Although war brought a decisive halt to cosmic ray research in Europe between 1939 and 1945, in India Debendra Mohan Bose and Bibha Chowdhuri, working at the Bose Institute, Kolkata, undertook a series of high altitude mountain-top experiments using photographic emulsion to detect and analyse cosmic rays. These measurements were notable for the first ever detection of muons by the photographic method: Chowdhuri's painstaking analysis of the observed tracks’ properties, including exposed halide grain densities with range and multiple-scattering correlations, revealing the detected particles to have a mass about 200 times that of the electron - the same ‘mesotron’ (later 'mu-meson' now muon) discovered in 1936 by Anderson and Neddermeyer using a Cloud Chamber. Distance and circumstances denied Bose and Chowdhuri the relatively easy access to manufacturers of photographic plates available to Blau and later, to Heitler, Powell et al.. It meant that Bose and Chowdhuri had to use standard commercial half-tone emulsions, rather than nuclear emulsions specifically designed for particle detection, which makes even more remarkable the quality of their work.