New simple approximate relativistic expressions for the Hermitian part of the dielectric tensor, the real part of the refractive index and the locations of cutoffs have recently been proposed [Phys. Fluids B4, 3460 (1992)]. It was suggested that this approximation would be of use to reflectometry as well as to a number of other applications. There are, however, significant limitations in the accuracy of this approximation, which are pointed out here.