1.A field ion microscope is used to image the surface of a conductive hemispherical tip as shown schematically in Figure 1. The screen is a phosphor maintained at a positive potential, if the resulting electric field at the tip exceeds the ionisation field strength (10⁸ Vm^-1) then electrons can overcome the work function and are accelerated to the screen following the green ray paths. The electrons striking the screen create a light pulse that can be detected.