Sculpture to stroll through

In 1661, the building of Bernini's colonnades around St. Peter's Square in Rome was well under way; as the Venetian ambassador reported: ''The building of the colonnades which encircle the Piazza di S. Pietro will be an achievement to recall the greatness of ancient Rome.''

It is certainly true that since their completion six years later, these semicircular ranks of noble columns, four deep, have stood as one of the most impressive monuments in a city not short on large-scale grandeur, from antiquity forward.

The sculptor Bernini is, of course, so central a figure of what we now think of as ''the baroque'' that he almost is the baroque. But it is interesting that these curved colonnades, which are among his finest architectural conceptions, have actually been described as self-effacing - scarcely a ''baroque'' trait.

Howard Hibbard wrote in 1965: ''Bernini's colonnade succeeds because of its self-effacement. The simple Tuscan order, crowned by a continuous Ionic frieze, is repeated in two pairs of colonnades framing a central walk. The sculptural travertine columns do not call attention to themselves but stand, sober and monumental, a penetrable passage as well as a guiding and moulding wall.''

Such discretion is not what we generally identify as ''baroque,'' thinking, perhaps, of the emotional, dramatic extravagance of which Bernini himself was certainly capable when he pulled out all the stops, visually lifting the roof with a mixture of color, opulence, illusion, and ecstasy. At his most extreme and overtly expressive he could bring all the arts into play, crossing conventional definitions that separated painting, sculpture, and architecture by means of a kind of glorious, unrestrained sense of pure theater.

The colonnades are theatrical, certainly, but in the manner of a stage set (and incidentally Bernini also designed for the theater) in which the predominant action and expression are reserved for the actors, in this case the milling crowds in front of St. Peter's. The two semicircular arms of the colonnade, drawn apart in order to define the piazza as an expansive oval, compose a backdrop, and suggest an arena. Bernini's original design intended a third section of colonnade to enclose the space even more, but it was not realized.

Primarily a sculptor, Bernini, like Michelangelo, was a superb architect on the side, as it were. It helps one to appreciate his colonnades to remember this , because they are not just giant, cold columns. There is a quality to them that is more than the merely useful or the merely decorative. The space they describe and the way they do so add up to an impressive sensation, both rather comfortable and warm. Nikolaus Pevsner points out that Bernini was from Naples, and brought to Rome a ''South Italian impetuosity.'' He sums up the sculptor's splendid colonnades by saying that they ''have something of the happy openness of Palladian villa architecture, in spite of their Roman weight and their Berninesque vigour.''

Historians have found few specific stylistic progenitors for the colonnades, although Hibbard names the vaulted ''hemicycle on the upper level of the Sanctuary Fortuna at Palestrina.'' Palestrina is 40 kilometers (25 miles) east of Rome, and this enormous hillside temple, which housed a much-consulted oracle in the late 2nd century BC, took up most of the area occupied by the modern town. The sanctuary is now reduced to imposing remains, but in Bernini's time it had been studied and copied with considerable interest.

Here, in the admirable, heroic terms of ancient Roman architecture, was a sacred-cum-popular ''theater'' of colonnades, and the unknown Republican architect had given his columns a role that was not simply constructional. They have their own life, their own expressive purpose apart from just holding up a roof. Like these Palestrina columns, Bernini made his own into a rhythmically directive procession, and had them surround a large open-air space as a screen which also implies the vitality of movement. The more one looks at them, in fact , the more ''baroque'' they seem, however self-effacing. And to experience them fully one really has to walk through them, to stroll (in company with a pigeon or two) along their interior curving walkways, and to gaze at them while standing in the middle of the piazza. Photographs cannot explain these extraordinary colonnades adequately.

Bernini's sculptures have been described as drawing the viewer into their action. His colonnades are like a large sculptural experience into which one can actually move. Impressive, but not as overwhelming as one might have imagined, they are environmental sculpture of a remarkably exciting 17th-century kind.