1. The mass defects of the lightest nuclei, particularly the ratio between that of the diplon and the α -particle, make it very probable that the range of the interaction force between proton and neutron is very small, of the order of about 10- 13 cm. Therefore, in all experiments in which scattering of neutrons by protons has been observed, their wave-length is larger than the range of the interaction force. In these circumstances it is well known that the scattering intensity will be independent of angle for that co-ordinate system in which the centre of gravity is at rest. For much higher energies, of course, this will no longer be true and one will expect then an anisotropy in the scattered intensity. This effect will become appreciable for energies for which the wave-length is of the same order as the range of the forces. Exact measurements of the angular distribution of scattered fast neutrons would therefore afford a direct check of the assumption of a short range and an estimate of this range. The existing experiments show an isotropic scattering within the limits of error, but are not accurate enough to allow very definite conclusions. 2. The importance of such experiments is further increased by the fact that, as was pointed out by Wick, the sign of the asymmetry in the scattering depends on whether the interaction is of the ordinary type or an exchange force as proposed by Heisenberg and Majorana, and observations of the asymmetry could therefore decide this question.