Last night's Wolf Trap concert by the National Symphony Orchestra was an unusual mixture of music and dance. Guest conductor Robert Irving, musical director of the New York City Ballet, presented a program of orchestral selections alternating with ballet excerpts, the latter danced by City Ballet principals Suzanne Farrell and Peter Martins.

Irving led the NSO in a vivid performance of Wagner's Overture to "Die Meistersinger von Nurnberg," Tchaikovsky's "Francesca da Rimini" and his own "Wits and Fancies," an orchestral suite concocted from a score he once wrote for a ballet.

Jerome Robbins' "The Afternoon of a Faun," shorn of its sets, is still a perfect miniature of the world of two dancers -- Martins languid and narcissistic, Farrell trance like -- whose only reality is their mirror.

The pas de deux from George Balanchine's "Chaconne" showed the two dancers as courtly and gracious partners; his "Tchaikovsky Pas de Deux" gave them a chance to cut loose. Farrell takes extraordinary chances, throwing herself into Martins' arms with nary a glance. Both were at their best in their solos: he tossing off the most difficult technical feats with incredible ease, his preparations effortless, his finishes rock-solid; she, dancing at daring speed, stretching each movement to the fullest, her endings intentionally off-balance.

It was a great performance by both dancers, but the evening belonged to Robert Irving, a conductor so sensitive to the needs of dancers that he can guide them with his back.