Abstract: The electron microscope has an evacuated housing 9 comprising a portion 14 containing an electron gun 10 from which a beam 11 of electrons is accelerated by accelerator 12 and directed into a microscopic column 16 forming part of housing 9. An ion-stopping member 33 is located in the accelerator 12 on the axis of column 16, to intercept ions which could otherwise pass to the specimen S. In order to avoid interception of the electron beam, the beam is directed along a path which avoids the member 33, either by a displacement or inclination of the gun or by electron beam deflection means (e.g. coils 35c, 36c), and the beam is deflected back on to the axis of the column 16 by electron beam deflection means (e.g. coils 33c, 34c).