  In two papers titled "On the so-called non-Euclidean geometry", I and II,
Felix Klein proposed a construction of the spaces of constant curvature -1, 0
and and 1 (that is, hyperbolic, Euclidean and spherical geometry) within the
realm of projective geometry. Klein's work was inspired by ideas of Cayley who
derived the distance between two points and the angle between two planes in
terms of an arbitrary fixed conic in projective space. We comment on these two
papers of Klein and we make relations with other works.
