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The photographic news
- Bandzählung
- 24.1880
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- 1880
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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. 1140, July 9, 1880
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Zeitschrift
The photographic news
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Band 24.1880
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- Register Index 631
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Band
Band 24.1880
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328 THE PHOTOGRAPHIC NEWS. [July 9, deem it worth their while to attend his studio from Lon don, leaving town by an early train, and returning in the evening. The “ At Home” next week will be “ Mr. P. M. Laws at Newcastle-on-Tyne—Photography by Gaslight.” AIDS TO THE WORKING OF THE GELATINO-BROMIDE PROCESS: OR, CURES FOR FOGGING AND FRILLING. BY OAPT. W. DE W. ABNEY, K.E., F.R.S.* I wish, in my short paper to-night, to bring to the notice of the Society three points which it seems should be of use in that fascinating, but still imperfect process the gelatino-bromide, process. The first is regarding foggy plates and foggy emulsion. Now foggy plates may be due to foggy emulsion, and the latter may be due to one or two causes ; either through making use of improper light in its preparation, through an alkaline re action of the gelatine, through prolonged cooking or boiling, through an excess of silver, or through the use of ammnoia in its preparation. Foggy plates may of course be due to exposure to light, accidental or otherwise, The question arises, can the fog in all these cases be eliminated ? Now there would be no need to answer this question at all if (may 1 be pardoned for saying it?) photographers would occasionally read dry matter. This they object to do, as a rule, and consequently often miss a point which might be of use. Among the dry matter which has been skipped, or we should have heard of it before, is a paper I read two years ago, on the “Destruction of the Photo graphic Image, and on the Elimination of Fog from Collodio- Bromide Plates.” I should not have referred to this probably now, had I not come across several persons who did not seem to know how to utilise gelatine plates, and emulsion which veiled strongly or lightly, it matters not which. In my previous papers I pointed out how nitric acids, permanganate of potash, hydroxyl ozone, potassium bichromate, &c., would eliminate fog, and also destroy the undeveloped photographic image in collodion plates, and I have to once more revert to these oxidising agents. Now all acids (unless a precaution be taken which I shall point out presently) are out of the question for using with gelatine. Permanganate is effective for destroying fog, but it stains the film yellowt; hydroxyl, or peroxide of hydrogen, attacks the gelatine, and not the silver salt, and therefore is useless, but bichromate of potash remains, and this is an effectual eliminator of fog, or a destroyer of the photo graphic image. If then an emulsion is fog producing, squeeze it through coarse canvas iuto water containing two or three grains to the ounce of bichromate, and allow it to rest there four or five hours, and thou wash it for an hour or more. The plates prepared with such a treated emulsion will be free from fog, and lose no sensitiveness. Plates, whether exposed or merely foggy, yield to the same treatment, and give pictures which are bright in the shadows and wonderfully brilliant. If the bichromate be not washed out, the plates will lose about one-third of their sensitiveness and not more, since the bichromate is inactive in a dry state when not absolutely exposed to light. No emulsion now need be discarded on account of foggy plates being produced by it. I have prepared a whole batch of plates from such au emulsion, and used them, and found them as good as any plates that can be made. Some gentlemen, I believe, prepare plates which fog in red light, but not in the dark, and the coating must be a nuisance under such circumstances. If they will take my advice they will coat them in red light, when dried immerse them in bichromate and wash the bichromate, out in the dark (though the better plan is certainly to make emulsions which do not fog in reasonable light). Their troubles will then end, since development in the dark is easy, as a plate will not fog in subdued light when once the developing action is set up. It has been my custom to throw away plates that frill; I shall do so no longer, because there is a remedy for frilling which is an absolute certainty, as far as my own experience goes. It is simple in the extreme : before developing, coat the film with plain collodion, wash under the tap till greasiness disappears, and develop as usual. The image comes out just as well as without the film, and there is an utter mipossibility, as far as I * Head before the Photographic Society of Great Britain t The yellowness can be got rid of by washing, after fixing, with dilute hydrochloric acid. know, for frilling to take place. I was led to the discovery by attempting to intensify half a plato, after fixing and drying, by coating it with collodion emulsion, and then using the alkaline developer. The part of the film that was coated remained firmly on the plate, the other frilled off in a most aggravating form : other experimentsconfirmed me in this, and I now offer my experience for the benefit of my brethren. It will now be seen that dilute acid may be used to clear plates from fog, if they be first coated with collodion. I trust that I have shown that neither foggy plates nor frilling plates should be consigned to the waste tub, and thus an economy has been introduced. Another wrinkle is, that sulphate of quinine may prove a substitute for chrome-alum—a mere trace of it renders the film totally insoluble. Whether it will be of value or not remains to bo proved. THE ELECTRIC SHUTTER. BY MB. H. DAB WIN. The electric shutter designed by Mr. H. Darwin consists of two parts; first the shutter itself, that is the arrangement for opening and closing the lens ; and, secondly, the electro-mag net. The shutter consists of two arms of flat steel, W, W, which turn about a common axis, and which are joined together by a spiral spring, S', wound round that axis. A separate draw ing is given of this part, which shows the shape of the openings which are cut in these arms. Just behind the slot, for the stops, another slot is cut right through the lens, L ; through this slot these arms are pushed past one another, the spring tending to force them out again. Thus the shutter is not in rigid connec tion with anything ; it simply rests in this slot. The electro magnet, M, which is seen in the elevation drawing of the com plete instrument, consists of a pair of electro-magnets, which, when the current passes, attract horizontally a piece of soft iron, A ; this latter is connected by a right angle joint, H, with a thin spring or point, P. Consequently the spring point I falls when the current passes through the coils of the magnets; this spring point works in the grooves C, 0, in the upper parts of the shutter, which come above the lens. When the shutter is set, the two arms are forced together, and the spring point working in the grooves, rises into the two little catches, which ' overlap one another in this position, but which are seen at the ends furthest apart of the grooves in the left hand drawing. The lens is then closed, as the holes in the arms do not overlap one another in that position; but when the current pusses, the spring point is forced down into the free part of the groove, and the arms being thus released, the spring forces them across one another until the two round openings 0, 0, come exactly ' opposite each other at the centre of the lens. At this point, i when the lens is fully open, the spring point catches in the
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