Light; a course of experimental optics, chiefly with the lantern . Fig. 151. these bases are screwed brass standards which carry separ-ately the polarising Nicol, p n, the stage and focal power,s, F, and convergent lens system when required (Fig. 152). The large Nicol, p n, is mounted in an inner tube, fittedin a larger tube—partly for appearance and partly to accom-modate a rather larger Nicol should such ever be save useless weight and space, its corners are cut off ina hexagonal form. The outer tube carries at the end nextthe lantern a flange, D (Figs. 151 and 152), divided irit
Light; a course of experimental optics, chiefly with the lantern . Fig. 151. these bases are screwed brass standards which carry separ-ately the polarising Nicol, p n, the stage and focal power,s, F, and convergent lens system when required (Fig. 152). The large Nicol, p n, is mounted in an inner tube, fittedin a larger tube—partly for appearance and partly to accom-modate a rather larger Nicol should such ever be save useless weight and space, its corners are cut off ina hexagonal form. The outer tube carries at the end nextthe lantern a flange, D (Figs. 151 and 152), divided iritoforty-five degrees, and at its middle four spokes, by which. X. o 12 og h u e, o K J 6* b < ea ^ lo !i; 4= M c fl) bl) H ^ hn ^ a 1 t O «2 ?s o a •2 1 s & bj] .s op. & 1 b 0 « d. 248 LIGHT. [chap. it can be rotated in collars screwed to the standards. Two ofthese spokes are bright, and two blackened, the latter alsohaving a groove turned round them ; so that the polarisingplanes can be known in an instant either by sight or in thedark. In the same end of the outer tube screws a fittingcarrying the concave meniscus lens, l, or a plane glass toprotect the spar, at pleasure. The object of the lens is,whenever the utmost illumination is required, to reconvertto parallelism the converging rays from an additional convexlens of similar focus, mounted in a frame as an ordinarysHde, and inserted in the slide-stage of the lantern. Thusa four-inch beam can be brought down to a parallel beamof two inches; but usually a plane glass is employed, andG is another plane glass cap slipped over the smaller tubeat the other end of the Nicol. The s
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Keywords: ., bookcentury1800, bookdecade1880, bookidcu3192403121, bookyear1882
