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POLARISED LIGHT 381 quarter-wave plate. The result is that a plate cut to exhibit one axis only of a bi-axial, shows one spiral figure surrounding this axis. A plate cut to show both axes, exhibits a few turns of a spiral round each axis, after which the two spirals em- brace each other, and proceed concentrically round the whole field. With a low-angled bi-axial cut thin, in highly conver- gent light, the two spirals enwrap each other nearly from the first. And finally, in a uni-axial the two separate spirals are visible, showing that both elements of the bi-axial remain and are combined in what is simply a limiting case. By applying the arrangement to Mitscherlich's experiment with a heated sslenite, the gradual drawing in of the figure, but preservation of the two spirals through all, can be simply demonstrated. A quartz plate alone in highly convergent circularly polarised light, projects a double spiral, as pointed out by Mr. Airy. The above experiments prove that this is owing to its peculiar properties enabling it to show its own spirals as a uni-axial crystal. A column of fluid of adequate rotary power, such as a column of oil of lemons 20 cm. long, employed with a crystal instead of the quartz plate, will exhibit exactly the same phenomena, thus affording proof that the molecular con- stitution of the fluid resembles that of the quartz. The polariscope I have described projects through the column of fluid, crystal, and convergent lenses, &c., brilliantly and with ease. It will be very interesting to point out, that by thus modifying the polarisation in different ways, rays of the same convergence can produce with the same plate of crystal, either rings with brushes, rings dislocated, unbroken rings without any brushes, or spirals. 222. Artificial Crystals.—It has been seen that a plate of crystal circularly polarised, roughly represents the rotary phenomena of quartz. Keusch, by employing a preparation built up of many thin films of mica successively rotated on