There is only a bare stage, the magic tricks are purposely corny, and the largest wild animal is made of chairs. The entire troupe, including costumes and apparatus, could fit into a clown car with room left over for a family of four. This is far too modest to be the greatest show on earth. How about something simpler: a one-ring wonder. The sweetest little circus this side of Barnum. Le Cirque Invisible has the wit and wonder of some half-remembered childhood reverie, as well as some of the contemporary sass of Penn and Teller. But Le Cirque is not quite invisible. To make it appear full-blown, in all its winsome glory, the audience must supplement the inventions of its two creator- performers, Victoria Chaplin and Jean Baptiste Thierree, with creativity of their own. It is an aerobic workout for the imagination.