  Recently, Zhang et al. (Phys. Rev. Lett. 91, 157404 (2003)) have demonstrated
that an amphoteric refraction, i. e. both positive and negative refraction, may
prevail at the interface of two uniaxial anisotropic crystals when their
optical axes are in different directions. The authors subsequently made a
correspondence between such a refraction with the negative refraction expected
for Left Handed Materials (LHMs). Here we comment that the amphoteric
refraction can be observed even with one uniaxial crystal, and the refraction
is not related to the negative refraction expected for the much debated LHM.
Rather, the phenomenon is a natural result of anisotropic media.
