Disable txt field

1

i need to disable selected txt fields


<html>
<head>
<script type="text/javascript">
function disableField(element_id))){
  document.getElementById(element_id))).disabled=true
 }
</script>
</head>
<body>

<form>
s <input type="text" id="startWhen" value="" onclick="disableField(hours)"/>
f <input type="text" id="finishWhen" value="" onclick="disableField("hours")"/>
-or- hr <input type="text" id="hours" value="" onclick="disableField("startWhen","finishWhen")"/>
</form>

</body>
</html>