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    Re: Centerless Sextant
    From: George Huxtable
    Date: 2008 Jul 1, 12:11 +0100

    D. Walden has done us a good turn by drawing our attention, in [5432], to 
    the Hughes "centerless sextant", held in Mystic museum, and to the related 
    US patent , 1,383.950, granted July 5 1921.
    
    To his posting he attached just one of the pages from that patent, but it's 
    useful to see it all, by going to Google Patents, and entering that patent 
    number, then click "download pdf", from which you can see, and print, 4 
    pages. This includes a ray diagram, which explains (nearly) all. That shows 
    up several curiosities about the instrument, and leads to some further 
    questions. Bill Morris ("engineer") has made perceptive comments in [5575], 
    copied below, but there's a bit more to discover yet, I hope.
    
    To follow Bill's comments, and mine, you need those 4 patent pages, text and 
    all. D. Walden first pointed out a link to the museum catalogue at Mystic, 
    http://mobius.mysticseaport.org/detail.php?t=objects&type=browse&f=maker&s=Hughes%2C+H.+%26+Son&record=2 
    but that is woefully uninformative. The photo is hard to make out, and the 
    instrument appears to lack its telescope. That insrument appears to differ 
    in some respects from the patent.
    
    There appears to be a silly error on page 2 of the patent's text , line 27. 
    It describes, sensibly, in line 31, the pitch of the screw as being equal in 
    pitch to a 720th of a circle, so that one revolution of the spindle moves 
    the carrier half a degree. But then, it goes on to say "To enable readings 
    to be taken of less than half a degree the head f4 of the spindle is divided 
    into 30 divisions ..." continuing, to say that these could be further 
    subdivided. But of course, that spindle-head should be divided into 60 
    minutes, just like the drum of a normal sextant, because the carrier 
    movement of half degree changes the viewed angle by 1 degree..
    
    We have to put this development into context. As far as I can discover, no 
    British micrometer sextant emerged until the very late 1920s, and this 1921 
    patent from Hughes is the earliest proposal that I have seen. This contrasts 
    with Plath of Hamburg, who had been offering micrometer instruments since 
    1908. My guess is that Plath had the international patents sewn up, and this 
    Hughes proposal may have been no more than an attempt to circumvent them by 
    attaching their first micrometer to a rather different instrument, which 
    (whether it actually was, or not) they could make out to be an improvement 
    on the traditional sextant. But I know little about the arcane world of 
    patent law.
    
    Some of the details are a bit odd, on close inspection. In place of a 
    horizon mirror, there's a prism with a very wide angle. Bill Morris states- 
    " The prism has an angle of 120 degrees and acts somewhat like a Porro prism 
    except that there is an extra internal reflection on the long face." What is 
    its advantage, over a simple mirror? As far as I can see, it allows the 
    reflected light ray to be displaced vertically, so allowing greater spacing 
    between the index mirror and the telescope. However, that spacing is far 
    more than really needed to accommodate the telescope that's shown, but it 
    might then allow the substitution of a larger-aperture telescope, for night 
    vision. Otherwise, to achieve a similar clearance with a single mirror, at a 
    similar angle of reflection, it would need to protrude way out in front of 
    the sextant.
    
    Presumably, the angled faces of that prism must be silvered; otherwise, at 
    the angles that the internal light ray strikes them, they would transmit, 
    and so lose, much of the light that needs to be internally reflected.
    
    The ray-diagram shows that when the index mirror is set to observe light 
    entering at 90 degrees, it's just on the verge of moving out of view from 
    the horizon prism, so I would expect part of the field of view to be already 
    starting to darken. By the time the carrier has moved another 5� or so, so 
    as to try to observe angles of 100�, there appears to be no field of view 
    left at all. This is a consequence of the centreless construction, which 
    causes the index mirror to shift in an unusual trajectory. So it seems 
    likely, to me, that far from the claims in the catalogue that this is a 
    quintant, it doesn't even qualify to be a sextant, and only just as an 
    octant. Really, though, that needs confirming by a visual inspection of the 
    instrument, before we can be sure.
    
    George.
    
    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.
    
    =================================
    ----- Original Message ----- 
    From: "engineer" 
    To: "NavList" 
    Sent: Monday, June 23, 2008 6:47 AM
    Subject: [NavList 5575] Re: Centerless Sextant
    
    
    
    
    Many thanks for bringing to our attention the patent document on
    Hughes and Baker�s centreless sextant. Their stated aim was to reduce
    the size of the instrument and lower its cost of manufacture. They
    suceeded in the former aim but presumably not in the latter, as it
    seems to have sunk almost without trace. I wonder if anyone who has
    seen the actual instrument noted whether it bore any serial number
    that might give a clue as to whether there was any production run?
    
    One of the main points of interest is the guiding of what the patent
    document calls the index mirror carrier, e.  We would nowadays call it
    the index arm. It is worth noting first that the index mirror is not
    mounted where the axis of rotation would have been if there had been a
    real rather than virtual axis. The mirror would be about half-way down
    the index arm of a conventional sextant.
    
    The carrier slides between two �curved and concentric� surfaces on the
    upper and lower flanges, a and j. The lower curved surface, a, is vee-
    shaped in section and is on the inside of the arc. A mating surface on
    the carrier engages with it. The upper curved flange, j, has a slot
    machined in it  and a bracket screwed to the carrier engages the
    slot.
    
    The micrometer assembly differs in detail from that of a modern
    sextant, but the principle is the same: a micrometer screw engages
    with a rack on the periphery of the limb and is held in engagement by
    a spring. The screw can be swung down out of engagement with the rack
    by overcoming the resistance of the spring, to allow rapid movement of
    the index arm. In the case of this instrument, the spring seems to
    have another function, to keep the sliding vee surfaces in contact and
    maintaining concentricity.
    
    George Huxtable wondered about the accuracy of guiding the index
    mirror carrier. This could only be found out for certain by actual
    trial, but I don�t suppose anyone is going to send  an example to New
    Zealand for me to check the calibration. However, there is nothing
    wrong with it in principle. Most lathes made since about 1930 have
    prismatic guides that follow the narrow guide principle. That is to
    say, the guiding surfaces should be narrow and as close as possible to
    the point of application of the force that moves them.
    
    The guides on the limb and carrier would have been relatively simple
    to machine and the matching surfaces on the index mirror carrier would
    not even have to match exactly.  If the angle of the vee on the
    carrier was no greater than the angle of the vee on the limb it would
    not rock at right angles to the plane of the instrument.  If the
    radius of the vee on the carrier was no less than the radius on the
    limb, the carrier would not  rock in the plane of the instrument. In
    practice, it would not be difficult to get a very close match  I
    guestimate that the vee guides are about 60 mm long and 15 mm wide,
    adequate no doubt, but longer would have been better. The upper
    flange, j, serves only to stop things falling apart when the
    micrometer screw is disengaged and is a �keeper� rather than a guider.
    
    The placing of the handle is unusual, but hold your closed hand up to
    your face and you will see that for a lightweight instrument about 120
    x 200 mm, it is placed entirely rationally.
    
    The light path, shown in Figure 6 of the patent document is certainly
    unusual in that the horizon is viewed beneath the horizon prism; and
    that a prism is used instead of a mirror. The prism has an angle of
    120 degrees and acts somewhat like a Porro prism except that there is
    an extra internal reflection on the long face.
    
    Hughes and Son showed a curious combination of following tradition
    combined with some brilliant departures from tradition. One sees this
    in the rather clunky, but entirely adequate design of Husun micrometer
    sextants, contrasted with the ingenious and ergonomically advanced
    design of the Mark IX series bubble sextants of WW II.  I for one
    greatly regret that their centreless sextant never took off. I would
    have enjoyed examining and possibly owning an example, something now
    denied to me by its extreme rarity.
    
    Bill Morris
    
    
    
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