File path from controller

I’m trying to load an .html file from my /public directory using Hpricot
in one of my controllers.

Ex: doc = Hpricot(open(’/template_1.htm’))

This can’t seem to find the file and it only works if I use the full,
absolute url which is unacceptable since there will be many different
sub-domains. How can I correctly reference a file in the /public
directory? Is there some type of root constant I can prepend to this?
Thanks.