Robert Clifford Magis, 50, an artist and illustrator for National Geographic magazine for 19 years before his retirement in June, died of leukemia Wednesday at Georgetown University Hospital.

Mr. Magis specialized in illustrations of both the ancient world and the space age. He traveled to Europe, Africa and the Middle East for his illustrations of antiquity and to space centers in this country for stories on moon landings and other space exploration. He also covered the 1964 earthquake in Alaska.

His illustrations for the magazine produced his favorite letter from a reader. His paintings, the reader said, were "too much like fine art" to be in a magazine.

Before joining the Geographic, Mr. Magis worked as an artist in Pennsylvania and for Vitro Laboratories of Silver Spring, a research firm affiliated with The Johns Hopkins University.

He also was an artist and illustrator while serving in the Air Force in Europe from 1951 to 1955.

Mr. Magis was born in Pittsburgh and graduated from the Philadelphia Museum College of Art.

Survivors include his wife, Lilibeth P. and a son, Matthew R., of the home in Alexandria; his mother, Anne S. Magis, of North Wildwood, N.J., and a brother, Erston Magis, of Alexandria.

The family suggests that expression of sympathy be in the form of contributions to the Bishop Ireton High School's art program in Alexandria.