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    From: John Rae
    Date: 2005 Oct 29, 17:03 -0700

    Over the last while, I have been an avid reader of the Navigation L list.


    Recent notes posted about the BRIS sextant, intrigued me, particularly those posted by Eremenko , and I decided to delve deeper.


    Two internet sites






    Firstly, I traced the rays on a square paper pad, with three reflecting planes touching at one end and angled outward towards the top, and assumed zero thickness.


    Initial try, each reflecting plane spaced at 10 degrees, (20 degrees total), second try, each reflecting plane spaced at 5 degrees, (10 degrees total).  It became clear that if the angle between the outer two reflecting surfaces was exactly equal, then an observer would see two superimposed images on the center glass, one transmitted, and one reflected, but if the angles were not identical, then additional images might be seen, but at about half the brightness.


    Then it was time to make one.  Using ordinary microscope slides, I lashed up a test rig.  Microscope slides seem to be high quality glass of uniform thickness size 25 x 75 x 1.0 mm. (available in a box of 72 for about $14).  I created a hinge at the bottom by using strips of sticky transparent tape, and wrapped an ordinary elastic band around the sandwich of the three microscope slides, and inserted ordinary wooden matches between the slides.  These two match sticks could be slid up and down to vary the angle between the glass slides held by the elastic.  The included angle between the slides can be calculated after measuring the gap at the top of the slides using a scale.


    When the sun came out, I put on my sunglasses and went outside.  Amazing!  Firstly I immediately obtained a view with several reflected sun images, and found that rotating the orientation of the apparatus in the up and down direction had no effect on the apparent location of the sun images.  This is just like the action of an ordinary marine sextant, and uses the same double reflection principle used since the early history of the sextant.


    I then held a polarizing filter between the sun and the test rig.  While the image was darkened somewhat, the polarizing filter did not interfere with the images in any observable way, no matter to what orientation the filter was rotated.  So the sun images appear not to be polarized to any great extent by being reflected back and forth by the glass.


    With a sun altitude of about 26 degrees, and playing around with the angles by moving the match sticks up and down, I was able to see a maximum of three large sun reflections plus four smaller reflections (a total of 7).  But the best arrangement seemed to give me 3 large and 3 small images ( a total of 5) while using angle pairs ranging from 5+9 to 3+3 degrees.


    Could anyone measure the angles on a commercial Bris Sextant, and report those angles?


    My next move is to make a proper instrument.  I am working up the design of a simple jig so I can grind the matching cementing angles on each of the three glass slides at the bottom of the instrument.  This jig will hold two slides at a time, so I can vary the angle between the front and back slides if I so choose.


    An initial comment.  Using my ordinary marine sextant, I can often obtain a sufficiently accurate reading from a sun that is partially obscured by light cloud cover.  But with my test rig, the sun had to be bright, as I couldn’t see the sun images with light cloud cover.  But the Bris Sextant appears to be a good cheap tool for instruction, since all of the other aspects of position finding apply.

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