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    Re: Irradiation; [was "Star sparkle in sextant image"]
    From: George Huxtable
    Date: 2004 Sep 28, 18:40 +0100

    Referring to the demonstration of the effect of irradiation, by watching a
    shadow jump across the gap between finger and thumb when the are held
    almost touching in front of a bright background, Jim Thompson wrote-
    >And conversely this must be part of the phenomenon of improving an image by
    >looking through the aperture created by one's fingers, placed as you
    >describe below for seeing the shadow that indicates irradiation.  When I do
    >that without my corrective lenses, bright images appear sharper, with less
    >"ghosting", if that's the right word.
    Response from George-
    Well, as I see it, these are two quite different effects.
    By peering through a small hole, smaller than the hole in the iris, you can
    improve the resolution of your eye by stopping the lens down, so as to
    exclude the outer parts of the lens which give rise to distortion and
    aberration. That certainly works. I have a "lazy" left eye, since
    childhood, which I have never bothered to focus. When I have an eye-test,
    all my left eye can make out is the enormous single letter at the top of
    the Snellen chart. Then they give me a "pinhole" to look through, and
    immediately I can see the next line, with its two letters, quite clearly.
    But no more than that.
    Tthe effect of the jumping shadow, which stems from the eye's tendency to
    shift the boundary between brighter and darker so as to make bright objects
    appear slightly larger, must (I think) lie in the retina and in the complex
    brain-processing of the retinal image, in determining an edge between light
    and dark.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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