IT is now twenty-five years since C. T. R. Wilson first succeeded in making visible and photographing the tracks of single ionizing particles by his condensation method. An exhibition has been arranged at the Science Museum, South Kensington, to illustrate the great variety of effects which have been investigated by means of Wilson chambers during the past twenty-five years. The exhibition will be open free to the public from November 19 until the end of February 1938. The centre-piece of the exhibition is Wilson's original apparatus with which the photographs published in 1912 were taken ; the apparatus has been kindly lent by the Cavendish Laboratory, Cambridge, where Wilson's pioneer work was carried out. The remainder of the exhibition consists of a collection of more than eighty photographs, which have been contributed by research workers from many countries. An introductory group of twelve photographs illustrates in as simple a way as possible some of the main properties of alpha and beta rays, X-rays and cosmic rays, for the benefit of those who are not familiar with them. The main collection of photographs is arranged in a series of groups showing typical effects produced by alpha, beta and gamma rays, X-rays, protons, deuterons, neutrons and cosmic rays, while a small group illustrates the phenomenon of induced radioactivity. The section devoted to cosmic rays includes a copy of the photograph taken by Anderson in 1932 which gave him conclusive evidence for the existence of the positive electron, while the discovery of cosmic-ray 'showers' is illustrated by copies of Skobelzyn's pioneer photographs, from which he showed that pairs and groups of 'straight' cosmic ray tracks occur more frequently than is to be expected by chance. In order to bring home to visitors to the exhibition the three-dimensional character of Wilson tracks a number of pairs of stereoscopic transparencies have been mounted for viewing in turn in a stereoscope. A small handbook (London: Science Museum, or H.M. Stationery Office, 6cL, by post Id.) has been prepared by Dr. F. A. B. Ward, an officer of the Museum, who has arranged the exhibition.