  We examine the relationship between the total X-ray and optical luminosities
of groups and clusters of galaxies taken from various samples in the
literature. The clusters and groups were drawn from four different catalogs:
(1) the Abell/ACO catalog, (2) the Edinburgh-Durham Cluster Catalog, (3) the
RASS Bright Cluster Sample, and (4) galaxy groups selected from the CfA
redshift survey. These catalogs represent a significant cross-section of
cluster selection tchniques as well as a wide range in mass scale and can be
considered statistically independent. We have calculated new cluster X-ray and
optical luminosities if not available in the literature. Based on 126 systems,
our analysis shows that the total optical luminosity of a cluster is directly
proportional to the total X-ray luminosity over a wide mass range and across
all four cluster samples studied herein. We also show that total cluster
optical luminosity is a good indicator of cluster virial mass, but richness is
not. Our results suggest that (1) the selection method of galaxy clusters may
not be crucial, providing wider latitude in assembling catalogs, (2) the
luminosity per baryon does not vary systematically between systems ranging from
groups to very rich clusters. We propose that future optical catalogs of
clusters use the total optical luminosity of a cluster, instead of galaxy
richness, as luminosity appears to be a better measure of cluster mass and can
be directly related to X-ray catalogs of clusters. This will also facilitate
the first direct comparison between the growing number of X-ray and optical
catalogs.
