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The photographic news
- Bandzählung
- 27.1883
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- 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. 1306, September 14, 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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brings us back to methylated spirit and its treatment. The methylated spirit sold by wholesale chemists may often be used for drying purposes just as obtained, but it is more desirable that it should be freed from any water it contains by distillation. After distillation it is equal to pure alcohol (ethylic alcohol is the scientific term) for many purposes for which photographers may require to use it,” and the great saving in cost, if much of it be used, is suffi cient compensation for the trouble taken. The best method is to place it in a retoit with a little more than an equal quantity of quicklime broken into small pieces, and allow it to digest for several hours before distillation. The best retort for the purpose is an ordinary tin can, such as work men carry their beer or tea in, with a piece of glass tubing leading out of the cork. A condenser is readily made with a yard or two of “ comro" gas-piping and an old tin pail or preserved meat tin. The retort is placed in a water bath, say a saucepan with a few pebbles in the bottom to prevent it coming into contact with the metal, and so receiving the heat direct. The cost of this simple piece of apparatus, and the trouble in using it, are very slight, and by its means the alcohol may be obtained in a highly rectified state. The methcd of obtaining its strength is exceedingly simple. Our readers have only to take a thin flask and make a mark on the neck with a file. This is placed in the scales (it is not necessary to use a chemical balance), and accurately weighed. It is then filled up to the scratch with the alcohol, and weighed, then filled up to the mark with distilled water, and again weighed. Deducting the weight of the flask in each case, the specific gravity of the spirit is its weight divided by the weight of the corresponding volume of water. The specific gravity thus obtained, the strength may be ascertained byareference to thetablss given at the end of any of the standard text-books. To dry a plate, after fixing and washing, we prefer to adopt the method by which we treat freshly coated plates, using a dipping bath in preference to a dish, the latter allowing too large a surface for evaporation to take place. The “ one thing necessary ” is to see that the plate is thoroughly washed. If this is not done, any hypo that may have been left in the film is far more perceptible, though it may not be more dangerous to the negative than in a plate which has been dried in the ordinary manner. After wash ing, the plate should be drained to free it from as much moisture as possible, and the back of the glass should be thoroughly wiped in order to keep back the deterioration of the alcohol as much as possible. If the operation is con ducted with care, this use of alcohol is a perfect boon. Another benefit accruing from the use of alcohol for dry ing plates has recently been put forward by Mr. Henderson, who expresses his opinion that density is thereby added to the image ; but this statement must be received with extreme caution. In the first place, as far as we at present know, there is nothing in the compositon and chemical properties of alcohol to lead us to believe that it can act as an intensi fier. Secondly, Mr. Henderson’s chief argument is not sufficiently convincing. This is bated on the fact, for it actually is the case in most instances, that if a plate be allowed to partially dry, and then be treated with alcohol, the part treated by alcohol is denser than the remainder. This experiment is not a sufficient test, for the part that tikes longest in drying is that which is most thickly coated, and which is naturally most likely to be densest. A fairer test is to take a plate that has been washed, immerse one half of it in alcohol, and then allow it to dry. We have done this both with iron and pyro development, and have failed to see that the alcoholically-dried part has gained density, and we believe that most of our readers who repeat this experiment will come to the same conclusion. different classes most used by photographers. At that time, as we wished our explanation to be as simple as possible, we omitted all mention of achromatism, and showed in the diagrams lenses without correction for chromatic aberration. It was our intention then to give shortly afterwards a set of diagrams of the same lenses, showing the means used to secure achromatism. Other matters intervened, and we left our diagrams on one side. The present time, when the admirable papers by Captain Abney are appearing in our columns on the subject of optics, seems a suitable one to place them before our readers. The papers of Captain Abney render it unneces sary fcr us to say more than a few words in explanation of the diagrams. Fig. 1. Figure 1 shows the bending of a pencil of light when it passes through a dense transparent medium, and shows how its course is changed when the surfaces of the medium are not parallel. As is well known to all, this bending causes those parallel rays which pass through a lens to meet together approximately at a point. Were the pencils of light only bent as a whole, chromatic aberration would not exist. They are, however, not only bent, but are dispersed—that is to say, are bent unequally. The violet rays are bent the most, the red the least. We see this in fig. 1, where the pencil, A B, instead of reaching H, is bent and dispersed so as to spread itself from C to D, where it gives rise to the beautiful pheno menon known as the spectrum. It is evident that this inequality of bending lenders it impossible that parallel rays passing through a lens formed of one glass shall focus at a point, but causes them, on the contrary, to focus in a series of concentric rings ; in fact, in a sort of circular spectrum. This is the defect known as chromatic aberration. Newton pronounced it incurable. He had, strange to say, over looked the fact that with different transparent media the degree of dispersion is not proportionate to the angle of refraction or bending. For example, flint and crown glass refract an average ray to about the same extent, but the H ACHROMATISM OF LENSES. About a year since we gave our readers a short descrip tion of what we consider lenses typical of the four Fig. 2. former disperses or splits up the ray to about double the extent that the latter does. It is thus possible to sorreet the defect. A lens is so constructed that each ray is rs bent far more than is required by a crown glass lens.
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