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Chuck Varney,
Thank you for all that info on your earlier work on this index arm add-on.

You wrote:
"The date and translation of your Knorre find is interesting. I had only been able to learn that Knorre’s revelation pre-dated 1872."

Yeah, I was astounded to discover it dated back to an article from 1829 ...until I considered that Chauvenet was the source. I think that he must have collected rather random notes over the decades leading up this great "tome" on "Spherical & Practical Astronomy". It's also interesting that everyone --Chauvenet, Lecky, and the editor of Hints-- more or less re-copied the original text with minimal editorial comment. Didn't anyone think about this device? The basic idea is that you point the horizon view of the sextant at the reflected image. Then you move the index arm until the bubble in the level "plays" (wobbles near the center). And at that point the direct image should be in the field of view. And the instructions note that you may have to rotate a little around the line of sight of the scope to bring it into view. But the only reason this last rotation would be necessary is because the sextant is significantly not vertical... So then... why not have a two-dimenional bubble?? I'm guessing that maybe they weren't common "off-the-shelf" when Knorre described his idea back in 1829. But surely by the early 20th century, when they were specifically using this as a marketing novelty, someone should have realized that a two-dimenional bubble would be more useful, right? If one is going to use a crutch to line up a star in an artificial horizon, then why not add a crutch that does the job properly? [this is a really a rhetorical question... I suspect we're looking mostly at marketing competition among sextant manufacturers in that era...]

And:
"My 2016 pursuit was prompted by just such a question.  A long-time correspondent of mine closed a blog post on astronomical sextants by jokingly offering a prize to anyone who could identify the purpose of the spirit levels shown mounted on them. The following link includes my response and his challenge post, with images of two astronomical sextants that had appeared in Brandis catalogs.
http://gardnerghost.blogspot.com/2016/12/ "

Fun! Thanks for the story. :)

You also wrote:
"I have attached the two sextant images from the blog post along with a Stackpole & Brother sextant shown at this Smithsonian link (which wrongly attributes introduction of the index arm spirit level to William Harkness)"

Well, in fairness to the Smithsonian folks, it seems that one of the advertisements also refers to this as "Prof. Harness' Pattern". I wonder what was going on there... Did Harkness independently suggest the idea? Or did he invent some small enhancement that briefly came to be associated with the original invention? This is small-time historical trivia, but it I admit I'm curious. :)

Frank Reed