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In reply to Alex's message about my dipmeter attachment-

>A theory of such periscope device is probably not hard to develop,
>but I doubt there is any need in it: you seem to be the only person
>who uses such device, and I conclude fom your messages that you do not
>worry much about its adjustment or accuracy:-)

It's rather better than that. I suggest that the way I use it, finding the
difference between upright and inverted readings, makes it INDEPENDENT of
such errors.

>> The backward-looking periscope was as-bought, part of some obsolete
>> military sighting equipment, at a guess. From the robust way it is
>> constructed, the angle between the two mirrors should indeed be very
>> stable.
>
>I conclude that the mirrors on your device do not have adjustment screws,
>correct?

Correct. Take a piece of solid Aluminium, 40mm sguare section, and machine
it away at 45 degrees, top and bottom, so that it ends up with one long
side and one short side. Now drill 3 holes 34 mm dia, one coaxially, the
others at right angles, to face the 45 degree bevels. Anodise it black. Now
fix, with some sort of glue or wax, two front-silvered mirrors on to those
surfaces at 45 degrees. And there you have it.

>This raises an interesting general issue: why do they never fix the
>mirrors rigidly on a sextant, so that they are strictly perpendicular
>to the frame and dispense with the adjustment screws?
>Apparently it cannot be guaranteed that they remain perpendicular...
>
>In your message of Sun Nov 21 2004 - 12:05:39 EST
>you criticized Dollond who proposed to make some rigid non-adjustable
>"gimmick" to permit index correction with back sights:
>
>>The flaw in that argument, as I see it, was that it relied on the
>>exactness of that 90-degree shift, over long periods, and any mechanical
>>derangement
>>would result in errors for all back observations.
>
>I think the same criticism applies to ANY device where the angle between
>mirrors is important. This angle should be adjustable.
>(Your periscope was probably not originally intended for precise angle
>measurements).

Maybe so. But I don't think that in the way I use the device (by
subtraction between normal and inverted positions), it's at all important
that this angle is exactly 90 degrees.
>
>> instrument is used normal way up and inverted.
>
>From your sketch of the device, it is hard for me to imagine
>how you can use it upside down.
>Do you have to bend yourself so that your head is in horizontal
>position?

Well, you have to tilt your head rather to the right, but not horizontal.
It's less difficult than it might appear. Certainly easier than looking
between your legs or drilling a hole in your chest to see through. I refer
Alex to my mailing of 16 Nov, in threadname "Dip of the horizon", where I
wrote-

"The inverted back-horizon, seen reflected via the two normal mirrors of
the sextant (and two extra reflections in the periscope), should coincide
with the normal horizon seen in the horizon mirror. The index arm is
adjusted until this happens, and a scale reading (of only a few
arc-minutes) is noted. Then the angle is measured again, but this time with
the sextant upside down. I hear you protest "but then it will be trying to
look back through the observer's chest", and so it would, if the observer
wasn't clever enough to tilt his head right over to one side, so the
back-facing view of the horizon gets to the periscope by passing below his
right ear (this is surprisingly easy to arrange). The scale reading, when
the two horizons coincide, is noted again, and the difference between those
two readings is, once again, four times the dip angle.

It was in association with the use of the sextant for this purpose that I
asked on Nav-l, a couple of years ago, for sextant users' experience of
whether a sextant itself could be trusted to read the same when used either
way up. This was the query to which Alex has recently responded. Clearly,
if there was any flexure resulting from its own weight when inverted, then
the periscope technique wouldn't work. The conclusion was that a sextant
could be inverted without any noticeable change in its angular reading."

George.

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contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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