From the extinction curve of a biaxial crystal mounted on a spindle stage (or one-axis stage goniometer), the related n₀ curve can be derived and drawn on the stereogram. The n₀ curve is the equivibration curve—or constant-refractive-index curve—that goes through the projection P₀ of the spindle-stage axis. After the principal diameters (maximum 2η and minimum 2ζ) 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.