If the selenite stage be employed in the examination of this object, one of the most gorgeous appearances in the whole domain of the polarising microscope is displayed: the black cross disappears, and is replaced by one consisting of two colours, and divided into a cross having a red and green fringe, whilst the four intermediate sectors are a gorgeous orange-yellow. These appearances alter on the revolution of the analysing-plate of tourmaline; when the blue stage is employed, the cross assumes a blue or yellow tint, varying according to the position of the analysing plate. These phenomena are analogous to those exhibited by certain circular crystals of boracic acid, and to circular discs of salicine (prepared by fusion), the difference being that the salts of quinidine have more intense depolarising powers than either of the other substances; the mode of preparation, however, excludes these from consideration. Quinine prepared in the same manner as quinidine has a very different mode of crystallisation; but it occasionally presents circular corneous plates, also exhibiting the black cross and white sectors, but not with one-tenth part of the brilliancy, which of course enables us readily to discriminate the two. [Illustration: Fig. 188.--Urinary Salts, seen under Polarised Light. _a_, Uric acid; _b_, Oxalate of lime, octahedral crystals of; _c_, Oxalate of lime allowed to dry, forming a black cube; _d_, Oxalate of lime as it occasionally appears, termed the dumb-bell crystal.] Urinary salts are more readily seen under polarised light than by white light. Ice doubly refracts, while water singly refracts. Ice takes the rhomboidic form; and snow in its crystalline forms may be regarded as the skeleton crystals of this system (Fig. 189). A sheet of clear ice, of about one inch thick, and slowly formed in still weather, shows circular rings with a cross by polarised light. [Illustration: Fig. 189.--Snow Crystals.]