Besides the problems considered here, one can find comparatively easily asymptotic solutions in a problem with a spherically symmetric exponential distribution of the primary sources, and also in the case when the primary sources are distributed uniformly within a sphere of finite radius. However, we do not give these solutions because they are cumbersome. Note that some of these problems may have direct astrophysical applications. For example, the problem of a point source can arise in the determination of the source function in the problem of the emission of a planetary nebula. The results of the present work, and also of the majority of the other investigations have been obtained under the assumption of complete frequency redistribution for scattering in the comoving coordinate system. In reality, this assumption is not strictly satisfied. However, calculations made recently for moving media [13, 14] have shown that the results found for true and complete redistribution differ little from one another, these differences being less than in the case of a medium which is not in motion.