Conductor Raymond Leppard brought out the best in the National Symphony Orchestra last night at the Kennedy Center, in a concert notable for its blend of variety and coherence, polish and vitality. In three works of the 20th century and one of the 18th, he displayed precise musicianship -- and, when an accident happened, a cool head.

A light bulb cracked loudly and fell to the stage near the climax of Ravel's "La Valse." It hit the floor near harpist Dotian Litton and tore the trousers of second violinist Desimont Alston on the rebound, but caused no injuries.

There was a brief flurry among the violins as the glass plummeted toward them, trailing smoke and sparks. But Leppard's beat remained steady -- at least, as steady as the beat should be when the musical peculiarities of the Viennese waltz are on the agenda.

The accident was premature; two minutes later, the music portrays the disintegration of a society, perhaps of the universe. But this miscue, plus a few moments of marginally square phrasing in Benjamin Britten's powerful, seldom-heard "Sinfonia da Requiem," were virtually the only problems.

Britten's work, composed in 1940, rings with the anger and anguish of a pacifist watching the world march into war. It is a technically brilliant, emotionally compelling score -- resonating powerfully at the beginning of the concert with "La Valse" at the end. In both works, Leppard and the orchestra sounded best when the music was most demanding.

The other two works were more joyful and, in their own way, no less brilliant: Ravel's Piano Concerto in G and Haydn's Symphony No. 104 ("London"). Bruno Leonardo Gelber, a pianist who should be heard in Washington more often, soloed superbly in the Ravel. The music's affinity to both Gershwin and Stravinsky was highlighted. But particularly in the limpid slow movement, Gelber also found and displayed its indebtedness to Mozart.

That association was probed in a "Prelude Concert" performance of Mozart's Clarinet Quintet by members of the orchestra. These enlightening chamber music preludes are beginning to find an audience and should be increased.

The Haydn symphony -- inventive, serene and beautifully proportioned -- received a finely polished reading.