When he left journalism to become a novelist, Ward Just gave a candid explanation for his change in careers. "I ceased to believe that facts could lead me to the truth," he said, "and when that happens, when you lose a certain essential respect for fact, you are useless as a journalist."

Many of us in the news business, myself included, share Just's concern about the limitations of the craft. We know all too well how increasingly difficult it becomes to determine "truth" out of a mass of available facts. In television terms especially that essential respect for fact matters less and less. We live in the time of "docudramas," a dreadful genre in which fiction masquerades as fact: If the historical dialogue doesn't fit the prescribed plot, well, make up other lines for the supposedly real-life figures to speak. The play's the thing, a literal record be damned. You can even make a case that fictional techniques enhance the meaning of grander themes. "Roots" was bad history, in many ways as one-dimensional and filled with cardboard characters as "Gone With the Wind" had been a generation before. But it was an immeasurably important rendering of a compelling topic, and represented the facing of brutal facts for the first time by millions of white Americans.

As someone who clings to the illusions of the trade, even though I know better, i'm pleased to report that the presentation of facts still has an honorable role to play -- on paper and over the tube. Tonight, in the first of a two-part special (Channel 9 at 8 p.m., both tonight and tommorow) CBS shows that the art of the old-fashioned documentary -- as notably distinguished from the docudrama -- remains an effective and at times powerful journalistic form.

The program, "Blacks in America: With All Deliberate Speed?" examines what's happened to blacks in the 25 years since the Supreme Court's Brown vs. Board of Education ruling outlawed segregation in public schools. CBS wisely recognizes the impossibility of answering the range of questions implicit in so sweeping a subject and limits its focus to two areas -- the state of Mississippi and the city of Philadelphia. The result is an absorbing production, a genuine public service.

What's most impressive about this effort is it's modesty. CBS knows better than to give us some ringing pronouncement, strong on emotion and short on analysis. And Ed Bradley, the narrator-correspondent, proves to be almost the perfect sort of guide to such a complicated subject. He's quiet, thoughtful, serious and low-keyed and we share in his own discoveries as the camera follows him on his journey South, where he's had little experience, and then back to his native Philadelphia.

No great message emerges from these two hours of television journalism, and none should be expected unless you've been absent from America during the last quarter-century of racial strife and change. For blacks, progress continues to be measured in inches. Depending on where you perch on the sociological-economic scale, the last decades have brought more of the proverbial best and worst of times.

In the South, 2 million blacks have been added to the voting rolls since Brown, leading to 2,700 more elected black officials in that region. At the same time rural poverty forces people to live in the same shacks as in the past, the Klan marches and threatens, and the flight of hundreds of thousands of white pupils from public schools into private segregationist academies adds greater strains to the South's public-school system. In the North racial conditions are worse: Four out of five schools, in Philadelphia are segregated, a third of the students in some elementary schools never t go beyond sixth-grade reading levels, and many are functionally illiterate. The generally has declined, despite the spectacular increase in numbers of blacks going on to college -- three times as many across the country as 10 years ago.

But statistics are not the point, and certainly not the reason for television. If it's figures you want, go back and read Myrdal or his successors. Tonight's program gives us real people in ways only television, at its best, can capture.

My favorite is the scene involving Bradley and an old black woman in a doctor's office in Lexington, Miss. The doctor, in violation of Title Six of the Civil Rights Act, maintains segregated waiting rooms for his patients. For blacks, the rooms are dark, shabby, windowless and small. No lamps, tables or magazines help the patient to pass the time. For whites, the rooms are sunny and airy, with serviceable furniture and much reading material.

Bradley asks the woman why she doesn't go into the white waiting room. Long, painful pause. "Have they always had one waiting room for whites and other ones for blacks here?" he asks. Another long pause. "It's always been that way?" he continues, trying elicit a response. Finally, a quiet "Yes, sir" comes forth.

"No change at all" Bradley says. "Do you think that's right?"

Again, a pause, and: "No i'm afraid to say what's right. I just comes to the doctor."

Some of the scenes in the North tommorow night are even more arresting. But there's a nice balance to them all. They are not strident, they are not cheap, they are not definitive, and, happily, they don't intend to be. They are thoughtful and worth your attention.

I could have done without some of the predictable scenes of black politicians and the staged black housing protests in Philadelphia, and i'd like to have seen more of these real people expressing how they felt their lives had changed -- or hadn't -- since the legal segregation barriers fell. A sharper edge to the concluding points would have strengthened the program, too.

But these are not fatal flaws. High drama it isn't. As reality, though, it rings true. These days that's rare enough to celebrate, however confining the stage. CAPTION: Picture, Martin Luther King Jr. in 1965