WHEN ASKED HOW his new costume--a beaded, bangled confection--feels, the young Nijinsky replies, "It weighs a ton." So does Nijinsky. Agonizingly slow, the movie staggers under the feathers and furbelows of opulent sets and exotic locales. Ironically, the director who did so much to popularize ballet in The Turning Point is afraid to show too much of it here. He consistently cuts away from a dance sequence to a spectator, or shows only the dancer's face. In The Turning Point, Ross appeared determined to educate the general public in the beauties of ballet; in Nijinsky, he assumes everyone recognizes Nijinsky, Diaghilev and the Ballets Russes.

Early this century, the Ballets Russes toured Europe and performed dances that helped lay the foundation for modern ballet; a young George Ballanchine honed his craft with this troupe. Under the direction of Russian impresario Diaghilev, the Ballets Russes featured the avant-garde: music by Stravinsky and Debussy, sets by Picasso and Matisse, choreography by Fokine and Nijinsky. The film opens in 1912, with Nijinsky (George de la Pena) at the height of his distinguished dancing career, and beginning to design his own ballets, encouraged by mentor and lover Diaghilev (Alan Bates). But as Nijinsky's innovative ballets meet with hostile receptions, the relationship deteriorates. Convinced of Diaghilev's rejection, Nijinsky impulsively marries Romola (Leslie Brown), an infatuated heiress who has taken up ballet just to be near him. Diaghilev promptly dismisses Nijinsky from the Ballets Russes company, and the already unstable dancer goes mad.

The film waxes ambiguous: Does Diaghilev's action depend on Nijinsky's choreographic incompetence--is he dropped "for the good of the company," as the previous choreographer was? Oris Diaghilev merely prone to the pangs of lost love? Unfortunately, any depiction of the company proves incidental; Nijinsky fails to convey much sense of excitement, or even of the life-style, of the Ballets Russes. Ross and screenwriter Hugh Wheeler seem determined not to tell a story about people who dance, but a love story about people who just happen to dance.

IF SO, then why cast two American Ballet theatre artistes, Leslie Browne and George de le Pena, in leading roles that demand precious little dancing, but require substantial emoting? Browne has lost much of her nasal whine of The Turning Point, and handles the dramatics fairly well. But the role swamps de la Pena; he acts like a dancer, relying on exaggerated expressions and quivering limbs to convey emotion. He performs several of Nijinsky's most famous ballets, including Afternoon of a Faun and Le Spectre de la Rose, but we see all too little of his dancing; Ross focuses the photography in Faun, for example, mainly on de la Pena's face.

Irritatingly active during the ballets, the camera becomes curiously fixed on the stage after the ballets--a few cheers off-screen (so feeble they sound as if supplied by the film crew) indicates a warm ovation. The sole exception, and the sole excitement, occurs during a disastrous premiere, Shocked by the unorthodox choreography, a raucous audience tosses programs at the stage; as the catcalls drown out the music, Nijinsky shouts the beats to dancers from the wings, while Diaghilev tries to calm the crowd out front.

Bates offers a modicum of relief. His performance not only provides the film with something that moves, but also contains the proper amounts of overt prissiness and veiled menace. He projects a well-rounded portrayal of the man who opts to be "the old monster" to his troupe, declaring "If I listened to my heart, it would break." Without resorting to flaming mannerisms, Bates suggests perfectly the character's homosexuality; he touches women, even when affectionate, with a reserved disinterest. Admittedly, Diaghilev has all the good lines; chiding Nijinsky for eating too much candy, he warns, "Nobody loves a fat faun."

EVEN IF THE other two principals performed on Bates' level, they probably couldn't dispel the moribund mood that suffuses Nijinsky.. The photography in each scene is beautiful, but the pace drags from one opulent set to another. This procession of stupefying splendor may be deliberate--Nijinsky cries at one point that he's tired of the "endless dressing rooms, hotel rooms," and as his insanity increases, he babbles of a simple life on the farm. But this theme of Nijinsky's fatigue with a decadent life remains sketchy, and the script in general botches character development. After painstaking suggestions of Nijinsky's growing interest in the opposite sex--he asks Diaghilev to describe what sleeping with women is like, indulges in a lengthy kiss with a ballerina--the film presents his marriage to Romola as hysterical revenge on his mentor.

The movie has its moments--a cute line here, a nice touch there. Mostly, however, Nijinsky offers a series of stuffed Edwardian interiors with little passion to enliven them. Herbert Ross has done the unthinkable: made a film about dance that's heavy on its feet.