CECILY CANNAN SELBY, Ph.D.
Division of Experimental Pathology, The Sloan-Kettering
Institute for Cancer Research, New York, N. Y.
ACKNOWLEDGMENT The author's work reported in this manuscript was conducted during the tenure of a postdoctoral research fellowship from the National Institutes of Health, Public Health Service. Additional support was obtiained from the Lillia Babbitt Hyde Foundation and the American Cancer Society.
Electron microscopy occupies a unique position among the various microscopic methods discussed in this volume as the only one which permits direct visualization of structure in the 1 to 100 mμ size range. Since dimensions larger than 100 mμ may also be observed, electron microscopy is able to contribute to analytical cytology in at least two directions: microscopic structures may be better understood by observation at higher resolving power, and hitherto invisible submicroscopic structures may be revealed. The uniquely high resolving power of a microscope using electrons as the illuminating beam is explained quite simply (recalling Abbe's rule) from the fact that electrons have an associated wavelength smaller than that of any other radiation practicable for use in a microscope system.
The periodic behavior of electrons has a wavelength given by the law:
where λ = the wavelength angstrom units (10-8 cm)
V = the potential in volts used to accelerate the electrons For instance, an electron moving down a potential gradient of 60 kv will have an associated wavelength of 0.0488 A, or about 100,000 × shorter than the wavelength of visible radiation. When it was shown