STEREOSCOPIC PHOTOGRAPHY The invention of the stereoscope being prior to the discovery of photography, the first pictures for this instrument consisted of line drawings. With the advent of photography, the stereoscope became increasingly popular, as this discovery provided the means of repro ducing the minutest details of a subject binocu larly with absolute precision. To secure a photographic slide for the stereoscope, it is necessary that two distinct photographs of the subject be taken from standpoints corresponding to the positions of the two eyes, and that these photographs be mounted side by side. If the subject is still-life, the two pictures may be taken by successive exposures, moving the camera between them a distance of 2f in. to 3 in. To facilitate the operation, various con trivances have been devised, an example of which is shown at A, in which A is a slab of wood having attached to its upper surface two narrower strips B C linked together at D parallel rule fashion. B is fixed to the base A, while C is free to swing. The camera to be used is secured to c and rests lightly on B. With the camera in the position indicated at x r, M N, the first exposure is made, which will give the picture belonging to the right eye. The strip c carrying the camera is now moved so that the camera occupies the position indicated at EI I j, when the second (left eye) picture is taken. Another method of stereo-photography with a single camera and by successive exposures is indicated in diagram B, in which R represents in plan a camera turnable on its axis at j. If the dark-slide, holding a plate, is furnished with an opaque card just half the size of the plate itself and loose enough to be shaken from one end of the dark-slide to the other without opening the slide, the two photographs of one subject may be obtained on a single plate. The picture for the right eye is secured with the camera in the position indicated in full line, covering the field between A B, half the plate only at El being at first exposed. The dark slide is then closed and withdrawn from the camera for the purpose of shaking the opaque card from G to H ; it is re-inserted in the camera and the shutter drawn out so that the portion of the plate at G may now be exposed. Before
uncapping the lens, however, the camera is turned slightly from left to right, so that it covers the field between C D. A stereoscopic pair of images will thus be impressed upon one plate. Further, if the operation be carried out in the order indicated no transposition of the finished prints will be required. Stereo-photo graphy by the successive exposures and displace ment system, whilst being useful for still-life or fixed subjects, cannot be adopted with success in cases where movement of any part of the subject is likely to occur, or when the illumination of the subject is liable to alter. The most perfect results for the stereoscope can only be obtained when the two images are received upon the plate simultaneously. It is not, however, impossible to do this with a single-lens camera. For example, in Brown's stereophoto-duplicon (C) there is a chamber forming a supplementary extension of an ordinary camera. In the chamber are placed four mirrors as at D E H G. Let A B represent the subject to be photographed. Taking the centre ray, the light coming from c strikes the surface of the two outermost mirrors G, is reflected to R a, thence through the lens j to the plate i s' at the back of the camera. Images from the binocular angle are thus secured simultaneously on one plate and at one exposure, and as the light crosses before reaching the plate the pictures are taken so that the negative yields a pair ready transposed for immediate inspection in the stereoscope.
A modification of the apparatus just described is that of the so-called stereoscopic transmitter, in which only two mirrors are employed, set at an angle to each other and placed at an angle of about in relation to the axis of the lens of an ordinary camera. Transposition of the images takes place before the light reaches the sensitive plate. The mirrors are surface-silvered and of the kind used in reflex cameras.