In 1937 Torbjörn Caspersson of Sweden sent him well-prepared samples of DNA from calf thymus. The fact that DNA produced a diffraction pattern showed it had a regular structure. Astbury reported that DNA's structure repeated every 2.7 nanometres and that the bases lay flat, stacked, 0.34 nanometres apart. At a symposium in 1938 at Cold Spring Harbor, Astbury pointed out that the 0.34 nanometre spacing was the same as amino acids in polypeptide chains. Actually, the spacing of the bases in the B-form of DNA is 0.332 nm.