1. Frehafer and Snow. Bureau of Standards Miscellaneous Publication No. 56.
2. Davis and Gibson. Bureau of Standards, Publication No. 114, Table 10.
3. Eastman Kodak Co. Book of Wratten filters, 1932.
4. The cross-cylinder tests, especially in relation to the astigmatic axis;Crisp;Amer. Jour. Ophth.,1932