On 2008-02-23, Christopher Tidy wrote:

[ ... ]

Thanks for those links. I took the reflector frame out of the microscope
today and measured it. Looks like the piece of glass needs to be 1" in
diameter and 1/64" thick, though a thinner piece could possibly be used.

According to some articles I've found online, certain metallurgical
microscopes use a plain glass sheet which works by total internal
reflection rather than a semi-silvered mirror. If that's the case with
my microscope, it would make the glass much easier to find. I might even
be able to use a circular glass cover slip.

The modern way of making semi-silvered mirrors is by vacuum
evaporation. You need a really good vacuum pump, some large diameter
stainless steel plumbing with flange couplings and knife-edges which
bite into copper washers -- replace the washer after every use.

Once you pump down to a low enough vacuum (the roughing pump
gets you most of the way, then you go to something like a "sorbtion"
pump or a turbo pump, or oil diffusion pump), then you put high current
through a filament around which is wrapped the metal which you wish to
deposit -- like perhaps aluminum wire. This boils off the metal, and
deposits it on the glass. You can control the thickness (and thus the
opaqueness) by timing, or the better way is with a crystal oscillator,
with one side of the crystal exposed adjacent to the glass being coated.
You can determine how thick the coating is by the change in frequency of
the oscillator from zeroing it before you start the plating.

Obviously -- this is a lot of stuff to do at home -- but you may
be able to find someone who retired from doing such work and is
continuing to play with it at home.

I don't know where you live, but it could be a possible choice.
Hmm ... if you are near someplace with a college which does Scientific
work, you might be able to find someone there to help you. Obviously --
cut the glass to shape *first*, and then coat it. The real trick is
coming up with a way to hold it down during plating without blocking
more than a bare minimum of the area -- say three tiny fingers just
barely sticking over the edge. And the whole fixture should be metal,
no plastics in the high vacuum.

Good Luck,
DoN.

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