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The photographic news
- Bandzählung
- 35.1891
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- 1891
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1691, January 30, 1891
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The photographic news
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January 30, 1891.] THE PHOTOGRAPHIC NEWS. 85 LANTERN SLIDE MAKING* BY EDGAR G. LEE. Confining my work almost entirely to the production of lantern slides, I have thought it expedient to take up a portion of your time to-night by contributing a few rough notes on this interesting and important branch of the photographic art. At the present day, by far the most popular process is the gelatino-bromide, although the collodion, both wet and dry, and other processes, give more latitude as regards the production of a good slide from an indifferent negative. First of all, it is necessary to consider what constitutes a good slide : one in which the highest lights are repre sented by clear glass, and the deepest shadows are of such a density as to be easily penetrated by a fair average light in the lantern. A good slide need not be composed of, say, equal proportions of clear glass and opaque deposit, like more than one make of commercial slides in the market. So long as we have a trace of clear glass to add brilliancy to the picture on the screen, it may with advantage, as seen out of the lantern, appear a trifle thin. To produce this goorl slide, having the characteristics which I have mentioned above, a negative is required showing considerable contrast. Personally, I prefer one of the type which yields a brilliant or plucky silver print. Whilst, on the one hand, a negative of the most extreme density will make a passable slide, thinness and want of contrast of the character which is suitable for bromide printing is, for slide work, absolutely hopeless. Coming to the actual working, we have two methods: we may reduce in the camera, or print by contact from small negatives. To the latter method I confine myself almost entirely, as nearly all my negatives are quarter plates. The first point to be considered is naturally ex posure, which may be made to a paraffin light, gas flame, or magnesium light —daylight, of course, being too strong and uncertain. Gaslight is, generally speaking, most useful, and exposures with the printing frame two feet from an average burner may run from thirty seconds to ten minutes, varying according to the density of the nega tive. For very dense negatives, exposure to magnesium light is to be decidedly recommended. With a negative verging on thinness, the exposure must be fairly exact, but with a dense negative and certain brands of lantern plates the latitude is simply enormous, and much greater than is usually supposed. With such a negative the exposure may be ten times the normal, and a perfect slide result from it—of course, varying in tone according to the ex tent of exposure. I have observed that the general ten dency of beginners is to under-expose, which is fatal, as the image either develops up patchy, or veils over before density is obtained. A lantern plate can not be forced in development. As to development, we have the choice of four reducing agents: ferrous oxalate, pyro, hydroquinone, and eikonogen. Each and all of these are capable of producing equally good results in the hands of careful workers. The easiest, by reason of the latitude it permits, is hydroquinone, the tone being dependent partly upon the negative, partly upon the exposure, and in no small degree upon whether the development is rapid or gradual, a remark which will be found to apply to any developing agent. It is not my intention to recommend any one formula, believing * A communication to the Newcastle-on-Tyne and Northern Counties Photographic Association, as I do that there is nothing magical in any prescribed combination of chemicals. My advice would therefore be to choose a well-tried and reliable formula, and stand by it. For warmth of tone pyro is, without doubt, the best developer, and as the tendency is now in that direction, it merits our attention. There is no difficulty in its use if we remember its staining properties, to counteract which it is always employed with a much larger propor tion of preservative. The image, when fully developed, has an over-done, sunk-in appearance, and unless this point is arrived at, the slide, when taken from the fixing bath, will be found much too thin. After fixing in a perfectly clean hypo bath, and a thorough washing, the slide will in all cases be the better for a short immersion in an alum and citric acid bath, which clears up the picture and improves its brilliancy. If too dense, or fogged beyond the capacity of the acid bath to reduce, the ferricyanide of potash and hypo reducer (made distinctly alkaline with ammonia) should be resorted to. Intensification at its best (which best may be represented by mercury and cyanide of silver) is but an unsatisfactory proceeding, and much the better alternative is to make another slide. Intensifying by mercury, followed by various alkalies, yields warm tones, varying with the agent used, many of them very fine, but all of doubtful perman ency. After fixing and washing, slides may be toned in a gold and sulphocyanide bath through a long range of bluish tones, useful for moonlight effects. Mounting calls for no particular mention, but I cannot refrain from adding a vigorous protest against the con tinued use of masks of circular and other eccentric shapes. Broadly speaking, the oblong form suits most subjects, and produces the best effect. A safe rule for guidance is to fit the subject to the mask when taking the negative, and not to fit the mask to the subject at the last moment before binding the slide. Increasing the Sensitiveness of PHOTOGRAPHIC Films.— The following process is by York Schwartz, of Hanover, Germany. It consists in mixing the materials for making the film with, or coating the photographic films with formalde hyde or a compound of formaldehyde with bisulphite salts. The process is based on the discovery that the sensitiveness of silver compounds to light is materially increased, or that the effect of light on the same is continued by the presence of form aldehyde, or a compound of formaldehyde with a bisulphite of an alkali metal, or of ammonia, or of substituted ammonia (capable of forming bisulphite), in the sensitive silver com pounds. For photographic purposes these discoveries may be utilised as follows:—First, by adding one or more of the specified compounds, either alone or in mixture with other substances, to the substances for making the films at the time of the production of the sensitive film ; second, by bathing the finished photographic film in a solution of the said compounds or mixtures before the film is exposed to light ; and third, by treating the photographic film after exposure with a solution of the said compounds or mixtures. In this case the changes produced in the film by the action of the light are continued, so as to obtain in this manner the same result as by a corres pondingly longer exposure to light. The application of the said compounds constitutes an important improvement in the manu facture of photographic films, because they possess in an increased measure all advantages of the substances hitherto employed for similar purposes, without sharing their faults— for instance, the tendency to form clouds or specks, or of impart ing a yellowish tint to the film. —Scientific American,
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