NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bris does not work
From: Alexandre Eremenko
Date: 2012 Jul 26, 12:40 -0400
From: Alexandre Eremenko
Date: 2012 Jul 26, 12:40 -0400
Greg, > The number of visible Sun images is dependent on the amount of shading. Yes, of course. Though I cannot say that I completely understand the theory. 3 images are obtained by 2 reflections. If you allow 4 reflections, you get 8 ADDITIONAL images. According to this count, I must have 3 bright and 8 dim images. While (few years ago) the total number of images was 8. That is 3 bright and 5 dim. (And this is what the manual says: 8 total). > Your original model may have gained shading properties > in the built in shade or become opaque with age. There are two logically possible explanation: a) the filter became more dense with age, and b) the reflecting power of the glasses decreased. I have no clue to any possible physical reason why could a) or b) happen. The "sextant" was permanently stored in its box since the last use. > Try cleaning with alcohol on all the surfaces. > Use a string soaked with > alcohol to clean the internal surfaces. Why alcohol? Usually a solution of ammonia (spirit of Hartshorn) is used to clean glass. Also sold as "optical glass cleaner". Sometimes they also recommend soap solution. But why alcohol? I understand that this kind of cleaning will remedy both a) and b) IF the reason is dirt. (How could dirt penetrate there, and what kind of dirt, I have no clue). The outer surfaces, which are frequently touched with fingers, I usually just wiped with a piece of cloth. But I don't see how dirt could penetrate inner surfaces. Technically this cleaning is not simple, because the gap between lenses in the original Bris is so narrow that no brush can penetrate there. And I will need to wipe the surface somehow. But with all my physics background (limited to what I was taught in high school) I cannot imagine how a dark glass could increase its darkness with age:-) That's why I initialy conjectured that the clear glass somehow lost its reflective power. By the way, I wrote an e-mail to the address shown in the manual. My e-mail returned:-( The manual says: "Copyright 1998" in it. The YouTube advertisement says that "you can have it for 200 pounds" but gives no hint where do you get it from... Searhing on Yrvind on internet gives some guy who explicitly says that he does not use e-mail:-( Alex.