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Frank,
I also noticed all these convenient IC adjustments
on the very old sextants/octants.

But still, I think there should be some reason
for re-adjusting the IC in the middle of
a series of shots. I could inderstand if the IC
after the adjustemnt became smaller.
But in this case it became bigger...
So I suspect that this was due to some other adjustment
that they found necessary.

For example, even a very small adjustment of
the big mirror changes IC very much.

Do you have any conjectures WHY they prefered
frequent adjustment approach to the modern no-touch
approach? Once I asked on this list why the adjustments
are necessary at all. Why not to fix both mirrors rigidly
in a factory. (As they do nowadays with colimation).
The answer was that an adjustment is necessary when you
replace mirrors. Sounds reasonable.
But why frequent adjustments?

By the way, the old sextant I have (late XIX century)
is surprisingly non-rigid. When I switch the hand
and hold it with my left hand by the frame
(this is sometimes convenient when doing Lunars)
the IC changes substantially.
I just see how well aligned stars become non-aligned
when I switch the hand. Nothing like this happens
with SNO.

Alex.

On Wed, 2 May 2007 navlist@historicalatlas.net wrote:

>
>
> Hello Alex. I haven't had little time recently to reply to your interesting
> messages in this thread, but this caught my eye:
> "And the thing I find most puzzling is
> that IC of their sextant used to take the Lunars
> changed from +1' to -3'42" (This is what they
> recorded !!!)
>
> Was it dropped??
> More likely is that the observer noticed a substantial
> side error/lack of parallelism or whatever while
> shooting the first series. So he adjusted the sextant
> and re-checked the IC."
>
> I have a distinct impression from reading old navigation manuals and
> journals that IC was regularly changed and re-measured in this period,
> rather than simply treated as a nearly fixed quantity that had to be
> measured. There is also evidence in the design of the instruments. Old
> sextants and octants especially (like the one I showed you) had large,
> easily-manipulated knobs whose only purpose was to change the IC. One day, I
> think it would be possible to contruct a history of the phrase "don't try
> this at home" just by measuring the size of screws and knobs! I'm not being
> entirely serious here, but clearly very small screws with small, unusual
> heads are intended for rare adjustment by knowledgeable users, while large
> "user-friendly" knobs that fit easily between thumb and forefinger are
> intended for frequent use by average users.
>
> I also notice something here that seems to be common from this period --a
> lack of understanding of significant digits. Or perhaps more likely, no
> means to distinguish varying levels of significance. So, for example,
> calculations for altitude corrections were often worked out to down to the
> second of arc when the observer surely knew that the observations were only
> accurate to the nearest minute of arc.
>
>  -FER
>
> >


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