Abstract
Measurements of X-ray optical rotary power have been made under conditions of simple transmission in nickel sulphate hexahydrate and in quartz at a wavelength of 1·540 Å. The specific rotary power is less than 2° cm−1. The contention that silicon rotates the plane of polarization in simple transmission is refuted by a complete analysis of the result of Cohen and Kuriyama (1978) using the conventional dynamical theory of X-ray diffraction. Measurements of the ferromagnetic Faraday rotation in iron, cobalt and nickel show that the conventional Drude theory is inadequate in the short-wavelength limit.