BORN, M. and P. JORDAN. Zur Quantenmechanik -- BORN, M.; W. HEISENBERG; and P. JORDAN. Zur Quantenmechanik II. Both offprints from Zeitschrift für Physik... Sonderabdruck Band 35, Heft 8/9 and Band 34, Heft 11/12. Both Berlin: Springer, 1925-26. 2 volumes, 8vo, original printed wrappers. (2)