"Come and go as you must," invited the program for yesterday's performance at the Foundry United Methodist Church. But it added, parenthetically: "Waiting, of course, until the conclusion of a piece." The audience, following this advice, came and went discreetly through a long afternoon of organ music. But more stayed than left.

The occasion was a "Franck-a-Thon," a nonstop performance of the complete organ works of Ce'sar Franck, played in chronological order of publication -- the Six Pieces of 1860, followed by the Three Pieces of 1878 and the Three Chorals of 1890. Twelve organists from Washington, Baltimore and Richmond performed; all of those heard by this critic (who stayed for approximately the first two hours) were good, and Randall Mullin of Baltimore was utterly brilliant in the "Grande pie ce symphonique."

But the real heroes of the occasion were Ce'sar Franck and the church's Casavant organ, which is having a brilliant inaugural year. Franck, the founder of the French school of organ music, was the instrument's most significant composer of the 19th century and arguably its finest composer since Bach. For most of his life, the public respected him more as a performer than a composer. But his music has an imaginative flair, an energy and a brilliance and an essential good taste (crucial for such a powerful instrument) that have been the hallmarks of the French organ tradition ever since, beginning with his pupils Tournemire and Vierne and continuing through such composers as Messiaen and Durufle' in our own time.

Sharing the spotlight with the composer and performers, the new organ was exactly right for the occasion. Built by a French-Canadian company that began building organs at about the time Franck began playing them, this Casavant is solidly in the French tradition and an ideal instrument for this music. Light and quick in response, rich and brilliant in its tonal palette, it has a beautifully balanced, transparent sound that can impress by its sheer power without losing the essential clarity. It sounded like a subtly different instrument in the hands (and feet) of each organist heard yesterday, but it maintained a consistent personality throughout.