diapositivas = new Array("");
  j = 0;

  function mostrar(direccion) {
    tabla = document.getElementById("visor");
    total = diapositivas.length;
    centinela = total - 1;
    if (centinela < 0) {
    	codigo_html = "&nbsp;";
    } else {
    	j = j + direccion;
    	if ((j > centinela) || (j < 0)) {
      	if (j < 0) {
        	j = centinela;
      	} else {
        	j = 0;
      	}
    	}
    	codigo_html  = "<img id=\"foto\" src=\"" + diapositivas[j] + "\"";
    	codigo_html += "width=\"290\" height=\"230\">";
    	tabla.rows[1].cells[1].innerHTML = (j + 1) + "/" + total;
    }
    tabla.rows[0].cells[0].innerHTML = codigo_html;
  }