PARALLAX STEREOGRAM An invention of P. E. Ives, of America. The parallax stereogram photograph consists of a single transparent image divided into lines ( roo to 15o to the inch), alternate lines forming one of a stereoscopic pair of images, and the inter mediate lines forming the other image, so that it has the appearance of a pair of stereoscopic images mechanically superposed. In order that each eye may see only the lines belonging to its respective view-point, the transparency is covered by a line-screen, with a definite separa tion from the surface of the photograph such that lines of the photograph covered by screen lines to one eye are seen by parallax of vision by the other eye. The combination of the photo graph and line-screen in suitable adjustment constitutes the parallax stereogram, whirl, when viewed from a suitable distance, directly in front, shows the object in stereoscopic relief. The method of production is by means of a camera having at the front a single plano-convex lens about 3 in. in diameter, behind which are two small openings about 2-1 in. apart in a horizontal plane (see A); thus an image is formed from two view-points, corresponding to the separation of the eyes. The image thus formed may be a perfectly sharp single image of objects at some one distance from the camera, in which case objects at every other camera distance will form two images laterally displaced and superposed, exactly as in two ordinary stereoscopic images mechanically superposed. It is then only neces sary to cover the plate with an opaque line screen suitably adjusted with reference to the spacing of the lines, separation of view-points, and camera extension, in order that the light coming from the two camera apertures may form sepa rate images in juxtaposed lines. A transparency from the negative thus obtained is covered by a line-screen with adjustment like that in the camera, and viewed from a point corresponding to the position of the lens apertures, in order to produce the effect of an ordinary double stereo gram in the stereoscope. If, however, these conditions are strictly adhered to, the result will be pseudoscopic instead of stereoscopic, just as in ordinary stereoscopic photography the results are pseudoscopic unless the photographic prints are cut apart and transposed before mounting them for inspection in the stereoscope. The stereoscopic effect is obtained by shifting the cover-screen laterally the width of one screen line, but with the result that the perspective is distorted when .1 moderately large angle view is embraced. In order that the path of the rays to or from every point of the stereogram may be absolutely identical in photographing and in viewing, the image formed through each aperture in the camera should be laterally inverted, so that the two pencils of light belonging to near objects bisect before reaching the screen, and pencils from far objects after passing through it.
This may be effected by placing laterally inverted prisms in front of the lens apertures, as shown at, B in which w represents the path of rays coming from an object more distant from the lens on one side than the screen and sensitive plate on the other. With this arrangement, the objects will be laterally reversed, unless photo graphed from an intervening mirror, but in other respects the results are much better than without the inverting prisms. The use of the inverting prisms also permits of the large lens being dis pensed with. The fact that the prisms may be disposed so as to direct the two pencils of rays towards the axis of the camera in the same manner as the prismatic edges of a single lens, permits of the use of two separate lenses C, or even of pinhole apertures without lenses D, and this method of controlling the parallax independently of the focal length of the lenses possesses certain practical advantages. Objects can be photographed so as to appear to be at the plane of the photograph, or within or beyond it, at will. When the arrangement shown at C is employed, it is advantageous to have pairs of lenses of various foci, in order to keep to one camera extension, and owing to the small size of the apertures, simple lenses may be used.
The most perfect screen for this work is a ioo line to the inch uncovered Levy single line screen with " hard filling," and opaque lines twice as broad as the clear spaces. A card mat separator is used, and the sensitive plate pressed flat by a thick plate-glass at the back in the dark-slide. The requisite thickness of the card separator depends upon the camera exten sion, which can be readily calculated. The cover screens for the parallax stereograms are made by contact printing on transparency plates from a negative made by contact printing from the original Levy screen, and intensified by mercury and ammonia. The lines should be quite opaque, and the spaces perfectly clear, and, while good enough for this purpose, such screens are not good enough for use in making the negatives. Owing to the fact that neither the transparencies nor the cover-screens are ordinarily flat, it is necessary, in mounting them together, to use a third glass, convex side (preferably ground) against the back of the transparency.