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On Oct 9, 2007, at 9:58 AM, George Huxtable wrote:

>
> I had written-
>
>  No matter
>> which side the glass is coated, the thickness of the glass does
>> not give
>> rise to multiple images, not at all. The refractive index and the
>> thickness
>> of the glass make no difference whatsoever. With rear silvering, some
>> light
>> will have been reflected from the front surface, some from the
>> back, but
>> to
>> the observer's eye the two images will EXACTLY coincide.
>
> and Nicolas de Hilster added-
>
> I played around with my David White & Co 1941 US Navy quintant. I
> removed the telescope and observed a high contrast image through the
> index-mirror only. In this way I saw two ghost images, one on either
> side of the main image (something Van Breen already mentioned in 1662
> when he described his spiegelboog [Dutch for mirror-staff] in his book
> Stiermans Gemack). Now when I put the telescope back in place and
> observe the same image through the horizon mirror and index mirror, so
> using double reflection, the two ghost images seem to have disappeared
> or at least have become too faint to distinguish. So it is the
> combination of two mirrors that makes the multiple images disappear
> for
> the eye (so they do not exactly coincide as George said). When
> using the
> spiegelboog one has to deal with those annoying multiple images
> (and so
> had Robert Hooke with his single reflecting instrument in 1666).
>
> ================
>
> I think that Nicolas has got the explanation wrong, however. The
> single
> image you see with a back-silvered mirror occurs even with a single
> reflection; it doesn't need double reflection. But it does need
> PARALLEL
> LI|GHT, such as you get from an object in the sky. Unless what you are
> looking at is a long way away, you will see two images, a bright
> one and a
> faint one, just as you get with your own reflection when looking in a
> domestic mirror.
>
> So I ask Nicolas how far away was the source of the "high-contrast
> image"contact George Huxtable at george@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. that he
> refers
> to?
>
> As for the Spiegelboog, I have read with interest Nicolas' own
> account of
> the instrument in Bulletin of the Scientific Instrument Society
> (though I no
> longer have that issue to hand), heard his lecture about it, and
> seen him
> demonstrate the replica he has made. As I recall, it involves
> aligning with
> the horizon the reflected image of a wooden sight-vane, which is just
> obscuring the Sun. And though the Sun is at infinity, the vane
> isn't, and
> that's why multiple images of its edge occur, when seen through the
> mirror.
> Have I remembered it right?
>
> George.
>

Hmm, I wonder if this shows up in artificial horizons, which is where
I recall (vaguely) having seen it?  No time to check now, but I'll see.



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