1. For a brief review, see Ref. 3.
2. Sheppard, S. E., U.S. Patent 1,602,592 (1927).
3. James. T. H., Paper given at the 1977 Moscow Symposium in honor of K. V. Chibisov.
4. Spencer. H. E., Paper VI-1, International Congress of Photographic Science, Rochester, New York. 1978.