The Optical Magic Lantern Journal (June 1895)

The Optical Magic Lantern Journal and Photographic Enlarger. 107 picture shall be sharply defined all over, the single lens is or will be discontinued except for objectives of long focus, when a single achromatic answers about as well as the more expensive double one. The form/of objective now in general use, and by far the best, is that of the Petzval portrait lens, as shown in Fig. V. This was designed to pass a large amount of light with the shortest possible exposure in the days when the actual taking of a photograph occupied fifty times as long as it does now. In lantern work a lens of this description can be used with full aperture, z.e., without any stop, and as it can be constructed to give a flat field, the picture will be in focus all over at the same time and sharply defined. But the same lens will not answer for taking a photograph, because the visual rays, or those seen upon the focusing screen and the chemical ones which act upon the salts on the plate, are not coincident unless made so by the lens maker, and this operation is liable to interfere with the flatness of the field ; especially as a lens for portraiture is not required to give a very flat field. Therefore it may be assumed that a portrait lens will not do well for a lantern, and neither can a good photograph be taken with a lantern objective if the best results are expected. Should the lenses of the objective be taken apart for cleaning purposes, the greatest care must be exercised to return them in their proper order, or the definition will suffer, The curves in Figs. TV. & V. are exaggerated to more clearly show their correct positions. In the better class of condensers and objectives the different lenses are loose and secured in their places by screwing a ring down on the top of them. The cheaper qualities are spun in by the thin end of the mound being turned over their edges, which keeps them permanently in position. If the lenses are properly mounted the latter is the better plan, except in the case of the condenser lens nearest the light. It is often possible for the amateur to improve upon the work of the lens mounter; for instance, the definition of a lens of portrait form may be considerably improved by readjusting the distance between the two lenses of the back combination, or the one nearest the light. This can be readily done by bending a strip of card into the form of a circle, and putting it in the place of the metal ring which separates the two lenses ; when the best results are obtained this temporary ring can be replaced by a metal one. There is as much difference of opinion with regard to the proper size of the diameter of the objectives as there is about the shape of con densers, the larger it is up to a certain point the greater will be the amount of light passing through it, as the condenser never does bring all the rays to a point, as it ought to do in theory ; on the other hand the greater the diameter ofalens the more difficult it is to construct and consequently more expensive. A lens two inches in diameter may cost five shillings; the same quality, four inches across, would be valued at something like twenty-five shillings. OC—— Editorial Table. BENETFINK’s CATALOGUE, — This time last year the catalogue issued by this firm (89-108, Cheapside) numbered 16 pages, and yet contained particulars of a good assortment of apparatus. The new catalogue just to hand contains 64 pages giving a choice selection of the cameras, stands, lenses, lanterns, and sundries made by various firms, Woop Bros., LivERPooL, CaTALOGUE.—This is a thoroughly up to date list of apparatus and appliances, and at their premises at 73, Lord Street, Liverpool, a good stock is kept. THE EastMAN PHOTOGRAPHIC AND MatERIats ComPANY, 115 and 117, Oxford Street, W. have commenced the issue of an eight page monthly, The Kodak News the first number of which is before us. Kodakers as well as others will find much interesting information in this new paper. ExposvurE Lapets.—From Geo. Wheeler & Co., 46, King Street West, Manchester, we have received a neat book of gummed and perforated labels marked “ Not exposed No...’ These are for attaching to the edge of dark slides after filling. After exposure, the word “ not” may be crossed out with a pencil or torn off. CycLopzpic PHOTOGRAPHY.—Received just before going to press, and will be noticed in our next. 70; Correspondence. BAGS v. CYLINDERS. To the Editor. Srr,—Your correspondent “The Showman” is very unwise in saying & word against the safety valves of retorts in making oxygen, as I have known even a bigger pipe than half inch choked by a rush of gas and debris. I should certainly advise—Never use a retort which has no safety valve. More than one sad accident has happened from this, and they are no nuisance in any way if properly constructed. The question of Bags v. Cylinders is being practically fought out, and will no doubt be decided by the majority of lanternists in favour of the latter. I, for one, kept on using bags long after the advent of cylinders, but I should not care now to go back to them, although they have one thing in their favour if properly weighted, and that is, the flow of gas is steady, and does not need the continual regulating the cylinder gas does. Very truly, HENRY W. TAUNT. Oxford, May 3rd, 1895.