1. Angenieux, P., 1954, French Pat. 1, 112, 979.
2. Back, F. G., 1948, U.S. Pat. 2, 454, 686.
3. General Theory of Optically Compensated Varifocal Systems*
4. Third-Order Aberration Theory for Varifocal Systems
5. Busch, E., 1934, Germ. Pat. 597, 354.