A device is provided for adjusting electron-optical apparatus. The specimen to be scanned is equipped with a marker emitting back-scattered electrons. A transducer, in response to these electrons, emits a signal which unblocks the modulating electrode of a cathode ray tube used for the display purpose. The tube screen has long persistance characteristics; the beam and the object holder being displaced in four mutually perpendicular directions by theoretically corresponding quantities, the adjusting errors are visible immediately.