Brian Andersen, age 15, of Birmingham, Ala., for his question:

HOW DOES AN ION MICROSCOPE WORK?

An ion microscope is man's most powerful magnifying instrument.

Also called a field ion microscope, it magnifies up to two million times.

Metallurgists and physicists use the ion microscope to learn how atoms of metals are arranged to form crystals. Scientists also use the powerful instrument to find out how metal crystals are affected by gases and by radiation.

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.

The needle 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. The dots show the arrangement of the metal atoms that form the tip of the needle.

An ion microscope works on the principle of electrical attraction and repulsion.

The needle and screen are enclosed in an airless glass tube that contains a small amount of helium gas. Up to 30,000 volts set up a strong electric field between the needle and the screen.

The needle has a positive charge and it attracts electrons from helium atoms that drift near the tip. When the helium atoms lose electrons, they become positively charged helium ions. Because of their positive charge, the ions are repelled from the atoms that form the tip of the positively charged needle.

The ions fly directly to the negatively charged screen from the needle's tip. Wherever they strike the screen, they cause it to glow.

As the ions stream from the needle to the screen, they fan out to cover the entire screen. In this way, they create an enlarged image of the tip surface,showing the arrangement of the atoms in the metal crystal.

Inventor of the ion microscope was Erwin W. Muller, a German physicist who moved to the United States in 1951.

Dr. Muller developed the ion microscope from the field emission microscope, which he had invented in 1936. In the field emission instrument, a high negative voltage causes electrons to form an image on the screen. But the images are too fuzzy to reveal individual atoms.

Dr. Miller took the first picture of the arrangement of atoms with his ion microscope in 1951.

The ordinary microscope goes back to ancient times when engravers used glass globes filled with water as magnifying glasses. And the ancient Romans made magnifying glasses from rock crystal.

A Dutch spectacle maker named Zacharias Janssen discovered the principle of the compound microscope in about 1590.