NavList:

Paul Bedel you wrote:  I have purchased a Freiberger sextant (N° 740720) with a prism wich can be fitted instead of the shades of the horizon mirror (see photos). I think that this prism is for measuring horizontal angles of more than 90 degrees. But is there an other use ?

With the prism in place in its mounting, is it the index view, the horizon view, or both that’s interfered with?  If it’s the first, it might be you can swing a marine sextant to find the vertical, but you can’t really do that to find the horizontal for survey work.  Could this rather deep prism arrangement split the index reflection into two halves and magically only present the two halves exactly inline when the sextant is horizontal?  Just a suggestion from an Air Navigator with negligible experience using a Hughes Mates sextant and only a schoolboy’s knowledge of optics.  Usual caveat applies.  I’m probably completely wrong. DaveP