The tunneling-current density is computed in the vacuum region between two planar metal electrodes, on each of which is an adsorbed atom. Scanning of one atom (taken as the tip) past the other (the sample) permits plotting of tip displacement versus lateral separation for constant current. The calculation shows the extent to which scanning-tunneling-microscope images of an individual atom are visible. It confirms that for low bias, the microscope images the Fermi-level local density of states of the sample at the position of the tip. © 1986 The American Physical Society.