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37 6 OPTICAL PROJECTION colour is visible ; but with a bi-quartz the effect is generally very evident with a moderate current. Thompson's mica sectors (fig. 200) may also be used. Dr. Kerr's experiment, showing that plane-polarised light is rotated if reflected from the polished pole of a magnet, is too sensitive for successful demonstration, unless with apparatus of the highest class and the arc light. 220. Rotation of Common Light.—If the light employed is powerful enough to exhibit interference-bands on the screen with Fresnel's bi-prism (§ 189) this can be exhibited after the manner of Profs. Abbe and Sohncke. It is only necessary to cover one-half of the bi-prism with a plate of left-handed, and the other with a plate of right-handed quartz, of 1-88 mm. in thickness. This thickness rotates yellow light 45°, and therefore if quartz rotates common light at all, the two interfering rays are brought into orbits differing by 90° in azimuth, when they cannot interfere. It is demonstrated that this is so, because the bands vanish accordingly. 221. Ring and Brushes in Crystals.—For use with the simpler forms of lantern polariscopes, like fig. 186, plates of crystals are cut transversely to their optic axes, and mounted in wooden sliders as repre- FIG. 207. .... _ sented in fig. 207. By having a frame made of the usual 4x2^ size, consisting ol two 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 strip between, this can be placed in the usual stage, when it will be seen that the plate of crystal (unless it be a circularly-polar- ising one) has no double refraction at all in this direction; the dark or light field, in the parallel beam of plane-polarised light, remains as it was before the crystal was inserted,