Abstract
Deals with the theory of T.E. Burlin published in Br. J. Radiol., vol.39, p.727, 1966, and in Radiation Dosimetry vol.1, 1968. It is pointed out that, in general, the average path length for radiation crossing the cavity need not equal the average path length for radiation created within the cavity. This inequality is proved for isotropically impinging radiation crossing the cavity and isotropic radiation homogeneously created within the cavity. Burlin's approach must therefore be modified to incorporate this inequality.