The magic lantern in its simplest form is represented in fig. 1. A is a dark box surmounted by a suitable chimney for carrying off the products of combustion from the source of light I., 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 combination 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 0 called the “objective” or “projecting lens.” At a suitable distance from O is placed the screen for receiving the 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 gasflame 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 excellent form recently introduced—a specially 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 on the screen. For the best effects, 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

(ç.v.); the magnesium light, in which two narrow ribands of magnesium are put through slits bya clock-work arrangement and burn as they advance; or, best of all,—unless when sunlight can be used with the aid of a heliostat (see Heliography), —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 transparent picture in the slit. For this purpose the condenser should subtend as large an angle as possible at the source of light. To secure this, the condenser 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.

Let f 1 and ∕ 2 be the focal lengths of two lenses, and f the focal length of their combination. Then, neglecting the thickness of the lenses, we have when the two are close toegther

7"⅛ + ⅛

From which we get/-a fraction which ie always less than either f 1 or∕ 2 provided these are not zero. Suppose, for example, two lenses of 6 and 10 inches focal length respectively; the focal length of their combination will be θ^θ =3f 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 condenser for ordinary work is that proposed by Herschel and represented 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 focus 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, 0) 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 photographic 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 other. By means of a lever the latter disk is moved in its own plane and in this way a cow for instance can be represented drinking, or a donkey cutting amusing capers. A lever slide is represented in fig. 6. In the chromatrope elide (fig. 7) two circular disks of glass are placed face to face, each containing a design radiating from the centre, and painted with brilliant transparent colours. By a small pinion gearing in toothed wheels or endless bands the disks are made to move in opposite directions in their own plane.

The effect produced is a singularly beautiful change of design and colour. In astronomical slides the motions of the heavenly bodies, eclipses, the phases of the moon, or the like are similarly represented by mechanical means. »Slides can also be made from narrow glass tanks with parallel sides. When these are filled with water containing delicate living organisms the forms and movements of the latter are beautifully seen. Such tanks can also be employed to show such phenomena as the gradual growth of crystals, the electrolysis of water between platinum electrodes, &c. A great variety of physical and chemical experiments can be shown in this way.

Dissolving Views.— For this purpose two magic lanterns are necessary, arranged either side by side or the one on the top of the other. The fronts of the lanterns are slightly inclined to each other so as to make the illuminated disks on the screen due to each lantern coincide. By means of a pair of thin metallic shutters terminating in comb-like teeth, and movable by a rack or lever, the light from either lantern can be gradually cut off at the same time that the light from the other is allowed gradually to fall on the screen. In this way one view appears to melt or dissolve into another. This arrangement was first adopted by Childe in 1811.

Phantasmagoria.— In this arrangement the pictures on the screen appear gradually to increase or diminish in size and brightness. To effect this a semi-transparent screen of cotton or other material is used, the lantern being behind and the audience in front. The lantern is mounted on wheels so that it can be rapidly moved up to or withdrawn from the screen; and an automatic arrangement is provided whereby simultaneously with this the objective is made to approach or recede from the slide so as to focus the picture on the screen in any position of the lantern. In this way a very small picture appears gradually to grow to enormous dimensions.

Lantern Polariscope.— This, perhaps the most beautiful modification of the magic lantern for scientific purposes, consists of an elbowshaped tube, containing mirrors, lenses, &c., and attached to the front of the lantern in place of the tube containing the objective. It is represented in section in fig. 8. C' is the usual condenser belonging to the lantern. G is a set of thin glass plates inclined at the polarizing angle 56° 45' to the axis of the tube. The beam of polarized light from I. reflected from G passes through the lenses

F and the analysing Nicol’s prism P, and falls on the screen. The objects to be examined by the polarized light are placed in the transverse slit 0. When thin plates of selenite or other doubly refracting crystals are placed in 0, a most beautiful display of complementary colours is produced on the screen by rotating the N icol’s prism. Almost all the experiments on polarized light can be well shown by this arrangement.

The sdopticon (fig. 3) is an excellent and convenient lantern, very suitable for all the requirements of the lecturer, as well as for school use in teaching geography, &c.

See Brewster's optics; Ganot's Physics; and Chadwick’s Manual of the Magic Lantern.