BERNARD @BERENSON, the connoisseur, called his clients "squillionaires." There were three things that he sold them: his knowledge, his example and a kind of immortality. Of course, he charged a lot.

A self-taught judge of pictures, he knew quattrocento Italian painting as well as any man alive. He was, writes John Pope-Hennessy, "the most sensitive precision-instrument that has ever been applied to the study of Italian art." He was also a pied piper, a creature of his own invention, a bit of a dandy and, in later life, something of a sage. His subject, when he came to it 90 years ago, was in a state of rank confusion. Authoritatively, briskly, he summoned it to order, applying to its chaos that same clarifying blend of passion and exactitude with which he forged the princely style of his extraordinary life.

It is fitting that the National Gallery of Art devote a show to Berenson. He was intimately familiar with its old Italian pictures -- one might even say that most of them would not be here without him. The Gallery's two directors, John Walker and Carter Brown, studied with the master (as did John Pope-Hennessy, now at the Metropolitan, Harvard's Sidney Freedberg and Kenneth Clark). The museum on the Mall, with its aristocratic aura, its august marble halls and its community of scholars, owes much of its style to Berenson's example. It is in many ways his shrine.

He began his life in poverty. He ended it a legend, a miraculous survival from an age that had long passed.

Berenson, at 10, was an immigrant outsider, a poor Jew in Yankee Boston, but he was beautiful, and brilliant, and while still in his teens he set the goals he adhered to until his death at 94 in 1959.

Goethe, Walter Pater and England's 19th-century esthetes, the pre-Raphaelites among them, were to be his guides. He would give himself to beauty and to cultivated living. He would make of his whole life a radiant work of art.

It is astonishing to see to what degree the boy succeeded. He conversed and corresponded with Henry James and Oscar Wilde. He filled his life with grace and luxury. "B.B." was his nickname, but little else about him was in any way informal. Pilgrims of the grandest sort -- J. Paul Getty, Edith Wharton, Prince Paul of Yugoslavia and the King of Sweden -- visited "i Tatti," his well-staffed villa outside Florence, to pay the master homage. Every detail of his existence -- the fresh flowers on his breakfast tray, his cashmere shawls, the rare books in his library -- seemed to be imbued with aristocratic grandeur and high esthetic purpose.

In the robber baron era when great wealth was not taxed -- and even more so later -- the rich who sought from life something more than money thought Berenson's example as precious and desirable as his expertise.

Their money came from slaughter houses, mines, railroads and chainstores, but through the services he sold them they might transcend the crass. He would share with them the genius of Giotto and Giorgione, he would authenticate their paintings (they trusted no one else), defend them against ridicule, protect them from mistakes. Berenson's certificates were seen by those who bought them as tickets of admission to the temple of esthetics. He alone, they thought, could guarantee the authorship of the Italian paintings which would, forever after, represent their buyers in highest realms of art.

"Berenson and the Connoisseurship of Italian Painting," which goes on view today in the East Building, is a didactic exhibition, chock full of books and photographs, telegrams and letters. An essential publication by David Alan Brown, the Gallery's curator of early Italian paintings, accompanies the show. It is rightly subtitled "a handbook," not a catalogue. This is a show that must be read.

Its impact is political. Berenson's connoisseurship is not its only subject. It treats commerce, too.

"Mr. Berenson," writes Kenneth Clark, "never mentioned money."

Most Gallery exhibitions are comparably discreet. This one, at least between the lines, is a bit of an exception. The Gallery here shows us that its old Italian pictures were sold to the squillionaires by dealers who depended on Berenson's assistance, among them Joseph Duveen, whose smug and crafty face looms large in this show.

Lord Duveen of Millbank was part charmer and part scoundrel (Berenson described him as "the king of the jungle"). Once, when Mary Berenson, the scholar's wife and partner, said she thought the dealer's stimulating visits were rather like champagne, Berenson objected. "Gin," was his reply.

When Kenneth Clark, some years ago, portrayed the connoisseur on television without referring to the dealer, he was asked by a reporter how could he discuss Berenson without mentioning Duveen? "What?" Lord Clark responded with what was obviously mock horror, "And say B.B. took bribes?"

Those "bribes," as Clark has noted elsewhere, made Berenson a millionaire. Meyer Schapiro tells us that "from 1907 to 1936 he received from the firm of Duveen 20,000 pounds a year and a cut of 10 per cent on the price of all pictures sold with his authentication." Though there are, of course, no price tags on this exhibition's labels, their presence is implied. The old Italian paintings in the Gallery's collection, the installation here tactfully reminds us, are not only Giottos and Mantegnas, Lottos and Robertis -- they are Mellons, Kresses, Wideners -- and Berensons as well.

Great wealth is a sub-theme of this exhibition. The walls are hung with photographs of Fenway Court and Lynewood Hall and other kingly mansions. Brown notes in his handbook that the privately printed catalogue in which Berenson surveyed the Wideners' Italian pictures was "bound in red leather and lined in green silk."

The subject of Berenson's presumed venality, which Lord Clark and Schapiro have discussed elsewhere with much candor, is treated by the Gallery with perhaps excessive tact. Its exhibit is neither a biographical exercise nor an expose. Rather than condemn the way Berenson made his fortune, the Gallery reminds us that the standards of his day (when, as Henry James observed, "there was money in the air, ever so much money") were no those of our own. Today's professional connoisseurs -- curators, we call them -- no longer are allowed to dabble in the market. Instead they are employed by non-profit museums. Berenson's finances trouble his admirers. And the the fault is his own.

Even if he did not lie outright -- and "there is no evidence," writes David Brown, "to suggest that he made an attribution he did not believe at the time it was made" -- there is no doubt at all that he exaggerated wildly. Some of his attributions read like fulsome ads. "One of his certifying letters," writes Meyer Schapiro, "gives the lay reader an idea of this type of art literature. It informs Messrs. Duveen that their panel of the Crucifixion by Piero della Francesca surpasses the frescoes at Arezzo in quality and condition and possesses the color of Giorgione and a handling like Cezanne's." By overpraising pictures, Berenson earned money. But when ascribing authorship he did not fudge the names. Not only would such lies have been against his grain. They would have cost him, in short order, his exalted reputation. He walked a narrow line.

Schapiro notes that "business, a distasteful, indelicate subject, was the concealed plumbing of his House of Life." Clark makes the same point: "Mr. Berenson," he writes, "was perched on a mountain of corruption. The air was purer up there. He would not have kept his position if he had done anything flagrantly dishonest; on the other hand, he could not get down."

The inevitable conflict between the high ideals that Berenson professed -- and the sources of his fortune -- distressed him all his life. "To have admitted the simple fact -- 'I was sick of being poor' -- would have undermined," writes Clark, "that skyscraper of high-mindedness he had been erecting in his mind since his boyhood."

This show is kind to Berenson. It does not stress his sins.

He burst upon the art world in 1895, when, not long out of Harvard, he savaged a London exhibition of early Venetian painting. There were 18 pictures by Giorgione listed in the catalogue. Much to the distress of their aristocratic owners, Berenson discussed the paintings in a pamphlet -- and dismissed them all.

One of those "Giogiones," now accepted as authentic, was given to the Gallery by Samuel H. Kress. It is in the show. Berenson believed it was the work of Giorgione's colleague, Vincenzo Catena. He held to that view for more than half a century, though in the 1950s, "exercising," Brown writes, "the right of a connoisseur to change his mind" he at last came around.

Connoisseurship is an art, it is not a science. Berenson's greatest contribution is that he placed it on a stronger footing than it had ever known.

"His sensibility," writes Schapiro, "became the instrument of a profession. Through it he performed an essential service in the diagnostic judgment of works of art. Without sure names, the Italian art that was the goal of travelers and collectors had an uncertain value. Berenson also told his readers and hearers what to enjoy and own and how to enjoy it.His early writings on pictures -- authoritative, resonant with lofty conviction and explanation -- presented a model of esthetic response, intense and refined, as well as a rational method of discerning the works of the great artists."

Though the pictures he discussed were painted centuries ago, determining "who painted what" with some degree of accuracy is a skill of recent times. Giovanni Morelli (1816-1891), Berenson's most distinguished predecessor, looked at how the masters painted hands, fingernails, ears, and based his connoisseurship on a study of such clues. Berenson's method was just as "scientific" -- but vastly more rhapsodic. He drank in the whole picture.

His memory was astonishing, though, of course, it was aided by photography. "Photographs! Photographs!," he wrote in 1932. "In our work one can never have enough."

Those who watched him, glass in hand, determining the authorship of some problematic painting, thought him some sort of magician. "I must add," writes Kenneth Clark, "that Mr. Berenson's procedure before a picture added to the effect of magic. He would come very close to it and tap its surface and then listen attentively, as if expecting some almost inaudible voice to reply. Then, after a long pause, he would murmur a name. Of course he had tapped the surface to see if the picture were on panel or if the canvas had been relined, but I realize that, to the lay eye, the whole performance looked rather like a conjuring trick."

To explain connoisseurship is one object of this show. So subtle is that art, that magical pursuit, that one need not be surprised that this exhibition only partially succeeds.

Connoisseurship, in a sense, involves educated guesses. Berenson, of course, was not always right. He once gave away a painting, which he had misattributed, only to discover, painfully, belatedly, that he'd parted with a costly work by Domenico Veneziano. It is in this show. Even the famous "Amico di Sandro," the "friend of Sandro" Botticelli, who Berenson invented, and to whom he ascribed a set of problematic pictures, has turned out to be the young Filippino Lippi. Additional ascriptions pronounced by the master, some of which appear on the National Gallery's own labels, are now the subjects of dispute, and may well be discarded as scholarship improves.

The long and well-known lists of "authenticated" pictures, which Bernard and Mary Berenson spent decades revising, have been shot full of holes. But as Brown's show makes clear, the connoisseurs now shooting are all in BB.'s debt.

So, too, are the rest of us. He was instrumental in bringing to this country the grand Italian pictures now displayed for our delight at the National Gallery, the Met, the Gardner and the Frick and in other fine museums.

"A serious judgment of authenticity," Clark writes, "involves one's whole faculties. It also keeps alive or revives the enjoyment of a work of art, which is the most defensible purpose of all criticism." Berenson believed that, too. There was no one else quite like him. He brought to the study of old Italian pictures an elegance of mind that lent his connoisseurship a quality of intellectual beauty. He linked the present to the past, the days of Carter Brown to those of William James and Santayana. He was one of the most influential, sensitive, convincing -- and intriguing -- art minds of his time.