  The H-T phase diagram of CuGeO$_3$ has been determined, for different values
of the hydrostatic pressure, utilizing optical absorption spectroscopy on the
Cu$^{2+}$ d-d transitions. It is shown that the intensity of the related zero
phonon line transition is very sensitive to the local environment of Cu$^{2+}$,
allowing for precise determination of all phase transitions. It is found that
the phase diagrams at various pressures do not scale according to the
Cross-Fischer theory. An alternative scaling is proposed.
