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The photographic news
- Bandzählung
- 24.1880
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- 1880
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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. 1130, April 30, 1880
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- Paginierfehler: S. XI als S. X gezählt.
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Zeitschrift
The photographic news
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Band 24.1880
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- Register Index 631
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Band
Band 24.1880
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214 THE PHOTOGRAPHIC NEWS. [APRIL 30, 1880. stud just in front of him. He may then be quite sure that his distant brothers can see them too. If he shows a light for a short time—say two seconds—he means a short signal ; if for six seconds, a long signal; and the alphabet is simply a combination of these long and short signals. PRINCIPLES OF OPTICS INVOLVED IN LANTERN CONST SUCTION; AND ON A NEW ENLARGING' LENS, ESPECIALLY DESIGNED FOR USE WITH THE MAGIC LANTERN. BY J. H. DALLMEYER, F.R.A.S., ETC.* The commercial condensers may be comprised under two heads, viz., the so-called “ Herschel ’’ form,+ composed of a plano-convex, ora concavo-convex combined with a double con vex, the flat or concave side facing the radiant, the lenses of 4 in. diameter and 3} in. equivalent focus; and the symme trical condenser of 3} in. and 3 in. focus respectively, and of the same diameters. I am disposed to give the preference to the latter form, resembling to some extent the Ramsden eye-piece, though this admits of improvement, as I shall show further on. The essentials, then, of a good condenser are “quantity and quality of light.’’ The former points to diameter and focal length, the latter to perfection of the glass and correction of aberrations of the lenses. The condenser constracted by me for Mr. Hardwich is of 4 in. effective diameter, and 21 in. equivalent focal length. As suming the radiant to be at a-safe distance of 21 in. from the flat surface of the first lens, this condenser collects an angular pencil of about 66°, i.e., about 20 per cent, more light than the shortest focus symmetrical. It is represented by Fig. 3, and consists of the two unsymmetrical lenses, A and B. A is a plano-convex of flint, 3§ in. diameter, and B is a double con vex of crown-glass of 4 in. diameter. The lenses are mounted at a certain interval apart, and their deep sides facing each other. Approximate correction of chromatic aberration for centrical pencils is obtained by a proper apportioning of their focal lengths and the distance at which they are placed. Thus: ray L R, after refraction by lens A, diverges into a pris matic beam ; this falls upon different parts of the lens B, which, while acting upon the two extremes—the red and violet—in contrary directions to A, causes them to emerge parallel, the condition of achromatism, when they converge to the conjugate focus f, about 9 inches removed from B. The spherical aberra tion is reduced to a minimum by the forms of the lenses em ployed, i.e , ray L refracted by the central portion of the lenses meets the axis at the same point, f, as the marginal rays, or nearly so. I have decided upon a 4 in. (effective) diameter condenser, since it fully illuminates the corners of a 24 in. square slide. Of course a circular slide of 3 inches only requires a 8} inch diameter condenser, of proportionately’ shorter focal length. I need hardly say that the glass composing this condenser has been selected with especial care. It is perfectly limpid or colourless, and will remain so; it is free from striae and air bubbles, and has a perfect polish. In fact, it is Chance’s best glass ; the only drawback being its cost. And here I would remark that any defects in the glass of the condenser are far more detrimental, as regards the purity and quality of the illuminated disc, than similar defects in the ob jective. I pass on to the enlarging lens, or objective; for however perfect the radiant and the condenser, the all-important thing Fig. 3. for a perfect picture on the screen is the lens by which it is produced. Of these I have the honour of submitting two specimens especially constructed for the purpose. Before 1 describe the new lens, 1 will briefly refer to those which have been used for the purpose up to the present time. There are the old so-called lantern “ fronts,” comprising various single or achromatised lenses, that are made to screw one upon the other, giving all sorts of sizes and shapes of pictures, but devoid of almost all correction. These “ toy ” lantern appliances are, however, no longer in vogue with photographers, or, indeed, with any one who appreciates definition, freedom from distor tion, &c., and hence came the substitution of portrait lenses for the old lantern “ fronts.” Those now supplied with the better class of instruments are generally French quarter-plate lenses ; most deficient, however, in respect of three very important particulars, viz., flatness of field, straightness of lines, and illu mination. The former are seen at a glance upon the screen, not so, however, the latter ; and in order to illustrate my mean ing, I must (not to multiply diagrams) once again refer you to Fig. 2, representing the commercial 4 inch condenser of 3 inch equivalent focus; the radiant at a distance of 3 inches with its subjugate of about 10 inches, as at f. To collect and to trans mit all the light at a distance of 4} inches from the slide, as at 0, requires a lens of 2 inches diameter in the clear. Upon examination of the French quarter-plate lenses of 4} inch back focus (6 inches equivalent) it will be found that the lenses are of barely If inch diameter, and this diameter, squared, affords the information that only about one-half of the light • Continued from page 196 + I deem it right to state that the late Sir John Herschel never designed the forms of lenses called by his name as a “ condenser.” He proposed it as a “ burning lens,” the spherical aberration being approximately cor rected only for the sun, or for parallel incident rays; as expressed in a letter addressed to the writer by the late Sir John, in February, 1867. from the condenser passes through the lens on to the screen. It may suggest itself, why not shorten the focus of the con denser by removing the radiant to a greater distance on the other side? The case is represented by the dotted lines, and it will be observed that for a shorter focus, as at f, the radiant moves to L'. It is true the lens O' now transmits all the light from the condenser, but this in turn receives less from the radiant, owing to increased distance ; hence, what is gained on one side is lost on the other, and the result is the same. Perhaps the balance of advantages is on the side of the latter expedient, since it excludes diffraction-phenomena. Then, again, it may be asked : why not use a longer focus objective which includes all the light from the condenser? Take an extreme case represented by O 2 , i.e., a lens of double the focal length of O. This, of course, includes all the light even from a larger radiant than a point; but observe, in order to get the same sized enlargement the screen must bo moved to double the distance, and the resulting picture appears dimly lighted. It possesses only one quarter of the illumination of tch former. (To be continued.') SILVER PRINTING. BY JOHN MATTHEWS. Mr. J. Spiller’s paper must have struck a new chord in the breasts of many photographers relative to the fading of silver prints. There are those who argue that sitters do not require permanent productions—that they are only too eager to “come again.” This sentiment, as far as I can discern, is confined to a very small proportion; for the most part certain young gentlemen who experiment
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