Deflection Focusing of Electron Microscopes

An image in an electron microscope is made to move in response to manipulation of a switch if the image is slightly out of focus. Because of the sensitivity of the eye to motion, even low contrast or dim images can thus be focused very accurately by noting lack of motion. This method is helpful in all cases, but with certain specimen types or with a biased electron gun it is essential. Construction information is given for a magnetic beam deflection unit that has performed well in regular use for over ten years. The electron optics of certain corrections are discussed briefly.