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THE PHOTOGRAPHIC NEWS. [MARCII 2, 1883. value of the reproduction by photography is lost. In copies of pictorial and ordinary subjects in which accuracy to scale is unimportant, this defect is not of much conse quence ; but in the reproduction of maps, plans, and other drawings to an exact scale, it is undoubtedly a great drawback. If the alteration of size were uniform all over the sheet, it could be allowed for in taking the negative ; but it is nearly always more in one direction than another, according to the make of the paper and the way the sheets are cut, some papers expanding in width and contracting in length, and vice versa, the distortion being tolerably uniform with the same paper and treatment. This defect may be minimised by careful selection of the paper and treatment of the transfers. When possible, the paper should be cut and laid on the negative in such a manner as shall be found by experience to give the transfer the least distortion. It is sometimes a good plan to soak the paper in water and dry it before coating with gelatine. In copying large scale plans, where the distortion may make a sensible difference in measurements taken in the length of the sheet or across it, it is a good plan to have two scales drawn on the original at right angles to one another. Tinfoil Transfers.—Senhor J. J. Rodriguez, of Lisbon, has proposed to overcome this defect in a very effectual and ingenious manner by using sheets of tinfoil coated with bichromated paper instead of paper. The tinfoil can be laid in very close contact with the negative, and in transfer it moulds itself round the ink, and prevents it from spreading on the zinc or stone. The tinfoil should not be thicker than thin paper—the thinner the better, provided that there are no holes in it, and that it is not too thin to handle conveniently. The sheets of tinfoil are first glazed with light pressure upon a lithographic stone, which should not be highly polished, but very finely-grained, and then very slightly pumiced. A very finely-grained zinc plate would also answer well. Too strong a pressure destroys the flexibility of the metal, and causes it to tear readily. If the stone is too highly polished, the adherence between the glazed sur face of the tin and the sensitive coating will be weakened; and if too rough, the delicacy of the drawing will be spoilt, and stains may be caused under the action of the inking roller. The glazed foil should be perfectly cleaned, and laid on a smooth hard surface, such as a lithographic stone ; or, better, a zinc plate of the kind used for engraving. The surface of the support is moistened with water, and the tin-foil is laid on it, carefully avoiding creases; the sheet is then smoothed down on to its support by gentle rubbing with a very soft, smooth, damp pad. If the surface of the metallic sheet seems clean, it will be sufficient to rub it lightly with a piece of rag moistened with a 10 per cent, solution of potash or soda ; if it should be necessary to have recourse to stronger measures, a little precipitated chalk or whiting may be added to the alkaline solution. The sheet is then well washed to re move all traces of the alkali, and the sensitive coating is brushed on. For large sizes, bichromated gelatine is preferable as the the sensitive material to asphaltum. The sensitizing liquid is composed of:— Good gelatine 40 parts Water 500 ,, dissolve with heat, and then add Bichromate of ammonia ... ... 20 parts Water ... ... ... ... 500 ,, also dissolve with heat. The two solutions are mixed while warm, and filtered at once through a double flannel or a clean sponge. The best gelatine to use is a kind which sets at about 95° F., without at the same timebeing too difficulty soluble. If the underlying support is properly plane, the mixture, when brushed on, should present a perfectly homogeneous coating of an amber tint, free from streaks or spots. The coating of the sheets may be done in daylight, but the drying must be in the dark, and should be effected quickly, so as to avoid differences of thickness in the coat ing and crystallisation of the chromic salt, which is, how ever, less to be feared if bichromate of ammonia be used. During the drying, the supporting plate of zinc is kept level, and may be heated with gas or in a stove. When the coating of the foil is dry, the sheet is separated from the zinc plate, being turned over on to a large sheet of cardboard, which is gently warmed till the water used for making contact between the two metals has completely evaporated. The sheet is then ready for exposure to light. The sensitized coating will keep good for two or three days, but it is preferable to expose it the same day as prepared. The above summary, though not complete, includes most of the best processes of preparing the photo-transfer paper as described by their authors ; and by combining the in formation thus given, the operator should be able after a few trials to select a mode of working that will suit his purposes. (To be continued.) NOTES ON PHOTOGRAPHY. BY E. HOWARD FARMER. Lecture XIII.—The Gelatine Process (Intensification) Continued. The Polytechnic Method ivith Silver.—A solution is pre pared as follows:— No. 1. Silver nitrate 1 ounce Water (distilled) 12 ounces No. 2. Potassium bromide ? ounce Water ... ... 2 ounces No. 3. Thiosulphate of soda (hypo.) ... 2 ounces Water ... ... 6 ounces Add No. 2 to No. 1, and after washing the precipitated bromide thoroughly by decantation, dissolve it with agita tion in No. 3. The muddy liquid thus obtained is either filtered perfectly clear, or placed aside for a day, and the clear solution syphoned off ; it is then made up to sixteen ounces with water, and kept for use. To intensify a plate, wash thoroughly after fixing, and, taking it on a pneu matic holder, flood with the following mixture :— Pyro (preserved in sulphite) ... 4 grains Water ... ... ... ... 2 ounces Silver solution 1 drachm to which is added, immediately before use, about half-a- drachm of dilute (1 to 8) ammonia. If the silver shows no tendency to reduction, add more ammonia, and if it be thrown down immediately, use less ; with a little experi ence a peculiar browning of the liquid shows when suffi cient ammonia is added. Rock the plate and apply fresh solution as the density gradually increases; if not suffi ciently dense and the solution be muddy, rinse the plate, and use fresh. Finally, place it for a short time in the fixing bath, and wash. Or immerse the washed plate in the silver solution, and leave it there for five minutes; take out, drain, and flood with an ordinary oxalate deve loper, when the image will rapidly increase in density. Rinse the plate, and place in the fixing bath as before. If the plate only requires slightly intensifying, dilute the silver solution more or less, as desired. Note.—Plates which in ordinary development show signs of fog setting in, can be successfully treated thus Immediately a trace of fog appears, wash and fix the plate, again wash and treat with the above intensifier, when the requisite detail and density can readily be obtained. deduction of Density.—There are three principal methods