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Gordon Talge wrote:

>The Davis plastic is by far harder to use and to read,
>than the others. The key word is read. It is a venier
>rather than a micro drum.
>
>

Actually, no. The keyword is "set". This is important to point out, lest
one might get the impression that the vernier itself is the culprit for
the bad operability of this device.

You are specifically talking about the Mark 3. Granted, the only use I
ever had for this model was to hand it out in class and have the
students try to set it to a given angle. This angle could be given
numerically (such as 20 deg 40') or physically (such as the angle
between a door frame and the edge of the ceiling above). After this
exercise, everyone immediately understood why the tangent screw HAD to
be invented. (Nota bene: I said "tangent screw"; not necessarily the
micrometer drum, which is a later development.)

On the other hand, the vernier scale per se has no impact on the ease of
reading, once the instrument is set correctly. In any case, all modern
sextant micrometer drums are equipped with a vernier. I actually find my
old Plath (c. 1950) with tangent screw, vernier (nominal 10") and
magnifying glass much easier to read than all the modern drum sextants:
If I wear my contact lenses I need reading glasses to read the
micrometer whereas on the old model I only need to adjust the magnifying
glass. If it were not for the lack of integrated illumination, this
model would be my favorite sextant for the said reason alone.

Herbert Prinz

   

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