American Light: The Luminist Movement, 1850-1875," which opened here last weekend at the National Gallery of Art, is a fortunate exhibit: fortunate in timing, and in its reception, and fortunate most of all in its curator, John Wilmerding.

Had he not been a sailor, and had be not been blessed with exactly the right pedigree and exactly the right colleagues, Wilmerding might never have organized the show.

"American Light" is a triumph for the luminists. It is a triumph too, for that small band of art scholars, all rebels of a sort, known as the Americanists. Wilmerding is one.

When he first proposed his show in 1977, the omens were not favorable. Wilmerding, now 41, was after all, a relative beginner, a Dartmouth art professor new to the museum. Never had he organized a large museum exhibition. And the quiet, glowing paintings that he propsed to show seemed peculiar choices for a Gallery exhibit. Most of them were pictures the public hardly knew.

The single famous painting in the luminist exhibit - the headline grabbing "Icebergs" of Frederic Edwin Church -- had not yet fetched a recrod price, had not yet been discovered, when the show was planned. The Gallery was doubtful. Insted of picking pictures sure to attract crowds, Wilmerding was borrowing Fitz Hugh Lanes and Kensetts, Heades and Sandord Giffords. wThe exhibit would be mounted in the Gallery's old building, in a space not used for years. His show might well have stumbled. Instead, it is a hit.

On the Sunday that it opened, 21,000 visitors viewed the exhibition. Never in its long run did the Tut show draw as well.

The credit for the exhibition is not his alone. Wilmerding has shared the luminist exhibit with the small group of Americanists who helped him write the catalogue, who ,teaching one another, helped to resurrect the gifted, long-forgotten landscape painters in the show.

Most of the Americanists are young; many went to Harvard. Together they have altered the fashions of art scholarship and the way we see and value America's old art.

A number of the best of them - William E. Gerdts, Joshua C. Taylor and Theodore E. Stebbins Jr. -- are well known in this city. Gredts, who taught for many years at the University of Maryland, has a mind like an encyclopedia; he does not forget a document, a picture or a date. Taylor, the director of the National Collection of Fine Arts, the senior rebel of the group, writes brilliantly. Ted Stebbins is Wilmerding's counterpart at the Boston Museum of Fine Arts as well as the discoverer of Martin Johnson Heade. All three of them, like Wilmerding are generous instructors and scholars of distinction.

But Wilmerding is blessed with one additional credential -- one given special weight at that most exclusive of museums, the National Gallery of Art. The fact that he's the great grandson of Louisine Havemeyer helped him get his job.

In 1907, to her family's dismay, she traveled to this city with a band of suffragettes to burn the president in effigy. She spent the night in jail. oBut it was not her politics that made her reputation. She was one of America's great collectors of old and modern art.

In the 1890's, at the instigation of her friend Mary Cassatt, she began to buy the pictures of Courbet, Manet and Degas. She brought Old Masters too -- Goyas and El Grecos, six Rembrants, five Cezannes. She cared about museums. She, her son and her daughters gave 1,972 works of art to the Metropolitan Museum of Art.

"You might suppose that I grew up loving works of art -- but you'd be wrong," says Wilmerding, "I was rather taught to hate them."

It is clear he loves them now. Looking at a picture, Wilmerding speaks quickly as if trying to provide you with everything he knows.

"I remember the Corots in my grandmother's apartment. What I remember even more is her saying how she hated all her mother's French impressionists. Instead she formed a wonderful Americal collection. My mother was defiant, too. She had a hatred for high culture. I sailed as a child -- I used to race with Carter Brown -- for sailing was acceptable. But I knew nothing about painting. Wen I arrived at Harvard, I had never even been inside the Methopolitan Museum of Art.

"My great love then was literature. I was sure I'd be a writer, maybe a professor. I was dating a girl from Gloucester. It was there I came across the marine paintings of Fitz Hugh Lane. And my whole life changed." aTed Stebbins was in Cambridge, too, at the Harvard Law Shool, when he discovered art. "The art expert, in those days, seemed to be to be a most exotic creature. How could he look at a picture and know who did it? I didn't trust him. I began by interviewing Jackob Rosenberg, the great connoisseur. He taught this famous course there -- he called it "Connoisseurship.' He'd show pairs of slides, one was always wrong. I was amazed to find that I could pick the right one. Wilmerding encouraged me. I turned from law to art."

Bill Gerdts, too, was trained -- if training is the word -- at the Fogg at Harvard. He was one of the first students there, or in America, to do a doctoral dissertation in American art history. The year was 1952, and though living New York painters already had made their city the center of the international art world, old American pictures were still being dismissed as hopelessly provincial.

"There was a prejudice against them," says Wilmerding. "Most scholars with Old Master training refused to acknowlege how beautiful they are. That is one reason I decided on this show. If you are going to define American art, you have to hold it up against the standards of everybody else's art."

While Gerdts, Wilmerding and Stebbins As well as Barbara Novak and Earl A. Powell, both of whom have written for the luminist exhibit) were studying at the Fogg, Harvard did not have even one full-time professor in American art history. "In my nine years at Harvard," Wilmerding remembers, "they offered just two courses in my field. When Bill Gredts began studying American 19th-Century still life and neoclassical sculpture, he did so on his own. When a group of Harvard students, Stebbins was one of them organized the first luminist exhibit there, they did so on their own initiative."

"We taught ourselves American art," say Wilmerding.

And they bought it, too. Wilmerding's Harvard students remember how surprised they were to discover in their instructor's rooms Fitz Hugh Lanes. In those days all such pictures still looked like calendar art. And the pop-art prints by Lichtenstein that Wilmerding had hanging in his bathroom put no one at ease. Stebbins then was busy buying pictures by the luminists, for he could still afford them. Gerdts's walls were crowded with an orchard full of apples, pears and cherries, still lifes the museums did not seem to want. Church was far from famous then. Heade and Lane and Gifford were even less well known.

"I'll tell you what has happened to the market since," say Washington art dealer Ted Cooper of Georgetown's Adams Davidson gallery. "I have a client who bought a Gifford in a gallery in Boston for $1,700 in 1962. Today I'd price it at $220,000. That's an increase of 12,850 percent."

Wilmerding had spent a dozen years teaching art at Dartmouth when he got the call from the National Gallery of Art. "The first person I telephoned was Joshua Taylor at the National Collection of Fine Arts. No other museum director in America has maintained a higher standard of scholarship. Taylor knows exactly what a museum of American art ought to be doing. His museum functions on the fringes of scholarly thought. Taylor has taken the initiative away from New York's Whitney Museum of American Art.We do not always agree with one another. But Taylor, more than anyone else, forged the climate that made it possible for me to come to Washington."

Wilmerding's precedessor as curator of American art at the National Gallery, William P. Campbell, had died in 1976. The American exhibits there during Campbell's reign had all been one-man shows, most given to the biggest names: Gilbert Stuart, Winslow Homer, Copley, Thomas Eakins.

Wilmerding, instead, decided it was time to call in his colleagues and with them organize a showing of the less familiar, not-yet-famous 19th-century painters they all had learned to love.

The Americanists, though allies, often disagree. Taylor often shows artists that his colleagues, especially those from Harvard, where connoisseurship rules, do not think top rung. "Bill Gerdts and John and I love objects most of all," says Stebbins. "Taylor loves scholarship -- an equally wonderful thing to love."

Wilmerding's luminist catalogue is a splendid book in part because it wirter's dispute one another. Powell, for example, sees luminist light as "a manifestation of the divine, a symbol of the throne on heaven," while Novak, in a far more formalist, and more turgid essay, never touches one the holy. Stebbins, for his part, begins by quoting Novak - "luminism is one of the most truly indigenous styles in the history of American art" - and then demolishes her claim.

"John's show," says Ted Stebbins, "is luminishm's apotheosis -- and perhaps its end. The market for these pictures is positively booming. I think that there's danger they'll be over-priced, over-studied, over-rated. But think of all the other shows that have yet to be done. Wilmerding is ideally placed. He has the respect of his colleagues, and the National Gallery's clout, and he loves the art he shows. His work has just begun."