A new and general method has been developed by which the optical activity of crystals can be measured along different directions from the optic axes. The principle of the method is to measure accurately the intensity of emergent light from parallel and crossed polarizers between which the specimen is properly oriented. The experimental apparatus is relatively simple and needs no elaborate machinery. By this method, gyration and birefringence can be obtained simultaneously as a function of wavelength. As a test of the applicability of the method, a component of the gyration tensorg11and birefringencene−n0of quartz have been observed as a function of wavelength, e.g.,g11=5.69×10−5, andne−n0=9.15×10−3at a wavelength of 550 nm. This method was also applied to the study of the optical activity of KH2PO4. It was found that the nonenantiomorphousD2dphase (paraelectric) of KH2PO4is optically active,g11=2.28×10−4at a wavelength of 506 nm.