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

94 THE PHOTOGRAPHIC NEWS. [January 30, 1891. of eighteen feet. If there was greater enlargement, or if the spectator were nearer, want of sharpness might be detected on the screen that was noticeable in the slide itself. He would like the slide to be defined as finely as possible, and for the image to bear closer examination on the screen if desired. At the next technical meeting, in February, views of Austra lian and American scenery will be shown by Mr. Addenbrooke. The Photographic Society of Ireland. A technical meeting was held at 15, Dawson Street, Dublin, on the 22nd inst., Mr. Herbert Bewley in the chair. Mr. H. M. Smith, of the Eastman Dry Plate and Film Co., exhibited the various forms of ‘‘ kodaks, ” including their new No. 4 folding kodak. He also explained the working of the rollable film, and gave instructions as to the best methods of enlarging on Eastman bromide paper, both by daylight and artificial light. He passed round some very fine enlargements from kodak negatives, and subsequently developed a 15 by 12 bromide print, and stated that the developei par excellence for the company’s bromide paper was ferrous oxalate. The North Middlesex Photographic Society. January 26th. — Mr. Goodhew, vice-president, in the chair. Mr. G. R. Martin, in opening a discussion on “ Halation and Light Fog,” said he did not pretend to have more know ledge of these phenomena and their causes than other mem bers of the Society, but proposed to give his opinions for comparison with the experience of others. He had, when the subject was first proposed some time ago, prepared negatives to illustrate his views. These, however, had been destroyed, and as he, most unfortunately, had not had time to repeat them, he would attempt to explain them with the aid of the blackboard. In the first place, he considered that the various kinds of halation and light fog were due to several causes, many or all of which were present at the same time, so that it would be futile to attempt to say to which cause any particular instance was due, and as light fog and halation were usually present at the same time, he proposed to take into consideration all species of fog which were impressed upon the plate during exposure in the camera. Halation in its usual form was a spreading of the high lights into the adjacent shadows, and was generally assumed to proceed from the reflection of these lights from the back of the plate. It had been objected that these rays would be reflected at an angle equal to the angle of their incidence, and so, in many cases, would appear in the form of a second image on one or the other side of the direct image. It was only necessary to remember that the film acted as a disperser of light, causing it to spread in all directions, and, consequently, to be reflected in the same way, to show that this objection was unfounded. The second cause might be attributed to the reflection of small portions of the light in all directions from the crystals of haloid salt in the film. This would probably be small in quantity and entirely on the surface, and could be removed by gentle rubbing with a rag moistened with methylated alcohol. Another cause of fog, when taking a negative with extreme contrasts, was that the light was sometimes reflected from the edges of a badly-finished lens, and even when the lens was perfect, uncondensed light entered and formed a halo on the plate. He then referred to ghost images, and showed by diagrams how they were formed. Prevention.—It had been suggested to back plates with some non-actinic matter, such as stained collodion, gamboge, and gum, or any medium which approached the refraction index of glass. He had in practice found that a piece of the Auto type Company’s black film wetted and applied to the back of the plate was as good as any, was easy to apply and remove, and did not in any way injure the plate, even when kept in contact for two or three weeks. It had also been suggested to mask the windows or other brilliant objects with a screen, and either to take two exposures on different plates, giving a long exposure for the shadows with the screen in position, and a short one for the lights with the screen removed, or to combine the two exposures on the one negative. He had not tried this method, but did not doubt that it was practicable in cer tain cases. He preferred to mask portions of the negative during the procfess of printing, and referred to successful instances of this method shown by a member of the Society at a previous meeting. It had been proposed to stain the film previous to exposure with a non-actinic colour, but he could not recommend the practice, as the exposure would be immoderately prolonged, or the shadow portions of the nega tive would be wanting in density. It had also been proposed to stain the film with orthochromatic dye, or to use orthochro matic plates. He failed to see that any advantage would accrue from the use of either. A cause of halation not usually taken note of would be found in the motes of dust floating in front of the brilliant object. These would tend to disperse the light and produce fog. His attention had been called to a theory that, when the haloid salts in the direct line of the light had been reduced, they would absorb bromine from the adjacent crystals, and these in turn from those farther off, thus causing a spreading of the light. He thought it impro bable, but understood that the Secretary had tested the theory. (The Secretary said his experiments were not conclusive.) Finally, he believed that the best method of preventing halation at the back of the plate, where it was impossible to remove it, was to use a slow, thickly-covered plate. The subject led to an animated discussion, in which Mr. Marchant thought the use of a screen in front of church windows could hardly be seriously proposed. Mr. Beadle had seen it adopted, several men with ladders being employed. Mr. Cox was of opinion that backing plates was next to useless, but had heard a salesman of celluloid films state at a society meeting that with those which had a matt surface hala tion was impossible. When the halation was slight it was no more than was palpable to the eyes when viewing a brilliant light crossed by bright lines. Mr. Mummery referred to the theory of the haloid salts being reduced by the mechanical action of the rapid vibrations of light waves, and thought it possible that the impact might be communicated to the adjacent crystals. Mr. Beadle had found that ground glass plates and the backing of ordinary plates with Brunswick black mitigated halation. The black might be removed with kerosine. He also deprecated the use of powerful developers when halation was suspected. Mr. Smith had found so much emulsion on the backs of his plates that he thought it would cost the manufacturers little to cover them entirely, when a piece of paper could easily be brought into optical contact. Mr. Gill developed his photographs till the high lights were well up, washed them thoroughly, applied vaseline to the parts where development was complete, and then proceeded with the development of the shadows. The vaseline was easily removable from the dried negative with spirit. Mr. Goodhew thought the amount of halation by reflection of light from the crystals of haloid salts would be but little. He preferred thickly-coated rapid plates for interiors, and had taken negatives in Canterbury Cathedral on them in twenty minutes, and had been little troubled with halations. He also deprecated rapid development, and although the practice had been condemned, he flooded his plates with water, believing that by doing so he obtained requisite density without develop ing the halated portions at the back of the film. He was of opinion that orthochromatic plates were useful for interiors, as they were super-sensitive to the yellow part of the spectrum, and the use of a coloured screen would hold back the violet and blue rays. Mr. Martin answered several questions, and received a vote of thanks. The West London PHOTOGRAPIIO Society. A meeting was held at Broadway Lecture Hall, Hammersmith, on January 23rd ; Mr. W. A. Brown (president) in the chair, The President read a letter from Mr. John A. Hodges, resigning his appointment as Hon. Secretary, on account of ill-health. A cordial vote of sympathy with Mr. Hodges was carried unanimously, and ordered to be entered on the minutes,