1. Airy, Sir George, Trans. Camb. Phil. Soc., 5, 283 (1834).
2. Martin, L. C., “Technical Optics”, 1, 125 (Pitman, London, 1948).
3. Hopkins, H. H., Proc. Roy. Soc., A, 217, 408 (1953).
4. Bracey, R. J. (publication confidential to B.S.I.R.A.). Naish, J. M., and Mathieson, B. M., R.A.E. Technical Notes Ph. 482 and 484, Fig. 11 (1953).
5. Ingelstam, E., Proc. Symp. on Image Evaluation, Nat. Bur. Stands. (1951).