function checkFields(f) {
  if ((f.loginName.value == "") || (f.loginName.value <= 0))
  {
   alert("invullen login is verplicht");
   f.loginName.value = "";
   f.loginName.focus();
   return false;
  }
  if ((f.password.value == "") || (f.password.value <= 0))
  {
   alert("invullen wachtwoord is verplicht");
   f.password.value = "";
   f.password.focus();
   return false;
  }
 return true;
}