In any magic lantern when the tube carrying the lens which is farthest from the lamp Is drawn out as much as possible, the magni tude of the image represented on a screen is the smallest ; the image increases in magnitude in proportion as the tubes are diminished in length, or as the lens is brought nearer to the body of the lantern. It increases also in proportion as the whole machine is made to recede from the screen ; and the variations of the length of the tubes must, by the exhibitor, be combined with the different distances of the lantern from the screen, in order that by the just convergence of the rays in each pencil, on the screen, the proper degree of distinctness of vision may be obtained under all the variations in the magnitude of the image. The consequence of this is the very serious defect that when the image is smallest, and apparently farthest off, it is brightest, and when large, and apparently near, the light is comparatively faint from being spread over a larger surface. This is partially remedied by intercepting a portion of the light, when the image is small and bright, by means of a gauze screen : an ingenious artificial iris has also been contrived for the purpose.
Care is taken that during the exhibition all light shall he excluded both before and behind the screen, except that which, in proceeding from the lantern, produces the image to be observed ; and the screen being itself invisible, the spectators can scarcely divest themselves of the idea that they are looking into a dark cavern, in which the objects, at first dimly visible as specks in its deepest recesses, appear to be gradually advancing towards them, increasing in magnitude as they seem to approach : after the objects have been some time apparently in the immediate presence of the company, perhaps surrounded by a blaze of light, they seem as gradually to retire, and at length they vanish, leaving the place in profound darkness.
Several of these particulars are derived from a Treatise on the Magic Lantern,' written in 1854 by Mr. Tomlinson, and published by a maker of the instrument for the benefit of his customers. This treatise lets us into the secret of many of the effects which always excite the admiration, if not the wonder, of the spectators. Tho following, for example, is the introduction to a description of dissolving views :— " Dissolving views are produced by means of two magic lanterns of the same dimensions and optical power. They are placed on the same table or horizontal plane, with their lens-tubes inclined towards each other in such a way as to produce a single disc of light upon the opposite wall or screen, instead of two discs, which would be formed if the lens-tubes were parallel. Now it is evident that if by the con joint action of these two lanterns a disc of light be formed on the screen, the dark shutter of either lantern may be let down, without destroying such disc, but only depriving it of half ito light. Suppose, further, that each lantern contained a slider, one representing the exterior of a building, and the other representing the interior, if both Lanterns were acting on the screen at the same time, the super-position of images would produce a confusion which the eye could not unravel ; but if the dark shutter were let down, so as to intercept the light of one lantern, the other would act with full effect. While the spectators are enjoying one view, the shutter of the other lantern is raised, and that of the first is gradually let down ; and it is during this process of moving the shutters that the dissolving effect takes place, because a portion of the light from both lanterns falls upon the screen at the same time ; but the moment one shutter is quite down and the other fully raised the dissolving effect ceases. There is one effect which
usually excites a great deal of surprise among the spectators—namely, the view of the interior of a large building, deserted and solitary, and while the spectator is gazing, the same building becomes filled with people ; this is produced by painting the same building ou two sliders of precisely the same dimensions, and with all the same details in both cases, only in one case empty and in the other full of people ; or as it is not necessary to repeat any part of the painting, one slider may have the building only, and the other the people on a ground of the deepest shade used in the former slide. While the lantern containing the slider representing the empty building is acting on the screen, the shutter of the lantern is slowly raised, and that of the first as slowly lowered. Thus without any necessary dissolving effect, since the two sliders represent the same object, the empty building appears gradually to fill with people. In the same way a leafless tree covered with snow, may quickly put on the verdure of summer." In the following s paasage the means of producing some of the effects before referred to are described :— " Many beautiful effects are produced by allowing both lanterns to act together ; one lantern being first employed to produce an effect, and the other being used to heighten it, or to produce some new feature. Thus, for example, a rainbow may be made to appear in the sky of a landscape, as represented by one lantern. For this purpose the landscape without the rainbow is shown by one lantern, and a slider containing only a rainbow, or a sky and a rainbow, being placed in the second lantern, the shade is removed from it. and the landscape is enlivened by this beautiful addition. By means of a second slider, lightning may also be represented, but in such case the effects of the flash are due to two glasses, both blackened, and containing the same flash of lightning, one of which is fixed and the other moveable. The latter is moved rapidly so as to allow the flash to be seen for an instant and then shaded from view. The dancing of the aurora borealis is pro duced in a similar way. The whirling of smoke issuing from a volcano is produced by a painting set in a circular brass frame. and capable of revolving round and round by means of a little cog-wheel fitting into the teeth of a circular rack. On turning a handle which projects from the side of the slider, the cog-wheel is made to revolve, and this gives motion to the ring of teeth to which the painting is attached. In this way the smoke appears to whirl out of the crater of the volcano, and to be dissipated by the wind. An effect which produces great surprise is that of falling snow. This is by a piece of black silk per forated with holes formed into a roll, mounted in a mahogany frame, and used as a slider, as the paintings are. By means of a small pro jecting handle at the side the silk can be wound upon another roller, and in its t &Eno will present the effect of falling mow. The land saps exhibited by the other lantern is a winter scene : but before the snow slider is adled into action, darkness is made to come over the landscape by means of a piece of black crape held over the aperture. As the lantern invert. the images, and the elide» are put in with what may be called the wrong aide up, so by winding the silk from the bottom roller on to the top one, the ascending holes appear to form a descending shower on the screen."