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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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- Englisch
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- F 135
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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/id1780948042-18830000
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- Bandzählung
- No. 1279, March 9, 1883
- Digitalisat
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Zeitschrift
The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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146 THE PHOTOGRAPHIC NEWS. [March 9, 1883. For portraits, also, the single lens may be used, and especially for large heads. In portraiture the limitation of angle is still more desirable than in landscape work, and the largest aperture which will define at all will also cover through the widest angle which ought to be used in portraiture. In the case of large heads it is by no means uncommon to use a portrait lens, and to stop down to a smaller aperture than a single lens will define with, the reason being that the long focus used necessitates a small aperture to give definition in more than one plane. With { the exposure in a studio should not exceed ten seconds. The aperture may occasionally in special cases be increased beyond Y,. In fact, it may be increased to give sufficient spherical aberration to produce exactly the same effect as is produced in a diffusion of focus lens. We have, it will be seen, been considering only such forms of single lenses as are not specially constructed for instan taneous work, but it is quite possible that by modification of the curves a single achromatic lens might be constructed which would give definition through a limited angle with even a larger aperture than the one which we have in dicated; in fact, we have lately noticed advertisements of a single lens to work at {,. We have not had an oppor tunity of judging of the definition given by this lens, but if it is good, certainly we have here a remarkable step in the right direction. THE BEHAVIOUR OF CHLORIDE AND BROMIDE OF SILVER WHEN BOILED IN MUCH WATER. AND THEIR PRECIPITATION FROM CONCEN TRATED SOLUTIONS. Lately a good deal of attention has been paid to the method of preparing emulsion by boiling it in a large quantity of water in the presence of a small quantity of gelatine. Very good results are obtained by this plan, which, as our readers are aware, is advocated by Mr. Burton, who carries the dilution to a very wide extent. What are the advantages of boiling in a large quantity of water ? Greater sensitiveness is usually attributed ; but there is another not less important point that we desire to touch upon, and upon which photographers set great store. In boiling with much water and little gelatine, there is less chance of producing fog than when we have to do with little water and a great deal of gelatine. In the Burton method, it is well known, the boiling goes on for hours without any tendency to fog, but it would be very unsafe to treat an emulsion rich in gelatine in the same manner. When bromide of silver is boiled under these circum stances—that is to say, with a good deal of water, and in the presence of a very little gelatine—the silver salt is reduced in a very slight degree; but how is it when only pure water is employed? Is the silver salt reduced at all when boiled in pure water? This is a question which has not yet been solved photographically. It is, namely, very difficult to produce a bromide of silver emulsion in pure water ; nearly always there is formed a curd-like or grain-like precipitate which refuses to emulsify under any circum stances. But we can nevertheless point to an analogy. We know, at any rate, that chloride of silver is reduced on boiling with pure water. Cooke has found that freshly-precipitated flocculent silver chloride gives up a little chloride of silver to boiling water, and that on repeated boiling less and less of the chloride is dissolved, until at last, when it has passed from the flocculent to the grain-like condition, all dissolution leaves off. Of 1-4561 grammes of chloride of silver employed by Cooke, 0 2241 of a gramme was dissolved in 66 litres of water. The residue obtained from this water, on being dis solved in ammonia, gave more than 0001 gramme of metallic silver. If, therefore, during the operation of boiling, all light was carefully cut off, we must conclude that hut water exerts a slight reducing action upon chloride of silcer, In all likelihood, however, the action of pure water is weaker than that of a solution of gelatine. Some little time ago, a practical photographer of experi ence told us that he obtained an accession of sensitiveness and vigour in emulsion if he dissolved the nitrate of silver in but very little water, instead of a large quantity, and then added it to the bromo-gelatine solution ; while some photographers there are who make a practice of adding the silver in crystals. Now we may point to an analogy in chemistry that seems to bear out this view. Ruyssen and Varenne have found that chloride of silver is dissolved the more easily in dilute hydrochloric acid, &c., the more readily it is precipitated from a concentrated solution of nitrate of silver. The same holds good also in the case of sub chloride of mercury and chloride of lead. So, it may well be inferred that the same also takes place with bromide of silver, and if more silver bromide is dissolved in an emulsion, it becomes more sensitive. Apparently, we find also in tbe realms of pure chemistry an explanation why it is not desirable to mix together perfectly equal equivalents of bromide of potassium and nitrate of silver. As a matter of fact, many chemists have shown that bromide of silver is most difficultly soluble in pure water, while in water containing very little soluble bromide or nitrate of silver, it is altogether insoluble. On the other hand, singularly enough, in water containing much soluble bromide or nitrate of silver, bromide of silver is soluble to a material extent. If, then, with this fact in our mind, we remember that photographers have a liking for preparing gelatine emnlsion with a large excess of solu ble bromide, and collodion emulsion with a large excess of silver nitrate, we can scarcely regard the concordance of the phenomena as accidental. PHOTOGRAPHIC ANALYSIS. Photographic analysis is just like spectrum analysis. It is a quick and ready means of examining bodies and dis covering their nature—or, rather, natures. It is con cerned, however, with the analysis of mind rather than matter. Still, in its main aspect, photographic analysis may well be compared to spectrum analysis, for in the latter, as in the former, it is necessary but to bring the object to the touchstone of ordeal, in order to learn the result. You heat a body in spirit flame or blow-pipe, and, presto I its composition is at once set before you in the spectroscope —in an instant the whole story of its composition or character is laid bare. So it is with photographic analysis; the sensitive plate is introduced into the camera, you expose, and at the instant of your uncapping, the result is apparent. Perhaps we had better explain ourselves a little more. As we have said, it is not in the analysis of matter that photography assists, but in the analysis of character. We maintain that, as in the case of the spectroscope, you have merely to bring the photographic lens to bear upon your chosen body or bodies, and, in an instant, we have valuable assistance in divining their character. In the studio, where there is but a single sitter, and he is under the hands of the photographer, no correct reading can be obtained: so many things tend to interfere with the truth of the research, that it is not worth having as a result of photographic analysis. But it is different in groups, and especially in the case of groups photographed out-of-doors ; then, if the photographer does not interfere, and has only time to give but a few rapid instructions as to posing—and he can seldom do more—we get valuable results in respect to the reading of character, which afford the psychologist infinite study. The best groups to examine are those of well-to-do peoPli In fact, photographic analysis cannot be applied to its fu
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