I have to thank the board of direction of the Bache fund of the National Academy of Sciences for their aid, and also the trustees of the Western Uni versity of Pennsylvania for their permission to use the means of the observatory under their charge in contributing to the same end, and I desire to acknowledge especially the constant and valued help of Mr. Frank W. Very, who has assisted me in all these experiments, and my further obligation to Mr. George E. Curtis.

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