have not now become public property. The following are some of the works that he has consulted: "Non-Euclidean Geometry," by Henry P. Manning; "The Elements of Non-Euclidean Geometry," by L. L. Coolidge; "The Fourth Dimension Simply Explained—a collection of Popular Essays with an Introduction and Editorial Notes by the Editor," Henry P. Manning, editor (published by the Scientific American): "The Fourth Dimension," by C. H. Hinton; "The Fourth Dimension," in "Mathematical Essays and Recreations," by Hermann Schubert, translated by T. J. McCormack.]