Abstract
The polarization properties responsible for the classical "green birefringence" of the amyloid-Congo red complex have been determined by a new optical method, phase modulation microspectrophotometry. This method now makes possible the measurement of one optical property at a time (birefringence, optical rotation, linear dichroism and circular dichroism throughout the visible spectrum) in complex specimens in which visible contrast in polarized light is the result of a mixture of polarization effects. The green birefringence is explained by a combination of optical effects, the strongest of which are dispersion of birefringence and linear dichroism superimposed on the smaller effects of circular dichroism and optical rotatory dispersion. The interaction of the planar dye molecules with the amyloid protein induces an extrinsic Cotton effect.