. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. izontal direction intothe instrument. This polariser is efficient, but two objectionsto it are the needless expense of so large a glass prism, andthat the absorption in so large a mass is very perceptible. A cheaper and upon the whole better arrangement wasadopted by the Kev. P. E. Sleeman. In this the parallel beamis first reflected by the silvered mirror s (fig. 193), and then LANTERN POLARISING APPARATUS 347 re-reflected into a horizontal direction and polarised bythe pile p with a black
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. izontal direction intothe instrument. This polariser is efficient, but two objectionsto it are the needless expense of so large a glass prism, andthat the absorption in so large a mass is very perceptible. A cheaper and upon the whole better arrangement wasadopted by the Kev. P. E. Sleeman. In this the parallel beamis first reflected by the silvered mirror s (fig. 193), and then LANTERN POLARISING APPARATUS 347 re-reflected into a horizontal direction and polarised bythe pile p with a black glass at the bottom. This arrange-ment can be made cheaply and readily of any size desired. It will be observed, that while the horizontal direction ofthe beam from the lantern is preserved in the Delezennepolarisers, it is necessarily deflected several inches to oneside of the axis of the flange-nozzle. In the first instrumentsmade on this principle, the beam was brought down belowthe nozzle; but this makes the apparatus rather deep, andrequires a deep case for packing. To avoid this the polariser. Fig. 194.—Direct Eeflecting Polariscope was turned sideways ; but that was found awkward, and alsonecessitated a large case. To meet these objections I advisedreversing the polariser so as to deflect the beam upwards, andMessrs. Newton & Co. now construct their instruments asin fig. 194. It will be seen that it is then rendered quitecompact, and requires no larger a case than the Nicol prismpolariscope. All the front portion is precisely the same as inthe instrument shown in section in fig. 190. Such polarisers are liable to the same objection as theelbow form, not being capable of rotating the polarised can, however, be effected through a simple expedientsuggested by Professor S. P. Thompson. If we cause the po-larised rays to pass through a mica quarter-wave plate (§ 211) 34S OPTICAL PROJECTION whose planes are at an angle of 45° with the plane of polarisa-tion, the
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Keywords: ., bookauthorwrightle, bookcentury1900, bookdecade1900, bookyear1906
