Ted Stewart, age 12, of Bennington, Vt., for his question:

HOW DOES A FLUOROSCOPE WORK?

A fluoroscope is an instrument that permits a doctor to see a silhouette of the bone structure and internal organs of a living body. The usual type of fluoroscope is made up of a large X ray machine and a fluorescent screen.

The patient stands in front of the X ray machine and the doctor places the fluorescent screen against the part of the body that he wishes to see. X rays are sent through the patient's body and strike the screen. The bones and thick organs partially block the invisible rays so that partial shadows of X rays are cast upon the screen.

The fluorescent material radiates light when it receives the invisible X rays. It reproduces the pattern of shadows and light, and the doctor can see the bones and organs.

Often a closed circuit television camera picks up an image from the sceen and the doctor studies the fluoroscope picture on a small television set.

Thomas A. Edison invented the fluoroscope in 1896.