The photographic news

• Bandzählung: 29.1885
• Erscheinungsdatum: 1885
• 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-id1780948042-188500006
• PURL: http://digital.slub-dresden.de/id1780948042-18850000
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• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Bemerkung: Seite I-II fehlen in der Vorlage. Paginierfehler: Seite 160 als Seite 144 gezählt.
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

AUGUsT 28, 1885.] Photographs for red prints are mostly always enlargements of the originals in the proportion of one to three. By reducing the drasving afterwards to the original size, the lines are rendered extremely sharp and delicate. Etchings and relief plates man aged in this manner are frequently of a beauty equal to any well executed wood engraving. The making of red prints for the engraver or draftsman, simple as it may appear, has its difficulties, as well as any other photographic process. The tone of the print is of great import ance ; if inclined to be black, or when printed too deep, the draftsman is unable to see the ink lines, especially when very fine. The negative must also be of a certain quality ; all details must be visible, and great contrasts between light and shade must be avoided; still the plate must be able to give a well- defined and somewhat brilliant print. Negatives for red prints, which are mostly made on collodion, are somewhat over-exposed and slightly intensified. Ordinary Rives paper is used for print ing, salted with chloride of ammonium or barium, from 10 to 30 grains to the ounce of water. Citric acid, the same quantity as the chloride, may be added to the salting bath, or the salted paper, when dried, may be floated for a minute, before silvering, upon an acid solution of the same strength. The strength of the silver should be regulated according to the strength of the salting bath ; weak solutions are, however, preferable. After the proof has been printed to the desired depth, it is washed in water acidulated with citric acid, and after all acidity has been washed away, fixed in a fresh hyposulphite-of-soda solution. 1'8. Organic matter, like gelatine or boiled starch, in the salting- bath, assists in keeping the red colour. A brick-red or a muddy yellowish tone is probably caused by too strong a silver solution or impure hypo. The quality of the negative has as much to do with a good tone of the print as in any other process. The washing of the print must be done quickly and thoroughly. Hypo in the print prevents the per fect bleaching of the print with mercury; too much washing softens the paper and makes its surface woolly ; hence, the addi tion of gelatine or starch is advisable also to impart strength to the paper. The red tone of the print is of great importance ; it assists greatly the engraver’s work. The fine lines he draws upon the photograph are more distinct and more visible to him than when made upon a bluish-bl ick ground. There is a great demand for silver prints in the engraving district of New York, and when they are made in the manner described, or by a similar process, they are always satisfactory to the engraver. These prints find a practical application. How to obtain beautiful and decided red tones has been de scribed repeatedly. The methods with bichromate, grape sugar, and silver development, and many others, give splendid results, and are highly interesting to the student as well as the practi tioner, but neither of them has gained such a popularity as the simple so-called silver print.—C. E., in the Photographic Times. PHOTOGRAPHY AND THE SPECTROSCOPE- Lecture II. BY CAPTAIN W. DKW. ABNEY, R.E., F.R.s.* PHOTOGRAPHIO spectroscopy is the easiest thing in the world when you know how to do it, but it requires a deal of patience to learn every dodge. As a rule, a photographer is a patient man ; indeed, there ought to be no class of men who have more patience than photographers; hence spectroscopy should not be difficult to them. Here is another piece of apparatus which is very useful in the spectroscope. It is an apparatus by which you c in take a great many spectra on one plate. I need not enter into its details ; it is simply a dark slide, by which a rack and pinion motion can be raised, so that the plate gives a fresh surface at each exposure. The only light accessible to the plate comes through an opening of about three-quarters of an inch wide, cut longitudinally in the shade. By this method we can get about sixteen dififerent spectra of different materials on the same plate. Here is another piece of apparatus which is also useful in investigations with photography. It is a slide in which you can expose plates in different gases or liquids—that is to say, in water, in alcohol, in nitrogen, and so on. It is essentially a glass cell which slips into a dark slide especially adapted for it; on the top there is an air-tight junction, which is screwed down, and there are two little tubes through which you can fill the cell • Continued from page 540. with gas or water, or whatever other material you wish to use. This is very useful in investigating the behaviour of different sensitive salts under different conditions of moisture, pressure, &c. This cell has been used in a great many hundred experi ments, and I hope it will be used in a great many more. Those who are going in for spectroscopy should not be without such an apparatus as this, for I do not believe much real investigation can be done without something of the kind. The sensitive salt of silver acts differently when isolated from its atmospheric surroundings, and the only way to ascertain how it does so is to expose it with other surroundings, and to differentiate the re sults one from another. There is no such thing as a perfect vacuum ; you cannot say you expose a plate in vacuo, and, for this reason, I say you have to differentiate between the different media in which you expose a plate, in order to get at the true result which would happen supposing you could expose the plate in vacuo. You saw last time how you could recombine a spectrum by means of a lens, to form white light. Now, I want to show you that it is not impossible to develop a plate in white light. I expose a plate behind a negative to the electric light, and in the cell which is placed in the patch of white light is some developing solution (which is quite colour less). The plate is dipped into it. The image comes out into it, although exposed to white light, without fog, which was sup posed to be an impossibility. I have another plate placed behind the same negative. I expose half of that plate for half a minute to the white light on the screen, and the other half to apparent darkness, but in the same position on the screen for a couple of minutes. The plate on development shows that the half which was exposed to what was presumably white light gives no image, while the half exposed in the dark shows a per fect picture. I dare say many of you have guessed my trick, for it is merely a trick ; but for those who have not, I will show you how it is done. It is perfectly easy, by mixing two ele ments of light of different refrangibility, to produce a colour which, at all events, to our eyes is a white light. But you must not take it for granted that wherever you can see white light you can photograph with it, because it is quite possible you may not. It is only a trick, but some of these tricks bear fruit in a very practical manner. I will re-form white light again, and wo will examine it by means of the colour-chart I showed you last time. You will see that when the red is placed in the white light there is blackness—no colour whatever—the yellow looks bright, as does the blue ; all the other colours are gone, except some few which are of a nondescript colour. The meaning of it is this: we have simply a combination of yellow and blue, which gives us the appearance of white light. [The blue and yellow rays were shown to be coming through two slits placed at the focussing screen of the camera.] The blue has no power of act ing on the iodide or chloride of silver, neither has the yellow, and, therefore, the white light, which is made by the combina tion of those two colours, is powerless to act on films made of such materials as those. We can also produce a white light, practically, by a red and green, and if we examine this (which is a very good imitation of white light) in the same way, you will not see the whole series of colours in the colour chart any better than you did before. The red comes out perfectly, but the blue is no longer visible ; the blue becomes green, and the violet becomes red; the yellow is also not intense. This is because we have only two colours present, viz., the red and the green. The apparent darkness to which we exposed the one- half of the plate was in reality the dark ultra-violet light, and I need say no more regarding that. I told you last time that this was a very interesting way of studying the spectrum. You see how, by combining two lights together, you may have a light which is perfectly safe for cer tain salts of silver. On the screen is the spectrum taken on the three ordinary salts of silver—chloride, iodide, and bromide. The iodide stops exactly at the violet. Below that light we have no action whatever, and we therefore may expose an iodide plate with impunity to any rays below the violet. A bromide plate, you see, is sensitive down as far as the yellow, and, therefore, it would be impossible to develop a bromide plate in such a light as I showed you just now, whereas iodide is per fectly capable of being developed in such white light; the chloride again stopped very nearly with the limits of violet, so that it would be safe to develop a chloride plate in such a light. [The lecturer concluded with a brief explanation of the diffrac tion spectrum.]