To the great majority of people the stereoscope is nothing more than a scientific toy or perhaps a rather troublesome means of looking at a number of photographs which have cost more than usual labour to produce. In consequence the instrument has been banished from its place in the drawing-room, and only a few enthusiasts who make their own slides venture to keep it in evidence. We have from time to time urged the claim of stereoscopy o be the one branch of photography with which the draughtsman or painter cannot compete, and, further, we have pointed out the great educational value of such pictures, which are as near faithful renderings of their subjects as it is possible to obtain upon a flat surface. When produced in an additive colour process such as that of F. E. Ives, and shown on a binocular Kromskop the illusion is almost perfect.
For the moment we are not concerned with the beauty or scientific interest of stereoscopic work so much as with its utility in various branches of science and industry. There are many subjects full of intricate detail lying in several planes which cannot be rendered satisfactorily in a monocular photograph or by the cleverest artist. In the one case we have a plan with a certain amount of shading to represent relief and in the other we get the impression of one person who if not an expert in the subject may omit important data, while if he is an expert may unconsciously emphasize such features as seem important to him. The stereoscope is impartial in such matters, and if the separation of the view-points for the two negatives has been properly adjusted the subject should appear exactly as in nature.
It is not necessary to give a detailed list of subjects suitable for stereoscopy, for once the question is raised any intelligent person will readily perceive in what way he can apply it to further his own work or studies. Recently we were glad to hear that the medical profession has shown considerable interest in this work. Many valuable records have been obtained, but there are still many branches of science and art in which development is possible. For example, crystalline fractures such as those of cast-iron or brass, can be photographed on an enlarged scale, and by a judicious separation of the lenses any desired amount of relief can be obtained. As this question of separation has not always been clearly understood, it may be well to point out that the degree of relief obtained is governed entirely by the separation between the lenses when the exposure is made. If only one lens be used and the exposures made successively, then the separation is the distance the lens has been moved. The distance by which the centres of prints is separated has no effect on the relief but only upon the ease with which they are combined in the stereoscope. Much of the eye strain which many people experienced is due to the separation in this respect being too great.
The error that is most likely to be made is that of using too wide a separation when working at close quarters, as when taking small objects on full or even quarter scale and in portraiture or ethnographical studies. The usual focal length of lenses supplied in pairs for stereoscopic work varies from three to six inches. Occasionally lengths up to eight inches are supplied, but this is unusual, except to special order. Even this is insufficient for close up work, as it will readily be understood that at a working distance of eighteen inches the disparity of the view-points of two lenses with a separation of three inches is very considerable, giving a drawn-
out appearance to any projecting details. For example, if we desired to take a stereograph of a lump of sugar, we should obtain quite a false rendering of the crystalline texture, the small crystals being drawn out into needlelike forms. It is, therefore, often desirable to avoid the use of paired lenses, and to make the exposures by successively moving the camera the necessary distance.
It will thus be teen that for stereographs of immobile subject, the possessor of a small camera needs no additional apparatus. All that is needed is a small board or platform on which the camera can be slidden laterally and secured at the proper point. Small deviors for this purpose are listed by most of the principal dealers, and can, we believe, still be supplied. An appliance which should prove of great value to the scientific photographer is the double mirror, introduced by Mr. Theodore Brown. In this apparatus two small mirror are hinged together like a book so that they may be placed either in one plane or at any angle to each other. When inclined together, be it ever so slightly, a dissimilar view of any object is reflected by each, and if these are photographed with an ordinary single-lens camera we have at one exposure a stereoscopic negative, no central partition or other modification of the apparatus being necessary. Although introduced mainly as a cheap and simple means of making stereo-negatives, the Brown transmitter possesses many great advantages. In the first place the limit as to the diameter of the lens is removed. Thus, rapid portrait lenses or large aperture anastigmats of any desired focal length may be used, and in the case of surgical work the simultaneous exposure minimises the risk of movement. A few experiments will be necessary to find the degree of inclination necessary for various distances, and if the mounting included a graduated arc such as is fitted to binocular field glasses, this could be registered for future use. There is one slight drawback to this method of working, and that is the fact that the images are laterally inverted, but for scientific work this would in many cases of no moment; if it were the prints can be made by a transfer process such as Transferotype or the single transfer carbon process or, if films are used, by printing from the reverse side. By this method the Datives may be of much larger dimensions than is, possible with a binocular camera even as large as 15 x 12 being practicable. The prints may then be viewed in the stone or reflecting stereoscope instead of the Brewster or box-form of instrument. Stereoscopic prints if not made in the form of transparencies, are best if I on gelatino-chloride paper, as there is less chance of loosing shadow detail than there is with developing papers. The negatives should be thin and fully exposed what would be called flat in ordinary work where the grapher relies only upon light and shade to give a semblance of relief.
For the moment we are not concerned with the beauty or scientific interest of stereoscopic work so much as with its utility in various branches of science and industry. There are many subjects full of intricate detail lying in several planes which cannot be rendered satisfactorily in a monocular photograph or by the cleverest artist. In the one case we have a plan with a certain amount of shading to represent relief and in the other we get the impression of one person who if not an expert in the subject may omit important data, while if he is an expert may unconsciously emphasize such features as seem important to him. The stereoscope is impartial in such matters, and if the separation of the view-points for the two negatives has been properly adjusted the subject should appear exactly as in nature.
It is not necessary to give a detailed list of subjects suitable for stereoscopy, for once the question is raised any intelligent person will readily perceive in what way he can apply it to further his own work or studies. Recently we were glad to hear that the medical profession has shown considerable interest in this work. Many valuable records have been obtained, but there are still many branches of science and art in which development is possible. For example, crystalline fractures such as those of cast-iron or brass, can be photographed on an enlarged scale, and by a judicious separation of the lenses any desired amount of relief can be obtained. As this question of separation has not always been clearly understood, it may be well to point out that the degree of relief obtained is governed entirely by the separation between the lenses when the exposure is made. If only one lens be used and the exposures made successively, then the separation is the distance the lens has been moved. The distance by which the centres of prints is separated has no effect on the relief but only upon the ease with which they are combined in the stereoscope. Much of the eye strain which many people experienced is due to the separation in this respect being too great.
The error that is most likely to be made is that of using too wide a separation when working at close quarters, as when taking small objects on full or even quarter scale and in portraiture or ethnographical studies. The usual focal length of lenses supplied in pairs for stereoscopic work varies from three to six inches. Occasionally lengths up to eight inches are supplied, but this is unusual, except to special order. Even this is insufficient for close up work, as it will readily be understood that at a working distance of eighteen inches the disparity of the view-points of two lenses with a separation of three inches is very considerable, giving a drawn-
out appearance to any projecting details. For example, if we desired to take a stereograph of a lump of sugar, we should obtain quite a false rendering of the crystalline texture, the small crystals being drawn out into needlelike forms. It is, therefore, often desirable to avoid the use of paired lenses, and to make the exposures by successively moving the camera the necessary distance.
It will thus be teen that for stereographs of immobile subject, the possessor of a small camera needs no additional apparatus. All that is needed is a small board or platform on which the camera can be slidden laterally and secured at the proper point. Small deviors for this purpose are listed by most of the principal dealers, and can, we believe, still be supplied. An appliance which should prove of great value to the scientific photographer is the double mirror, introduced by Mr. Theodore Brown. In this apparatus two small mirror are hinged together like a book so that they may be placed either in one plane or at any angle to each other. When inclined together, be it ever so slightly, a dissimilar view of any object is reflected by each, and if these are photographed with an ordinary single-lens camera we have at one exposure a stereoscopic negative, no central partition or other modification of the apparatus being necessary. Although introduced mainly as a cheap and simple means of making stereo-negatives, the Brown transmitter possesses many great advantages. In the first place the limit as to the diameter of the lens is removed. Thus, rapid portrait lenses or large aperture anastigmats of any desired focal length may be used, and in the case of surgical work the simultaneous exposure minimises the risk of movement. A few experiments will be necessary to find the degree of inclination necessary for various distances, and if the mounting included a graduated arc such as is fitted to binocular field glasses, this could be registered for future use. There is one slight drawback to this method of working, and that is the fact that the images are laterally inverted, but for scientific work this would in many cases of no moment; if it were the prints can be made by a transfer process such as Transferotype or the single transfer carbon process or, if films are used, by printing from the reverse side. By this method the Datives may be of much larger dimensions than is, possible with a binocular camera even as large as 15 x 12 being practicable. The prints may then be viewed in the stone or reflecting stereoscope instead of the Brewster or box-form of instrument. Stereoscopic prints if not made in the form of transparencies, are best if I on gelatino-chloride paper, as there is less chance of loosing shadow detail than there is with developing papers. The negatives should be thin and fully exposed what would be called flat in ordinary work where the grapher relies only upon light and shade to give a semblance of relief.
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