Referencing HTML object in javascript

I'm trying to access an object in javascript in a content page, but I can't seem to do it. 
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<script type="text/javascript">
function OnClear()
{
document.SigPlus1.ClearTablet(); //Clears the signature, in case of error or mistake
}
</script>
<asp:Table ID="Table1" runat="server">
</asp:TableRow><asp:TableRow ID="TableRow4" Go to the complete details ...