AMERICAN LIGHT: The Luminist Movement, 1850-1875," which opens here this morning, is the best American painting show ever offered to the public by the National Gallery of Art. It is beautiful and useful. Its pictures portray light, light that fills the air, light that, glowing everywhere calls the mind to stillness. It is show about reflection -- both optical and mental.

Once-forgotten painters, still far-from-famous now -- Fitz Hughe Lane, Martin Johnson Heade, Sanford Gifford, John R. Kensett -- made the polished hymms to quietrude that rule this exhibition. These men, and their strange ally, Frederic Edwin Church, around whose active paintings their more gently ones revolve, did not see as we do. They were something more than analysts of light, more than careful students of the look seaside things. The croses that we see in Heade's "Twilight on the Marshes," or those formed by the masts in these Churches and these Lanes, and Gifford's virgin Moons are more than incidental. For these painters were at once scientists -- and seers. Their impulse was religous. They sensed divinity in skies and sanctity in light.

Because something in out age blinds us to the sacred, curator John Wilmerding's exhibit is bound to be misread. Seas and skies and Marshes are not its central subject. Modern viewers who love facts, who think the holy to be hokum, may not notice, or believe in, the radiance at its core.

"American light" focuses attention on a surprisingly small number of fine and subtle pictures made mostly on the East Coast by a surprisingly small number of gifted lanscape painters who flourished in the few years just before, and after, the American Civil War.

None of them were "luminists" throughout the careers. None of the believed, as this show contends, that that mode of painting known today as "Luminism" was crucially important, much less that it was, in fact, the "conclusive development" of early American landscape art.

Wilmerding's luminist exhibit is a show so well selected, has a beauty so seductive, that it seems, at least to me, to justify that otherwise questionable claim. But all "ism" shows are dangerous -- they edit, they distort, they polish down complexities -- and this one, like the others, should not be swallowed whole.

Like so many pictures in it, Wilmerding's exhibit is brilliant at the center, dimmer at the edge.

In the paintings it begins with -- Robert Salmon's Boston harborscapes, John Caleb Bingham's "Jolly Flatboatmen," Asher Durand's landscapes, W.S. Mount's genre scenes -- luminism's germ perhaps may be detected, but only at a distance.In the pictures that it ends with -- a half Barbizon Blacklock seascape, a riverscape by Eakins, and a pair of wonderful masterworks by Winslow Homer -- it is possible to argue that luminism lingers, but by now it has so faded, it is almost out of sight.

True luminist paintings are extremely rare. The Viewer who attempts to define the style strictly will discover that in doing so he has managed to exclude two-thirds of the paintings included in this show. The exemplary luminist picture should, the scholars tell us, glow with suffused light, should be panoramic, open, egoless in brushwork, and should manage to suggest a mood of frozen time. Lane's calm seascapes qualify, so do the marshscapes of Martin Johnson Heade, and a few of the Kensett beachscapes, and some of Gifford's skies. But Frederick Edwin Church, although his example influenced the others, is almost never a pure luminist. Nor, in fact, are most of the painters in this show.

In Church's most dramatic works, his icerberg's and volcanos, on feels luminism's quiet, its sense of sublime stillness, framed by swirl and bombast. Some glimpse of the apocalypse, perhaps a painted echo of the Civil War, also seems to trouble the darkest of the heades.

Much about this show is peculiarly elusive. No single image here lingers in the memory as strongly as the mood, that sense of all-suffusing sanctity in nature, that to some degree or other all these pictures share.

"In the woods." wrote Emerson, "we return to reason and faith . . . Standing on the bare ground -- my head bathed by the blithe air, the uplifted into infinite space -- all mean egotism vanishess. I become a transparent eyeball; I am nothing; I see all; the currents of the Universal Being circulate through me; I am part of parcel of God." But unless you read your Emerson, your Thoreau and your Kant, you may not apprehend how well these paintings illustrate the distinctly protestant, transcedental spirit abroad in their time.

These calm and lucid skyscapes, these placid panoramas, only a few years ago, were seen as mere antiques -- decorative, charming, but essentially unimportant. That is no longer so. Today they fetch huge prices. One painting in this show. Frederic Church's "Icebergs," sold only a few months ago for $2.75 million, an auction record for a picture made by an American. Washington's museums, the Corcoran expecially, and the National Collection of Fine Arts, are chock full of such pictures; they once were undervalued and may be overvalued now. The paintings in this show ared, in some ways, less originial than this shining exhibition might lead one to believe.

Unless you know art history, you may not perceive how much the painters here have borrowed from the past, from the seascapes of the Dutch, from the turbulent and churning atmostpheres of Turner, from the meticulous precisionism proclaimed by John Ruskin. Unless you read the essay of Theodore E. Stebbins Jr. in the exhibition catalogue is splendid) you may not be aware that "luminism, or something very like it" appeared in Russia and in Denmark and "everywhere in Europe" while it flourished here. The bright new Cadmium paints, which in the late 1840s helped the artists here lend fire to their sunsets, were available to other painters of the time.

The American Luminist movement may not be American, may not be a movement -- yet still this exhibition convinces and compels. That is John Wilmerding's accomplishment. He, and the Americanists who helped him write his catalogue, have shown the public something we had not seen before.

Their exhibit seems to me specially important for at least three reasons.

Though it is not the first luminist exhibition (the first apparently was held at Harvard in 1966, one was seen last year in a commercial New York gallery, a third is here on view now at the Adams Davidson gallery in George town), the Galler's display is the first to bring together the finest paintings of the mode. It gathers in one place a group of landscape pictures -- paintings, drawings, photographs -- that though mostly unfamilar are as beautiful as any we have seen before.

Secondly, it calls overdue attention to a period of art History too long misapprehended. But even more significant is the central compound image, the core idea it summons. The paintings on display, the seascapes and the landscapes, the moonrises and sunsets, gather in the memory until they cast their light far beyond this show.

This show, it seems to me, will change the way we see the landscapes found on calenders, and Hudson River pictures, and the modern light-filled space of color field paintings. This exhibition glows. The ideas at its center will light the way we read writers as diverse as Wallace Stevens, James Fenimore Cooper. Emerson and Eliot, and change the way we see Ansel Adam's photographs, Jules Olitski's fogs, Agnes Martin's grids, and a thousand other pictures, new as well as old.

Washington is fortunate. Other thoughtful studies of the painting of America have been placed on view here, but the best of them -- "America as Art," the Academy exhibit, the Barbizon display -- have been offered to the public by Joshua C. Taylor's National Collection of Fine Arts.It is good to see this American painting shoe is comparable usefulness mounted here by the National Gallery of Art. May there be many more. It will remain on view, in the Galler's old West Building unitl June 15.