a. One of the three optic axes (x,y, and z) in a biaxial crystal. The y-axis is
the intermediate optic axis, at right angles to the plane containing optic axes
x and z. Light vibrating parallel to the y-axis is called the intermediate ray,
the y-ray, and the beta -ray. The middle-value index of refraction nbeta
in biaxial minerals is the index of the intermediate ray vibrating parallel to
the y-axis.
