Opera was always supposed to be an equal mixture of words, singing and dance. Last night, opening its annual visit to the Kennedy Center, the New York City Opera married the three in a double bill that began with Purcell's opera "Dido and Aeneas" and closed with a new ballet on "Le Bourgeois Gentilhomme."

On the musical side in the opera, things were rarely more than adequate, which is not what Purcell deserves. Janice Hall as Belinda's chief attendant, was a model in all things: manner, singing, style, technique. Neither Sandra Browne as Dido nor David Holloway as Aeneas brought any real distinction to their roles. If you think of the singing of Tourel, Flagstad, Schwarzkopf and similar artists in these roles, you glimpse what they can and should be.

Cal Stewart Kellogg conducted stylishly, and had his chorus well in hand for the often very tricky passages. The echo chorus should be much, much softer and more distant. The harpsichord was excellent. Rosemarie Freni's undisciplined mezzo sounded good as the sorceress but Robert Sapolsky's Spirit was weak as was much of the rest of the singing.

Purcell's "Dido" could not exist without substantial amounts of dance. It was written around 1689 for the tender abilities of Josias Priest's School for Young Gentlewomen and its demands can be decently met by any good collection of choristers, dancers and leading singers.

These were all on hand last night, with the New York company's full resources in a production designed by Frank Corsaro, with choreography by Peter Martins and collaboration from George Balanchine on some of the pantomime. It is a happy paradox that such grand names are not inappropriate for Purcell's brief opera. For, while it is filled with amusingly absurd lines by Nahum Tate-"Our plot has took, the Queen's forsook!"-and while, for the most part, it moves through light or dark moods of no special depth, suddenly in its final moments it becomes truly great. For those moments, all the genius of today's theatrical arts is not too much.

Things were wildly mixed in last night's production, which is new to the company this season. One group of Dido's attendants, fitted out with long gold trumpets, would not be out of place at the chorus line at the Sands in Las Vegas-body suits, white beads and all.

The dance varied from aptly happy witches plotting the ruin of Dido ("the Queen of Carthage whom we hate") to less pointed work in more general scenes. CAPTION: Picture, Rudolf Nureyev in "Le Bourgeois Gentilhomme"