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Alex Eremenko wrote, referring to my use of a modified sextant as dipmeter-

>On the other hand, this argument does not apply to the
>periscope device which George made.
>(Contrary toi what I said in my first message on this
>topic). Because with this
>device,
>the sextant index error can be found in the usual
>way. The only question is how one adjusts the mirrors
>or the prisms of the telescope itself.
>They need two checks if I understand correctly:
>a) that the angle between the mirrors is exactly 90 degrees,
>and
>b) that both poeriscope mirrors are perpendicular to the frame.
>But this is the question to George, how he does this.

I'm sorry, but I don't have a precise analytical answer to give to Alex.

The best I can do is to explain as follows-

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. But whether it's exactly 90 degrees, so that light would be
reflected right back on itself, I have no idea, and have made no attempts
to check it (and wouldn't know how to).

I suggest that any discrepancy from exactly 180 degrees reversal would be
compensated for in the subtraction of the measurement process, in which the
instrument is used normal way up and inverted. Comments from others would
be welcome.

As for the mounting of the periscope on the sextant, I don't think it
matters a damn, really. If it was tilted, keeping in the plane of the
sextant (i.e. in a "nodding" direction) then it would still reflect light
through EXACTLY the same angle of (nominally) 180 degrees. It could even be
mounted on an upper extension to the index arm, and it would still work.
Tilting it slightly out of the plane implies no more than looking at a bit
of the horizon behind your head, that's a bit displaced to one side or
another of the azimuth opposite to the forward view. But the horizon is
uninterestingly flat, so I don't think there would be anything but a
second-order error. Again, comments welcome.

==================

Such a prism or backwards-periscope has a useful application of its own,
when parted from the sextant, as Blish pointed out. Say you want to keep to
a transit, shown by a line between two posts or landmarks. It's easy to
line them up, before you have reached them. But if you wish to keep to a
track when BETWEEN two such landmarks, when one is ahead and one astern
(which Blash called an "inner transit"), how do you do it? This comes up
often in harbour pilotage, where you might well invent your own useful
transit line between two marks on opposite sides of the harbour, in shallow
water with narrow channels such as my home harbour at Poole.

Most of us find we can do such a trick by looking ahead and astern, and
guessing when the marks are about 180 degrees apart. But with a Blish prism
or periscope you can do it precisely! Hold the prism horizontally, so that
when looking into it you see a view behind, past your ear. Hold it up so
that over the top of it you see one of the marks, and then in the prism
view you should see an image of the other, if the directions are anywhere
near opposite. Now you just have to steer so that those images line up with
each other, and you are exactly on the line between them. Simple as that.
It works!

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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