利用折射率椭球理论研究了各向异性晶体的平板负折射成像。研究结果表明：单轴晶体的负折射现象是由晶体的光学各向异性所引起的，属于单轴晶体的本质特性；单轴晶体的切角和双折射率是产生负折射的关键因素；各向异性晶体在一定的前提条件下可以实现平板负折射二次成像。最后通过钒酸钇（YVO4）和方解石（CaCO3）晶体对这套理论进行了实验验证。

Negative refraction and imaging properties of uniaxial crystal slabs are investigated by the refractive index ellipsoid analyses. It is found that the concept of negative refraction can be extended to be an intrinsic property of all uinaxial crystals. The negative refraction effect of uinaxial crystals is mainly due to the anisotropic refractive index. The angular range for incident light to yield negative refraction attains its maximum which only depends on the difference of two refractive indices and the orientation of the crystals with their optic axes at a certain angle to the normal of the light incoming surface. The negative refraction imaging of uniaxial crystal slabs is restricted within some special conditions. These conclusions are verified by the experiments on positive uniaxial crystal YVO4 and negative unaxial crystal CaCO3.