  We reassess the importance of photon splitting to spectral formation in
cosmic X-ray and $\gamma$-ray sources with magnetic fields of several $10^{12}$
gauss. Our analysis is based on the recent numerically accurate evaluation of
the full relativistic expression for photon splitting at neutron star magnetic
field strengths (Mentzel et al. 1994). Using these results we demonstrate that
Adler's (1971) widely used approximation formula for photon splitting
underestimates the correct splitting rate by {\em several} orders of magnitude,
at these magnetic field strengths. Even down at X-ray energies of a few tens of
keV magnetic photon splitting is found to be an efficient photon conversion
process, producing mean free paths as short as several tens of centimeters for
fields on the order of $5 \times 10^{12}$ gauss. We conclude that previous
analyses of the contributions of magnetic photon splitting to spectral
formation based on Adler's approximation formula are physically unreliable,
and, in particular, that photon splitting must be included in realistic
modelings of spectra of both strongly magnetized cosmic X-ray and $\gamma$-ray
sources.
