Tonight and tomorrow night at the Hirshhorn (and for free, no less), you can take a delightful trip back to an America that operated on "the principle that nothing was impossible." "The World of Tomorrow," Tom Johnson and Lance Bird's nostalgic but unsentimental documentary of the 1939 World's Fair, vividly and often humorously evokes that time when America was poised between the past of the Depression and the future of world war.

The World's Fair of 1939 was conceived as boosterism, a way to bring in tourists and jobs and advertise New York City. But it became much more than that -- a showcase for modernist architecture and "scientific" planning, featuring a model city called "Democracity" and an ideal world called "Futurama." Built on a vast ash-heap in Flushing, the fair was a sort of model city of its own. Even the streets were color-coded: Designed like the spokes of a wheel, each had a different color, and they darkened from pastels to deep colors as they fanned out from the center.

Reading a narration by John Crowley, Jason Robards tells the story with gentle irony. The fair was dominated by the globe and obelisk at its center, "only it wasn't a globe -- it was a perisphere," and "it wasn't a pyramid -- it was a trylon." The movie is filled with the campy pleasures of America's cheery, inflated faith in progress: You get to see "the torture test for tires, featuring Jimmy Lynch and his Death Dodgers," and "Electro, the Moto-Man, moving up the ramp on his own power," and a dishwashing contest between Mrs. Modern (armed with dishwasher) and Mrs. Drudge that evokes the classic old "Honeymooners" routine, "the Chef of the Future."

Johnson and Bird orchestrate their found footage and newsreels around a now-hilarious movie-within-a-movie, a film made by Westinghouse to promote the fair, in which a young Midwestern wastrel travels to New York and learns that "prosperity and pessimism don't mix." And an animated sequence by Max Fleischer, also created to promote the fair, almost steals the show. It is here, when the narration falls silent, that "The World of Tomorrow," with its cameos of Ethel Merman (braying "It's the dawn of a new day!"), Fiorello La Guardia, Franklin Roosevelt, Mickey Rooney, Judy Garland and Henry Ford, actually transports you to another era, an effect that can happen only in the theater (the movie aired this past Monday on PBS).

In the opening scenes, the administrators sink a time capsule into the Flushing turf; filled with microfilms of Mickey Mouse, messages from Thomas Mann and Albert Einstein, a pack of Camels, a golf tee and a Kewpie doll, the capsule was intended for the people of 6939. Only a half-century later, "The World of Tomorrow" shows us a civilization that in many ways already seems as strange to us as the ancient Hittites. "The World of Tomorrow" is unrated, but contains no nudity, violence or profanity.