  The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters is
providing a major breakthrough in our knowledge of Globular Clusters (GCs) and
their stellar populations. Among the main results, we discovered that all the
studied GCs host two main discrete groups consisting of first generation (1G)
and second generation (2G) stars. We exploit the multiwavelength photometry
from this project to investigate, for the first time, the Red Giant Branch Bump
(RGBB) of the two generations in a large sample of GCs. We identified, with
high statistical significance, the RGBB of 1G and 2G stars in 26 GCs and found
that their magnitude separation as a function of the filter wavelength follows
comparable trends. The comparison of observations to synthetic spectra reveals
that the RGBB luminosity depends on the stellar chemical composition and that
the 2G RGBB is consistent with stars enhanced in He and N and depleted in C and
O with respect to 1G stars. For metal-poor GCs the 1G and 2G RGBB relative
luminosity in optical bands mostly depends on helium content, Y. We used the
RGBB observations in F606W and F814W bands to infer the relative helium
abundance of 1G and 2G stars in 18 GCs, finding an average helium enhancement
$\Delta$Y=0.011$\pm$0.002 of 2G stars with respect to 1G stars. This is the
first determination of the average difference in helium abundance of multiple
populations in a large number of clusters and provides a lower limit to the
maximum internal variation of helium in GCs.
