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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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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. 1299, July 27, 1883
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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
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IHE PHOTOGRAPHIC NEWS. [July 27, 1883. of water, and the potassium bromide, potassium iodide’ and No. 1 gelatine with other 4 ounces ; the hard gelatine* again, with the least quantity of water that will swell it. Boiling is performed as before, the hard gelatine is added, the whole is either heated or cooled to 160°F. as is found necessary, and 5 drams of the strongest ammcnia (about the same percentage as before) are added. Stirring is performed, and it is so arranged that the solution will take an hour to cool to 100°F. When it has reached that temperature, 200 grains more of hard gelatine soaked in water are added. There will be just enough heat left in the emulsion to melt this, after which the whole will set very quickly. It may with advantage be left for twenty- four hours, as probably a slight increase of sensitiveness will result. After this it is washed as usual. It will probably make about 25 ounces of emulsion. The result of our experiments, conducted as recorded above, was an emulsion very fairly opaque, not granular above the average, of the very highest sensitiveness (25° of the sensitometer), and giving so great density of image that one-half to three-quarters of a grain of pyro to the ounce of developer was sufficient to give a plucky negative. Both the emulsions which we made gave plates showing green fog to a quite appreciable extent, but not sufficiently to spoil the printing qualities of the negatives. The opacity of the last described emulsion was very fairly good, but hot such as would be got by the boiling process. With the quantity made—400 grains of silver being used—sixteen plates, 12 by 10, were coated. With a boiled emulsion, even of the highest degree of sensitive ness, twenty-four could have been coated. A fact is worth noting in connection with the ratio of silver bromide to gelatine in emulsions. When the quantity of silver bromide is comparatively large in pro portion to the gelatine, so that plates dry with a matt sur face, they get less transparent in drying. If this propor tion of gelatine is, on the other hand, so high that a glazed surface results, the plates get more transparent on drying, or do not alter. The proportions which we mentioned above give a matt surface. * o HOT WEATHER AND GELATINO-BROMIDE WORK. The present season being the time of year when the photo grapher may expect the most abundant crop of blisters, frills, and other troubles of like character, it may be well to note a few points bearing on this subject. It is, however, highly gratifying to compare the earlier period of gelatino-bromide work with the present, and to find a most decided improvement as regards the stability of the film; this result is due to the circumstance that emulsion is generally so made in the present time as to guard the gelatine against such deteriorations as serve to bring about frilling and blistering. In addition to this, it is a common practice to treat the emulsion with a small proportion of chrome alum—a proceeding not alto gether without objection, as films prepared from such an emulsion gradually become increasingly impervious to aqueous liquids, and are consequently liable to be very sluggish under the action of the developer. Mere immer sion in a bath of chrome alum before development is fre quently useful as a preventive of frilling; but this pro ceeding alone is nearly useless when the gelatine has become thoroughly softened or deteriorated by long emulsification. In such a case it is advisable to adopt Captain Abney’s method of coating the film with plain collodion. The collodion should be applied to the plate precisely as if a glass were being coated for the bath process, and as soon as the collodion has thoroughly set, the film is immerse 1 in a ten-grain solution of chrome alum. It is well to allow the plate to remain in this bath for about twice the time required for the thorough disappearance of repulsion lines on withdrawing the plate. A collodion which is neither decidedly horny nor extremely weak should be employed, a sample well adapted for the ordinary wet pro cess being generally suitable; and before proceeding to development it is well to rinse the plate once with water. Such precautions as working in the coolest place available, and making use of moderately hard and cold water when it can be obtained, scarcely require mention ; but in any case, it is as well to harden the water which is to be used, by the addition of a small proportion of Epsom salts—one ounce to a bucket of water being generally sufficient. After fixation, the chrome alum bath may be used again, or a saturated solution of common alum may be employed; this second hardening being a good preliminary to the long washing which is always needful. In spite of the precautions referred to above, it sometimes happens that a plate will frill; and should the frilling be so decided as to indicate that it will extend to an essential part of the picture, it generally becomes advisable to take no further precautions against it, but rather to encourage it, with the view of completely removing the film from the glass, and mounting it upon a fresh plate. Frilling can generally be encouraged by gently pressing with the palm of the hand on those portions of the film which immediately adjoin the frilled parts ; the plate being meanwhile under water. By very little careful work of this kind, combined with a slight tendency towards a twisting action during the time pressure is exercised, it ordinarily becomes quite easy to completely and rapidly detach the film. It should be allowed to remain in water until those parts which have separated most recently from the glass have swelled suffi ciently to attain the same scale of magnitude as those parts which frilled up at an earlier period; when it may be caught on a fresh glass (which must of course be somewhat larger than the original one), and allowed to dry. It is obvious that it can be so placed on the fresh glass as to form either a direct negative or a reversed negative ; but inconvenience may possibly arise from the size of the transferred negative being considerably larger than that of the original plate; this enlargement being of course accompanied with a corres ponding reduction of vigour or intensity. It is, however, easy to reduce the floating film to its original dimensions, or even much smaller, before putting it upon a fresh glass, this end being attained either by adding dilute sulphuric acid or alcohol to the water in which the film is floating. The amount added will naturally depend on the degree of reduc tion wished for, and in order to economise material, it is well to pour off the greater part of thewater before adding the alco hol or the sulphuric acid. If sulphuric acid is used, it is neces sary to well rinse the film with water when it is on the fresh plate, and for this reason alcohol is generally to be preferred. STAINED SENSITIVE FILMS. Bromide of silver films stained with eosine are no novelty. Dr. Hermann Vogel, of Berlin, Major Waterhouse, of Cal cutta, and others, have made experiment with sensitive films stained with this rosy pigment, the haloid salt of silver being contained, however, in collodion, and not gelatine. Dr. Vogel, it maybe remembered, distinctly avowed that in the case of certain colours, this staining of the film was bene ficial, in so far as the eosine acted in the capacity of an “optical sensitizer.” A stained film, according to Dr. Vogel, was sensitive to certain rays, which, in its unstained condition, had but little or no action; and Major Water house, on several occasions, obtained results which certainly bore out the theory. Those interested in Dr. Vogel’s experiments, published nine or ten years since, will find them described in previous columns of the News ; but in considering the subject they must remember two things. In the first place, the use of “optical sensitizers ’’ is very limited, and their action, even according to Vogel and Waterhouse, of so trifling a nature that hardly any practical good can come of their employ
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