if (document.images) {
  image1on = new Image();
  image1on.src = "images/home_but1.gif";

  image2on = new Image();
  image2on.src = "images/about_but1.gif";

  image3on = new Image();
  image3on.src = "images/vision_but1.gif";

  image4on = new Image();
  image4on.src = "images/portfolio_but1.gif";

  image5on = new Image();
  image5on.src = "images/contact_but1.gif";
  
  image1off = new Image();
  image1off.src = "images/home_but.gif";

  image2off = new Image();
  image2off.src = "images/about_but.gif";

  image3off = new Image();
  image3off.src = "images/vision_but.gif";

  image4off = new Image();
  image4off.src = "images/portfolio_but.gif";

  image5off = new Image();
  image5off.src = "images/contact_but.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");
  }
}