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

March 27, 1891.] THE PHOTOGRAPHIC NEWS. 251 PHOTOGRAPHIC CHEMISTRY. in. Last Monday night Professor Meldola delivered his third and last lecture on the above subject at the Society of Arts. The speaker stated that when light falls on silver bro mide or silver chloride it darkens in colour, and gives off gas which acts upon paper prepared with potassium iodide and starch ; in these cases it is evident that chemical de composition has taken place, but silver iodide does not undergo decomposition under like circumstances except in the presence of an iodine absorbent. When the silver haloids, free from nitrate, are spread upon paper, different substances can be applied to the paper with them, which substances act either as accelera tors or retarders; reducing agents accelerate, and oxidis ing agents retard or stop the action of light altogether. Sodium nitrate is sometimes an oxidising and sometimes a reducing agent; when applied to a silver haloid it acts as a reducer. Sodium sulphite is also a reducing agent. Silver nitrate is usually considered to be an oxidising agent, but in this case it is a reducing agent because it acts as a halogen absorbent. Mercuric chloride is an oxidising agent and a retarder, so also is a solution of bichromate of potash. In all cases in which a visible image is formed there has been photo-chemical decomposition, but in the example of an ordinary plate in the camera, the exposure is too short to permit the formation of a visible image, and the ques tion arises whether the action is the same in both cases. He was personally inclined to think that the deposits are the same in both cases; still, in the way of direct proof, there is a gap, and the conclusion cannot be treated as an established fact; in relation to this matter our knowledge is in the same state as it is in relation to the coloured products dealt with in the previous lecture. As regards the darkened products, there is proof that they contain a little less chlorine than the unaltered silver haloids, but they are not definite chemical compounds; they contain also a great excess of unaltered haloid ; they are a mixture of silver haloids with unstable coloured pro ducts of photo-chemical decomposition. These coloured solid products may be compared, though it may seem strange to do so, to saturated solutions. Dr. Otto Witt is also of this opinion. After this saturating point is reached, light produces no additional effect unless a reducing agent be present to absorb the halogen as fast as it is liberated. Forinstance, if silver chloride be put in a tube, and submitted to the action of light, while moist hydrogen is passed over it, and that hydrogen be then made to bubble through silver nitrate, in the latter solution a precipitate of silver chloride will fall, and much more than if air had been used instead of hydrogen. This experiment was originally devised by Robert Hunt. The dark deposit in the case of chloride of silver is certainly not metallic silver, for it can be formed in the light while the chloride is under nitric acid. Whether the deposits contain oxygen is a problem demanding additional research. The speaker, after showing a few experiments to prove that water in contact with silver haloids acts as a weak sensitiser, said that silver chloride will darken in a high vacuum, but when the bulb is taken into the dark, the deposit in its interior soon absorbs the chlorine again, as Mr. Crookes has experimentally proved. He next spoke of the chemical and physical theories of the latent image, and argued that the probabilities are in favour of the former being the true one. He incidentally mentioned that Dr. Hurter does not hold this view. As regards the ripening af emulsions, he said that some experiments he was making tend to show that some amount of chemical reaction is set up between the silver haloid and the gelatine. In investigating the problem, less atten tion should be given to the state of molecular aggregation of the haloid, and more to the sensitive substance formed, upon which a developable image is afterwards produced. A very slow reducer acting upon a photographic film for a short time has the same effect upon it as a short exposure to light, and can be made to form a developable image. Glucose, with a little alkali, is a feeble reducing agent, and can be used for this purpose. That the kind of pressure called “shearing stress” will do the same was first revealed by Carey Lea, but, by pressure, the lecturer had been unable to make developable images upon any of the pure haloids ; it was necessary that some foreign substance should be present to act as a sensitiser. Mr. Spring, of Liege, has proved that by sufficient pres sure chemical reactions can be set up ; so in the case of an invisible image produced on a gelatine plate by shear ing stress, it is probable that the pressure has produced a chemical effect. The Chairman, Mr. Francis Cobb, moved a vote of thanks to Professor Meldola for his three lectures, and said that it is his personal opinion that the invisible image is not of a physical nature. Last week, in our summary of Professor Meldola’s second lecture, we, by an obvious slip, described corrosive sublimate as mercuric oxide instead of mercuric chloride. When the full text of Professor Meldola’s utterances on the invisible image is published, we may make some criticisms thereupon. PICTORIAL COMPOSITION.* BY P. H. NEWMAN. The successful application of light and shade is but a poor achievement if unaccompanied by sentiment, although the flat surface of the picture be entirely overcome. We demand the magic influence of mid-day, sparkling in abrupt transitions ; the mingling shades of dawn and twilight; by the powerful influence of light and shade we crouch at the storm and shiver in the breeze ; its poetical influence is independent of subject. In proportion as a subject is poor, compensation must be made by the interest given to the sky. It is the sky which unites the several parts of a landscape into one harmonious whole—it enriches poverty by supplying new forms, or by affording contrast to those which the subject itself possesses. It is to the sky that the landscape painter must look as the great source of pictorial effect worth his recording. Varying for ever in form and colour, its fitful changes are communicated to the landscape—now obscure, now bright. Terra firma may, indeed, afford a good subject; but from cloudland we must derive impressive sentiments. Often when the student has roamed far and wide in search of something “ worthy of his pencil,” he would have done better to have remained at home, and besides looking to the earth for subjects, have looked upwards to the sky for sentiments in which to clothe them. Without senti ment the painter of landscape would make his mountains high, but not inaccessible to all but thought; large, but not immense ; his gulfs might be deep, but they would not be profound or terrible ; he might paint a storm or a shipwreck, but he would to fail suggest “ the remorseless dash of the * Concluded from page 231.