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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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- Englisch
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- F 135
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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. 1690, January 23, 1891
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The photographic news
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Band 35.1891
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Band 35.1891
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January 23, 1891.] THE PHOTOGRAPHIC NEWS. 65 The author expresses the following opinions about condensers:— Condensers are usually made of crown glass, density about 1’516. This answers perfectly for ordinary purposes ; but for the highest glass of modern exhibitions, which have to cover large screens of twenty to thirty feet diameter, sometimes at great distances, it is objectionable, as the green colour absorbs light which can ill be spared in such circumstances. Chance’s “optical” flints, of course, leave nothing to be desired in this respect, but condensers so made are very expensive ; in some cases, however, they are worth while, as the glass is both more colourless, and the lenses are thinner for the same focus. A more common flint would answer practical purposes, and it is desirable that some colourless glass should be introduced if possible, rather than green crown. This need not make them so very much more expensive ; for while “optical ” flint must be homogeneous, and is usually wanted dense for a condenser, a perceptible amount of strife is of little practical importance, and density is not required, only colourlessness. It is utterly useless to pay the cost of optical perfection in a condenser for any ordinary purposes. A perceptible bubble in the lens next the slide, however, would be a defect of importance, probably showing as a black spot. It is very likely that many purchasers would reject a colourless lens with perceptible stria), rather than a crown lens which showed none ; nevertheless there can be no question that the first would be the better condenser, unless the stria) were excessive. The point most commercial condensers chiefly fail in, how ever, is that the lenses are not ground thin at the edges ; nearly all lantern makers being far too careless in this respect. Not only is a needlessly thick lens much more likely to crack, and more absorbent of light, but it is distinctly worse in optical performance. The lenses should be ground to as nearly knife-edges as possible, being only just edged down for fixing into their cells. Both lenses should be mounted so loosdy in their brass cells that they can be turned round with the fingers, else they may crack merely from expansion when healed, and they often become hotter than the hand can bear. Holes must be pierced in the margin of the cell, to allow of the escape of the aqueous vapour which always forms when the lantern is first lit. Only the back lens ever cracks from heat alone. If the crack be irregular, the lens must be replaced ; but it often happens that the crack is quite straight across a diameter, in which case the cracked lens will answer perfectly well if the crack is arranged perpendicularly, and the operator may feel certain that his lens will never crack again. In speaking of single lenses as compared with photo graphic portrait lenses—or modifications thereof—for pro jections, Mr. Wright gives useful information about a French single combination, consisting of a concave lens of flint cemented between two double convex crown glass lenses, the whole then assuming a double convex form. Mr. Wright says:— Some of these lenses perform exceedingly well. They were used a great deal by Mr. Dancer for his lanterns, and I possess a pair of them, 6-inches focus, whose performance can hardly be distinguished from that of the best of the construction next to be described. They appear to me to combine flatness of field, evenness and sharpness of definition, and ortho-symmetry of image, in a greater degree than any other single lenses, and I think it is to be desired that more attention should be directed to this construction for long focus work. My pair of 6-inch lenses have a clear diameter of 1J inch, and require a 1-inch stop placed about 2} inches in front to produce their best effect. In this position the stop cuts off scarcely any rays of serious importance, and the image of a slide of printed matter is exceedingly good. For lenses of 9 inches focus and upwards no stop whatever would be required, and such lenses would be much cheaper, and pass more light than double com binations. The chief points to be attended to in projection lenses are to see that they are of sufficiently large diameter to take in all the cone of rays coming from the condensers, and that they give a flat field combined with good definition. About the petroleum oil for use in lamps, Mr. Wright says that the “common trash” often sold as “paraffin oil” should not be used, and that much of it “is not safe to use in any lamp, and none of it gives a good light; ” only the best refined paraffins should be used. If such dangerous oils are in market, and in weather, too, when there is a run upon them for household purposes, the Government should be made aware of the infringe ment of the law as regards the flashing-point of the said oils. Notwithstanding the hundreds of tons of such oils burnt in London for heating purposes during the recent severe weather, we are not aware of a single accident therefrom having been recorded in the newspapers. In relation to oil lamps, a word of warning might have been uttered in the book, about those of faulty construc tion, so made that the light cannot, in certain cases, be brought so near to the condenser as when the limelight is employed. Information also might, with advantage, have been given about the merits of circular wick lights, such as yielded by Trinity House two-ring burners, as compared with the ordinary three-wick lamps, and the slight uneven ness of illumination of the screen given by the latter. The fact that oxygen gas passes through india-rubber, but less rapidly than the nitrogen, does not seem to be generally known. Attempts have been made to obtain, by this method, air rich in oxygen available for practical purposes, but as yet it has not been done to commer cial advantage. In relation to this passage of oxygen through india-rubber, Mr. Wright states :— Gas should never be kept in a bag any length of time. Oxygen acts rapidly upon the material ; but independently of this, no gas can remain pure, owing to that wonderful process of diffusion through the material which physicists call osmosis. A bag filled with perfectly pure oxygen will contain a portion of air in some hours’ time, and any bag tight in the morning is perceptibly slackened by night. Supposing gas to be left over one night, to any amount, there is no reason, if con venient, why it should not be kept, and merely filled up for the next night; but beyond this gas should never be kept, or the light will be perceptibly affected. If in photographic research the experimenter could get coloured glasses to give all the more pronounced colours of the spectrum quite pure, such a set of glasses would be extremely useful, and the author of the book under notice says that Mr. Madan has discovered that, by superposing Chance’s signal green on a rich cobalt blue glass, the two only transmit rays between F and G. This, therefore, is the way to obtain a true blue by means of flat glass plates, and is an interesting fact worth notice. The work contains useful practical information about lime-light jets, photo-micrographic instruments, and the application of the magic lantern to purposes of scientific demonstration. The book contains practically nothing about the history of the magic lantern. Those inte rested in the latter branch of the subject will find much about it scattered up and down in pages of this journal during the last five or six years. Taking Mr. Wright’s book altogether, it is about the best we have seen on the subject, but is defective in not going into sufficient detail about the optical system of the lantern, the relative merits of various condensers, and the focal lengths and other points about the lenses of which they are composed. It also is too meagre in information about oil lamps.
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