var base = "images/";
if (document.images) {
        img11on = new Image();   img11on.src = base + "nav_obs_on.gif";
        img12on = new Image();   img12on.src = base + "nav_ext_on.gif";
        img13on = new Image();   img13on.src = base + "nav_eval_on.gif";
        img14on = new Image();   img14on.src = base + "nav_proc_on.gif";
        img15on = new Image();   img15on.src = base + "nav_dire_on.gif";

        // Inactive images
        img11off = new Image();   img11off.src = base + "nav_obs_off.gif";
        img12off = new Image();   img12off.src = base + "nav_ext_off.gif";
        img13off = new Image();   img13off.src = base + "nav_eval_off.gif";
        img14off = new Image();   img14off.src = base + "nav_proc_off.gif";
        img15off = new Image();   img15off.src = base + "nav_dire_off.gif";
        }
        
        function imgOn(imgName) {
            if (document.images) {
                document[imgName].src = eval(imgName + "on.src");       
            }
    }

    function imgOff(imgName) {
            if (document.images) {
                document[imgName].src = eval(imgName + "off.src");        
            }
    }