  The incoherent approximation for the determination of the vibrational density
of states of glasses from inelastic neutron or x-ray scattering data is
extended to treat the coherent scattering. The method is applied to new room
temperature measurements of vitreous silica and germania on the thermal
time-of-flight spectrometer IN4 at the High Flux Reactor in Grenoble. The
inelastic dynamic structure factor at the boson peak turns out to be reasonably
well described in terms of a mixture of rotation and translation of practically
undistorted SiO4 or GeO4 tetrahedra. The translational component exceeds the
expectation of the Debye model by a factor of two. A possible relation of this
excess to the phonon shift and broadening observed in x-ray Brillouin
scattering experiments is discussed.
