LANTERN STEREOSCOPIC PROJECTION.
The principle of stereoscopy, which has already been set forth, shows that the Iroblem to be solved in lantern stereo scopic projection is to so arrange matters that each eye sees only one image, and that belonging to it, or corresponding to the one it would see in nature. The re searches of Dore (1841) were productive of many suggestions in this direction, and seem to have foreshadowed the recently revived anaglyph of Ducos du Hauron, which consists of a composite picture printed on a sheet of paper from twu relief accompanied by magnification. This phenomenon, it has been suggested, is due to the non-achromatic lens causing over lapping of the different rays. In the "Verant " of Dr. von Rohr, a single photograph is held in a special viewing instrument for inspection by one eye only ; there is also a double Verant de signed on the same principle, in which two prints are used In the " Stereo factor " of Mr. A. Lockett, two exactly similar, non - stereoscopic prints are mounted side by side and viewed in an ordinary stereoscope, in a special holder, inclined to each other at an angle of 140°. None of these contrivances, however, although interesting and capable of giving a very fair appreciation of relief, can equal the effect produced by the examina tion of a good stereoscopic slide in the stereoscope.
process blocks made from the two elements of a stereograph, one impression printed in blue and the other in red, such pictures being viewed with a pair of red and blue glasses. In 1858, J. Ch. D'Almeida, experimenting in the same direction, devised similar means, but used red and green discriminating glasses in stead of red and blue, as in the former case. When lantern projections were involved, the inventor placed in the course of the luminous rays the two coloured glasses, which have no element, or scarcely any element of the spectrum in common. By means of these coloured glasses, one of the images projected upon the screen is rendered green and the other red, and, similar glasses being placed before the eyes, the green image alone will be perceived by the eye covered by the red glass, and the other by the eye furnished with the green glass. This principle is shown by Fig. 848.