2.—VARIOUS KINDS OF LANTERNS.
The magic-lantern, as now constructed, consists of a box or chamber of japanned tin-plate, with a lamp for the source of light, a large lens to converge the light, and a smaller lens to magnify the object to be exhibited. The large lens is called the condenser, and the smaller lens is designated the objective. Between the condenser and the objective, immediately in front of the former, an aperture is provided for the reception of the painted slide. This aperture is called the slide-holder. We may separate magic-lanterns into three great divisions, the distinguishing difference between them being the kind of light employed. In the first division the lamps are constructed to burn colza or sperm oil; in the second division a mineral oil (purified paraffin) takes the place of the sperm or colza oil; and in the third division some form of lime-light is employed. We will describe these varieties in order.
First, then, let us deal with the lanterns illuminated by a lamp burning colza or sperm oil. Of these lanterns there are two subdivisions, those whose condensing lenses are of a size suitable for exhibiting the standard slides (slides three and a quarter inches in diameter), called phantasmagoria lanterns, and lanterns of a smaller size, which may be described as toys, and for which but a very limited variety of slides can be obtained. We give an illustration ([Fig. 1]) of one of these small lanterns, from which it will be seen that the lamp is of very simple construction, and provided with a silvered reflector behind it. Magic-lanterns, the condensing lenses of which are from one and a half inch to two and a half inches in diameter, are calculated to produce discs of light from four feet to six feet in diameter. Such discs are usually obtained when the distance of the lantern from the screen is about one-third more than the desired diameter of the disc. Lanterns such as these are described, according to their size, by number, and include all from Nos. 1 to 6.
Fig. 1.
In preparing this lantern for use the lenses should be carefully cleaned with a soft cloth, and the lamp freshly trimmed. The cotton must be long enough to reach to the bottom of the lamp, and be freshly and evenly cut; the oil should be supplied to the lamp an hour before it is lighted, that the wick may become thoroughly saturated. The best oil to use is sperm oil in which camphor has been dissolved in the proportion of one ounce to one pint of oil. The object of the addition of the camphor is to increase the brilliancy of the light. If, however, sperm oil cannot be obtained, colza oil may be substituted, and this should be treated with camphor in the same way.
The room in which the exhibition is to take place should be entirely darkened, and a clean white screen or sheet hung up for the reception of the magnified pictures. The lantern is to be placed in front of the screen, upon a table or other suitable support, and at such a distance as will produce the required disc or circle of light.
The painted slide must be placed in the slide-holder upside down, and if the representation on the screen be not clear and sharp, the objective (the front lens) must be moved towards or away from the slide-holder, until the picture is well defined, or, as it is called, properly focused.