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"Royer, Doug" wrote:
>
> observer would then reverse the scope 180* on the verticle plane and repeat
> the proceedure to cancel certain errors and allow corrections to be computed
> for others.I cannot visualize this scope.Can anyone help?


I'm not sure what instrument you're describing, but my guess is they
used a meridian circle:

http://oacosf.na.astro.it/MUSEO/INGLESE/01i.html

The scope is only able to tilt up/down, and always stays in the plane
of the meridian. The west and east trunnions are supported on piers
anchored in the ground.

To reverse the instrument, it's lifted off those piers and turned
around using the "reversal undercarriage" depicted on the Web page.
After completion of the operation, the trunnion that was resting on
the east pier has moved to the west pier, and vice versa.

With a portable meridian instrument like a Bamberg (used for years
by the U.S. Coast & Geodetic Survey), the scope assembly could be
lifted and reversed by hand. An instrument this small left no room
for an observer to sit comfortably beneath the scope, so the line of
sight was bent 90 degrees by a mirror. The eyepiece was on the end of
the horizontal axis (the left side, in the picture).

http://oacosf.na.astro.it/MUSEO/INGLESE/08i.html

A surveyor's theodolite is even easier to reverse. After making an
observation and recording the angles, turn the alidade (the part that
rotates about a vertical axis like a carousel) 180 degrees. The
eyepiece now points away from you. Pull it up and over, rotating the
scope about the instrument's *horizontal* axis so the eyepiece
momentarily points straight up then comes back around to you. Now the
instrument has been reversed. Observe the object again in this
position. If you average the readings from both positions, any
out-of-square conditions among the instrument's vertical, horizontal,
and optical axes are cancelled. Also, the index error of a vertical
angle is cancelled.

   

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