Bray, Richard Nicholas
Arnold, 1997
Buku Referensi
Singleton, W. Ralph (Willard Ralph), 1900-, editor
D Van Nostrand, 1962
Buku Teks
Blodgett, Ralp H., author
New York, N.Y. Rinehart & Co 1951,
Buku Teks
Phillips, Ralph W.
United states Government Printing Office, 1945
Buku Teks
Carson, Ralph S.
Wiley, , 1982.
Buku Teks