var isMinNS4 = (document.layers) ? 1 : 0;
var isMinIE4 = (document.all)    ? 1 : 0;
var isMinNS6 = (document.getElementById && !isMinIE4) ? 1 : 0;
// in netscape6, the style.left, style.top values are strings!!

function hideLayer(layer) {
    if (isMinNS6)
     layer.style.visibility = "hidden";
   else if (isMinNS4)
     layer.visibility = "hide";
   else if (isMinIE4)
     layer.style.visibility = "hidden";
}

function showLayer(layer) {
  if ( isMinNS6)
    layer.style.visibility = "visible";
  else  if (isMinNS4)
    layer.visibility = "show";
  else if (isMinIE4)
    layer.style.visibility = "visible";
}

var imageAt;

function updateScreen() {
  hideLayer(display_images[imageAt]);
  imageAt++;
  if (imageAt >= display_images.length)
    imageAt=0;
  showLayer(display_images[imageAt]); 
  setTimeout('updateScreen()', 7000); // delay before next update (ms?)
}