Great acting is difficult of definition: it has to be perceived to be understood. Boston audiences have a chance this week and next to see and appreciate-- a great actor and great acting in the largest sense of the terms, for John Gielgud is demonstrating beyond all doubt at the Plymouth the accomplishments and general finesse both of his own tradition and that of the entire English stage.

Gielgud has chosen for his first American season of comedy a play which has the peculiar double aspect of a period piece and also of a classic achievement in language and satire. The revival of two Wilde plays in the United States this season may betoken better treatment for this greatest of wits: if so, the American theater will be adding immeasurably to its richness.

Around himself in this production Gielgud has built a competent but not always sparkling supporting cast. In what is really the play's most significant role, that of lady Bracknell, Margaret Rutherford tries to create her own interpretation one of saga city rather than more overwhelming personality but she does not seem able to escape completely the characterization by Edith Evans, made famous in London and on records, and she does not, therefore, entirely succeed in her own.

Robert Flemming and Jane Baxter are more than perfect as Algy and Cecily, but the brave attempts of Pamela Brown, miscast in the part of Gwendolyn, are somewhat negated by her unfittingly low voice. The decor by Motley is more than suitable: it rivals the more famous work of Cecil Beaton on this year's "Lady Windermere's Fan."

Behind the play's success, however, is the performance and direction of Mr. Gielgud. Whether he is dressed in morning for his undeceased and indeed imaginary brother Earnest or merely struggling with the force of Lady Bracknell, Gielgud is worth an evening at the theater.