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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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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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- http://digital.slub-dresden.de/id1780948042-18910000
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- Bandzählung
- No. 1726, October 2, 1891
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The photographic news
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other halogen and acid radicals, with the hydroxyl go to the unexposed parts, forming the positive pole. On the other hand, with the alkaline thio-carbamide developers, at the same time that silver is reduced on the exposed parts, silver sulphide is formed on the unexposed parts, which then become the negative pole, and attract the sulphur, the hydrogen, and some of the silver from the exposed parts ; while the halogen and hydroxyl pass to the positive pole, and transform part of the remaining silver into silver haloid, which is dissolved in the fixing bath. Although this theory, as stated in a rough way, seems to agree fairly well with the facts,and,from the experiments I have made, seems probably the correct explanation of the reversals, I do not feel myself yet able to put it forward authoritatively, and a great deal of further investigation is required. The subject is a very difficult one, beset with uncertainties, and requires more time and close attention than I have been able to give for working it out fully. The theory that photographic action is accompanied by electrical phenomena is no new one. Becquerel found that if silver plates, coated with silver bromide or other haloid silver salts, were electrically connected and immersed in dilute acid, and light was allowed to fall upon one plate while the other was screened, the effect of the light was marked by distinct electrical action capable of deflecting the needle of a galvanometer. Prof. Minchin has recently found that the same effect is produced on silver plates coated with silver haloids in emulsions of gelatine or collodion, and immersed in very dilute solutions of alkaline bromides, iodides, or chlorides. He has also found that if silver plates coated with a silver bromide emulsion in gelatine are attached to the poles of a battery, and half immersed in a weak solution of potassium bromide, the film attached to the carbon pole is visibly blackened on its immersed part, while no visible effect is produced on the other; but on developing this plate with pyrogallic acid and ammonia, its immersed part also becomes dark, exactly as if it had been exposed to light for a few seconds. There is a good deal of other more or less direct evidence that an electrolytic action takes place during development, although, so far as I can ascertain, no observations of the currents with a galvanometer are recorded. Comparatively little attention has, however, been given to the question, and electrical or electrochemical action has never been looked upon as a principal factor in the production of the photographic image. From these new results it would appear that—at any rate, as regards the silver haloids—the formation and development of the photographic image is to a very great extent influenced by electrical action, if not actually caused by it. It has lately been found that electrochemical reactions have explained many obscure points in ordinary chemistry, and it seems likely, therefore, that further investigation of photographic action by the light of the most recent electrochemical theories would also throw light on much that is now obscure and uncertain as regards the formation and development of the invisible photographic image. Prof. Meldola, in his “ Chemistry of Photography,” notes these micro-electrical phenomena in photography as subjects for further investigation, and I hope these experiments may prove a useful contribution to the investigation. Addendum, September 2M, 1891. In some recent experiments, I have obtained indications of a current with an E. M. F. amounting to •09 volt. during the development, in ferrous oxalate, of an ordinary celluloid dry-plate film (Thomas’sthickly-coated landscape) exposed in the ordinary way upon a landscape, with sky and trees; the sky ( — ) end being attached to one pole of the galvanometer, and the tree ( + ) end to the other. The film was previously moistened with a five per cent, solution of potassium bromide, and the ends were held in silver clips attached to the conducting wires, but not allowed to touch the developing solution. THE LIGHT-FOG DEMON AND THE BEGINNER, BY “SENEX.” Akter a little experience, most beginners in photography are able to steer clear of chemical fog, but the light-fog demon is one that pursues an amateur in a most exasperat ing way, for it hoodwinks him so completely that he blames any but the right cause of his failures. Looking over one’s friend’s negatives, one is very much struck by the fact of the large majority being light-fogged, and if mention be made of this, one is almost invariably told that this is due to over-exposure. However, since most excellent silver or bromide prints can be made from these negatives, the conclusion one naturally arrives at is that a small amount of light-fog does not in any way injure the printing capabilities of a negative. Moreover, the brightly- lit dark rooms of professional photographers certainly seem to produce a little of this result; for, in looking at professional portrait negatives, one is often surprised at detecting light-fog even in these ; yet the quality of the prints leave nothing to be desired. If you should happen to indulge in platinotype printing, then it is that light-fog makes itself felt, by producing a muddiness and want of detail, when probably the erring amateur will give up in desperation this artistic method of producing pictures. Eliminate, however, all the causes of light-fog from one’s attempts at negative production, and one is surprised with what ease most beautiful results can be obtained; and unless an amateur does not exclude them, the probability is that in a short time he will have discarded photography. Let us, then, enquire when it is that this demon works his wicked will upon our plates. Taking it for granted that apparatus, camera, and slides are light-tight, when is it that light-fogging takes place? It is either in the dark room or during the exposure. Concerning the dark room, it must be absolutely light tight, so that after fifteen minutes shut up in it without a light, one can nowhere discern chinks through which white light enters; if one’s dark room is, therefore, not perfect, work on in it at night, and do not believe the excuse that, because an infinitesimally small stream is Well away from the plate, it cannot hurt the sensitive film. It does, and will do so when you are filling your slides, or taking out an exposed plate for transference to the devel oping dish. Again, what sort of a lamp have you got ? Is it a lantern made of tin, and more or less complicated in design, with a view of preventing white light from shining on the plate ? That is exactly where the fault lies. What we want is a lantern which shall not only prevent white light from entering the plate, but shall also exclude any such quality of light from entering the dark room by even one tittle of an actinic ray. In addition, test your lantern when you are testing your dark room, and after your eyes have become as sensitive as those of a nocturnal animal, examine your lantern, top, bottom, front, sides, and back, and look out for chinks or light
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