var BtName = new Array('ma','employ','principle','company');
var OnBt   = new Array();
var OffBt  = new Array();

for(i in BtName){
   var name = BtName[i];
   OnBt[name]  = new Image();
   OffBt[name] = new Image();
   OnBt[name].src  = '/img/bt_' + name + '_on.gif'; 
   OffBt[name].src = '/img/bt_' + name + '_off.gif';
}

function on_bt(name){
   if(load) document.images['bt_' + name].src = OnBt[name].src;
}
function off_bt(name){
   if(load) document.images['bt_' + name].src = OffBt[name].src;
}