212 of light L, which is placed in the focus of a spherical reflector. On the side opposite the reflector the box is pierced by a round hole, into which is fitted the metal tube D, which may either be cylindrical or slightly tapered. To the inner end of the tube D is fitted a lens or combina tion of lenses C called the " condenser." Immediately in front of the condenser, and at right angles to the axis of the tube, is a vertical slit S for receiving the transparent pictures usually called " slides." To the outer end of D is fitted a lens or combination of lenses O called the "objective" or "project ing lens." At a suitable distance from /-> O is placed the screen for receiving the Vv magnified picture the screen and slit ^ being in the conjugate foci of the lens O. Since the objective reverses, the picture must be inserted in the slit upside down. The objective can be moved backwards and forwards in the tube D by means of a rack and pinion. Since the screen and slide are in the conjugate foci of O the diameter of the picture on the screen has the same ratio to the screen s distance from O that the diameter of the picture on the slide has to the slide s distance from O. The ratio adopted is generally 1 to 3 or 4. Source of Light. Almost any good source of light can be used in the magic lantern. In the earlier forms a simple oil light was used ; and in the toy forms either an oil light or simple gas- flame is still employed. Better effects are obtained from the Argand fountain lamp (fig. 2) or from the common Argand gas burner with a glass chimney. In the sciopticon (fig. 3) a handy and ex cellent form recently introduced a speci ally constructed paraffin lamp is employed with three parallel flat wicks set edgeways to the condenser. With this lamp a clear, well-defined picture varying from 6 to 10 feet in diameter can be readily thrown FIG. 2. Argand on the screen. For the best effects, Fountain Lamp, however, recourse must be had to the oxy-calcium light (fig. 4), in which a small cylinder of lime is heated to intense luminosity in the flame of a spirit lamp through which a jet of oxygen plays; the OXY-HYDROGEN LIGHT FIG. 3. Sciopticon. (q.v.) ; the magnesium light, in which two narrow ribands of magnesium are put through slits by a clock-work arrange ment and burn as they advance; or, best of all, unless when sunlight can be used with the aid of a heliostat (see HELIO- GRAPHY), the electric light. Condenser and Objective. The object of the condenser (fig. 5, C) is to collect as much light as possible from the source, and pass it through the trans parent picture in the slit. For this purpose the con denser should sub tend as large an angle as possible at the source of light. To secure this, the condenser Should be tolerably FlG " 4.-Oxy-calcium Lamp. large, and its distance from the light, that is, its focal length, small. Since effective single lenses of large diameter are neces- 1 sarily of long focus, a really good con denser of consider able diameter and yet of short focus must be a com bination of two or more lenses. Let/! and/ 2 be the focal lengths of two lenses, and/ the focal Q Q length of their com- r <-, > n ,-.. ,- bination. Then, ne- FIG. 5. -Condenser and Objective. glecting the thickness of the lenses, we have when the two are close toegther From which weget/ 1- / ., a fraction which is always less than /1+/2 either / x or/, provided these are not zero. Suppose, for example, two lenses of" 6 and 10 inches focal length respectively ; the focal lenth of their combination will be ~ = 3 f inches. In the earlier lanterns, as still in the cheaper forms, only a single plano-convex lens or bull s-eye was employed as a condenser. Better effects are produced by two such lenses. Perhaps the best con denser for ordinary work is that proposed by Herschel and repre sented in fig. 5, C, consisting of a biconvex lens and a meniscus mounted together with the concave side of the meniscus next the light. The diameter of such a condenser is about 6 inches, and its focal length a little over 3 inches. The focns must not be so short as to bring the lens too near the light, and render it liable to crack from the intense heat. In some lanterns this is guarded against by placing a plate of thin glass between the condenser and the light. In the sciopticon (fig. 3) the condenser consists of two plane-convex lenses, each about 4 inches in diameter. Condensers of large diameter are not so essential now as formerly, seeing that small pictures can easily be produced by photography. The function of the objective (fig. 5, O) is to produce a magnified inverted image of the picture on the screen. In toy lanterns it is a simple double-convex lens of short focus. This, however, can only produce a small picture, and that not very distinct at the edges. The best objective is the portrait combination lens used in ordinary photo graphic cameras. These are carefully corrected both for spherical and chromatic aberration, which is absolutely essential in the objective, although not so necessary in the condenser. It is essential, however, that the condenser be free from cracks or flaws, as these would appear and mar the picture. Slides. These are pictures, painted with transparent water or oil colours, or photographed on pieces of glass. The pieces of glass are usually fixed in small wooden frames for insertion in the slit. If parts of the picture are to be movable, two disks of glass are employed, the one movable in front of the other, the fixed part of the picture being painted on the fixed disk and the movable part on the
otlur. By means of a lever the latter disk is moved in its