  Case's method obtains solutions to the radiative transport equation as
superpositions of elementary solutions when the specific intensity depends on
one spatial variable. In this paper, we find elementary solutions when the
specific intensity depends on three spatial variables in three-dimensional
space. By using the reference frame whose z-axis lies in the direction of the
wave vector, the angular part of each elementary solution becomes the singular
eigenfunction for the one-dimensional radiative transport equation. Thus Case's
method is generalized.
