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The photographic news
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- 27.1883
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- 1883
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- Bandzählung
- No. 1293, June 15, 1883
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The photographic news
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- Titelblatt Titelblatt I
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THE PHOTOGRAPHIC NEWS, Vol. XXVII. No. 1293.—June 15, 1883. CONTENTS. PAGE Abney’s Experiments on the Effects of Pressure on Sensitive Compounds 369 Inequality of Illumination of the Image Given by Different Lenses . 370 A Multiplex Camera Back. 370 Selatine.zersus Collodion 371 A FEty Non-Actinic Colour. By A. Percy Smith, F.C.S., Half-a-Dozen 370 News from the Eclipse Party 373 Review 374 PAGE French Correspondence. By Leon Vidal 374 Notes on the Development of Gelatine Plates. By W. B. Allison 375 Notes 375 Patent Intelligence 378 A Tour in Kent. By Photo-Cyclist 379 Bristol International Photographic Exhibition (Triennial) 380 Correspondence 380 Proceedings of Societies 381 Talk in the Studio 384 To Correspondents 384 ABNEY’S EXPERIMENTS ON THE EFFECTS OF f PRESSURE ON SENSITIVE COMPOUNDS. t The results obtained by Captain Abney, and communi- € cated by him to the Photographic Society of Great Britain ' on Tuesday last, possess unusual interest on theoretical I grounds; and, indeed, also from a practical point of view. : Captain Abney finds that when a gelatino-bromide film is 1 rubbed by a hard body—as, for example, a glass rod— 8 each line of friction comes up in development, just as if the < film had been exposed to light at the points of contact. 1 The similarity of the results produced by friction on the one hand, and exposure to light on the other hand, holds ' good so far that an oxidising agent, such as bichromate of I potassium, serves to destroy the latent image in either ’ case. I While Captain Abney was reading his paper, it struck us that the results might in reality be due to the action of light, as a phosphorescent light is well known to be generated when two white pebbles or other silicious bodies are rubbed against each other, and many organic bodies, such as loaf sugar or gelatine, become phosphor escent when rubbed ; this phenomenon being easily demonstrated in a dark room. In the first place, we proceeded to repeat Captain Abney’s experiment, and we found the results to be precisely as be describes, every line traced on the film with a glass rod coming out dis tinctly under the action of the developer. In order to obtain as full a realisation of the effect of pressure as possible, we continued to rub a portion of the film side of a plate until a considerable patch—perhaps half-an-inch by two inches—was thoroughly covered with the friction- lines, and on development this patch came out nearly black and opaque. When, however, the back of the plate was inspected, we noticed that lightening of the general tint or incipient reversal which is almost always to be noticed when a plate is partially exposed to light; and this circum stance served to confirm our view that light, and not pressure, had been the active agent. It need scarcely be said that the pressure on both sides of the film during friction with a glass rod would be practically equal, as the rigidity of the film must be regarded as being so small as to have no influence worth mentioning ; and if the change in the condition of the bromide had really been brought abont by pressure, one would expect to find a dark image developed all through the film. During the time we were occupied in rubbing the film with the glass rod, traces of light were visible ; but this could only be seen after the eye had become thoroughly accustomed to the darkness. Assuming our phosphorescent theory to be true, it should certainly be practicable to impress the film by friction on the reverse side of the glass, and the experiment was accordingly made. A very much brighter phosphorescent light was visible in this case, and, after a few strokes, the plate was developed ; but no effect was traceable on the film. Knowing, however, that phosphor escent light does affect the sensitive gelatino-bromide film, we repeated the experiment, rubbing vigorously for nearly half-an-hour. In this case, a distinct effect was traceable, although considerably less than could have been produced in a few seconds by direct friction on the film. In a similar way, we rubbed the back of a plate with a piece of gelatine, and found that the light developed was sufficient to affect the film. Some substances which phosphoresce energetically when rubbed—such as fluorspar, mother-of-pearl, and gypsum—were next employed instead of the glass rod, and the effect on the film was found to be proportionately greater. It is quite possible that friction or pressure, as well as light, may be efficient in producing the effect described by Abney, but our experiments seem to point clearly to the circumstance that the result is due, at any rate partially, to the action of light. The circumstance that so enor mous a difference exists between the action when the film itself is rubbed, and when the back of the glass is subjected to friction, seems at first sight rather to indicate an essential difference between the two cases; but it is quite possible that the molecular impulses which consti tute light may affect the bromide much more powerfully when they originate in the sensitive film itself ; the light being in this case comparable to an element in a nascent or active condition. Our readers will remember that Mr. A. L. Henderson added phosphorescent sulphide of calcium to a gelatine emulsion, and found that the sensitiveness of the prepara tion was so much increased as to render it almost—if not quite—impracticable to work with the mixture. It is possible that in this case the sulphide may have absorbed the light during exposure, and transmitted the impulse to . the bromide in such a manner as to enormously increase । the practical efficiency of the original radiations. The : persistency with which phosphorescent bodies retain s traces of light is surprising, sulphide of calcium i having been known to shine in the dark two months ! after exposure ; and, indeed, after being kept in the dark t for two years, a sample was found to evolve light when i heated to 150° Centigrade. We would, therefore, suggest ) that any persons making experiments on the addition of , the sulphide to emulsion, should heat it to redness, and i allow it to cool in absolute darkness before using it. • When the sulphide is thus exhausted of light, it will be e found to be recharged, up to a certain point, even by the i red light of the developing room. e It may be mentioned that light may be driven from r most phosphorescent bodies by heating them and cooling
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