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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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- Englisch
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- F 135
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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. 1694, February 20, 1891
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The photographic news
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140 THE PHOTOGRAPHIC NEWS. [FEBRUARY 20, 1891. latter and projects a blow-pipe flame on to the lime. It is easy enough to manage, and, do what you will, you cannot make the flame go out with that sharp crack which sounds so ominously of coming disaster to nervous listeners. Far better is that form in which the oxygen outlet tube is embraced by the wider hydrogen tube, so that when you look down the latter you see the oxygen pipe hidden about one-eighth of an inch from the general outlet of the jet. In this case the gases mix, to some extent, before the lime cylinder is reached, and the gain of light from this circumstance is great. Another point to which we may direct attention is the necessity for seeing that the bore of the 0 tube is something more than a pinhole. It should be suffi cient to pass not less than four and a-half feet of gas per hour under moderate pressure. A less amount than that cannot give a good light. But, with all these things provided for, some operators do not get the best out of their jets because they do not place the lime cylinder close enough to the outlet pipe. The distance between jet and pipe should never be more than one-eighth of an inch. Careless workers fall into another fault in neglecting to turn the lime cylinder every few minutes as it ought to be turned. The loss of light from this common error is enormous, with the farther serious risk of breaking the condenser of the lantern; for if a pit be formed in the lime, the flame will often be bent back towards the glass, and will fracture it. The successful operator with the lime-light must be continually on the alert to see that he is getting due efficiency from his jet. The gases must be regulated to a nicety until a point is reached when they give the best possible light without noise, and this point Can only be ascertained by watching the effect upon the screen as the taps are manipulated. The lime must be turned to a small extent, but often; and the jet should have an arrangement by which this can be done easily, and without opening the door of the lantern. There are jets in the market which possess all the faults against which we have warned our readers, and we can only suppose that they are constructed from time-honoured models by those who can have had no practical experience of the lime-light. In another article we shall treat of the more powerful mixed jet for lantern use. The New Discovery in Heliochromy.—In an article in the Daily News of last Monday, its special correspondent at Paris says:—“ I have bad another conversation with Professor Lipp mann, of the Sorbonne. The colours are permanent—he made use of the word “ fixed ”—and they are only seen by reflection in looking at the plate, and not through it. One sees the colours well in daylight or lamplight, but better in reflected than in direct artificial light. Thus, the professor covered the back of a glass plate on which he photographed a spectrum, and held the face towards the white side of a paper lamp-shade. In the light it threw back on them the colours took such a brightness as to be only comparable to the prismatic hues in a well-cut Golconda diamond. When he held the plate between my eye and the light I did not see a trace of colour on it. I never saw any effects more neat and perfect than those he has obtained. M. Lipp mann has been at the Sorbonne five years.” PHOTOGRAPHY IN COLOURS.* BY PROFESSOR G. LIPPMANN. I have set myself to the task of obtaining upon a photo graphic plate the image of the spectrum with its colours, in such a fashion that the image shall be permanently fixed, and last indefinitely in full daylight without alteration. I have resolved this problem by operating with the ordinary sensitive substances, and photographic developers and fixing solutions, modifying simply the physical con ditions of the experiment. The conditions necessary to obtain colours in photography are two: —1. The continuity of the sensitive film. 2. The presence of a reflecting surface behind this film. I mean by continuity the absence of granularity; it is necessary that the iodide, the bromide of silver, &c., shall be disseminated in a film of albumen, gelatine, or other substance transparent and inert, in a uniform manner, without forming particles visible even under the microscope; if particles be present, it is necessary that they should be of neglegible dimensions in relation to the length of a luminous wave. The coarse emulsions hitherto used must be excluded. A continuous couch is transparent, but usually has a slight blue opalescence. As vehicles to the sensitive salts I have used albumen, collodion, and gelatine ; as sensitive agents, iodide and bromide of silver ; all these combina tions give good results. The dry plate is placed in a grooved frame into which mercury is poured ; the mercury forms a reflecting surface in contact with the sensitive film. Exposure, development, and fixing are the same as with an ordinary black negative, but the result is different; when the negative is finished and dried, the colours appear. The image obtained is negative by transmitted light, that is to say, every colour is represented by its comple mentary tint. By reflection it is positive, the colour itself can be seen, and it can be obtained very brilliantly. To turn it into a positive, the image must be redeveloped or intensified, until the photographic deposit has a bright colour, such as can be obtained, it is said, by the employ ment of liquid acids. Fixing is done by means of hyposulphite of soda, followed by careful washings. I have verified that the colours resist the action of the most intense electric light. The theory of the experiment is very simple. The incident light, which forms the image in the camera, interferes with the light reflected by the mercury. It forms, consequently, in the interior of the sensitive film, a system of fringes, that is to say, at the maximum of luminosity and the minimum of obscurity. The maxima alone impress the plate ; as a result of the photographic operations, these maxima remain indicated by deposits of silver more or less reflecting, which occupy their place. The sensitive film is thus divided by these deposits into a series of thin plates, equalling in thickness the interval which separates the two maxima ; that is to say, half a wave length of the incident light. These thin films possess, then, precisely the thickness necessary to reproduce by reflection the incident colour. The colours visible are thus of the same nature as those of the soap-bubble; they are only more pure and more brilliant—at least, when the photographic operations have given a good reflecting film. This results from the for mation in the thickness of the sensitive couch of a great * Read at the Academy of Sciences, Rafis, February 2nd, 1891.
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