origin, progress, and principles of chemical printing or lithography, illustrating his remarks by a series of lithographs showing successive improvements, commencing with a proof from a stone which had belonged to Senefelder, the inventor of the art, and adding that in recent years zinc has been found to possess the properties of the lithographic stone, with the advantage of occupying less space, being more portable, and less fragile. The anastatic transfer process, and the mode of making drawings on lithographic transfer paper, were then explained, and examples shown of facsimiles and other drawings done by this useful process, which requires no reversal of the design, and demands little more than a sharp eye and a steady hand, with care not to use any more ink than just necessary. Mr. WILSON next gave an account of the important steps in early photography, illustrating the subject with proofs made by the early and imperfect processes, and then continued:-Fox Talbot found that paper covered with silver chloride darkened in the daylight, and consequently could be impressed with the picture of flat and partly translucent objects superposed. Herschel found that hyposulphite of soda dissolved the unaltered chloride which had been protected by the opaque parts of the object, but not the darkened silver chloride, and consequently that the pictures might be fixed: this simple plan is still used to obtain a few copies of sketches or plans, etc. This is the ordinary silver print without the "toning," an operation by which minute particles of gold are precipitated from solution upon the image, and give it the rich purple tone with which we are familiar, together with greater permanency. The permanency of such prints is, however, much contested, prints only ten years old being often faint or yellow like the example shown. Silver has great affinity for sulphur, and traces of this substance are apt to remain from the hyposulphite, and are sure always to prove fatal if not entirely eliminated by careful washing of the prints. In 1830, Daguerre introduced iodide of silver. Bromide of silver was found to be even more sensitive, especially to green and some other colours. The adoption of these haloid salts of silver, together with the treatment of the plate, after exposure in the camera-obscura, with gallic acid, or sulphate of iron, was an immense advance. The last-named substances have the property of determining a change in the portions of the silver salt which have been predisposed to that change, by exposure to light, thus evoking into visibility the previously latent image. Niepce de St. Victor substituted glass for paper, retaining the silver salt in a thin coating of albumen. Instead of albumen, Archer adopted collodion, an adhesive, limpid, and very volatile solution of cotton, which, poured out over glass, rapidly dries, leaving a fine transparent substratum for the image. This image is of course negative; the strongest lights producing the greatest effect, the light and shade of the model are exactly reversed. "Candida de nigris et de candentibus atra." The negative serves as a matrix from which copies can be multiplied by exposing paper under it to the light.

The collodion film is nowadays often removed from the glass, and mounted on gelatine. You see how light and pliant this example is; not only is it not brittle like a glass plate, but the gelatine is so thin, that prints can be made from either side without perceptible loss of sharpness, and thus reversed pictures needed for transfer to stone, metal, or wood can be easily obtained.

Let me here mention two other processes which rather belong to photography than photo-mechanical multiplication. Among the numerous substances sensible to light, one of the most important is potassic bichromate. Added to gelatine, and exposed to daylight, it converts the gelatine into a kind of insoluble leather, while the unexposed portions remain as before, soluble in warm water. This was discovered by Talbot, in 1852. Poitevin mixed finely ground pigments with the sensitized gelatine, and coated paper with the mixture; the result, after exposure and development in warm water, was an image in the insoluble pigment, greatly superior to the silver print. Boaun, of Dornach in Alsace, made large numbers of facsimiles of manuscripts in this carbon-photography, so called because the pigment first used was finely subdivided carbon.

Here also let me introduce to your notice a new mode of photography not yet so well known as it deserves. It is called the platinotype, and was invented by Mr. Willis, who found that suboxalate of iron in a hot solution of potassic oxalate precipitated metallic platinum from salts of that metal; it had long been known that oxalate of iron was reduced to the suboxalate by light.

Here are some pale yellow prints on paper, which has been coated with iron oxalate and a salt of platinum. The pictures are so faint as to be hardly visible; the brownish grey parts are those where the light has reduced the light from the ferric to the ferrous salt. I draw a print over this hot solution of potassic oxalate, and you see the picture comes out in full, vigorous black tones. The platinum is precipitated upon the parts where the light has acted. This newest photographic improvement is one of the most easy, rapid, and permanent processes. The tone suits engravings. It is, however, very often the case that the contrasts in the negative lack the vigour of those in the original; this is especially the case when the models are in black and white, and are without half tones (or uniform tints not produced by lines or dots), and are so near the camera that the light reflected from them is sensibly diminished. In these cases, strengthening of the image is resorted to-that is, rendering the lights more opaque to chemical rays. This may be done by treating the negative with various solutions, so as to colour the lights black or scarlet, or by thickening them with a further addition of silver, which readily precipitates upon them from solution. The same can be effected as recently proposed with lead solutions. This plan of heaping up metallic deposit is, however, not free from the danger of lateral extension, and consequent encroachment upon the transparent contours of the image, so as even to entirely choke up the finer lines. Still it may be done with care, and it is the basis of one of the finest processes of

mechanical reproduction. Scamoni, the heliographer of the Russian Government, succeeded in obtaining directly on the negative, a relief almost equal to the depth of the incised lines in a copperplate engraving, and then took electrotypes from such negatives. These electrotypes served as copperplates for printing from. The process has been much used for the production of bank-notes, reductions of maps, &c. Its value as a mode of art reproduction is of the highest, as you may judge by these beautiful specimens, for which I am indebted to the kindness of the Typographic Etching Company, who do not hesitate to display their own productions by the side of them. Here is a reproduction of a Rembrandt etching executed by this Company. I am glad to be able to shew you a facsimile of this sort by an English house, which may fairly vie with the better known productions of Paris firms, of which specimens are also on the table. In considering Scamoni's plan of obtaining electrotypes directly from the negative, we have come by a sort of natural transition from photography proper to the photo-mechanical processes, that is processes in which the printing is not chemical as in photography, but where the block or plate from which impressions are taken is obtained by photography. Mr. WILSON then gave a description of some of the modes of photographic etching, illustrating his remarks by plates and proofs, and continued:-It is often difficult to tell whether these proofs were produced by electrotype or by etching, and there is naturally a tendency on the part of inventors and workers of successful processes to keep the details of their operations secret, as patents of this sort are peculiarly hard to protect.

In 1854, Pretsch, an Austrian, obtained plates by exposing a film of bichromated gelatine beneath a negative, such as the chromogelatine film you have seen, and developed in hot water which dissolves the unaffected gelatine, and leaves what has been acted on by light through the transparent parts of the picture remaining in relief; this is dusted with graphite, and an electrotype procured from it, which serves as the printing-plate.

Here is a fine volume of facsimiles of departmental archives, published by the Ministry of the Interior, in connection with the International Exhibition, 1878: they are by Dujardin's héliogravure. You are almost ready to pick up some of these charters and fold them in the well-worn creases. Here also is a large map by the same house, and some facsimiles of more artistic subjects. Writing of Dujardin's reproductions of Boucher's drawings and etchings, the Marquis de Chenevières says in the Gazette des Beaux-Arts, Janvier, 1880: "Ce sont les seules que nous regardons avec confance, les seules qui puissent faire apprécier les qualités et les défauts du maître."

Numerous and excellent as are the different ways of printing from metal plates with the ink contained in the incised portions, they have not the advantage of wood-engraving, where the blocks are in high relief, and can be printed with ordinary letterpress. It has accordingly been sought to imitate wood-engraving by pro

ducing blocks in high relief. As in stereotyping from engraved wood blocks, gypsum casts are taken-not, indeed, from an engraved block, but from the gelatine image. The unaltered parts which the light has not reached are not dissolved in warm water, but the impression is immersed in cold water. The yellow chromate is dissolved out, and the unaltered parts swell, the parts rendered insoluble by light remaining depressed. Considerable relief thus results, from which gypsum casts can be made, and from these again casts in type metal, wax, sulphur, or other subtances, can be obtained, or electrotypes, from which to print as wood blocks with the letterpress. Gillot, of Paris, has carried this relief-printing to great perfection, as you will see from these specimens; and on account of the great convenience of printing in the typographic press, and by any printer, its use is gaining ground. Here is a block and its impression. The process is useful in reducing copies to a smaller size. Here is another block of Mr. Dallas's; it shows that where there are large blank spaces, clearing with a scorper is necessary to obtain sufficient depth, or the ink would be caught and transferred to paper, as in this proof, pulled before this scraping. Here is a block by the Typographic Etching Company, for typographic printing, and its print. Such blocks are sometimes electrotypes from etched zinc. Such reproductions are good substitutes for wood in many cases. Mr. Dallas has recently applied the anastatic process in this way: a tracing or drawing, made as for a lithograph, can be turned into a metal relief for typographic printing. This would be an inexpensive and easy plan, for instance, for reproducing the facsimiles of signatures given in picture catalogues, &c.

In the processes hitherto mentioned, there has been no attempt to give homogeneous half-tones. The image consists of black and white, half-tones being got by numbers of fine lines or points near each other. But in 1865, Mr. Woodbury, in England, invented a way of doing this. A gelatine relief, when dry, is so strong and hard as to impart an impression on lead under great pressure. This lead is used as the printing-plate. The ink is not fat, but gelatinous and semi-transparent; and as its thickness varies with the depth of the depressions on the metal plate, the gradation of tint is almost as perfect as in a photograph. It is a suitable process for illustrating books where great numbers of copies are required; very perfect imitations of photographs are obtained by it, which have the merit of being in permanent pigment. The glossy surface, partly a result of the nature of the ink, is popular, but scarcely artistic. Here are some recent examples where this gloss is reduced to a minimum. I have known an instance, but only one, of the ink peeling off the paper, from an impression made when the process was younger.

Here is a reproduction of Goupil's from an oil painting: it seems printed like a copperplate. Some say that the impression is made upon the plate in a way analogous to that of the Woodburytype lead plate. But M. Rousselon, the manager of the technical

department, is sagaciously retentive about all such secrets. Here are also some specimens of a half-tone process by Dallas, intended for the typographic press, but it may be doubted whether such blocks will ever prove successful in the hands of the ordinary printer.

Here is another process giving half-tone very successfully; it has lately come into extensive use. Poitevin found that if an exposed film of bichromate and gelatine, such as the one you have seen, is damped with a sponge, and then gently rolled with a printer's inking roller, the printer's ink adheres only to the parts where the light has rendered the gelatine insoluble. Here, then, is a simple way of multiplying impressions mechanically, as long as the bichromate film holds good; by inking the image, and then pressing on it paper to which in turn the ink is transferred. This process is known as Albert-type, from Albert of Munich, who was the first to bring it up to practical efficiency. It is also known as the autotype, mechanical, or collotype process, and also as Licht-druck. The facsimiles of the Paleographical Society are produced in this way from original negatives by Mr. Prætorius, to whose kindness I am indebted for these and many of the examples before you. As you will easily understand, instead of taking these Licht-druck impressions upon paper, stone or zinc may be substituted, and thus the photographic image transferred to stone-that is photo-lithography; but half-tones are very unsatisfactory on zinc or stone. Such impressions are also printed on woodblocks for engraving. Reproductions in different colours are either very costly or very unsatisfactory. We may point to the reproductions of the Arundel Society, the facsimiles of ancient French MSS. undertaken by Comte Bastard, the pages of the "Paléographie Universelle," and some recent reproductions of water-colour drawings. Chromo-lithography, or chromo-lithography seconded by hand, is the all but universal process for such productions. The processes we have passed in review have nearly all been closely connected with photography, and to colours photography is not kind. Here is a "photo-chromo-lithographic facsimile by Mr. Prætorius, in which the area and position of the respective colours have been obtained from photographic negatives. Something analogous to this has been done in pigment photography by Ducos, du Hauron, and M. Cros, by combining separate prints of different colours, but the results have been pronounced hard and unnatural, and are little known beyond photographic circles. A plan which has succeeded better is to superpose a pigment or Licht-druck picture upon colours previously printed by chromo-lithography.

In spite of occasional announcements by enthusiasts or charlatans, the question of photography in natural colours seems to be not much further advanced than it was years ago. Niepce obtained coloured images on subchloride of silver, so did Becquerel, Zenker, Chastaing, and others. Abney obtained coloured images of the solar spectrum, and expressed confidence that a fixative for such pictures would be found. Eder, in his recent contribution to the subject, scarcely encourages such a conclusion.