The rapture of young ballerina Heather's first opening night--charmingly conveyed through touches of young-girlishness. We see Heather tugging her mother; see that, as her mother says, ""The house lights are just going on."" A glowingly colonnaded hall appears, on that frosty night, through a clump of dark trees (all more magic-Russian than American). Heather leaves a package for her ""favorite ballerina,"" Laura--leg-warmers she's knitted, we later learn. And she and fellow""bug"" Libby prance about, playing off against one another (the book's strongest feature). In the pictures, Heather is transformed from baggy-pants sprite to stately young maiden on point. Then the bugs, clustered backstage, make their entrance; Heather has her solo, the cast bows. And when last seen in mufflers, Libby and Heather are heading homeward with rose bouquets: ""See you tomorrow, bug."" ""Good night, bug."" Reverberant, but not overdramatized.