A white horse and its rider disappear in midair at your left only to reappear seconds later at your right.

An air bubble, inches in diameter, floats down from a starry sky to enlarge, enlarge, enlarge until it bursts, and there stands a full-sized man in flowing medieval robes.

From hunks of metal, a green-eyed monster materializes.

Those are some of the marvels concocted by Doug Henning for "Merlin," a wonder-filled musical spectacular, the sort that grandpa might have seen in a fabled, long-gone Hippodrome at 44th and Sixth Avenue.

Forget all the fuss about "Merlin"--the $4 million cost and the many delays in the opening at the Mark Hellinger. Forget its battle to hold off New York's critics, who revolted at the eight weeks of previews and reviewed it ahead of its official opening Feb. 13.

"Merlin" is merely a toy, glittering as a Christmas bauble, trivial as a paper valentine and no more substantial. But while you're watching Henning's tricks you are sucked into his carefree cosmos.

Henning, who achieved a five-season run for a far simpler little stage offering, "The Magic Show," is here paying tribute to the magician's patron saint, Merlin, King Arthur's goodly guide. This is pre-Camelot, before Arthur learned to fly, though Merlin shows how.

Writers Richard Levinson and William Link aren't in the T.H. White league nor do they seem set on reaching the shining heights of White's masterpiece of fancy on the same subject, "The Once and Future King." They have created a sharp antagonist, a wicked queen worthy of Chita Rivera's whimsical talents. Energetic, high-kicking Rivera enjoys four big production numbers. In one, she becomes six clones of herself. In another, she creates the green-eyed monster. Altogether, Rivera is a welcome fiend to have around, an outrageously formidable foe for Merlin and the Good Guys.

Henning, who looks quite like a rabbit, plays Merlin in his prime. There are two other Merlins, of youth and age, and they are all guided by a nameless tutor, known only as the wizard, played with glaring authority by Edmund Lyndeck.

Henning's magic, alas, can't cover everything, including his inability to sing, dance or act, but that little matters.

More important is the failure of a pivotal character, the queen's reluctant son who is being pushed toward Arthur's throne, to come to life either in the writing or Nathan Lane's indecisive acting. This lapse causes two full production numbers to go down the drain.

Nor, alas, is it possible to find kind words for the score by Elmer Bernstein. His overture warns of what little is to follow. It's all ruffles and flourishes. What begin as possible melodies disappear into thin air like that white horse. And Don Black has not determined what style his lyrics are to be--satiric, serious, romantic, what?

Robin Wagner's sets are a flashily lean suggestion of pre-Arthurian mystery, making clever use of the black all magicians require. The Theoni Aldredge costumes are puckishly outsized, and Tharon Musser's lighting aids in the magic of the whole.

Four organizations have banded resources to produce this by now legendary labor. How the bookkeepers will tot it all up eventually need not concern us. The bottom line for theatergoers is that Doug Henning's expensive wonders can be a pack of fun.