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The photographic news
- Bandzählung
- 29.1885
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- 1885
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- Bandzählung
- No. 1425, December 24, 1885
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The photographic news
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Band 29.1885
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THE PHOTOGRAPHIC NEWS. Vol. XXIX. No. 1425.—December 24, 1885. CONTENTS. PAGE Some Optical Matters in Connection with Stereoscopic Photo graphy 817 Captain Abney’s Lecture 818 Practical Chemistry for Photographers. By Ralph W• Robin- g18 Communications from the Photo-Chemical Laboratory of the Imperial High School of Berlin. By Dr. W. II. Vogel... 810 The New Winter Health Resort. By “ Jean Ville ” 820 The Appearance of the Image During Development. By F. C. Beach 821 PAGE Photographic Apparatus at the English Patent Office. By Chapman Jones 823 Notes 824 Patent Intelligence 825 A Tale of a Dry Plate. By W. S. Gilbert 828 Whore Art Begins. By II. Nisbet 829 Bleaching Drawings made upon a Photograph, and the Making of a Suitable Photograph. By W. W. Bode 830 Proceedings of Societies 830 Talk in the Studio—Answers to Correspondents 832 SOME OPTICAL MATTERS IN CONNECTION WITH STEREOSCOPIC PHOTOGRAPHY. It has always been a matter of some surprise to us that the stereoscope has fallen so much into disrepute during the last few years. That it has so fallen is, however, in disputable. Every effort to raise it into popular favour again seems, moreover, to meet with total failure more or less nearly complete. In the Photographisches WocheMatt, Von F. Stolze has a long disquisition on the instrument, in the course of which he certainly points out very many defects possessed by it in its present form, which have, so far as we are aware, always been more or less overlooked. We cannot go so thoroughly into the matter as he has, but will give an out line of what he says on the subject. We think it will be useful to our readers, especially as in the end Herr Stolze describes an arrangement whereby the defects are over come. The main defect of the stereoscope is that it shows the objects not as they are seen—barring colour, of course—but as minute models of these objects ; models standing out in bold reliefs, it is true, still, only models. The principle whereby the stereoscope is enabled to produce the appearance of solidity is, that it presents to each eye a slightly different picture. As each eye, in look ing at an actual object, being a little distant from the other, sees the object from a somewhat different point of view, the mind, either in the case of the actual object, or of the stereoscopic reproduction of it, fills in, by what may be termed a reflex trigonometrical effort, the relative dis tances of the various parts. To produce the stereoscopic effect, it is of course neces sary that similar points of the true photographs should fall on corresponding parts on the retinas of the two eyes. “ Hereby the size of each picture is at once limited to a maximum breadth of 2} inches, or even a little less,” that being the average distance between a pair of human eyes. This limitation at once produces liability to a certain error. A stereoscopic picture must always be so arranged that on looking at it through the stereoscope, the impres sion is conveyed that the object is seen through a framing, and the framing should naturally be made to appear between the object and the observer. For this it is necessary, however, that, in the case of the right hand picture, a little more should be visible on the left hand edge, than on the same edge of the left band picture. It is a common practice, however, to so trim prints that the reverse is the case, that there is a little more on the out side edges than on the inside. The effect of this is to cause the object to appear between the framing and the ob server, which, of course, does not tend to magnify its apparent size. The next point enlarged upon is one of very consider able importance. It is pointed out that it is quite neces sary, if the stereoscopic delusion is to approach complete ness, that the simulations of the objects in the instru ment subtend the same angle as did the objects themselves to the camera when they were being photographed. According to Herr Von Stolze, it very seldom occurs that, with the ordinary stereoscope, the arrangement is such that the picture is seen under the correct angle. Further, he states that in the case of pictures taken with the wide angle stereoscopic objectives, the magnifying power of the lenses of the spectroscope could require to be so great, if the pictures therein are to subtend to the eye, the correct angle that the grain, even of albumenized paper, would become very visible, and materially deteriorate the effect. It is an indistinct knowledge of this fact that has caused stereoscope makers rather to increase the widths of their prints than the power of their lenses ; but this has availed nothing, as lenses of longer focus have not been used in making the pictures, so that the scale is not in creased, but only the amount in subject. The maximum angle of view properly admissible with the ordinary stereoscope is one of 30°. Another fault in the usual stereoscopic apparatus is caused by the fact that the objectives with which the photo graphs are produced are always placed too far apart. Our idea of the distance of an object is gained from the amount of difference in the images received by our two eyes. The greater the difference, the nearer the object appears. By placing the objectives with which the photographs are taken farther apart than the eyes, the difference is in creased, so that on this account alone the objects, as seen in steroscope, appear nearer than they should, and con sequently smaller. So far various reasons have been given for the appear ance of diminutiveness in the image as seen in the stereo scope, but followed by the final one they seem somewhat unnecessary—as unnecessary, perhaps, as the preliminary excuses of the soldier who was asked why he did not fire on the enemy. He gave sundry trifling reasons first, but finally explained that he had neither powder nor shot to fire with. We quote Herr Stolze's first reason. “As is well-known, the position of any object is of necessity assigned to that point where the lines drawn from the two eyes to it cross each other. The same thing follows in observing with the Brewster stereoscope. It is well known, however, that in looking through the pris matic glasses of this instrument, parallel rays emanating from corresponding points are so deflected that they appear to cross at a point about 28 c.m. from both eyes. This uncommonly weighty circumstance induces us, then, to place these points at this distance, not of course quite pre cisely, because the adjustment of the axis of our eye is not
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