FOR many purposes, such as tracing networks of neural structure in the brain, a normal microscope has the serious limitation that only a thin layer of the specimen can be studied at one time, owing to the limited depth of focus of microscope objectives. When a thin section is examined, only those structures which happen to lie in the plane of the section can be observed. This plane may be confused by such objects as fibres coming away from or toward the plane of the section. A microscope which gave a large depth of field, and which presented the image as a solid in a luminous block in which the structures could be observed in depth would have important advantages over existing optical microscopes. We have designed and constructed a primitive prototype of an instrument giving such a ‘solid image’. It is possible to observe this from any position and to see the structures with appropriate parallax.