  We present observational estimates of the magnitude difference between the
luminosity function RGB bump and the HB (Delta(bump-zahb)), and of star counts
in the bump region (R_bump), for a sample of 54 Galactic globular clusters
(GCs) observed by the HST. The large sample of stars resolved in each cluster,
and the high photometric accuracy of the data allowed us to detect the bump
also in a number of metal poor clusters. To reduce the photometric
uncertainties, empirical values are compared with theoretical predictions
obtained from a set of updated canonical stellar evolution models which have
been transformed directly into the HST flight system. We found an overall
qualitative agreement between theory and observations. Quantitative estimates
of the confidence level are hampered by current uncertainties on the GCs
metallicity scale, and by the strong dependence of Delta(bump-zahb) on the
cluster metallicity. In case of the R_bump parameter, which is only weakly
affected by the metallicity, we find a very good quantitative agreement between
theoretical canonical models and observations. For our full cluster sample the
average difference between predicted and observed R_bump values is practically
negligible, and ranges from -0.002 to -0.028, depending on the employed
metallicity scale. The observed dispersion around these values is entirely
consistent with the observational errors on R_bump. As a comparison, the value
of R_bump predicted by theory in case of spurious bump detections would be
\~0.10 smaller than the observed ones. We have also tested the influence on the
predicted Delta(bump-zahb) and R_bump values of an He-enriched component in the
cluster stellar population. We find that the predicted Delta(bump-zahb) and
R_bump values are only marginally affected.

Astrophysics