Mixture of outgoing and incoming electromagnetic radiation. Change of mass of the source of radiation

In a previous paper the author, using a method of successive approximations, verified by means of the Einstein-Maxwell equations of general relativity the well-known result that outgoing electromagnetic radiation from a source conveys energy, so that the source loses gravitational mass corresponding to this energy. The purpose of this work is to show a similar result for the general case of any mixture of outgoing and incoming radiation.This work is included in a thesis submitted by the author (1964) to the University of London for the degree of Ph.D.