Used with
permission of Maiken Naylor, University at Buffalo, Buffalo, NY, USA,
http://ublib.buffalo.edu/libraries/units/sel/exhibits/stamps
Hideki Yukawa's (1907-1981) 1934 theoretical calculations on the forces binding the atomic nucleus predicted the existence of new subatomic particles, now called mesons. These particles were expected to be 200 to 300 times as heavy as the electron and because of their mass act at very short intranuclear distances. Many different types of these highly unstable particles which may be charged or neutral were later observed experimentally, including resulting from cosmic rays, and their study has proved to be of fundamental importance for the understanding of the forces acting in the atomic nucleus. Yukawa won the 1949 Nobel prize in physics.