ATM -- Thickness Measurement

["Richard F.L.R. Snashall" <rflrs@javanet.com>]

I came up with the following method of measuring the thickness of a
lens.

       In the wedge tester, position the balls more accurately relative
to the
       center, so that the zone radius y is known.  Put a flat plate on
the
       tester and null the indicator, placed over the center of the
tester.
       Replace the flat plate with the lens, with a measured radius of
curvature
       on the side contacting the balls.

       The central thickness is then:

                  t = M - d - S

        where M is the indicator reading, d is the thickness of the flat
plate and
        S is the sagitta computed from the radius of curvature, the zone
radius,
        and the ball size.

Any problems with this method?  What source of errors would be
encountered?

                                          Rick S.