Abstract
It is shown that the beam-tilt device of the Philips EM300 electron microscope may be used as a diffraction standard to superimpose an accurately known pattern of spots on the photograph of a diffraction pattern. A general method is described by means of which elliptical distortion of the diffraction pattern due to astigmatism, and the distortion arising from the finite radius of the Ewald sphere, may be treated. The determination of interplanar spacings to about +or-0.1% is demonstrated. The techniques described should be applicable to other microscopes.