also incapable of absorbing water where the action of the light has had full play. It is this last phenomenon which occupies such an important place in photo-lithography. In the spring of 1859 E. J. Asser of Amsterdam produced photographs on a paper basis in printer’s ink. Being anxious to produce copies of such prints mechanically, he conceived the idea of transferring the greasy ink impression to stone, and multiplying the impressions by mechanical lithography. Following very closely upon Asser, J. W. Osborne of Melbourne made a similar application; his process is described by himself in the Photographic Journal for April 1860 as follows: “A negative is produce in the usual way, bearing to the original the desired ratio A positive is printed from this negative upon a sheet of (gelatinized) paper, so prepared that the image can be transferred to stone, it having been previously covered with greasy printer’s ink. The impression is developed by washing away the soluble matter with hot water, which leaves the ink on the lines of print of the map or engraving.” The process of transferring is accomplished in the ordinary way. Early in 1860 Colonel Sir H. James, R.E., F.R.S., brought forward the Southampton method of photo-lithography, which had been carefully worked out by Captain de Courcy Scott, R.E. The “papyrotype process” was published by Abney in 1870 (see and ).

Photographs in Natural Colours.

The first notice on record of coloured light impressing its own colours on a sensitive surface is in the passage already quoted from the Farbenlehre of Goethe, where T. J. Seebeck of Jena (1810) describes the impression he obtained on paper impregnated with moist silver chloride. In 1839 Sir J. Herschel (Athenaeum, No. 621) gave a. somewhat similar description. In 1848 Edmond Becquerel succeeded in reproducing upon a daguerreotype plate not only the colours of the spectrum but also, up to a certain point, the colours of drawings and objects. His method of proceeding was to give the silver plate a thin coating of silver chloride by immersing it in ferric or cupric chlorides. It may also be immersed in chlorine water till it takes a feeble rose tint. Becquerel preferred to chlorinize the plate by immersion in a solution of hydrochloric acid in water, attaching it to the positive pole of a voltaic couple, whilst the other pole he attached to a platinum plate also immersed in the acid solution. After a minute’s subjection to the current the plate took successively a grey, a yellow, a violet and a blue tint, which order was again repeated. When the violet tint appeared for the second time the plate was withdrawn and washed and dried over a spirit-lamp. In this state it 