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The photographic news
- Bandzählung
- 24.1880
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- 1880
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- Englisch
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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. 1138, June 25, 1880
- Digitalisat
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Zeitschrift
The photographic news
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Band
Band 24.1880
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- Register Index 631
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Band
Band 24.1880
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304 THE PHOTOGRAPHIC NEWS. [June 25, 1880. degrees, and the whole when, developed gives a duplicate negative. If the exposure be continued for a still longer time, from six to nine minutes, the solarisation will become extended over the whole plate, and what we may call the “solarisation negative’’ gradually disappears under ordinary circumstances. Duplicate negatives of this kind are so highly fogged as to be practically useless. In order, therefore, to make this process workable, we must endeavour as far as possible to augment the solarisa tion. Captain Abney has shown that solarisation is really due to oxidation, and that by the addition of oxidising substances the solarisation can be materially increased. Mr. Bolas, who has pursued the subject in practice, dips an ordinary gelatino-bromide plate in a bath of a 5 per cent, solution of potassium bichromate, and, after drying it, he exposes it under a negative. Under the influence of light the potassium bichromate gives off oxygen, and hence the oxidation of the silver salt, by the action of the rays of light (or, in other words, solarisation), is promoted. The parts that have been exposed oxidise much more rapidly than is the case with those of an ordinary gelatine plate, and the transformation of the positive to a negative is accomplished much quicker and more perfectly. By this method, there fore, the multipication of negative by means of solarisation is materially improved, in that the oxidising action of sun light is furthered by the addition of potassium bichromate. Solarisation, which up to the present has always been considered as an evil, and which on that account has been a cause of dread, may now be itself converted to serve a useful purpose in photography. It may possibly be largely taken advantage of in practice, and in the future it may be called on to play a not unimportant part therein. THE REPRODUCTION OF NEGATIVES IN A REVERSED DIRECTION, BY THE INVERSE ACTION OF LIGHT ON GELATINE PLATES. BY T. BOLAS, P.C.S.* The reproduction of negatives in a perfectly satisfactory manner is not by any means easy, as the various gradations of opacity which exist in a good negative can only be fairly reproduced by a most accurate proportioning of the development to the ex posure, and the fulfilment of some other conditions. The conditions of satisfactory reproduction are rendered much more difficult to fulfil when a transparency has to be first made, and the negative is then to be produced by a second operation. In fact, those who have had most experience in the reproduction of negatives generally admit that a first-class picture, almost in variably, is deteriorated by reproduction ; while a second-class one, containing a less complete range of tone, may be reproduced without any deterioration. Reversed negatives are now a necessity for several processes of considerable importance, such as the single transfer carbon pro cess, collotypic printing, and the new Woodbury method in which tinfoil is the moulding material ; and this circumstance very much lessens the usefulness of these processes. A photographer who has produced a negative naturally objects to running the risk of having it stripped, and, on account of the uncertainty of obtaining a good result by reproduction, he often rejects a printing method which he would otherwise avail himself of. The process about to be described arose out of some experi ments in connection with the Pretsch process. Plates containing bichromate and bromide of silver, having been treated, after ex posure, with ferrous oxalate, it was found that the inverse action of light had so far changed the silver bromide as to lead to the development of a reproduction of the original picture: those portions of the plate which had received most exposure remaining clear under the developer, and those portions which had been exposed under the opaque parts of the original became opaque. These results accord well with some experiments which Capt. Abney has recently made on the oxidising action which a bichromate exercises on those portions of the bromide in an ordinary gelatino-bromide plate which have been acted on by light in the usual manner. It is well known that the inverse action of light sets in the * Bead before the Photographic Society of Great Britaid case of an ordinary gelatino-bromide plate, but the action is not sufficiently vigorous to render it practicable to reproduce a nega tive by taking advantage of it. A gelatino-bromide plate is soaked for a few minutes in a four per cent, solution of potassium bichromate, and after this it is rinsed for a few seconds in a bath composed of equal volumes of alcohol and water. On removal from the alcohol bath, it is laid down on its back, a piece of clean blotting-paper is laid on its face, the paper being pressed gently into contact with the plate by means of a cloth. The paper being now removed, the surface of the plate will be found to be free from any superfluous moisture, which might tend to accumulate into drops during the drying; and it should be next placed in a warmish place to dry. When dry the plate is to be exposed under the negative which is to be reproduced, the exposure being about the same as one would give in making a carbon print from the same negative—say two, three, or four minutes in moderate sunshine, or ten to fifteen minutes in a good diffused light. After exposure the plate will be seen to be impressed with a very delicate and perfect positive impression, owing to the darkening action of light on the argentic bromide, and also to its action on the bichromatised gelatine. The plate must next be soaked in a few changes of cold water, in order to remove the excess of potassium bichromate ; and this having been done, any developer which is suitable for gelatino- bromide plates is to be poured on, but a pyrogallic acid and ammonia developer appears to be, on the whole, the most suitable. Under the action of the developer the nature of the picture rapidly changes, the light parts becoming dark and opaque; while the parts already tinted by the action of light either be come actually clearer, or appear to do so by contrast. The posi tive having been thus converted into a sufficiently dense negative, the plate is rinsed with water and cleared with hyposulphite of sodium in the usual manner. In the case of the above described process, undue exposure tends to give a flat picture which develops rapidly, while over- exposure results in the production of a hard picture which develops slowly. In demonstrating his process, Mr. Bolas said : Here is a gelatino-bromide plate on which an impression has been printed sufficiently long to darken the bromide. Previous to the printing the plate was soaked in a solution of bichromate of potash, and dried. What Capt. Abney has told us teaches us that for this purpose we may use a plate that has been exposed to light either in or out of the camera. After the printing of the plate all traces of bichromate were removed by washing. Here is another plate similar to the first. This I will pass round for examination ; the other I treat with the ordinary developer, pyrogallic and am monia. I have noticed that photographers, generally speaking, use glass beakers for development, these being remarkably invisi ble in the dingy light of the dark room. Porcelain cups, how ever, are quite the reverse, and I therefore use them for the purpose. If the developers are poured into the same vessel from the two bottles, imperfect mixture of the fluids often leads to the production of streaks on the plate. There is an American prac tice of mixing drinks, which consists in pouring from one vessel into another and back again, and this process of manufacturing a “corpse reviver,” or “ brandy smash,” can be imitated with advantage in mixing the developer. The positive in the dish is now rapidly changing to a negative, the black parts turning white. It is now converted into a negative, though not suffi ciently dense for printing, but density will be gained by allowing it to remain in the dish. In this mode of reproduction I see very little difference in detail between the original and the dupli cate, though they can be easily distinguished, because one is developed by ferrous oxalate, and the other by pyrogallic. It should be also mentioned that in the reproduction the position is reversed, the right hand becoming the left hand, but this is the especial advantage of the process, as it affords a negative suitable for collotypic printing or the new Woodbury type process. A NEW INSTANTANEOUS SHUTTER. BY G. L. ADDENBROOKE. So many instantaneous shutters have been invented within the last year or two, that one cannot help feeling some diffi dence in introducing another to the notice of photographers. I’he one about to be described is, however, I believe, new in principle, and has these advantages : it works just in front of the plate; gives a longer exposure to the fore-
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