Interstellar polarization from a medium with changing grain alignment.

If the direction of grain alignment in the interstellar medium changes along the line of sight to a star, circular polarization can be produced. Models are developed for the polarization from such a medium, as the basis for interpreting existing observations. Emphasis is placed on the numerous relationships between circular polarization, linear polarization, and extinction from which information about the interstellar grains can be derived. Detailed applications are made to both stars and clusters of stars. Some new measurements of circular polarization are reported and observational prospects for the future are evaluated. Subject headings: interstellar matter - interstellar reddening - open clusters - polarization