THE thirty-ninth Bedson Lecture was delivered on October 25 in King's College, Newcastle-on-Tyne, by Prof. G. I. Finch, on "Electron Diffraction and Surface Structure". Prof. Finch stated that patterns are obtained on a photographic plate placed in the path of an electron stream which has been allowed to graze the surface of a solid body, and from these diffraction patterns an accurate indication of the surface structure is obtained. The diffraction patterns tend more and more towards well-defined rings with decreasing crystal size and more random orientation, until eventually an effect similar to the Debye-Scherrer pattern is obtained. Experiments with thin films of nickel deposited on a copper surface show that the nickel crystals follow, up to a certain thickness, the orientation and size of the original copper crystals. As the thickness of the nickel film increases to about 30,000 A., the crystals orient themselves in directions independent of the original copper crystals, although ordinary microscopic examination indicates a continuance of the original orientation. Electron diffraction experiments have also been used for determining the chemical composition of thin surface films, where ordinary chemical analysis has failed ; for example, the composition of the blue film on tempered steel razor-blades was successfully determined in this manner. Sir George Beilby's theory of surface liquefaction of solids during polishing has received experimental proof from electron diffraction experiments on polished surfaces, and extremely important work is being carried out in this field in connexion with the 'running-in' of machinery.