I have YAHOO mail classic. How do I activate the "PREVIEW PANE" so I can scan an e-mails. without opening them?

I have YAHOO mail classic. How do I activate the "PREVIEW PANE" so I can scan an e-mails content without opening them. I remember this was an option but I can't seem to find out how to do it. If anyone can help please e-mail me. romiki63@yahoo.com