Page "Field ion microscope" Paragraph 1

It was the first technique by which individual atoms could be spatially resolved.

On October 11, 1955, Muller & Bahadur ( Pennsylvania State University ) observed individual tungsten ( W ) atoms on the surface of a sharply pointed W tip by cooling it to 78 K and employing helium as the imaging gas.

Muller & Bahadur were the first persons to observe individual atoms directly ; to do so, they used an FIM, which Muller had invented in 1951.