Creating a mask with "uncover and stay visible" effect

Hello,

I've been looking for a solution of a problem which may seem pretty easy for you. I want to create a 'magnifying glass' effect (unconvering an image with a mask following the cursor) - the problem is, I'd like to leave already uncovered parts of the image visible.

It's like erasing the upper layer and uncovering the bottom layer. I'd be grateful if you could give me a hand. I've got Flash CS3 Professional

Image: http://www.fotodruk.com.pl/miller/

cheers