For nearly half a century, the cloud chamber invented by C. T. R. Wilson in 1911 was about the only method for observing the tracks of very small particles such as alpha and beta particles, protons and deuterons. The cloud chamber operates on the principle that a particle passing through a supersaturated vapor will cause condensation on ions formed along its path. The condensed liquid can then be observed directly or photographed for further study. For all its power as a detection device, however, the cloud chamber has some serious limitations. Most important is the fact that the gaseous atoms and molecules within the chamber are relatively far apart. If a nuclear event occurs in which a particle travels only a very short distance--less than the distance between two atoms or molecules--that event will not be recorded by the cloud chamber. It was this problem to which...