The history of modern art is a history of how people laughed at the new, and at what they did not understand. Over a century ago pregnant women were actually warned against impressionists, lest they be upset by such affronts to their sensibilities.

The subsequent controversies over the Armory Show in 1913 (was it art?); the law suit over the Brancusi "Bird in Flight" in 1928 (was it art?) and, more recently, the placement of an Alexander Calder stabile in Grand Rapids (was it art?) are all part of the same legend.

"ISIS," the 43-foot-high, 35-ton scrap-metal sculpture by Mark di Suvero dedicated on the Hirshhorn plaza yesterday at noon, threatens, absurdly, to be the most recent chapter in the saga. The issue is not whether this is art. It is only whether this is a first-or second-class example of one of America's finest sculptors.

In fact, "ISIS" is still not finished. It will be painted, Di Suvero said yesterday, but he has not yet decided what color. Until it is painted industrial red or orange or yellow, or whatever color he chooses, neither the mood of the piece nor its relationship with its surroundings can be finally assessed.

The formal qualities other than color, however, are now firmly established, and "ISIS" is cleary the first of the Hirshorn's outdoor sculptures to take on and challenger the scale of the building itself. As a result, the sculptures standing less assertively nearby, such as the Manzu figure of a girl, the small Snelson and even Oldenburg's "Geometric Mouse," seem dwarfed and almost decorative by comparison. One suddenly realizes that most of these works were originally purchased to grace Joseph Hirshhorn's lawn in Connecticut, not the plaza of Bunshaft's concrete donut.

Di Suvero's work, the first commission done especially for the museum, and the gift of the Institute of Scrap Iron and Steel, is a grand gesture, a benevolent act of defiance designed to triumph over the surrounding architectural bouillvabaisse, and it almost succeeds. Almost.

Di Suvero is an intuitive sculptor who works out the forms as he goes along, and that takes time. The piece as it was sketched originally at his California sculpture-yard, was considerably taller, but the stance had to be adjusted here so that each bearing point was located precisely over the Hirshhorn's underground caissons.

If a film made by Suzanne Simpson tells the tale correctly, the originally piece had far greater height over a smaller base, and considerably more drama than it has been able to sustain in its final form. The Hirshhorn piece, in the end, seems a somewhat tamed example of the artist's work.

To those who dismiss this as yet another manifestation of a straining avant-grade, however, it must be said that this fine example of Di Suvero's are not only stands in a tradition now nearly a century old, but also right in the mainstream of American sculpture.

The idea of constructed sculpture, as opposed to traditionally carved or modelled work began in the first decade of this century with synthetic cubism and collage - the idea of putting parts of things together to make new expressive combinations.

That idea was carried forward by the Constructivists in Russia after the Revolution, and was tken up again after World War II in America by David Smith and Alexander Calder. Both took abstract elements and synthesized them in different ways and to very different effect, but their works stand allied as "constructed" sculpture.

In the '60s, as America began gagging on its own trash, artists began making assemblages using urban discards. Some did it for purely formal reasons; others, to make political and social statements.

What Di Suvero does it basically assemblage, but the "junk" element of his work is used heroically, romantically. If there is a political statement it is about man against the odds, and his actual working by hand with the tools of technology, such as cranes and welding arcs, reinforces this theme. No one who saw Di Suvero at work over the last week could fail to feel some sense of triumph at its completion.

If there is a formal problem with the completed work, the final balancing of the forms, it is the ease with which it can be read figuratively - as the "bashful horse" noted by one editorial writer.

It has never before Di Suvero's intent to allude to specific human or animal forms, and no doubt was not his intention here. The fact that such a reading is so accessible from one of the chief vantage points of the work, however, must lead to the conclusion that Di Suvero did not sustain the degree of abstraction he no doubt sought. This must remain a failing of the work, and one which the sculptor would have corrected had there been time. But by Wednesday his time was up.

"Deadlines are wrong," he said. "The sculpture should dictate how it grows up and when it grows up. Working against the clock is like grabbing a little baby and saying 'grow up, tomorrow you've got to go to college.' I like my pieces to have their own time." The cab of a diesel engine, to the disappointment of the artist and other onlookers, was ultimately left out "because there wasn't enough time."

But as Hirshhorn director Al Lerner pointed out, Michangelo was under time pressure when he painted the Sistine Ceiling, and that turned out to be a more than satisfactory job. Di Suvero's work must now be judged on the basis of what is there.

What is there stands as a major example of the work of a major American sculptor. It took guts for the Institute of Scrap Iron and Steel to make the $85,000 commission; it took guts for the Hirshhorn to accept the piece, sight unseen. And Washington is the richer for it.