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On 23 Apr 2004 at 9:01, Jim Thompson wrote:

> 4a. Joel and George proposed that the sun's image could be reflecting
> off the clear portion of the split mirror, since it grows dimmer
> toward the left side of the field of view.

If this was the case, you would see something entirely different. At
the moment that you rotated the reflected image off the mirror, you
would then see two equally bright reflected images that were both
much, much dimmer than the mirror reflection. With a 5mm thick
glass/mirror, these two images would only partly overlap and you
would have no chance using the sextant properly because you wouldn't
know which one to place on the horizon (or whatever object you're
bringing them toward). I'm not saying these reflections don't exist,
they certainly do. For example, if you're out in the dark taking a
sight and somebody suddenly switches a porch light on behind you,
then you will suddenly see a bright reflection of your telescope and
face in the clear horizon glass (two reflections, actually, you can
try it and see) and your observation will have to wait until the
light is switched off. But normally, the glass reflections have a
brightness that is only a few percent of the mirror reflections, so
you don't notice them at all. If you did, the sextant would be a far
more difficult instrument to use.

Ken Muldrew.

   

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