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Herbert Prinz and I seem to be at cross-purposes about perpendicularity,
though I don't think there's any real disagreement between us..

He said-

>George Huxtable wrote:
>
>> 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?
>
>Maybe my message sent 4 hours before yours didn't get through to you. This
>seems to happen a lot on this list. But it is on the server. Check the2nd
>paragraph.

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

Yes, Herbert's message did get through to me, if it's the message I think it is.

If so, the 2nd paragraph reads-

"You are describing a parallactic effect which can only be caused by a
different distance of
the individual cylinders from the front edge of the index mirror. As long
as the pivot of
the index arm (= centre of the arch) is in the plane of the mirror and one
cylinder
coincides with the image of the other, the front edge of the mirror is
equidistant from both
cylinders and you can therefore look at them at any vertical angle. If, on
the other hand
the mirror surface is off centre, you will get the effect that you
describe. In this case,
you must look at the cylinders in a plane parallel to the limb."

That refers to the two-cylinders method of alignment, doesn't it, not the
method where you look at the reflected edge of the arc, which is what I was
considering. Or have I missed something?

However, Herbert does attribute alignment problems to a displacement of the
reflecting surface of the index mirror from the axis of the index arm,
which is indeed the nub of the problem, as I see it.

The important factor, arising from Doug's Tamaya manual, that I wanted to
emphasise, was this-

In a sextant which has been properly made and set up by the maker, whether
front or rear silvered, the index axis thould pass along the reflecting
surface of the index mirror. If so, the align-the-arc method should work
just as well, whichever surface has been silvered. It's only if the
reflecting plane has been shifted, by replacing a rear-silvered mirror with
a front-silvered version, and nothing has been done to return its plane to
its correct position, that the Tamaya warning applies. But the notion that
the align-the arc method can't apply to ANY sextant with a front-silvered
mirror is incorrect.

It all depends on thether the maker has taken the trouble to put the
reflecting plane, appropriate to each model, on to the pivot axis, and I
wonder whether those on this list who are professionally involved with
sextants, can tell us whether they do, or don't, bother.

As far as actual measurement of celestial angles is concerned, I don't
think the precise fore-and aft positioning of the plane of the index mirror
matters a damn, so makers may perhaps not take that positioning very
seriously.

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

Cliff Sojourner asked-

"is there any effect from index of refraction of the glass in
front of the silvered surface?"

Yes, it has to be allowed for. In the case of a front-silvered mirror, then
of course the front surface should be aligned with the pivot. When it's
rear-silvered, the effective reflecting surface should be aligned with the
pivot, but that is not the actual rear surface of the glass. Because of its
refraction, the reflecting surface "seems" to be only about 2/3 of the
glass thickness back from the front face, and that's the surface the pivot
should align with.

It's the same effect working as causes a stick to appear to "bend" when you
poke it through a water surface, and makes the water seem shallower than it
really is.

George.

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