var button1_off = "images/tab1.jpg";
   var button1_on = "images/tab1_over.jpg";

   var button2_off = "images/tab2.jpg";
   var button2_on = "images/tab2_over.jpg";

   var button3_off = "images/tab3.jpg";
   var button3_on = "images/tab3_over.jpg";
   
   var button4_off = "images/tab4.jpg";
   var button4_on = "images/tab4_over.jpg";



   function activate(imgName) {
      document.images[imgName].src = eval(imgName + "_on");
   }

   function deactivate(imgName) {
      document.images[imgName].src = eval(imgName + "_off");
   }
   function press(imgName) {
      document.images[imgName].src = eval(imgName + "_off");
   }