ATM Berry's Foucault tester

["Jamaal" <t0kx@unb.ca>]

This is about the Foucault tester in Richard Berry's (BYOT) book.  It seems 
that when you start looking at shadows, it matters which vertical 
position you place your eye at.  Move your eye up and down and the 
shadows seem to change.  I think it's because the "slit" of light 
coming back from the mirror doesn't exactly line up with the knife 
edge.

Has anyone else noticed this problem?  It's highly annoying.  What do 
you suggest?  Thanks!

Jamaal
NB Canada Earth