Shadow casting; a technique to show surface texture in microscopical material.

Objects seen through the microscope by transmitted light are translucent in differing degrees. Staining gives a measure of differential opacity and contrast to biological material but, even here, the appearance, like that of an x-ray picture, is a record of relative opacities. Varying the focal plane, of course, allows a range of optical sections, and from this the surface appearance can be inferred to some extent. Surfaces are actually seen with the microscope when opaque objects are viewed by oblique illumination, but this method is illadapted for the higher powers of microscopical work and is little used in standard microscopy save as an occasional adjunct or for special purposes where low powers suffice. The method presented here allows surface texture to be detected over the whole range of magnification of the microscope. The technique was originally developed for work with the electron microscope (Williams and Wyckoff, ’44) as a. means of measuring thickness of objects and of increasing image contrast. Williams and Wyckoff also applied it in revealing the surface detail of bactcria ( ’45a), viruses ( ’45b, c, d) and minute chemical aggregations ( ’45e). A special “replica” method using colloidin films was developed for use on opaque material so that metal and mineral surfaces could be studied ( ’46). Through use of the technique in electron microscopy, features fa r beyond the range of resolution of the light microscope have been revealed.