The photographic news

• Bandzählung: 7.1863
• Erscheinungsdatum: 1863
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
• Rechtehinweis: Public Domain Mark 1.0
• URN: urn:nbn:de:bsz:14-db-id1780948042-186300004
• PURL: http://digital.slub-dresden.de/id1780948042-18630000
• OAI: oai:de:slub-dresden:db:id-1780948042-18630000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

January 23, 1863.] THE PHOTOGRAPHIC NEWS. 39 discovery of the latter of these, as our readers also know, is due to Mr. William Crookes, a gentleman to whom the photographic world generally is indebted for much informa tion on the chemistry of photography. In the beginning of 1861, being engaged in spectrum examinations, Mr. Crookes discovered, in the flame of some residues of selenium, a single bright green line in a portion of the spectrum where such a line had not been seen before. Further investigations convinced him that this was a new element, which he shortly succeeded in isolating. The name of thallium was given to the new element—derived from a Greek word, signifying a young twig, the vivid green spectral line suggesting the name. The exact nature of the new body was not at first decided, but it was supposed to be a metalloid. In September of the same year, however, Mr. Crookes satisfied himself of its nature by obtaining the substance as a metal, which was shown to various persons in his laboratory. Mr. Crookes seems destined to meet with injustice in regard to this discovery. In the International Exhibition, notwithstanding that he exhibited the metal and its com pounds, and that his discovery of it was well known to the jurors, his name was not mentioned in the first published list of awards ; whilst a French gentleman, M. Lamy, was rewarded with a medal for the discovery of new and abun dant sources of the metal, the original discoverer of which was unnoticed. A proper and energetic exposure of this grievance brought some reparation. But recently Mr. Crookes has suffered another not less grievous injustice. M. Dumas, in a memoir just presented to the Academy of Sciences, denies to Mr. Crookes the discovery of thallium as a metal at all, and awards that honour to M. Lamy. The facts appear to us, however, to lie in a nut-shell. The credit is given to M. Lamy on assumed prior publication ; and this publication of the metallic character of the new element is stated to have been made in a communication to the Socit Imperiale of Lille on the 15th of May, 1862. Now setting aside (as non-publication or insufficient publication) explanations and exhibitions to friends in the laboratory here is the fact patent to all the world, that on the 1st of May, 1862, a case was exhibited by Mr. Crookes in the International Exhibition, containing samples of the metal and its compounds, in which it was distinctly labelled as a “new metallic element,” and further described as a “heavy metal.” Publication to a larger audience could not well have been made; and it is to be noted that the contributions of thallium to the International Exhibition by M. Lanny were not made until some time after it was opened. The comparison of the two dates appears to us to settle beyond a cavil the priority of the discovery ; and whilst M. Lanny has had the good fortune and sagacity to discover abundant sources of the metal, it is clear that to an English investiga tor belongs the honour of discovering the element and deciding its metallic character. :—• PHOTOGRAPHIC CHEMICALS: Their Manufacture, Adulteration, and Analysis. The other compounds of alumina are of too slight importance to the photographic chemist to render any separate account of them of interest. The next element, the compounds of which demand our attention, is one which can scarcely be classed among the metals at all, although for the sake of convenience it is frequently so classified in chemical works. We allude to silicium, the oxide of which is the well known silica or rock crystal. In the natural state silica occurs under many forms ; it is almost chemically pure under the form of fine white sand, immense quantities of which are brought from the Isle of Wight, for the purposes of glass making. Another form of pure silica is known under the name of rock crystal. This is met with in hexagonal crystals, surmounted by hexagonal pyramids ; the colourless trans parent variety is largely used for optical purposes. Although not strictly coming under the category of a photographic chemical, the properties of rock crystal are sufficiently in teresting to our readers for us to devote a small space here to their description. Pure rock crystal possesses one great advantage over other solid transparent media, in being perfectly transparent to all the chemical rays of light. A photograph of the solar spectrum, taken with a complete quartz train—that is, with prisms and lenses of quartz—will show lines and bands in the spectrum three or four times as high as when taken with a glass train. In scientific re searches upon the photographic action of light and colour, it is, therefore, necessary to employ rock crystal instead of glass wherever light has to be transmitted, as it has been shown by Mr. Crookes that the interposition of a piece of colourless glass, the hundredth of an inch thick, in the path of a beam of light, passing through a quartz train, is suffi cient to cut off very considerable quantities of the chemically acting rays of light. By employing a meniscus lens of rock crystal for photographic purposes, very excellent results are obtained, owing to the large number of actinic rays which are transmitted by it on to the plate. A quartz lens can, however, be used only with a small aperture in front of it, as it cannot be achromatized without diminishing its transparency down to that of glass. Another objection to a quartz lens is its double refracting power, by reason of which the image formed by it is double when viewed under a high magnifying power. This defect may be obviated by taking care in the selection of the lens, or in having it cut in a particular direction, with respect to the axis of the crystal. When a beam of light passes through a crystal of quartz in such a way that it forms an angle with the axis of the crystal, it is doubly refracted, or split up into two rays slightly divergent. A lens cut from a crystal, in such a manner that the light would pass through, forming an angle with the axis, would therefore give a double image. When, however, a ray of light passes through a plate of quartz, parallel to the axis of the crystal, it is not doubly refracted ; a lens, therefore, cut so that its axis coincides with the axis of the crystal, will only give one image. It may be of use if we instruct our readers in a simple way of ascertaining whether a lens is made of quartz or glass. The latter is much the softer of the two, and therefore, by a comparison of one with the other, the sharp point of a file will show the difference without further trouble. This, how ever, is not a plan to be recommended, as it involves per manent injury to the lens. The plan we recommend is to employ polarized light for this purpose. A small tourma line, which can be purchased mounted for a few shillings, is the only special apparatus required. A dark polished surface —such as French-polished mahogany, a japanned tray, black marble, or similar material—is placed on a table in front of a window. The photographer stations himself about four feet from this, so that the bright light from the sky is reflected upwards to him from the polished surface. The tourmaline is now brought up to the eye, and slowly rotated on its axis. As the crystal turns, the polished reflecting surface will be seen to become alternately light and dark; the tourmaline is kept in the position in which the reflecting surface appears darkest. The lens to be examined is now held between the tourmaline and the reflecting surface, and in its turn moved round on its axis. If the lens is of glass, no change will be produced by this movement; but if it is of rock crystal, it will become luminous during one part of its rotation, and then dark again, presenting a similar appearance to what took place when the tourmaline was turned round. It is possible, by this means, to ascertain in which direction, as regards the axis of the crystal, the lens is cut; and at the risk of not being clearly understood by some of our readers, we will endeavour to point out how this is dis covered. Turn the tourmaline so that the reflecting surface appears quite dark, and move the lens on its axis until it appears luminous over its whole surface. Now turn the lens about sideways, so as to look through it in different directions diagonally, keeping the relative positions of top and bottom unchanged. As it is being moved about,