The technique of "shadowing" by metal evaporation in a vacuum has been adapted to the production of plane "negative" images of dust deposits on microscope slides at natural size by vacuum evaporation of metal at right angles to the slide, and subsequent removal of the dust. The slide receives a precise impression of the geometric projections of the particles. The correspondence of image and object is exact. The thickness of the deposited film can be suited to the contrast requirements of an automatic particle counter; further, the film lies in one plane and is only a few hundred ångström units thick. Many of the optical problems arising from the association of automatic counting techniques with high power light microscopy, namely, depth of focus and variable contrast between particles and background and between different parts of single particles, are avoided by substituting the shadow for the original specimen, as suggested by Whitaker.
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Hamilton et al. (1956) studied this question.