function check(f_name) {
    //veld-namen
    var fields = new Array()
      fields[0] = "naam"
      fields[1] = "telefoonnummer"
   //veld-naam in de melding
    var fieldnames = new Array()
      fieldnames[0] = "naam"
      fieldnames[1] = "telefoon nummer"
    res = true;
    var msg = '';
    for (i in fieldnames) {
      if (eval('document.Form1.' + fields[i] + '.value') == "") {
        eval('document.Form1.' + fields[i] + '.focus()');
        res = false;
        msg = ("U dient uw " + fieldnames[i] + " in te voeren");
        break;
      }
    }
    if (msg != '') {
      window.alert(msg);
    }
    return res;
  }