The stereoscopic " Chinese shadows " (incor rectly called anaglyphs on the programmes of the music-halls exhibiting this attraction), due to L. Hammond (1923), are based on this type of projection. Two ordinary projectors situated behind the stage illuminate a transparent screen, one with red and the other with green light. A person or object situated between the projectors and the screen appears on the screen as two shadows, one in red and the other green. If the left-hand projector, relative to the audience, has a red filter, and if the bi-coloured spectacles are worn so that the left eye is covered by the red filter, the observer sees in front of the screen a single shadow of the object in silhouette. This shadow will appear nearer the audience the nearer the object is to the projector, and a scene will have a greater exaggeration of depth the greater the separation of the projectors. By reason of the known laws governing the de formation of reconstructed objects when viewed stereoscopically under abnormal conditions, any object thrown towards the projectors will appear to be thrown towards each individual of the audience whatever his position in the theatre.
86o. Projection of Anaglyphs. At the same time as he applied the anaglyphic method to the production of stereoscopic pairs of large size on opaque supports (§ 833), Ducos du Hauron applied the same principle to the production of transparencies for projection.
The application of the anaglyph to cinemato graphy was first pointed out, we believe, by 0. Gersacsevics and E. Franzos (1907), who described the production of anaglyphic film by superposition, after suitable toning, of two positive films placed gelatine to gelatine, one having been printed from a negative back to front. Many patents taken out since then claim the idea, but make use of films coated on both sides, as currently used in various two-colour cinematograph processes.
861. Projections in Polarized Light. The use of polarized light (§r22a) for stereoscopic pro jections seems to have been suggested first by Otto Wiener. It was first put into practice in 1891 by John Anderton, who showed the pro cess in public in London for some time. The projection was made on a screen which did not depolarize the light (ground glass, or silver paper mounted on calico). Two lanterns were used, one beam being polarized horizontally and the other vertically, by piles of plates the con struction of which was very minutely described in the patent. The projected images were examined by means of binoculars fitted with similar piles of plates.
Although this process has been described in various periodicals and in a great number of treatises on stereoscopy, it has been re-invented many times. Its application to cinematography
was claimed in 1908 by Boris Weinberg.
862. Geometrical Separation of the Two Images. About 1860 Claudet pointed out a. process for stereoscopic projection, which, un fortunately was only visible to one person in a definite position. The two images are projected obliquely on a slightly ground glass, each in the direction of the eye to which it corresponds. Each eye sees the two images, but one of them appears much clearer than the other.
A more exact arrangement, but of equally limited application, was described under the name of the " concentration stereoscope " by G. Jager (1905). The images were projected on a lens in such a way that the eyes could occupy the conjugate points of the exit pupils of the projecting lenses.
A process suggested in 1908 by E. Estanave is a more complete solution of the problem, a large number of positions for stereoscopic view ing being possible, but the practical difficulties of the stereoscopic screen, viz., a transparent diffuser having in front of it a vertical line grating (§834), have not permitted of its practical use.
863. Stereoscopic Viewing of a Pair of Images Thrown on a Screen Side by Side. The various arrangements already described for viewing large size stereograms are applicable to a pair of images thrown side by side on the screen.
A stereoscope having two mirrors in front of one eye (§ 830) was described in 1899 by J. H. Knight for viewing stereoscopic projections_ The use of Galilean binoculars with variable separation of the objectives was proposed in 1903 by A. Papigny for viewing stereogranis of large size and for stereoscopic projections. The use of this idea for cinematography was claimed by Prepognot (1904).
The use of pairs of prisms of adjustable orientation was described by Moessard for view ing stereoscopic projections, the two images being either one above the other or side by side. It is simpler, as was worked out by J. Mat-6 de Lepinav, to distribute to each row of the audience (or to the people in several consecutive rows), a pair of prisms adjusted to the angle subtended at each seat by the centres of the two images projected on the screen side by side.
Finally, there is the very simple apparatus used in Brussels in 1891 by M. Moulin. Each spectator held at some distance in front of the eyes a card having a rectangular hole cut in the centre through which each eye could only see its corresponding picture (§ 826).