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The photographic news
- Bandzählung
- 7.1863
- Erscheinungsdatum
- 1863
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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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- http://digital.slub-dresden.de/id1780948042-18630000
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- Bandzählung
- No. 255, July 24, 1863
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Zeitschrift
The photographic news
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Band
Band 7.1863
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- Register Index 619
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Band
Band 7.1863
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350 THE PHOTOGRAPHIC NEWS. [July 24,1868. which, at his request, we stood, only occupied about as many minutes, although the exposure was fourteen seconds. The lens employed was a French one of about twelve inches focus. The distance of the sitter for card portraits, full-length figures was, so far as we could measure by the eye, about thirty feet. We may here remark, however, that a custom is beginning to prevail in Paris of avoiding full- length figures in portraits of gentlemen. Instead of the full-length, about three-quarters of the figure are given, the great difficulty of arranging, with pictorial effect, the trousered legs of the male figure is thus avoided. The dis tance for these sizes was about twenty feet. One lens only was used, but the plates were twelve by ten, and contained eight on one plate. To English photographers it will pro bably appear odd that plates of this size should be used whilst two or three negatives only of each sitter are taken, several different persons being thus taken on one plate. Notwithstanding the celerity of the operations, it must often happen that the arrangement and posing of two or three different persons, must occupy a space of from twenty minutes to half an hour, during which time the sensitive plate, in very hot weather, apparently keeps in good condi tion, as we saw no signs of stains or other bad results. Entering the dark room, another circumstance strikes us as novel to English photographers : the nitrate bath is not upright, but a large flat dish. The solution is tilted to one end, and the plate placed, face upwards, on the bottom of the dish, which is then lowered into the horizontal position, and the solution brought in an even wave over its face. After standing a few seconds the dish is well rocked, so as to equalize the action, and prevent the settlement of any loose particles of dust, &c., from becoming attached to the film. The collodion is freely bromized, more so, however, in winter than in summer. The silver bath for summer use is about thirty grains to the ounce, or six per cent. In winter it is used stronger. The developing solution used contains twenty grains of protosulphate of iron, and twenty minims of acetic acid to the ounce of water. The negative produced by these solutions give a tolerably vigorous image with the iron alone ; but when further intensifying is required it is effected by an application of the same iron solution, to which a few drops of a fifteen-grain silver solution is added. M. Disderi informed us, referring to the developer, that he had never used pyrogallic acid for development at all, having em ployed iron from his first trial of the collodion process. The negatives we saw were all clean, brilliant, soft and well modelled, possessing, in fact, those qualities which have made the name of Disderi celebrated all over the world. • Stientitir Gossip. WHITE LEAD AND ZINO WHITE IS ANYTHING OPAQUE ?—LIGHT ONLY EXISTS IN THE EYE. Every artist who has been troubled with the rapid sulphura tion of white lead, and has been obliged to resort to the employment of zinc white in order to prevent the high lights of his painting from gradually darkening, has noticed that whilst equally white in appearance the white lead possesses considerably more body than the zinc white. Whence arises this difference? What is the meaning of the term body as applied to pigments ? How is it that of two powders, appa rently equal in their colour and brilliancy, one looks feeble and transparent when employed as a paint, whilst the other shines out with redoubled vigour ? Such questions as these cannot fail to have suggested themselves to many persons, and have elicited a variety of answers. Mr. Barnard 8. Proctor has lately made these and other similar questions the text for some very interesting as well as philosophical reflections upon some phenomena of light which have struck us as being so valuable, both on account of the recondite facts they deal with and tie popular way in which they are discussed, that weare induced to give a brief outline of them to our readers. “ As white as fine linen, flour, chalk,” “as white as snow,” are frequent comparisons, but they are all dull examples as compared to many chemical precipitates. Precipitated chalk far outshines the natural varieties, and fine qualities of magnesia carbonate surpass this. Microscopic examination indicates that this latter consists of particles, clear and colourless, but very minute. White lead consists of particles equally minute and also transparent, but of a yellow brown colour by transmitted light; consequently, when seen in bulk it appears of a less pure white. But magnesia cannot be used as a pigment because it possesses no body, and the difference between the white lead and the magnesia in this respect depends upon the different refractive powers of the individual particles which compose the separate powders. They are both transparent in their individual particles, but the magnesia is more so. They are both bodies possessed of considerable refractive power, but the lead is more so. When air intervenes between their particles the reflective power of both so much exceeds that of air that they are highly reflecting and very slightly transmitting ; but the less absorbing power of the magnesia makes it the whitest—the more reflecting of the two. But when oil in tervenes, as would be the case if they were used for pigments, the refractive power of the magnesia so nearly coincides with that of the oil that much transmission and little reflection is the result, and this constitutes what painters call want of body. But the lead so greatly exceeds the oil in refracting power that its reflective property is not much interfered with, and even with its greater absorbing power it reflects much and transmits little light; and this is what painters call great body. Another question which must often occur to our readers, and which has been ingeniously discussed by Mr. Proctor, is, whether anything is opaque. This can only be answered inductively from the results of numerous observations. Glass is only approximately transparent, for, if looked through edgeways, we find it stops a great deal of light. It is not so transparent as pure water, and even this, as has been shown by Tyndall, has a blue-green colour when light is passed through fifteen feet of it. Even air is far from trans mitting all the light which enters it. A comet is almost incalculably more transparent than the earth’s atmosphere. The light of a star passing through hundreds of thousands of miles of a comet’s atmosphere and nucleus, loses less light than in passing through the thin stratum of air which covers the earth ; yet even the comet is imperfectly transparent, and wo do not know whether even the luminiferous ether itself allows the passage of light without some loss, but we know that glass is as much opaque compared with it as gold is when compared with glass; and from this we readily learn to believe that transparency and opacity arc only comparative terms— that nothing transmits all the light, and nothing is entirely impervious to light; and this supposition is confirmed by experience. Even metals, which are usually taken as types of opacity, transmit light in thin films; and each metal has a proper colour of its own : thus, gold leaf viewed by trans mitted light looks green, brown, violet, red, purple, or blue, according to the thickness of the film; silver leaf is grey violet, purple, or brown; copper is green, antimony is grey, arsenic is brown, platinum grey, palladium grey, rhodium blue or brown, and charcoal grey. These illustrations show that most bodies transmit a coloured light, the colour deep ening as the thickness increases, until it is so dark that we call it opaque. Not long ago light was believed to be reflected from the surfaces of bodies. And now it is only when we are on our guard that we bear in mind the thick ness of matter required to reflect a ray. We know that in a soap-bubble we often see patches so thin that they do not reflect light, though they arc still possessed of two surfaces. Faraday observed that some of the gold films he experi mented with when reduced very thin by chemical means, lost part of their reflecting power, though they continued to be free from any material injury to their surface or integrity,
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