A study of birefringence in the interstellar medium in the direction of the Crab Nebula

The interstellar medium may be regarded as a weak wave plate, the linear birefringence arising from the aligned grains which produce interstellar linear polarization. Using the Crab Nebula as a background source of linearly polarized light we have investigated this birefringence by measurements of circular polarization. The circular component is found to vary with the intrinsic linear polarization in a sinusoidal fashion characteristic of a wave plate with the orientation expected from independent measurements of the interstellar linear polarization in the same direction. Measurements of the wavelength dependence, together with the sense and magnitude of the circular polarization are interpreted as evidence for the dielectric nature of the interstellar grain materials. These observations provide a firm basis for a similar interpretation of the circular polarization of reddened stars. The observations of the stars can then be used to study the grain composition and the structure of the magnetic field in many directions in the Galaxy.