In 1939, when the electron optics laboratory of Siemens & Halske Inc. began to manufacture the first electron microscopes, the biological and medical profes sions had an unexpected instrument at their disposal which exceeded the reso lution of the light microscope by more than a hundredfold. The immediate and broad application of this new tool was complicated by the overwhelming prob lems inherent in specimen preparation for the investigation of cellular struc tures. The microtechniques applied in light microscopy were no longer appli cable, since even the thinnest paraffin layers could not...