Abstract: The invention relates to a raster electron microscope having a specimen chamber and a detector for electrons mounted in the specimen chamber. The raster electron microscope also includes a specimen table having a specimen holder and the specimen table is mounted in the specimen chamber. A diaphragm system is provided on the specimen table and has a diaphragm between the specimen holder and the detector. The diaphragm system is adjustable relative to the specimen holder. The invention can be configured especially as an ancillary module which is accommodated on the specimen table of a conventional raster electron microscope. The system of raster electron microscope and ancillary unit serves for generating special contrastings, especially a dark-field contrast in transmission.