Multicolumn indexes and ORDER BY

I have a multicolumn index on two columns. If i use the columns in ORDER BY
like this:
  ORDER BY col1, col2;

The index is used. But, if one column is sorted DESC it is not used:
  ORDER BY col1 DESC, col2;

How can i make this work ?

Regards,
Jernej Kos.
--
Kostko <kostko@jweb-network.net>
JWeb-Network