NavList:

Brian Villmoare, you wrote:
"If you have a sextant and a friend it is pretty easy, using an artificial horizon."

I'm not entirely sure I've read your comment here correctly... 
But if you do have a friend with a sextant and you each have an artificial horizon (bowl of water or some more stable fluid), then, in fact, this is the easiest way to calibrate a Bris sextant since you can both shoot exactly the same thing. Grab your Bris sextant and wait for the direct Sun image (direct meaning the one you see in the AH) to be exactly superimposed with one of the reflected images. Then your friend does exactly the same with a "proper" sextant. Read the angle. Correct for any index correction in the proper sextant, and you're done. That angle is then the angular calibration of the Bris sextant for that reflected image. And no corrections (apart from index error) are required since you have shot the same angle.

There's yet another "tabletop" approach if you have a proper sextant available which can also be used to test any sextant. The good sextant can "feed" a beam of light deflected by any dialed-in angle to any sextant being tested or calibrated. 

You wrote:
"I'm not sure why it would have to be 4" square."

Oh. If you go back to my earlier post, I was talking about making an experimental demo version of a Bris sextant --something that one can experiment with to see different angles and try out different geometries. I suggested 4-inches square simply as something "hand-sized" that's easy to manipulate, easy to assemble/disassemble/re-assemble in an experimental rendering of the Bris sextant geometry.

Frank Reed