Chuck Barnhardt, age 14, of Nashville, Tenn., for his question:

HOW DOES AN ULTRAMICROSCOPE WORK?

An ultramicroscope is an instrument that allows a person to see objects that are much smaller than those he can see under any ordinary microscope. The secret in the equipment is that it is a compound microscope with several lenses.

It differs from other compound microscopes in that 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 instrument.

The particles shine out as bright "stars" against a dark background. Like the stars in the sky, these particles appear as points of light without structural detail.

Objects as small as six one millionth of a millimeter can be seen with an ultramicroscope.