API-RP-576

In the above example the fringes
appear as a series of rings which
indicate the surface under test is
either concave or convex the
simple way to determine which, is
to apply finger pressure to the
center fringe, if fringes reduce in
number then the surface is concave
The two Slip Gages above are of
notionally the same length, both
show a series of fringes indicating
there is a variation in height in the
direction at right angles to the
fringes http://philipgee.com/OptFlt-01.html
Fion Zhang/ Charlie Chong