Gheorghe Titeica and affine differential geometry

In 1908 Titeica showed that for a surface in Euclidean 3-space the ratio of the Gaussian curvature to the fourth power of the distance from a fixed point to the tangent plane is invariant under an affine transformation fixing O. He defined an S-surface to be any surface for which this ratio is constant. These S-surfaces turn out to be what are now called proper affine spheres with center at O.