SDERTON'S POLARISING SYSTEM.
By far the best method of stereoscopic projection in which intermediate aids are necessary is that clue to the inventive genius of Mr. John Andertou, of Birming ham. One of two slides, prepared from pictures taken in the stereoscopic camera, is placed in one lantern, and the other slide in a second lantern. Into the optical system of each lantern is intro duced, either between the condenser and objective, before the objective, or be tween the radiant and condenser, but preferably between the lenses of a Petzval objective, a Nicol's prism plate of tour maline. The plates are so arranged in each lantern that the light from one will emerge polarised in a plane at right angles to that of the light emerging from the other. The images of the two slides or pictures are superimposed upon each other when projected upon the screen, but, on account of the light being polar ised, one image will be erect while the other is lying on its side, the resulting image or composite being a confusion of the two. These superimposed images are
viewed through a pair of analysers, having their planes at right angles to each other. One analyser permits one eye to see one of the projected pictures, and the other analyser permits the other eye to see its companion, this resulting in stereoscopic effect in the mind of the observer. The analysers designed for this purpose are made of a number of thin glass plates, and the necessary polarisa tion is effected by refraction. These glass plates are set at a proper angle in a mount resembling a very small opera glass. It should be stated that in carry ing out the Anderton method it is neces sary that the screen should have a metallic surface, otherwise the light would be de polarised. Mr. Anderton employs a flexible screen covered with silver leaf and subsequently lacquered.