Last night opened with audience participation at Wolf Trap: a capacity crowd standing up and singing "Happy Birthday, dear Wolf Trap" under the firm direction of Kay Shouse, the guiding spirit of the place. Even the weather, which managed to dampen everything else, could not dampen the enthusiasm. Nor could the absence of Eubie Blake, who is considerably older than Wolf Trap and was prevented by illness from performing at its 10th birthday celebration.

Warned of the weather well ahead of time, the audience made the lawn a colorful sea of umbrellas -- but they filled it anyway, they stayed to the end and the weather improved after some initial tantrums. The only remaining problem was the music, and that remained a problem.

Minus Blake, the evening was basically a meeting of two violinists -- Yehudi Menuhin and Stephane Grappelli -- who had met before on records but had never performed together on stage. A few minutes of joint performance showed why it was probably a wise policy to collaborate only in the more controlled environment of a recording studio. Grappelli was right at home, doing his own thing and doing it with the same polish he has shown since the 1930s, making his bow dance through "Fascinatin' Rhythm" and dissolving the melody of "Honeysuckle Rose" into a shower of ornamentation. His violin was wired for sound, and he was fully at ease, in complete control. Menuhin seemed much less comfortable and not quite in the swing of things; his unamplified violin sounded tinny, and his taste for jazz improvisation seemed half-hearted until he got to a tune he could do in his own style -- Gershwin's "Summertime." With that, he did his best playing of the evening.

But jazz was only part of Menuhin's problem. In the first half of the program, with the Wolf Trap Chamber Players, he performed Vivaldi's "Four Seasons," which is very much his kind of music, with ragged phrasing and often unreliable pitch. If they had wanted merely a good performance rather than an international star name, the Wolf Trap management could have picked from several violinists closer to home -- Jody Gatwood, for example, or Miran Kojian, who gave a much better performance of the "Spring Concerto" last spring with the Alexandria Chamber Orchestra.But of course those names would not have filled the place.Apparently, judging by the applause, the capacity crowd got what it wanted.