a. One of three orthogonal optic directions in biaxial crystals. Light with its
electric vector (vibration direction in early terminology) parallel to the
y-direction has a unique intermediate refractive index (n beta ) for
a given crystal and is called the fast ray, relative to light with its electric
vector parallel to the z-direction, and the slow ray, relative to light with
its electric vector parallel to the x-direction. In orthorhombic crystals, the
y-direction is constrained by symmetry to correspond to one of the
crystallographic directions, the correspondence determined being empirically.
In monoclinic crystals, one optic direction, commonly the y-direction, is
constrained by symmetry to correspond to the unique diad. In triclinic
crystals, there is no symmetrical constraint relating optic directions to
crystallographic axes.
