  Approximate expressions for X-ray resonant and M\"ossbauer reflectivity in
the total external reflection region are developed for the limiting cases of a
semiinfinite mirror with a small resonant addition to the total susceptibility
and for the case of an ultrathin resonant layer. It is shown that in this
region the reflectivity can really show the linear dependence on the imaginary
part of resonant susceptibility, therefore in these cases the consideration of
reflectivity spectra (R-spectra) as absorption resonant spectra, undertaken in
several experimental works, can be justified. However, we have selected several
effects producing essential dispersive distortions of the R-spectrum shape even
for very small grazing angles. It has been shown that the dispersive
corrections to the R-spectrum shape are mostly essential if the nonresonant
absorption is relatively large. Model calculations demonstrate that the
quantitative spectroscopic information extracted from R-spectra by using the
software developed for absorption spectra can be inaccurate.
