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    Re: Back sights
    From: Brad Morris
    Date: 2010 Mar 19, 14:27 -0400

    Hi George


    Thank you for encouraging my concept.  It will have to wait until I can get an Octant with a backsight, in working order.  Perhaps there is a community member with such an instrument, who would be willing to try.  I do recognize the great difficulty in an altitude measurement near the zenith.    An extended arc would be better.  The fact that this method uses the exact same section of the horizon will help to null dip anomalies.


    I have been doing a bit more digging.   In the 1823 Encyclopaedia Britannica, we find an article entitled Adjustment of Hadley's Quadrant.  In case this doesn’t work, search for “Britannica Backsight Index Error” and it should pop out near the top of the found items.   The article details 3 methods of adjusting the backsight for index error (adjustment V)!  I will summarize them here, but for real fun, I suggest going right to the original.  The figures referenced will be found near page 706 (most annoyingly, you cannot go directly to the figures.  Start at 706 and page down 7 times).


    Method 1:  Fore & Back Horizon Method


                Set the nonius of the vernier to twice the dip, “to the right” of zero of the arc.  I believe that to be off the arc for a normal quadrant, essentially 180 degrees plus twice the dip.

                Align the horizons.  The article makes note that the back horizon will be inverted when compared to the fore horizon.

                Also makes note that this is difficult to accomplish at sea, without further comment as to why.


    Method 2: Dollond’s Method


                Requires a 90 degree face polished for reflection on the index mirror.  This is the method you previously mentioned.  I suspect that the Dollond mentioned is the same Dollond as

                repeating circle fame and other optics.  {The same Dollond name was used until very recently in the UK for eyeglass outlets, a very long run if you ask me.}


    Method 3: Wright’s Method


                Uses his patent quadrant (time to look at those figures).   Essentially, the index mirror reflects from both sides as near as I can tell from the article.  The Backsight peep is now on the

                same side as his Foresight vane.  Demands parallelism of the index glass faces.  Not particularly helpful, unless you just so happen to have one of these quadrant / octants


    And that’s it.  No further advice is offered on how to align your instrument.


    I do have a small quibble.  You wrote:

    A normal quadrant / octant, for forward observation,  uses some sort of telescope or at least a peep-tube to put the eye in the right place. In the back mode, there's no such guidance.


    Perhaps this was just a slip of the keyboard.  In the octants/quadrants I have looked at, there very much seems to be a peep sight for backsights, just as there is a special backsight mirror.  Your eye is forced into location.  This is similar to the arrangement for a foresight observation, albeit that the peep and mirrors are in different locations.


    Best Regards






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