The startling impact of Schiele's portraits

The art critic's real rewards are those very special exhibitions that occur three or four times during a season, at best. Most of them are major museum shows, possibly full-scale retrospectives of important artists, or broad surveys devoted to the art of particular countries, movements, or periods. Every once in a while they may be about such things as historical costumes or antique musical instruments.

Occasionally, however, a private gallery will mount a truly first-rate exhibition, and when it does, it merits one's fullest attention. Such an exhibition is taking place at the Serge Sabarsky Gallery here. It consists of 34 magnificent portrait studies by Egon Schiele (1890-1918), one of the three great draftsmen this century has so far produced (the others being Picasso and Matisse).

The show doesn't take up much space - only one medium-size room - but then, good things have been known to come in small packages. And the works themselves, all watercolors and drawings on paper, are quite small.

But what a stupendous impact these studies make! They present all periods of Schiele's art, from a few rather academic but brilliant works he did as a very young student to some wiry and acidic drawings executed not long before his passing at 28. Every one is superb, and at least a dozen are among the great drawings of the 20th century.

Now, as I've said in other articles, I very much dislike using the word ''great'' when discussing art. It's too easy to leap from momentary enthusiasm for something exciting or excellent to a declaration that the work in question is great. And then to find oneself rather apologetically trying to reclaim that opinion, once the enthusiasm has cooled down.

The word ''great,'' however, fits Schiele's watercolors and drawings perfectly. After decades of studying them, I can as easily describe them as great as I can say that two and two are four, or that the sun is very hot. And I've never once felt afterward that I should have used a more conservative word.

For one thing, they were executed with absolute authority. For another, they speak the truth with shattering impact. We would never question the truth of Schiele's art; only, possibly, its undeviating starkness. If a human perception can be described as ruthless, his was. Not, I hasten to add, in a cruel manner, only in that he was uncompromising in his pursuit of any clues to individual character.

When he found them - and they could be something as simple as the way his subject tilted his face and pursed his lips, or the way he leaned languidly against the door - he was also ''ruthless'' at laying them out for all to see. He laid bare the unique living truth of whatever he was drawing - an individual, landscape, or thing. It was an uncanny gift given to only a few, and it meant he could cause us to see more deeply than we normally would.

Schiele's genius lay in his ability to make us see more clearly; his greatness, in his ability to make us see more deeply. Like every great artist, he presents us with another pair of eyes with which to see - as well as a hint of another human being's spiritual universe. It's an incredible gift which provides insights of the deepest kinds, helping many whose lives would otherwise remain like small darkened rooms with only one or two tiny windows looking out.

Now, I'm not claiming that Schiele was necessarily aware of all these dimensions. Nor that art for him was a deeply devotional act aimed specifically at raising his viewers' levels of consciousness. Far from it. As an artist, he was painting and drawing whatever intrigued or challenged him.

His job was to look carefully, to see clearly and deeply, and to translate what he saw into the appropriate lines, colors, textures, and shapes. Beyond that he couldn't go, but that was far enough. If we have any sensibility at all, and the willingness to apply it, we can easily bridge the differences between his perceptions and insights and ours through his art. And in doing so, enrich our lives and, perhaps, ability to see.

This truly exceptional show will remain on view at the Sabarsky Gallery, 987 Madison Avenue, through Feb. 12 - which should be good news for anyone the least bit interested in drawing. An extraordinary collector

Every form of art has a few special individuals whose behind-the-scenes activities contributed greatly to making the public aware of that form's new ideas, or to helping its artists survive by supporting their work. Thus, 20 th-century music has a special niche reserved for Elizabeth Sprague Coolidge, the music patron, and contemporary literature owes a great debt to Sylvia Beach, the book dealer and publisher.

Modern painting and sculpture have also had their share of such individuals, including art champions Gertrude and Leo Stein, collector George Costakis, and museum curator Dorothy Miller. But no one has achieved quite the reputation as all-around champion and patron of 20th-century modernism as Peggy Guggenheim ( 1898-1979).

She was an extraordinary collector - as well as a close friend and patron of many of modernism's outstanding figures. Although she began her collection during the late 1930s with European avant-garde art, she shifted much of her attention to American art after her return to New York in 1941. Between that year and 1949 (when she returned permanently to Venice), she was an important force in the art world's acceptance of what would later be known as Abstract Expressionism.

To celebrate her collection and her contribution to art, the Guggenheim Museum here is showing 60 outstanding works from that collection. Included are major (in some instances absolutely first-rate) paintings and works on paper by (among others) Picasso, Leger, Duchamp, Tanguy, Kandinsky, Mondrian, de Chirico, Max Ernst, Miro, Gorky, Pollock, and Still.

The selection is broad, and the quality is high. I was especially taken with de Chirico's ''The Gentle Afternoon,'' Picasso's magnificent ''Cubist the Poet, '' Miro's ''Seated Woman II,'' and the early Pollocks. But then, almost everything on view is superb.

After this exhibition's closing at the Guggenheim on March 13, the paintings will return to their permanent home in the Palazzo Venier dei Leoni in Venice. Modern masterpieces

''Impressions of Excellence'' is another exhibition of modern masterpieces. It consists of 54 major modern prints on loan from the McNay Art Institute in San Antonio, Texas, and is currently on display here at Aldis Browne Fine Arts Ltd.

Although the word ''modern'' has been stretched a bit to include Goya and Delacroix, no one can argue with the quality of the prints by those artists included in this show. Of particular interest are four early Goya etchings after Velazquez paintings, two prints from Goya's series ''Los Caprichos,'' Delacroix's ''Tigre Royal,'' and Lautrec's color lithograph ''Le Jockey.'' Other outstanding prints are by Braque, Heckel, Prendergast, and Bellows. In many ways , however, the stars of the show are Mary Cassatt's 10 color aquatints. They are magnificent, and it's a pleasure to see them together for once.

At Aldis Browne Fine Arts Ltd., 1018 Madison Avenue, through Dec. 23.