Washington has a sizable audience for contempory music, but only a smattering of them were at the Library of Congress last night for one of the most challenging programs of the season. The Contemporary Chamber Ensemble under the direction of Arthur Weisberg performed music by Joan Tower, Elliot Carter, Roberto Gerhard and Richard Wernick -- all of it music in a post-serial vein, highly intellectual and sufficiently complex to tax even the most devoted concentration.

The biggest work was Carter's "A Mirror on which to Dwell" written in 1976 for soprano and nine instruments on texts by Elizabeth Bishop.

Susan Belling gave an assured and well-focused reading of lines that were expressive but without a vestige of lyricism. The vocal patterns were woven entirely of intervals, the other dimensions of dynamic variation, and tone shading was left to the instruments. They, in turn, provided a background aura, a sensuous context for the poetry in agitated outbursts, languid clusters and soft instrumental moans.

Wernick's "Introits and Canons," written in 1977, utilizes familiar contrapuntal structures and some literal quotations from hymns and other sacred music sources.

Its episodes display a variety of intensities from the soft sounds of muted bells to a tough and aggressive blare.

"Breakfast Rhymes I and II" for clarinet and five instruments by Tower may have had a program suggested by its title, but if so, it was well disguised. It required everything that a clarinet could humanly be expected to do, and Anand Devendra did it well.

Gerhard's "Libra" for guitar and five instruments is a series of short excursions that seem never to be resolved or fully developed. The piece begins with a bang but ends indeterminantly. The guitar sound was a nice touch but a little light to compete with the rest of the ensemble.

Weisberg, who led his ensemble with convincing economy of gesture, provided the crowning tour de force in an evening of them by conducting one extended section of the Wernick piece in three beats to a measure with one hand and, simultaneously, five with the other.