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We present absorption line indices measured in the integrated spectra ofglobular clusters both from the Galaxy and from M 31. Our samples include 41Galactic globular clusters, and more than 300 clusters in M 31. The conversionof instrumental equivalent widths into the Lick system is described, andzero-point uncertainties are provided. Comparison of line indices of old M 31clusters and Galactic globular clusters suggests an absence of importantdifferences in chemical composition between the two cluster systems. Inparticular, CN indices in the spectra of M 31 and Galactic clusters areessentially consistent with each other, in disagreement with several previousworks. We reanalyze some of the previous data, and conclude that reported CNdifferences between M 31 and Galactic clusters were mostly due to datacalibration uncertainties. Our data support the conclusion that the chemicalcompositions of Milky Way and M 31 globular clusters are not substantiallydifferent, and that there is no need to resort to enhanced nitrogen abundancesto account for the optical spectra of M 31 globular clusters.

Star Clusters in M 31. IV. A Comparative Analysis of Absorption Line Indices in Old M 31 and Milky Way Clusters