America, in the 19th century, was in love with nature, commonly spelled with a capital, as in Nature; consequently our landscape painters were held in high repute. But the 20th century has not been so kind. Only in recent years have we ceased looking upon these artistic achievements as quaint, provincial embarrassments.

Scholarly reassessment began more than 25 years ago. Popular taste, and the market, caught up during the 1970s, when a spate of high-powered museum exhibitions drew big crowds and sent people scurrying to attics in search of yellowing treasures. Research and reassessment continue, along with exhibitions, the latest of which is "George Inness," opening today at the National Gallery of Art's West Building.

The show is a tribute to a resolute loner. Inness, a mystic and an epileptic given to introspection and occasional fits of depression, did not habitually go on sketching outings with artist pals, as did so many of his contemporaries. He was a hard customer at times, and impetuous. As a young man he was expelled from Rome for batting a soldier on the head with a walking stick; in later years he would banish admiring collectors from his studio at the hint of a stupid question.

More importantly, he was resolutely independent in his art. That Inness was a superb painter has perhaps been better appreciated in Washington than in most places because several of his better works have long been on public view here: "The Lackawanna Valley" at the National Gallery, "Niagara" at the National Museum of American Art and "Sunset in the Woods" at the Corcoran Gallery of Art.

But even in Washington this show is a revelation. An hour spent in the company of these 43 paintings -- scaled down from the 63 works displayed at the Metropolitan Museum of Art, where the exhibit originated last year -- is an hour of intense concentration and pleasure. A gentleman standing in front of a particularly beautiful canvas the other day encapsulated the feeling when he announced, unbidden, "You know, if Inness had lived in Europe and painted like this, he would be considered one of the greatest in the world."

Alas, the provincial label sticks, and something should be said in its defense. Landscape painting almost by definition pays close attention to local topography, vegetation, color and light. And even though Inness (1825-1894) was more in touch than his generational colleagues with traditional and contemporary European art, there remains something particularly North American about his painting.

"However American his work would have looked in France, it looked very French in America," wrote historian James Thomas Flexner, an early digger in the field of 19th-century Only in recent years have we ceased looking upon these artistic achievements as quaint, provincial embarrassments. American art. Flexner admired, at Inness' expense, painters whose subjects often were more identifiably American (hence the group title, the Hudson River School), but his statement sums up the Inness position between two worlds.

That Inness shared certain distinctly modern attitudes with the European avant-garde is persuasively argued by National Gallery American art curator Nicolai Cikovsky Jr. in a catalogue essay. This is most obvious in the late paintings, which, while usually retaining some sense of particular place, invariably are formed according to recognizably abstract principles of structure and color. In them, Cikovsky aptly writes, "weight is suspended, time is slowed, sound is stilled, substance is softened, atmosphere is unbreathably substantial and thick with color."

At the same time, Inness clearly was part of a continuum in American painting. Emerson's passage -- that in the woods "I become a transparent eyeball; I am nothing; I see all; the currents of the Universal Being circulate through me; I am part and parcel of God" -- is but the most famous statement of a belief in the religious significance of nature that was a leitmotif of American culture through most of the century and that formed the intellectual context for much American landscape painting.

This belief was as flexible as it was widespread; it had undertones that were patriotic, political and morally uplifting as well as religious. Inness' version of it was mystical. In the mid-1860s he became intensely interested in the thought of Emanuel Swedenborg, the 18th-century Swedish mystical theologian, and though the painter never developed an explicitly religious symbolism, he devoutly believed that "the highest art is where has been most perfectly breathed the sentiment of humanity. Rivers, streams, the rippling brook, the hillside, the sky, clouds -- all things that we see -- can convey that sentiment if we are in the love of God and the desire of truth."

The test, of course, is in the paintings. As we observe Inness' development in the chronological installation at the National Gallery, we begin to appreciate not only increasing technical mastery but the artist's deepening conviction that his own observation of nature, transformed into art, was at once a quest and a spiritual reward.

The exhibit emphasizes the late works, but it also demonstrates thatIn Inness' late paintings, "weight is suspended, time is slowed, sound is stilled, substance is softened, atmosphere is unbreathably substantial and thick with color." National Gallery American art curator Nicolai Cikovsky Jr., in the catalogue essay Inness was an experimenter and an innovator, and that he created beautiful paintings in all mature periods of his career. "The Lackawanna Valley," for instance, was painted in 1855 (for a fee of $75!), when Inness was 30. With its locomotive belching smoke in the middle ground, it is quite different from the more bucolic or the more heroic paintings of his contemporaries, and it has rightly become an icon of America at mid-century.

Inness, who showed a pronounced respect for the European landscape tradition at a time when American painters were being urged to focus upon the uniqueness of the native terrain, came into contact with the Barbizon painters during a trip to France in the early 1850s. The experience reinforced his penchant for humble subjects, it enriched and lightened his palette, it confirmed his predilection for lively, expressive brushwork and, once fully mastered, it contributed to a sequence of masterful paintings in the 1860s.

My favorite painting in the exhibition -- a favorite among favorites -- is an 1860 painting titled "Clearing Up." This magnificent, though small, canvas (it measures 15 by 25 inches) is a scintillating example of an artist coming to full maturity. The subtle richness of its colors, the whole soft scheme set off by the touch of red in the vest of a lone figure in a middle-ground field, is not matched by any American painting I know of the period. Most important, though, is the light, which in quality is neither so hard nor so even as the light of contemporary American luminists, but, filtered through rolling clouds, is every bit as transcendent.

Inness went through many changes on his way to the late style. In the 1870s, for instance, he painted a number of really wild, romantic landscapes, such as "The Coming Storm" (1878), in which the agitation of the brushwork, in a manner somewhat similar to Constable's sketches, is the perfect metaphor for the event being depicted and, at the same time, is exciting all on its own.

He even flirted with overt symbolism in "The Monk" (1873), a moody picture that, in its flat and rigid patterning anticipates the linearity of Art Nouveau. We are perhaps fortunate that Inness was still too much a realist to follow this tantalizing lead; his genius was not that of a tight, patterning draftsman, nor was he a creator of extraordinary images.

The Inness star was clearly rising as those of his Hudson River School contemporaries, such as Frederick Edwin Church, were falling. In 1884 he was given a retrospective exhibition of 57 paintings in New York. The event seems to have had an exhilarating effect on his creative powers; in any case, not long after the exhibit he began an almost entirely new phase that would last until he died 10 years later.

It was a late flowering of the kind that rarely happens, in which Inness seemed able to place all that he had learned in the service of, for his time and circumstance, a radically altered vision.

These late works are resolutely geometrical in their underlying structure; they are poetic and evocative although, even in this limited selection, somewhat repetitive. At their best, as in, for instance, "A Breezy Autumn" (1887) or "Early Autumn, Montclair" (1888), they retain an indelible freshness of observation while provoking almost free-associational responses.

The late Joshua Taylor, while director of the National Collection of Fine Arts, wrote that, when viewing Inness paintings of this period, "we are thrown back upon our emotional responses, set free to wander not only in the undefined washes of color in the landscape, but also pleasurably in the reaches of the mind."

The Inness exhibition as a whole is as stimulating to the mind as to the eye. First credit goes, of course, to the painter, but there is ample residue for Cikovsky, who stumbled upon his first Inness some three decades ago while an undergraduate at Harvard and who subsequently made the artist a subject of in-depth inquiry, and for Michael Quick, curator of the Los Angeles County Museum of Art, who organized the exhibition.

It continues through Sept. 17.