A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Peter Monta
Date: 2022 May 20, 23:47 +0000
... I think that this prism is for measuring horizontal angles of more than 90 degrees. But is there an other use ?
None that I know of, except the self-calibration trick of reversing the prism direction, as described in the sextantbook.com article linked to by David Pike.
See also this page on Nicolàs de Hilster's web site: