The method is a modification of one used originally by Voigt for the identification of elliptically polarized light. The light under examination passes first through an arrangement of quartz wedges acting as a Babinet compensator, then through a rotator consisting of another pair of quartz wedges cut perpendicular to the axis, one from right-handed, the other from left-handed quartz; and finally through a large Nicol prism. The result is that the field is filled with rows of black spots in regular arrangement; and from the location of these spots with reference to the cross-hairs, as photographed, the exact character of the elliptic vibration can be readily calculated
