Christy Perkins, age 14, of Nashville, Tenn., for her question.

HOW SMALL CAN AN OBJECT BE AND STILL BE SEEN BY AN ULTRAMICROSCOPE?~

Ultramicroscope is an instrument that allows a person to see objects that are much smaller than those he can see under an ordinary microscope. With an ultramicroscope, objects as small as six one millionths of a millimeter can be seen.

An ultramicroscope is a compound microscope with several lenses. But it differs from other compound microscopes because it uses a strong horizontal beam of light to illuminate the particles to be seen.

This light beam can be brought into intense focus. Usually a powerful arc lamp supplies the light. The rays of light are focused by means of a system of condensing lenses. The last lens brings the rays together into very small, intensely brilliant focus.

Scientists use this microscope to study colloidal particles or bacteria floating in liquid or in the air. They set the microscope so that it receives only the light scattered by the particles themselves.

No part of the direct light that illuminates the objects can enter the instruments. Therefore, the particles shine out as bright "stars" against a dark background.