The National Gallery of Art is finally finished. With the opening of eight new galleries, all main-floor gallery spaces have been delineated and refurbished.

Over the 45 years from the building's opening in 1941 to 1986, these eight spaces -- and indeed many others -- were not galleries at all; there wasn't enough art to fill all those galleries in 1941. Instead, they served as auxiliary support spaces for the museum growing up around them.

The transformation of the previously undeveloped West Building spaces into galleries is a fascinating story, revealing the constant growth and change of a major institution. The process is complex and time consuming and requires the work of exhibition designers, architects and engineers as well as curators and other museum staff. And it is a very expensive proposition.

Incongruous as it may seem in the context of the National Gallery of Art, in the 1940s these spaces were set aside as the staff's gymnasium and were outfitted with basketball courts. The museum administration took great pains to utilize all rooms so that space would not be taken from the museum and allocated to the war effort, according to gallery Director J. Carter Brown. Later, as the gallery's needs changed, the gymnasium was converted to office space.

Because architect John Russell Pope's galleries are 21 feet high, and today's offices can have ceilings as low as 8 or 9 feet, two floors of offices fit easily into the space. But the offices did not exist for long. As the gallery grew, so did its need for storage, and once again these spaces were transformed.

Finally, just in time to house the most recent exhibition of paintings, the old gym-storage-office area has been converted into spectacular but refined exhibition spaces, in the best National Gallery tradition.

The visitor more than simply sees these galleries. One hears them, feels their beat, senses their power and sometimes succumbs to it, slouching into the comfortable couches provided, just to relax and enjoy the space.

Andrew Mellon, when first shown plans for the gallery's interiors in 1937, replied: "I don't care how expensive they are, if they don't look expensive." In today's world of budget cuts, Brown can endorse only the latter phrase of Mellon's comment. "We do care about costs," he said. "The galleries were paid for by hard-won federal appropriations."

As in the gallery's infancy, a full-scale mock-up of one new gallery and its detailing was constructed. Although such stage sets are enormously expensive, they help avoid more costly errors by catching mistakes in the planning stages.

The gallery's design staff developed rough plans, but it quickly became apparent, said exhibition designer Elroy Quenroe, that they would need the aid of an architect. "We needed a firm which would carry out the spirit of architect John Russell Pope's design," Quenroe said. "The Vitteta Group/Studio Four of Philadelphia was selected because we liked their historic preservation work. They had sensitively restored Frank Furness' Pennsylvania Academy of the Fine Arts, as well as Philadelphia's City Hall," originally designed by architect John McArthur with help from Thomas U. Walter, the architect of the U.S. Capitol.

"We were trying to reclaim the exuberance of some of the main-floor galleries," explained Quenroe. "We felt that substantiality was lacking in the galleries now housing the 19th-century French Impressionist paintings, so we returned to other parts of the building for inspiration."

The careful observer can confirm Quenroe's observation. The galleries currently housing the permanent collection of 19th-century French painting seem flat and dry compared with the new galleries.

Enough details in these new galleries have been subtly changed to increase the play of light and shadow. For example, the cove cornice that joins the ceiling to the wall is deeper in these galleries than elsewhere, to accommodate the classic egg-and-dart motif. The profile of the moldings is accentuated and amplified, adding greater luxuriance to the overall effect.

Lighting effects seen elsewhere on the main floor were duplicated, but also updated. The lay lights (skylights) above each gallery were originally made of plain glass. Later, with increased knowledge about harmful ultraviolet rays, an ultraviolet filter was laid atop each lay light. Today's technology allows the ultraviolet filter to be laminated into the glass.

The roof had to be finished and skylit like the rest of the building. Again, new technology offered improvements. Instead of ripple glass strengthened by chicken wire, found in most of the building, the new glass is thermal, has no light-blocking chicken wire and is strengthened by lamination. Furthermore, by doing away with the chicken wire and by painting the attic above the lay lights white, to reflect light, the gallery can increase light levels from daylight, thus saving money on supplementary electric light over the lay lights.

However, such savings will probably be offset by an ongoing program to install track lighting in galleries where it does not now exist. The staff believes strongly that this lighting enhances the paintings by highlighting them, while the lay lights and daylight provide general ambiant lighting, so that one still perceives the changing cloud patterns and their effect on the paintings as light levels change in the room. The difference is immediately evident when galleries without track lighting are seen in comparison with those that have it.

One feature of the galleries is entirely new, but quite hidden. The door panels inside each gallery opening are now moving doors, not fixed panels. Now it is a simple task for the staff to close off a gallery: They simply close the double doors. Since the doors are paneled on each side, they look like any other gallery wall. This feature eventually will be duplicated throughout the gallery, thus eliminating the unsightly panels that sometimes appear in front of doorways, shutting the visitor out of certain spaces.

Although the public may perceive the galleries that house the museum's permanent collection as static, many small but significant changes are being made all the time. In designing these eight new galleries, the designers wanted to capture the best features of the other galleries and make modifications that would result in improvements. The wall paneling is one example.

When the galleries first opened, David Finley, the first director, and John Walker, the first curator, decided that the rooms should be paneled. The intent was to isolate each painting.

Today, paneling gallery walls is considered restrictive. When a gallery is paneled, moldings are applied to the wall as decorative divisions of the wall space. They can help to focus attention on a given work of art. However, they can also restrict the creativity of a curator or exhibition designer who may wish to put three or four works instead of two into the paneled space or shift a painting to the right or left but is unable to do so because of the molding. Therefore, in the eight new galleries a special molding technique has been used -- moldings stretch the entire length of any given wall, and do not divide the wall into small units. The moldings are carried from corner to corner, allowing greater flexibility in placing paintings.

Ironically, the curators were struggling during the latest installation to find the best placement of paintings on one of the largest walls. It seemed to this observer that the very freedom they had sought in removing restrictive paneling was causing problems; the wall seemed too large for the number and sizes of paintings to be hung. Paneling likely would have made the installation easier by giving the curators something against which to frame the paintings.

In his book "A Standard of Excellence," Finley describes one prominent visitor's reaction to the new building in 1941: She loved everything about the new National Gallery, especially the beautiful bare walls. "Couldn't you keep them this way and not clutter them up with pictures?" she is said to have asked.

It is easy to understand this reaction. The building is more than a subtle interplay of pragmatism and style; it transcends both to become a philosophic statement and esthetic experience in its own right.

Walker, who became the gallery's second director, states in his book that "the major purpose of the National Gallery of Art was to allow each painting, and piece of sculpture . . . to communicate to the spectator with as little interference as possible the enjoyment it was designed to give."

Strolling through the new galleries, one follows an uninterrupted continuum from the old gallery spaces to the new. So complete is their transformation and integration into the whole that few will be able to recognize where the old galleries end and the new ones begin.

Even the marble baseboard that extends throughout the building was quarried from the same place for the new galleries. Only the informed eye can spot where the match is imperfect.

Although every attempt was made to duplicate the older galleries' floors of random wide oak boards, this proved quite difficult because lumber companies were unable to supply such a floor. Finally, an expert in forestry and lumber was cajoled into taking on the task. (Two paintings by Caillebotte in the Impressionist exhibition show similar floors being scraped in the 19th century.)

One of the wonderful things about the rooms at the National Gallery is their expressive detailing, which conjures up the different eras of the paintings on display. These new galleries continue that tradition, expressing architectural continuity despite a 45-year span of time and many subtle design improvements. That is why the National Gallery not only is complete, but feels complete.