THOUGH nothing appears to have been published, it is now fairly generally known that linear particle accelerators of the standing-wave type cannot be satisfactorily stabilized with electron lenses of conventional design for energies much exceeding about 10 MeV. I have confirmed this in an investigation starting from first principles, which showed also that lenses terminated at the exit end by a thin metal foil, such as the beryllium windows proposed by L. Alvarez of Berkeley University, promise satisfactory stability up to at least 1,000 MeV. On the suggestion of Prof. M. L. Oliphant, I extended the theory also to lenses in which the metal foil is replaced by a wire mesh grid or the like. 'Grid lenses' were investigated experimentally by Knoll and Ruska in the early days of electron optics, but they have largely fallen into oblivion. It appears now that they may well be revived for the concentration of the beam in linear accelerators, as they offer advantages equal if not superior to beryllium foils.