The viola -- the middle voice of the string family -- has a relatively small repertoire, not nearly as spectacular or well-known as those of the violin or cello. So it is a pleasure to report that a significant addition to that repertoire, "Evocation No. III" for viola and piano, had its first Washington hearing Saturday afternoon at the Library of Congress -- and even more of a pleasure to note that the composer is Ralph Shapey, a Schoenberg disciple whose work has been more noted for thoroughness and complexity than for charm or easy listening.

"Evocation" explores a variety of styles within a basically atonal idiom, before reaching a resolution in a gentle, romantic melody, almost an elegy. It was commissioned last year by the Naumburg Foundation as part of its award to violist Thomas Riebl, who gave it a splendid performance at the Library with pianist Susan Tomes.

The program also included Schubert's "Arpeggione" Sonata, Schumann's "Ma rchenbilder" ("Scenes From Fairy Tales") and the Sonata, Op. 11 No. 4, of Hindemith, who played the viola and knew how to make it sing. Throughout, Riebl showed a high level of technical finesse, a tone of pure gold and a fine musical intelligence, all of which were well-matched by his partner at the piano. In a dazzling encore, he proved that he could become the James Galway of the viola, but apparently he prefers to be a musician.