On May 17, 1910, under the signatures of J.G. Cannon, speaker of the House of Representatives, and J.S. Sherman, vice president of the United States, a statute creating a permanent Commission of Fine Arts "to advise upon the location of statues, fountains, and monuments in the public squares, streets, and parks in the District of Columbia" became law.

Sherman signed the measure in his capacity as president of the Senate and as agent for President William Howard Taft, a strong supporter of the bill. Cannon's signature on the document is ironic -- he had fiercely opposed the legislation, just as, in the preceding years, he had fought one by one the recommendations of the 1902 Senate Park Commission Plan for the Improvement of the Park System of the District of Columbia, better known as the McMillan plan after its sponsor, Sen. James McMillan of Michigan.

"Uncle Joe" Cannon did not lose many fights during his powerhouse eight-year term as speaker, but for 75 years the city of Washington has been the fortunate inheritor of this major defeat. In its advisory role as arbiter of the McMillan plan and, by implication, of the original L'Enfant plan for the city, the Commission of Fine Arts has had an extraordinary hand in making Washington the rare and beautiful place it is.

Even though the scope of its authority has been expanded by presidential decree and congressional legislation in subsequent years, how a small body (the commission has seven appointed members and a professional staff of four) with purely advisory responsibilities has for three quarters of a century exercised such beneficent power remains intriguing. One large reason is the continuity of its leadership -- the commission has had but seven chairmen -- and its quality.

J. Carter Brown, the distinguished connoisseur (he's also director of the National Gallery of Art) with the fine eye for architecture and urban design, has held the chairmanship since 1971. When he stated recently that "Washington is quite simply the most beautiful city in the world," he was carrying on a notable tradition: Each of his predecessors has held similarly educated, impassioned beliefs concerning the uniqueness and importance of design to the reality and image of the capital city.

The first commission chairman was none other than Daniel H. Burnham, the Chicago architect who, with the assistance of architect Charles Follen McKim, landscape architect Frederick Law Olmsted Jr. and sculptor Augustus Saint-Gaudens, devised the McMillan plan. Burnham served officially for only two years, but for eight years before the official appointment he and McKim had acted as staunch guardians of the plan.

These formidable talents were, in effect, commissioners before there was a commission. The McMillan plan was of course much more than a treatise on the city's parks. It was an exemplary, broad vision that foresaw the rescue of the nobility of L'Enfant's concept from the destructive, predatory raids that had been conducted on it as the city grew during the 19th century, and it built upon the L'Enfant scheme in important ways.

The need for a commission to oversee the McMillan plan was apparent to its authors and supporters from the beginning. In 1903 the Agriculture Department proposed to build a new structure on the south side of the Mall near Independence Avenue and 12th Street. This was fine, except that the building was to be situated 300 feet from the center line of the Mall instead of the 400 feet envisioned in the McMillan plan.

Today, with the magnificent greensward and its bordering "panels" of elm trees in place, it is not hard to appreciate the wise foresight of the planners. At the time, when the Mall was a forest of trees and curving pathways, obstructed by several buildings, it wasn't so easy to see the difference a hundred feet can make. Burnham and McKim twice had to intercede to get the building placed correctly and at the proper height, each time taking the case all the way to President Theodore Roosevelt.

The bitterness of the struggle to maintain the plan can be gauged by a comment made years later, during the House debate on the bill to establish the commission, when an opponent testified that "a future place will never be hot enough to properly singe a man for the present Agriculture Department constructed as it is."

"Uncle Joe" Cannon was much less polite in a later fight over the location of the Lincoln Memorial. "So long as I live," he told Elihu Root, another strong supporter of the McMillan plan, "I'll never let a memorial to Abraham Lincoln be erected in that God damned swamp." He lost that battle, too. In one of its first acts the newly constituted Commission of Fine Arts argued persuasively for the isolated site in then-swampy Potomac Park.

These two crucial victories set the stage for the monumental core of the city as we know it today -- the one established the building line for the stretch of the Mall from the Capitol to 15th Street, and thereby permitted the elm trees to be planted in the 1930s. The other locked into place the western terminus of the spacious, symbolic ensemble that extends from the equestrian statue of Gen. Grant in the east, past the Washington Monument to the Lincoln Memorial.

The commission's great achievements in ensuing years, under the leadership of chairmen Daniel Chester French (1912-1915), Charles Moore (1915-1937), Gilmore D. Clarke (1937-1950), David E. Finley (1950-1963), William Walton 1963-1971) and Brown, have been accomplished with the same kind of high public spirit.

There have been notable defeats, of course, the most recent being the construction of high-rise buildings on the Virginia side of the Potomac and the building of the Washington Harbour complex on the Georgetown Waterfront, both of which are being done over the strenuous, principled objections of the Commission of Fine Arts. There have been famous architectural duds okayed by the commission -- the FBI building being the most remarkable of recent vintage -- and numerous fiascoes of lesser importance.

The degree to which a design review commission can affect architectural design for the better is of course a pertinent question. Such an agency can alter design, but it does not and cannot initiate it. Its powers in some ways are negative, as Brown stresses: "That's the fun of the Fine Arts Commission, the hidden part of the process that you never see -- all the half-baked proposals that don't get built."

But the commission's power to do good for the city has been far greater than this modest assessment implies. Over the years it has not proven inflexible in its judgments -- Finley and Walton, Brown's immediate predecessors, both effectively fought the demolition of the row houses surrounding Lafayette Park, which was a major feature of the McMillan plan. And Brown's handling of the bitter fight over the design of the Vietnam Veterans Memorial was indisputably sagacious.

With the exception of sculptor Frederick Hart, the creator of the "three soldiers" statue for the memorial who was appointed last month by President Reagan to serve a four-year term, the current commission members, including the brilliant Brown, are lame ducks. Hart was an excellent choice. One hopes that the president takes advantage of his opportunity to give the commission a superb 75th-birthday present by appointing (or reappointing) members of similar, or even greater, stature.

The Commission of Fine Arts deserves a great gift. For 75 years it has acted, in the apt accolade of Commission Secretary Charles Atherton, as the true "guardian of the quality of public spaces in this city."