A scanning electron microscope comprising a chamber, a specimen holder located within the chamber for holding a specimen thereon at high voltage to produce a retardation field thereat, an electron detector disposed adjacent to specimen holder, and a microtome, the microtome including a knife arranged to be carried into engagement with the specimen on the specimen holder to slice a portion of the specimen away to expose a new face of the specimen, the knife being mounted so that after having engaged the specimen to expose a new face of the specimen it is moved to a new position, whereupon it does not interfere with the retardation field.