Roger Mudd is an exceptional news writer whose delivery on the air enhances the impact of what he is saying -- hence, he is the consummate broadcast journalist, if hardly the most personable. Tonight Mudd anchors his first documentary since joining NBC News, a "White Paper" called "Reagan: The First Hundred Days," at 9 Channel 4.

After Mudd says that the first act of the Reagan administration has been " a period unlike any other in almost 50 years" in terms of rolling up sleeves and trying to change the direction of the country, highlights from it flash by on a briskly effective electronic desk calendar. The assassination attempt on the president is recalled in still photos (sparing us the sight of another slow-motion replay); because of that incident, the "White Paper," originally to be 90 minutes, was extended to two full hours.

Mudd mades the rounds of NBC correspondents in a style too similar to the "Nightly News." Judy Woodruff at the White House does her stand-up from Lafayette Park. Diplomatic correspondent Marvin Kalb does his a short time later from the Ellipse. Pentagon correspondent George Lewis stands in front of the Pentagon. And so on.

There is, however, an all-star lineup of administration officials interviewed for the report (the interview with Reagan himself was called off on doctor's orders). Reagan's three top advisers -- Michael Deaver, James Baker and Ed Meese -- talk about the day of the shooting and about Reagan's presidential mind-set.

Woodruff calls him "a sheltered president" by nature, one who delegates authority readily and prefers for himself what Baker calls "The Big Picture," by which he does not mean "Heaven's Gate."

The matter of Secretary of State Alexander (I-am-in-control-here") Haig gets considerable time on the report. America has a "herky-jerky foreign policy," Mudd says, because of the confusions and politicking that surround Haig and his elusive role. Haig himself is interviewed; he looks rather sad beneath the surface, like a character who thinks he is about to be written out of a play. Henry Kissinger says of him, "I suspect he wants to be president."

The two hours are mercifully short on gimmicks and gadgetry. One attempt at visual punctuation, animated cartoons by Pulitzer Prize-winner Mike Peters, comes so close to working that it passes for a good idea.

Although Reagan could not be interviewed, he keeps popping up genially throughout the report, in candid White House footage and in public ceremonial situations taped prior to the shooting. He has the invaluable capacity, expecially rare for a head of state, of provoking a smile at every appearance, and for all the misgivings some people have about his programs, he seems to have become personally beloved in record time. Roger Mudd says, "More than any other president since Kennedy, he has been able to laugh at himself without looking stricken or grim or furious."

This is terribly important to a society that gets most of its impressions and much of its morale from television. Between the lines of this thorough and pungent report, there's a melancholy sense of temporal loss. What the assailant's bullet has taken from the assailant's bullet has taken from us, for most of the past month, is the ingratiating style of a reassuring presence. We wait now for television to give that back to us again.

All of NBC's prime-time schedule tonight is given over to news broadcasts; the evening begins at 8 on Channel 4 with the recently relocated "NBC Magazine with David Binkley," formerly the lamb slaughtered weekly by the CBS Friday-night smash "Dallas."

The first edition, however, contains a report that is disturbing in more ways than were probably intended. Reporter Jack Perkins visits a Syrna, Ga., ad agency specializing in young and precocious teen-age models -- "Lolitas," as a German magazine calls them. It evolves that the photographer taking the pictures, and acclaiming the girls for their "innocent sexiness," was convicted in another county on three counts of child molestation and was released on five years' probation.

Perkins and cameria trot about to the mothers of girls being photographed and break the news to them. "How could I let it happen?!" one sobs. Then Perkins goes to the photographer -- who is unaware that NBC has gotten the goods on him -- and sashays from an innocuous interview into confrontation about the convictions.

The photographer, with camera trained on him, gets up from his desk, leaves his office and then runs down the street, with Perkins and crew in breathless pursuit. Perkins ends the report with a tsk-tsk to law enforcement agencies for allowing the photographer to set up shop again. But the tactics used by NBC News seem cruel, almost sadistic, and in the end viewers may end up feeling sorrier for the child molester than for anybody else.

It's really a local story, not of national significance, and no attempt is made within the report itself (though something may be added to the final air version) to set it in any kind of context. It isn't suggested that the photographer is merely America's current infatuation with baby fat taken to the logical, if abhorrent, extreme. Society is let off the hook; the photographer gets the video equivalent of the stocks and the pillories.

"This isn't fair to me, is it?" he had asked Perkins. Maybe Brinkley will tackle that question on the air tonight.