NavList:

The reason I understand is that normally there is clear glass in the horizon path because there is a slight induced error in the optical path thru the horizon mirror (the 2 faces of the mirror glass are not perfectly parallel.  By having the same piece of glass in both optical paths that error cancels out.  You still have the error of the index mirror that isn't cancelled out, and I suspect the error is pretty small with currenty glass making technology, but if you want to do high precision work you take every error reducing step you can.

For the Mark-III cost is prime consideration and accuracy around 1' is expected so the cost of this error correcting step is not worth the money.

Re: Perpendicularity Error and the Davis Mk 3 Sextant
From: Peter Monta
Date: 2017 Nov 13, 13:19 -0800

. . . 

Incidentally, and this is off-topic to the question, but what is the reasoning behind a horizon mirror with glass in the horizon path?  The Mark 3 has just a mirror on the index-mirror path and empty air on the horizon path.  It seems perfectly usable this way.  Do "normal" sextants provide a split mirror to get the benefit of the 4% reflection from the plain glass?  Or they don't want to expose the edge of the mirror to the elements?  Or is there some other reason?

Cheers,

Peter