Magic Lantern, an optical instrument in tended for exhibiting, by means of lenses, magnified images of pictures painted in variously colored transparent gums, on glass slides. It is constructed upon the simple dioptrical principle of conjugate foci (see Optics), in accordance with which, when any object, as a picture, is brought upon one side of a convex lens, and at a distance slightly greater than its focal length, such object or picture will be reproduced upon a white screen placed at a certain distance on the opposite side of the lens. In the common form, used for schools or scientific purposes, the instrument consists of a large dark lantern, having at top a bent chimney for the escape of smoke or heated air, and an opening on one side containing a convex lens on a level with the flame of a strong lamp within; the side of the lantern opposite the opening being furnished with a parabolic metallic reflector, for the purpose of collecting the light and throwing it upon the lens. Beyond this lens, within a horizontal tube, the picture is introduced, and beyond this is a second convex lens, a little further than its focal length from the picture, and the distance of which from the latter is regulated by an arrangement for sliding it within the tube.
By the action of the reflector and the first lens, a strong light is condensed upon the picture; and its pencils, being converged and made to cross by the second lens, form at their several foci the image, which, being received at that place by the screen, is rendered visible. The inversion of the image is corrected by placing the slides inverted. For exhibitions before large audiences, the lime light, obtained by keeping a cone of lime ignited in the flame of the oxyhydrogen blowpipe, and revolving at the same time, has been much used; but the apparatus now prepared by M. Dubose and others (see Electric Light) enables the experimenter to employ the most intense artificial light known, that of the galvanic current passing between charcoal points; and some of its forms have two reflectors so placed as to throw the images of two pictures at the same time on the same part of the screen, as is required for the effects known as "dissolving views." These effects consist in gradually covering one slide, while the other is uncovered, thus causing one scene to fade or melt into another, as a day into a moonlight scene.