This felicitous double bill served up by the highly regarded French period-instrument crusaders Les Arts Florissants places two Baroque masters of different nationalities side by side. Contemporaries Henry Purcell and Marc-Antoine Charpentier both made key contributions to late-17th-century music and eased the transition from modality to tonality, and both were half-forgotten for almost two centuries. Purcell's revival came sooner; he's now indisputably acknowledged as the greatest of all English composers. His only opera, Dido and Aeneas (1689), based on the doomed love between the Carthage queen and the Trojan prince, was exceptional in its time for its lyrical simplicity, delicate touch, and psychological intensity. Les Arts will perform the hour-long opera in its entirety, with Veronique Gens and Jerome Correas in the title roles. Charpentier, rediscovered as recently as 1945, is now regarded as a first-rank composer of the grand siecle, especially for his vivid sense of color in his use of harmony; Les Arts Florissants takes its name from one of his idylls. The group will give his pastorale Acteon (1695) its first Chicago performance; it tells the story of the Peeping Tom who's caught spying on the goddess Diana and is turned into a stag as punishment. (Interestingly, the tale of Actaeon is also a masque in Purcell's Dido.) The music in Acteon is expressive and enchanting, and thoroughly French in its attention to formality. William Christie conducts the ensemble in both works. Thursday, August 12, 8 PM, Murray Theatre, Ravinia Festival, Green Bay and Lake Cook Rds., Highland Park; 728-4642.