Sunday brings a frightful anniversary of an act of terrorism that served as a sober and sad reminder, a half century ago, of the limits of good will in America's long struggle to create a society worthy of its founding aspirations. It was 50 years ago that an explosion at the 16th Street Baptist Church in Birmingham, Ala., killed four black girls, three of them 14 and one 11.It took 14 years for a Klansman to be indicted for the murder of the Birmingham girls, with another member of the invisible empire convicted after the turn of the century, more than two decades later in 2001, and a third sentenced to life in prison only in 2002. The wheels of justice turned slowly, but the wheels of injustice moved with astonishing speed; only hours after the bombing, two black boys, one 13 and one 16, also were shot in Birmingham.Only days before the Birmingham incident, Martin Luther King Jr., had spoken of Alabama and its "vicious racists, with its governor having his lips dripping with the words of interposition and nullification," and indeed only a week before the bombing Gov.