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344 OPTICAL PROJECTION between H and K. The whole instrument is about 24 inches long. For convenience at different distances, it is well to have a polariscope fitted with several powers. I have a second back lens of 6-inch focus, as well as the usual 5-inch back lens; then, by removing the front lens of the power when needed, there will be a range of powers of about 3|, 4, 5, and 6 inches. They are easily adjusted by screwing in a lengthen- ing ring or adapter. Another plan often adopted is to have a concave amplifying lens mounted in front of the instrument, which amplifies the image precisely as described in ยง 93, Chapter XIII. An alum-cell may be used in the ordinary lantern-stage with this polariscope, but is not really necessary. The glas? caps protecting the Nicol should be removed for a few minutes after lighting up, in order to allow the dew to escape which will at first form on the end surfaces. In ordinary work the stage and power s F is pushed close up to the end of the polariser, and not kept apart from it, as here shown for clearness. Occasionally a tube of fluid may be inserted between the polariser and the stage, then the stage with power is drawn forward sufficiently to allow of this. It is very easy to adapt a low microscopic power to this polariscope, by fitting a tube into the nozzle of F with a slot or slide-stage at the point where the lenses F condense the light into the smallest area. In the other end of this tube slides (for focussing) a power of about two inches, and the analyser. Such a power will exhibit many fine rock sections or ordinary micro slides, which the ordinary 3^-inch power is unable to do. But for anything like real microscopic work, resort must be had to the polarising microscope described in Chapter XIII. Great care should be taken of fine calcite prisms. The ends should be cleansed only with a clean, round-nosed camel- hair brush, often called a * sky-brush ' at artists' shops. If