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    Re: "Vernier acuity" of horizon IC tests
    From: Frank Reed
    Date: 2009 Jul 6, 20:19 -0700

    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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