"Show down, professor, I've always had trouble with math," Albert Einstein cautioned Robert A. Thornton at Princeton in the late 1940s.

Thornton, visiting professor of physical sciences at the University of the District of Columbia, fondly remembers his first meeting with the great scientist.

"I tried to impress him with my knowledge of physics and mathematics. That's when he told me to slow down.

"I thought I was going to meet a godlike presence when I walked into his house. But he quickly dispelled that. He was casually dressed and calm, very down to earth."

Now 83 and still lively of step, Thornton is editing his correspondence with the physicist for Princeton University, which is publishing the complete Einstein letters and papers.

Thornton first wrote Einstein in 1944, asking for a brief statment on the methodolgy and philosophy of science for a liberal arts program the professor was setting up in the University of Puerto Rico.

This started a correspondence that lasted several years. The two men also met seven times, by Thornton's count.

"I had first seen Einstein in 1921 when I was a student at Howard," recalls Thornton. "He lectured at the Belasco Theater and spoke in German. I didn't understand everything . . .

"Years later when I saw him, I'd always formulate a problem. I wanted to know about his notations and his mathematical concepts.

"The conversations were always about some aspect of theory. I had been calling theories true and false. He corrected that. He told me that any theory is tentative. A theory can't be wrong, he said. It's just like grammatical nonsense. The grammar itself is not wrong. The person makes it wrong."

In replying to Thornton's first letter, Einstein sent a one-paragraph, hand-written note. Many people, including professional scientists, he wrote, have seen thousands of trees but not a forest. Einstein suggested philosophical insight as the way to gain independence from prejudice.

"He always listened very intently to what I said," Thornton says. "He always seemed relaxed. He smoked a pipe a lot and sometimes his secretary would bring in coffee."

Thornton maintained contact with the scientist until 1953, two years before Einstein's death.

"I wish I had spent more time with him," the professor says, looking wistfully out the window of his office. "I could have, but I didn't. He'd always say, 'Do you have anything else to ask, professor?"

Thornton came here in January after teaching 16 years at San Francisco State and the University of San Francisco.

"I never wanted to be a great physicist," he says. "I've alway wanted to be a master teacher."

The professor first became interested in science as a youngster at the Houston Colored High School. After graduation he attended Los Angeles Polytechnic High, a school that filled a modern language and math requirement for college.

When he arrived at Howard in 1918, he was sure about science. Sometimes he got carried away with it. Like the time he was working as an on-call waiter at the Cosmos Club and struck up a conversation with David Todd, an Amherst astronomer. "He was amazed that I knew about the inclination of earth's axis," Thornton recalls with a quiet chuckle. "He asked me to sit down and we talked for 45 minutes. And he ended up suggesting that I come to Amherst to study.

"Well, I went back to the kitchen and no one said anything. But the next time I showed up for work, they told me to go home."

Thornton also cut enough of a figure in music to be given a tryout to replace Paul Robeson in "Shuffle Along," a black musical on Broadway in 1921.

He didn't make it. "I was too stilted," he says. "I wasn't flamboyant or theatrical. But I continued singing after the rejection."

Thornton picked up musical tips by hobnobbing with singers Roland Hayes, Harry T. Burleigh and Mme. Schumann - Heink. He gave recitals and sang in groups.

"I can't sing like I used to," he says with a big laugh. "But music is one of my most comforting memories."

Thornton and his wife of 54 years, Jessie, 80, live in an Upper Northwest apartment.

Jessie Thornton, a former home economics professor, stays busy making her own clothes and decorating the house. But her husband's interests have remained fixed. He gets up every day at 5 a.m. to read physics.

"I used to try to run my home by the scientific method," he says. "But my wife cut that out: She didn't want things run by a stopwatch.