Note:  Do not rely on this information. It is very old.

Biaxial Crystals

Biaxial Crystals are such as possess a certain definite optical property. A wave of light emanating from a point within the crystal, which must be transparent for such waves, will divide into two parts, as is usual with all substances exhibiting the phenomenon of double refraction (q.v.). For any given direction in the crystal, each part of the wave will have a definite wave-velocity, and as a rule the wave-velocities for the two parts will be different. In biaxial crystals there are two definite directions in which the wave-velocities are equal; in uniaxial crystals there is only one direction in which the wave-velocities for the two parts are equal. As examples of the biaxial type we have borax, sugar, felspar, and nitre.