FOR some years it has been possible to focus beams of electrons into fine probes with diameters in the range 10 µm–100 A. These have been used for electron beam machining, electron probe analysis and scanning electron microscopy. Progress in focusing ion beams has been more limited. Notably, Gabbay et al.1 have used a beam of lithium ions from a coated tungsten hair-pin filament and have reported a resolution of the order of 3 µm in a scanning ion microscope. The only work in which very fine beams of rare gas ions have been used appears to be that of Saparin et al.2, who have reported a final beam diameter of 10–15 µm at 30 kV. von Ardenne3 has described two systems, one using a canal-ray source and the other a capillary source in which ions were focused to about 5 µm, but in this case the final lens used was of very short focal length and the focus was not readily accessible.