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    Re: Bris Sextant
    From: George Huxtable
    Date: 2005 Nov 7, 10:00 -0000

    Responding to John Rae's
    >> I then held the apparatus in front of one side of a 7 x 35 binocular.
    Alex replied-
    > As I said before, this is dangerous!
    Yes, it is, but so is looking at the Sun with the naked eye.
    We have discussed this matter within Nav-l recently (within the last year or
    Let me rehearse some of that discussion-
    Looking at the Sun with the naked-eye, straight-on, is indeed dangerous. On
    the retina (which is presumably the part of the eye that's most sensitive to
    damage) the spot caused by an unmagnified Sun is about 0.175mm across. You
    get that by taking the angular diameter of the Sun (half a degree) and the
    focal length of the eye (about 20 mm). On that spot falls all the incoming
    energy (heat and light) from the Sun that has passed through the pupil,
    which in the circumstances will be contracted to its minimum diameter of
    about 2 mm.
    Now compare that with the situation where a telescope has been placed in
    front of the eye. Assume its magnification is x5. The telescope can now
    collect light that falls on a 10mm disc at the middle of its objective lens,
    and compresses it into a light-pencil of 2mm dia., which will all pass
    through that contracted pupil. That compression, by a factor of 5, goes
    inevitably with an angular  magnification of 5; it's a law of optics. Light
    falling on outer parts of the objective will be obstructed by the iris
    diaphragm. So, from the point of view of the retina, light reaching it from
    the Sun is increased by 25 times. Even if the objective is bigger than 10mm
    diameter, that will not further increase the amount of light passing through
    the pupil, though it will increase the total light falling on the iris.
    So that telescope has caused an increase of x25 in the amount of light
    entering the eye. But because it has magnified the Sun's image by a factor
    of 5, that image now paints a Sun-disc on the retina which is x5 greater in
    diameter, 0.875 mm dia., and 25x greater in area. So we can deduce that the
    light energy falling on any point of the retina that's within the Sun's
    image disc is no greater with the telescope than without it. The only
    difference is that the image disc is much larger in size.
    That means that the retina can be burned just as readily and as quickly by
    the naked-eye Sun as when the Sun is seen through a telescope. The only
    difference is that with the telescope, such burns will be much bigger.
    So I would qualify Alex's warning about the dangers of looking at the Sun
    through a telescope, to say that looking at the Sun without a telescope is
    almost equally dangerous. The moral is; don't do either!
    On a slightly different topic, I have been told (but can't vouch for its
    truth) that damage to the retina is mainly caused by heat energy rather than
    light energy. For that reason, filters made from highly-exposed colour film
    are somewhat dangerous, as the dyes they rely on, while obstructing light to
    give the appearance of blackness, do little to reduce heat transmission. If
    that is indeed the case, welders' glass would be much safer. However, those
    that I've looked through seem to be overkill even for looking at the Sun,
    and there would be no hope of seeing a horizon through that glass.
    About the geometry of the Bris device (gonioscope?).
    It's important, for a sextant, that both lines of view (to Sun and to
    horizon) are in the plane of its frame, which is at right angles to the axis
    of the pivot. In the Bris device, it's the line of intersection between the
    glass plates that corresponds to that pivot axis. It seems to me that two
    aspects become important. First, that the intersection line between  middle
    plate and outer should be carefully parallel to the intersection line
    between middle and inner, when the device is assembled. Second, that the
    view lines between eye and Sun. and between eye and horizon, are in the
    plane at right angles to that intersection line, to avoid "collimation
    error". How does this work out when a Bris is used? Clearly, you have to
    rotate it so that that intersection axis appears to be  parallel to the
    horixon. But it has two other axes of freedom, in "yaw" and in "pitch". It
    ought to be insensitive to pitch, just as a sextant is.  But how do you get
    the adjustment right, in yaw? How sensitive is it to such misalignment.?
    Unlike a sextant, it gives no clue to exactly where your eye is to go, with
    respect to the intersection axis of the glasses, and to the direction of the

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