One could hardly ask for a more enjoyable midsummer treat than an oasis within an oasis: seven peaceful, well-lit rooms in the beautiful West Building of the National Gallery of Art, each room thoughtfully hung with pictures of the most pleasing sort, each picture speaking in friendly dialogue with its neighbors, each saying something about the persons who put it there.

The occasion, starting today, is a tribute from a grateful institution to its former leader and great benefactor, and his wife: "Gifts to the Nation -- Selected Acquisitions from the Collections of Mr. and Mrs. Paul Mellon." It is a fitting gesture that makes a wonderful exhibition.

During more than half a century, Mellon has given more than generously in time, money and art to the museum founded by his father, Andrew Mellon, but the art he has donated has been inserted, usually with little fanfare, into galleries replete with treasures acquired from other sources. This show of 85 works, chosen from more than 800 Mellon donations and including quite a few not before exhibited publicly, offers a welcome chance to experience the collector's taste more directly and more sympathetically.

The opening room, devoted entirely to works on paper, establishes a proper domestic note and is a connoisseur's delight -- it prepares the eyes. The first images one sees, turning left at the entrance, are three prints by Jacques Villon that make one wish (not for the first time) to be young and in Paris at the turn of the century. At least as attractive as the women they portray, with such youthful humor and longing, are the soft, rich, lively surfaces and the simplicity of composition. One learns to expect such qualities -- pretty women, evocations of time and place, and top quality -- as one progresses through the exhibition.

One learns to expect neat surprises, too. Villon, like so many artists in Paris at the time, made an artistic journey in the following decade from Japanese prints to cubism, from Toulouse-Lautrec to Picasso, and one is ready for his 1913 print, also of a woman but in deep dry-point cubist hatchings. But the placement of this nice work next to a classical-period Picasso of more than a decade later -- a superb pencil portrait of a woman made all in correct, elegant, simple lines -- is the kind of playful jolt one hardly ever sees in museums, where chronology, style, nationality or some other organizing rule usually pertains. It's almost as if the collector himself had said, "Well, let's just put the Picasso lady here for a while, and see how she gets along with the Villon."

Another great juxtaposition, also in this room: two superior Ce'zanne watercolors of trees flanking a terrific brush-and-ink drawing by Vuillard of a public square, also prominently featuring trees. Only the subjects are similar, of course, but the placement makes one see the differences with fresh eyes -- Ce'zanne as an older artist, intent upon breaking a bucolic scene into its essentials; Vuillard as a young man, creating an atmosphere with seemingly effortless elegance.

And others: boxers by Gericault and Bellows, a century and an ocean apart in time and place, separated by but a few inches in this room; self-portraits by Ce'zanne and Matisse, Ce'zanne intense as always and a little awkward, Matisse absolutely commanding, as if daring his mirrored image to express the slightest doubt; an American group (Hassam, Prendergast, Harnett and Cassatt -- what a Cassatt!). If the exhibition ended here it would repay a trip through the muggy heat.

The next two stops are England in the 18th and early 19th centuries, a period in which, as a collector, Paul Mellon reigns supreme. In "In Honor of Paul Mellon," a celebratory book published to accompany the exhibition, London art dealer Geoffrey Agnew comments, "It took an American collector to make the English look again at their own painting" -- hard to top as a tribute, and true. Mellon began to buy English works in the late 1950s. This passion, culminating with the Yale Center for British Art and British Studies, which was conceived, built and stocked by Mellon, is his most substantial single achievement as a collector.

The National Gallery rooms hardly do justice to the depth and scope of Mellon's interests in this area, but they do cast light upon his relationship, as a patron, to the gallery. Since the institution already was well supplied with works by Gainsborough, Romney, Reynolds and other well-known English masters, the paintings he owned by these artists mostly went to Yale. But the gallery had no great Fuseli, so Mellon bought one and gave it: "Oedipus Cursing His Son, Polynices," a disturbing subject and vision altogether typical of the Swiss-English master. Other pictures here -- by Joseph Wright of Derby, Richard Wilson, John Crome, Arthur Devis -- similarly fill major and minor holes in the gallery's English field, making it much more interesting and comprehensive than it otherwise would be.

In the English rooms is a solitary American, Robert Salmon, and his painting hangs there, one supposes, for its subject: "The Ship 'Favorite' Maneuvering off Greenock." One can look from Salmon's boat, so meticulously rendered in shining light, through doorways framing George Bellows' "New York, February, 1911" -- as cacophonous and heavy a vision (there is but a patch of dirty sky) as Salmon's is harmonious and light, or as Devis' little country estate is gently civilized.

This definitively abrupt transition between old and new worlds is matched, almost, by the next, which is back to the Old World made new with color and light: France! Matisse! His "Still Life with Sleeping Woman" is perhaps the most distinguished of the Mellons' recent gifts to the gallery. Certainly, as Assistant Director John Wilmerding said the other day, it sums up the couple's shared taste in French art: Can she be having anything but sweet dreams, this napping woman, so much a part of her setting, which unites, in soft washes of color, the man-made and natural worlds?

This is a gallery made beautiful by strangeness. The brilliant Matisses (another recent gift is the superb "Pianist with Checker Players") and the fauvist Derain play against the rich, elusive subtleties of Vuillard's "Woman in Striped Dress," and each contrasts strongly with Vallotton's "The Wind" (a cold one for sure) and Magritte's "The Blank Signature." Mellon, the quiet, self-assured horseman-collector, may never have felt the tinge of dark mystery Magritte suggests in his visual conundrum -- a horse and rider sliced surgically by a forest of trees -- but one can hear him chuckle at the possibility.

The painters everybody likes best, of course, are the impressionists, and the big gallery that comes next is full of treasures, a mix of familiar pictures and new gifts from late 19th-century France: Boudin, Manet, Monet, Sisley, Degas, Cassatt, Caillebotte, Seurat, Gauguin. It's a great, nervous room in which no particular painting dominates, the obvious lesson being, of course, that these painters are remarkably different from one another. There is a Winslow Homer here, too, and he holds up well in the formidable competition.

Ce'zanne is the finale -- not Ce'zanne alone, though he might as well be. One enters the gallery and faces his early masterpiece, the giant portrait of his father, whom the artist said he disliked at the time. Ce'zanne was always one to hide a deep emotional life, from himself and from others, but it came out in his work. This painting is one of history's powerful statements about the coexistence of love and hate and so on within a single breast -- not exactly a friendly picture, but a great one.

The exhibition continues until Oct. 16. Congratulations to all involved, especially to the Mellons.