There have been many attempts to improve the resolution of electron microscopes. Transmission electron microscopes are normally limited in resolution by a balance between the diffraction limit (which could be overcome by the use of a large objective aperture) and the spherical aberration of the lenses used (which could be overcome by the use of a small objective aperture). In recent years successful attempts have been made to achieve resolutions beyond this limit by holography and also by the development of aberration correctors.