if (document.images) {

  image1on = new Image();image1on.src = "../img/navi/1b.gif";image1off = new Image();image1off.src ="../img/navi/1a.gif";
  image2on = new Image();image2on.src = "../img/navi/2b.gif";image2off = new Image();image2off.src ="../img/navi/2a.gif";
  image3on = new Image();image3on.src = "../img/navi/3b.gif";image3off = new Image();image3off.src ="../img/navi/3a.gif";
  image4on = new Image();image4on.src = "../img/navi/4b.gif";image4off = new Image();image4off.src ="../img/navi/4a.gif";
  image5on = new Image();image5on.src = "../img/navi/5b.gif";image5off = new Image();image5off.src ="../img/navi/5a.gif";
}
function turnOn(imageName) {
  if (document.images) {
    document[imageName].src = eval(imageName + "on.src");
  }
}

function turnOff(imageName) {
  if (document.images) {
    document[imageName].src = eval(imageName + "off.src");
  }
}