This beautiful experiment is now frequently performed, as gypsum is perhaps the best example yet known which exhibits the phenomenon of crossed-axial-plane dispersion by change of temperature alone. A considerable number of other cases are known, such as brookite, the rhombic form of titanium dioxide TiO_{2}, and the triple tartrate of potassium, sodium, and ammonium, but these are more sensitive to change of wave-length in the illuminating light than to change of temperature. [Illustration: FIG. 51.—The Mitscherlich Experiment with Gypsum. ] [Illustration: _PLATE XII._ FIG. 52.—Appearance of the Interference Figure half a Minute after commencing the Experiment. Temperature of Crystal about 40° C. ] [Illustration: FIG. 53.—Appearance a Minute or so later, the Axes approaching the Centre. Temperature of Crystal about 85° C. ] [Illustration: FIG. 54.—The Two Optic Axes coincident in the Centre of the Figure, two or three Minutes from the commencement. Temperature of Crystal 106° C. ] [Illustration: FIG. 55.—The Axes re-separated in the Vertical Plane a Minute or two later. Temperature of Crystal about 125° C. THE MITSCHERLICH EXPERIMENT WITH GYPSUM. FOUR STAGES IN THE TRANSFORMATION OF THE INTERFERENCE FIGURE IN CONVERGENT POLARISED LIGHT, FROM HORIZONTALLY BIAXIAL THROUGH UNIAXIAL TO VERTICALLY BIAXIAL, ON RAISING THE TEMPERATURE TO 125° C. (From Photographs by the author.) ] The author has recently exhibited the “Mitscherlich experiment” to the Royal Society,[2] and also in his Evening Discourse to the British Association at their 1909 meeting in Winnipeg, in a new and more elegant manner, employing the large Nicolprism projection polariscope shown in Fig. 51, and a special arrangement of lenses for the convergence of the light, which is so effective that no extraneous heating of the crystal is required. The convergence of the rays is so true on a single spot in the centre of the crystal plate about two millimetres diameter, that a crystal plate not exceeding 6 mm. is of adequate size, mounted in a miniature holder-frame of platinum or brass with an aperture not more than 3 mm; the thickness of the crystal should remain about 2 mm., in order that the rings round the axes may not be too large and diffuse, the crystal being endowed with very feeble double refraction, which is one of the causes of the phenomenon. Such a small crystal heats up so rapidly in the heat rays accompanying the converging light rays—even with the essential cold water cell two inches thick between the lantern condenser and the polarising Nicol, for the protection of the balsam of the latter—that any extraneous heating by a spirit or other lamp is entirely unnecessary. The moment the electric arc of the lantern is switched on, the optic axial rings appear at the right and left margins of the screen, when the crystal is properly adjusted and the arc