Magnitudes and colors in ultraviolet, blue, and yellow light have been measured for 133 physical members of the Praesepe cluster and for 17 nonmembers in the same field A comparison with the photographic photometry of Haffner and Heckmann shows the latter work to be extraordinarily good. Two color-magnitude diagrams are given, and the relation between the two color indices is discussed.
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Harold L. Johnson (1952) studied this question.