Fig. 186.—In this figure heraldic lines are adopted to denote colour. The dotted parts indicate yellow, the straight lines red, the horizontal lines blue, and the diagonal, or oblique lines, green. The arrows show the plane of the tourmaline, a, blue stage; b, red stage of selenite employed.
To render these crystals evident, it merely remains to bring the glass-slide upon the field of the microscope, with the selenite stage and single tourmaline, or Nicol’s prism, beneath it; instantly the crystals assume the two complementary colours of the stage: red and green, supposing that the pink stage is employed; or blue and yellow, provided the blue selenite is made use of. All those crystals at right angles to the plane of the tourmaline produce that tint which an analysing-plate of tourmaline would produce when at right angles to the polarising-plate; whilst those at 90° to these educe the complementary tint, as the analysing-plate would also have done if revolved through an arc of 90°.
This test is a delicate one for quinine ([Fig. 186], a and b); not only do these peculiar crystals act in the way just related, but they may be easily proved to possess the optical properties of that remarkable salt, the sulphate of iodo-quinine.
Fig. 187.—Polarised Crystals of Quinidine.
To test for quinidine, it is merely necessary to allow a drop of acid solution to evaporate to dryness upon the slide, and to examine the crystalline mass by two tourmalines, crossed at right angles, and without the stage. Immediately little circular discs of white, with a well-defined black cross, start into existence, should quinidine be present even in very minute traces. These crystals are represented in [Fig. 187].
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.
Fig. 188.—Urinary Salts, seen under Polarised Light.