THERE ARE only two ways to get very excited about the possibilities of television: look into its past or look into its future. It is the present that's a bore. With a new videotape produced for home use by CBS News, it's possible to look both ways at the same time.

The tape is part of the home-video revolution that is changing the nature of television and promises, for the future, greatly expanded alternatives to the weary networks and clattering local stations that dominate the system now. But the subject of the tape is from television of nearly two decades ago: the presidency of John F. Kennedy, and the sorrowful way it ended.

The tape, first in the CBS News Collectors Series (next release: a history of the Iranian hostage crisis), is called "Great Figures in History: John F. Kennedy," and consists of edited-down excerpts from past CBS News specials produced by Leslie Midgley and Don "60 Minutes" Hewitt. The tape was dramatic and transporting enough when released a few weeks ago; since then, with the attempt on President Reagan's life, it has become additionally fascinating to watch.

Whatever materializes on a television screen always appears to be happening "now," so that to watch the "Four Dark Days" segment of the tape is to relive the anguish and shock of November 1963, when President Kennedy was assassinated in Dallas. You still find yourself thinking, "This couldn't have happened. This can't be happening."

Technically, the news of President Kennedy's death was given us secondhand; the motorcade in Dallas was not carried live on any network, and pictures were not immediately available. Mini-cam technology was not what it is today, so there were no near-instant tapes available from the scene of the crime comparable to those of the shooting of President Reagan. (For all the wonders of '80s TV technology, NBC News was last on the air with the Reagan tapes because its Washington offices are located farther from the scene of the shooting than ABC's and CBS's are.)

The news of Walter Cronkite reacting to the Kennedy tragedy came firsthand, however, because he was talking directly to us.

Soon after a "Bulletin" in which Cronkite's voice is heard saying the president has been shot, Cronkite appears on camera in his shirt sleeves. He is giving the latest news on the situation in Dallas when someone hands him a piece of paper and he stops in mid-sentence and puts on his glasses to read it.

Clearly stunned by what he has read, Cronkite looks into the camera. "From Dallas, Texas, the flash, apparently official: President Kennedy died at 1 p.m. Central Standard Time, 2 o'clock Eastern Time," he says, glancing up at a clock on the wall, "some 38 minutes ago." His voice cracks and he is silent. Something so horrible and momentous has happened that, just for a moment, not even Walter Cronkite knows what to say. An expression crosses his face as the bearer of bad tidings has become too much for him. It is still a moving, alarming thing to see.

(Eighteen years later, March 30, 1981, on CBS News, Dan Rather reporting: "Everyone's conern, and rightly so, is for the president's condition. Now we said earlier that the president was reported to have been unscathed and unhurt. What you have had in the last minute, in the last few seconds, is a change, and that Lyn Nofziger, who is the presidential assistant at the White House, is quoted as saying that the president was struck or at least grazed -- we don't know the extent of it -- in the area of the chest").

The Kennedy tape, marketed through the MGM-CBS video arrangement, begins with a segment called "The Presidential Years," devoted to a totally unsentimental look at Kennedy's 1,000 days in the White House and concentrating almost exclusively on foreign affairs. New Frontier domestic policies are largely ignored. We see Kennedy tell Walter Cronkite, on the subject of growing American military involvement in Vietnam, "I don't agree with those who say we should withdraw. That would be a great mistake."

The material in the first section comes from a CBS News report on the Kennedy presidency that aired in 1973. "Four Dark Days" was culled from a news special produced on the first anniversary of the assassination. The final segment is an extract from "Rose Kennedy Remembers," which aired in 1968.

In that segment, Mrs. Kennedy recalls how she told the president to keep his hands out of his coat pockets when he appeared on television so that his suits wouldn'tg look rumpled. And she remembers JFK asking her to stop writing requests to world leaders for autographed pictures of themselves.

The tape is also a history of CBS News. The correspondents heard include both Roger Mudd, who covered part of the funeral procession and last year joined NBC News, and the man who beat him out for the CBS anchor chair, Dan Rather, who overdramatizes in the style of young reporters, which he was; this was one of the ambitious Texan's first big stories. John Hart, now with NBC, narrates the presidency segment ("In those days, he said, and we believed, there wasn't anything we could not do," Hart says of Kennedy). Harry Reasoner -- who later left for ABC News, and the perils of being teamed with Barbara Walters, then returned to CBS -- interviewed Mrs. Kennedy.

Also heard are Robert Pierpoint, Charles Collingwood and the late Wells Church. And the voice from Arlington National Cemetery belongs to William Leonard, no longer a reporter but president of CBS News. A young and thin Sander Vanocur, then of NBC News, rises at a press conference to ask Kennedy about the "lambasting" America was taking around the world over the Bay of Pigs fiasco.

Pat Shevlin, associate producer of the tape, says it took six weeks of rummaging through CBS News archives (in such remote locales of Fort Lee, N.J.) to assemble the material. The assassination footage was on early, "archaic" videotape, she says, that had to be tinkered with electronically in order to transfer it to today's technology. CBS does not have tape of the shooting of Lee Harvey Oswald by Jack Ruby in its files, so a still picture is used.

Kennedy was our last black-and-white president; the network news departments all went to full color by 1965. There are only two color segments in the tape; one is the Rose Kennedy interview, the other some candid, touching film of the president playfully feeding a pony on a farm in Virginia. It was the Sunday before he went to Dallas.

Other planned tapes in the CBS News series include a detailed chronicle of the Vietnam war. But the Kennedy tape may have the broadest appeal. In the future, our scrapbooks will be electronic, and they will include such national shared moments as those preserved here.

Shevlin says she was a freshman in high school when Kennedy died and can remember, like many of us, being stationed before the television set for that long, terrible weekend. "It was the beginning of growing up for me, as I remember," she says, and she found herself getting "choked up" even after seeing the tape replayed many times.

"I think," she says of the Kennedy tape, "people will want to have it forever." How often would one play a tape like this if it were added to a household's electronic library? Perhaps only once in a great while, perhaps just once or twice for the children, so they can see what it was like. It's more than a sober, if futile, history lesson about violence in America; the tape brings back proud times, too -- a myth still terribly attractive many years, and several national traumas, later.

In effect, the promise of Edward R. Murrow's brilliant "See It Now" is upheld by such projects. See it and smile, see it and weep, see it and remember.