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POLARISED LIGHT 379 medium which will preserve these crystals clear is much to be desired. When clear, by placing in the stage gelatines carefully selected to absorb all but the blue and red rays, the two systems of rings can be shown crossed on the screen if the light is brilliant. The axes are shown easily. Airy's Spirals are produced by a right-handed quartz, cut perpendicularly to the optic axis, superposed upon a left-handed one. The two need not be of equal thickness, and plates containing cross-crystallisation will show the spirals naturally, wherever the two forms happen to overlap each other. It ought to be noted, that crystals which can be shown in both forms of polariscope, require to be much thinner for highly convergent light than for the low convergence method. A thick plate of nitre will fill the field, with low convergence ; for high convergence it must be very thin, and the two axes will appear very close together in the centre of the field. Most mica commonly obtainable has an angle of about 45°. Mitscherlich's Experiment, showing that a crystal of selenite, when heated, gradually becomes uni-axial, and with further heat becomes again bi-axial with the axes at right angles to the former direction, is easily demonstrated with the apparatus described. I employ a slide devised for the purpose ; but it is sufficient to procure a strip of brass or copper about 4x1 inches, the same thickness as the crystal, drill and file a hole through the centre which just admits the latter loosely, and bend up rather more than an inch of each end, so that these ends stand well away from the crystal stage like ears, when the strip is held by the springs. Then bend a piece of thin card round one edge, so as to cover both sides of the centre of the strip, and cut through both cards a hole rather smaller than the crystal; thus the card, as it embraces the brass with the crystal in it, keeps the latter in place. All this being adjusted on the stage, and the rings focussed as usual, a lighted spirit-lamp is taken in hand and applied alternately to the projecting ears. At first a slight mist generally appears,