This radiant anecdotal history, of the legendary troupes that first popularized ballet for American audiences in the ’30s and ’40s, could very easily serve as a rebuke-by-example to today’s overflowing bandwagon of slavishly self-aggrandized documentaries—were it not, by nature, so modest and genial. Ballets Russes offers a surprisingly rare thing in contemporary nonfiction film: a portrait of an art form in the process of discovering itself and its audience. Plus, many of its most alert and reflective participants—all grins and shining eyes—are just great fun to be with. Even the proudest and most regal of the performers seem strongly disinclined to take themselves too seriously. Actually, it’s a sort of coup that filmmakers Dan Geller and Dayna Goldfine have so instinctively done the complex work of explaining how proto-hipsters of the avant-garde turned out to be adorable old-timers you’d want for grandparents.