The photographic news

• Bandzählung: 35.1891
• Erscheinungsdatum: 1891
• 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-id1780948042-189100009
• PURL: http://digital.slub-dresden.de/id1780948042-18910000
• OAI: oai:de:slub-dresden:db:id-1780948042-18910000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

September 11, 1891.] THE PHOTOGRAPHIC NEWS. 645 ON SOME CONDITIONS AFFECTING REVERSAL OF THE PHOTOGRAPHIC IMAGE. BY HENRY SUTTON. In drawing to a conclusion the discussion on Professor Minchin’s paper at the last Conference of the Camera Club, Captain Abney remarked that the blue rays have a reducing and the red rays an oxidising effect. Of course Captain Abney taught us these facts many years ago, but I mention their expression at the Conference because it immediately occurred to me—though why I cannot say, for Professor Minchin’s entirely different subject was dis tinctly before my mind—that reversal of the photographic image must be a result of an over-balance of the two effects, by the exposure being continued for such a time as to allow the reducing action of the blue rays to act all over the plate ; in short, to produce fog, even in the pro tected parts, it would allow the red and yellow-red rays time to re-oxidise such parts as are more or less unpro tected. It occurred to me that, by exposing a plate to ordinary light, so that the blue rays may exert their power of re duction, and then subsequently exposing such reduced (or fogged) plate, in contact with a negative or positive, to red rays, by interposition of a red screen, the result on development should be reversal. On putting the matter to the test, I found such apparently to be the case. Captain Abney, Colonel Waterhouse, and Fox Talbot have noted the advantage of a preliminary exposure in assisting the reversal of image, but I have not noticed attention drawn to the connection between the preliminary and subsequent exposures, or the manner in which such a reversal is possibly produced by the relative action of the reducing and oxidising rays. So that in this paper no claim is set up how to produce reversal, nor yet to the effect that the red and yellow are the active rays in so doing (for, in the language of the patent office, “I am aware all this has been done before ”), but what I desire to call attention to is how they probably act, and that in producing reversal a proper relation should exist between the reducing and oxidising rays. By a considera tion of the subject from these points, an every-day method of producing reversed negatives suitable for collotype and carbon work may result. From the influence fog seems to have in producing the results, I am afraid it will be impossible to produce reversed negatives suited to ordinary relief block processes, in which clear glass is of more im portance than density. In the following experiments very powerful exposures are used, and the results, except where otherwise stated, developed with Wood’s hydrokinone developer used in strength as directed on the label, and therefore normally developed. It is interesting to note that development required about the same time as a nor mally exposed plate, and hence contrasts very markedly with the results obtained by Mr. Humphery when using hydrokinone, as stated in his paper read at the Club last October. In my experiments, judging a plate by its colour, I note such as appear to contain iodide produce the best results; this, I believe, is in agreement with the experiments of Captain Abney and others. The theory of reduction and re-oxidisation suggested by the following experiments, it will be noticed, explains the alternate reversal, such as negative, positive, negative, positive, sometimes mentioned in the journals. These alternate reversals have not come under my notice ex perimentally. Experiment 1.—Expose an Edwards’ landscape plate to white light for one minute, then, still in light, placed in printing frame in contact with a negative, but separating them by a tissue of red stained gelatine. This was ex posed to direct sunlight (July) for five minutes. On examination in the dark room by ruby light, it showed a delicately printed-out positive. It was then developed with Wood’s hydrokinone developer, to which a small portion of same firm’s eikonogen solution was added. liesuit.—The plate developed in the ordinary manner, except that the positive print-out image disappeared shortly after immersion in the developer, and in less than one minute the reversed negative had begun to appear, rapidly gained density, and for ordinary work little inferior to the original. Experiment 2.—Edwards’ transparency plate, prelim inary exposure to white light, then in contact with positive exposed for twenty minutes to diffused sunlight through red gelatine tissue. Result. —Fog all over ; no reversal. Experiment 3. —Same conditions and result as Experi ment 2. Experiment 4.—Preliminary exposed Edwards’ landscape plate to direct sunlight half minute; then exposed to direct sun through red tissue for five minutes. Result.—Good reversal. Note.—Unless otherwise stated, Edwards’ landscape plates are used. Experiment 5.—As Experiment 4, but exposed ten minutes through red tissue. Result.—Reversal, but rather thin. Experiment 6.—No preliminary exposure ; then exposed through red tissue five minutes’ direct sun. Result.—Developed thin negative, which gradually re versed to thin positive on front surface, and remained thin negative on back surface of film, On fixing, the result is barely visible. This experiment should be compared with Experiment 4, being under similar conditions, except that Experiment 4 had half a minute preliminary sunning, and produced a good reversal. Experiment 7.—In order to have some idea of the deve loping value of the preliminary exposure, each side of a plate was exposed to candle-light at C in. for ten seconds; total exposure, twenty seconds; then exposed with red tissue to sun (haze) for four minutes. Result.—Very good reversal, and would indicate that the preliminary exposure should be such as to produce, if developed, a good density. In this experiment an extra minute of exposure to sun would have produced clearer glass. Experiment 8.—Preliminary exposed plate to candle light, as in Experiment 7 ; then exposed ten minutes to diffused sunlight without red tissue. Result.—Reversal, but not so good as 1, 4, 5, 7. Experiment 9.—Preliminary exposed Edwards’ trans parency plate, as in Experiment 8; then exposed, without red tissue, to ten minutes’ diffused sunlight. Result.—Exceedingly weak reversal. Note.—Where transparency plates are used, they are given an increased exposure, on account of the lower sen sitiveness. They are not suited for reversal, but the landscape plates are particularly well adapted for it. Experiment 10.—Preliminary exposed landscape plate to candle-light; then covered half with red tissue, and ex posed to 12 in. of magnesium at 12 in. distance. Result.— No reversal under tissue, but reversal where