The art of singing was radiantly manifested last night at the Library of Congress where Sir Peter Pears sang, both with chamber orchestra and entirely alone.

In an evening enriched by poetry from Shakespeare and Milton, Dryden and Wordsworth, to Keats, Coleridge, and Wilfrid Owen, Pears sang and read. His initial appearance was in the Nocturne which Benjamin Britten wrote for him in 1958. Each of its eight poems is accompanied by different combination of instruments, seven of them giving prominence to solosits on the harp, oboe, English horn, flute, clarinet, bassoon and timpani.

As in nearly all his major works, Britten fashioned the Nocturne so as to employ at their height the special gifts Pears has always brought to singing; his exquisite enunciation, expressive shading within phrases, his fluent way with notes in filigree and always his immaculate musicianship.

As he sang Coleridge's "The moon was bright, the air was free," and later Owens' "She sleeps on soft, last breaths," Sir Peter was giving a demonstration of the art of song that was a model for all singers. And this elegant singer is now approaching his 70th birthday!

After intermission, Pears put his distincitve feeling for words at the service of Milton and Dryden in famous poems about music. To these he added songs that reached back to Perotin in the 12th centruy and up to Percy Grainger's adaptation of English folksongs like "The Pretty Girl Milking A Cow" and "Once I Had A Sprig of Thyme."

Frequently sounding irrespressible, Sir Peter dropped his own touches of wit, as, when introducing the folksong, "Six Dukes Went A-fishing," he commented drily, "An unlikely situation."

Hearing him sing with no accompaniment whatsoever, his voice shaping long phrases, the ends of which died away in lustrous wisps of sound, you had to wonder who needed accompaniment. Earlier, however, the chamber orchestra conducted by Frederik Prausnitz had played the difficult and subtle Britten score with commendable effect.