The human eye is a very fine OPTICAL INSTRUMENT. However, the eye cannot distinguish objects smaller than about four one-thousandths of an inch.
The power of an instrument to enlarge and form a distinct image of small details is its resolving power. The limit of resolution of an optical instrument is the smallest distance between two objects for which the instrument can form two distinct images of these objects.
The magnifying power of an optical microscope is thus limited by the fact that objects cannot be distinguished unless they are somewhat larger than the waves of light reflected from them.
In 1932 Ruska, a German, constructed an electron microscope. He allowed a beam of electrons to be reflected from an object. (Since electrons are charged they can be controlled by electric and magnetic fields.) The reflected electrons were directed through a magnetic field and then focused on a screen (as electrons are focused on a television screen to make a visible image) or on a photographic plate so that the image would be recorded.
The superiority of the electron microscope over the optical microscope depends on the fact that fast-moving electrons have a wave length a thousand times smaller than the wave length of visible light.
In most cases an electron microscope must be used with objects which are very thin. Thus, stray electrons will pass through them rather easily. Only recently has it been possible to investigate living matter with an electron microscope.
The electron microscope can be used to investigate a wide variety of materials. Many applications have been made in chemistry, biology, metallurgy and other fields. Many new structures have been discovered in insects. Details which occur in chemical changes have been seen.
