AN OPTICAL microscope is unable to form a clear image of any object smaller than about 0.2 μ (a μ is 0.001 mm.). The electron microscope,1 utilizing a beam of electrons with much shorter wave length than visible light, is able to resolve particles less than 20 A. apart (an A. is 0.0001 μ). A beam of electrons differs from visible light in the extreme readiness with which it is stopped by all forms of matter, including air. As a result, electron microscopy must be carried out in a vacuum with desiccated and necessarily nonliving specimens. Because specimens must be sufficiently thin for beam penetration, effective histological work was delayed until practical techniques of preparing ultrathin tissue sections were developed.* The origin of contrast in the electron microscope is entirely different from that in the light microscope. For this reason, together with the increased order of