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247 Negatives likely to be much used—as, for instance, Read berore the Photographic Society ot London, May 10th, 1809. graphy, which are only capable of rendering line or stipple ; while the original process of Poitevin enables us to obtain considerable half-tone, thus resembling the modern processes of Tessie du Motay and Albert. A fact of greater importance to our present purpose is his application of the insolubility of albumen under the action of light and the bichromate, to fix or imprison a pig ment mixed with it, and thus to obtain a coloured image. Hence, all our subsequent efforts at pigment printing, for all start from this point. To effect this, he simply mixed a finely ground pigment with the sensitive agents, albumen and bichromate of potash, and spread the mixture upon the surface of paper in an extremely thin layer. When this was dry, it was exposed under a negative, and, after exposure, was well washed and sponged in water, when the parts unacted upon by light washed away, and those acted upon remained attached to the paper. If the subject be in line or stipple, considerable success may be achieved, but little or none if the subject be in graduated tint, that is, possess half-tone. talking here of portrait negatives) requiring to be retouched. This very finely-divided powder is now rubbed over the surface—as, for instance, the head of the portrait—with the finger until a sufficiently matt surface has been produced, and upon this the pencil may be worked very effectually in the usual manner. When held against an intense light and viewed as a transparency, fine hair-lines are visible upon this part of the negative, but these are not seen in the least degree during printing. For some considerable time I have employed this mode of proceeding, and can, therefore, safely recommend it, more especially as the shadows in the face are by its means rendered more delicate, a very welcome and important consideration in the manipulation of lady portraits. The practice of treating the varnish with oil of turpentine immediately before retouching, in order to facilitate the employment of the pencil, is a very good mode of proceed ing, but possesses the disadvantage that the application of indian ink to which gum has been added, in order to give greater opacity and body, is rendered impossible. should be preserved, as much as possible, from all external risk of injury. To effect this, it is well to coat the negative in the first place with thin amber varnish, as Bedford recom mended some time since, taking 1 part of ordinary amber varnish (the liner qualities of amber varnish dry too slowly, ou account of the large quantity of oil they contain) and 8 to 10 parts of oil of turpentine, and filtering the mixture carefully before use. Let the negative treated with this varnish stand till the next day in a locality sheltered from dust, and then manipulate very carefully with an exceed ingly soft pencil. Alter drying thoroughly, the surface will receive water-colours with the same readiness as the black- lead. When the retouching upon the amber varnish has been completed, the negative is varnished in the ordinary man ner, the plate being slightly warmed previously to the operation ; further retouching can afterwards be effected if Poitevin, to be successful, must have a minutely thin layer, which will be penetrated by the light transmitted through the finest line of the negative: while Swan lays on so thick a coaling that it is practically impenetrable by light passing through the barest portion of the negative. Pouncy, whose first patent (never completed) follows that I of Poitevin, succeeded more perfectly than his predecessor towards the production of half-tone. He operated precisely like Poitevin, but used a porous paper, and brushed the sensitive coating into its pores. By the action of light the mixture becomes insoluble under the transparent parts of the negative, and even those least acted upon were more or less retained, being in contact with the paper instead of having a coating of soluble gelatine interposed between them. Burnett and Blair are still more worthy of note, having introduced a new feature into their operations—that of print ing from behind the paper, so that the parts rendered in soluble may be in contact with the support, and the soluble gelatine unacted upon by light on the surface may be easily removed by water without undermining the insoluble por tions forming the picture. By rendering the paper support ing the sensitive coating translucent by means of oil or var nish, Hr. Blair obtained a considerable amount of success; but there were certain objections which induced him to abandon it for other and more complicated methods. We would, however, here again point out the wide difference between the prepared paper of Mr. Blair and the paper tissue of Mr. Swan, which is the basis of the process I am about to describe. In its perfected state Mr. Blair's paper is trans parent and impervious to water, a quality which would entirely unfit it for use in Swan’s and analogous processes. The next important step was made by Faigier, who coated a glass-plate or other plane surface with the sensitive compound, and exposed it under a negative. When exposed, he coated it with a layer of collodion, and then plunged it into warm water. The fluid penetrated the collodion, dissolved the gelatine unacted upon by light, when the pellicle of collodion floated, bearing with it the insoluble-pigment picture. Most delicate results were thus obtained ; but the manipulations were found tedious and uncertain. We now come to the most important step in pigment printing which had yet been made—that of Mr. Swan. Most unworthy attempts have lately been made by interested persons and their agents to decry the labours of this gentle man ; but the fact remains undisputed, that by his modifi cations alone of the processes I have alluded to, carbon printing has been made a practical and commercial process, instead of one merely experimental and tentative. It has been worked on a larger scale than any other process of printing by Mr. Braun, of Dornach ; and the.two. folio albums on the table, part of a series of thirty-five similar necessary. The method of coating unvarnished plates with a solution I of gum or gelatine, and then retouching them, 1 haveprac- 1 tised for a considerable period, but have found that nega tives of this description, when subjected to extremes of tem perature or to the influence of damp, do not exhibit those permanent qualities which a valuable negative should possess; amber varnish used in the manner described forms a much more permanent protection. ON A GREATLY SIMPLIFIED PROCESS OF PRINTING IN CARBON OR OTHER PERMANENI PIGMENT. BY J. R. JOHNSON* I. — PaELIINARY Observations. The process about to be described, and all those alluded to in the course of these observations, are based upon the fact discovered by Mungo Ponton, viz., that a solution of a salt of chromic acid, and particularly the bichromate of potash, becomes sensitive to light when spread Upon paper. Becquerel showed that the action was much more ener getic upon the size of the paper than upon the paper itself. Roitevin showed us that albumen and other analogous sub stances have their properties entirely changed by the joint action of light and the bichromates; thus, instead of mixing with water in all proportions, they become insoluble in that menstruum, and acquire an affinity for fatty bodies. He thus laid the basis of all subsequent processes of photo litho graphy ; indeed his process, which has ever since been successfully worked by MM. Lemercier and Co., of Paris, is superior to most of the modern processes of photo-litho- nnogauvus aney • mo muuuu ubuu—ao, upuauce. If any one repeat this mode of operation with Swan's stereoscopic views, portraits, &c.—is very essential, and I gelatine tissue, he will find that neither line nor stipple is obtainable; in fact, the conditions are totally dissimilar.