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

THE PARTS OF A LANTERN 19 The condenser at E, consisting of two plano-convex lenses, was also used by Herschel, was adopted by Mr. Marcy for his Sciopticon lamp, and is the best form for all large and hot radiants. Such lamps have to be placed at a greater distance from the condensers to avoid cracking them, and are generally used with short focus objectives ; the two cones of light being thus more alike in angle, two such lenses represent pretty well the optical conditions. When properly modified, this form is equally adapted for the lime-light, and is now the most usual condenser found in good lanterns. 10. Correction of the Condenser.—For oil lanterns the plain form shown at E, of two similar lenses, cannot be improved. Optically, it would be better that the lens next the lamp should be rather deeper in curve, but the great heat makes this undesirable. With the lime-light it is different, and in considering the very best form, it is well to understand the principles upon which the correction of aberrations depends. This has been very familiarly explained in the diagram, fig. 11, by Mr. Eobert Bow. Here D E re- presents the upper half of a plano-convex lens, the faint line h e the outline of a double convex lens in contact with it, and de a meniscus lens of the same focus. Consider now the different effect of these two latter upon the spherical aberrations, due to the fact that the marginal rays are brought to a shorter or closer focus than the central rays. It is quite plain that the focus F of the central rays, A A, will be almost exactly the same in the two arrangements—we may practically consider that a fixed point. But with the marginal rays, B B, it is different. Tracing the top one, B D, let us suppose that the lens h re- 02