The appearance in the Philosophical Magazine for September of a translation of Professor F. Eisenlohr's paper in the 104th volume of Poggendorff's Annalen, induces me to offer some remarks on the subject there treated of.

Had my paper “On the Dynamical Theory of Diffraction” been accessible to M. Eisenlohr at the time when he wrote, he would have seen that I did not content myself with merely resolving the vibrations of the incident light in directions parallel and perpendicular to the diffracted ray, and neglecting the former component, as competent to produce only normal vibrations, but that I gave a rigorous dynamical solution of the problem, in which the normal vibrations, or their imaginary representatives, as well as the transversal vibrations, were fully taken into account, though the result of the investigation showed that, in case of diffraction in one and the same medium (the only case investigated), the state of polarization of the diffracted ray was independent of the normal vibrations. M. Eisenlohr's result, on the other hand, confessedly rests on the assumption that the diffracted ray may be regarded as produced by an incident ray agreeing in direction of propagation with an incident ray which would produce the diffracted ray by regular refraction, but in direction of vibration (in the immediate neighbourhood of the surface at which the diffraction takes place) with the actual incident ray.