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Wed, 26 Mar 2003 15:45:27 -0700, Richard M. Pisko
 wrote:

>The Barr and Stroud rangefinder, the same article goes on to
>say, used a thin wedge of glass which slid along one arm and
>changed the angle through only 3 degrees.
>
After thinking that over I worked the figures instead of
just going by what I thought I read; because something
seemed very wrong.  It was, I screwed up.

I should have written 1/3 of a degree, because that's what
the encyclopedia article had.  Even that is too much for the
other information I have on the minimum range of 400 yards
on the 1 meter model.

For a one meter base, the maximum reading being infinity
corresponding to 90 degrees, and the minimum reading in the
order of 400 meters, the angle would be 89.85 or so for a
reading of 382 meters or 418 yards.  This is a deflection of
only 0.15 degrees or 9 minutes.

 A third of a degree is 20 minutes, and would give a minimum
range of 172 meters, or 188 yards.   I will have to do some
more checking ... hopefully with the real instrument ... to
be sure.

Many thanks to Paul Hirose for bringing this interesting
encyclopedia to my attention.  He did warn: " It's a bit
rough due to misreads by the optical character reader used
to transcribe the text." but that's no excuse, I should have
done my homework.

>>>> Quote  best read in Times New Roman >>>>>>
 1 The length of tube varies from ~ ft. in the smaller to 9
ft. in the larrer instruments.
read off against a pointer from the scale seen with the left
eye. Fot night use, means are provided for illuminating the
scale. The rangi to lights may be ascertained by the use of
the astigmatizer, an optica device by which a point of light
is drawn out into a vertical streak A beam of light from the
objective falls on each reflector (fig. 2)

and passing through the object-glasses, each is received by
an arrange ment of prisms about the centre of the tube, and
reflected throug] the right eye-piece. Two partial images
are thus seen. The image could be united by the rotation of
one of the reflectors, but owin~ to the small base used the
necessary movement would be so extremeb small that it would
be practically impossible to measure it. Tb difficulty has
been surmounted by utilizing fixed reflectors and effect ing
coincidence by means of a prism of small angle. The
deflectin~ prism is situated in the line of the beam of
light from the reflecto at the right-hand end of the tube.
Its multiplying action is a great delicacy. The angle
available for subdivision, to measur ranges between infinity
and 250 yds., is only one-third of a degree In a travel of 6
in. the prism renders accurate measurements possibl within
the required limits.
>>>>>>>>> end quote >>>>>>>>>>

Take care,


--
Richard ...

   

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