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Extra subdivisions to the
right of 0 and to the left of 10'
are for your convenience.

When you look at the main scale, you may see
that 0 of the vernier is almost against
some main scale division (for example, against zero
on the main scale).

You are not sure, whether the angle is actually a bit
less than zero or a bit more than that.

Now you look at the vernier through the magnifying glass.
If the true angle is really close to zero,
you do not have to examine all vernier. You just look
a bit left and right of zero.

Alex.

On Sat, 6 Jan 2007, Jim wrote:

>
>
> Sirs,
>       I have recently purchased an Heath&Co Vernier sextant?.The
> Vernier has the usual 10' divisions,each with
five 12"subdivisions.My
> problem is,
that there are three 12"divisions to the left of 10', and
> three to right of 0'.What is the purpose for this?.Incidentally the
> main arc reads to 150deg.,hence my query against the use of sextant.The
> certificate is dated 1940
>
>                            Jim Martin.
>
>
> >
>


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