EVERY PRESENTATION of full-fledged classical ballet is to some degree an exercise in anachronism. Performer and audience alike must willfully assume the aesthetic judgement of another age. Offered wholeheartedly, a classical performance works on us with the poignant clarity of emotional truth; done poorly, it ranges from the banal to the ridiculous. The problem is one of authenticity, and has less to do with execution of movement than with a state of mind.

Michel Fokine's "Les Sylphides," one of two works performed last weekend by the Boston Ballet, is itself a loving tribute to a style and spirit already of the past when Fokine created his choreography in 1908. He distilled the essence of Romantic ballet--a series of dreamy reveries suffused with moonlight and white-clad sylphs floating to the music of Chopin. We might be witnessing the animation of an 1840s watercolor, so fluent is Fokine in his chosen language.

The Boston Ballet offered its own tribute to Romanticism in the authentic perfection of slight gestures: the demurely downcast eyes, the drooping curve of the arms, a dancer's hand to her cheek like a sprig of flowers. The four soloists on opening night (Thursday) were a study in the company's uneven strength. David Brown floundered, as though classical technique were a suit of clothes three sizes too big, while Anamarie Sarazin wandered dutifully through a colorless waltz. But dainty Stephanie Moy, who has improved noticeably over the past couple of years, darted about deft as a hummingbird. And Elaine Bauer proved once again that she is the company's best female dancer, less by her limber elegance than by the harmony of each detail. For a moment her wrist, flickering like a tip of flame, became the point of focus framed by everything else onstage.

Yet Fokine's "Sylphides" is more than a brilliant evocation of a bygone era's art; it is a superbly-integrated work of art in its own right, a choreographic realization of the mood of lyric poetry. Even the form is that of lyric: a sequence of expressive meditations, a personal dream-world made vivid in the ephemeral moment. Like the shapes in a dreaming mind, the dancers echo a single identity. All save the one man are dressed exactly alike in flowering tulle, and their interaction is a matter of motion, not of differing feelings. No sexual tension develops in the male-female pas de deux.

Each gesture unfolds from within, like ripples from a stone cast on the water. The translation of feeling into movement is as effortless as the flow of breath, and as unbroken. For even the shape of the dance's impulse is that of subjective emotional experience; the rhythm moves as a seamless whole, each suspended pose not a break but a pulse-point, the peak of the wing-beat of a soaring bird. The realm of personal feeling is a continuum, and so are these forms: the body lines smoothed clean into curves, all weight belied by the tracery of pointe-work, every arabesque hovering at the edge of flight.

But if this choreography is as delicate as lacework, it is every bit as intricate. Fokine uses the ensemble dancers--the corps de ballet--with deft economy of rich imagination. At different times they function both as a choral commentary on the soloists, mirroring the angle of a ballerina's body, and as architecture, a fluid linkage of arches and trelliswork. The Boston Ballet's corps rose to the occasion, offering Fokine's masterpiece with devotion and care.

It was perhaps the authenticity in their presentation of "Les Sylphides" which made the evening's other offering, "Cinderella," such a disappointment by contrast. To dance a fairy-tale well, one must make it believable. Yet this "Cinderella" by resident choreographer Ron Cunningham distances the audience from empathy and belief. He sets up a series of cardboard figures, and proceeds to comment on them.

The ballet is basically a good-will-be-rewarded morality tale, and the characters are conceived on this level. There are the ugly stepsisters (David Drummond and Larry Robertson), cavorting with bovine vulgarity, the shrewish stepmother (Elaine Bauer), and Cinderella herself (Laura Young), a painfully angelic victim. We can't be expected to take these people seriously, and Cunningham doesn't either. Large chunks of the ballet are given over to slapstick--the stepsisters squabble tug-of-war fashion over a shawl, or trip over each other to greet the Prince (Woytek Lowski). The liveliest moments are high comedy having nothing to do with ballet, and the work becomes difficult to approach on any other level. The happily-ever-after final tableau, signaled by the descent of a festooned crown from the ceiling, is unintentionally hilarious.

The humor is one kind of comment offered here, and the predominance of narrative dance is another. The problem is that almost all the movement is figurative, a vehicle of the story rather than the other way round. There are lengthy sequences of mime, and even the symbolism of some of the dance patterns comes close to the verbally explicit. You can't mistake the Fairy of Autumn when she sweeps her arms like a scythe.

Worse, the actual dancing is largely incidental to both story and spectacle. Like the ballet interludes in a 19th-century opera, dance merely embroiders diverting decorations. Dancers dance only when one would expect the characters to do so--Cinderella daydreaming with her broomstick, or the guests waltzing at the royal ball--and the content of the movement is a trite and anonymous classical pattern. Cunningham's choreographic vocabulary is limited, ignoring both the music (a beguiling Prokofiev score) and any place of space outside the lateral extension of center stage.

The climactic love scene at the ball is a case in point. It doggedly explores the mechanical possibilities of lift-and-stretch, put-down-and-release, back and forth. It is a rationally conceived design rather than an organic development informed by dramatic feeling. Laura Young's eloquent face is an incongruity here; her body's range of emotion is simplified to a perfunctory embrace or two, and authentic feeling is wasted in this brash cartoon anyway.

Ironically, Nicolas Pacana's Jester spun out the ballet's only truly arresting dance--capering, cartwheeling, bursting up in a jangle of lines knocked askew. As a clown, the Jester can spoof and comment like the ballet itself, exercising the only mode of feeling and moving, which is neither blunted nor ridiculed.

FOR AT ITS BEST, this is good zesty comedy, and it should not pretend to aspire to the emotional delicacy and technical subtlety of the great classical tradition. Dressed up in bright crayon colors, the ample scenery chunky as a child's blocks, "Cinderella" paints a tongue-in-cheek fable in a collection of surfaces; as spectacle charming, but trivial as art.