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    Re: "Vernier acuity" of horizon IC tests
    From: Greg Rudzinski
    Date: 2009 Jul 7, 20:46 -0700

    As you have suggested I did a series of 30 index error measurements
    with no sight tube then 30 with the sight tube. The sextant used is a
    US Navy mark 3 M.Low with a horizon mirror silvered on the back
    leaving a clean line between the clear half and silvered half. Holding
    the sextant two feet from the eye for 30 consecutive observations was
    quite punishing on the arm but steadiness was maintained. The horizon
    was good showing only a very slight fuzziness. This time I wore
    sunglasses, blinked, then squinted to get an improved focus of the
    horizon (5.1 NM away from a height of eye of 20ft.). The sunglasses
    removed some glare and most importantly kept the wind out of my eyes.
    The sight tube observations presented a sharper horizon and an
    overlapping field (acuity not in effect). The observations without the
    sight tube did not have an overlapping field (acuity in effect).
    Results:   Without sight tube          With sight tube
    Average     2.05' off arc                  1.85' off arc
    SD           .40'                       .44'
    Spread      1.6'                       1.6'
    Conclusion: Results are very close so if acuity has an effect then it
    is subtle in nature. And I had a good vision day ;-)
    On Jul 6, 8:19�pm,  wrote:
    > Hi Greg.
    > You should try this without the sight tube. The sight tube serves two 
    purposes: it maintains the collimation of the line of sight so that you are 
    looking along a line of sight parallel to the frame, and by providing a tiny 
    aperture to look through, it generally improves the focus of the eye. We 
    don't need to worry about collimation for IC tests (the error resulting from 
    collimation error goes to zero at zero degrees). The focus of the observer's 
    eye can be equally well improved by wearing eyeglasses. With the sight tube 
    removed, hold the instrument at arm's length. Then you will see the straight 
    line of the horizon on each side of the horizon glass with no "merging" or 
    "fading" zone across the middle. By the way, this possibility of a 
    "hyper-acuity task" only applies to standard style horizon glasses, mirrored 
    on one side, clear on the other, with no dividing line between the sides 
    (just the edge of the silvering itself).
    > -FER
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