NavList:

Brad,
Thank you for your comments.
You wrote:

"It is the degree of precision in creating the worm that separates the good from the bad"

That most accurate and precise of threads, of the familiar engineer's micrometer, has a permissible range of error of traverse of 0.003 mm and, while there should be no perceptible backlash between the screw and the nut, every beginner is taught to wind back the thimble a full turn before repeating a measurement. I cut the worm of one of the Husuns tested on an ordinary lathe that does not pretend to be of toolroom quality, and was pleasantly surprised at the results.

And you asked:

"In the early days of micrometer sextants, was there ever a sextant created which had both a vernier AND a micrometer?"

In a way, yes. Both Heath and Hughes had a vernier instrument fitted with a worm and rack, but no micrometer drum, as the initial pitch chosen for the worms was too small. Later they reduced the radius of the arc a little and used a pitch of 18 turns per inch. See my blog post on the "Evolution of the Sextant Micrometer"

and you wrote:

"With this type of instrument, a remaining error
will be the centering error in the placement of the index arm, something that may permit you to separate it out."

I think centring errors would be minute over a range of only a degree or so, but I will one day look at vernier sextant errors over a small range. I guess that they would be made up of dividing and centring errors of the arc and dividing errors of the vernier scale.

As you imply, there are two ways makers of vernier sextants adjusted the reading. The first is the common tangent screw and nut. Some makers split the nut so it could spring in and reduce clearances between it and the screw, though backlash here is more of a minor annoyance than anything else.

Posher instruments, like the C Plath Dreikreis whose restoration I describe in a blog post(thanks again, Mike)and an up-market Hughes used the same arrangement found in theodolites, of a screw working against the opposition of a compression spring. This type works well so long as nothing sticks. It is very annoying when making an observation to reverse the direction of the screw to find that the spring has not moved the index arm backwards. It is even worse if it becomes unstuck between making the observation and reading the vernier.

A mid-nineteenth century writer wrote about allowing a little while for the movement of the vernier sextant index arm to settle before reading it. He thought it was due to the release of stresses in the instrument. At least, that is my interpretation of what he said. I think it more likely that what he
thinks he observed was due to stick-slip at the index arm bearing. Note my caution in the post about being heavy handed when fiddling with this latter.

Bill Morris
Pukenui
New Zealand

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