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Alex-
 I read this and did some thinking over it:
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I think the "translation" of this, and the crux of the front/rear silvering
issue, is that it really does not matter if the mirror is silvered on the
front or rear. What matters, is that the silvered surface is on the axis of
rotation, or whatever you want to call that particular plane where the
silvering "should" be.

Given a mirror that is installed with the rear side of the mirror sitting
"in" the mirror holder, the sextant manufacturer can easily design that
holder to place the silvering in the right position. But given a
front-silvered mirror, and an unknown thickness of glass, the manufacturer
cannot ensure this and loses control of the positioning. Unless, the
manufacturer were to use a front-surface mirror AND a corresponding mirror
holder DESIGNED to place the front surface, instead of the rear surface, at
the correct plane in the axis of rotation.

Without having access to lots of sextants to research this  I can only
speculate aloud. But the issue of front/vs/rear surfacing, per se, just
makes no sense. And if my thinking is right on this, it does make no
sense--until you add in the context of the mirror holder, and whether/how
the maker has designed the mirror holder and the instrument as a whole to be
used with front or rear silvered glass.

Does that sound logical?

   

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