光轴角近90°时二轴晶光性正负的判别

The Identification of Optic Sign of Biaxial Crystal at Optic Angle Near 90°

二轴晶光率体光性正负有两种判别方法：一种是根据光轴角（２Ｖ）的锐角平分线（Ｂｘａ）与Ｎｇ一致还是与Ｎｐ一致来判别，其临界点是２ｙ＝９０°；另一种是根据三个主拆射率的相对大小来判别．由于二轴晶光率体为三轴椭球体，在其临界点Ｎｇ一Ｎｍ＝Ｎｍ一Ｎｐ时，光轴角并不等于９０°。就是说，两种判别方法的临界点不一致。本文从光率体几何特性研究入手，通过几何及数学方法，着重讨论上述两种判别方法在其临界点时的相对关系，并结合实际例证，认为二轴晶光率体光性正负的临界点不能只确定为一个点，而应限定在一个范围，即２Ｖ＝９０°到Ｎｇ－Ｎｍ＝Ｎｍ－Ｎｐ之间。

There are two methods to identify optic sign of biaxial indicatrix. 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 indexes:when Ng-Nm>Nm- Np, the optic sign is positive; when Ng-Nm<Nm-Np,the sign is negative. It is generally held that the critical point for the two methods is identical,i.e,when Ng-Nm=Nm-Np , the optic angle is 90°. Nevertheless, the author considers that the critical points of the two methods are not consistent with each other in that the biaxial indicatrix is a tria一xial ellipsoid. The paper deals emphatically with the relationship between the methods at the critical point in terms of geometrical property of the opti-cal indicatrix and mathematic analysis, and holds that, instead of being a fixed point, the critical point for determining the optic sign of the biaxial indicatrix varies in a range from a point at which the acute optical angle 90°to the point at which Ng- Nm equals Nm-Np.