. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. riscope, 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 f
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. riscope, 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 in a horizontaldirection. To keepit from turninground in the flangefrom its own weight,there is either a slotfitting over a pin,or a simple bayonetjoint. The Nicol prismanalyser will per-form all ordinary experiments, but its performance shouldbe examined. A Nicol of proper proportions will, with theobjective described in Chapter XII., just cover { give apolarised field over) slides of the standard London pattern, z2. Via. 186.—Elbow r-olariscopo 340 OPTICAL PROJECTION which have a disc lf-inch diameter in 4 x 2| wooden shade more may be sometimes covered. Some improvements have lately been made upon the Nicolprism. Prazmowskis prism, as further improved by & Eeuter, is a rectangular parallelopiped, cut fromedge to edge instead of from corner to corner (thereby givinga pencil of double the area) and with the section at rightangles to the optic axis. This utilises better the two refrac-tive indices, and moreover the joint is made with linseed-oilinstead of balsam : the total result being a shorter prism, ofwider area and more angular field. It, however, requiresmore than double the spar, and on account of the work, isabout three times the price of a Nicol of similar area. Asimpler and cheaper improved prism was devised by Professor S. P. Thompson, and is made bytaking an or
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Keywords: ., bookauthorwrightle, bookcentury1900, bookdecade1900, bookyear1906
