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POLARISED LIGHT 373 which rotate the colours in opposite directions, if a plate be composed half of one, and half of the other, of the proper thick- ness, it is a very sensitive test of any optical rotation, as the purple transition-tint seen all over in one position of the analyser, changes towards blue in one half and red in the other, on the least rotation. Such is therefore used to demon- strate the rotation of fluids. Another useful and more sensitive bi-quartz preparation is that shown in fig. 205. In the portion B, the wedge B is of say right-handed, and A of left-handed quartz, the effect of which is a black band across the centre where the thicknesses are equal (i.e. in the dark field) and coloured bands on each side. In the half c D the wedges are reversed. The consequence is, that on the least additional rotation in one direction by any substance used with the wedges, or the least rotation of the analyser, the bands move in opposite directions, and the distance the analyser has to be rotated to bring them back, is a measure of the rotation due to the PIQ 2 o5.-Bi-quartz wedges substance examined. Plates of quartz may be obtained in which both right- and left-handed crystallisation occurs, and such are very beautiful objects. Amethyst is a quartz crystal with the contrary cry- stals arranged in narrow parallel bands, and such are still more beautiful, but a large one is very difficult to find : one that will cover a standard-sized disc is a gorgeous object. Plates about | inch diameter can be obtained without difficulty. Any clean quartz crystal of good size which is a violet colour, is almost certain to exhibit either amethyst or cross-crystallisation, and should be cut up into polariscope specimens. Other crystals can be obtained which rotate the beam, and most of them in bi-quartz form. They are, however, all