MAGIC LANTERN. An instrument for exhibiting magnified images of transparent pictures upon a screen. The arrangement of the different parts of the apparatus will be understood from the following figure.
A is the source of light ; which may be either an argand lamp with a reflector behind it, or better still, a lime-ball rendered incan descent by passing a jet of oxygen through the flame of a spirit lamp, and causing the flame to act upon it. (See " Oxycalcium light.") B C are the lenses of the condenser, (See " Condenser,") which collect all the rays of light which proceed from the lamp, within the angle FAB, and cause them to converge in such a way as to pass through the transparent picture ED. The front lens FB is plano-convex, the back lens C a " Crossed lens ;" q. v. ED is the transparent picture, which must be placed in an inverted position. L is a combination of lenses having short negative focal length ; and de the screen on which the magnified image of ED is received, and rendered visible.
The distance of the slide ED from the lens at L is rather greater than the principal focal length of the combination, and then the screen, which is in the other conjugate focus of the lens, is at a much greater distance from it. A pencil diverging from E is brought to a focus at e, and a pencil from D at d ; and so on. The combination of lenses at L which the photographer will find by far the best is the common portrait combination ; and when this is used, the posterior lens of the combination should be placed next to the picture, and a stop should be placed between the lenses, as occasion may require. The focus of the condenser should fall pretty
nearly on the lens at L which is nearest to the screen.
In order to obtain a perfectly flat field upon the screen, the picture ED should be painted, or photographed, upon a curved surface like a watch glass. (See " Micro-photography.") Magic lantern slides are painted in transparent colours ground in Canada balsam.
Photographic slides for the magic lantern may either be printed upon dry collodionized or albumenized glass plates, by superposition of the negative, or they may be printed by the wet collodion process, by means of a Copying Camera ; q. v. In this case a little chloride of magnesium added to the iodizing solution is an improvement, for it gives greater vigour and a better colour to the blacks. The glass should be perfectly transparent in the light parts ; but stereoscopic slides backed with ground glass may be exhibited in the magic lantern, and produce a tolerably good effect.
Sometimes the screen, or sheet, on which the image is thrown is wetted in order to render it semi-transparent, and the spectators are then placed behind it, and consequently see nothing of the apparatus.
Dissolving views are produced by means of two lanterns, the axes of which are directed towards the same part of the screen, and each of which contains a slide. When a view is to be changed, the cap which closes one lantern is gradually opened, while the other lantern is being shut off. This is done by turning a handle which puts in motion a piece of mechanism contrived for the purpose.