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The photographic news
- Bandzählung
- 24.1880
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- 1880
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1142, July 23, 1880
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Zeitschrift
The photographic news
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Band 24.1880
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- Register Index 631
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Band 24.1880
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354 THE PHOTOGRAPHIC NEWS. [July 23, 1880. bge Pyotograpkir Uebs, Vol; XXIV. No. 1142.—JULY 23, 1880. CONTENTS. page Photography In and Out of the Studio 349 At Home 350 French Correspondence. By Leon Vidal 351 Gelatine Development. By II. Bruyere 352 The Changes which Gelatine Undergoes in Emulsifying. By Dr. J. M. Eder 352 Gelatino-Bromide Negatives for Photo-Mechanical Printing... 354 Notes 355 Topics of the Day 356 Nelson’s Gelatine. By J. R. Johnson 356 Prints from Old Negatives. By the Author of “Looking Back” 358 Correspondence 359 Proceedings of Societies 359 Talk in the Studio 360 To Correspondents 360 GELATINO-BROMIDE NEGATIVES FOR PHOTO MECHANICAL PRINTING. The gelatino-bromide process is undoubtedly pressing itself forward with great strides; and though it may not be destined to completely rout and expel the bath and wet plates, there can be no doubt that the new method is, at the very least, able to take a firm stand side by side with its elder and finger-blackening brother. Two years ago the gelatino- bromide process was in the hands of a few, and the excellence of the results which they obtained was commonly attributed to special experience rather than to the inherent merits of the process itself. Now the case is altogether different, and a pushing London photographer who is not capable of obtaining first-class results on gelatino-bromide plates would perhaps be hard to find. As soon as the general run of photographic workers discovered that excellent results were yielded by the new process, even when in the hands of men who, like themselves, had but little time for experimental work, they went in for the new thing, and succeeded. All kinds of dismal forebodings were heard as to the ruthless manner in which the hot weather would punish those progressionists who were so daring as to throw over the bath and take to gelatine; yet, although it is now summer, the difficulties incident to the new order of things are steadily diminishing. Frilling or blistering, which was looked on as such aformidable enemy, is now rarely heard of, as the conditions for preventing it are now pretty thoroughly understood, both by plate makers and plate users. Those who practise the various photo-mechanical printing processes appear to be rather slow to adopt the new mode of working as regards the production of negatives; but the purpose of the present article is to point out the special adaptability of the gelatino-bromide process to the require ments of the photo-mechanical printer. For the ordinary photo-lithographic processes, in which the original fatty image is prepared on a sheet of gelatinised paper, a direct negative of considerable vigour and intensity is required. To obtain a suitable negative of a sharp or clean line subject is a matter of the greatest ease. A sufficiently long exposure to render all forcing with ammonia unnecessary should be given; and either a normal developer, or one containing a slight preponderance of pyrogallic acid, should be employed. If the plates employed are of a kind tending towards the production of thin images, the proportion of pyrogallic acid must be somewhat increased ; but in such a case it may be necessary to pro long the exposure to a certain extent, lest the transparent portions of the negative should become tinged by the stain almost invariably produced when the development is unduly prolonged. As a general rule, three or four minutes is suffi cient for the development of a dense negative of a line subject. Apart from chemical causes, there are three circumstances which may tend to mar that extreme clearness of the lines which is to be desired. These are, reflection of light from the back of the plate, commonly called halation ; reflection of light by the black lines of the subject ; and reflection from the surfaces of the lens employed. The first of these, or hala tion, rises into importance on account of the large propor tion which white bears to black in the case of most sub jects intended for reproduction by photo-lithography; and it not unfrequently happens that very fine hair-like lines extending over a white ground are almost totally obliterated by this agency. It is therefore advisable that backing of the plates should be resorted to whenever fine work is to be reproduced, or when the scale of reduction is considerable. The amount of light reflected from the black lines of the original is often considerable, cs the blacks of an engraving frequently lack solidity, and show as a kind of stipple under a magnifying glass, but in such a case the operator can hardly mend matters. When an ordinary actinised meniscus is employed, the reflection inside i the lens is altogether insignificant, but it occasionally [ happens that reflection from some portion of the tube for mount degrades the image. In such a case the remedy is obvious, and it is a good precaution to varnish the edges of the lens with black varnish. Nowadays, however, the old-fashioned view lens is not so much used for copying as it deserves to be ; as in order to reduce the distortion to a minimum one must employ an objective of very long focus in relation to the working area. The short focus doublets and triplets now so generally employed present numerous reflecting surfaces, and in some cases the light so reflected considerably degrades the purity of the image. All that a person using such a lens can do is to blacken the edges of the glasses, and to provide against any possible reflection from the metal-work of the instrument, let us say by lining the tube with velvet or other means. The diaphragm slit must of course be looked to, as during a long exposure a very small trace of light entering here might do much mischief. Negatives intended to be printed by the collotype pro- cessmustbe reversed, and when line subjects are to be repro duced by the method in question no unusual density is requisite, so that our remarks on this point may be confined to the production of the negatives in a reversed form. Should the operator be possessed of a reflecting prism or true surfaced mirror which lie can adapt to the front of the lens when he takes the original negative, the production of a re versed image is effected without trouble, only a slightly increased exposure being needed. Supposing that the operator does not possess a suitable reflecting instrument, either one of two course! is open to him for the production of a reversed image. He may place the plate with the glass side next the lens, or he may strip his negative from the glass plate after it is finished. Should the former plan be adopted, a plate quite free from irregularities should be selected, and it is needless to say that the back of the plate ought tobe very carefully cleaned. Fairly satisfactory results may generally be attained by this method ; but when results of the first order are required it is better to strip the gelatine film. We have found the following plan toanswer perfectly for this purpose. A plate of flashed ruby glass is cleaned on the coloured side just as one would clean a plate for the ordinary wet process, and after this a little powdered French chalk is gently rubbed over the surface by means of a soft cloth. All excess of the French chalk having been wiped away, the prepared surface is bordered with a solution of india-rubber in chloroform ; six grains to an ounce of solvent being a convenient strength for this latter. The next step is to coat the plate with plain collodion contain ing about five grains of pyroxyline to an ounce ; and if this collodion leaves a film of a horny character, so much the better. The collodion film being dry, all is ready for coat ing the plates with emulsion, and the emulsion films may either be allowed to become dry, or they may be exposed in the moist Condition as soon as the gelatine has set; but in this latter case there is no necessity to wash the emulsion unless the greatest extreme of sensitiveness is required. In either case the negative is developed, fixed, and washed in
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