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    Re: Finding Distance - The Barr & Stroud Rangefinders by Peter Ifland
    From: George Huxtable
    Date: 2007 Jun 29, 12:23 +0100

    Has anyone read this article by Peter Ifland?
    Journal of Navigation (2003), 56: 315-321 Cambridge University Press
    Copyright (c) 2003 The Royal Institute of Navigation
    Published online by Cambridge University Press 13May2003
    Richard asked this question.-
    "I would like to know if Peter has explained how the optics in the B&S
    range-finder seem to convert what is a mechanically a variable base
    range-finder into what appears to be a variable angle range-finder with a
    more or less fixed base?
    Is it possible the traveling wedge (between the eyepiece and the
    plano-convex objective) displacing a trace (laser) ray in a series of
    *parallel* positions will exit the objective in a *fan* of possible
    rays?   Trying to shine a laser through the objective and watching the two
    dots converge and diverge on a building half a block away (as I turn the
    actuating knob) doesn't work as well as I had hoped.  Maybe if I use two
    tripods . . . .
    I do have the Barr and Stroud operators manual, a working Barr and Stroud
    one meter range-finder from the WWI era, and library access to the book
    "Range and Distance" about the Barr and Stroud company.   I can find
    nothing, so far, that will make clear the actual physical operation.
    I will get the article if that is explained; but I hate to buy a pig in a
    reply from George-
    Peter Ifland is a long-standing member of this list and its predecessor, and
    an occasional contributor. Perhaps he still casts a benevolent eye on our
    proceedings from time to time. I will forward a copy of this message to
    him., backstage, just in case he doesn't.. I've held off responding, for a
    couple of days, to give Peter a chance to jump in first
    The 8-page article Richard refers to is the fourth of a series, by Peter, on
    rangefinding, in Journal of Navigation (of the Royal Institute of
    Navigation, based in London).
    I have scanned and attached extracts which are relevant to Richard's
    question, which should minimise download time, and I hope will keep within
    letter and spirit of the rules of copyright. Please tell me if they fall
    Richard wrote-
    "Trying to shine a laser through the objective and watching the two
    dots converge and diverge on a building half a block away (as I turn the
    actuating knob) doesn't work as well as I had hoped."
    I wonder if he meant "trying to shine a laser through the eyepiece ...",
    which should provide a useful test, as light will follow exactly the same
    path, in either direction.
    I have spent a bit of time studying that diagram and explanation, which has
    puzzled me, as it seems to have puzzled Richard.
    At first, I was wrongly thinking of it like a device that follows a small
    telescope, at the observer's end, just as in a sextant, in which case all
    the deflecting optics would be handling parallel light-rays, coming from
    nearly infinity. In which case, because moving the wedge sideways would do
    nothing to alter the direction of any light, it would have no effect.
    But it isn't like that, as Richard has probably realised, and as the
    positioning of the objective on the diagram makes quite clear. The movable
    wedge system is placed between the objective and eyepiece of the telescope.
    If we look at Richard's setup, with a laser shining axially through the
    centre of the eyepiece, along the line of the split between the two views,
    we can see what happens to the thin laser light-pencil that's shown by the
    solid line in the right half of Peter's illustration, but with light
    travelling in the opposite way to what was originally intended. For our
    purpose, we will here ignore the astigmatisers, that just spread a
    point-focus into a line.
    Because the two mirrors, that split the image, are deliberately placed
    slightly different from 90 degrees apart, the right hand beam intentionally
    travels a few degrees off-axis, where it meets the wedge. For simplicity, I
    will assume that the intention is for that ray to be angled to pass
    symmetrically through the prism, so that it emerges on the other side, now
    approaching the axis at the same angle. That equality of angles doesn't seem
    to be crucial, to me.
    The light pencil meets the objective at some point across its diameter, and
    as the wedge moves along its track, that point will shift also.
    What happens to our light-pencil, as it passes through the objective lens at
    these different points? To such a narror ray, the bit of the lens that it
    crosses looks itself like another wedge of glass, of differing angle. If the
    ray happened to pass through the pole of the lens (that is, its
    centre-point) the lens will look like a bit of plane-parallel glass, and the
    ray will pass through undeflected. If it passes above the pole (as seen in
    the view of the diagram), then that bit of lens acts as a glass wedge with
    point-up and base-down, just like the main wedge, and the light will be bent
    further off-axis. If it passes below (on the diagram) the pole, it will get
    bent toward the axis. This, I suggest, corresponds to the fanning of the
    laser light that Richard sees.
    Here, the diagram is somewhat deceptive, maybe even wrong. It shows, as a
    solid line, a ray passing off-centre of the objective, but which appears to
    be undeflected in angle by the lens. Maybe that's because the defdlection
    would be small, but as it seems to play an important role, it may have been
    better to play-up that bending.
    Of course, we have been looking at the path of just one single ray, and in
    reality the situation is more complex, forming an image by light from every
    point of the scene, occupying the whole area of the objective. I haven't
    really attempted to explain the action of the shifting wedge in that
    real-life situation.
    There's another aspect that puzzles me, here. Next, the light passes through
    a pentaprism, which deflects it through another 90 degrees. But if you look
    at the way that on-axis light, shown by the dotted line, passes through the
    prism, and look at the way in which the off-axis laser beam, shown by the
    solid line differs from it, if that solid line is to be deflected through 90
    also, it would emerge from the pentaprism diverging from the left-hand beam,
    not converging as shown.
    My final quibble is in the view of the flagpole. Presumably the lenses of
    the telescope make up a proper high-magnification inverting telescope, not a
    Galilean with its inherent limits. In which case the image should be an
    inverting one, which would not affect the operation of the instrument. But
    the view of the flagpole would be upside down. Am I right about those
    assumptions? Richard, with his rangefinder, may be able to tell us.
    So, all in all, I can't claim to understand all of Peter Ifland's diagram.
    Far from it. So where does that diagram come from? His caption states
    "Redrawn from Moss and Russell, 1988, and Anonymous, 1917", and in his
    references he cites-
    Moss & Russell's book "Range and Vision - The first Hundred years of Barr &
    Stroud " (1988), which Richard has already seen in a library.
    There are two references listed as "Anonymous (1917)", which are-
    Description of the Barr & Stroud 9 foot horizontal base Self-Contained
    Rangefinder Type F. Q. 2 No 1905 of May 16y, 1911. Government Printing
    Office, washington DC.
    Handbook of Range-Finders, 70CM. and 80 CM.Base For Use Of Infantry and
    Cavalry. No 1797 of December 9, 1915, Government Printing Office, Washington
    I hope this, with its admitted gaps, helps Richards quest for understanding.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@googlegroups.com
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