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    Re: 1911 encyclopedia
    From: Richard M Pisko
    Date: 2003 Mar 26, 23:01 -0700

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