Caption: Field ion micrograph of atoms of iridium. The tiny dots are the locations of individual atoms; the ring-like patterns are facets of a single crystal of the metal. The image was made by superimposing successive micrographs, taken with different colour filters, to show small changes on the surface. The red dots are atoms which have evaporated or corroded away, while the green ones are probably atoms of gas which have been absorbed. Field-ion microscopy involves placing a tiny needle of a substance like iridium in a gas-filled chamber & passing a high voltage through it. The drifting gas ions hit the charged atoms & are repelled at right angles to form this pattern on a screen.
Credit: PROF. ERWIN MUELLER/SCIENCE PHOTO LIBRARY