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The photographic news
- Bandzählung
- 7.1863
- Erscheinungsdatum
- 1863
- Sprache
- Englisch
- Signatur
- F 135
- Vorlage
- Hochschule für Grafik und Buchkunst Leipzig
- Digitalisat
- Hochschule für Grafik und Buchkunst Leipzig
- Digitalisat
- SLUB Dresden
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- Public Domain Mark 1.0
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- urn:nbn:de:bsz:14-db-id1780948042-186300004
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- http://digital.slub-dresden.de/id1780948042-18630000
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- oai:de:slub-dresden:db:id-1780948042-18630000
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- LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
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- Parlamentsperiode
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- Bandzählung
- No. 249, June 12, 1863
- Digitalisat
- SLUB Dresden
- Strukturtyp
- Ausgabe
- Parlamentsperiode
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- Wahlperiode
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Zeitschrift
The photographic news
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Band
Band 7.1863
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- Register Index 619
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Band
Band 7.1863
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- Titel
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the result we must obtain should be the power of strengthen ing this or that portion of a picture at pleasure; The artistic work of which we speak in this place can be produced neither by pencil nor palette, it must remain purely chemical, so as not to alter the purity of the lines, and have the picture in a perfectly homogeneous condition, with all its finish and delicacy of execution; We must discover an agent so subtle as to be able to rival light, or, still better, have recourse to light itself. The problem to be solved will therefore assume the following form :— Bring the light to continue, and to modify in its effect, at the will of the operator, the image formed in the camera. When a layer of iodized collodion is moistened with a solution of nitrate of silver, there is an exchange of bases, and formation of iodide of silver with excess of silver. This is what in practice we term the sensitive film. Exposing the sensitive film in the camera, all the points more or less luminous of the image act upon the sensitive film in proportion to their intensity, and a complete image is formed, but invisible after an ordinary exposure. It might be thought that this image would be visible by transmitted light, but nothing is visible to the naked eye. It is necessary for the latent image to be revealed, to be submitted to the action of one of the three developing agents usually employed—gallic acid, pyrogallic acid, or sulphate of iron. The picture then manifests itself under the form of a metallic precipitate, weak in those parts where the action of the light has been feeble, abundant where it has been intense. It is from this variety of transparency or opacity in the parts of the image that results that beautiful gradation of light and shade which forms the charm of successful pictures. The abundance of the precipitate produced by the action of the reducing salt upon the portions of silver impressed by the light is therefore proportional to the intensity of that impression. If the image is deficient in opacity on account of the precipitate being too weak, it shows that the luminous action has been insufficient. We must in that case submit the negative to a fresh exposure in the camera. But this result, good in theory, has no practical value, and only in very rare cases can we resume the pose of a model, under conditions precisely identical; and, moreover, if the thing were easy, we should only possess a new method of strengthening, which is not the object of our inquiry. It would be entirely different if we could succeed in isolating the image from its medium, or, in other words, if we could preserve to the elements of which it is composed, their photogenic properties, and render insensible to a new exposure the metal, which, not having been impressed at first, remains useless to the formation of the picture. To arrive at the solution of this interesting problem, let us first study the conditions of the formation of the picture upon the collodion negative. Contrary to what takes place upon a paper negative, where the image is deeply impressed in the whole tex ture of the paper, the picture of a collodion negative is formed only by the reduction of silver precipitated in the state of a metallic powder, without cohesion, upon the col lodion film, from whence it is removed upon the least contact, without leaving any trace of the image. Evidently the reductions arise only from the salts of silver in excess in the formation of iodide of silver. Eor, if by washing in abundant water we remove from the sensitized collodion the greater portion of the excess of silver which, in ordinary practice, is found on the surface of the film after exposure in the camera, the developer- will, after the exposure, yield only a scarcely visible image. If we then pour upon the negative a solution of nitrate of silver, the image will take what it requires from the silver, and manifest itself strongly. Now, if instead of limiting ourselves to washing the film in abundance of water, we re-cover it with a solution of the reducing salt, which, combining with the silver in excess, is carried away with it during the washings, and we afterwards expose it in the camera, we should have to submit it anew, after sub mitting it to the action of the reducing salt, and no trace of the picture will be revealed, although the principle of this image exists, since it appears, as in the preceding case, in presence of a solution of nitrate of silver. Let us carefully consider the difference of the two results. In the first case, a washing in abundance of water, the production of a thin picture ; in the second case, a washing followed by immersion in a solution of the reducing salt, and entire absence of a picture. It is, therefore, well established, 1st. That no picture manifests itself without the presence of an excess of silver upon iodide of silver. 2nd. That if water alone is insufficient to remove all this excess of silver, it is not the same with the reducing salt, which combines with it, and removes it during the washings in water, which must always follow the immersion of the negative in the developing baths. Now that it is demonstrated that the developer removes from the iodized film all the salts of silver in excess, and that then this iodide, although sensitive to light, manifests evident alteration only after it is put, subsequent to expo sure, in presence of a fresh solution of silver—we have the solution of our problem. For, if, on the one hand, as we have stated above, the picture manifests itself only imperfectly under the action of the developing salt when the impression by light is weak, and that in such case it suffices to strengthen this image, to prolong or renew the exposure ; if, on the other hand, the image formed by the silver impressed by light and preci pitated by the developer, retains its photogenic properties, while they are not stopped and destroyed by the de-iodizing of the collodion by means of hyposulphite of soda, or of cyanide of potassium, it will be sufficient for the operator to impress his negative by light, and afterwards bring out the image by a reducing salt and washing the negative many times in abundance of water. The negative in this state must be kept in the dark until the moment we are ready to modify the design obtained. If we desire to lighten certain parts to give them more brilliancy or import ance, as we must in those parts increase the opacity of the negative, the operator must expose it to the light, taking care to intercept the action of the light by means of screens more or less directly interposed, from those parts of the design which must remain in their primitive state. He can. at pleasure, interrupt, resume, or displace this action of light slowly enough to be able to follow it with the eye. When he has obtained the desired effect, it will suffice, to stop it definitely, to immerse the negative in cyanide or hyposulphite, to remove from it its photogenic properties.— Le Moniteur de la Photographic. (To be continued.') • THE INFLUENCE OF PHOTOGRAPHY.* Some day, when mankind has grown wiser, our descendants will smile at the folly which distinguished and decorated the so- called “ reformers,” who make a noise in the world—and not much more—and will transfer their admiration and their honours together to the true revolutionists, the mon of science. It is from the closet and cabinet that the movements originate which shake the earth. The chemist, compelling some new element to the service of man, or explaining for him the occult operations of nature; the mechanician, reinforcing his weak muscles with arms of iron and sinews of steel; the geollogist, anticipating over his map storehouses of glittering gold, destined to become tlie origin of empires; the aeronaut, launching his silken bark upon a sea to which the “ unvoyaged Atlantic ” of Columbus was a mill-pond ; the anatomist, questioning the mind of the Creator himself by comparison of all his works together; the antiquarian, making' the dead eloquent, and eliciting from * Daily Telegraph.
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