function lightup(imgName)
 {
   if (document.images)
    {
      imgOn = eval(imgName + "_on.src");
      document[imgName].src = imgOn;
    }
 }

function turnoff(imgName)
 {
   if (document.images)
    {
      imgOff = eval(imgName + "_off.src");
      document[imgName].src = imgOff;
    }
 }