STEREOSCOPIC PROJECTION The projection of a pair of stereoscopic pictures in such a manner that each eye of the observer shall see but one image, the one belong ing to it. In 1841 Dove showed that if one of a pair of stereoscopic pictures is outlined in blue on a white ground, and the other element in red, the two being approximately superposed upon the same sheet, a spectator furnished with red and blue glasses will see the outlines as a single solid image. In demonstrating this fact Dove obviously foreshadowed the work of Ducos du Hauron. (See " Anaglyph.") De la Blanchere and Claudet, some years later, at tempted to eliminate the use of the spectacles.
J. Ch. D'Almeida, in his communication (1858) to the French Academy of Sciences, described how he placed in the course of the luminous rays two coloured glasses (red and green) ; the observer views the projections through glasses of similar colours, the fusion being seen as a black and white combination in stereoscopic relief. D'Almeida's system is shown at A. A represents the green image on the lantern screen, and B the red image ; the former is blotted out with the green spectacle glass so that the right eye R sees only the red image B. Likewise, the red image is blotted out by the red glass so that the left eye r., sees only the green picture A. Many modern experimenters have modified the above system. It has been suggested that, instead of using two separate projections, similar results may be obtained by making a composite slide and pro jecting the same by a single lantern.
D'Almeida also invented the eclipse system in which a perforated shutter rotates in front of side-by-side lanterns. Another form of eclipse system, but of small practical value, consists in using a box apparatus similar to Elliot's stereoscope.
There are numerous ways in which stereoscopic elements when projected side by side upon a screen may be united so as to produce the desired results. Drouin, of France, suggested the system shown at B. The stereoscopic elements C n being projected by a lantern, D is seen by the right eye, R, direct, and C by reflection in the prism. The light emanating from C
impinges at B, is deflected to A and thence to the left eye L. The left eye, therefore, sees the image of C, superposed on the image D, where unison takes place and stereoscopic effect results.
Moessard used two prisms, mounted opera glass fashion. C shows an arrangement by which a single mirror M suffices to superpose a second image of the necessary pair upon its complement image D. Two lanterns A B are sed to project their respective elements upon lantern screens, C D. The observer, with his eyes situated at 1, R, sees one element, D, by direct vision, and the companion element, c, by reflection in the mirror, stereoscopic relief resulting. There is a number of similar systems.
In r89o, Anderson, of Birmingham, invented stereoscopic lantern projection by means of polarised light. Two lanterns project the elements in superposition upon the screen s, in diagram D. Before each lantern is a polarises P Pl. The picture received by the screen is thus formed of two polarised pictures ; for instance, one in a vertical, and the other in a horizontal position. The observer looking at the composite picture through analysers A placed before his eyes, B Br, will see with each eye its proper element, and fusion results. The fact that light is lost in polarisation and depolari sation somewhat discounted Anderson's method ; but to rectify matters a flexible screen having a metallic reflecting surface was introduced, and this also had the merit of preventing depolari sation apart from the analysers. Anderson also constructed his polarisers and analysers of bundles of thin glass, like the microscopic cover glass, and set at about the polarising angle. The glasses in the bundles are not set quite parallel to each other ; otherwise ill defined images would result, owing to disturbing reflections. Many of the systems above described are obviously applicable to the kinematograph, and, d indeed, almost all have been tried with varying success.