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Hello Igor.

Good to hear from you again! You wondered, "Was that specially designed for people with a poor sight ?" In a way... Arguably, every modern sextant is designed for the inferior vision of our species. :) With a vernier sextant, decent accuracy in reading required a small magnifier on a swing arm over the scales. That was a real problem! Common, traditional vernier sextants were hard to read. The micrometer and worm gear was the eventual solution.

I'll speculate that this was an early attempt at a similar micrometer-like solution. The gear teeth along the arc here turn that dial pointer in its glass-protected (though probably not sealed) housing. It's not quite obvious in the photo, but the dial scale inside that housing is readable to at least two minutes of arc all through the 5° cycle around the circle. Also of note, the main scale is only marked every five degrees. That's consistent with the dial scale range.

Frank Reed

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