Abstract: An apparatus and method for determining optical properties of a substance by passing a beam of linearly polarized light through the substance. The polarization vector of the light rotates at a definite frequency but the intensity does not depend upon the orientation of the vector. The relative phase shift and/or modulation coefficient of this beam is compared with a reference beam to effect measurements of light absorption, light scattering, linear and circular birefringency, and linear and circular dichroism, all of which can be measured separately or simultaneously by a single instrument.