NavList:

The Bris is a unique example of a "fixed angle sextant" (yet another stretch of the original meaning of the word sextant since this instrument technically has a range of zero degrees --but it's still quite useful for navigation and fundamentally it's a sextant!). The key feature of the ordinary sextant and any of the instruments of reflection is double reflection. If you have two mirrored surfaces mounted in some way with an angle between them and perpendicular to some other surface, then you have the ability to make accurate manual observations of one angular separation.

Think of it this way:
1) take an ordinary cheap sextant and glue the index arm in place at some random angle.
2) erase the scale on the arc.
3) remove all the other bits on the instrument including the telescope and the shades and also those components that no longer serve any purpose with the index arm glued in place (so delete the micrometer and even the pivot of the index arm).

What are you left with? It's basically two mirrors mounted on a flat surface, and in fact you can make an instrument like this by sticking two small mirrors on a block of wood. It can still measure the altitude of the Sun with all the ease and accuracy of a true sextant, but only at one specific angle. Calibrate it with any sight from a known location, and you can navigate the globe with it. The catch is that it's probably only useful twice a day (for the Sun's altitude). But it's an extremely simple, low maintenance device, so one solution to its single-angle limitation would be to make a bunch of them: ten blocks of wood with ten fixed angles.

The Bris sextant simplified the fixed angle sextant even further. The mirrors become ordinary panes of glass (glass slides) which are reflective enough to act as mirrors, and rather than being glued to a "block of wood" to serve as a frame, they are glued directly to each other. And since the mirrors are now transparent, we can include a second "index mirror" and glue three panes of glass together instantly doubly its angle-measuring options. And finally, thanks to multiple internal reflections, instead of measuring a single fixed angle, the Bris sextant becomes capable of measuring four or more angles. It's the "instrument of reflection" rendered down to its bare bones.

There have been NavList discussions about the Bris sextant roughly every other year since the year 2000. Here's an index to all posts with the word "Bris" in the subject: http://www.fer3.com/arc/sort2.aspx?subj=bris&y=1996&y2=2015.

Frank Reed
ReedNavigation.com
Conanicut Island USA