Following the discovery of mesotrons (intermediate-mass particles) in cosmic radiation in the 1930s, a group of physicists originating in Italy participated in a series of experiments designed to permit the observation of the spontaneous decay of elementary particles. The experimental results were classified as "indirect observations" of the microphysical process of decay, and the development of experimental methods was regarded as a progression toward increasing observational directness. This paper traces the activities of the cosmic-ray experimenters, viewing them as part of a stream in the international current of interest and research on the natural β radioactivity of the mesotrons. The paper pays particular attention to those aspects of experimental practice that the researchers associated with observational directness. I argue that the attribution of degrees of directness depended on the elimination of intrusive additional assumptions in the phenomenological models of the experiments. My study thus contributes to the analysis of experimental observation in microphysics.

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