NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Perpendicularity
From: George Huxtable
Date: 2004 Oct 12, 19:13 +0100
From: George Huxtable
Date: 2004 Oct 12, 19:13 +0100
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. 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. ================================================================
