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POLARISED LIGHT 351 and the other black; but on rotating the crystal, this will reverse as at B ; or it will be also reversed by rotating the prisni to a position at 90° angle with the first. 201. Piles of Glass.—The effect of these is most conveni- ently demonstrated by mounting a pile of from twelve to twenty plates, B c (fig. 198), of the thinnest and most colourless glass that can be procured, in a frame screwed to the edges B c of two triangular pieces of board BCD, which are screwed to a base-board B D, and whose angles are such that a horizontal ray E F striking on the glass, is reflected at the polarising angle to G, while the unreflected portion (i.e. of common light) is trans- mitted to H. The thin glass analyser (fig. 188) would answer the same purpose, but its arrangements cannot be seen in the same way, and moreover it should be used with the present pile. First placing A the glass pile in the position of the figure, the base-boards Don E atable-stand, and the rotating tourmaline F m-Giass pile in the stage of the optical front, it will be shown that when in one position the tourmaline image is black on the screen by transmitted light, and transparent on the ceiling or overhead screen by reflected light; and that rotating the tourmaline 90° reverses this. Rotating the pile 90° (easily done by laying the pile on its side BCD) the images are also reversed. Using an aperture and the double-image prism, in the same way, the pair of discs will be alternately light and dark according to the positions; and then substituting the Nicol, it will be shown how one half the phenomena are suppressed or turned aside by this piece of apparatus. Finally, substituting the glass analyser (fig. 188) still with the aperture in the stage, it will be seen how the transmitted beam is either reflected or again