Columbia subject headings:
Harvard subject headings:
Humboldt, Alexander von, 1769-1859--Correspondence.
Ritter, Carl, 1779-1859--Correspondence.
Princeton subject headings:
Explorers--Germany--Correspondence.
Geographers--Germany--Correspondence.
Humboldt, Alexander von, 1769-1859--Correspondence.
Personal correspondence.
Ritter, Carl, 1779-1859--Correspondence.