The photographic news

• Bandzählung: 6.1862
• Erscheinungsdatum: 1862
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
• Rechtehinweis: Public Domain Mark 1.0
• URN: urn:nbn:de:bsz:14-db-id1780948042-186200003
• PURL: http://digital.slub-dresden.de/id1780948042-18620000
• OAI: oai:de:slub-dresden:db:id-1780948042-18620000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Bemerkung: Seite 1-72 fehlen in der Vorlage. Vorlagebedingter Textverlust.
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

242 THE PHOTOGRAPHIC NEWS. [May 23,1862. In this case, with iodide alone, the negative is thin and poor, and devoid of detail, requiring an enormous exposure, and diffi cult to intensify, so that things do not work well at all; but when a bromide is added, a thin picture can be got with a fair exposure, with a good deal of detail, which can be intensified into a printing negative by treatment with pyro and silver, after the details have been first brought out with the iron de veloper. “ We now seo why the common opinion has gained ground that bromide is an accelerator in the collodion process. It is so when the chemicals are bad, but not so when the chemicals are pure. “ When the chemicals are bad they contain an acid or highly oxydized principle which interferes with obtaining a dense negative in the ordinary way; and we have now to endeavour to explain how the addition of bromide to the collodion counter acts that effect. “ In the preceding article on this subject it was stated that the film of bromide of silver is very slowly formed in the nitrate bath, requiring sometimes as much as a couple of hours to con vert the whole of the bromide of cadmium into bromide of silver. But the film of iodide of silver is fully formed after the plate has been immersed a few minutes in the bath. If, then, a plate is removed from the bath after it has only remained in it a few minutes, some undecomposed bromide of cadmium will remain in the film, and this will no doubt render it less sensi tive. But we are now speaking of a pure neutral nitrate bath. Suppose the bath or the film to contain traces of a strong acid, such as nitric or oxalic, it is then easy to see that this will attack the cadmium, and liberate the bromine, allowing it at once and promptly to combine with the silver and form bromide of silver in the film. The proof of this is, that if you add nitric acid to a solution of bromide of cadmium, you at once liberate bromine and form nitrate of cadmium in the solution. It appears, therefore, that a strong acid in the film (produced by free iodine or oxalic acid in the collodion, or nitric acid in the bath), assists the rapid formation of the bromide of silver, and thus renders the bromo-iodized collodion more sensi tive.” Some very interesting remarks on the value of bromides in the dry processes follow, from which it will be seen that Mr. Sutton’s views on this subject have undergone consider able modification:— “ We have next to determine, by careful experiment, the effects of bromide in the dry processes, and here some remark able results are obtained, so remarkable that we must defer for a time the completion of this article, until we are able to speak more confidently on the subject. Those only who have experi mented much in photographic processes can form an idea of the difficulty of arriving at a constant result and safe conclusion, and of the time these experiments take, and the multiplicity of little accidents which throw you out, however well arranged your system may be, and however carefully you may per form the manipulations. It appears, however, to be nearly certain that bromide is an accelerator in some of the dry pro cesses, giving, with a wonderfully short exposure, details to which the preservative seems to impart greater power of acquiring density. When a bromo-iodized collodion film is exposed in its wet state it is not highly sensitive, probably, as we have shown, from its containing free unconverted bromide of cadmium ; but when this film has been washed nearly free from that bromide and also from the free nitrate, and then allowed to get dry, the small quantity of free nitrate which re mains entangled with the pyroxyline and preservative, becomes concentrated by parting with its water, and the dry film then acquires exquisite sensitiveness. In order, however, to produce this state of things, certain proportions must be observed among the various substances used, and when these are pro perly determined, a highly sensitive dry plate may be produced —as sensitive, in fact, as the best wet collodion, or possibly even more so.” To this we may add that in all the most rapid dry pro cesses of which we have any knowledge, a collodion con taining a large proportion of bromide is used. Mr. England, who contemplates competing for the Marseilles prize for an instantaneous dry process, uses four parts of bromide to three of iodide. From a conversation we had with Dr. Hill Norris, we glean that a bromo-iodized collodion was used in his rapid dry plates ; and in our own experiments dry plates with bromo-iodized collodion have been at least two 01 three times as sensitive as those with iodide only. REMARKS UPON SOME OF THE APPARATUS EMPLOYED IN PHOTOGRAPHY* BY F. B. WINDOW. TRAYS and Dishes. Trays and dishes may be separated into two classes, accord ing to the use for which they are required. 1st. Those which are intended for developing paper or glass negatives 2nd. Those which are intended for washing, sensitizing, or any other purposes. Trays intended to be used for developing pictures with any reducing agent and nitrate of silver, must be made in glass or porcelain; because the vitreous surface of these materials affords a greater facility of ensuring that absolul cleanliness which is indispensable in this operation. The most minute particle of organic matter, or metallic deposit! adhering to the bottom of the tray, will, under the action of the developing agent, and in the presence of free nitrate of silver, prove the cause of a reduction of metallic silver upon the surface of the dish, which will speedily spread itself, fint in isolated patches, and gradually over all that portion of the dish which is covered with the solution. This action i of course produced at the the expense of the picture which is at the moment being developed in the dish, and upon which alone the reduction of silver should take place; but I in addition to this loss, the picture is more frequently than I not spoiled by contamination with the irregular deposit and by detached particles of this latter floating in the liquid and settling upon the face of the picture. To the beginner these remarks may appear to be hypet critical, as it will seem to him an easy matter to wash a dish of any material perfectly clean. He must remember, ho" ever, that not only ordinary and apparent cleanliness : necessary, but that a vessel in which a reducing age®' and free nitrate of silver are to sojourn together must 1* absolutely and chemically clean. The following experiment will go far to convince him of the truth of this. Clean A porcelain tray thoroughly with whiting and cyanide ot potassium, and having washed and dried it, develop in if 8 collodion plate of any kind—say a collodio-albumen witl gallic acid. If the proper quantity of free silver only I* ? added, it will be found that the liquid will remain to the - end of the operation quite clear and nearly white, and th® > dish will remain perfectly clean. When the picture i finished encourage a deposit of silver upon the dish b) placing it with the refuse developing liquid still in it iu the day light. The bottom of the tray will soon be blackened. When i ; is in this state pour off the liquid, and let the deposit dry : on the dish, still under the influence of light, for a few days Then wash the dish with a piece of rag and water only, unti it appears perfectly clean, and after wiping it, proceed W develop in it another plate similar to the first, with a lik. solution. It will be found, in all probability, that the developing liquid will soon become turbid, and that the bottom of the tray will be covered in patches with reduce 11 silver, which will gradually spread over its surface to the detriment of the picture. These effects are produced b! some chemical impurities which the simple washing was un able to remove, and that the eye could not detect. . H A glass or porcelain tray can always be rendered chem cally clean by rubbing them with whiting, or some suCI substance, and cyanide of potassium. But the porous grant lated surface of such materials as gutta-percha, india-rubbsn ebonite, &c., renders it impossible to apply mechanical fn tion to every part of their surfaces, and unfits them cons" quently for the purposes of development. . or For sensitizing positive paper, for washing, dipping, * Continued from page 232,