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A few mimute ago, I wrote-

"Two correspondents (Doug Royer, Gary Harkins), have informed us that the
simple test for perpendicularity of the index mirror of a sextant (by
checking that the arc aligns with its reflection) will work only if the
mirror is back-silvered.

They have given no references or reasons for the failure of this method
with a front-silvered mirror.

I have been racking my brains to imagine what could cause either of the
methods that were described to be any better or worse depending on which
surface the mirror was silvered. Just because I can't think of such a
reason, that doesn't inply that there isn't one.

So I ask: is that claim actually true? Has anyone tried it out with a
front-silvered mirror? Did it fail, and what were the symptoms? Has this
failure been explained, or even mentioned, in a publication? Can any list
member explain the cause, please, for our benefit?

I'm not (at present) challenging the truth or otherwise of that claim, but
I would like to be convinced, if only to distinguish between fact and
folklore."

==========================

And now, thanks to a communication just received from Doug, " Tamaya MS-733
manual", I am starting to see light!

He quotes from the Tamaya manual, about front silvered index mirrors-

"FRONT SURFACE MIRRORS
If you need to replace your Index mirror with a mirror that is silvered on
the front surface,the silvering will be located forward of the axis of
rotation of the sextant arm.With this configuration,the normal method of
useing the reflected arc will not be adequate.You will notice that as the
eye is moved above the sextant,the appearance of the reflected arc
changes.The only correct observation point is with the eyes on a level with
the sextant frame itself.however,in this position,nothing of the reflected
arc can be observed..."

Note that he is NOT referring to ALL front-silvered mirror sextants, ONLY
those in which an index mirror (presumably rear-silvered) has been replaced
by a front-silvered mirror. In that case, if the glass occupied the same
position as before, the effective reflecting surface would be shifted
forward, by something less than the thickness of the glass. And no longer
would the rotation axis of the sextant lie exactly in that reflecting
plane. I can see how that might change the appearance of the arc as
reflected in the index mirror, so as to spoil the exact alignment.

Presumably a responsible sextant-maker, introducing a design with
front-silvered mirrors from new, would take steps to ensure that the axis
passed through the reflecting surface, in which case either method of
aligning the index mirror would suffice. Would our sextant experts care to
comment?

Doug's original mailing said-

"It is stated that all front coated index mirrors can not be checked for
perpendicularity in the old way of useing the arc's relected image."

In the light of this more recent information, perhaps than now needs some
qualification.

George..

================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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