function  show(a){
  var b=a;
    switch(b){
   case 2: Layer2.style.visibility="visible"; 
 Layer3.style.visibility="hidden";
 Layer4.style.visibility="hidden"; Layer7.style.visibility="hidden";
   break; 
  case 3: Layer3.style.visibility="visible"; 
 Layer2.style.visibility="hidden";
 Layer4.style.visibility="hidden";
 Layer7.style.visibility="hidden";
   break; 
  case 4: Layer4.style.visibility="visible"; 
 Layer2.style.visibility="hidden";
 Layer3.style.visibility="hidden";
 Layer7.style.visibility="hidden";
   break; 
 case 5: Layer5.style.visibility="visible"; 
 Layer2.style.visibility="hidden";
 Layer3.style.visibility="hidden";
  Layer4.style.visibility="hidden";
 Layer7.style.visibility="hidden";
   break; 
   case 7: Layer7.style.visibility="visible"; 
 Layer2.style.visibility="hidden";
 Layer3.style.visibility="hidden";
 Layer4.style.visibility="hidden";
   break; 
  }
 }