Venice's genius is captured triumphantly in London

To stage an exhibition of the art of Venice's golden age is no mean feat. That, unforgettably, is what the Royal Academy of Arts has triumphantly done, here. (''The Genius of Venice'' continues until March 11.)

''Very few requests for loans have been turned down,'' academy president Hugh Casson says in his foreword to the exhibit catalog. The academy has good reason for gratitude to an international roll call of lenders. Mr. Casson's only voiced regret is that some of the mainland, or terra firma, Italian artists are not shown with ''full justice'' because of the delicate state of their paintings.

Dazzled by the extraordinary range and quality of the paintings (as well as sculpture and a considerable array of drawings and prints) brought together here , the visitor wonders for a moment whose absence he could possibly regret. ''Mainland'' artists such as Jacopo Bassano and Lorenzo Lotto (not to mention lesser names from Bergamo and Brescia) are given a lot of room, and emerge as painters of extraordinary interest.

The highly individualistic and various character of Lotto's art, his dramatic use of light and color, his startling break with iconographic tradition in such a picture as the expressively ''primitive'' spirit of ''The Annunciation'' from Recanti, or his straight realism and sincerity in the ''Double Portrait'' from Leningrad must surely be a revelation to anyone who thinks of Venetian art mainly in terms of the more sumptuous subtleties of Giorgione, Titian, Veronese, and Tintoretto.

These four are, of course, the big guns of Venetian art, and they, rather than the less central figures, are surely the ones to have presented the most difficulties to the organizers of a show of such ambition. Museums and galleries are naturally reluctant to part even temporarily with their most prized works. The show's happiest successes are the finest works assembled by Titian, Veronese , and Tintoretto. Its saddest failure (though understandable) is the absence of any works by Giorgione.

The display of Titians is quite simply stunning: a thrilling tribute to this long-lived genius which would amount to a memorable exhibition on its own account. More than 20 paintings present his range and development, beginning with his early Giorgionesque pictures, with quiet spaces and balanced color. His portraits are informed by a rich appreciation of character and of his sitters' dignity and self-esteem, while gentled by the wholeness and humanity of his touch. His religious subjects are given deep piety, deep tone, and deep color. His late paintings explore potentialities of expressive color, particularly reds with a fierceness of intent that takes the breath away - a case of the opulent glory of sheer painting achieving its own splendor even while describing the most vicious subject or dramatic brutality.

No group of artists rivals the Venetians in the sensuous relish of moments from classical myth, and Veronese, even more than Titian, was a master of this. His ''Venus and Adonis,'' from the Prado in Madrid, is a gorgeous example here. A recent, but still incomplete, cleaning adds to its impressiveness, disclosing the warmth of hue in Veronese's painting of flesh, his capacity to suggest the fleeting brightness of sunlight, and his delight in a large area of pure, saturated color - in this case the orange-red of the sleeping Adonis's garment. For me, this marvelously clear, high note of color, singing out with absolute confidence, was one of the most striking experiences of the entire show.

Velasquez was responsible for this Veronese painting being taken to Spain. Velasquez also acquired for his royal master, King Philip IV, the companion picture, ''Cephalus and Procris.'' Time and war have separated them, but this exhibition pleasantly brings them together again for a while - the latter work on loan from Strasbourg, France.

Some visitors have felt that Veronese could have been better represented. Perhaps this is a justifiable grouse, although the size of many of his works is an undoubted obstacle. One unusually small painting shown is a ''crucifixion'' from the Louvre, the crosses set obliquely to the left, allowing for a great expanse of sky to loom over the mourners. It is a painting described in the catalog, with justice, as ''deeply moving.''

There are also at least two of Veronese's lush, direct portraits included which demonstrate his remarkable skill in this area and his magnificent appreciation of color, enriched by restriction to a few intense harmonies. Naturally one could have enjoyed even more of this kind of thing without complaint. . . .

The most difficult artist for the organizers to represent, however, was not the ebullient and productive Veronese, but the shadowy, though historically crucial, Giorgione. Not one of the 10 rare pictures agreed by a majority of scholars to be by this artist has been lent to the exhibition. His new style - the very foundation of Venetian 16-century art - can only be shown by means of four works from his circle of followers - two of them attributed to Titian, one to Sebastiano del Piombo. These are beautiful in their own right (at least one, when at home in Glasgow, is persistently called a ''Giorgione''), and the Giorgione ''atmosphere'' is in them, so far as this vaguely felt phenomenon can be pinpointed.

But it seems a great shame that this prestigious exhibition has not been graced by one ''real'' Giorgione. The Kunsthist-orisches Museum in Vienna owns two of these agreed Giorgiones. It happens that for the almost exact duration of the London show these two paintings are not on view in Vienna because the museum is reorganizing its Italian galleries.

Couldn't house rules have been bent for once and the pictures sent to London? ''No,'' a museum official in Vienna said on the phone, ''the Giorgiones are on our list of works which never travel.'' And that appears to have been that. A shame.

This omission apart, however, ''The Genius of Venice'' is a supremely special exhibition. And not the least of its pleasures - in size as well as drama of event, space, and color - is last of the four stars of Venetian art, Tintoretto.

In two of his paintings shown here, the scale of the public side of Venetian art is at least hinted.

One is a modello, or sketch, for his gigantic painting in the Great Hall of the Doge's Palace in Venice, his ''paradise.''' It is a vast and amazing cloudscape of thousands of blissful figures (a kind of Venetian ''mannerist'' answer, perhaps, to Michelangelo's ''Last Judgment''), endlessly vigorous, endlessly fantastic, the nearest thing imaginable to a visualized choir of angels, a heavenly host, in decorative terms of staggering magnitude. This feat of bravura has only recently been included by scholars in the corpus of Tintoretto's work.

The story behind his enormous ''Washing of the Feet,'' lent from Newcastle-upon-Tyne, is also one of recent art-historical re-evaluation. This is a wonderful picture, an immediately accessible drama, staged brilliantly with an eye to human motive and emotion, airy and compelling, with deep visual perspective. And it is almost identical to a Tintoretto in the Prado.

Until 1976 it was thought to be a copy or only a partly autographical replica of the Madrid work. But cleaning has convinced scholars that it is completely the master's work. Certainly it is a most impressive and exciting item in a quite exceptional exhibition.