MR. ROBT. B. HAYWARD has written to me that some of the statements in my article, “The Axioms of Geometry,” in NATURE, March 13 (p. 453), are too sweeping, and that in particular Euclid I. 16 does not necessarily hold for the geometry of the eye-being, or, to use the more familiar language of spherical geometry, that this theorem does not hold unless the median line of the triangle on the side on which the exterior angle lies is less than a quadrant.