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98 MODEEN MAGIO LANTBENS. Geometrical designs, butterflies, etc., are built up of pieces of the substance in question, mounted up as slides, and are obtainable through the dealers. A more satisfactory method with many people will be to make them them- selves, the operation not being a very difficult one, consisting, as it does, of a building up of the design on a glass slide with pieces of mica cemented together with Canada balsam. Crystallised benzoic acid, saUcine, etc., also give very beautiful projected images with polarised light. Slabs of glass held in a clamp in such a way that, while the image of the glass is on the screen, stress can be set up within the glass by means of a screw, exhibit in a very clear manner the changes set up by the stress which the polariscope reveals—changes which in no other way can be rendered visible. Anderton's stereoscopic lantern, introduced a few years ago, is an example of an ingenious application of polar- ised light, with a view to enabling an audience to see the image on the screen stereoscopically or in relief. In order to effect this it is essential that two distinct images shall be seen by the two eyes, the two slides for the purpose being made from two negatives taken from different stand- points, side by side, so that one differs from the other by seeing a little more round one side or the other of the object. The two slides, which in all other respects are identical, are shown simultaneously upon a screen, the surface of which is composed of metallic foil, by means of a biunial lantern. After leaving the objective, the light from each of these lanterns passes through a polariser, these polarisers in the two lanterns being so arranged that the planes of polarisa- tion are at right angles to one another. Each observer is provided with a little pair of analysers mounted like opera- glasses, but in each of which the analysers are arranged with reference to each other as are the polarisers in the lanterns, the effect of which is thaS only one of the blended pictures on the screen is seen with one eye, and the other picture with the other eye. The result, on viewing the two images on the screen through such an apparatus, is to eliminate from the field of view of one eye the picture from