SUITABLE SUBJECTS FOR THE POLAHISCOPE.
Doubly refracting substances such as starches, horn, hoofs, fish scales, and crystals of tourmaline, selenite, quartz, aragonite, etc., give very fine effects with polarised light. Starch granules are brilliantly illuminated, and show black crosses (illustrated by one of the Plates), while many of the crystals yield very beautiful colours through " interfer ence." Crystals of selenite, for instance, appear alternately red and green or blue and yellow. Many beautiful effects can be produced with sections of various rocks ; also with sections of horn, muscu lar tissue, hairs, and crystals, and these form exceptionally good subjects for colour photography.
INfoux.rtsG THE SPECIMEN.
The microscope is universally applicable to the examination of minerals, and of animal and vegetable structures ; there is scarcely a single substance, however com mon, which, when examined in the proper way, will not reveal sc.me more or less minute hidden structure which will well repay the trouble ; in many cases the ap pearance of substances under magnifica tion is so characteristic, that the micro scope is one of the most useful instru ments in analytical research. To give an
adequate idea of the methods of mounting different objects would require a special treatise, therefore it will not be possible to do more than give a sketch in outline. Many objects, for instance the majority of water anitualculx, cannot be mounted in any but liquid media, because they would otherwise lose their shape ; they are usually examined in a drop of water with out any treatment ; moving animalculte arc killed by osmic acid or cocaine. Bacteria contained in fluids are mounted by placing a drop of the liquid upon a thin glass circle and, drying down by a gentle heat ; the circle is then passed three times quickly through a Bunsen burner to fix them, and, after cooling, the circle is placed in a solution of carbol-fuchsin or other aniline dye to stain the bacteria. The glass is then cleared of excess of colour by dipping in methylated spirit or other fluid, and, after drying, it is ready to be mounted permanently.