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The photographic news
- Bandzählung
- 12.1868
- Erscheinungsdatum
- 1868
- 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
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- urn:nbn:de:bsz:14-db-id1780948042-186800009
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- http://digital.slub-dresden.de/id1780948042-18680000
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- oai:de:slub-dresden:db:id-1780948042-18680000
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- LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 530, October 30, 1868
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Zeitschrift
The photographic news
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Band
Band 12.1868
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- Titelblatt Titelblatt I
- Kapitel Preface III
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- Register The Index To Volume XII 619
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520 THE PHOTOGRAPHIC NEWS [October 30, 1868. struetions as to their management imparted, as far as it is possible in so small a space ; the finishing and varnishing of the pictures is also touched upon. As M. Grasshoff rightly remarks at the outset, he does not profess to teach anybody to paint solely with the help of his book, but that, in all the manipulations he describes, some amount of skill is necessary on the part of the opera tor. The author’s object in publishing his remarks is to simplify the process as much as possible, so as to enable photographers and amateurs of ordinary ability to produce coloured pictures in the easiest and best manner, and to un derstand the principles upon which they work. That M. Grasshoff has been successful in doing this there can be little doubt; and we feel sure that many aspirants will be greatly aided in their work by a reference to the volume now before us. We hope the time may not be far distant when the author will again resume his pen to tell us something of photo graphic backgrounds and accessories, a subject upon which his remarks would be equally valuable with those at pre sent under discussion. A TREATISE OX OPTICS; or, Light and Sight, Theo retically and Practically Treated, with the Application to Fine Art and Industrial Pursuits. By E. Nugent, C.E. (London: Virtue and Co.) This is a comprehensive and popular treatise on optics, which, in the dearth of such works, will be welcomed by the student. That an elementary work intended for a text-book for schools and colleges should be a compila tion from recognized authorities was not unnatural, and accordingly we find that Sir David Brewster’s work on optics has furnished the basis of the present work, and that the author has fallen into an error, only too common, of omitting to mention the authorities on whose pages he has drawn. Nevertheless, the work is a very valuable one, containing much information on photographic optics and theapparatus connected therewith, the information on these subjects being brought to a later date, and being therefore fuller, than in any other work on the subject. The work is well printed and copiously illustrated, and it contains that valuable but rare adjunct, a very complete index. Altogether, we can recommend this volume to those of our readers who may desire to know something of the prin ciples which underlie their art. The work is clearly written and carefully arranged, and is at once comprehen sive and popular. PHOTO-MICROGRAPHS Executed for the Army Medi cal Museum, United States. We have been favoured by the Surgeon-General of the United States Army with a series of photo-micrographs executed by Brevet-Major Curtis, Assistant-Surgeon of the Army. Several of these photographs have become familiar to English photographers by the examples sent over to this country by Col. Woodward, which have been exhibited at meetings of some of the societies. Criticism on such work is altogether superfluous ; it appears to be simply perfect, both in definition and brilliancy. We have examples of amplification varying from 30 diameters to 2,100 diameters with perfectly satisfactory definition. A Podura scale, for instance, taken with Powell and Lea- land’s l-50th inch objective, with the latter amount of enlargement, is marvellously well defined. The same test object, however, taken with a l-25th inch objective by the same maker, magnified 756 diameters, is, in our estimation, preferable. The following extract from a description of the operations in producing these enlargements, contributed by Col. Woodward to our Philadelphia contemporary, will interest many of our readers:— The essential parts of the process are, the use of object-glasses corrected so as to bring the chemical rays to a focus, and illumi nation by violet light. The steps are, briefly, as follows:— The microscope is used in a dark room, the windows of which face to the south. Outside of one of these windows is a shelf, on which stands a Silberman’s heliostat, so arranged as to reflect the direct rays of the sun upon the mirror of the microscope, which is fixed on an arm outside of the window, and, in its turn, reflects the sunlight through a short tube in the shutter. The microscope is placed at the inner extremity of this tube in a horizoncal posi tion, while at the outer end hangs a plate-glass cell, filled with a saturated solution of ammonio-sulphate of copper. Through this solution the sunlight must pass to enter the tube, and in so doing all but the violet ray is absorbed. Two steel rods, attached to the mirror, permit it to be adjusted without opening the window, and a black velvet hood thrown around the stage of the microscope prevents any leakage of light into the room from the space between the condenser and the objective of the instrument. With high powers, an ordinary achromatic condenser is used; or, in some cases, a pair of simple plano-convex lenses, with a large central stop, may be advantageously substituted, to give greater obliquity to the illuminating pencil. Objectives properly corrected for illumination with violet light have been manufactured for the Museum by Mr. W. Wales, of Fort Lee, N. J., who has produced a three-inch, four-tenths, one-fifth, and one-eighth, the quality of which, in my opinion, is all that can be desired. The plate holder, properly centred, slides on a horizontal walnut frame, by which it is held perpendicular to the axis of the microscope, and can be clamped at any distance not exceeding nine feet from the stage. By the side of this frame is a round rod. on the extremity of which, next to the microscope, is a grooved wheel. The milled head of the fine adjustment of the microscope is also grooved, and a silk cord over the two enables the operator to focus the micro scope by means of the rod, no matter how far from the instrument the plate-holder may be. To arrange the illumination, position of the object, &c., the operator stands by the microscope, puts in an eye-piece, and focusses in the usual way, adjusting the mirror by means of the steel rods. The violet light is readily borne by the eye, even when the whole power of the sun is employed in the illumination. This adjustment completed, the eye-piece is withdrawn, and, going to the plate-holder, the final adjustment is made by turning the rod. In this final focussing, the object is viewed with a 'focussing glass on a piece of plate-glass held in the plate-holder. The sensitive plate is then exposed in the usual way, the time required being from less than a second to twenty minutes, according to the power employed. For low powers, the objective alone is relied on to give the necessary size. To obtain the highest powers, however, an achromatic concave is placed at the upper extremity of the micro scope body. The one now employed gives a perfectly flat field, and increases the number of diameters between six and seven times. This concave is about half an inch in transverse diameter, and has an angle of aperture of 28°. Many objects—as, for example, some of the tissues, certain diatoms, &c.—present interference lines when illuminated with a powerful pencil of parallel rays, and in order to prevent this false appearance, it is necessary to interpose a piece of ground glass in the solar pencil to disperse the light: in this case the time of exposure necessary is, of course, much increased. By these contrivances we have been enabled to produce pictures of the utmost sharpness, and perfectly satisfactory in every other respect, with powers up to 2,500 diameters; and’ these pictures bear a further enlargement of from six to eight diameters in a copying camera. We have thus obtained excellent pictures with no less than 19,000 diameters. If the foregoing points are duly attended to, it matters com paratively little which of the very many excellent photographic processes in use for ordinary work is employed. As to collodion, many different samples have been used with good results—among others, for example, one containing two and a half grains each of the iodide of ammonium, the iodide of cadmium, anil the bromide of ammonium, to the ounce of collcdion. With the highest powers, a collodion containing two grains of bromide and five of the iodide of magnesium to the ounce has been found advantageous, the re sulting nitrate of magnesia preventing the plate from drying during the long exposure necessary. The nitrate bath is used of the strength of forty grains to the ounce, and is acidulated with nitric acid. We use the ordinary iron developer, restrained by the gelatine solution, on the plan proposed by Brevet-Major W: Thomson in a recent number of your journal. ’ The picture is fixed with hyposulphite of soda or cyanide of potassium, and afterwards intensified with iodide of mercury dissolved in a solution of iodide of potassium, and, when necessary, still further with Schlippe’s salt. Great intensity is especially required in pictures of the diatomacea, and other lined objects. The negative is finally varnished, and prints taken on albumen-paper in the usual way I send you samples of these prints. It is to be remarked, how ever, that for the Museum we prefer transparent positives on glass, mounted before a piece of ground-glass. A fac-simile 01 the field of the microscope is thus obtained. I should take plea-
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