ns4 = (document.layers)? true:false  
ie4 = (document.all)? true:false  


function MostrarCapa(capa){
  if(document.layers)document.layers[capa].visibility='show'     // NS
  if(document.all)document.all(capa).style.visibility='visible'  // IE
  document.getElementById(capa).style.visibility = 'visible'     // firefox
}

function OcultarCapa(capa){
  if(document.layers)document.layers[capa].visibility='hide'     //  NS
  if(document.all)document.all(capa).style.visibility='hidden'   //  IE
  document.getElementById(capa).style.visibility = 'hidden'      // firefox
}