Some recent contributions from the electron microscope laboratories.

In the past few years many interesting developments have come from studies using the electron microscope. Some of these new methods and discoveries already have a clinical application, others offer a hope of clinical applications and developments in the future. This account will be restricted to work published since 1965. Capabilities and Limitations Though its appearance is impressive and its cost considerable , an electron microscope is basically simple: like an optical microscope tumed upside-down, using electrons instead of light. The source of electrons is a V-shaped tungsten filament, which is heated to incandescence. Any incandescent filament gives off both light and electrons. In the conventional microscope lamp the light is used and the electrons just sit in a close cloud around the filament; in the electron microscope the light goes to waste and the electrons are propelled downwards by the application of a high voltage, usually between 25 and 100 kV. Electrons are soon stopped by air, so the microscope has to be evacuated to a vacuum of 10-torr. Instead of glass lenses, the electron beam is focused and the image formed by strong magnetic fields. Most electron microscopes have five or six lenses: one corresponding to the lamp lens, in the optical microscope, and one to the substage condenser; an objective, and a projector (corresponding to the eye-piece in the optical microscope); and one or two others, between the objective and projector, are used to vary the magnification-between about 500 and 200,000 times-of the final image on the fluorescent screen (Figs. 1 and 2). The high voltage and lens currents have to be very closely stabilized, to better than one part in 100,000: and it is largely this requirement for very high electronic stability, together with the requirement for-the high vacuum, which make the instrument so expensive. Nevertheless, despite their cost, electron microscopes are now commonplace and no longer the status symbol they once were. A limitation, overcome by the development about twenty years ago of techniques for cutting ultra-thin sections, is the poor penetrating power of electrons; to show fine detail the object must be less than 0-1 ,um thick. Recently, megavolt electron microscopes of vast cost, with a beam capable of penetrating a 1 ,um section, have become available, but, though useful to metallurgists, their contribution to biological knowledge has so far been small. RESOLVING POWER The resolving power of a microscope is limited to about …