Não informado. (1960). Educators Guide to Free Science Materials. Educators Progress Service.
Chicago-referens (17:e uppl.)Não informado. Educators Guide to Free Science Materials. Randolph, Wis: Educators Progress Service, 1960.
MLA-referens (8:e uppl.)Não informado. Educators Guide to Free Science Materials. Educators Progress Service, 1960.
Varning: dessa hänvisningar är inte alltid fullständigt riktiga.