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    The Blish prism and the sextant.
    From: George Huxtable
    Date: 2000 Jul 08, 5:45 PM

    Toward the end of October 99 there was a discussion thread on this mailing
    list which concerned the ultimate limits on accuracy achievable in a
    sextant sight.
    The consensus of opinion was that the limiting factor was the unknown
    effect on the dip of the horizon caused by the variation in the refraction
    of light in its path between the horizon and the eye of the observer. If
    the dip could somehow be measured, that error could be allowed for.
    I have just been reading "A history of nautical astronomy", by Charles H
    Cotter (Hollis & Carter, 1967), which I can thoroughly recommend. On page
    91 he describes the Blish prism as follows-
    " To eliminate the uncertainty of the effect of refraction on the dip of
    the sea horizon, Commander Blish of the United States Navy invented, in the
    early part of the 20th century, an attachment for a sextant known as the
    Blish prism. This device has the top and bottom faces bevelled at 45
    degrees. It is fitted to the sextant so that the longer of the front and
    back surfaces faces the observer. This face is provided with two polished
    surfaces, the lower of which is directly opposite the top of the index
    mirror, and the higher of which faces over the observer's head towards that
    part of the horizon 180 degrees away from the part the observer is facing.
    With the index of the sextant set to zero on the arc the observer looks
    directly at the sea horizon in front of him and sees, at the same time, the
    back horizon reflecting from the prism. When the fore and back horizons are
    brought into line, the sextant reading is twice the angle of dip, assuming
    that the sextant is free from index error."
    I wonder if any subscriber to this list has ever come across or read about
    such a device, or seen an account of it in use. It doesn't seem to have
    become a popular fitting, and I wonder why this should be? Was it because
    in real life the variations in dip, that it was designed to show, turned
    out to be always insignificant, so that the measurement was not worth the
    Paul Hirose, back on 30 October 99, pointed out-
    >a statistical study of dip error has been done.
    >From the 1984 Bowditch Vol 1, chapter on sextant altitude corrections:
    >"An investigation by the Carnegie Institution of Washington showed
    >that of 5,000 measurements of dip at sea, no value differed from the
    >tabulated value by more than 2.5', except for one difference of 10.6'.
    >Extreme values of more than 30' have been reported and even values of
    >several DEGREES have been encountered in polar regions."
    >Unfortunately, I have no other details.  I wonder how one goes about
    >measuring dip at sea.  The only method that occurs to me is to set up
    >a theodolite on shore, oil rig or other steady platform and measure
    >the sea horizon with respect to the spirit bubble.
    In my 1984 Bowditch, this quotation appears in Vol. 2, article 805. I
    wonder, were the measurements of dip at sea in the Carnegie study made
    using the Blish prism? Does anyone have a reference to when and where this
    work was published?
    The Blish prism doesn't appear to present serious difficulties in
    construction, installation, or use. As I see it, it acts as a sort-of
    backward-looking periscope mounted on top of the sextant, and needs to be
    tall enough so that the light arriving from the horizon, behind the
    observer's back, can pass clear of the top of his head, down through the
    prism, into the index mirror. It could be simulated using a pair of
    mirrors, rigidly mounted at 90 degrees to each other, but this would be
    nowhere near as stable as a solid glass-block prism.
    Allowing a clearance of 155mm above the eye-line would give clearance over
    the head for all but the most dome-headed of navigators, though a bouffant
    hairstyle may require scissors. From this, you can subtract the vertical
    distance in the sextant between the telescope line and the pivot point, to
    deduce a suitable extra height for the periscope prism to provide.
    I am seriously considering fitting a Blish prism to my own Ebbco plastic
    sextant, just to see what it will tell me. The Ebbco has a 180mm. arm
    (unusually long), the vertical offset within the sextant being about 75mm.,
    leaving another 80mm. of vertical displacement for the periscope prism to
    provide. Suggested dimensions for the glass prism would be, say 20mm x 20mm
    x 100mm overall length on the longer side, with the 45-degree bevelled
    corners reducing the shorter side to 60mm. It should be easy to mount to
    the Ebbco frame so that it can be swung into place to face the index
    mirror, then out of the way again. Alignment is not at all crucial, as I
    see it. The sextant would no longer fit into its box, unfortunately.
    Checking the zero-setting ought not to be a difficult matter. It would
    involve measuring the dip as described above, then inverting the sextant
    and measuring again, from a similar height-of-eye. To do this, the observer
    would have to incline his head sideways through 90 degrees or so to provide
    a light path. Gymnastics, such as bending over to make an observation
    between the legs, are not called for!
    Before I start to install any extra bits to my sextant, I would welcome any
    comments from readers, especially critical ones. Have I thought it out
    right? Would it work?
    Finally, can anyone suggest a source of supply for such a prism, from an
    optical supplier or optical workshop in the UK or elsewhere? Accuracy is
    required only in setting the 90-degree angle between the bevelled
    reflecting surfaces. What I need is almost identical to a Porro roof-prism,
    and any prismatic binocular contains 4 such prisms. Unfortunately, the
    standard prism from binoculars doesn't give enough offset for the 80mm.
    that is required, so I expect to require something specially made. Any
    suggestions would be most welcome.
    George Huxtable
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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