NavList:

"The screw adjuster 'micrometer' is an interesting idea, but I can't see how it achieves better accuracy."

I think I can answer my own 'can't see'. Here is the URL of the 1783 Ramsden sextant 'with vernier and micrometer'

http://www.nmm.ac.uk/collections/explore/object.cfm?ID=NAV1103&picture=1#content

Unfortunately, the vernier seems to be missing in the image, but there are two similar pieces on the site that have them above the scale in the arm's window. The other pieces don't have 'micrometers'.

The text says that the vernier goes down to 30 arc seconds and the micrometer goes down to 10 arc seconds. I think that the touch is made, and if the vernier isn't dead on a mark, the micrometer moves the arm until it is. The amount of a turn to make the vernier touch a mark is added or subtracted.

Later, it appears that Ramsden took a different approach. Other Ramsden sextants in the collection have no micrometer and longer or finer verniers, one having a magnifier for a vernier down to 5 arc seconds.

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