Schlieren optical imaging of weak objects

Abstract

It is shown that with the aid of a schlieren optical mode described in this paper in real electron microscopes, the amplitude contrast of weak objects can be enhanced up to two-fold in comparison with the bright-field mode, if a part of the unscattered beam strikes the objective aperture. This remains true also in the case of thicker objects imaged (for example, unstained sections of biological matter). Futhermore, evidence is given for the existence of an optimum schlieren image in the transition range between strict schlieren optics and conventional bright-field microscopy. Such images are characterized by high amplitude contrast without significant distortion. Theory and experiment are shown to be in good agreement. As an application, unstained nucleosomes have been investigated schlieren-optically. Model calculations show that the ‘hole’ in their centre, which has been demonstrated from dark field images, is probably not due to a structural void (cavity), but is a consequence of the lower density of the protein core as compared with DNA.