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Replying to Kieran Kelly's question about a binocular attachment to a Plath
sextant, as follows-

>6. BINOCULARS The sextant came with a small pair of binoculars which had a
>post for fitting to the rising piece. I cannot for the life of me imagine
>how a binocular could improve sights over the monocular telescope, although
>Watts says that many sextants came with binoculars in addition to the
>standard telescope monocular, although they were not popular with seamen. He
>doesn't say what they were used for.

I speculated-

I can find no mention in Cotter of a binocular attachment, but I can
imagine it might be very useful in finding a particular star, because its
feld of view in the left eye would presumably be much wider than the field
of view in the right eye, which is limited by the small solid-angle of the
index mirror of those earlier days. It would presumably allow the observer
to see a whole constellation.

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

But on second thoughts, I'm no longer clear that was likely.

Such a binocular would would provide a wider-angle view of what the
observer could see in the straight-through path, not in the reflected path.
That view wasn't limited by the size of the index mirror (as I suggested),
but by the aperture of the clear part of the horizon mirror. In the way a
sextant is normally held for altitude measurements, the left eye would see
a wider view of the horizon (not the constellation), which would not be
very useful at all.

However, for star altitudes, many observers find that it's easier to locate
a star by inverting the sextant, putting the star in the horizon mirror and
the horizon in the index mirror. Used in that way, then the binocular could
help in identifying the right star. But the binoc. would have to be
suitable for using that way up. Usually, you can't use binocs upside-down
because they clang the bridge of your nose. Is that the case with those on
this sextant?

On the other hand, for lunars, it's often recommended that the Moon is put
in the index mirror and the star in the horizon mirror, to preserve as much
star light as possible. So the wider other-eye field of view would be
useful in that case. But used for a lunar, not only would the sextant have
to be tilted, as usual, to an awkward oblique angle, but so also would the
observer's head, to align both eyes with the eyepieces in the binoc. It
doesn't seem very convincing any more. There must be another reason for the
binocs...

By the way, I presume that the binocs were "opera-glass" type instruments,
rather than prismatics, at that date. Perhaps Kieran can confirm that.

Does it look kike a manufacturer's fitting, or is more like an individual
mariner's try-out?

George.

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