function flip(imgName){
    theImg = document.images[imgName]; 
    srcStr = theImg.src;
    if (srcStr.search("_off") != -1){
	theImg.src = eval ("'"+srcStr.replace("_off","_on")+"'");
    }
    if (srcStr.search("_on") != -1){
	theImg.src = eval ("'"+srcStr.replace("_on","_off")+"'");
    }
  }