function turnOn(imageName) {
  if (document.images != null) {
      document[imageName].src = eval(imageName.substring(0, imageName.length - 1) + "on.src");
  }
}
function turnOff(imageName) {
  if (document.images != null) {
      document[imageName].src = eval(imageName.substring(0, imageName.length - 1) + "off.src");
  }
}