var cislo = '1';
var id,
    mp = 0.3, 
    op = 30,    
    step = 5;

function Start()
{
    id = window.setInterval("Move()", 100);
}

function Stop()
{
    window.clearInterval(id);
}

function Move()
{
    op += step;
    if (op < 30){
      op = 30;
      step = -step;
    }
    if (op > 100){
      op = 100;
      step = -step;
    }
    mp = op/100;
    
    if (document.getElementById("save_kc") && document.getElementById("save_kc").style.MozOpacity) //if Firefox/ NS6+
      document.getElementById("save_kc").style.MozOpacity = mp;
    else document.getElementById("save_kc").filters.Alpha.opacity = op;
}