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The photographic news
- Bandzählung
- 7.1863
- Erscheinungsdatum
- 1863
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- Englisch
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- F 135
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- http://digital.slub-dresden.de/id1780948042-18630000
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- Bandzählung
- No. 250, June 19, 1863
- Digitalisat
- SLUB Dresden
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Zeitschrift
The photographic news
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Band
Band 7.1863
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- Titelblatt Titelblatt -
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- Register Index 619
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Band
Band 7.1863
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THE PHOTOGRAPHIC NEWS. 293 A sample of recently prepared gun cotton is immersed in alcoholized ether, with which it is left in contact several hours; when it is dissolved, iodide of cadmium is added. We perceive that at first the solution becomes red, and then colourless, and remains so. (It is well understood that the gun cotton gives no acid reaction). Cover a clean glass with such a collodion, sensitize the film, awl carefully wash it with distilled, water. Expose it to the light while it is still moist, then cover it with the deve loping solution mixed with nitrate of silver; a picture will be well developed, but quite fogged, without any intensity, and even when the exposure is prolonged the results are scarcely better. If, instead of exposing the plate while it is moist, it is not exposed until dry, the same results are observed, but the picture is still more defective. But if the gun cotton employed, instead of giving an amber-coloured collodion gives one that is bright red, then we observe a reverse state of things. Certainly, by washing with water the excess of nitrate of silver from the film after sensitizing, we remove from it a portion of its sensibility, and still more by leaving it to dry entirely; but the picture obtained is at least not veiled, and acquires great intensity under the influence of the developer. There exists, then, a cause for the opinion admitted by some that collodion sensitized and simply washed will yield pictures, and also for the contrary opinion. It is a very remarkable fact that collodions made with decomposed gun cotton, and old collodions, may be used dry without any preservative coating; and also, as a general rule, a good collodion (i. e. quick, and yielding not very intense images), cannot be worked dry. The same thing applies to paper. Make use of a very pure paper, prepared with iodide of potassium and nitrate of silver. Wash it in water and dry it; no image will be obtained, or at least only a very bad proof. But if sized paper be used, then success results. From this set of facts it results :— 1. That pure iodide of silver is incapable of yielding pictures. 2. That when in presence of an organic substance capable of combining with nitrate of silver, such as albumen, gela tine, resin, &c., it will, on the contrary, yield results when dry. The proof lies in these two rules :— A. When the collodion is new, as I have said, it does not ozonize the ether (that is to say, it does not liberate the iodine from the iodides), we shall then have, after sensitizing, a film exempt from organic matters, capable of combining with nitrate of silver, and the image obtained in the usual manner will be entirely soluble in pure nitric acid. The same result occurs with paper. B. But when the gun cotton ozonizes the ether, it is because it contains a foreign organic matter ; the same theory results when the collodion is kept a long time. The gun cotton is decomposed into matters hitherto unknown ; but it is certain that there is no longer a simple solution of alkaline or metallic iodide, but rather a solution containing also an organic sub stance, capable of combining with nitrate of silver, and furnishing a picture independently of the iodide of silver. The proof of this is, that a picture obtained in the ordi- nary way by the aid of an old collodion, and treated by nitric acid does not wholly disappear, the argentico-organic combination on the contrary remains, and it is so much the more visible the more the collodion is decomposed. Moreover, take a good collodion, and employ it dry, with an exaggerated exposure. Then examine the plate, and no picture will be visible, just as if iodide of silver prepared in glass vessels had been used. (Washed iodide of silver does not blacken in the light.) But take an old collodion, employ it dry, and the picture will become visible (at least in the high lights, corresponding with the darkest portions of the negative) on removal from the camera. However, this experiment proves that the iodide of silver prepared by double decomposition, and well washed, does not blacken. It is then quite evident that the picture is due rather to the organic matter. To operate with dry collodion, therefore, it suffices, accord ing to this theory, to ascertain if the image disappears or not under nitric acid (after development), if traces of collo dion remain, it can be employed dry, if not, it cannot. Such is, in fact, the truth. The following is the method to operate with certainty in dry collodion :—Cover the plate with the collodion to be dried, sensitize the film, expose and develop either with sulphate of iron or pyrogallic acid. Wash in pure water when the picture has come out suffi ciently, and cover the plate with pure nitric acid (this acid should remain clear after the addition of nitrate of silver). The picture disappears instantaneously. Remove the acid by immersing the plate for a few moments in -water, then carry it into a full light. Next examine the film attentively, especially in those parts which correspond to the sky or light parts of the model; with good collodions no image at all will be per ceptible, but with those which have been made with bad or old gun cotton, or even old collodions, the images will be very perceptible, and the better the more the collodions are decomposed. Collodions prepared with acid gun cotton do not give rise to this phenomenon, for it is sufficient to add an acid to the collodion; for the best prepared gun cotton decomposes slowly, both when it is dissolved and when it is kept in bottles. The newer it is the more the film is composed of pure iodide of silver, and the more it yields quick pictures ; but the older' it is the more it ozonizes the ether, the more organic matter it contains capable of combining with nitrate of silver, and the better it is suited for working dry. To work with dry collodion, then, it suffices to add to the collodion a substance capable of combining with nitrate of silver. This is the purely practical side of the question, which we shall examine in our next article.—Le Moniteur de la Photographic. MICROPHOTOGRAPHY, OR THE ART OF TAKING ENLARGED COPIES OF PHOTOGRAPHS.® Tins branch comprehends the mode of taking photographs of microscopic or almost invisible objects, as also of amplification by means of the solar camera. In either case means are resorted to by which light can be concentrated or condensed on the object or collodion positive to be copied and enlarged. These means are combination of plain reflectors, concave reflectors, double convex or plano-convex lenses. The appendages to the solar microscope and the solar camera are fac-similes of each other; but the solar microscope existed before photography had been elicited from chaos; the solar camera, therefore, is a mere imitation of its antecedent; the patentees of the latter instrument, then, can make claim to no originality of design; their only claim can bo the application of the instrument to photography. Solar MICROSCOPE. The appendages to the solar microscope, that is, the condens ing part of the apparatus, consists in the first place of a plane mirror in the form of a rectangle, whose width is at least equal to the diameter of the plano-convex or double convex lens which condenses the light received from the mirror. The length ot the mirror must bo about four times its width. At one end there is a hinge-joint, which allows the mirror to swing on the same like a door. The hinge is fixed to a circle of brass or other metal, which, by moans of a dentated periphery, admits of a circular motion. By this contrivance it will be seen that the mirror has two motions at right angles to each other; for instance, supposing the back of the mirror faced the sun at noon, and were perpendicular to the horizon, then one of the motions mentioned would cause the mirror to incline towards the sun until finally it would lie flat on the horizon. The other motion permits the mirror to move either towards the East or the West; so that, as it now stands, if moved towards the West * From Humphrey's Journal.
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