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Kieran Kelly wrote:
>
> All very good but how did they see the wires in the telescopes in the days
> before illumination? The book admits that scholars don't know as M and D
> left no record of this mundane procedure. However Danson speculates that
> they held a candle obliquely up to the object lens of the scope allowing
> just enough light in to see the wires. Sounds dodgy to me. Wouldn't this
> have obliterated the light from the star especially the dimmer stars.

If the star is dim enough then you'll lose it when the field is
illuminated. However, a relatively bright star like Polaris is easy.
The lamp is held just outside the field of view, so all you see is
scattered light. It gives the sky a twilight appearance. Moving the
lamp more to the side reduces the illumination.

Another technique, which I like better, is to hold a small flashlight
so the tip of your index finger is illuminated. Then put the fingertip
close to the scope objective and poke it into the field of view a
little. I think this gives a more comfortable body position, and the
flashlight is at right angles to the line of sight, reducing the
chance of accidentally shining the beam into your eyes.

Both methods work well; in fact, the main advantage of built-in
reticle illumination is simply the physical convenience. The
illuminated theodolites I've seen all used a tiny "lollipop" in the
center of the optical path, behind the objective. Light came in via
the hollow elevation axis and was reflected toward the eyepiece by the
lollipop. When not needed, a knob on the scope tube allowed you to
turn it edge-on to minimize the obstruction, which wasn't much in any
event.

Last year Professional Surveyor magazine ran an article analyzing the
effect of zenith sector errors during the Mason & Dixon survey.

http://www.profsurv.com/ps_scripts/article.idc?id=1106

And this April's issue has an article about a team of volunteers who
replaced a damaged monument stone on the line.

http://www.profsurv.com/psarchiv.htm

   

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