Achter de horizon . . . de grote ontdekkingen, book, 1965/1973; (https://digital.library.unt.edu/ark:/67531/metadc659/m1/2/: accessed June 9, 2024), University of North Texas Libraries, UNT Digital Library, digital.library.unt.edu; .
Achter de horizon . . . de grote ontdekkingen, book, 1965/1973; (https://digital.library.unt.edu/ark:/67531/metadc659/m1/2/: accessed June 9, 2024), University of North Texas Libraries, UNT Digital Library, digital.library.unt.edu; .