REVIEW ARTICLE: Clusters of galaxies

Determinations of the mass/light ratio of galactic clusters, red-shift surveys of the galactic distributions in space, and the discovery of superclusters of galaxies are discussed. Attention is given to observed astronomical groupings, including the Virgo cluster, compact groups, and the Coma cluster. The M/L problem, which is suggestive of missing mass in galaxies, is considered in terms of the virial theorem, dynamical friction, loose groups, and binary galaxies. Cosmography is reviewed, including the surface distribution of galaxies, space distributions in the Coma and Hercules regions, galaxy-free voids, etc. Finally, enigmatic observational facts are considered for the anomalous redshifts in compact groups, quasars in galaxy groups and superclusters, and bands in the redshift-magnitude diagram of the Coma cluster, as well as periodicity in the histogram of redshift differences for binary galaxies.