where a and b are the lengths of the semi-axes perpendicular to the rotation axis, and n and n^{\prime} are the "numbers of principal curvature" that terminate semi-axes a and \(b\...where a and b are the lengths of the semi-axes perpendicular to the rotation axis, and n and n^{\prime} are the "numbers of principal curvature" that terminate semi-axes a and b, respectively ( n, n^{\prime}=0 for a flat surface, 1 for a cylindrical surface, or 2 for a spherical surface).