MAGIC LANTERN, an optical instru ment for the projecting of enlarged images upon a screen or other adapted surface. This title is commonly given to the type of instru ment employed for home amusement, the more elaborate types used for public exhibition and educational work being known as stereopticons or optical lanterns (q.v.). In its simplest form the instrument consists of a lantern, generally of tin, and cubical in form, having in the in terior a powerful Argand lamp, the pencils of light issuing from which pass through a con vex lens to a screen or a white wall a few feet away. It is most commonly used as a toy. The principle of its construction is very simple. A lamp is placed within the closed lantern with its burner in the focus of a con cave parabolic mirror, the reflected light from which passes through a horizontal tube on a level with the flame. This tube contains two lenses, the one a hemispherical illuminating lens, of short focus, to condense a strong light on tha picture, and the other a double convex lens, which receives the rays after they have passed through the picture, and throws them on the screen. The picture, known as the lan tern slide, is inserted through a transverse slit into the tube between the lenses. The screen must not be too far removed from the lantern, otherwise the image will become indistinct and distorted. The tube is made to slide in and out, so that the distance of the lens from the slide being capable of being increased or dimin ished, an image of any moderate size, larger or smaller, may be focused after the desired size is secured by increasing or diminishing the distance between the lantern and the screen.
Where an electric lighting current is avail able the pictures may be much improved in illumination by the use of a 60-watt Mazda lamp in place of the Argand lamp (which is often dangerous from overheating if run too long without cooling down). A still more bril
liant illumination may be had with the nitrogen lamp. Further improvement may be secured by using a regular condenser, formed of two plano-convex lenses of four or five inches diameter, set with the convex sides toward each other and quite close together. Far better re sults as to sharpness and clearness on the screen will be secured by the use of a lens of the standard *portraits type as the projector. For a small house picture, say from six to nine feet in diameter, the lens known as size' will give very satisfactory results. The accompanying diagram represents convention ally a longitudinal section through a magic lan tern fitted with a nitrogen lamp (N), attached to the house lighting wires (E), and provided with a pair of plano-convex condensing lenses (C). The reflector is indicated at R; the slot in which the slides are inserted at S; the double.
convex projecting lens at P; and the ventilating chimney, to carry off excessive heat, at V.
The objects exhibited with the magic lan tern are usually pictures drawn, painted or photographed on glass strips or plates of appro priate size, the light passing through them from the condenser and thence through the project ing lens. The size of the image upon the screen depends chiefly upon the brilliancy of the light, and hence the circle which it is capable of illuminating. Secondarily the size will depend upon the distance of the lantern from the screen, and the ability to focus the image thereon. These three conditions must be skil fully adjusted to secure the best results. See