References
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- L. A. Sayce, Phot. J., 80, 1940, p. 454.
- E. W. H. Selwyn. Phot. J., 88B, Jan. -Feb., 1948, 6. and Phot. J. 88B, May-June, 1948, p. 46.
- J. W. Coltman, J.O.S.A., 44, No. 6. 1954. p. 468.