login on the “read more”

Hi,

I was wondering if it is possible to put a login on the “Read more” link in a post. What i mean is, a vistitor can read the first few lines on a post, if he wants to read more, he/she needs to login to read the rest of the entry. Is this possible? And where can I find more information on that subject. I have searched the plugin page, but there I can only find topics on the main login page.
Anyone?

Thanks a lot!