var first_opac = 20;
var sec_opac =100;
var opac = first_opac;
var speed = 6;
var si;
var cleared = true;

function showimage(what,doshow) {

if (document.all) {
  cleared = false;
  (doshow) ? opac+=speed : opac-=speed;
  eval(what+".filters.alpha.opacity="+opac);
  if (opac <= first_opac || opac >= sec_opac) {
    clearInterval(si);
    cleared = true;
  }
}

if (document.layers) clearInterval(si);
}

function ClearInt() {
if (!cleared) clearInterval(si);
}