Not many artists leave behind fabulously entertaining notebooks. That seems a lot to ask even of a genius. But Charles Chaplin did. He made his notes on film, and through some miracle, a good deal of the footage survived the cruel years. To see it now is to look into a great man's mind, and perhaps to go even further than that.

"This program consists almost entirely of film never before seen in public," the late James Mason says at the onset of "Unknown Chaplin," a three-part Thames Television production imported by PBS. It airs tonight and for the next two Mondays as part of the "American Masters" series, at 9 on Channel 26 and other public TV stations. Revelations don't come along every day in television, or anywhere else, but this program is one.

The first two-thirds are the best, starting with tonight's episode, "My Happiest Years," consisting of outtakes and filmed rehearsals from the two-reelers Chaplin made under contract ($670,000 a year!) to the Mutual Film Corp. in 1916 and 1917. Mason says the filmed notes constitute "a kind of archeology of the cinema"; certainly we get to see the archeology of a gag. Chaplin filmed his gags over and over until he got them the way he wanted them, which is to say, right.

Thus in rushes from "The Floorwalker" one can follow the evolution of jokes built around the presence on the set of an escalator, then a high-tech piece of equipment. Chaplin liked the idea of using it, and so he ordered the prop first and worried about the situations later. Brainstorms come and go as the numbers on the slate at the beginning of each take advance. Chaplin had limitless patience when it came to satisfying himself.

"Behind the Screen" was a movie about moviemaking, and Chaplin invented a bit of business involving an executioner and a huge falling ax that just barely misses, in take after take, Chaplin's toes. In fact, as we learn, the whole thing was shot backward so as to heighten the effect. There's an affecting sadness in Mason's voice when he says, "And after all this effort, the scene was cut and never shown."

Later, from another film, comes a scene involving a radiator and a Spanish dancer. After more than 100 takes, the Spanish dancer goes, the radiator goes, and Chaplin salvages wit from the remains. The way Chaplin worked was uniquely extravagant. He could get away with it because, among other reasons, he was, at the time, easily the most celebrated person in the world.

For all that, there's a dogged humility in the way he plods on and on and on until he achieves something sufficiently Chaplinesque (one of the slates reads, "Chas. Chaplin, #257"). Surely he tried the patience of those who worked with him, but you can sense that people tolerated much for the privilege of being around during these wildly productive times. To their credit, Kevin Brownlow and David Gill, the dedicated film historians who wrote and produced "Unknown Chaplin," didn't merely assemble the material; they gave it shape and purpose.

In Part 2, next week, some of those who worked with (and/or married) Chaplin in the '20s and '30s reminisce about the experience, supplementing clips from home movies and discarded bits from Chaplin features. Jackie Coogan, child costar of "The Kid," says Chaplin's array of ideas was held together "by a small gossamer web." Lita Grey Chaplin, one of Charlie's many wives, recalls the effect his daily arrivals would have at the new studio he had built for himself: "Someone would shout, 'He's here!' " Virginia Cherrill, the leading lady in "City Lights," says Chaplin told her, "I like to work my own way, and it's not the way anyone else works," and remembers that "he didn't care how many takes it took."

That becomes evident in rushes from "City Lights" that show Chaplin repeatedly changing and revising the scene in which The Little Fellow (Chaplin, of course) meets the blind flower girl (Cherrill). Chaplin fired Cherrill in the middle of filming, dissatisfied with her performance, and shot scenes with her replacement, Georgia Hale, costar of "The Gold Rush." Then he rehired Cherrill and finished the film. "I don't think Charlie really liked me very much," says Cherrill, ethereally pretty still.

Winston Churchill visits the set of one film, and Chaplin dances for him. Studio logs tell the story of the agonized delays in the filming of "City Lights": "Mr. Chaplin ill," it says, though one suspects he was home sitting out a very bad case of auteur's block. Dean Riesner, a child player in "The Pilgrim" who was related to Chaplin, remembers protesting on the set, "I don't want to hit Uncle Charlie," even though this was his role in the scene.

Part 2 is called "The Great Director." Part 3, "Hidden Treasures," doesn't seem as fascinating as the others. However, it does include such home-movie sights as Sir Harry Lauter's visit, in kilts, to Chaplin's studio, and a shot of Charlie cavorting at a party at Douglas Fairbanks' house, improvising a routine he would refine, later, for "The Great Dictator."

Among the illuminations of the creative process in "Unknown Chaplin" is that knowing what to exclude may be what separates the great from the ordinary. A viewing, in Part 3, of the seven-minute sequence that was to open "City Lights" proves mainly how correct Chaplin was in throwing it out. Until his later, bloated, self-indulgent talkies, Chaplin was a merciless editor of himself.

However distinctive and privileged Chaplin's filmmaking style was, the collected snippets of "Unknown Chaplin" make up an invaluable study in human creativity, not just Chaplin's creativity. Perhaps eventually, the leftovers and filmed memorabilia rushes of other directors -- say, Orson Welles -- will get the same treatment, but it's problematic whether the discards and ephemera of other filmmakers would be as revelatory as Chaplin's are.

There is some irony in Chaplin's being included as an "American" master. He was born in England and, in the early '50s, he was forbidden reentry into the United States after a trip abroad by a nervous and paranoid administration (the kind we have again now). And yet there is no question that in their ebullience and bravado and daring, Chaplin's films are distinctly American works.

Chaplin's reputation has been subject to radical fluctuations over the years -- for reasons other than his off-screen political and amorous adventures. "Unknown Chaplin" restores luster to that reputation by showing not only his work, but how he worked. And from that you get a sense of why he worked, and of what words like "artist" and "genius" really mean, or ought to mean.

Some would deny him such titles because his art was so entertaining, and appealed to those of all social strata. In "Moments With Chaplin," Lillian Ross recounts all the developments and crises of motion picture history -- from the founding of Hollywood to the tyranny of television -- and writes, "Through everything, Charlie Chaplin has persisted as a gigantic, incomparable figure." Virtually everything in "Unknown Chaplin" supports that view. He wasn't just funny, he was brilliant. He wasn't just brilliant, he was funny.