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The photographic news
- Bandzählung
- 7.1863
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- 1863
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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. 250, June 19, 1863
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Zeitschrift
The photographic news
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Band 7.1863
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- Register Index 619
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Band 7.1863
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292 THE PHOTOGRAPHIC NEWS. [June 19, 1863. the plate ; then we pour on a third dose of the iron solution and allow the greater portion of it to flow off again, at the same time rapidly mixing the two solutions. The action is very energetic, the negative gains much in vigour, and it must be carefully watched so as to stop it in time. This method, a little more expensive than the former, is very advantageous for large negatives, we employ it also for plates 12x10 which we can cover sufficiently with from five to seven ounces of the preceding silver solution. We have in this manner obtained very fine negatives, soft and beautifully modelled, only the lighting must be appro priate, and also present a character of uniformity in the light tones as in the more luminous parts, which must, neverthe less, receive only very little direct light. We must not allow the solutions to remain too long upon the plate after they have begun to thicken, because they deposit particles of metallic silver which impair the purity of the picture. In most operating-rooms another method, slower in its action, is practised, the superiority of which, however, is undoubted; it consists in first developing the image with sulphate of iron, exactly in the same manner as previously indicated, only we must take care to arrest the action of the developer as soon as the picture appears in all its parts, and before it has begun to fog : it is at this moment, if we clearly understand the process, that we can obtain the wished-for result by modifying the effect, if we perceive that the ex posure in the camera has been too long, or not long enough, or the lighting badly distributed, &c. In fact, before going further, let us say that if the exposure, for example, has been exaggerated, it will suffice, to avoid the inconvenience which must necessarily result, to stop the development im mediately the shadows are sufficiently brought out, without waiting the action continuing itself, darkening all the details in one uniform grey tint. If the contrary is the case we prolong as much as possible the action of the sulphate of iron to gain on the side of the shadows and light tones. The same thing takes place in an unfavourable lighting of the object; if it be too harsh, we continue the action of the developer a long time, in order to equalise the impression ; while if it be uniform and without depth, we endeavour to preserve more opposition or contrast in the picture than it naturally possesses by stopping the reaction sooner than we should do in ordinary cases. Returning to our own mode of development, let us say, that as we have stopped the coming of the image without any regard to its degree of intensity, we must communicate to it the vigour required for printing positives by means which, no longer developing the image, simply communicate to it more strength in the opaque portions. To this end, after well washing the plate on both sides, to stop the action of the ferric liquid and remove the last traces of it, we effect the intensifying with pyrogallic acid, mixed with some hundredths of nitrate of silver in acqueous solution. Into a glass measure we put a sufficient quantity of pyrogallic acid prepared according to the formula previously given; then pour the liquid on the plate, in the same manner as with the first developing solution ; but accidents being less liable to occur, it is not necessary to pour it on so largely or in so regular a manner. We keep the pyrogallic solution a few seconds upon the picture, and if there be reason to fear that the plate has not been sufficiently freed from the sulphate of iron, we reject the first layer and replace it by a second. In every case, while the solution covers the plate, we put into the glass measure a small quantity of nitrate of silver solution, and collecting the pyrogallic acid that flows off upon inclining the plate, the whole is immediately poured on to it. We follow the action attentively by viewing the picture as a transparency, by lifting up the plate from time to time, from which we collect in the glass measure the liquid which flows from the lower angle. It is useful to repeat this opera tion often, by pouring on the pyrogallic acid collected each time in the measure. If this intensifying liquid becomes dark and muddy before the negative is quite finished, the collodioned plate is carefully washed under a stream of pure water, and the action may be continued with a fresh quan tity of pyrogallic acid, to which must be added one or two hundredths of nitrate of silver, prepared according to the formula given above. It is always good to wash the negative with pure water towards the middle of the operation of intensifying, because the pyrogallic acid becomes oxygenated during the reduction, it forms, as we have remarked above, even in contact with the collodioned surface, a film, rendered greasy in its aspects by the acids, composed chiefly of insoluble ellagic acid, a film which opposes the contact of the liquid surfaces with that of the negative, which slackens considerably the effect, at the same time these impurities attach themselves more and more to the negative. The picture should retain all its purity; under the action of the pyrogallic acid it acquires much vigour and relief, but it must not be carried too far. It is not the most vigorous negatives that yield the best proofs. We must, on the con trary, preserve a certain transparency in the darks, for there are shades in those parts as in all the others. When the negative has arrived at the desired point, it is cautiously washed so as not to remove the collodion film from the glass, its adherence to which is greatly diminished after all these manipulations. If we provide a stream of water from a tap, we must receive the jet that flows upon the middle of the plate, and incline it in different directions to cause it to flow off the edges, so that we incur no risk of raising the collodion by the infiltration of the water at the sides of the plate, where it is always more or less detached. The undoubted advantage of this process is, that it per mits of stopping the development of the picture when it has arrived at a determined point, and in retaining the faculty of augmenting it afterwards to the degree of vigour desired. The sulphate of iron appears to possess the remarkable property of completely destroying the effect produced by the light upon iodide of silver, as soon as it comes into contact with the latter; either because it destroys it by the reduction of the soluble salts of silver necessary to the sensibility, or from another cause, it acts in such manner, that the pyro gallic acid develops no detail, however weak it may be; be, its operation is limited to strengthening each of the parts already apparent by the metallic molecules which this organic reducing agent slowly liberates, and which fix them selves upon the various tones, in masses more or less consider able according to the extent of the first deposit effected by the sulphate of iron. It thus results that the entire image increases in intensity, or preserves a constant relation be tween its lights and shades, since the molecular attraction of the silvery particles is effected in the order determined by the luminous image, and the first reaction is exercised by virtue even of these lights and shades. In treating of the development of collodion negatives, wo have not thought of broaching a new subject, but we believe that the complete details of a good method of operating would be found very useful to many photographers. We have no doubt that the method we have here indicated is already more or less known ; but when we see how few useful details are published in similar articles, we venture to hope that what we have written will be found of some service.— Bulletin Belye de la Photographie. • DRY COLLODION AND DRY PROCESSES. BY DR. D. VAN MONCKHOVEN. It is a common error to suppose that dry iodide of silver will not yield pictures in the camera. It is, nevertheless, certain that this substance yields very good results with albumen, waxed paper, and gelatined paper : but in paper quite exempt from albumen and gelatine, and especially in collodion, iodide of silver sometimes yields pictures, and sometimes not. We propose in this place to elucidate the cause of this fact.
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