AN apparatus has been constructed which permits ultra-microscopic observation of individual condensation nuclei in an expansion chamber. The chamber may be cooled by immersion in a low-temperature bath, and the air in it supersaturated on further cooling by sudden small expansions. The object has been to study cloud formation at temperatures below 0° C., particularly in order to discover under what conditions ice crystals appear and whether they are the result of direct sublimation from vapour to ice or of the condensation and subsequent freezing of water droplets. This question has considerable meteorological importance, since the presence of ice crystals in a cloud is often supposed to be essential to the development of rain.