Abstract
An alkali amphibole crystal on a thin section of nepheline syenite from Poços de Caldas, Brazil is examined using an elliptically polarizing microscope equipped with a mercury lamp as a light source and a pair of rotatable quarter wave plates placed in the optical path. The nature of the light transmitted through the crystal is revealed to be an elliptically polarized light with an ellipticity of 0.23. This result is concordant with the works of T. Shoda (1957; 1958; 1961), who revealed the existence of elliptically polarized light not only in optically active crystals but also in some absorbing crystals. The difference between the nature of the elliptically polarized light resulting from absorption and that due to optical activity is discussed.