If you ever wondered why they invented spotlights, you can find out at Ford's Theatre: They were made for Barbara Cook to sing in.

Give her a ballad like "The Man I Love," caress her with a circle of twilight blue, and the rest of the world disappears. Or more accurately, Cook turns that isolated circle of light and song into a world unto itself. I know of no simpler recipe for theatrical magic.

The Broadway musical comedy veteran and cabaret star tackles considerably more in the revue entitled "Barbara Cook: A Broadway Evening," which opened a two-week engagement last night at Ford's. But nothing is so potent as the singer, looking svelter and prettier than she has in years, alone in the spotlight, making her extraordinary music. Her soprano voice, tinged perhaps with a little more velvet than in the past, remains a glory. Her warmth and sense of fun are undimmed. But her presence seems enriched by a new confidence, a new radiance. Those who, like myself, have been shameless fans for decades will find the reburnishing thrilling.

Most of the songs in the revue are already familiar items from her cabaret act or her various record albums: the tumultuous renditions of "Wait 'til You See Him" and "Carolina"; the jazzy "Foggy Day"; the beguiling "Change Partners and Dance"; that tremulous medley of "What'll I Do," "Remember" and "Come in From the Rain"; and, of course, "Sweet Georgia Brown," which invariably brings out the sassy southern vamp in her.

It's not the material but the format that's original -- or certainly trying to be. More than a concert, the evening aims to be, I suspect, a response to Lena Horne's triumphant Broadway outing. But right now, it's a wildly uneven affair, held together only by Cook's ebullience. Perhaps out of self-protection, Ford's is describing it as a "work in progress."

The chief addition is three female singers -- Nicholas, Glover and Wray -- who do close harmonies in the manner of the sisters Boswell, Andrew and deCastro. In Las Vegas, they'd be called a warm-up act and be dispensed with at the top of the show. Here, they get a spot to themselves in the second act for such musical eccentricities as "Crazy People" and "Bubbles."

Mostly, though, they function as a backup group for the star, provide the barbershop counterpoint to Cook's lilting reprise of "Dream" (from "The Music Man") and join her in a tap dance to "Broadway Rhythm," during which all four performers stay happily seated. You can see what director Thommie Walsh wants to do: surround Cook with support troops, add razzle to her dazzle. The thought may occur to you more than once, however, that Cook just doesn't need surrounding.

You can imagine her, perhaps, paired with another star of contrasting talents. (There was heady talk, once upon a time, of Cook and Tommy Tune joining forces in a revue.) But too often you wish that Nicholas, Glover and Wray would just slip away into the darkness. They are better heard than seen.

Nor do I see much point in the comic monologue with which Cook opens the second act. Just when she's looking so good, why dress her up like a garish Miami Beach widow in a rainbow-colored serape, dark glasses and fox furs and have her natter on about her dear departed husband, the aches and pains of old age, and her visits to the cemetery and the safety deposit vault? Even Lily Tomlin couldn't redeem this pointless character. Eventually the sketch leads into "What Did I Have That I Don't Have" (from "On a Clear Day"), but by then it's too late.

Cook is on much safer -- and ingratiating -- ground when she reminisces about growing up in Atlanta and running off to the Fox Theatre to catch the films of Ginger and Fred, Mickey and Judy. "I never wanted to be in the movies," she confesses. "I wanted to be in the shows that were in the movies." She always has had the gift of intimacy, and while she could use a writer to sharpen some of the autobiographical memories, she shares them with a great deal of charm. The joyful simplicity with which she talks about her pregnancy and the birth of her son (to whom she dedicates "Wait 'til You See Him") is disarming.

Sure, there can be dangers in too much self-confession (to wit, Peggy Lee's misguided one-woman Broadway show), but Cook's biographical asides dry up midway through the show. And her Broadway career, surely a source of rich material, is pretty much passed over, although the few strains of "Till There Was You" (from "The Music Man" again) that she does sing will make you catch your breath.

Cook's longtime accompanist, pianist Wally Harper, heads the three-piece combo and continues to provide the arrangements that emphasize the performer's range and her instinctive dramatic sense. He's also hatched a zippy novelty number, "Cold Hard Cash," which allows her, wrapped in a white feather boa, to play gold digger, which she does with abundant good humor.

Toward the end of the show, she wanders over to sit by him on the piano bench. The blue spotlight narrows in on her lovely face. And she sings. My how she sings! With such clarity of tone and emotion that you will forgive the mishaps that have cropped up before. This part of Barbara Cook's Broadway evening you'll want to go on forever.

Barbara Cook. A Broadway Evening. Directed by Thommie Walsh;l musical director, Wally Harper; associate director, Baayork Lee. Lighting, Richard Winkles; costumes, Dona Granata. With Barbara Cook, Julie Nickolas, Sheilah Glover; Willow Wray. At Ford's Theatre through Oct. 27.