By 1933 Anderson had looked at 1300 photographs of cosmic ray tracks of which 15 were of positive tracks that could not have a mass as great as that of a proton. He found that they occur in groups of other tracks so were secondary particles and appeared to have the same charge as a proton and a mass less than 20 times the mass of an electron. Dirac's theory of the electron played little part in the initial interpretation of the discovery of what Anderson called positrons. This was a serendipitous discovery not led by theory.