A discussion of the diffraction and scattering of particles by a grating shows that the experiment discussed by H. Hönl and by L. Rosenfeld in 1965 and again in 1981 does not reveal any contradiction between general relativity and quantum theory. Moreover, these theories, in principle, cannot refute one another because the (weak) principle of equivalence, underlying general relativity theory, entails that gravitation does not alter the laws of microphysics.