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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. 1277, February 23, 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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- Titel
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FEBRUARY 23, 1883.] THE PHOTOGRAPHIC NEWS. 125 is required, which gives a bronze tint, like that seen cover ing plaster figures. This bi-sulphide of tin consists of 35-55 of sulphur and 64-45 of tin. As this substance really contains such a large amount of what is most per nicious to photographs, one cannot wonder at its action. Added to which, the combination itself is not stable, so that the sulphur can readily attack silver pictures. Curiously enough, the spots caused by this bi-sulphide of tin are very similar to those formed by the decomposi tion of paste. They are small, not sharply defined, and follow the strokes of the brush. The reason for this appearance is that the card receiving more paste in some places than others slightly raises the picture in these parts, and the eminences when the pictures are stacked up together detach a small quantity of the bronze card touch ing them, as may be seen through a magnifying glass. It is useless to try and rub off all traces of the powder, because it enters into the surface of the paper. Let this stand as a warning to all photographers to shun cheap bronze cards. Although this aurum musivum is so easily distinguish able from all others by its colour, it is better to have a chemical test for it. The most simple are nitric acid and alkalis. Bronzes containing copper and zinc, and some having tin in their composition, are very soluble in the former, while the bi-sulphide of tin withstands concentrated nitric acid. On the other hand, it is soluble in alkaline solutions, while the bronzes are not. In testing, care must be taken not to cofuse the action of the adhering substance with that of the bronze itself when using the printed card. The best plan is to place a drop of the testing solution upon the bronze card and observe the result through a magnifying glass ; the dissolving of real bronze in nitric action will infallibly evolve a gas. PHOTO-MICROGRAPHY. BY O. J. JOHNSON.* Actinic and Visual Foci.—In photographic lenses by good makers great pains are taken to make the actinic and visual rays meet in the same point; but, as microscopic lenses as a rule are constructed solely for giving the best definition to the human eye, a different formula is adopted for the curves of the glasses. For the sake of your junior members, some of whom may be imper fectly acquainted with optics, you will, perhaps, grant me a short indulgence while I explain the principle of achromatism. 'Diagrams were thrown upon the screen illustrative of the con struction of achromatic lenses and the over-correction of micro scopic objectives.] As it is generally accepted that the best definition is obtained in photo-micrography without the use of the eyepiece, you will at once see the necessity for allowing readily for this discrepancy between the visual and actinic foci. In the instrument before you I have determined by experiment that, when a two-inch object-glass by Baker is employed, the screen being thirty-seven inches distant from the object which has been visually focussed, the objective must be withdrawn 2186 inches, or the rod turned till the pointer previously placed at zero passes through 15°, to the right (at half the focal distance 740 must be allowed), the chemical rays will then make a sharp picture on the film, whilst the outline appears blurred to ordinary vision. In the more delicate correc tions required by the higher powers recourse may be had to extension of the bellows instead of altering the fine adjustment screw. The allowance required for each objective can readily be ascertained by placing an ordinary micrometer scale ruled to Tenths and JDogths inch on the stage of the microscope at an angle of 10° to 15°, so that each line has a varying focal point. On the screen is a photograph of such a scale taken at thirty-seven inches by the two-inch lens, the visual focus having been adjusted at the first line of the thousandths, and the best photographic defiinition coming out about the fifth or sixth lino of the hundredths. The difference in the visual foci of these two lines was represented by 15° on the scale, which was readily found by viewing the scale through the eyepiece and swinging the index between the two focal points. Instead of the micrometer scale an object not • Continued from page 109. pressed too flat—such as a fly which has delicate hairs lying in different planes—may be employed in a like manner. If a specially-sharp picture be required of some difficult object, trial plates should be taken with determinate variations in the length of the bellows, either by cutting a dry plate into slips— which should be numbered and exposed separately—or a diaphragm can be placed immedately behind the focussing-screen, having a revolving disc three inches in diameter fixed in the centre, a quadrant being cut out of the same to allow of sucessive ex posure, the disc being revolved and the shutter lowered between each variation of the bellows. Four results will thus be exhibited on the same plate. No tables can be given for the actinic allowance required by the various powers. A one-inch by Dancer, lent by a friend, was found to produce sharp pictures without any such allowance, whilst another by Swift required 2° on my scale. Powers above one- quarter of an inch seldom require compensation, and some of the photographic lenses of English make, when stopped down, per form admirably on large objects of half or one inch diameter. Many of the pictures to be seen to-night were produced by Dallmeyer’s stereoscopic lens, kindly lent by Mr. Pollitt. The tyro is recommended to ascertain definitely once for all the exact allowance required for each lens, and to keep a careful record of the same. In order to produce satisfactory negatives no pains should be spared to obtain a flat and even picture, by levelling the object by means of the set screws referred to. I have seen many otherwise commendable photographs utterly spoiled by one-half of the picture being out of focus. Illumination.—The most preferable source of light, as far as my experience goes, is the sun ; but the electric arc, lime-light, gas, and paraffine lamps have all been used. Now that sensitive dry plates are within the reach of all, the paraffine lamp is usually employed, either naked or with a bull’s-eye condenser interposed. If the latter be employed, difficulty is often experiencedin obtain ing even illumination all ever the field, the thickness of the glass breaking up the rays into prismatic colours. Mr. Dancer recom mends a double combination quarter-plate lens used as a con denser, an image of the lamp flame being formed thereby a little behind the object, so that the rays just cross before arrival. A disc of light should be produced which amply covers the size of the object. It will be found convenient to ascertain the correct ness of the illumination by placing a slip of white writing paper on the object slide, and observing whether the image of the flame fully envelops the field. In using the higher power, when it becomes necessary to obtain more intense light upon a small space, the paraffine flame may be placed edgewise. With a good lamp, powers up to the one-fifteenth of an inch may be employed with dry plates. For low magnifications ordinary daylight from a white cloud will suffice. In using sunlight it is sometimes necessary to interpose a glass cell containing a solution of common alum to arrest the heat rays, otherwise the object or the lens may be injured. With naked sunlight, also, diffraction and interference lines are apt to appear around the image, when a p late of ground glass should be fixed an inch or so behind the object to soften the light. Dr. Woodward, in using very high powers, has some times found it imperative to make use of monochromatic light. This can be obtained by causing the sun’s rays to pass through a solution of sulphate of copper to which is added strong ammonia. In photographing the delicate markings on some diatoms the light must impinge on these transparent objects at a considerable angle, or the direct flood of rays will drown all detail. In photo graphing the proboscis of a blow-fly I have found a superabundance of light fatal to the fine delineation of the false trachea. With objects difficult of resolution the ordinary achromatic condenser of the microscope is often employed. Definition.—In the lower powers definition is rendered much more perfect by the introduction of a stop behind the back lens. For instance : in portraying a section of the stem of the dog rose, a stop a-quarter of an inch in diameter materially improved the definition of the delicate cells, and in many histological specimens, where great penetration is required, the tissues being compara tively thick, a reduction of the aperture of the lens is imperative. Stops of cardboard or turned wood may be employed, or Davis’s iris aperture shutter, which gives all variations in size from a pin point upwards. Objects.—All microscopical objects are not equally suitable on account of either colour or thickness. Tissues stained light blue or purple give faint images, whilst dense brown objects will not allow light to penetrate the detail. Preparations of insects (such as fleas, which make capital subjects for the beginner) should
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