Because it has a captive ausience, not just because it's good, H.W. Janson's "History of Art: A Survey of the major Visual Arts from the Dawn of History to the Present Day" is the world's best-selling art book. It's sold 21&Day" is the world's best-selling art book. It's sold 21/2 million copies at $15.95, and it has made its author, a 63-year-old New York University professor, the most influential art historian working in America today.

Two thousand college art departments use Janson as a base map. In countless darkened lecture rooms the slides shown on the screen are of works of art he's picked. Millions of undergraduates have lugged that book around. Their Janson is the gate through which they first gain entry to the history of art.

H. (for Horst) W. (for Woldemar) Janson will speak tonight on "Donatello and the Antique" at the Folger Shakespeare Library (the 8 p.m. lecture is free and open to the public). He will not rely on notes. Prof. Janson (called Peter by colleagues) carries countless images, measurements, anecdotes and dates - a vast encyclopedia of art information - in his ordered mind.%THis sentences are chiseled, his speech is eloquent, precise. You cannot place his accent. He was born in St. Petersburg> but Riga, on the Baltic> is the city of his people. Janson is a Swedish name, he is part German, too.

In Hamburg> after high school, he went to see the great Panofsky. "I asked him," Janson says, "whether one could earn a living as an art historian. He told me, "there's always room at the tap."

Janson has been there for years. He will not say how many, but his text has earned him millions of dollars. His scholarship wins prizes. He is a teacher, an administrator, an academic politician.

Most of all he's tough.

One sees his toughness in the strength of mind with which he wields his information. The schedule he follows would exhause lesser mortals, the Picayunes he smokes would defeat normal lungs.

"Peter Janson," says colleague Joshua Taylor of the National Collection of Fine Arts, "is enormously engaging, tolerant, entertaining. He has a well-stocked mind. His energy is endless. You get this extraordinary sense of being goaded towards total knowledge."

H.W. Janson is a Germanic pedagogue with a Viking's drive.

Of the art historians living, two have reached the minds of millions. Both are students of the Renaissance. One is Peter Janson, the other is Kenneth Clark.

"There is one attitude we share," says Janson. "When Clark's Civilization films were first shown in New York, I was invited to introduce them. First I telephoned Lord ClarK. I asked him for his major premise. he said, "to keep them from turning to another channel."

Like other master teachers, Janson is at once scholar and performer. On recent Monday afternoons he has been teaching Donatello, and much more than Donatello, to students at the Folger. In the semi-darkness, Janson looms abover the glowing slide projector. His voice is gentle, sure. He is in complete command.

As alone, in pairs, in ordered sequence, the bronzes of the Renaisance appear upon the screen, Janson lets a glitter of anecdotes and footnotes decorate his theme.

He speaks of Plato's disapproval of the visual arts, of how mercury evaporates when used in fire-gilding, of the Virgin and St. Thomas, "the 'doubting Thomas' for those sake she renacted her assumption." He speaks of Donatello's homosexuality ("a factor which I introduced into the literature"), of the sculptor's doctor, of Florence and the plague.

He says, "France escaped Islamic occupation because her horsemen had the stirrup, which, when first invented, in India at the time of Christ, was only large enough to encircle the big toe. The Mongolians much improved it. They made it big enough for booties. In Mongolia it's cold."

A rather awkward statue of Can Grande della Scala is cast upon the screen. Janson weaves a story of the 14th-century soldier who, though he ruled Verona, styled himself a kind of Asiatic ruler, of how his title was mistranslated "not Great Khan but Great Dog." Janson speaks of Marco Polo, and footnotes his story by mentioning the silken Chinese robe in which Can Grande's corpes was laid.

It's true Can Grande's statue looks a little silly. It is there on the screen, and it is in Janson's book, not because it's beautiful. It is there because it is a work that lets Janson teach.

Two differing traditions, one British, one Germanic, are apparent in the way our colleges convey the history of art.

Kenneth Clark, seen on the tube, is less a teacher than a charming and exquisite host. He never lists, he never bores, sensibility is all. His effort is concealed. "Historical truth," Clark rightly notes, "is usually complex and frequently dull," but his programs are never that. he prefers to delight us, to offer us surprises, to profer delectation. His lectures are as pleasing as high tea in some great house.

Janson is a scholar. His lectures, and his writings, suggest some mighty library, ordered and complete. he tends to stress chronology. "All study of history is predicated,' says Janson, "on seeing patterns where there is only chaos." Pattern rules his textbook. One leaves its pages feeling that the Baroque flows from the Renaissance as inevitably as Chapter Seven follows Chapter Six.

The book for it is fine. Its reproductions are impressive, it does not issue rules. It opens minds, it does not close them. Janson calls it "the base of the pyramid."

he pattern of the volume," writes Janson, "has the defects of its virtue."

Survey texts, even those as good as Janson's are easily misued, for they suggest that there is such a thing as linear development. The danger is that historical system may overrule artistic content, that objects may gain entrance to the "standard repertory of masterpieces" because they nicely prove a point or demonstrate a system. Textbooks tend to overlook ambiguous works or art.

"Well, how should one begin? Is the historical survey the best entry to the field? Or is any survey course bound to be too superficial? Janson asks. "That's debate," he answers, "cannot be resolved."

Janson's text, which first appeared in 1962, has just been revised. The new edition has more pictures, and more of them in color, and it has been improved at the beginning and the end. "The Dawn Of History" has been pushed back 13,000 years, and "The Present Day" now takes us to Rauschenberg and Morris Louis.

No one could accuse Janson's other writings of being superficial. Take his ape book for example. It's called "Ape and Ape Lore in the Middle Ages and the Renaissance" and it won, as did his Donatello study, the College Art Association's award for "the most distinguished work of art historical scholarship. Learned people may recall one or three or seven Medieval monkey pictures.Janson dug up hundreds. The 384-page book includes 1,000 footnotes.

"In 1941, my wife picked up an inexpensive, unrecorded 16-century engraving of monkeys among ruins. It was a birthday present. I began to think."

And to search and sift Janson is a whiz when it comes to writing on what art is about. "The book, though rather specialized, is cultural history in minature. How did we look on our poor cousins before Darwin came along? We thought, "there but for the grace of God go I.' We thought monkeys descended from people, rather than the other way around. Darwin did upset us. His theories seemed to fly in the face of common sense."

He's part scientist, part connoisseur. He grazes through his knowledge. The success of "Roots" is mentioned. Janson starts to muse.

If Arabs were the slavers, why are there not black masses living in Arabia? He pauses to discuss castration in the Eastern Mediterranean. "Testimony, testify, testament, the root is shared with testes. You grab them and you swear by what you hold most sacred." He speaks of Mozart, of 19th-century sculpture, of the harems of the Turks, and his thoughts flow on.

"Art history is useful for the student, say> in Podunk, says Janson who, though trained at Harvard, taught for years in the Midwest. "Where else do students learn anything at all about the Egyptians and the Greeks? History does not begun in 1976."