  It is found that the amphoteric refraction, i.e. the refraction can be either
positive or negative depending on the incident angles, could occur at a planar
interface associated with a uniaxially anisotropic positive index media (PIM)
or an anisotropic negative index media (NIM). Particularly, the anomalous
negative refraction can occur at a planar interface from an isotropic PIM to an
anisotropic PIM, whereas the anomalous positive refraction occurs at the
interface from an isotropic PIM to an anisotropic NIM. The optimal conditions
to yield the two unusual refractions are obtained. The difference of the two
types of amphoteric refraction is discussed.
