Some time ago a series of papers 1 was published on the constitution of clusters of galaxies (extragalactic nebulae). This series, interrupted by the war, is now being continued. In addition to observations with the 18-inch schmidt telescope, some preliminary data are also available from work with the 48-inch schmidt telescope on Palomar Mountain. The Cancer cluster (R.A. 8 h 17*6, Deck +21° 16' [1950], gal. long. 170°, lat. +30°) according to E. P. Hubble and M. L. Humason, 2 consists of about

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... 150 galaxies distributed over an area of about one square degree near NGC 2562 and 2563 whose apparent photographic magnitudes according to unpublished determinations by E. Pettit are respectively m p = 14.1 and 13.8. The distance of the cluster is estimated to be nine million parsecs. Counts of nebulae with the 18-inch schmidt telescope.-As was found in all clusters previously investigated 1 the schmidt telescopes reveal the Cancer cluster as of much larger angular dimensions than those found with the large reflectors, whose very restricted fields make them little suited for the investigation of extended objects. Various emulsions were tried in photographing the field of the Cancer cluster. Unfortunately none of these was found with the speed, uniformity, and fine grain of some of the prewar emulsions, such as the Agfa supersensitive Panchromatic. Actually the best results were obtained with Agfa commercial films, ten years old. These results are therefore not quite as good as analogous ones reported on before the war for other clusters. It is estimated that most of the galaxies brighter than about m p -16.5 were reached with the 18-inch telescope. The radial distribution from the center of the Cancer cluster outward is given in Table I . The number of field galaxies per i R Zwicky, Proc. Nat Acad.,