FIG. 73.—Crystal Plate cut perpendicularly to the Bisectrix of the Acute Optic Axial Angle. Biaxial Interference Figure afforded by Aragonite (Rhombic) in Convergent Polarised Light, with Nicols crossed and parallel to the Vibration Directions of the Crystal. ] [Illustration: FIG. 74.—The same Plate as for the previous Figure. The same when the two Nicols have been rotated in the same direction for 45°, still remaining crossed. CHARACTERISTIC UNIAXIAL AND BIAXIAL INTERFERENCE FIGURES IN CONVERGENT POLARISED LIGHT. (Reproductions of direct Photographs by the author.) ] On rotating the crystal plate in its own plane, while no change occurs with the calcite, the aragonite figure changes as regards the black cross, which breaks up into hyperbolic curves currently spoken of as “brushes,” until when the plate has been rotated 45° the appearance is that shown in Fig. 74, the eye being supposed to have followed the rotation. Or, keeping the eye still, the effect shown in Fig. 74 is equally produced by the simultaneous rotation of both Nicols for 45°. The vertices of the hyperbolæ now mark the positions of the optic axes, and the angle between them is the apparent angle of the optic axes as seen in air, which is considerably different from the true angle between the optic axes within the crystal, owing to the very different refraction of light in air and in the crystal substance. Now some crystals exhibit a very different optic axial angle at different temperatures, and one of the most beautiful experiments which have ever been performed is the Mitscherlich experiment with gypsum, which has already been described in Chapter VII. in connection with the work of Mitscherlich, and illustrated in Plate XII., Figs. 52 to 55. Other substances, on the other hand, show a marked change of optic axial angle as the wave-length of the light is changed, and such a case has already been described in Chapter VIII. and illustrated in Plate XIII., Fig. 58. The figure afforded by such a substance in ordinary white light is, however, a complicated one, quite different from the normal one of Fig. 73 afforded by aragonite, as will be clear on reference to the interference figure shown at _f_ in Fig. 58, which represents the figure given by ethyl triphenyl pyrrholone in white light.