Who can forget Dick Cavett, wading ashore with MacArthur in the Philippines, or Dick Cavett, merrily pouncing around on the moon with American astronauts, or Dick Cavett, peering out the window of The Spirit of St. Louis as it taxied down a runway?

Of course it really happened. Would television lie? As a matter of fact, it would, but sometimes it lies in extremely entertaining ways. This is the case with "Time Was," as series of six one-hour specials on decades from the '20s through the '70s that premieres tomorrow not on any of the three networks but on Home Box Offices, Time Inc.'s hugely successful pay TV network.

From now through mid-December, viewers in 3 million HBO households will have several chances to catch the first show of the series, "The Twenties," on the 1,200 cable and microwave pay TV systems that receive HBO programming. In the Washington area, that includes Arlington's cable TV system and, come December, Marquee Television, which beams pay TV programming by microwave to 12,000 subscribers, most of them in suburban high-rise apartments.

"Time Was," is not only a topflight tour through days gone by, but a breakthrough for TV documentaries besides. It crosses a threshold you can actually see on the screen, because producer Bruce Cohn used a relatively new video technique that allows host Cavett to saunter into scenes from the past as if he were being physically projected back in time.

In the program on the '20s, Cavett hops up on a Model T assembly line and peeks into the window of a car, walks a tightrope stretched across a city light that has long since changed and, perhaps most impressively, strolls on stage in a Harlem night club, tap dances briefly with the chorus line walks down a few stairs and proceeds to mingle with the revelers. They are probably all dead by now.

The process that makes it possible to defy time and trick the eye is a sophisticated version of Chroma-Key, a gizmo used for such simple superimpositions as say, a weatherman against a map or radar screen, or the Taj Mahal if a director's in the mood. HBO calls the new refinement shadow Chroma-Key, because the superimposed figure casts lifelike Shadows against the background and in fact interacts with the picture to such a degree that it no longer looks like a background.

The figure becomes part of the other picture, and can interact with people or objects in it, instead of just being pasted on to its surface. This is literally and artistically a new dimension for television and it opens up a world of imaginative, impudent and provocative possibilities.

"It actually draws the viewer in," says Cohn, who first used the technique trade name, Ultra-Mat -- for "A Crack in Time," an ABC News documentary about 1968. "The show's not just a history lesson any more. It intensifies the whole experience of seeing a piece of historical film."

Cohn and staff went through thousands of miles of historical film to compile the footage, much of it rare, for the series. At one point, 10 film editors were working simultaneously on the six decades. "It was incredible roaming the halls," says Cohn, "because from one room you'd hear 'Oh,d the humanity!" -- the Hindenburg disastger, and from another MacArthur saying "I have returned," and from another Nixon's 'I am not a crook.'"

Yes, "Time Was" is time warped, but gracefully and felicitously.

There are 60 separate Cavett inserts done up in Ultra-Mat, and they took seven 10-hour days to put on tape. In the '50s, Cavett will be able to appear to hand perspiring gangster Frank Costello a handkerchief at the Kefauver hearings.

We asked Cohn to explain how the thing works technically.

"I don't know how the thing works technically," he said. "The engineers just go crazy when you start to play with it. They say, 'Why don't you try this?' and 'Why don't you try that?'" Eventually, he says, it will be possible to subtract one figure from an old piece of film and replace it with another. Perhaps you will one day be able to cast yourself as Scarlett O'Hara in "Gone with the Wind" or, as an overweight TV critic suggested to Cohn, dance The Carioca with Ginger Rogers.

That is indeed feasible, said Cohn, so long as the overweight TV critic was able to duplicate precisely the original movements made by Fred Astaire. Thus is the technology willing but the flesh a flop.

"Time Was" represents HBO's most substantial original programming effort yet. The system has made most of its money off recent movies shown without cuts or commercials. One problem, though, is that a large number of recent movies that were not worth going out for aren't worth staying home for, either. m

HBO's November offerings include such Seconal cinema as "Ashanti," "Ice Castles," "The Magic of Lassie," "Tommy," "Magic," and "Pocket Money," along with more glittering prizes like "Agatha," "Last Embrace" and "Enforcer." HBO sells itself to customers as "Something Else," but in July its programming included Universal's laughing-stock, schlock "The Seekers," which was made to be shown with and not without commercials and aired on free TV this week.

"Time Was" compares with but can't come close to equalling such elaborate backward glances as NBC's splendid "Project XX" documentaries, a project long abandoned but fondly remembered. Cohn obviously didn't have network resources, and his budget was probably under $2 million for all six shows.

Among the amenities done without is an original musical score. Though the soundtrack for the '20s show includes more authentic tunes of the period, much of the score was lifted from a single out-of-print record album, Jack Shaindlin's "50 Years of Movie Music."

Also, during the sequence on hoodlumism in Chicago, a brief shot from a theatrical feature is passed off as a piece of newsreel footage. At this point the program becomes Time Wasn't. Basically, though, it has the superficial appeal and aura of authority that -- appropriately enough -- characterized Life Magazine during its lifetime as a weekly.

There is the distinct danger that HBO will merely develop into a source of alternative television but of alternative mediocrity. Cohn says dealing with the networks (he has worked for NPACT here as well as ABC) but then he notes parentheticlly that he and HBO executives are still "fighting" over what the closing theme music on the show will be.

If HBO executives are going to be meddlesome little woodpeckers, then HBO will differ from the commercial networks only because of the four-letter words and bared breasts in its R-rated movies, and because you have to pay for it.

For now, though, "Time Was" qualifies as a good idea done well, pop history of a very palatable sort. The ghosts and echos in "The Twenties" are strikingly evocative, from Gershwin at the piano to Bessie Smith at the bar to Lindbergh at the controls to Babe Ruth at the bat.

The special effects make it truly unusual and arresting, starting with the moment near the show's opening when Cavett fades from color to black-and-white and from now into then. Too seldom is there cause to look at a TV picture and wonder How-They-Did-That; "Time Was" offers that opportunity to be entranced and bedazzled. Visually and nostalgically, it is a clear-cut case of sheer delight.