var ns6 = (!document.all && document.getElementById);
var ie4 = (document.all);
var ns4 = (document.layers);

function showLayer(layerName) {

       var layerImageName = layerName + "Image"
       if (ns6) {
              if ( document.getElementById(layerName).style.display == '' ) {
                     document.getElementById(layerName).style.display = 'none';
              } else {
                     document.getElementById(layerName).style.display = '';
              }
       } else if (ie4) {
              if ( document.all[layerName].style.display == '' ) {
                     document.all[layerName].style.display = 'none';
              } else {
                     document.all[layerName].style.display = '';
              }
       } else if (ns4) {
              if ( document.layers[layerName].display == '' ) {
                     document.layers[layerName].display == 'none';
              } else {
                     document.layers[layerName].display == ''; 
              }
       }
}