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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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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/id1780948042-18830000
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- Bandzählung
- No. 1314, November 9, 1883
- Digitalisat
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Zeitschrift
The photographic news
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Band
Band 27.1883
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- Titelblatt Titelblatt I
- Register Index III
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Band
Band 27.1883
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- Titel
- The photographic news
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706 THE PHOTOGRAPHIC NEWS. LNOvEAIBLR 9, 1883. good conductor by treatment in a chloride of gold solation, is the best that can be employed for the purpose. More over, it is desirable frequently to shift the plate in the trough and to turn it over, as otherwise there will not be an equal deposition of copper if the battery is a powerful one. The depositing action may be accelerated by hanging inside the trough a little bag of sal-ammoniac crystals. The cleaning of the relief plates, when of sufficient thick ness, and freeing them from the gelatine film in hot water, is done by Scamoni with the aid of caustic potash, prepared chalk, ci), and charcoal powder ; or in the case of very delicate objects by means of benzole and india-rubber. In conclusion, it may be remarked that generally speak ing, notwithstanding the enormous progress that has lately been made in electrotyping, and in the deposition of various metals, the moulding of fine gelatine reliefs still requires much experience, great skill, and extraordinary patience. DEVELOPMENT-SHOULD THE DEVELOPING DISH BE ROCKED, OR NOT? When dry plates first became aiticles of commerce, the earliest manufacturer in his instructions directed that after the solution was poured over the plate, it should be kept in constant motion by rocking. A second manu facturer directed that the solution be kept quite still till development was complete, and moreover declared that any motion or rocking of the solution was not only useless, but was detrimental to the result. Since that time the matter has been but little discussed, probably because it was considered to be of little conse quence which course was taken. Photographers have followed their own sweet wills in the matter, most, how ever, we believe, adopting the rocking motion. That there is in the result produced not only a differ ence, but a very considerable one according as the solu tion is kept in motion or not, is very easily proved. Any photographer who performs for the first time the following experiment will, we think, be somewhat astonished at the result. A considerable quantity of developing solution is pre pared—we will say six ounces—of the strength that would give a good result with the plates to be worked if the method of rocking the dish were adopted. Two plates are exposed for the same length of time. It is best in this case, as indeed in almost any case where it is desired to compare the results of developers, to slightly under expose. Two developing dishes are taken, and into each is put one of the plates. Over one is poured four ounces of the developing solution, and it is afterwards allowed to stand quite still, no motion at all being given to it. Over the other is poured the remaining two ounces of the solution. This time the solution is kept moving as rapidly over the film as possible. The first plate and dish are placed in such a position that the progress of development may be watched. When the second plate appears to be completely developed, the first will not be nearly so, but this is little other than might be expected. What is not at first quite so readily accounted for is that, even when the first plate has been allowed to develop as long as is required, the result is by no means equal to what has been produced in the second case. When the first plate appears to be completely deve loped, it is lifted from the solution, and both arc fixed. They may now be compared. If the developer have been such that it gave a plucky image in the second case, when the dish was rocked, it will be found that in the other case it has given a foggy-looking negative, with quite in sufficient contrast. At first sight this appears somewhat inexplicable, but a little consideration will show that it is only what is to be anticipated. If we considerthe process of development, we shall see that it consists of a reduction of silver bromide to the condition of metallic silver, bromine being freed. Now it is evident that where a certain portion of a plate blackens under the developer, there is given off bromine, which, probably, combining with the first ammonia which itmeets, forms bromide of ammonium. This, as we well know, is a powerful restrainer. Now it will be seen that in the development of a plate, if the solution be not kept in motion, we have the following state of affairs. Wherever the image has commenced to appear, there will be over the darkening part a thin film of solution of bromide of ammo nia, and probably the interstices of the gelatine will be filled up with the same. It is natural that the accretion of density is almost entirely stopped. At the same time, the developer is acting with its full energy on that por tion of the film which has not yet been reduced. So long does it take for density to be gained in the high lights, that before this takes place the deeper shadows are sure to be more or less fogged. There are several other indications of the same action to be met with at times, and they are very instructive. Where we have a case of a very bright object in close juxta position with a dark object—for example, a piece of white drapery against a dark curtain—the whiteness not being so intense as to produce halation, we have seen in the case of a gelatine negative the very reverse of what is often seen in the case of a collodion negative under the same con ditions. The appearance is well known in the case of a collodion negative. The silver solution is floating over it during development, and if the motion bo not quite enough, that which comes from a part not acted upon by light is quickly reduced on its first passing the edge of a part which is. Consequently, around the deep shadows there will be a portion of the high light denser in the negative and whiter in the print than the rest. In the gelatine negative it is the very reverse. If the solution be left without motion, the bromide of ammonia, as explained, already causes the high lights to acquire density very slowly. At the same time, a small quantity of the solution spreads itself later ally, and as a consequence, it may be found that there is a narrow portion of the shadow close to the high lights, where no detail appears; in fact, a narrow strip of transparent glass fringing the high light. There is still another case of similar action. It must have been noticed by many that in the case of certain plates, and indeed in the case of almost any plates if a strong enough developer be used, there appears on the back of the negative, before it is fixed, a positive image. It may not have been also noticed, but it is a fact, that this ap pearance is much more liable to show itself when the plate is held motionless than when it is rocked. The cause of the appearance is readily understood if we consider that the film, wherever reduction has taken place, becomes saturated with bromide of ammonia, probably quite to the back of the film. Here the silver bromide has scarcely been acted upon by light, and if it were to be reduced at all, it would be by molecular action. In the presence of the bromide of ammonia, and, be it noted, the absence of ammonia, the reduction cannot take place. On the other hand, the developer has penetrated to the back of the rest of the film with at least its normal energy. We sy at least, because we believe that as a matter of fact it reaches the back of the film with its energy increased—that is to say, with the ratio of the ammonia and bromide of ammonium altered. We will at a future time give our reasons for believing that ammonia diffuses itself through a gelatine film more rapidly than does bromide of ammonia ; in the meantime we may assume this. It follows that there reaches the back of the film, those portions of the plate which are not acted upon y light, a developer with reduced quantity of rcstraine , the natural result being that fog is produced.
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