The Baltimore Museum of Art celebrated the opening of a long-awaited addition over the weekend, but people who flocked to the inaugural events quickly realized that the new wing was simply the end of a much larger story, for the entire museum looks crisp, clean and new.

In part this is due to the esthetic tact and practical skill of the architects, Bower Lewis Thrower of Philadelphia, who had the good sense to set the addition back from, and in other ways honor, the original neoclassical structure designed by John Russell Pope 55 years ago.

Also, it was part of the program from the very start. When museum officials first considered the project a decade ago they decided that the need to renovate Pope's building and the sizable additions tacked on to it over the years would be given as much weight as the need for additional space.

As it turns out, just one-third of the $13.5 million package was devoted to design and construction of the new wing and an adjoining sculpture garden. The rest of the money was applied to the older spaces with dramatic results.

More of the museum's permanent collections are on view than ever before, and they are exhibited in galleries that are more secure, better lit and generally more fitting. In addition, visual and pedestrian connections between parts of the museum that had been obscured by happenstance construction over the years have been opened. The final effect is that for the first time in many years visitors get not only a clear picture of where they are at a given moment but also an exhilarating sense of the museum as a whole.

The collections of the Baltimore Museum, as those of many such civic art institutions, have grown like Topsy over the years, following the interests of prominent individual collectors as well as the guidance of its professional staff. Architectural additions have followed suit: the Jacobs Wing and Antioch Court in the 1930s, the Cone, Woodward and May Wings in the 1950s, the Wurtzburger Sculpture Garden (actually the first phase of the new plan) in 1980.

In addition to providing extensive, if hidden, improvements to the physical plant (all new climate-control and security systems, for instance), the firm of Bower Lewis Thrower has taken this hodgepodge of add-ons and, in the spirit of axial planning that Pope followed in his original temple in the park, made sense out of it. Probably the most dramatic instance of this is the spectacular, shadowed view looking west from the entrance to the Wurtzburger Sculpture Garden all the way through the ground floor of the museum to the little Greek Revival temple (a white-painted wood-and-stucco structure designed by Benjamin Henry Latrobe for a Baltimore estate around 1800) on the other side of the building. But there are other views almost equally grand.

The new wing is not attached directly to Pope's building but to the east wall of one of the older wings in back of the original structure. This is important, for it means that there is no teeth-gnashing competition between old and new: From front and center Pope's ionic portico with symmetrical granite wings is still the signature structure of the Baltimore Museum. The new building only comes into its own when you face it directly.

In style the addition is a circumspect sort of modernism with three distinctive elements: a two-story International Style curtain wall of tinted glass attached to, and set off by, a Corbusier-like "piano curve" wall of an ashen-color ceramic tile, with a book-end flat wall made of granite blocks from the same Tennessee quarry that provided the stones for Pope's building half a century ago.

A skillful composition, it is not, in itself, thoroughly satisfying. It does not possess the brainy intensity of I.M. Pei's East Building of the National Gallery of Art (another Pope temple-museum), for instance, but then I suppose that is the point, or part of it. Pei's building, separated from its parent by a vast urban plaza, attempts to grasp the geometric essentials of Pope's ideas and play them back in an abstract language of the strongest possible elegance. The Bower Lewis Thrower structure, attached to a less prepossessing parent, plays a similar game but in a minor key.

Modesty is the special virtue of this pleasant, if not inspiring, new building -- modesty and the fact that on the inside it works extremely well, providing two excellent entryways, a superb 400-seat auditorium, an enticing cafe' and several needed service rooms.

And then again the new structure is only part of the story, and perhaps not the most important part at that. The rest of the story is told in the superbly articulated new circulation system, the beautiful exhibition spaces, both new and old, and the art in them.

Opening exhibitions include a special show of four modern artists -- Grace Hartigan, Morris Louis, Clyfford Still and Anne Truitt -- whose lives and work have been intimately involved with Maryland; a new exposition of 19th- and 20th-century drawings and watercolors, culled from the permanent collection; renovated galleries for African, Oceanic, pre-Columbian and North American Indian Art, and a series of spacious galleries devoted, for the first time in the museum's history, to its permanent collection of 20th-century art.

The renovation and new construction was financed jointly by private donations, state and city appropriations and federal funds. To help the city defray some of the operating costs the museum for the first time has instituted an admission fee ($2 for those 19 and older), and it also has expanded its hours: Tuesday through Friday from 10 a.m. to 4 p.m.; Saturday and Sunday from 11 a.m. to 6 p.m., and Thursday evening from 6 to 10 p.m. Thursdays are free.