Abstract
IF we place, on the stage of a microscope, a slide coated with a thin but opaque metal film pierced by small apertures of distinctive shapes and sizes, there is no doubt as to how its image should appear. We expect to see a dark field surrounding bright areas geometrically similar to the apertures, though on a larger scale. Suppose, however, that the object is a very thin section of some biological structure, for example, a nerve with many component fibres. The various parts of the cellular structure have little difference in their relative transmission for ordinary light, and only differ optically in their refractive indices. What should the image look like ?
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