) principal section主截面 78 The principal section is consist of the normal to crystal surface and optic axIS。 注意: 78° OAny section in the crystal parallel to this section is 光轴 also the principal section @when incident ray lie in the principal section, the vibration of o-ray normal to the principal section and the vibration of e-ray parallel to the principal section.光轴 4）principal section 主截面 The principal section is consist of the normal to crystal surface and optic axis. Any section in the crystal parallel to this section is also the principal section. 注意： when incident ray lie in the principal section, the vibration of O-ray normal to the principal section and the vibration of e-ray parallel to the principal section. n+ ˆ 102º 78º 78º