From: Mark Holm 
To: atm{at}shore.net
Reply-To: Mark Holm 


Over the years I have read a couple of articles by fellows who used water
as a reference surface to test, and thus figure, flats.  There are some
disadvantages: You have to be a bit fanatical about keeping dust off of the
water surface.  Water mounds up around a dust particle.  Water tends to
ripple in response to even tiny vibrations.  You aren't going to use this
test in an upper story apartment.  The vibration problem is lessened
somewhat as the water layer over the test piece thins down.  You adjust the
thickness of the water by letting water in or out of the pan.  The test
piece needs to be perfectly level,
but you have the interference fringes to use to measure level.  There have to be
fine threaded leveling screws on the setup, perhaps with some sort of
reducing levers to get the small amount of leveling travel needed.  I
recall there is some problem with the fact that the refractive index
difference between water and glass is relatively small.  It didn't seem to
be insurmountable.

Never actually tried it myself, but the fellows who have seemed reasonably
positive about it.

Mark Holm
mdholm{at}telerama.com

--- BBBS/NT v4.00 MP