3 pages, 4to, written on rectos only in dark ink, slight fold separation; EINSTEIN. Typed letter signed ("A. Einstein"), [Princeton, N.J.], 23 April 1936. 1 page, 4to.; EINSTEIN. Typed letter signed ("A. Einstein") the text incorporating autograph formulas and equations, [Princeton, N.J.], 12 May 1936. 2 pages, 4to; EINSTEIN. Autograph letter signed ("A. Einstein"), Princeton, N.J., 23 November 1936. 1 page, on stationery with Einstein's deleted Berlin address; DUKAS, Helen, Einstein's secretary. Three typed letters signed to Mandl, Princeton, 1 April, 25 May & 28 November 1936. Each 1 page, 4to; Together 10 pages, 4to, all in German." /> EINSTEIN, Albert. A collection of letters to Rudi W. Mandel ON THE CONCEPT OF THE GRAVITATIONAL LENS, comprising: EINSTEIN. Autograph letter signed ("Albert Einstein") to Rudi W. Mandel, incorporating many formulas and equations, [Princeton, N.J.], 2 June 1936. <I>3 pages, 4to, written on rectos only in dark ink, slight fold separation</I>; EINSTEIN. Typed letter signed ("A. Einstein"), [Princeton, N.J.], 23 April 1936. <I>1 page, 4to.</I>; EINSTEIN. Typed letter signed ("A. Einstein") the text incorporating autograph formulas and equations, [Princeton, N.J.], 12 May 1936. <I>2 pages, 4to</I>; EINSTEIN. Autograph letter signed ("A. Einstein"), Princeton, N.J., 23 November 1936. <I>1 page, on stationery with Einstein's deleted Berlin address</I>; DUKAS, Helen, <I>Einstein's secretary</I>. Three typed letters signed to Mandl, Princeton, 1 April, 25 May & 28 November 1936. <I>Each 1 page, 4to</I>; <I>Together 10 pages, 4to</I>, all in German. | Christie's