  We present absorption line indices measured in the integrated spectra of
globular clusters both from the Galaxy and from M 31. Our samples include 41
Galactic globular clusters, and more than 300 clusters in M 31. The conversion
of instrumental equivalent widths into the Lick system is described, and
zero-point uncertainties are provided. Comparison of line indices of old M 31
clusters and Galactic globular clusters suggests an absence of important
differences in chemical composition between the two cluster systems. In
particular, CN indices in the spectra of M 31 and Galactic clusters are
essentially consistent with each other, in disagreement with several previous
works. We reanalyze some of the previous data, and conclude that reported CN
differences between M 31 and Galactic clusters were mostly due to data
calibration uncertainties. Our data support the conclusion that the chemical
compositions of Milky Way and M 31 globular clusters are not substantially
different, and that there is no need to resort to enhanced nitrogen abundances
to account for the optical spectra of M 31 globular clusters.
