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    Re: Troughton and Simms
    From: Frank Reed
    Date: 2013 Jan 12, 12:41 -0800

    Norm, you wrote:
    "I just realized how TRULY wonderful the sextant vernier is. 10 seconds is a very small angle..."

    Yes, the instrument takes great advantage of the capabilities of the human eye and human visual perception (which is not entirely contained within that biological camera).

    You had mentioned in another post how small this angle is. But don't forget that you're observing it through a telescope. We cannot otherwise perceive an angle that small. The human eye had a normal resolution of just about 1 minute of arc (a bit better for people with great eyes under excellent observing conditions). In fact, if you haven't heard of this, it may interest you to know that the "20:20" standard for effectively "perfect" vision is defined as a visual resolution of exactly one minute of arc. Of course, if you have a sextant with a 7x telescope on it, then you can see angles (as measured without the telescope) that are seven times finer. You can readily resolve an angle of just about 8.5 seconds through a telescope magnifying by a factor of 7. So that's no miracle. We SHOULD be able to measure angles that small if the instrument is otherwise well-made and properly adjusted.

    On the vernier, this still seems like an extraordinary thing. How can we read those fine lines? And in fact, it is. Beyond the "normal" camera-like vision of the human eye, bound by the rules of optics, there is a capability of human vision that goes beyond the standard resolution limits by a factor of five or ten. It's called "hyper-acuity" and indeed a synonym for it is "vernier acuity". Our vision processing system (somewhere behind the eyeball) is capable of detecting very small defects in the alignment of straight lines. In these specific "hyper-acuity" tasks, somehow we can resolve better than the laws of optics would appear to require. Without this remarkable capability, reading a vernier would be much less successful.


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