NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant mirrors
From: George Huxtable
Date: 2007 Oct 9, 14:58 +0100
From: George Huxtable
Date: 2007 Oct 9, 14:58 +0100
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. 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. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---