## Determination of 2V from One Extinction Curve and Its Related n_{0} Curve

N. Joel

Centro de Investigaciones de Cristalografía, Instituto de Física y Matemáticas, Universidad de Chile, Casilla 2777, Santiago, Chile

**Summary:** From the extinction curve of a biaxial crystal mounted on a spindle stage (or one-axis stage goniometer), the related *n*_{0} curve can be derived and drawn on the stereogram. The *n*_{0} curve is the equivibration curve—or constant-refractive-index curve—that goes through the projection *P*_{0} of the spindle-stage axis. After the principal diameters (maximum 2*η* and minimum 2*ζ*) of the *n*_{o} curve have been measured, the angle 2V may be calculated by means of the formula cos V = sin *ζ*/sin *η*. This method can also be used for determining—or refining—the directions of the principal axes **α**, **β**, **γ**.