The Classical Quartet achieved the ideal in authenticity at its concert last night in the Smithsonian's Hall of Musical Instruments -- the level of musicianship proved so stisfying that thoughts of historical accuracy only occasionally flitted through the listener's mind. Music came first, and the means was, as it always should be, secondary.

Formed to re-create the performance conditions of the second half of the 18th century, the ensemble applies playing practices of the time to period instruments. In this particular concert they were using late-18th-century Italian instruments with classical bows, producing a sound somewhere between the shadowed antique quality of the Baroque and the even brilliance of the 19th century. This approach produced surprising shifts among the lines and textures in quartets of Mozart, Haydn and Boccherini, imparting a pleasing freshness to the familiar.

All four members of the ensemble displayed a keen interpretive sense and lively expressive spirit. Violinist Klinda Quan consistently brought an extra propulsive edge to her part as did cellist Fortunato Arico, laying to rest any questions about the possibilities of forceful projection on earlier instruments. Though less flamboyant in style, both Nancy Wilson, who alternated on first violin with Quan, and violist David Miller contributed sensitive interpretations to the program, which included a particularly infectious rendering of Haydn's joyous F-major Quartet, Op. 77, No. 2.