AN AMERICAN Perspective: 19th-Century Art from the Collection of Jo Ann and Julian Ganz Jr.," which opens today at the National Gallery of Art, is a show of lustrous smoothness. The Gallery's John Wilmerding calls these 100 objects -- these magically clear paintings, these carvings of white marble -- "the finest private collection in the country of 19th-century American art." It is easy to see why. They have been chosen with such wisdom and affection, this show seems nearly flawless. That's its only flaw.

Nothing in it jangles, disrupts or offends. With refinement as their beacon, Jo Ann and Julian Ganz have fashioned a scholarly yet personal historical collection that misleads in one way only. The tranquil objects in it are all so highly finished -- and so kind to one another -- that they lull the mind away from all the mystery and anguish of the dark side of our art.

The Ganzes shun the troubled, the painful and the scary. They do not like the violence of cowboy art, the roughness of the primitives or the Impressionists' free brushwork. They like Ingres more than Rembrandt. The Ganzes champion calm.

They are natives of Los Angeles who met, in 1950, as college kids at Stanford, married a year later, sold furniture and blue jeans and then made a lot of money from the chain store called The Gap. They occasionally buy pictures by the famous, but they are not intent on capturing big names. The masterworks they've purchased -- Winslow Homer's "Blackboard" (1877) with its minimalist prophecies, Joseph Mozier's marble "Undine" (1867), Fitz Hugh Lane's "Boston Harbor" (1850-55), and extraordinary still lifes by Raphaelie Peale and John Frederick Peto -- do not jump out in this show. They've been bought because they fit.

Hiram Powers' Greek Revival marble "Bust of the Greek Slave" and Lilly Martin Spencer's corny jewel of a painting of a "Mother and Child" could not have less in common. The Powers, though its nudity once shocked, is austere; the Spencer; with its cloying domesticity, is sentimental. But a third work on display, acquired by the Ganzes with the help of William Gerdts, that most knowledgeable of scholars, locks the other two in place. It is the little carving of his baby daughter's hand that Powers gave his wife on Christmas 1839. Does that dimpled hand, emerging from a daisy, suggest classical antiquity or kitchy-kitchykoo? It does a bit of both. Part genre piece, part still life, it is a work that crosses boundaries. There are others in this show.

The fine essays in the catalog (by Wilmerding and Linda Ayres, who both work at the Gallery, and by Earl A. Rusty Powell, who used to, but is now director of the Los Angeles County Museum of Art) divide the show by subject, but many of these works ignore such divisions. Is Samuel S. Carr's charming "Beach at Coney Island" (1879) genre scene or seascape? Is J. G. Brown's delightful "Girl Under a Tree" (1866) a sweet Victorian fable, or a hard-nosed nature study inspired by John Ruskin? It is rare to find a landscape here that does not include figures, that is not, in part, a genre scene. This show has no rough edges -- all its boundaries are blurred.

It is both one show and many. Within it are a dozen interlocking smaller exhibitions.

Of these, perhaps the finest is devoted to meticulous paintings of the flower -- though if the viewer wants to move from Peale's painted walnuts to John F. Francis' chestnuts, or from Joseph Decker's feasting squirrels to S. S. David's fine, eye-fooling almonds, he may also trace the ways 19th-century painters chose other artists -- who pose like antique statues, in flowing gowns, near columns. They do their best to look half American, half Greek. Suddenly we recognize odd linkages between Powers' marble slave and Homer's pensive schoolmarm that were not seen before.

This show, though not large, casts to portray nuts. A third show-within-the-show is devoted to the oil sketches of Sanford Robinson Gifford -- the Ganzes like his smaller studies better than his larger, finished works. A fourth explores the ways American Victorians chose to view their children. A fifth concerns the ways they idealized the virgin. The show is full of women -- by Frederick Dielman, Albert Herter, Francis David Millet and many a fine-meshed net that manages to capture a good part of the range of 19th-century American art. The century's delight in precisionist recording of exotic fruits and flowers, of daylight and of landscape, is abundantly apparent. So, too, is its concern with uplifting moral content, whether expressed through the landscape or through sweet domestic scenes of children at their prayers. That purity is flawed, of course: A partially concealed flirtation with the sexual throbs throughout this show.

Though Rev. Orville Dewey claimed Powers' marble slave was "clothed all over with sentiment," still her breasts are bare. Seymour J. Guy's "Making Believe" (1870) is as subtly exotic. A prepubescent girl is playing in the attic, trying on her mother's clothes amidst dark, suggestive shadows; we see her corset clearly, she wears an off-the-shoulder blouse and a skirt of flaming red. Joseph Mozier's "Undine" is about to loosen her long see-through veil, and Williams Henry Rinehart's antique nymph waiting for her lover wears no clothes at all. John Singer Sargent's wanton gypsy leans back in her chair as if anticipating passion (in case you miss the point, her dark companion strikes a match).

Though many art collectors, like the misers of the fables, love nothing more than watching their holdings grow and grow, the Ganzes do not work that way. They are as quick to sell or trade as they are to buy.

Twenty years ago, when they began collecting, they bought late 19th-century pictures -- by Robert Henri and the painters of New York's Ash Can School. But then, as their taste sharpened and their knowledge grew, they progressed into the past. They have jettisoned more paintings than they have retained. Of the 36 they showed at the Los Angeles County Museum in 1969, only two are in this show. Of the 75 objects they exhibited in Santa Barbara in 1973, they now own but 18. The collection that we see here is astonishingly new. Most of it was purchased in the past decade, and although the Ganzes had help, the viewer sees at once that their choices were their own.

"What I admire most about Jo Ann and Julian Ganz," says curator John Wilmerding, who organized last year's Luminist exhibit and sees "An American Perspective" as a sequel to that show, "is that in their field they are as well-educated -- and self-educated, too -- as any curator I know. They've been to every museum storeroom, visited every dealer, studied every catalog. They have formed a superb library. They've done their homework, and it shows."

The exhibition catalog, though its entries are irritatingly unnumbered, is filled with unusually fine color reproductions. The show, which has been beautifully installed on dark Victorian walls in the Gallery's East Building, will travel to the Amon Carter Museum in Fort Worth and to the Los Angeles County Museum of Art (Julian Ganz is chairman of that institution's acquisitions committee) after closing here Jan. 31.