The slide should now be placed upon the stage of the microscope, and the progress of formation of the crystals closely watched. When these are seen to be large enough, and it is deemed necessary to stop their further development, the slide must be quickly transferred to the palm of the hand, the warmth of which will be found sufficient to stop further crystallisation. These crystals attract moisture, deliquesce, and should therefore be kept in a perfectly dry place. [Illustration: 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. [Illustration: 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.