. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. herefore if quartz rotates common light at all, the twointerfering rays are brought into orbits differing by 90° inazimuth, when they cannot interfere. It is demonstratedthat this is so, because the bands vanish accordingly. 221. Ring and Brushes in Crystals.—For use with thesimpler forms of lantern polariscopes, like fig. 186, plates of crystals are cuttransversely to theiroptic axes, andmounted in woodensliders as repre-sented in fig. having a frame made of the usual 4 x 2£ size, co
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. herefore if quartz rotates common light at all, the twointerfering rays are brought into orbits differing by 90° inazimuth, when they cannot interfere. It is demonstratedthat this is so, because the bands vanish accordingly. 221. Ring and Brushes in Crystals.—For use with thesimpler forms of lantern polariscopes, like fig. 186, plates of crystals are cuttransversely to theiroptic axes, andmounted in woodensliders as repre-sented in fig. having a frame made of the usual 4 x 2£ size, consisting oltwo thin metal plates with a circular aperture in the centres,separated by a strip of wood along the top and bottom edges,and leaving a space for the small slide as a centre stripbetween, this can be placed in the usual stage, when it will beseen that the plate of crystal (unless it be a circularly-polar-ising one) has no double refraction at all in this direction; thedark or light field, in the parallel beam of plane-polarisedlight, remains as it was before the crystal was Fig. 207. POLARISED LIGHT 377 With convergent light it is of course different. For thosimpler polariscopes a crystal-stage is provided, which consistsof a tubular fitting like fig. 208. One end a fits into thenozzle of the objective, from which the analyser is withdrawn,the latter now fitting into the other end b of this crystal-stage. In the centre at s is a slot or stage with springs, largeenough to receive the crystal sliders, which are about an inchwide ; and the stage is so placed that the crystal occupiesthe spot where the cone of light is of the smallest the low convergence from the usual power alone,the convergent (or divergent) light passes through thecrystal and analyser straight to the screen, and needs nofocussing lens whatever, the fringes appearing simply asshadows. The rings and brushes are shown as perfectly inthis simple and inexpensive way,as in any o
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