ABSTRACT
A crystal is an anisotropic medium, so that its equation of motion in the linear approximation must be expressed in the general form. The laws of propagation of a wave in a particular characteristic direction in the crystal will be the same as those for a wave with the same polarization in an isotropic body, and the corresponding equations for it can be written in a scalar form. The latter, in their turn, can be determined by measuring the velocity of ultrasound in some “sections” of the crystal, i.e., in crystal samples whose faces are cut perpendicular to a chosen direction. Arbitrary directions in the crystal relative to the crystallographic axes are indicated in brackets, using Miller’s notation, in terms of the relative magnitudes of the projections of the elementary segments. Since in crystals several waves can propagate in any direction, the general equations for the reflection and refraction coefficients, even for a specific crystal, are very complicated.
