Masterpieces by the square yard

Very few museums in the world have as many masterpieces per square yard as the National Gallery of Art here. Other museums may have collections in greater depth and detail, may span more cultures and societies, and may be more specialized. But none can offer the visitor more artistic greatness in such relatively little space.

During an hour's leisurely stroll, he can encounter Botticelli's ''The Adoration of the Magi,'' Bellini's ''The Feast of the Gods,'' Leonardo's only accredited painting in the United States, ''Ginevra de' Benci,'' Giorgione's ''The Adoration of the Shepherds,'' Raphael's ''The Alba Madonna,'' Titian's ''Doge Andrea Gritti,'' and Veronese's ''The Finding of Moses'' - as well as dozens of only slightly less important works by these Italian Renaissance artists and their contemporaries.

The visitor can continue on to view some of the greatest works by El Greco, Rembrandt, Vermeer, Poussin, and Constable - as well as important examples by Holbein, Cranach, Durer, Van Dyck, Ruisdael, Rubens, Hals, and many others.

But that's not all. The National Gallery's collection of 18th- and 19 th-century Am-erican art is first-rate - as are its collections of Impressionist and Post-Impression

ist European painting and its holdings in sculpture, drawing, the graphic arts, and photography.

The National Gallery's East Building has received a great deal of attention since its 1978 opening, both for the building itself (designed by I.M. Pei) and for the contemporary art it houses. It is also the location of numerous exhibitions devoted to the art of earlier periods and cultures, as well as to the art of the very recent past. The John Hay Whitney collection

A good example of the latter is ''The John Hay Whitney Collection'' on view in the East Building. Its 73 paintings include French Pre-Impressionist, Impressionist, Post-Impressionist, Fauve, and Cubist works, a sprinkling of late 19th-century American paintings, and a few more recent pieces. All were assembled by the late John Hay Whitney and his wife. Some have been presented to the Yale University Art Gallery, the Museum of Modern Art, and to the National Gallery of Art, and are included in this exhibition as loans from those institutions.

It's an extremely handsome show that includes several paintings worth a trip to Washington all by themselves. Van Gogh's ''Olive Trees'' and ''Self-Portrait, '' Cezanne's ''Still Life With Apples, Pears, and a Gray Jug,'' Lautrec's ''Marcelle Lender Dancing the Bolero,'' Corot's ''Cottages and Mill by a Torrent ,'' and Seurat's ''Grandcamp, Evening'' are all superb. And other works by Gericault, Gauguin, Degas, Redon, Henri Rousseau, Vuillard, Matisse, and Picasso are of only marginally lower quality.

And yet, I must admit I found the collection somewhat disappointing. Its overall level of quality, considering the stature of the artists included, is surprisingly low. Almost everything on view is well, if not brilliantly, executed. And for sheer, exquisite painterliness, this show is a delight. But when it comes to the best of what the leading Impressionists and Post-Impressionists were capable, it falls somewhat short.

I spent the better part of two hours in the galleries trying to convince myself otherwise. But I could not, and left with the odd feeling that this show is almost ''stolen'' by the works of Eakins, Whistler, Sargent, and Balthus.

While I respect these four artists highly, I would not put them in the same class with Degas, Manet, Van Gogh, Seurat, or most of the others represented in this exhibition. And yet, on the basis of quality rather than reputation, I'd rather own Eakins's ''The Oarsmen,'' Balthus's ''Le Salon,'' Whistler's ''Wapping on the Thames,'' and Sargent's ''Robert Louis Stevenson'' than many, if not most, of the other paintings. First-rate examples by artists of slightly lower rank are often superior to second- and third-level works by artists of great genius. Or so it would seem.

At any rate, it is an outstanding exhibition, and will remain on view through Oct. 2. Five surrealists

To the public, Salador Dali may be the Surrealist, but to the art world, he's only one of several important Surrealists - and not too high on the list at that.

At best, he'd rank third after Giorgio de Chirico and Max Ernst, and possibly even behind Rene Magritte, Yves Tanguy, and Victor Brauner. The reasons are complex, but focus largely on his tendency toward slickness and commercialism. No one would deny his talent, but many would question his dedication to Surrealist theory or ideal.

These were first formulated by Andre Breton, Surrealism's chief spokesman and philosopher, in 1924. According to him, Surrealism's main objective was the freeing of the human unconscious, and this was to be accomplished by means of free association and automatic writing. An artist or writer using these methods properly could expect the release of spontaneous imagery or writings from the so-called unconscious.

What resulted is now an important part of recent art history. It is also the subject of a small but fascinating exhibition in the East Building. ''Five Surrealists From the Menil Collections'' shows examples of the art of de Chirico , Ernst, Magritte, Tanguy, and Brauner (but not Dali), and does so in a manner that should make Surrealism understandable to everyone.

It should also give the viewer a clear insight into what these artists were attempting. Brauner and Ernst, in particular, are well represented, and Magritte and Tanguy are also shown to good advantage. Only de Chirico falls somewhat short - not because the work shown isn't typical and good, but because it doesn't show him at his very best. The viewer can see what he did, but not really how magnificent he was at it.

At the National Gallery through Sept. 28. Lucas van Leyden and contemporaries

In some ways, the most important of the special shows now on view at the National Gallery is ''Lucas van Leyden and His Contemporaries.'' It is the first major exhibition in the United States of this Dutch artist who was Durer's contemporary and one of Europe's most important printmakers.

At his best, Van Leyden was second only to Durer in his mastery of the woodcut and engraving. Such prints as ''Esther Before Ahasuerus,'' ''Golgotha,'' ''The Dance of St. Mary Magdalene,'' and ''The Milkmaid'' rank among the world's graphic masterpieces. And even at his most ''ordinary,'' his prints have a clarity and directness found only in the work of a handful of major printmakers.

It's a special treat, then, to have excellent impressions of most of his prints assembled in one place, and to be able to view them in conjunction with outstanding graphic work by some of his contemporaries. But almost as welcome is the fact that the entire exhibition of 140 prints has been documented and beautifully illustrated in the exhibition catalog. This is one catalog that will go up on my bookshelf - and stay there.

After its Aug. 14 closing at the National Gallery, this exceptional and important show travels to the Museum of Fine Arts in Boston. It opens there on Sept. 14 and will run through Nov. 20. Night prints

Another excellent print show concerns itself with depictions of darkness and dramatic light/dark contrasts. The 100 prints that make up ''Night Prints'' were drawn primarily from the National Gallery's holdings, and include works by Rembrandt, Goya, Delacroix, Degas, Whistler, Kollwitz, and many others. Particularly outstanding are Rembrandt's ''The Descent From the Cross by Torchlight,'' Durer's ''The Kiss of Judas,'' and Kollwitz's ''The Peasant Rebellion: Death.''

It's a beautiful exhibition and a wonderful complement to the show of Van Leyden's prints. It closes Oct. 9.