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382 OPTICAL PROJECTION each other by an aliquot part of a circle, found that all the phenomena of quartz, either in parallel or convergent light, are perfectly reproduced. Such a preparation treated as a crystal gives the crystal fringes of quartz exactly, and two of opposite rotations superposed, give Airy's spirals. Plates of mica of equal thickness crossed at right angles, after Norremberg, as the plates become thinner and more numerous, give a gradual transition in convergent light from bi-axial rings, to the circular rings and four-armed cross of a uni-axial. Two crossed give of course the four rings of a crossed bi-axial; twenty-four of about J wave thickness, give effects quite undistinguishable from those of a calcite crystal. Plates of mica of various thicknesses crossed in different ways 1 give very beautiful and complicated figures in con- vergent light, which may still more be varied by crossing very thin films between much thicker ones, or interposing plates at an angle of 45° to the others. Mica-selenite combinations after Norremberg, built up of elements consisting of two parallel micas, with a selenite between them either parallel or across, these triple' elements ' being superposed in various ways and number, crossed or parallel, give in highly-convergent light the most beautiful projections. With no geometrical design whatever from the hand of man (except the crossing of films in various ways), the most exquisite coloured patterns are produced by con- vergent polarised light, some of them looking more like the most brilliant designs in squares of Turkey carpet than any- thing elso. 223. Polarisation by Small Particles.—The blue colour and polarisation produced by all reflection of light from sufficiently small particles (as in the sky), may be easily demonstrated by Tyndall's method, a large glass tube with 1 For details of such crossed preparations, see my paper in Proceedings of the Physical Society, entitled ' Optical Combinations of Crystalline Films, 1 reprinted in Phil. Mag. May 1883.