Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

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THE PARTS OF A LANTERN 17 warming his slides. If the plate itself is too hot to lay them upon, they can be readily supported on three small slabs of wood or cork. Opticians have long been behindhand in this respect, and such a low flat top has only to be known to be universally approved of. In the United States, lanterns are sometimes made with no rigid body at all. One such which was purchased there, and used in this country by the late Mr. E. A. Proctor for his lectures, had for its base a light metal quadrangle supported on four short pillars at the corners. The front alone, carrying the lenses, was hinged to this, and a light body fitted on the base behind like the tilt of a waggon. Another common American plan (shown hereafter in Chapter XII.), is to hinge the front to the base in the same way as just described, to add two light pillars behind, on which and the front is supported a sheet-iron top, and to form the sides of black cloth curtains. The arrangement may occasionally be useful for its lightness, and for packing into a small space ; but so far I have met with none who liked it in this country, in comparison with the more solid English bodies. 9. The Condenser.—The use of this has been explained. A single lens is, however, only used in toy lanterns. The object is to take up as wide an angular pencil of rays from the radiant as possible, and send them through the objective. A single lens, to do this, must be of such thickness as to lose much light by absorption; and moreover the chromatic and spherical aberration would be excessive. The use of two or more lenses accomplishes the object with a moderate thick- ness of glass, and also allows a large part of the aberrations to be corrected. Fig. 10 shows the principal forms of double condensers which have been used. Two double convex lenses, c, were used by Messrs. Carpenter & Westley in the early phantas- magoria lanterns, and for many years afterwards, till very lately, in the electric lanterns of M. Duboscq. They aro C