The method worked out previously for correcting distortions which affect the apparent shape of a continuous beta‐spectrum has been further extended. A general solution of the integral equation governing the relation between the true and the observed spectra is given, and is applied to the correction of the experimental spectra from a 1‐mg/cm2source of Cu64. The kernel of the distortion integral equation is determined from a study of the internal conversion lines of I131(47 and 330 kev) using sources of various thicknesses from 0.042 to 0.840 mg/cm2. It is found that the discrepancy between the experimental Cu64negatron and positron spectra and those predicted by the Fermi (``allowed'') theory can be completely understood as a thick source effect.