Peter Chase, age 15, of Columbus, Ohio, for his question:

WHEN WAS THE ION MICROSCOPE INVENTED?

The ion microscope is man's most powerful magnifying instrument. It magnifies up to two million times. Scientists use it to learn how atoms of metal are arranged.

A German physicist named Erwin Muller invented the ion microscope in 1951 while living in the United States. He developed the ion microscope from the field emission microscope which he had invented in 1936. In this instrument, a high negative voltage causes electrons to form an image on the screen.

The main part of an ion microscope is a thin needle made of the metal to be examined. This needle is 1,000 times sharper than the tip of an ordinary pin. It points toward a fluorescent screen positioned nearby.

When the microscope is in operation, the screen shows an enlarged image of the tip of the needle. This image is a pattern of bright dots which show the arrangement of the metal atoms that form the needle's tip.