Soprano Amy Burton made her Washington Opera debut at the Kennedy Center Friday night in the role of Emmeline, which she will sing in this season's five remaining performances of Purcell's "King Arthur." She replaces Sylvia McNair, who had one of the best voices in the show (it is not really an opera), but patrons with tickets for the last half of the run need not feel slighted.

Burton is more than a promising young singer; in the past five years, with more than a dozen opera companies from San Diego to Zurich, she has been fulfilling promises in such light soprano roles as Gilda ("Rigoletto"), Zerlina ("Don Giovanni") and Adele ("Die Fledermaus"). Vocally, the role of Emmeline fits well into that range, which Burton clearly finds comfortable; dramatically, she is the only one in "King Arthur" who evokes any real pathos -- if you can swallow the plot.

She is a blind princess and in love with Arthur, who is struggling with Oswald, the Saxon king, for her hand and the rule of England. The wicked sorcerer Guillamar (ostensibly allied with Oswald, but really out for himself) abducts her and, in league with the loathsome demon Grimbald, attempts to win her love -- by displays of magic if possible, by force if necessary.

This is not really much to build on, but Burton has moments of true eloquence. Her voice sounds small, though not too small for the Eisenhower Theater; the tone is light, the diction generally clear and the emotional expression effective.

Burton is a substantial asset to a show that still evokes mixed reactions but has clearly become more tightly coordinated and richer in texture during the first half of its run.