if (document.images) {
  image1on = new Image();
  image1on.src = "graphics/gramophone_a.jpg";

  image2on = new Image();
  image2on.src = "graphics/radio_a.jpg";

  image3on = new Image();
  image3on.src = "graphics/projector_a.jpg";

  image4on = new Image();
  image4on.src = "graphics/typewriter_a.jpg";

  image5on = new Image();
  image5on.src = "graphics/microphone_a.jpg";

  image6on = new Image();
  image6on.src = "graphics/book_sm2a.gif";

  image1off = new Image();
  image1off.src = "graphics/gramophone.gif";

  image2off = new Image();
  image2off.src = "graphics/radio.gif";

  image3off = new Image();
  image3off.src = "graphics/projector.gif";

  image4off = new Image();
  image4off.src = "graphics/typewriter.gif";

  image5off = new Image();
  image5off.src = "graphics/microphone.gif";

  image6off = new Image();
  image6off.src = "graphics/book_sm2.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");
  }
}