DANCE happens so fast that even during a performance it's hard to be sure you just saw what you think you saw. People who watched Nijinsky always had this problem. He would leap up and pause in the air. He could become whatever role he was performing so completely that people swore he changed his size and shape.What is left when the dancing stops? We will never have the first moment Nijinsky leaped offstage, disappearing as he rose in the air. But we do have a glorious paper trail left by his company, the Ballets Russes de Serge Diaghilev: sketches, photographs, letters, reviews, ticket stubs, angry surrealist manifestos handed out at one concert, and lovingly autographed photos given at stage doors to favored fans, among them Howard D. Rothschild, a young man who almost saw Nijinsky. It was 1916. Rothschild, then a young boy, was taken to see the Ballets Russes during its first tour of the United States. Nijinsky's performance was cancelled. Rothschild never got over missing Nijinsky, and a collector was born. He recently left his collection to the Harvard Theatre Collection. Some items from the bequest, exhibited there this summer, appear on this page. For every step a dancer in Diaghilev's Ballets Russes took onstage, there was a wealth of dancing the audience didn't see. It was not just the ballet classes at the Imperial Ballet School in Saint Petersburg and the sessions with choreographers like Fokine, Massine, Nijinsky, Nijinska, and Balanchine. It was the dance of history, a to-ing and fro-ing of ideas that, thanks to Rothschild, can now be retraced more fully. During the 20 years when the Ballets Russes performed - from 1909 to 1929, and almost everywhere but in Russia - there was a prodigious gathering of artists behind the scenes as costumes, scenery, choreography, and music came together. "Diaghilev was a creative genius, although he neither composed nor painted, nor made choreography," writes Richard Buckle in his book "Diaghilev." His gift was for assembling artists, getting them to collaborate, and transforming the results into a spectacle. The sketch for a stage design for the ballet "Le Bal" painted in 1929 by Giorgio di Chirico gives us a window into the process. Boris Kochno, a young poet who had left Russia and joined up with the ballet company in Paris, choreographed "Le Bal." Richard Buckle quotes Kochno's description: "It was a poetic episode set in the Romantic period, having a mysterious and unearthly character which few people appreciated at the time." Di Chirico painted with appropriate mystery. The cloud, the wing on top of the columns, and the lines that could be mountains, set a scene that is full of possibilities. The architectural elements are classical, but the pillars lean. Are they becoming a Romantic ruin before our eyes or taking off? The wing is frisky but poignant. It is as if the picture were a visual pun: "Time flies!" Perhaps some historical action has passed this place by, and the scene waits to be filled with dancers who will, in thei r turn, disappear when the dance ends. Di Chirico was only one of the artists Diaghilev mustered. Picasso, Matisse, Bakst, Derain, Braque, and Utrillo also designed sets and costumes for the ballets. And they didn't always stay in the background. The Russian painter Mikhail Larionov sketched Diaghilev with Stravinsky and Prokofiev as casually as one would if such a constellation of talent was just part of the landscape. It was. Larionov became the ballet's artistic director. As the drawing suggests, artists rubbed elbows and so did their idea s. Larionov didn't just stick to scene-painting and costume designs. As Kochno learned to choreograph, Larionov would guide him, telling him which elements, according to his design, should be central in a particular ballet. IT is hard to imagine any time in history riper for a new amalgamation of the arts than the period when Stravinsky and others were experimenting with dissonance; Surrealism, Constructivism, and Dadaism were making painters into revolutionaries; and Isadora Duncan was blasting dance off the bedrock of 19th-century ballet technique. But it is also hard to imagine how much would have happened if Diaghilev had not brought the Russian dancers to Paris with their elegant schooling, a repertoire that was a trea sure-trove of the great classics, and an appetite for invention. They were a hit. When Tamara Karsavina and Nijinsky appeared in 1909, the company's courier recalled, "I have never seen such a public. You would have thought their seats were on fire." Karsavina may be one of the dancers kicking up their heels at what looks like an intermission break on the balcony of Paris's Thtre du Chatelet in the photograph. She was an international star, dancing for Diaghilev while under contract to the czar's company, the Maryinsky. Once, she was to perform with Diaghilev's compan y in Dresden, Germany, but she was also due to dance in St. Petersburg. Neither Diaghilev nor the Maryinsky would let her off. Karsavina recalled in her autobiography, "Theatre Street," that in Dresden, "I rushed off the stage immediately after the performance, threw a shawl over my ringlet wig, a coat over my Egyptian dress, and arrived at the station just in time to catch the last train... . The express sped on till, on the second day, when a snowdrift caused a delay, the train was six hours late. From the station, I hurried to the theater. My understudy had been got ready, but there still remained 10 minutes of interval. When , in the costume of Sugar Plum Fairy, I came on the stage, the overture had finished and the curtain was going up." That was in 1912. It was a time when only a snowdrift lay in the way of a talented young woman moving between "Cleoptre," cooked up by Diaghilev for the Parisians, and "The Nutcracker." She had the best of both worlds: The Russian classic danced in the theater it was made for and a modern work to show off in in the West. She might have considered that particular train trip the opposite of artistic freedom. But even today, as the barriers that went up soon thereafter are dissolving, Karsavina's voyag e is tantalizing for its disregard of artistic and political boundaries. It is the kind of freedom usually only available to historians, who can follow an idea wherever it goes. The Harvard Theatre Collection must be the starting point of many such voyages. Curator Jeanne Newlin says that while it is impossible to put an actual dance in mothballs to be studied later, the collection documents the "ripples," the "cultural response" to those fleeting events. The dancing of Diaghilev's Ballets Russes is over, but its ripples lap at our time. The way we enjoy dance today - present ed on an equal footing with music and the visual arts, and always ripe for experiment - is a survival of the Ballets Russes' artistic outburst. And though we, like Rothschild, have missed Nijinsky's leap, the knowledge of it lifts our hope and sharpens our eyes as we look to see who may leap next.