function showArrow(e) {
   if (e.layerY < (canvas_height/2)) {
        document.field_lyr.document.arrow_lyr.document.arrow_img.src = uparrow.src;  }
            else if (e.layerY > (canvas_height/2)) {
                document.field_lyr.document.arrow_lyr.document.arrow_img.src = dnarrow.src;   }
                document.field_lyr.document.arrow_lyr.visibility = "SHOW";
                document.field_lyr.document.arrow_lyr.moveTo(e.layerX-10, e.layerY+25);
                return true;        }