A method for measuring local atom or ion densities, based on the scattering of resonance line radiation, is discussed for the case of an optically thin barium plasma confined by a strong magnetic field. The anisotropy of the flux of scattered resonance radiation due to anisotropic illumination and the presence of a magnetic field is examined. Barium ion density measurements on a laboratory plasma are described. It is noteworthy that this method is capable of high spatial resolution, and that light intensity ratio measurements suffice to obtain absolute ion densities. No absolute photometric measurements are required.