In the years before the Civil War, Frederic E. Church (1826-1900) was the most famous landscape painter in America. A close look at "Close Observation," an exhibition of 112 oil sketches by Church that opened last night at the Corcoran, reveals why.

Church had a spectacular talent that did not go unnoticed for long. While he was in his teens, his wealthy Connecticut parents sent him to study with Thomas Cole, father of American landscape painting and founder of the so-called "Hudson River School." Church was, in fact, the only real pupil Cole ever had.

By age 35, Church was a sensation both in America and abroad for his huge, panoramic paintings of the wilderness: from the Catskills and Maine to South America. 'Niagara," painted in 1857 and now in the Corcoran's permanent collection, was hailed as "the finest oil-picture ever painted on this side of the Alantic." Two years later, he sold "The Heart of the Andes" for $10,000, then the highest price ever paid a living American artist.

But those giant panoramic paintings are not the subject of the Corcoran show. Rather, these are 112 small oil sketches made "en plein air." A naturalist and world traveler, Church worked on the site, often in difficult circumstances, from the Arctic to the Amazon, and from Jamaica to Jerusalem. These sketches span his lifetime.

Although most of his contemporaries also were out looking at nature -- for these were the days of the nature-worshipping Transcendentalists, Thoreau and von Humboldt -- Church was one of the few who sketched on the site in oil rather than in pencil, and "with a rapidity and precision which were simply inconceivable by one who had not seen him at work," one observer wrote.

Photographically precise or loosely brushed, these small-scale paintings miraculously sum up the light, atmosphere, color and form of what the artist chose to observe -- from looming icebergs to jungle rainbows, mountain mists and stormy sunsets. Here, far better than in his highly finished spectaculars, one is in touch with the intensity of Church's response and with the deftness of brush by which he transmits the essence of that response.

These are little wonders, too many to enumerate. The earliest sketch, "Apotheosis to Thomas Cole," should be observed for the melodramatic style it feigns as an hommage to the style of his recently deceased master. An awesome landscape dwarfs two figures as a cross glows in a stormy sky. This is the romantic landscape tradition Church inherited. How far he moved away from it toward precise observation becomes immediately clear in the works that follow.

Church was 41 before he traveled to Europe and the Near East (surprisingly, he left the Far West to Bierstadt and others), and it is clear that the closer he came to civilization, the less he painted.

Rome he found "cheap and vulgar." His rendering of Greece and even the Alps seem to show loss of originality. Church obviously took his energy from the "New Eden," where all of nature was a "happening," with God at the controls.

At the end of his life, his reputation faded into the mists of Impressionism and he returned to "Olana," his great Victorian manse by the Hudson. There he rendered much that was ordinary, along with a series of late "Cloud Studies" which bridge the century, from Constable to Steiglitz.

Photography, in fact, obviously played some role in Church's compositions, and several of these sketches could be mistaken as color work in the then newfangled medium. One study for "Niagara" is, in fact, painted over a photograph.This issue is dealt with in the fine catalogue of the exhibition, written by Boston Museum scholar Theodore E. Stebbins and published by the Smithsonian Traveling Exhibition Service, which is circulating this show.

Happily, these "sketches" were not considered finished enough for sale or exhibition in the 19th century, and the artist's son gave 500 of them to what is now the Cooper Hewitt Museum (the Smithsonian's New York Design branch), from which this show has been lent.

The Corcoran also has mounted a show of drawings by Church contemporaries from their own collections, which makes him stand is even higher relief.

Both exhibitions continue through September 2. CAPTION: Picture, A detail from "Rocks and Moss off Grand Manan," by Frederic E. Church