It is shown that multiple scattering by the atoms of the gas in a cloud chamber can cause large apparent curvature of tracks. This effect makes the measurement of curvature in a magnetic field meaningless if the energy of the particle is small and the magnetic field low. An analysis shows that all published meson tracks are compatible with a unique mass of about 200 electron masses.

• Received 20 September 1946

DOI:https://doi.org/10.1103/PhysRev.70.821