By the time Handel wrote his opera, "Semele," he was at the height of his powers. He filled the score with vivid picture-paintings of winged dragons, Grecian hills and lovers mourning for one another. Into it he also put some of his finest choral writing and solo arias of unsurpassed richness.

All of these Stephen Simon spread before his Kennedy Center audience yesterday afternoon, enjoying inspired, exciting singing from the chorus and singing of varying kinds from his large cast of solo artists. Through it all, the thread of Martin Isepp's harpsichord playing was a mosaic of glorious invention.

Every Handel opera is a reminder of the kind of singers and singing the composer knew and took for granted. Its arts were realized splendidly yesterday when Linda Mabbs sang Iris' first scene and stopped the show even when Simon indicated that he wanted to go right ahead. When opera audiences hear what they heard from Mabbs, they are not about to let a conductor overrule them.

Henry Price as Jupiter displayed fine agility and tone, but he tends to slide into notes more frequently than Handel permits. Mariana Paunova's Juno had the right weight in the upper range but was often barely audible in the lower octave.

Rita Shane sang the title role, handling the excellent ornamentation with ease and brilliance. Her role includes one of the opera's finest gems, "Oh sleep, why dost thou leave me?" While Shane sang this well, it lacked the long line and soft climax that make it wonderful.