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George wrote:
"Now, yaw the instrument about,  so
that first its right-hand edge, then its left, is closest to the eye.  Only
at one position, when it's exactly at right angles, so the edges  are
equidistant from the eye, will the viewline be in exactly the  right
direction to avoid collimation error. What I ask is this; in use, is  that
optimum orientation obvious to the observer?"

Alex wrote way back  on October 11th:
"Rocking. When you slightly rotate the devise about VERTICAL  axis,
you will see that the reflected "Suns" move up and down slightly.
You want to measure the altitudes when the reflected Sun is in the LOWEST
position. This happens when the horizontal lines in the planes of your
glasses are perpendicular to the line of your sight.
This rotation plays  the role of rocking the usual sextant. "

He's clearly describing the same  thing here. This rotation about the
vertical axis is designed to eliminate  collimation error --as much as possible. This
is *not* the same thing as  'rocking the usual sextant'. So here's the tricky
part: can you rock the Bris  "sextant" about its vertical axis to eliminate
collimation error and then  independently rock it about the line of sight axis
to ensure that the  measurement is perpendicular to the horizon? I think it
would be difficult to do  these two motions correctly and indepenently, and I
think this is the biggest  problem with the accuracy of the Bris "sextant".

A note on semantics: is  it a sextant? Technically, no. But then again, I
have a David White Co. sextant  (US Navy, 1944) in front of me that can measure
angles up to 142 degrees. So  it's not a sextant --it's a quintant. Of course,
if I call it that, it just  generates confusion. In the present era, if the
word sextant means anything at  all (*), it means angle-measuring instrument.
But there's a distinction that I  think is worth making anyway. The biggest
difference between the Bris "sextant"  and a standard sextant is that a standard
sextant can measure ANY angle within  the limits of the arc. So maybe we could
call it a "Bris fixed-angle  sextant".

I've been entertained by the discussions people have had about  improving the
Bris sextant by attaching a telescope, different shades, and so  on. Myself,
I've been thinking about mounting the glass pieces on a frame.  Perhaps one of
the mirrors should be allowed to rotate. And as long as I've got  a frame, I
could permanently attach a small telescope. Hmmm... I'll need some  sort of
handle to rotate the movable mirror... And maybe I could record the  angles on a
table affixed directly to the frame indicated by the position of the
rotating mirror handle! I'll call it a "Variable Angle Bris device" You  can see
where I'm going with this. This all leads me to wonder whether the early
inventors of the sextant came upon the idea by playing with homemade devices  very
similar to the Bris sextant and went on to invent the complete instrument  by
considering the very issues that have been discussed on the list in the past
couple of weeks.

*(continuing a thought from above) DOES the word  "sextant" mean anything
today? Twenty years ago, I could say "sextant" and most  people with some
education were aware that the word refered to some instrument  for looking at the sky
from the deck of a ship. Today, I find that people of  similar education
stare blankly when I use this word, even giggling a bit  because it sounds like
"sex tent".

-FER
42.0N 87.7W, or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

   

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