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    Re: Troughton and Simms: Vernier acuity
    From: Bill Morris
    Date: 2013 Jan 12, 16:23 -0800

    Norm and Frank

    I have yet to come across a vernier sextant divided to read to 10 seconds on which I can decide with certainty which two or even three lines coincide.

    Optimal vernier acuity in the best of conditions, in which there is adequate light and, also importantly, high contrast, is said to be 8 arcseconds. The degree divisions on a sextant are 30 minutes apart and, if my reasoning is correct, when trying to read a vernier to 10 seconds, one is attempting a vernier discrimination of 5 arc seconds. This seems to accord with my experience. I once owned a thodolite with 4 inch circles and a vernier reading to 20 seconds. I was just able to discriminate to 20 seconds (equivalent to 10 seconds on a sextant) in full daylight using the fitted x 10 Ramsden-type magnifier.

    This had nothing to do with the accuracy of the dividing. At a time when vernier sextants were still being made, the most accurate dividing engine in the world was said to be at E.R. Watts, completed in 1905. It was capable of dividing a master circle to +/- 1 second. Lesser machines were probably used to divide sextant scales.

    Bill Morris
    Pukenui
    New Zealand
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