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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
- 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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- Public Domain Mark 1.0
- URN
- urn:nbn:de:bsz:14-db-id1780948042-188300004
- PURL
- http://digital.slub-dresden.de/id1780948042-18830000
- OAI-Identifier
- oai:de:slub-dresden:db:id-1780948042-18830000
- Sammlungen
- Fotografie
- LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
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- Parlamentsperiode
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- Wahlperiode
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- Bandzählung
- No. 1294, June 22, 1883
- Digitalisat
- SLUB Dresden
- Strukturtyp
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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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- Titel
- The photographic news
- Autor
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June 22, 1883. | still adhering to the photograph ; the liquid is completely removed by wiping off and rubbing the photograph either with a cloth or with soft paper, and, after exposing it to the air for some minutes, the photograph is ready to receive the paint, which will give it the required colours or shades. It is at this stage that the use of the complementary apparatus called an “ Oven,” and hereinafter described, becomes indispensable. This oven is constructed with double walls, and is heated to a suit able temperature by means of hot water, steam, or gas, and permits the complete desiccation of the paint in a relatively short time, and establishes, by a slight fusion, the most intimate combination of the paint and the material of which the photo graph is composed. This oven is of cylindrical form, and has two casings, and the hot water, steam, or hot gas circulates between the wall of the oven and its first casing ; between the first casing and the second casing is placed a covering of felt or other non-conducting material. At each end are doors which can be opened or shut as desired, and which, in order to avoid loss of heat, are lined inside and outside with non-conducting material. In the interior of the oven are fixed, horizontally, partitions of perforated sheet iron, or of wire-gauze designed to receive the photographs to be dried. In order to extract the volatile matters contained in the paint or in the photograph, tubes are passed through the upper part of the two casings to establish communication between the interior of the oven and the external atmosphere. In order to cause a current of air in the. interior of the oven, other tubes are passed through the lower part of the two casings. If the oven is heated by hot water, the latter is caused to enter at the bottom, and be dis charged at the top, by means of pipes provided with stop-cocks, and passes only between the two casings. If it is heated by steam or gas, the latter is introduced at the top, and discharged at the bottom. In this oven the photographs ate dried, and, by a sufficient temperature, the material of which they are com posed is softened to such an extent that the paint mixes with it so intimately that the two are entirely blended at the point of contact. It is this intimate combination produced by the heat which ensues (after the photograph has been a suitable length of time in the oven) the success of the operation, and produces the permanency or durability of the photograph, notwithstanding variations of temperature and climate. It is only necessary to put it under a press and finish it by the ordinary processes, I can dispense with the oven, if desired, and arrange, within a chamber, a heating and ventilating apparatus, by means of which the air can be raised to a very high temperature. The different parts of the apparatus above described can be made of any suit able material, and of any desired form and dimensions. Having thus fully described the said invention as communicated to me by my foreign correspondents, and the manner of performing the same, I wish it understood that I claim : First. The above described improved method or process con sisting in the employment of the bath, the table, and the oven, with the aid of heat, for the production of coloured photographs, all substantially as, and for the purposes, set forth. Second. The above described improved apparatus for the pro duction of coloured photographs, which apparatus consists of the bath, the table, and the oven, all constructed substantially as set forth for the purposes specified. Third. The employment of heat for fixing the photograph, and rendering the oil colour permanent by a rapid artificial desiccation, substantially as set forth. LESSONS IN OPTICS FOR PHOTOGRAPHERS. BY CAPTAIN w. DE W. ABNEY, R.E., F.R.S. Lesson I. It may be considered axiomatic that a ray of light travels in a straight line when the substance through which it travels is homogeneous—that is, does not vary in density. When the medium through which it travels varies in density, then the ray will be bent from its original course; and even when the medium is uniform, part of it may still be bent by diffraction, which will be explained subse quently. It may appear that I am upsetting all ideas of geometrical optics in stating this, but it is a fact neverthe less, and a fact that is usually ignored when treating of the subject of lenses. The reader must not go away with the idea that high mathematics are necessary to understand dif ¬ fraction. I hope to be able to put it in such a way that a beginner may grasp it. The simplest case of the formation of an image is one in which a lens is absent, where it is formed by a pin-hole in an opaque plate. The experiment is worth trying. Replace the ordinary lens of a camera by a card, and with a pin prick a hole in it, and then view the image of a landscape on the focussing screen. No doubt it will be a very dull image, but it will be there, and be tolerably sharp, but not quite. Let us stop for a moment to see how the image is formed. Fig. 1. Suppose we examine a candle flame, the ray of light coming from the point of the flame will pass through the hole, and completely fill the hole, and instead of a point of light from the tip, we shall have a disc of light, wider, of course, than the hole. The same happens when we take any other point in the candle flame ; it, too, will be represented by a circular disc of light. Now all these discs of light will overlap one another, and we shall have an image of the candle and its flame; but instead of the edges being sharp, they will be slightly fuzzy, owing to the overlap of these discs. The fuzziness in the first place will evidently depend upon the diameter of the pinhole used, but besides this, there is another cause for it in the fact that the rays striking the edge of the hole are bent to a small extent, the amount of which will be shown in a sub sequent lesson. I mention this, as it would leave a wrong impression to state that the diameter of the hole is the only cause of want of sharpness in an image formed by a pin-hole. We shall see by-and-bye that when the pin-hole is large, this is of very little consequence ; but when it is minute, the want of sharpness may be principally due to this bending. Now another point to attend to in this pin-hole formed image is, that it is always in focus ; that is to say, that the candle and the screen on which the image is thrown may be shifted further, and that there is no very great alteration in the sharpness—-or, as photo graphers say, the definition—of the image. A little thought will show that there is no reason why it should ; more particularly if the image thrown on the screen be a land scape, as then the points of light are so far off that the discs representing each point of light remain of very nearly the same diameter at all feasible distances. When we make a large image by withdrawing the focussing screen a good distance, the sharpness may appear to gain, because the discs of light will become proportionally smaller to the image which is depicted. Thus, if we had a flagstaff in the distance, which if focussed by a lens would be shown as % of an inch across at a distance of one foot from the lens, a pin-hole of 2% of an inch at the same distance would make it appear 1 of an inch broad on the whole. If, then, the lens were changed, and an image thrown on the focuss ing screen at two feet distance, the breadth of the flagstaff would naturally appear 1 of an inch, and in using the same pin-hole it would be % and * of an inch, or 2 of an inch. In the first case the fuzziness would extend half the breadth of the flagstaff, and in the latter only | the breadth. Thus there is a slight gain in sharpness by making the distance from the pin-hole to the focussing screen as large as possible. The drawback to this is of course the great diminution in light. Let us see how this diminution arises. Let C be a candle-flame, H a card with a hole in it, A and B two screens on which to receive the image, the latter twice as far away from the hole as the former ; then it is evident that the light passing through the hole in each case is always the
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