TO LOOK into the light might not have been a specifically religious metaphor for Sanford Gifford, but it was clearly an undertaking of a transcendent variety. For while the National Gallery's beautiful retrospective of his work is titled "Hudson River School Visions: The Landscapes of Sanford R. Gifford," the view is often only a pretense for what might better be called lightscapes or skyscapes or -- in both senses -- illuminations. Gifford's portrayal of light, his mastery of atmosphere, made his paintings mysterious, moving and critical to the emerging sense of American nature as the American nation.

Gifford might have been destined to join the Hudson River movement. He was born in 1823, just as the Catskills were catching the popular imagination -- James Fenimore Cooper published "The Pioneers," the first resort hotel opened and Thomas Cole, who would give the Hudson River School its name, began to sketch and paint the mountains -- and he was born into the light itself, in the heart of the Hudson River Valley, and into a family wealthy enough to allow him to pursue his artistic ambitions.

By his mid-twenties he was already showing his paintings in the major academies of New York. He spent a couple of years touring Europe and was particularly taken with J.M.W. Turner's sweeping manifestations of illumination and reflection; gradually he became converted to the pursuit of beauty for its own sake, not as "truth" but as a sublime unity that could evoke the highest, purest emotion in the human heart. He spent every summer in a sort of aesthetic quest, roaming from the Catskills to the Adirondacks, the Green Mountains to the White Mountains, even to Maine and Nova Scotia, searching out the scenes that set his senses, and his spirit, afire. Lakes, waterfalls, chasms, sunrises, sunsets -- the world he painted was so vast it was almost ungraspable. It was, so to speak, our higher nature.

Despite the vastness of these landscapes, and the seeming impenetrability of the light, Gifford laid down his lines of perspective with painstaking exactitude. He knew Vermeer's trick of the white: Again and again, he uses dots of the purest paint to draw the eye down the angle of the sun, dabbing it diamond-hard onto the backs of cows and on bits of cloth, whitewashing marble domes and stucco walls, gilding the peaks of sails, sand and snow. But where Vermeer's whites are cool, like secret pearls, Gifford's are hot, wincingly brilliant; to look into the light requires a strength of character, a surrender to the ineffable.

Witnessing Gifford's obsession with light, his almost physical ecstasy and his drive to re-create it, one thinks he must have experienced at least some degree of synesthesia, palpably absorbing the intricacies of color and reflection in a sensual fashion that went far beyond the merely visual. When he does indulge in rich colors, in the ember-hot reds and oranges of sunsets and autumn leaves, he seems positively intoxicated; standing before "Kauterskill Falls," which depicts a Gothic arch of blaze-leafed trees over a breaking stream, is like being drowned in a flood of light from a stained-glass window. In "A Winter Twilight," as the layers of dusk and gleam and rose fall across the countryside like a prism, the ice skaters seem to be enveloped by flame.

But for Gifford, light was anything but ethereal. In his canvases the air is permeated with color as heavy as mist, and on his canvases the glazes are so thickly layered they seem almost oppressive -- and a moment later, translucent. Again and again he doubles his efforts, painting the mountains as reflected across water, exploring the shifting qualities of air and water. He seems to be struggling with man's place in the scale, and not only when he places a human figure in the landscape. The sky is sometimes more ceiling than heaven. (This sense of the finite actually increases the foreboding power of his paintings of approaching storms; the iron-gray thunderclouds swoop down the curve of the sky like hooded specters.)

That somehow tangible air also divides his paintings into the attainable and the unattainable, between the boats and branches and crags of the foreground, extravagantly detailed, and the soft-edged, partially obscured gorges and ridges in the distance. (In a different sense, the sunlight breaking through a line of rain clouds in "Camp of the Seventh Regiment, Near Frederick, Maryland, in July 1863," falls as a barrier between the bivouacking National Guard and the mountains beyond; depicting the aftermath of Gettysburg, it may also suggest a sort of blessing, a "coming of the light," separating the soldiers from the darkness of the battle.)

Oddly, as he aged -- and as he experienced the depthless clarity of the Mediterranean sky, particularly on a tour of the Middle East after the Civil War -- Gifford seems to have found the weight of the heavens lessening. He breathes more easily. The ruins of the Parthenon are immense, elegiac, monumental. The gypsy sails of the Venetian boats are like jewels in a boundless blue; and his views of Mount Rainier, though swirled in mist, reach up beyond the fog into a fine clear sky.

Though the exhibit will travel to the Metropolitan Museum in New York and to the Amon Carter Museum in Fort Worth, Washington is the only venue that will feature three of Gifford's most important paintings, the oval-shaped pair "Morning in the Adirondacks" and "Sunset in the Shawangunk Mountains," which led to his being named a member of the National Academy of Design in 1854; and the sweeping "View From South Mountain in the Catskills" from 1873.

As informative and expansive as the exhibit catalogue is, the plates literally pale in comparison to the paintings. And sadly, there are no posters or reproductions from the show on sale at the gallery shop.

HUDSON RIVER SCHOOL VISIONS: THE LANDSCAPES OF SANFORD R. GIFFORD -- Through Sept. 26 at the National Gallery of Art, West Building, Sixth Street and Constitution Avenue NW. 202-737-4215. Open Monday-Saturday 10-5 and Sundays 11-6.

Foreboding clouds fill Sanford R. Gifford's 1877 work "A Sudden Storm, Lake George (Coming Rain, Lake George)," part of the National Gallery of Art's retrospective on the artist. Gifford combined light and exacting perspective in his works, including his 1878 "Galleries of the Stelvio -- Lake Como."