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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1276, February 16, 1883
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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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98 THE PHOTOGRAPHIC NEWS. [FEBRUARY 16, 1883, that no hard lines exist in the image; the highest light 8 graduate imperceptibly into the darkest shadows. This encroachment upon the shadows of the reduced particles of silver becomes of importance when images of fine lines are required. Dr. Eder has shown, by careful microscopic measurements, that a line, originally measuring -28 m.m., contracted to *198 m.m. after a prolonged development. This point is of especial importance in connection with photo-zincography. It shows that in developing images of sharply defined lines, lateral reduction must be prevented as far as possible by rapid development. Doubtless, also, some developers will be found more prone to produce lateral reduction than others; and this is another point which it is of importance to note in careful comparisons of various developers. —o PHOTO - SPECTROGRAPHIC RESEARCHES ON REFLECTED LIGHT. Some recent investigations made by M. de Chardonnet have an especially important bearing on the every-day woik of the photographer, and perhaps more especially on that of the portraitist. In the case of all ordinary photographic operations, the light which enters the camera and affects the sensitive plate does not consist of the direct rays as they leave the source of light, but of radiations which have been reflected from the surfaces of the various solid or liquid bodies which enter into the composition of the scene or subject to be depicted ; indeed, it occasionally happens that no light is utilised which has not been reflected twice or oftener. Strangely enough, the composition of light thus reflected has scarcely been studied until quite recently ; it being vaguely stated in books that substances possess certain selective powers as regards the rays which they reflect on the one hand, or absorb on the other hand. Chardonnet finds, however, that this assumption is altogether incorrect, selective absorption being a myth. Indeed, absolutely the same spectrum was obtained with sunlight reflected from solid bodies so various as speculum-metal, green leaves, Arcet’s fusible alloy, hmatite, diamond, Prussian blue, mercury, various enamels, uranium-glass, charcoal, gold, lead, nickel, steel, and copper. Experiments were also made with light reflected from liquids—potassium bi-chro- mate solution, milk, ink, ammonio-copper sulphate solution, water, magenta solution, and other fluids being used ; and it was found that, as in the case of the solids, the same spectrum was obtained in every instance. Although every grade, or vibration-rate, of the incident light is reflected, it must, of course, be understood that the relative proportion in which the various rays are reflected varies extremely; the ratio in which they are present determining what the author calls the actinic colour of the reflecting surface. Reflection spectra, then, do not exist in the same sense as absorption spectra, the former being only capable of expres sion by a scale showing the relative ratios of the intensity of the various portions; while the latter may be expressed by the usual diagrammatic method. Chardonnet’s researches teach us the reason why the folds and structural details of the deepest red drapery can be so perfectly delineated by the aid of photography, and will ' serve to indicate to the photographer the importance of ' bearing in mind the great difference which may exist beween 1 light which has been transmitted through a medium, and ' that which may be reflected from its surface. NOTE ON GELATINO-CHLORIDE PRINTS. BY DR. J. M. EDER. GELATINO-CHLORIDE paper, when developed with a citrate developer, gives prints of a light or dark brown. The tint becomes darker when a mixture of oxalate and citrate solutions is employed for development, and at the same time there is the advantage that the time of exposure is shortened. When this citro-oxalate developer is employed for the treatment of paper prints, it will be found that the best form of emulsion to use for coating the paper is one in which silver chloride is predominant. The emulsion may be produced either by the ammonia, or boiling method, so long as it is prepared with care. By using a chloro- bromide emulsion, the printing by gaslight is considerably shortened, and the tone of the pictures is darker. I cannot recommend any formula for preparing the emulsion better than that published by Captain Pizzighelli and myself a year or so back. To the chlorine salt is simply added 10 per cent, of bromide of potassium, and the preparation of the emulsion proceeded with in the ordinary manner. As to preparing the citro-oxalate developer, I may mention that I prefer to do so according to the following formula:— Acid citrate of ammonia... ... 1 part Oxalate of potash... 1 ,, Water 6 to 8 parts Before use, I mix three volumes of this solution with one volume of a solution of sulphate of iron (strength 1 to 4), and add a little chloride of sodium, as restrainer, to pro duce more contrast and brilliancy in the prints. GLASS. FoURTII Article. Our third article, on page 3, explained the means adopted to obtain nearly colourless glass—or, at least, to rectify the inherent tendency in all ordinary commercial mate rials to produce a bad coloured glass. From a purely technical point of view, dozens of pages of the News might be occupied with details of the processes adopted in various glass manufactories to prepare the materials and com ponents, but probably this would be somewhat tedious to the general reader, and we must now press on with our subject, in order to arrive at what is, perhaps, more inte resting, even to those who may have witnessed the sight—• i. e., the actual making of the glass. But, before we do so, we must detain the reader while we explain that glass cannot be made without crucibles and furnaces. In glass making, as in every other complicated and elaborate process, if one branch or department is defec tive or inefficient, the result is marred. Take photography as au example. However perfect the chemicals and the manipulation, if the photographer has a bad camera or a poor lens, he cannot turn out good work. Just so in glass making ; if a manufacturer has bad crucibles, he cannot make good glass ; and, in short, his commercial existence will come to an end unless he has good crucibles and furnaces. Glass can be made without crucibles, viz., in tanks, or what are known as tank-furnaces, of which we shall treat further on, but at the present day the tank furnaces in existence do not exceed five per cent, or ten per cent, of the whole number actually in operation. Crucibles, commonly termed Pots.—We can melt lead in a common iron ladle, and we can boil water in a glass flask ; but if we desire to employ the agency of heat for the fusion or liquefaction of substances which require a higher degree of temperature, such as iron or glass, we must pro cure some vessel to retain it whose properties are such as to resist the fiercest heat of our furnaces. In the melting of metals, crucibles or pots of plumbago (graphite) are largely used, and, on the score of economy, are probably to be preferred to those of fire-clay ; but in the melting of glass, plumbago pots are not equally advantageous, inas much as the alkalies eat into the sides of the pots ; there fore, in the operation of glass-making, the manufacturer is forced to use clay. Clay in the form of a vessel in which to submit substances to heat has been employed for thousands of years. In the Scriptures frequent allusion is made thereto, and no doubt man’s first want—food—sug gested the employment of some vessel in which to cook it,
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