Lattice fringes from amorphous ge: fact or artefact?

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Abstract
For electron micrographs taken with the offset bright-field configuration (interference microscopy), a transfer function is derived which includes the effects of angular and energy spread in the illumination on spherical aberration, defocussing and chromatic aberration of the objective lens. It is shown that this transfer function varies markedly across the plane of the objective aperture, acting like a 'fourier filter' which suppresses certain periodicities while leaving others relatively unaffected. The results support the view that 'lattice fringes' which have been observed in micrographs of amorphous ge may in certain circumstances be artefacts of the imaging process.