Fig 12. Barrel torus showing location of a toric lens surface KLMN on torus. XX" is axis of revolution of torus. Circle AZB is equator of barrel lotus, and PZP" is a principal meridian. YY", normal to surface at Z, is axis of toric lens surface whose meridional principal curve KZM has a greater curvature than equatorial principal curve LZN. Note that YY" is perpendicular to XX" at point O, which is geometric center of barrel torus.