There are two methods to identify optic sign of biaxial indicatriz. One is based on whether the bisector of the acute angle is parallel with Ng (positi-ve) or with Np (negative)，with the critical angle 2V being 90°; the other is based on the comparison of three principal refractive indezesc when Ng-Nm>Nm-Np, the optic sign is positivef when Ng一Nm