. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. condenser-lenses themselves, at a distance which gives aluminous disc 6 to 10 feet in diameter. Now hold a pieceof perforated card or zinc (very coarse is best) in a positioneasily found, either in front of or behind the second lens ; theresult will be most gorgeous chromatic patterns. Instead ofa simple plate a revolving Kaleidotrope (p. 144) may be used,when the effect will be magnificent if the zinc is of coarsepattern. As already stated, however, it is only partially due todiffraction. O
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. condenser-lenses themselves, at a distance which gives aluminous disc 6 to 10 feet in diameter. Now hold a pieceof perforated card or zinc (very coarse is best) in a positioneasily found, either in front of or behind the second lens ; theresult will be most gorgeous chromatic patterns. Instead ofa simple plate a revolving Kaleidotrope (p. 144) may be used,when the effect will be magnificent if the zinc is of coarsepattern. As already stated, however, it is only partially due todiffraction. OHAPTEE XXI IiANTBBN POLAEISING APPARATUS 194. The Elbow Polariscope.—The simplest and cheapestpolariscope for lantern work consists of a bundle of thin glassplates as a polariser, arranged at the back of an elbow, as infig. 185. The end n of the elbow is made to fit on the flange-nozzle of the lantern, and the elbow is of course so constructedthat nearly parallel light from the condensers entering at nfalls upon the glass plates G at the polarising angle of 56°. LANTERN POLARISING APPARATUS 339. Fia. 185 The other end of the elbow at b has a screw-collar, intowhich screws the b collar of the optical front (fig. 94), withstage, objective, and nozzle. Into this nozzle fits, so as torotate easily, an analyser, usually a Nicol prism. The wholeapparatus is shown in fig. 186, and is commonly known asthe lantern polariscope, or elbow polariscope. It is alsouseful and convenient as atable instrument for manypurposes, if a plate of finely-ground glass is fitted intothe end which fits into thelantern ; and will perform ina most efficient and satisfac-tory manner all ordinary ex-periments, which do not re-quire the rotation of the polariser, at a very moderate using this instrument, it is placed on or in the flange-nozzle with the elbow lying horizontally, so that the lanternhas to be deflected from the screen; because the opticalportion of the instrument must be preserved i
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Keywords: ., bookauthorwrightle, bookcentury1900, bookdecade1900, bookyear1906
