The invention of the magic lantern is usually attributed to Athanasius Kircher, who described it in the first edition (1646) of his Ars magna lucis et umbrae, but it is very probably of earlier discovery. Another application of the optical lantern is found in the cinematograph (q.v.).
The optical lantern, in its simpler forms, consists of the follow ing parts: (I) the lantern body, (2) a source of light, (3) an optical system for projecting the images. The lantern body is usually made of iron, provided with the openings necessary to the insertion of the source of light, windows for viewing the same, a chimney for conveying away the products of combustion, fittings to carry the slides and the optical system. Petroleum oil lamps, gas, light from a cylinder of lime rendered incandescent by an impinging oxy-hydrogen or oxy-gas flame (the lime-light) and incandescent thorium mantles served as sources of light in the earlier optical lanterns, but these have been superseded— except in remote districts—by electrical illuminants. Various types of the electric arc—more especially the carbon arc—are undoubtedly among the best illuminants for use in the projecting lantern, and of recent years wire filament lamps have been introduced for this purpose.
Optical System.—In the ordinary projecting lantern the rays are transmitted through a lens termed the "condenser," then through the object, and finally through another lens termed the "objective." In another type the light, after passing through the condenser, is reflected vertically by a plane mirror inclined at to the direction of the light; it then traverses another lens, then the object, then the objective, and is finally projected horizontally by a plane mirror inclined at or by a right angled glass prism. In episcopic projection, the light, having traversed the condenser, is reflected on to the 'object, placed horizontally, by an inclined mirror. The rays reflected from the object then traverse the objective, and are then projected horizontally by a mirror or prism. The object of the condenser is to collect as much light as possible from the source, and pass it through the object in a uniform beam. For this purpose the condenser should subtend as large an angle as possible at the source of light. To secure this, it should be tolerably large, and its distance from the light, that is, its focal length, small. Since
effective single lenses of large diameter are necessarily of long focus, a really good condenser of considerable diameter and yet of short focus must be a combination of two or more lenses. It is essential that the condenser be white and limpid and free from defects or striae.
In the earlier lanterns, as still in the cheaper forms, only a single plano-convex lens or bull's-eye was employed as a con denser. A good compound condenser for ordinary work is that proposed by Herschel, consisting of a biconvex lens and a meniscus mounted together with the concave side of the meniscus next the light. Other types make use of two plano-convex lenses, with the curved surfaces nearly in contact; or a concavo-convex and a plano-convex lens. Or it may be a triple combination, the object always being to increase the aperture. The focus must not be so short as to bring the lens too near the light, and render it liable to crack from the heat. This is sometimes guarded against by a plate of thin glass between the condenser and the light.
The function of the objective is to produce a magnified inverted image of the picture on the screen. The best objective is the portrait combination lens usually of the Petzval type as used in ordinary photographic cameras. (See PHOTOGRAPHY : Apparatus.) These are carefully corrected both for spherical and chromatic aberration, which is absolutely essential in the objective, although not so necessary in the condenser. The photographic lantern slide used for projection in an optical lantern consists of a photograph printed in glass and the films used in motion picture (q.v.) work are photographs printed on a transparent film.
The projecting lantern has been considerably improved of recent years both in general construction and in design of the optical system. As examples of modern projecting lanterns the Epidiascope and Cinematograph may be instanced. The Epidia scope is a projection lantern designed for both episcopic and diascopic projection and is constructed so as to permit easy projection of lantern slides, book illustrations, scientific experi ments and apparatus, microscopic slides and so on. The intention of the designers of the Epidiascope is to flood the object which is to be projected with light and so effect better projection and obtain more detail in the image on the screen. To accomplish this a powerful carbon arc fitted with a self-regulating device is used. A large glass cell containing water for absorbing the heat rays from the arc is attached to the lamp house. The cinemato graph is a projecting lantern fitted with a mechanical device whereby the light issuing from the objective is interrupted periodically (see MOTION PICTURES : Technology).