function popup(url,width_var,height_var) {
    if (!width_var) {
	width = 550;
    } else {
	width = width_var;
    }
    
    if (!height_var) {
	height = 350;
    } else {
	height = height_var;
    }
    
    
    var left   = (screen.width  - width)/2;
    var top    = (screen.height - height)/2;
    var params = 'width='+width+', height='+height;
    params += ', top='+top+', left='+left;
    params += ', directories=no';
    params += ', location=no';
    params += ', menubar=no';
    params += ', resizable=no';
    params += ', scrollbars=no';
    params += ', status=no';
    params += ', toolbar=no';
    newwin=window.open(url,'windowname5', params);
    if (window.focus) {newwin.focus()}
    return false;
}