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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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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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- Bandzählung
- No. 1724, September 18, 1891
- Digitalisat
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The photographic news
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Band 35.1891
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Band 35.1891
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September 18, 1891:] THE PHOTOGRAPHIC NEWS. 651 they will be astonished at the vast amount of detail it will bring out; and when I say detail I do not mean fog, but detail exactly the same as would have been obtained by a prolonged exposure. Think of this when a cross baby or a restive animal is to be taken, and any negative of which you have the slightest suspicion as regards fulness of exposure, hold up to your dark room window for twenty or thirty seconds before you develop, and then proceed as for a fully exposed plate. Sulphite of soda in the developer acts, to a certain extent, as a restrainer, and whenever it is used care should be taken in apportioning the bromide, or the result will be a negative which, while it assimilates in colour to the old wet plate, will have very harsh lights and clear shadows, very much after the style of a bad flash-light picture. The mention of flash-light pictures reminds me that winter is at hand, and that ere many months—or very many weeks, for matter of that—are gone, the flash-lamp will be in use again. Already the shades of evening come upon us so early that those of us who have no flash-light apparatus are often compelled to send a late sitter away. However, I opine that the coming winter will see the apparatus in nearly every studio except those which have an electric light installation; and, if not out of place in such an article as this, I would advise all intending pur chasers to avoid spirit lamps. I have used James’s lamp, in which the magnesium is burned in a gas flame, for the past three winters, and have been more than satisfied with the results. One reason for speaking of flash-lamps is that pictures taken by their aid require some considerable modification of the developer, and probably the easiest way of effecting this is to use just half the usual quantity of bromide in the stock solution. A formula which I have found very useful for this class of picture is the following: A.—-Pyrogallic acid (Schering) (Bromide ammonium - or (Bromide of potassium ... Water ... ... ... B.—Ammonia ... Water C A Water To develop, take equal parts of B and C, which will, in ordinary cases, ensure a soft and well graduated negative, although different proportions of B and C will be required for under-exposed or over-exposed plates, or for subjects introducing white draperies, or dead black dresses, &c. The judgment of the operator will, in most cases, tell him what proportions are required, and if the first appearance of the image should prove he has miscalculated, he should not try to improve the solution he is using, but throw it away and commence again with fresh. One last word, and that is, to obtain clean negatives, the measures and dishes in use for developing must be kept clean ; water alone is not sufficient for this purpose. They should be occasionally cleansed with dilute hydro chloric or nitric acid, preferably the former. The Photochronograph. — Bev. George A. Fargis, S.J., assistant director of the Georgetown College Observatory, at Washington, D.C., has invented an instrument which he calls the photochronograph, and which he claims to be capable of registering the time of star transits to the one-thousandth part of a second. Should his instrument prove thoroughly accurate and reliable, as is believed it will, he will have the satisfaction of having taken a long stride toward the perfection of photographic records.—Anthony's Bulletin. THE RATE OF EXPLOSIONS IN GASES.* The use of compressed gases is now so much identified with photographic work, that the following abstract of a paper brought recently before the Royal Institution of Great Britain by Professor Dixon, F.R.S., of Owen’s College, Manchester, cannot fail to be of interest to many of our readers. The rapid act of chemical change which follows the kindling of an explosive mixture of gases has of late years attracted the interest both of practical engineers and of theoretical chemists. To utilise for motive power the expansive force of ignited gases; to minimise the chance of disastrous conflagrations of fire-damp in coal-mines ; to follow the progress of chemical changes under the simplest conditions, are some among the problems presented to us in industry or science, demanding for their solution a knowledge of the phenomena of the explosions of gases. To understand the nature of explosions in gases, it is necessary to know certain fundamental properties of the explosive mixture. With this object in view, experi menters have sought to determine for various mixtures of gases—the heat of chemical combination, the temperature of inflammation, the pressure developed, and, lastly, the rate at which the explosion is propagated under different conditions. It is on the last of these problems—the determination of the velocity with which the flame travels through the gas—that I have been asked to speak. Twenty-four years ago Bunsen described a method of measuring the rapidity of the flame in gas explosions. Passing a mixture of explosive gases through an orifice at the end of a tube, and igniting the gases as they issued into the air, he determined the rate at which the gases must be driven through the tube to prevent the flame passing back through the opening and exploding inside the tube. By this method he found that the rate of pro pagation of the ignition of hydrogen and oxygen was thirty-four metres per second, while the rate of ignition of carbonic oxide and oxygen was less than one metre per second. Bunsen applied these results to the rate of explosion of gases in closed vessels, and his results were accepted without cavil for fourteen years. By 1880 facts began to accumulate which seemed in consistent with Bunsen’s conclusions. For instance, between 1876-80 I had several times observed that the flame of carbonic oxide and oxygen travelled in a long eudiometer too quickly to be followed by the eye. Mr. A. V. Harcourt, in his investigation of an explosion which happened in a large gas main near the Tottenham Court Road in 1880, was led to the conclusion that the flame travelled at a rate exceeding one hundred yards per second. In the winter of 1880-1 I noticed the rapid increase of velocity as a flame of carbon bisulphide with nitric oxide travelled down a long glass vessel ; and shortly afterwards I attempted to measure the rate of explosion of carbonic oxide and oxygen by photographing on a moving plate the flash at the beginning and end of a long tube. The two flashes appeared to be simultaneous to the eye, but no record of the rate was obtained, for the apparatus was broken to pieces by the violence of the explosion. In July, 1881, two papers appeared in the Comptes Hendus, one by M. Berthelot, the other by MM. Mallard and Le Chatelier. Both papers announced the discovery of the enormous velocity of explosion of gaseous mixtures. * Chemical Nets. .. 1 ounce .. } » .. I , .. 7 ounces .. 1 ounce .. 40 ounces .. 1 ounce .. 20 ounces
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