The Conformal Transformations of a Space of Four Dimensions and Their Applications to Geometrical Optics

Koenig's La Geometrie r&gUe, p. 125, and to an article by Borel on the " Transformations of Geometry in Niewenglowsky's Solid Geometry."t This is equivalent to a rotation through an angle 2IT/TC just as successive reflexions in two planes are equivalent to a rotation about their line of intersection.