and, indeed, is difficult to remove without grinding the stone down to some depth.

This process requires care in adjusting the amount of moisture to be applied to soften the gum, so that it may not be squeezed under the lines and block them up, and it has not, I believe, come into general use.

There are two disadvantages which militate against the employment of the transfer processes of photolithography for the finer and better class of maps. The first, is the difficulty of obtaining reproductions perfectly true to scale, owing to the unequal expansion of the transfer paper in the various washings and squeezings it has to undergo. Although this unequal expansion and contraction is very slight, and for most practical purposes may be disregarded, it has greatly hindered the more universal adoption of this valuable method for the reproduction of the official maps in England and foreign

countries.

Mr. Rodriguez, of Lisbon, has, however, lately introduced an improvement into the transfer process with the object of doing away with the possibility of stretching in the course of any of the operations.* Instead of using paper as the support of the coating of gelatine on which the photographic image is impressed, he uses a sheet of tinfoil about the thickness of thin paper. This is first smoothed on a very finely grained lithographic stone and then laid down quite flat on a sheet of zinc. After being cleaned with alkali and well washed, the tinfoil is brushed over with a solution of gelatine and bichromate, dried rapidly, and is then ready to be exposed under a negative in the usual manner. To ink the print, the sheet of tin is first plunged into water, and then carefully laid down wet on a lithographic stone so as to avoid folds, the gelatine side being uppermost. The film is then inked in with a roller. After the first inking in the print is left for about a couple of hours and is then inked in again and afterwards washed with a sponge and water. It may then be lifted off the stone and dried. The operations of transfer are the same as usual.

The second disadvantage of the transfer methods is the almost unavoidable spreading of the lines under the operation of transferring, which makes a photolithographed map look heavy and unsightly compared with a lithographed one. This defect may, however, be diminished very much by skilful manipulation and taking care to have as thin a coating as possible of gelatine on the paper, and to use a good hard transfer ink in small quantity. With these precautions and with a suitable original, results may be obtained from photolithographic transfers which will well compare with ordinary lithography, or even engraving, in sharpness and delicacy.

These special defects of the transfer methods may be in great part obviated by impressing the photographic image direct on the stone, as origi

**British Journal of Photography,' Vol. XXV, p. 232.

nally proposed by Poitevin, but this plan has again other disadvantages of its own which render it less suitable for map work than the transfer process. It has, however, been used extensively, and very successfully, in the production of the Belgian topographical maps on the scale of 1: 20,000.

In the process used for the Belgian maps, the stone is covered with a very thin coating of a mixture of gelatine and bichromate of potash, rapidly dried and exposed to light under a reversed negative, which is obtained by reversing the position of a dry tannin plate in the camera and allowing the light to act through the glass on the underside of the collodion film. A thin coating of printing ink is then applied all over the stone with a roller, and the surface is afterwards washed with warm water in which a little starch has been dissolved. This gradually removes all the soluble parts of the gelatine coating, leaving on the stone a clear image of the map. The stone is then covered with gum and after drying and remaining for a short time is ready for printing and capable of yielding 1500 good impressions.*

way.

For line-work zinc plates are also used and prepared in much the same

This process has undoubtedly some advantages as regards accuracy of scale, and the quickness and cheapness of the operations, but on the other hand it has disadvantages as regards the difficulty of securing perfect contact between the stone and the negatives, the necessity for a reversed negative, the prints being limited within a single negative and the inconveniences of working with heavy stones.

Besides the foregoing, many methods of photolithography have been proposed, but as for the most part they are only modifications of the processes I have described, which are all good and may be considered typical, it will be unnecessary for me to go further into details regarding them.

VI. PHOTOCOLLOTYPE.

The great defect of all the processes of photolithography described in the last section is, that they can only be applied with advantage to the reproduction of drawings or subjects in which the gradation of shade is shown by lines or dots separated by white spaces of varying sizes and at different intervals apart, as in line or stipple engravings and lithographs in line or chalk. Even such drawings to be successfully reproduced must be in a good bold open style and have all the lines or points composing them of an equal and perfect blackness. In the many attempts that have been made to reproduce photographs from nature by photolithography or photoengraving, or to copy paintings and brush-shaded drawings in which gradation of shade is continuous, success, only partial at best, has been secured by

ar

*Macs and Hannot's' Traité de Topographie, et de Reproduction des Cartes aus moyen de la Photographie'; also Hannot's 'La Photographie dans les Armées,'

breaking up and destroying the continuity of gradation. By the processes of photocollotype, so called from the printing surface being of gelatine, these defects are entirely obviated, and absolutely permanent photographic prints may be produced in the printing press equal to silver prints in perfect delineation of detail and delicate gradation of shade, but vastly superior to them in permanence and cheapness of production.

Poitevin was the first to recognise, so early as 1855, the fact that the half-tones were better preserved on stones that had been treated with a chromated colloid mixture if, after exposure to light under a negative, instead of being inked all over and then washed with water to remove the superfluous ink, they were first moistened and then inked in with a lithographic roller charged with printing ink*. He seems, however, to have always regarded the stone as the principal printing surface and treated it. by the ordinary methods of lithography. Only a few impressions could be obtained from stones thus treated.

In 1866, Messrs. Tessié du Mothay and Marechal, of Metz, discovered that the stone or metal plate hitherto used as a printing surface might be replaced by a mixture of isinglass, gelatine and gum, treated with an acid chromate, and evenly spread upon a well polished metal surface; because if, after exposure to light under a photographic negative, such a gelatinous surface were moistened, greasy ink applied upon it with a roller would adhere well to the parts of it that had been acted upon by light, and would be taken up by those parts in proportionate quantities, according to the intensity of the gradations of light and shade produced on them by the action of light, and their consequent impermeability to water. Photographic prints in fatty ink reproducing the most delicate gradations of shade without any apparent grain or break of continuity could thus be produced.†

It will be seen that this process was based on exactly the same principle as Poitevin's photolithography, but differed from it in the distinct recognition of the colloid film as the printing surface. Messrs. Tessié de Mothay and Marechal were also the first to recognise the necessity of adding a certain proportion of acid or of oxydising or reducing agents to the chromate salt used for sensitising the gelatine, with the object of rendering the colloid surface more apt to receive the greasy ink and also of hardening the film so as to enable it to withstand the wear and tear of printing. This they did by exposing the sensitive plates to a high temperature before using, but the effect was produced in great measure by the decomposition of the chromate salts by the acids or other substances added to the colloid mixture.

Messrs. Tessié du Mothay and Marechal printed off their 'phototype' plates in a lithographic press in much the same way as ordinary lithographs, * Traité de l'impression photographique sans sels d'argent,' p. 78. †' Photographic News,' Vol. XI, p. 260.

but with certain modifications due to the peculiar nature of the printing surface. The principal of these was the use of two inks, one stiff, for giving force to the shadows, the other thin, for bringing out the more delicate half tones.

The phototype' process as at first proposed laboured under the defect of not being able to yield a large number of prints from a single plate, but, in 1869, it was improved upon in this respect by Albert, of Munich, who substituted a thick glass plate for the metal plate used by Tessié du Mothay and Marechal as a support for the colloid film. His films consisted of albumen, gelatine and bichromate of potash alone, and he gave them the required solidity and adherence to the glass by first coating the plate with a sensitive colloid mixture containing a large proportion of albumen, and then giving the under side of this first coating a preliminary exposure to light through the glass. The second coating containing more gelatine was then applied, and after it had dried, and the photographic image had been impressed upon it, the plate was again exposed from the back, in order to thoroughly solidify and combine the under part of the compound film. The gelatine films so prepared were capable of yielding some hundreds, or even, it is said, thousands of perfect copies. This process. is still largely used by its inventor and is known by the name of Alberttype.*

According to some authorities, Messrs. Ohm, Grossmann and Gemoser, of Berlin, took out a patent, in 1867, for a method of photocollographic printing comprising, in addition to the use of glass as the support of the gelatine film, of the double coating of the plate and of the hardening of the film by exposure of the back surface, the introduction into the sensitive gelatine mixture of certain resinous compounds dissolved in spirit, by which the gelatine film is rendered quite insoluble and admirably adapted to form a fine printing surface. It is said on the other hand that the credit of all these improvements is due to Albert; but, in any case, it is certain that until after the publication of Albert's process early in 1869, Ohm and Grossmann's was almost unknown and had not come into general use. In October 1869, the Autotype Company in London acquired the patent, and have since worked the process with the greatest success.

About a year after the publication of Albert's method, Mr. Ernest Edwards, of London, introduced, under the name of 'Heliotype,' a very important modification of the photocollotype process.

He first waxed a glass plate and then coated it with a substantial layer of gelatine and bichromate of potash, containing a small quantity of chrome alum, with the object of hardening the gelatine and rendering it insoluble, without destroying its impermeability to water. When dry, the gelatine

* 'Photographic News, Vol. XIII, p. 121.

film was removed from the waxed glass plate, and the side which had been next to the glass was exposed under a reversed negative in the usual way, and, then, as in Albert's process, the back surface of the film was hardened by exposure to light. After this, the film was attached under water to a metal plate, preferably pewter, coated with india-rubber, and 'squeegeed' into perfect contact with it. The bichromate salt was then removed by washing and the plate was ready to be printed in an ordinary Albion printing press.

In this process the peculiarities were the use of chrome alum for hardening the gelatine; the separation of the colloid film from its original support, by which perfect contact with the negative was secured, as well as less risk of breakage of the latter; the subsequent transference of the film to a metal plate, by which the liability to breakage of glass plates in the progress of printing was obviated, and, lastly, the substitution of vertical instead of a scraping pressure in printing, by which the gelatine films were not exposed to injury by wear and scraping of the surface.

This process is still, I believe, largely practised and full details of it, with various improvements suggested by Capt Abney, R. E., will be found in the latter's excellent little work—“ Instruction in Photography.”

About the same time, Herr Obernetter, of Munich, proposed another process of the same kind offering some peculiarities, and said to produce very satisfactory results.

A sheet of glass is coated with a mixture of gelatine, albumen, sugar and bichromate of potash, dried and exposed to light under a negative. The plate is then dusted over with finely powdered zinc, which attaches itself only to the parts protected from the light and in proportion to the amount of protection they have received. The plate is then heated to about 369° F., or exposed to light till the whole surface of the film has been rendered insoluble. Before printing, the plates are treated with dilute muriatic or sulphuric acid. By this operation the parts of the gelatine film covered with zinc, are rendered, by the formation of hydrogen, susceptible of attracting water to a greater or less degree, while the other portions, upon which no zine has settled, are capable of receiving a fatty ink. The printing is then proceeded with in the usual manner.*

Since 1869, when these processes first began to come into practical use, many methods of working have been introduced, chiefly in Germany and France, but so far as known they are nearly all of them more or less modifications of one or other of the above, merely differing in the manner of preparing and hardening the gelatine film. A good deal of information on the subject will be found in Husnik's "Gesammtgebiet des Lichtdrucks,' Geymet's "Phototypie," Moock's "Traité pratique d'impressions photo* 'Photographic News,' Vol. XIII, p. 483.