It was an evening of Faure, Piston, Barber and Mozart at the Library of Congress last night. And most of all it was an evening of beauty, as baritone William Parker and pianist William Huckaby joined the Juilliard String Quartet in one of its loveliest concerts in memory.

Late in his life, Paul Verlaine wrote that the "finest song is a vague song, where indecision joins precision." It is this calculated vagueness that Gabriel Faure brought to his own setting of "La bonne chanson," nine youthful poems by Verlaine. The melancholy echoes of the music mirror the ambiguity of the gay poet's love for his wife, and in Faure's melodies the indeterminate feelings are no less true, the momentary passion no less eternal. Parker, Huckaby and the Juilliard were splendid in this evocation of the fragile happiness of love.

Parker's voice has grown to an autumnal color in the past seasons. It is warm and never heavy, and at times possessed of such gentle dynamics that it seems to float away in light pianissimi. If the voice still tends toward yawning when caught in downward passages, it is also much freer now; and the top has a fine bloom. His phrasing can be lazy but his diction is natural and unforced. "La bonne chanson" found him at his best.

Huckaby as always listened to the heartbeat of the score as he played. And the Juilliard strings embraced the voice from the first page of the score.

The same quartet whose individual voices rang out in Barber's"Dove Beach" then turned around and gave a lesson in classical purity with Mozart's String Quartet in B-Flat, K. 589. The concert, which is repeated tonight, also included an intense and ravishing performance of Piston's 1933 String Quartet No. 1.