An instrument for studying the surface of a solid, usually a metal, by subjecting it to field ionization. The form is identical to the field-emission microscope but the voltage is applied in the opposite direction. Low-pressure helium is allowed into the microscope and helium ions, formed at the surface of the tip, are accelerated to the fluorescent screen to form the image. Atomic vibrations, which affect the resolution, are minimized by holding the tip at liquid helium or hydrogen temperature; individual atoms of the metal can be resolved. The structure of alloys, the structure and behaviour of surfaces, and adsorption on the metal surface can be studied.