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The photographic news
- Bandzählung
- 29.1885
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- 1885
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- Bandzählung
- No. 1384, March 13, 1885
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The photographic news
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Band
Band 29.1885
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- Register Index III
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165 Iodide of Silver in Gelatine Emulsion.—When iodide of silver is precipitated with excess of potassium iodide, it is several hundred times less sensitive than the bromide. With very long exposure, and development by alkaline pyrogallic acid, a weak image in the neighbourhood of G is obtained, and by a still longer exposure this stretches out towards H and F ; the maximum being between G and F, curve 6 (Fig. 7). If ready-washed bromide emulsion is mixed with from ten to fifty per cent, of washed iodide emulsion, the sensi tiveness of the mixture to white light sinks somewhat, but, on the other hand, the clearness of the plates becomes greater, and the tendency towards halation becomes less. In the spectrum there now appear two maxima; one between G and H, and another between G and F, this latter being generally the stronger of the two. Between these maxima is found a point of minimum action. The action of the spectrum on the mixture of gelatino-bromide and gelatino-iodide is shown by curve 7 (Fig. 7). The behaviour of mixed iodide and bromide, pointed out by Abney, and clearly made apparent in the course of my experiments, left it an open question whether these two maxima could be made to merge into one by prolonged digestion, or whether they would retain their individu ality. My own experiments show that the separated maxima made with the mixture of the two emulsions (bromide and iodide) lose their identity when the mixed emulsion is digested for a period of half-an-hour to one hour. A similar result is obtained when the two haloids are formed together by adding silver nitrate to a gelatinous mixture containing potassium bromide, with potassium iodide, and digesting the emulsion. The sensitiveness of such an emulsion towards the less refrangible rays of the spectrum was found to be greater than of a pure bromide emulsion, and the maximum of action is shifted a little towards F ; this being shown in curve 8 (fig. 7). The best mixture to use is one containing the haloids, in the proportion of one molecule of iodide to from eight to twenty molecules of the bromide. Although the emulsion, rich in iodide, may be more sensitive to the green, it must be remembered that the emulsions poor in iodide give more vigorous and better images. The photographic behaviour of the iodo-bromide emul sion forces us to the supposition that a kind of double salt containing the two haloids is formed ; a supposition made more reasonable by the fact that a mixture of equal mole cules of chloride of silver and iodide of silver melts at a much lower temperature than its constituents. According to Kohlrausch, the melting-point of silver chloride is 485° Centigrade, that of silver iodide 540°, and that of chlor iodide 260°. The action of heat and that of light thus jointly point to the probability of a true combination. It must also be remembered that the existence of the crystal lized bromo-chloride of silver, which is known to miner alogists as embolite, must be taken into consideration. Chloride of Silver in Gelatine Emulsion.—When ferrous citrate or ferrous oxalate is used as a developer, this salt is less sensitive than bromide of silver to white light. Curve 1 (fig. 6) shows the spectrum image on a gelatino- chloride emulsion, the photograph being produced in the large Steinheil spectrograph. The maximum is close to the boundary of the visible violet near K H, and the action extends from N to F. A mixture of chloride of silver emulsion with ten to thirty per cent, of iodide emulsion gives two separate maxima ; one of these is near II (chloride), the other between G and F (iodide), and the whole will be made clear by an inspection of the curve 9 (fig. 7). By digestion or boiling, these maxima become united as in the case of the brom-iodide emulsion, only not so easily, and not in every case. Chlor-iodide emulsion bears stronger developers than a simple chloride emulsion, and the sensitiveness of the chlor-iodide approximates more nearly to the sensitiveness of bromide, but without reach ing that of the latter. (To be Continued.) WALTER B. WOODBURY. One of the familiar names in the history of photography is that of Woodbury ; in a work by a French writer, Davanne, this name is placed next in rotation to those of Talbot, Daguerre, and Niepce ; and Mr. Woodbury’s most notable achievement is the invention of the permanent printing process which bears bis name. For a score of years, photographic and other scientific literature, also the records of the Patent Office, bear witness to the wide range of his knowledge, and to the ingenious activity of his inventive powers. It is doubtful, indeed, if the inventive faculty has not been too powerful for his material interest, impelling him on new lines of research before garnering a harvest from seed earlier sown. Photographic inventors have rarely won any conspicuous reward; the field open to them scarcely admits of great financial success. Such a com plete invention as the Woodburytype, for example, became practical only through a long series of experiments, by which each stage of operations is perfected ; the labour employed must be trained to manipulative skill, and half the term of a patent may expire before success is suf ficiently assured to attract capital, by which the process may be developed into a profitable industry. The subject of notice has had his successes and his dis appointments, and taken each with equanimity. Recently, however, his health became seriously undermined before its cause was suspected ; and without health, the prosecu tion of his business enterprises was impossible. The crisis of affairs being serious, and becoming known to his friends, the regard, sympathy, and good will towards Mr. Woodbury found expression in the shape of an influential committee, determined to appeal to the photographic and literary world for subscriptions to a fund which should afford him means and leisure to recover his health; and therefore the opportunity of establishing his stannotype process, and completing improvements in block-printing, which he has begun. In the advertising columns of this journal will be found the names of the gentlemen forming the committee, and the first list of subscriptions received. Mr. Woodbury was early attached to photography, and at about the age of seventeen determined to carve out a career for himself, and started, with a modest photographic outfit, for Australia, in 1849. Here he acquired for him self mastery over his materials, and in a few years we hear of him in Java, single-handed, inprovising a studio, struggling with the difficulties incident to a tropical climate, and creating quite a sensation with excellent por traits, produced, from beginning to end, with his own hands. The first month, working twelve hours a day, realised a considerable sum of money, but this eventuated in a serious illness of three months. In 1859 he returned to England with a series of exquisite views taken in the Island of Java; these were purchased by Negretti and Zambra. In 1860 Mr. W oodbury returned to Java, and estab lished himself in the capital, Batavia. Those who remem ber the difficulties that beset the collodion process five- and-twenty years ago will appreciate the obstacles to be overcome in a hot and humid climate, with a vertical sun in the midday hours. For success a fund of resource was necessary, and Woodbury was equal to the task. Health giving way under the climate, he returned to Europe, and having already in his mind the genius of the process which bears his name, he determined to give his faculties free play, and devote himself to invention. From 1864 to 1884, Mr. Woodbury took out no less than twenty patents, some, of course, dealing with the
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