This folder contains documents written during the period of summer to autumn of 1937. Inspired by the discovery of the new particle in cosmic rays in 1936, Yukawa worked on the theory of heavy particle (meson) that he developed in 1934 for the purpose of explaining the nuclear force. The documents show the process of summarizing his research results for the announcement of his paper, “On the Interaction of Elementary Particles II.” The folder also contains a note written by his co-researcher, Shoichi Sakata, and a postcard sent from Mitsuo Taketani.