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THE PHOTOGRAPHIC NEWS, Vol. XXVII. No. 1294.—June 22, 1883. CONTENTS. PAGE The Potash Developer 385 Photo-Block Printing Twenty-Three Years Ago 385 Eder’s Recent Work on Gelatino-Bromide 387 By-the-Bye.—Photographers’ Prospects 388 ■French Correspondence. By Leon Vidal 389 Review 389 Alkaline Pyro. Developer all in One Solution. By G. Cramer 391 Notes 392 Patent Intelligence 394 PAGE Lessons in Optics for Photographers. By Capt. W. de W. Abney, R E., F.R.S 395 No Filtering for the Printing Bath. By R. H. Chapman 396 Photo-Lithography and Photo-Zincography. By Major J. Waterhouse, B.S.C 397 Correspondence 397 Proceedings of Societies 398 Talk in the Studio 399 To Correspondents....... 400 THE POTASH DEVELOPER. The pyrogallic developer with ammonia has many impor tant advantages. More particularly is it of value from the fact that it is possible to impart to a negative by its means any desirable character or aspect. Long or short exposure may equally be treated with success, and in the hands of a skilful photographer hard and soft negatives may be produced at will. On the other hand, there is the disadvantage, which can not be gainsaid, that the ammonia employed in the formula gradually alters in respect to its specific gravity, and grows weaker in time. Moreover, there are certain emulsion plates, that one comes across occasionally, which refuse to be developed, at any rate satisfactorily, with ammonia. Notably is this the case with emulsion plates produced by the ammonia method, which are given to yielding fog of different characters, and those films frequently appear of two colours, according as you look at them by trans mitted or reflected light. For this reason photographic chemists cast about for a substitute for ammonia, and selected the soda developer. A solution of soda, unlike that of ammonia, is constant, and often yields clear negatives in cases where ammonia gives nothing but fogged plates. Unfortunately, soda is not very soluble in water. Only 16 parts of it dissolve in 100 parts of water, and for this reason it is necessary to deal with large quantities of solution. For instance, for one ounce of developer, something like half an ounce of soda solution must be added. In travelling, it is especially inconvenient to have to do with large quantities of liquid, if not in the ordinary studio, so that, again, the soda developer is not everything to be wished. Of carbonate of potash, on the other hand (K,COa), it is possible to obtain a much more concentrated solution. Potash is six times more soluble than soda; therefore, for the travelling photographer especially, a developer made up with carbonate of potash has many ad vantages, as Dr. Stolze of Berlin has recently pointed out. Dr. Stolze makes up the following solutions for his pot ash developer ;— A.—25 parts of sulphite of soda dissolved in 100 parts of water. B.—90 parts of pure carbonate of potash in 100 parts of water. C.—10 parts of pyrogallol, 10 parts of alcohol, and 1 part of salicylic acid, in 100 parts of water. Equal parts of solutions A and B are mixed together, and labelled “ Potash solution,” while solution C receives the label “ Pyro solution.” In developing, one hundred parts of water are mixed with two parts of the “ pyro solution,” and from one to three parts of the “ potash solution.” P We have obtained very good results in working with 100 parts of water, 2 to 4 parts of “ pyro solution,” and 4 to 5 parts of “potash solution.” The developer is about as rapid as that with ammonia when tested by the sensi tometer ; but in ordinary work with the camera, the pot ash developer frequently appears to more advantage. The reason of this is to be found in the fact that the higher numbers on the sensitometer become very vigorous. The potash developer, we may mention, assumes a reddish-brown colour by contact with the air; while the ammonia developer, as our readers know very well, has a brownish yellow tint. As regards the colour of the resulting negative, this depends upon the quantity of potash that happens to be in the developer. Equal parts of “ pyro solution ” and of “ potash solution” give a dark brown tint; while an excess of potash yields negatives of a pronounced olive-green character. The use of an alum bath is here very desirable, since it removes this greenish hue entirely, and its action will be found to act best if employed between the opera tions of developing and fixing. If you put the plate into the alum bath after fixing, the green colour, it is true, disappears; but, strange to say, it is not unlikely to make its appearance again, by degrees, as the negative is being washed. For intensifying plates developed by means of potash, Dr. Eder recommends a mixture of sulphate of iron and alum, the solution to be applied after fixing. Immersed in a bath of this description, the greenish-brown parts of the image become of a dark brown. In a word, a kind of ink is produced possessed of good covering properties. To make use of this intensifier with advantage, the plate should be withdrawn from the bath before the whole of the gelatine film has dissolved. In the case, however, of badly-washed emulsion, or plates liable to fog, this system of intensifying is inapplicable, since it only adds more intensity to the fog. PHOTO-BLOCK PRINTING TWENTY-THREE YEARS AGO. Much has been said of late concerning the advantages to be realised by the application of photography to the require ments of the typographic printer, and over twenty-three years ago, many confidently predicted that the art of the wood-engraver was soon to be altogether superseded by the rapid and economical production of stipple or photo-tint blocks from negatives after nature. Those taking such an optimist view generally fail to perceive the real difficulties which have hitherto prevented photo-block printing from competing to any extent with wood-engraving as a means of illustrating such papers as the Graphic or the lllus ruled London News ; and as we have no doubt that the best way of