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THE PHOTOGRAPHIC NEWS. Vol. XXVII. No. 1321.—December 28, 1883. Testing Photograglic Lenses 817 Some Suggest.uns as to the Future of Photo-Mechanical Prir49bg Methods 818 Lantern Slides 818 The International Exhibition of the Bristol and West of England Photographic Association 820 “ By-tbe-Bye ” —Too Scientific 821 French Correspondence. By Leon Vidal 822 The Past, Present, and Future of Photography in Natural Colours. By A. A. Campbell Swinton 823 Notes 824 Patent Intelligence 826 PAGE Lessons in Optics for Photographers. By Capt. W. de W. Abney, R.E., F.R.S 82G Review 827 On Rational Development: Notes for the Guidance of Photo graphers. By Herbert B. Berkeley 827 Photo-Lithography and Photo-Zincography. By Major J. Waterhouse, B.S.C 830 Correspondence 831 Proceedings of Societies 831 Talk in the Studio 832 To Correspondents 832 The Photographic News Registry 832 CONTENTS. TESTING PHOTOGRAPHIC LENSES. The names of certain opticians, when affixed to photo graphic lenses, form a guarantee that the instruments are up to a certain very high standard of excellence ; and the photographer who can afford and is willing to purchase his lenses direct from one of these opticians need not trouble himself about testing them, but may take it for granted that they are all that could be desired. There are often, however, times when, could the photo grapher but discover that a lens offered for sale is a good one, he might get it very much cheaper than by purchasing it direct from the maker. Perhaps it is an instrument offered second-hand. It has a good name on it, but the would-be purchaser is afraid that possibly the name may be a forgery, or that, although the name was truly engraved on the brass-work by the optician bearing it, the original lenses have been removed, and inferior ones have been substituted for them. Again, it is well known that a certain percentage of the very cheapest lenses made come up in quality to the most expensive. The photographer who could test these lenses might get an excellent instrument for half the price he would otherwise have to pay for it. Altogether, it is most advisable that every photographer should have the means of telling whether any lens which he may think of purchasing is a good one or not. The method used by opticians for testing the lenses which they make is generally somewhat complicated ; but it must be remembered that the optician wishes to dis cover something more than the photographer does. The optician wishes to know exactly what is wrong with a lens, so that, if possible, a remedy may be applied. The photo grapher merely wishes to know whether a lens is or is not efficient as a photographic tool, so that he may make up his mind whether to purchase it or not. He may therefore use the simplest of appliances. The first thing to be discovered is of what quality the definition given is. To discover this it is always necessary to use an eye-piece. The only question is whether the eye-piece should be used to examine directly the image as formed in the air, or whether it should be used to exa mine that thrown on a piece of ground-glass. At first it would appear that the former method would be the best, and certainly it is if we have the proper appliances for adjusting the eye-piece; but we think we can show that for ordinary purposes of judgment, and where no special apparatus is at hand, the use of the ground-glass is advisable. If we use an ordinary eye-piece of inch and a half or two-inch focus, such as is commonly used when examin ing an image on the ground-glass, it is in most cases quite useless when applied to the aerial image. This is a matter somewhat difficult to explain fully without a diagram, but we may state that with such an eye-piece and with a lens of considerable angular aperture, we cannot examine the image as formed by the whole of the lens, but only that formed by a portion of it. To take a definite example. If we are examining a portrait lens of three inches diameter, and about twelve inches focus, no ground-glass being used, but an examination being made of the image by the eye-piece direct, we can only examine that portion of the image which is formed by a part of the lens of perhaps one inch diameter—this part being at the centre or at one side of the lens, according to the position in which the eye piece is held. In other words, we are examining the image which would be given were a stop of one inch dia meter used, Evidently we have no criterion of how the lens would work full aperture. With an eye-piece of two inches focus, the image formed in the air by an ordinary meniscus lens without any stop will appear quite sharp, whilst we know that no image of any degree of sharpness would be formed by such a lens on a piece of ground glass. As we reduce tire focal length of our eye-piece, we take in an image formed by a larger and larger part of the photographic lens; and when we come to an eye-piece of half-an-inch focus, or, perhaps, a little less, we take in an image formed by the whole of any ordinary photographic lens. The test with such an eye-piece is therefore per fectly efficient; but to use so powerful a magnifier without any apparatus to hold and adjust it, is very difficult. With an ordinary good focussing magnifier, and a piece of smooth ground glass, a very fair examination may be made, whilst, on account of the fact that the ground glass diffuses the light falling on it, the image examined will always be one formed by the whole of the lens. Of course, the easiest way to examine an image on ground glass is, to fix the lens to a camera, and to place the whole opposite some object with a bold outline. The first thing which we require to do is to discover whether the rays parallel to the axis focus nearly at a point; that is to say, whether the definition of the image immediately behind the lens is good. If it be not, then there is no use of going farther with our test. The very best object to focus upon for this test is a sign-board, with sharply-painted white letters on a black ground, or black letters on a white ground. The lens is used full aperture ; that is to say, the largest aperture with which it is intended that it should ever be worked in practice. When focussing is performed, the image should' appear absolutely clean cut under the ordinary focussing magnifier. If it does not, the lens is worth nothing; if it does, we may proceed to test for flatness of field. If the sign-board be of sufficient magnitude to subtend a considerable angle to the lens, we may use it in testing