the diamonds even when the Nicols were parallel. It is obvious, then, that both a transparent non-crystalline substance such as glass, and a cubic crystal, must be free from strain in order that it shall exhibit no colour in polarised light and, indeed, no polarisation effects whatever, and behave as an isotropic substance. [Illustration: _PLATE XVI._ FIG. 82.—Ten Diamonds exhibiting Natural Faces, mounted for the Lantern Polariscope, to show Polarisation Colours due to Internal Strain. ] [Illustration: FIG. 121.—Doubly Refracting Liquid Crystals of Cholesteryl Acetate, projected on the Screen in the Act of Growth (see p. 281). TWO FIGURES ILLUSTRATING THE HARDEST (DIAMOND) AND THE SOFTEST (LIQUID CRYSTALS) OF CRYSTALS. ] The second special case to which attention may be called, that of a plate of an ordinary uniaxial crystal such as calcite, cut perpendicularly to the optic axis, is also obviously subject to the same proviso, that the crystal must be free from strain in order to exhibit the normal phenomena. Such a perfectly normal plate remains quite obscure in the dark field in parallel light, producing neither colour nor interference figure, even on rotation of the object stage with the crystal, in its own plane. For the light traverses the crystal along the optic axis, the axis of single refraction, and the vibrations occur with equal velocity in all directions perpendicular to it. Hence there is no division into two rays, one retarded behind the other on account of less velocity of vibration, and therefore no interference colour.