Rich Text Box

Question

Hi All Can anyone help?? I'm developing an app which has a vertically scrollable rich text box. What I want to know is how to find out the range of lines which are currently displayed. i.e. if the text is too large to fit into the box and the user has scrolled down the page somewhat to view some latter text, how do I find out which line number of the text is the one that appears at the top (or bottom) of the box. Yours hopefully Nick James