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I have recently added a Plath sextant - circa 1920 - to my collection and I
would appreciate some advice from list members regarding its features.

1. RISING PIECE The sextant is fitted with a rising piece which moves the
telescope in or out while maintaining its parallelism with the frame of the
instrument. This is designed to allow more of less of the horizon/ celestial
body in the telescope. Why was this necessary? Why not just have the
telescope a fixed distance from the instrument and which bisects the horizon
mirror as in modern sextants. Is it because the horizon mirrors were so
small in days gone by?

2. COLLIMATION RING The sextant is fitted with a collimation ring which
allows adjustment to the axis of the telescope bringing it into exact
parallelism with the instrument. Why was this necessary? The amount of error
in the observation produced by collimation error must have been very small,
as the telescope is not prima facie a measuring device. One suggestion I
could make is that the Plath sextant in question can be completely taken to
pieces, down to is constituent parts and then reassembled, making it very
convenient to clean and service. However the flip side of this is that it
must be completely readjusted when re-assembled.

I wonder why the rising piece/collimation ring system was abandoned? Its
great advantage is that if the sextant telescope  mount is given a hard
knock it can easily be re-calibrated by the observer. On a modern Plath the
telescope post is fixed and if it was bent, or worse still broken, the
sextant would be ruined. With the old models if the mount/rising price was
bent you simply bought a new one. This seems very sensible to me.

3. INVERTING SCOPE      The sextant comes with an inverting telescope with 4
wires for making collimation adjustments. I have looked up Oswald M Watts'
excellent "The Sextant Simplified" for instruction on using this scope for
collimation error, but would appreciate advice on other books dealing with
the subject. Is anyone aware of a good book circa 1900-1925 that  deals with
the care and adjustment of sextants?

4. PLATH SERIAL NUMBERS The sextant has no papers but its serial number is
8368. I know that by end of 1925 the Plath numbers had reached 10,500. I
presume they made about 400 items per year so that 8368 indicates date of
production about 1920 or not long after the First World War. Could anyone
help me out here?

Does anyone else on the list own a Plath in the 8000 numbers?

5. MIRRORS The sextant in question has rear surfaced mirrors. Does anyone
know when Plath went to front surfaced mirrors or when manufacturers in
general went to front surfaced mirrors? It also has the very small, square
horizon mirror. Does anyone know when Plath went to its now typical large
horizon mirrors?

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.

Any help would be greatly appreciated.

Kieran Kelly
Sydney
Australia

   

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