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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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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. 1695, February 27, 1891
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The photographic news
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the branch of a tree to protect himself, but the branch broke, and, by some mysterious law of nature or art, left by the New Zealanders unexplained, instead of the moon falling upon him, he fell himself upon the moon, and-there remains with the broken bough up to this present. It will be clearly discovered in a photograph of the moon whether this story has any real or apparent, primary or secondary truth. (To be continued.) PHOTOGRAPHING IN COLOURS. The invention which has recently been announced from Paris, of a method of communicating colour to a sensitive photographic film by the action of a ray of coloured light, will, if confirmed, awaken very widespread interest, for it is impossible to foresee all the applications to which such a discovery might be expected eventually to lend itself. The idea is credited to M. Lippmann, a professor of the Sorbonne, and a physicist whose name is already widely and favourably known in connection with some highly ingenious instruments of physical research. The theory which he is said to have successfully embodied in experiment is that the interference of two rays of light of the same wave length may be caused to produce a characteristic effect upon a sensitive film such as amounts to dyeing the film of the particu lar hue in question. So far as has at present been explained, the process appears to depend upon the doctrine that the chemical action, like the chromatic properties of a ray of light, is capable of being affected by the conditions which give rise to the phenomena of interference. In itself this seems likely enough. It is well known that if two rays of light having the same wave length travel along the same path simultaneously, they interfere with one another. It may and does happen that at certain points along the path, depending for their position on the wave length, the two impulses tend to counteract, and at other points to reinforce one another. A luminiferous particle—whatever that may be—which is acted upon by two opposing forces will remain at rest if the forces are equal as well as opposite; while if the forces be not equal, or not directly opposed, there will be some resulting disturbance, but still a disturbance different from that which would have been produced by either of the coincident forces acting alone. From this cause arise all those beautiful optical effects of interfer ence which so strikingly illustrate the undulatory theory of light. Now, if it be a fact that the chemical activity of a ray of light is subject to the same law, and can be either exalted or depressed by causing a second ray of the same wave length to act upon the sensitive reagent, it is quite conceivable that a pair of rays of light might be so directed through a photographic film as to cause, not a uniform, but a sporadic deposit of the precipitated element. These minute deposits would, in that case, be distributed at definite distances apart along the line of the common path of the active rays, because they would mark the points of co-operating and therefore exalted activity. Similarly, their intervals would occupy the spaces in which the antagonism of the two rays had reduced or even destroyed the actinic effect. Now, such a film would apparently be opaque to any light not having this particular wave length, because at some point it would necessarily be absorbed by the opaque deposit. But if a ray of the same wave length impinged, it might be able to pick its way successfully between the deposits. Professor Lippmann is said to have produced such a film by the expedient of placing a mirror at the back of his sensitive plate, and causing the focussed ray to return along its own path. The mirror is produced by making the plate one side of a mercury trough, which, when filled, constitutes a mirror in exactly the desired position. The expedient, if successful, is certainly charmingly simple, and, seeing that the healing art would undoubtedly benefit in as high a degree as any by a practicable system of chromatic photography, we look with lively interest for some further information upon the theory and practice so ingeniously originated.—The Lancet. A ZINC SULPHIDE COMPOUND SENSITIVE TO LIGHT* BY JOHN CAWLEY, ASSOCIATE OF THE ROYAL SCHOOL OF MINES. Zinc sulphate, or chloride freed from iron, manganese, lead, tin, and so on, is precipitated by an alkaline or alkaline-earthy sulphide ; the precipitate, consisting of hydrated zinc sulphide alone, or in admixture with a sulphate of an alkaline earth, is dried and ignited at a red heat ; the red-hot product is thrown into water, levigated, washed, dried again, and reduced to fine powder. The pigment is then ready for grinding in oil as a paint. This process, simple in outline, requires considerable skill and experience to carry it out successfully, but if well carried out it produces a pigment superior to the best white-lead in respect of whiteness, body, and covering power. It is, more over, relatively innocuous, and is not discoloured by gases con taining SH 2 . In general, however, it is subject to the grave defect of darkening when exposed to sunlight, the darkening apparently being due not only to light, but also dependent on the condi tion of the atmosphere in respect of moisture-contents and other influences. I have prepared pigments so sensitive as to be turned almost black when exposed to bright sunlight for one or two minutes. A curious thing to be noted in this connection is that the darkened pigment will comp’etely regain its whiteness when placed in the dark for a few hours, and when so bleached it appears to be somewhat less sensitive than it was originally. It would be too long a story to go into all the details of the experiments made by me in connection with this matter. I will, therefore, content myself by giving a brief account of the more important observations, interpolating here and there remarks of an explanatory character. 1. The dried hydrated zinc sulphide, before ignition, is not altered by exposure to light ; after ignition it darkens readily. 2. The whitest and purest zinc blende obtainable was levi gated and exposed to light. No darkening was observed, but the same blende calcined, so as to slightly oxidise it, was readily darkened. 3. Zinc sulphide was prepared in a very finely-divided con dition, and freed from hydration by burning zinc in an atmo- sphere of sulphur vapour, and collecting the product in a chamber the temperature of which was high enough to pre vent the condensation of sulphur. This sulphide was unchanged by light, but, like the blende, darkened readily after slight calcination. In connection with the observations above-named, I may say I had been struck by the resemblance which the colour developed in the pigment by light bore to that of the finely- divided metal deposited from certain metallic solutions. Noting that the pigment moistened with water was more sensitive than when in a dry condition, a sample was dried at 120° C., and while warm was placed in a glass tube, which was sealed before the blowpipe. Result, no discolouration after one year’s exposure. A sample placed in a sealed tube without extra drying was readily darkened. Taking a sample of pigment not very sensitive, per se, it was noted that its sensitiveness was enormously increased by moist ening it with a weak solution of zinc sulphate, from which it was inferred that the sensitiveness might be due to imperfect washing. The most thorough washing, however, would not remove the basic zinc sulphate formed in the process of ignition. Attempts were made to decompose this basic sulphate into zinc oxide and an inert sulphate by treating the pigment with solutions of the hydrates of potassium, sodium, barium, and calcium ; the first three were rejected, as they appeared to act on the zinc sulphide itself. Lime-water exercised for a short time a decidedly protective effect ; afterwards it seemed to lose its power of making the pigment resistant to light. BRIXTOX and Clapham Camera Club.—The proceedings on the 21st inst. included a limelight lantern display, discussion of the proposal in favour of cheap fares for phote graphers, and a flash-light demonstration. * Abridged from last week’s Chemical Neics,
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