Louis Sullivan, though rightfully acclaimed as one of America's greatest architects for nearly a century, nonetheless has always given critics fits.

To categorize or pigeonhole his work remains extremely difficult. Modernists have hailed him as an important precursor. While praising his exemplary use of materials such as steel and glass, and adopting his famous dictum, "form follows function," as a sort of motto, they have denigrated or ignored his achievements as an ornamentalist. Traditionalists have seen it in reverse and, of course, there have been many shades of opinion in between.

Fortunately, any encounter with Sullivan's best buildings, even one so secondhand as with the photographs, models, drawings and ornamental fragments making up "Louis Sullivan: Ornament and the Skyscraper," an exhibition currently on view at the Octagon Museum, puts such questions in proper perspective.

The works are exhilarating; one can see and feel in them an artist creating at peak concentration and intensity. The fact that Sullivan was striving to join in harmony aspects of his art we've been taught to think of as contradictory -- trying simultaneously to express structure and to decorate it -- simply makes the results that much more pleasing, and even astonishing.

Sullivan was born in 1856 and he died, penurious, in a Chicago hotel room in 1924. His worldly and to a large extent his architectural triumphs were confined to a decade and a half beginning in 1886 when, in partnership with Dankmar Adler, he commenced working on the Chicago Auditorium Building, and ending with the Carson Pirie Scott store, also in Chicago.

Afterward his commissions were few and small; he produced buildings mainly in little Midwestern towns for businessmen who, observes biographer Robert Twombly, thought of him "as if he were still the Louis Sullivan of the mid-1890s, America's foremost architect at the forefront of design." The personal tragedy of the world passing Sullivan by is of no mean dimension -- it is heart-rending to read in Twombly's book the proud man's letters to his former pupil Frank Lloyd Wright and to others begging handouts in order to stave off creditors. (Wright responded with checks, to his credit, even when his own finances were in baleful shape.)

But the great cultural irony is that after 1900 Sullivan designed no more large-scale commercial structures -- his passion for ornament of a certain kind (it can only be called Sullivanesque) had gone out of fashion among captains of industry. This is the man who had given the skyscraper -- "something new under the sun" in his own apt, excited phrase -- an intellectual rationale as well as impressive physical form. What he would have done had he been given further opportunity remains a tantalizing might-have-been.

Even so, his contributions remain immense, though not literally so. Another irony of Sullivan's career is that the modern skyscraper is implicit in his buildings -- its skeletal steel superstructure with relatively lightweight skin attached -- but their actual size is limited. The highest building he designed (the unbuilt Fraternity Temple in Chicago) contained 36 stories. The highest of his designs to be built contained 18 stories. None of his undisputed masterpieces (Carson Pirie Scott, the Wainwright Building in St. Louis and the Guaranty Building in Buffalo) would exceed the height limit in downtown Washington.

Sullivan's buildings, in other words, soar only in the metaphorical sense. Though each is unique, each was designed according to a tripartite formula he worked out as "a logical and poetic expression of metallic frame construction" -- each had a two-story base, an intervening, "indefinite number of stories of offices piled tier upon tier," and an attic where "the circulatory system completes itself and makes its grand turn, ascending and descending ..."

Sullivan's expression of height -- such a building "must be tall," he had written, "every inch of it tall" -- is nowhere more thrilling than in the Wainwright and Guaranty structures, each with delicate vertical piers rising from solid, elegant bases. But projecting cornices bring each of the buildings to a conclusive termination at the top, turning vertical forces downward once again. And Sullivan carefully counterbalanced vertical and horizontal forces here; the emphatic, recessed spandrels between each of the piers make clear the underlying structural grid.

This is the sort of poetry modernist architects and critics comprehended and approved, but to their confusion, Sullivan did not stop there: Both of these buildings, but especially the Guaranty with its all-over terra-cotta skin, are richly ornamented. For Sullivan ornament is an essential aspect of the process of designing and it, too, had a function: Ornament was an expression of the humane intentions of architect, builder and client. It expressed a social as well as an esthetic purpose. In these two buildings, and a few others, he was able to bring ornament and structure, spirit and commerce, together as parts of an indelible whole.

As David Van Zanten explains in his essay in a recent book, "Louis Sullivan: The Function of Ornament," Sullivan as a young man had absorbed the latest French theories concerning ornament. As a result, his own ornament, although extremely personal, was also very systematic and was based upon abstracting designs from nature rather than copying or adapting motifs from architectural history. One cannot look for long at Sullivan's sketches for ornamentation -- soft, shadowed, deeply personal, even romantic, and at the same time precise, rational -- without realizing that in some measure ornament represents the poetic core of his creative personality.

We feel no contradictions in his best works because Sullivan himself saw none, and perhaps, because the time has arrived when it is possible to see Sullivan whole rather than as a tragically split artistic personality. He was, after all, a very great, very complex artist.

The Octagon exhibition is a much condensed version of "The Function of Ornament," organized in 1986 by the Chicago Historical Society and seen previously there and in St. Louis and New York. Originally, we were to see it in its entirety at the National Building Museum; the location was changed because of continuing construction there. It continues at the Octagon through March 13.

"There was something about the low, horizontal landscape of Long Island that provided fertile ground for the cultivation of European Modernism," writes curator Alastair Gordon in the catalogue to "Long Island Modern: 1925-1960," an exhibition on view through Feb. 28 at the American Institute of Architects headquarters building adjacent to the Octagon Museum. The show proves the thesis in smashing style. Scale models of certain buildings, some famous, some not, are a special attraction.