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    Re: Vernier explanation
    From: Don Seltzer
    Date: 2022 Jun 27, 16:36 -0400
    The owner replies, “On the question of parallax, I can guarantee that the vernier units are exactly 1/20 of the main units, as originally stated. In my photograph of the two scales I could not align the camera precisely above the centre because my shadow would have fallen just where I needed the best lighting, and in addition there is now a slight gap between the scales; this may have given a misleading impression in the photograph.”

    Don Seltzer

    On Mon, Jun 27, 2022 at 11:12 AM Peter Monta <NoReply_PeterMonta@fer3.com> wrote:
    Hi Don,


    Parallax in the camera view is not likely. The owner posted this inquiry specifically because he could not detect any difference in the spacing of the two scales by eye.

    Hmm.  Could the vernier have been mis-engraved, perhaps by configuring the dividing engine with settings intended for the arc?  How would that have gotten past final quality control?

    I'm tempted to do some analysis of the image, but that would have to estimate the camera pose and a good subset of the tick marks.  Four engraved circular arcs are also available as constraints.  Alternatively, could the owner supply a few more photos, for example with the camera directly above the vernier's 0, 2, 4, 6, 8, and 10 (using a fixed index-arm setting of course)?

    Cheers,
    Peter

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