To: "COLJA, Mike" 
From: mdholm{at}telerama.com
Cc: "'Mark Holm'" , david_margrave{at}yahoo.com,
   atm{at}shore.net
Reply-To: mdholm{at}telerama.com


Quoting "COLJA, Mike" :

> Would it be possible to use a mercury flat?
> Mike Colja
>
Probably, BUT I think there would be some serious problems.

1.  Water has a big advantage, it is transparent.  The test surface can be
submerged.  Light passes through the water and reflects from the surface
under test.  Since the water bouys up the test piece, the problem of
supporting the test piece to prevent sag is reduced, and the test piece can
be supported from behind with a flotation system if required.  Mercury (Hg)
is opaque.  The test piece will have to be transparent.  The test piece
will further have to be suspended a very small, and finely adjustable,
distance above the Hg.  It will not be possible to support the test piece
except by the edge.  This will limit the test to rather small apertures.

2.  Hg is toxic and has a fairly high vapor pressure.  This means that Hg
vapor is in the air wherever Hg is in use.  Breathing the vapor is a well
know route of toxic exposure.  Hg is easy to spill and difficult to clean
up. (Personal experience.)  This makes it very likely that any place Hg is
used will become contaminated with small amounts of Hg that will give off
vapor for years. Hg is also expensive, and rather hard to keep clean.
(Personal experience.) Also, hard to dispose of in an environmentally
responsible manner.

3. Hg is rather highly reflective.  Glass is not.  The inequality of
reflections is supposed to make the diffraction patterns rather weak.
Actually I think I recall reading that this is something of a problem with
the water test too, only if you are testing a metallized surface, but there
is a fairly straightforward fix as I recall. I think it is putting a small
amount of food coloring in the water to absorb some of the light.

Many people have had the experience of playing with small amounts of Hg.
Although this is fascinating, and a great introduction to the mysteries of
material science, it is not a good idea given what we know now about Hg
toxicity.  It can get into the body by skin absorbtion, inhalation of vapor
and by contamination of food by small amounts on the skin or surfaces.  All
in all, it is best to forgo the educational benefit to avoid the hazards.

I have the S&T and ATMJ references on water flat testing at home.  I
will try to post them to the list this evening.

Mark Holm
mdholm{at}telerama.com

--- BBBS/NT v4.00 MP