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    Re: sextant precision.
    From: Bill B
    Date: 2005 Jun 23, 17:45 -0500

    Bill you wrote:
    "My contention is that a  optically perfect shade with faces parallel will
    still offset the ray if not  perfectly perpendicular to the ray path.  You
    are probably correct that  the offset is negligible in practice, but
    nonetheless does exist."
    Frank responded:
    "So  experiment! Get a piece of flat clear glass, and hold it in the light
    path.  Rotate it. Tilt it 45 degrees to the light path. Then try
    perpendicular to the  rays. Does it change the alignment of direct and
    reflected images in your  sextant? There's nothing like experimental
    evidence to constrain one's theoretical musings.
    Here's another: No sextant involved. Hold a piece of  flat glass close to
    some text on a page at normal reading distance. Rotate it.  Does the text
    appear to shift from refraction (it should)? Now hold that same  piece of
    flat glass up in front of some distant scenery. When you rotate the  glass
    as before, do objects shift back and forth (they shouldn't)? TRY this
    experiment and see if you agree with my should/shouldn't comments. Then work
    out the theory...
    Evidence constrains theory."
    Tried the "no sextant" version of the experiment you proposed.  I used a
    1/2"-thick piece of float glass with polished edges (I generally us it with
    sandpaper adhered to it to lap plane blades and bottoms, and chisel backs).
    The glass thickness near the edges was checked with a micrometer to verify
    that they were parallel.
    I then viewed a table edge at about 3 feet, a sidewalk edge about 10 feet
    away, a rooftop about 100 yards away, and later the moon.  I used both the
    left and right vertical edges of the glass, then flipped the glass end for
    end and repeated for each viewing.
    The glass was rotated from approx. perpendicular to the line of sight to
    approx. 15d to the line of sight.  I attempted to keep the position on the
    glass where the images were equidistant from my eye, and used the dominant
    eye only.
    I preface the following with the caution that there were no reasonable
    measurement practices in place.  Displacement was judged by eye, and the
    angle of the glass to the line of sight was hand held and at best
    Trial 1, Wednesday eve
    In all cases, from the table edge to the moon, there was marked displacement
    of the line/object halves viewed through the glass vs. the direct view
    (table edge, sidewalk edge, and moon) as the glass approached a severe angle
    (closer to parallel than perpendicular) to the line of sight.  The moon
    halves were displaced by about 1/4 of its diameter (approx. 7') as the glass
    reached a severe angle (approx. 15-20d off the line of sight).
    Gauging displacement by the difference between the glass and air images by
    distance along the glass edge, the amount of displacement appeared to be
    about the same for far and near objects.  I again caution that angles were
    approximate, and foreshortening was compensated for by eye.
    Trial 2, Thursday afternoon
    Same methodology as trial 1, but paid closer attention to the final angle of
    the glass in relation to the line of sight.   A number of objects from 3
    feet to 200 yards were observed, including a table edge, patio edge, roof
    lines, and power lines.  In these observations the displacement appeared to
    diminish with distance, but was still easily visible at all distances.
    All I can safely conclude is that refraction is still with us (thank
    goodness, I have a lot invested in camera lenses ;-)  Currently I am
    considering how I can use photo clamps, articulated arms, tripods etc. to
    set the angle of the glass to the line of sight consistently for subjects at
    different distance, as well as a method to measure the offset.
    So at the moment, my muse, the "should" does, and the "shouldn't" does (to a
    magnitude related to the "should" yet be be determined).  Theory will follow
    experiments with greater controls in place.

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