  We compare our compilation of the W' calcium index for 71 Galactic globular
clusters to the widely used Zinn and West (1984 ApJS, 55, 45) [Fe/H] scale and
to Carretta and Gratton's (1997 A&A Supplement 121, 95) scale from
high-dispersion spectra analyzed with Kurucz (1992, private communication)
model atmospheres. We find our calcium ranking to be tightly correlated with
each comparison set, in a non-linear and a linear fashion, respectively. By
combining our calcium index information with the Zinn and West ranking, we are
able to rank the globular clusters in our sample with a typical precision of
+/- 0.05 dex for [Fe/H] < -0.5 on the Zinn and West scale; for clusters more
metal rich than this, the ranking is less precise. The significant differences
between these metallicity scales raise important questions about our
understanding of Galactic formation and chemical enrichment processes.
Furthermore, in spite of the apparent improvement in metallicity ranking for
the Galactic globular clusters that results from our addition of information
from the Ca II triplet lines to the potpourri of other metallicity indicators,
caution -- perhaps considerable -- may be advisable when using W' as a
surrogate for metallicity, especially for systems where ranges in age and
metallicity are likely.
