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OUR KNOWLEDGE of the large-scale structure of the Universe is limited by our ability to recognize and observe remote objects, and to determine their distances and physical properties. Clusters of galaxies may be the most fundamental condensations of matter in space. In any case, they are the systems which we can most conveniently recognize, and for which estimates of at least relative distances are possible. The study of clusters of galaxies, therefore, in addition to its own intrinsic interest, may well provide the clues to our understanding of the Universe as a whole.