The photographic news

• Bandzählung: 29.1885
• Erscheinungsdatum: 1885
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
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• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
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164 THE PHOTOGRAPHIC NEWS. [March 13, 1885. spectrograph is used, the dotted line showing the result when a short exposure is given ; while in the case of the small direct vision instrument the maximum lies between G and F. Indeed, the difference between the two curves Fig. 6. SPECTRA ON SILVER GELATTNO-CHLORIDE. 1. With large Steinheil spectrograph. 2. With small direct-vision in strument. is so great that one can scarcely believe that the same silver salt is being dealt with. Even when the large spectrograph is used, the maximum of action shifts to a very notable extent, according as the violet or the bluish-green is in the middle of the field, and this result throws doubt upon some of the distinctions which have been drawn between various modifications of bromide and chloride of silver. It is, however, a striking circumstance that, as far as my own observations go, the sensitizing action of red, yellow, and green colouring matters is so energetic that the maximum is shown in the same place, whichever apparatus is used ; but as experi ments about to be described were all made with the large instrument, they are unquestionably comparable among themselves. Bromide of Silver in a Gelatine Emulsion.—When a solu tion of ammonia silver nitrate is mixed with excess of ammonium bromide in the presence of gelatine, a finely- divided form of silver bromide is formed, and this silver bromide is red when viewed by transmitted light. A plate coated with this emulsion and dried, shows, after short exposure to the solar spectrum and development with ferrous oxalate, or with alkaline pyrogallic acid, an action from the violet to the bluish green (from H to a point near F), and the maximum of action between G and F. If, however, a longer exposure is given, the action will extend in both directions; as, for example, from M to E. The curve 3 (Fig. 7) gives a representation of the action of light on such a plate as that referred to ; the dotted line showing the action with short exposure, and the solid line showing the action when a longer exposure is given. When the above-mentioned ammoniacal emulsion is digested at a temperature of 30® to 40% Centigrade for half an hour, or it is boiled, the silver bromide passes into the finely-divided granular condition. Under these circumstances, the grains become larger, and a thin layer of the emulsion allows blue light to pass through, while the general sensitiveness to white light is considerably ex alted. At the same time, the sensitiveness to the violet and green of the spectrum increases, while the maximum of sensitiveness shifts a little towards F, taking a position about midway between G and F. Notwithstanding this, the action between G and F appears to be less energetic than before ; the curve representing the action of the spectrum becoming flattened down in this locality. Curve 4 (tig. 7) shows this, and it will be seen how the action extends from D to N (or even farther) when the exposure is prolonged. The dotted line shows the result of a short exposure. When the digestion is prolonged to the extent of from three to ten times that referred to as necessary to pro duce the cited result, the condition of the silver bromide approaches towards decomposition, the curve obtained on its exposure to the spectrum changing at the same time. It becomes still more flattened, gives no easily recognizable maximum, the photographic image lacks vigour, and a notable exaltation of sensitiveness is the result, not only with respect to the white light, but also towards the less refrangible rays of the spectrum. Curve 5 (fig. 7) shows these differences.* Gelatino-bromide emulsions having the capability of giving results corresponding to the curves 3, 4, and 5 may be readily produced at will. That corres ponding to curve 2 is produced with most certainty when the silver bromide is formed in the presence of a large proportion of gelatine (e. g., 5 per cent, of the fluid), and a minimum of ammonia is used in conjunction with short digestion. An emulsion corresponding to curve 4 is pro duced by a moderate digestion of that giving curve 3. Most commercial gelatino-bromide plates give analogous results in the spectrum, and such plates are best adapted for general spectral work, such as is detailed in this paper. It is therefore unnecessary for me to apologize for giving working details for making the required emulsion. Thirty grammes (463 grains) of silver nitrate are dissolved in 250 cubic centimeters (8 oz. 7 drachms) of water, as much ammonia is added as will serve just to redissolve the precipitate first formed. In another vessel 22 grammes (339-5 grains) of ammonium bromide, 0-3 grammes (4} grains) of potassium iodide, and 40 grammes (617 grains) of hard Winterthur gela tine, are dissolved in 250 cubic centimeters (8 oz. 7 drachms) of warm water. The first solution (silver) is now added gradually to the gelatine solution, and care must he taken to thoroughly agitate during the time of mixing, and during the operation the temperature of the solutions should not exceed 30° Centigrade (86° F.) The emulsion is now digested for half an hour at a temperature of from 30° to 40° Centigrade (86° to 104° F.), after- which it is poured out into a dish and allowed to cool. In about six or twelve hours time it is cut up into small shreds, and then washed for ten hours in running water. It can now be melted, and plates may be coated; but if it should be necessary to remove some of the water, this may be done by soaking the shreds of emulsion in alcohol. The emulsion will keep months if immersed in alcohol. The small amount of iodide present makes the images clearer than they would other wise be, and prevents the spectral lines running into each other ; but no drawback seems to result from this use of iodide. The first experiments on the action of colouring matters were made without iodide in order not to complicate the result. A gelatino-bromide giving results corresponding to curve 5 may, as already mentioned, be made by a pro longed digestion of the preparation just referred to, but more easily when the amount of gelatine present during the time of the digestion is reduced to lhe proportion of half per cent, of the fluid, the rest being added immedi ately before the cooling. A silver bromide in a similar condition is contained in emulsion made by Henderson’s “cold emulsification method.” • Photometric experiments describe.’ in my Ausfuhi-liches Handtuch dor Photographie t yo\A; page 183, prove that bromide of silver in this condition gives a developable image with a very short exposure, but the intensity of the image does not increase proportionally with the action of the light. The action soon reaches a maximum which cannot be exceeded, consequently modelling is lost in the high lights,