Columbia subject headings:
Harvard subject headings:
Du Bois-Reymond, Emil, 1818-1896.
Helmholtz, Hermann von, 1821-1894.
Science--Philosophy
Scientists--Correspondence
Princeton subject headings:
Du Bois-Reymond, Emil, 1818-1896--Correspondence.
Helmholtz, Hermann von, 1821-1894--Correspondence.
Physiologists--Germany--Correspondence.
Scientists--Germany--Correspondence.