IT'S EMBARRASSING, you understand, to begin your career in opera just when all the other guys your age are ending theirs. Dick Gill is brave, and perhaps it helped that he didn't know exactly what he was doing.

At 50, Gill makes his Washington debut tonight at the Kennedy Center in Mozart's "The Magic Flute," but I am not here to praise him nor bury at the debut of another 50-year-old (the painter Titian, if memory serves, began to paint at the age of 97, and I have heard it said George Blanda may go into tennis) as to inquire what prompted the switch to opera.

It is true, as one had assumed, that Gill had not just sat meditating all those years but had been gainfully employed. He was an economist, and wrote books and was assistant dean of Harvard College and master of Leverett House (which has 500 students and enormous chandeliers in its commons and early on had stimulating figures like Malcolm X for dinner).

His life was full. A wife and three sons. No complaints. More than that, he had had some success as a writer. He won the Atlantic's prize for best short story and sold several stories to the New Yorker.

Some of my best friends are economists. I think, since one always has a few friends that one is not sure exactly what they do, if anything. Still, a life dedicated to economics is bound to leave part of the soul unflowered.

Gill knew that. He is loyal to economics. Once you invest a few decades in a trade, you aren't likely to say it's worthless. Sometimes a person simply outgrows a subject. (Many who once regarded "Peter Rabbit" as the ultimate novel can now take it or leave it alone.)

Darwin got to the point that he no longer liked to read Shakespeare. Shakespeare was as magnificient as ever, but somehow the excitement was all gone.

"I have read that, about Darwin," said Gill. "But it was not like that with me and economics. I still like it, still like to write about it.

"Don't you think a lot more people could have second careers, if they wished? I've always had a lot of interests, and I don't think our society needs the intense specialization in one subject that people seem to think. Maybe when the continent was being developed we needed that intense concentration on one subject, but I'm not sure that's what we need now."

When he was a boy (in New Jersey) his mother was a choir director. (She was a member of the Dutch Reformed Church, but made do with the Presbyterians, who are more common up there.) Gill sang in the choir. His father, not at all musical himself, loved and encouraged music in others, but it never occurred to young Gill to make a living with music. Economics is respectable, at least among economists.

So there he was at Harvard.

"In my world then, the writing and the singing got lost."

Sometimes the overwhelming conversion occures in a burst of flame along the Damascus Road.

More often, it springs from a sense of play, just trying something out for laughs, as a pup may bounce around with a toad (once only, as a rule) or bit his own tail to see what happens.

"At age 38 I thought it would be good to learn how to sing a song," he said, "so I took voice lessons. It was extremely difficult to work them in, with the schedule I was on, and sometimes I would be up at dawn parctically racing to Dedham or somewhere, and back to teach the same morning." His audition song, "Drink to Me Only With Thine Eyes," was too much, and his voice gave out halfway through. But he went on with lessons.

His wife, Betty, could always get a job as principal of a school, and she had attended the New England Conservatory (piano) and thus loved music as well as education. The day came - "I don't know if Betty or I said it, but one of us said, 'Why not?' and that was when I became an opera singer. It was rash, since I had no contract, no job."

First there was a year in England, on a leave granted by Harvard to work on economics. On the side, Gill sand:

"The English are pretty tacky about work permits," he said, "but I got to sing a little. Something inspired me to write the New York City Opera asking for an audition.

"So I went to audition (in 1971) and Julius Rudel said 'Bravo,' which it turns out he does not say very often, and they offered me a contract.

"But I said, 'Oh, no. I couldn't possibly work anything like that into my schedule at Harvard."

"Now I see how absured it was, but they had to tell me, look, if I didn't want a job singing, then why had I requested an audition in the first place?"

But he turned it donw. It began to occur to Gill, with more conviction, that he did want to sing, and that that would make differences in his life.

Somehow (and this is common amongst us all) it had never quite got through to him that a career in grand opera could not really be fitted into a deanship in Cambridge.

"Nothing except may be pro football could have been a more extraordinary switch from what I was doing," he said, and yet it was only after he resigned from Harvard that it really sank in. (He took a leave of absence and finally severed his connection with the school in the summer of 1973).

"They said I could not possibly mean to resign, and a man phoned Betty and said, 'Now your are not to worry about this singing thing. Dick will get over it."

But he didn't, or hasn't. He started his voice lessons about 12 years ago but only got his City Opera contract late in 1971, and after that sang for the Met, and now is a sort of freelance bass, singing in the opera here and in five other cities, and planning to sing with the New York Philharmonic under Bernstein next month.

Startling changes of direction are easy enough, no doubt, for the few who see the light in one blinding glory and who can thereafter date all life from the blazing trauma.

It is different for others, who have no such fiery crisis, but start modestly enough "to learn to sing a song" and only much later can look back and see what that entrained. Now he sings the priest in Mozart, and eats God knows what and stays in hotels and thinks about plane schedules.

He stays busy now and makes a perfectly good living. He missed some of the trauma of finding this new life because his wife and sons supported him enthusiastically in his decision to sing.

"Daddy, you have one of the finest voices I ever heard," said a son at the time. "I have not heard many voices," he added.

Gill once had rather a contempt for singers (as his wife said, because wives love to Taunt the Beast at vulnerable moments) and, he thought, the world of intellect and order.

He knew in October what he'd be doing in May, in his former life. Now he is not quite sure when he is due in California or if he has a hotel reservation yet.

Sometimes when he sings he is overcome with anxiety and thinks he was very bad (he is getting over that sensation now) and cannot believe his ears when someone says he was great, just great.

People used to say to him he would be perfect in such-and-such an opera, and that was embarrassing since he never heard of it, and was middleaged before he discovered Bellini, Donizetti, Verdi and Wagner even.

"How many roles is it you have now learned?" asks his wife (sorry she had taunted him about his earlier scorn of singers) and thus allowing him to say: "Sixty-Six."

His role is crucial in "The Magic Flute." The hero of the piece is young, gallant and not stupid, but nearly, and sets off to risk his life for the wrong thing, having confused evil with good.

It is Sarastro, the dark bass, who sets him straight.

In the opera, there are people who just want to get through their day and are happy.

The hero is obliged to see he was dead wrong about the most important things. He has to walk through fire, not knowing how. They give him a magic talisman to carry, and he has to take it on faith that he will come out alive. Knowing that the last time he took something on faith he was dead wrong.

Sarastro turns to the audience and sings his hymn to Isis - the goddess who gathered up every lost fragment - and sings of the peace inside his temple.The hero listens and learns.

All this opera, before Sarastro, is a kaleidoscope of mixed-up nonsense. It is life, of course. We walked it out, we drink it out, we sleep it out (as a great English wit once said) to avoid the change of that when Sarastro sings.

To sing such a role, even to touch it at its outer possibilities, is the sort of experience a singer might well say good bye to Harvard for.

"We should have started earlier in the day," he said. We had talked for six hours. The day is not long enough.