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Chuck,

I've often seen the same explanation in other books -- namely, that a half-mirror results in "the reflected object [being] suddenly cut out of the field of view." My problem with that explanation is that, in most cases, it's simply not true. I own two sextants, one with a traditional half-silvered horizon mirror, and one with a fully-silvered half-mirror, and the view through both is basically the same: In both, there is a zone where the view of the horizon and the view of the reflected object fully overlap. Even with a fully-silvered half-mirror, there is no "sudden cut-off," and it's possible to make full contact between the horizon and the reflected object.

The reason for this is the use of a telescope to view the horizon. It's true that, without a telescope, there is a "sudden cut-off" with a half-mirror. But a telescope changes that: Its optical properties result in a partial overlap between the direct view of the horizon and the reflected view in the half-mirror. The result is that, so long as a telescope is used, there's no meaningful difference between a half-silvered horizon mirror and a fully-silvered half-mirror. At least, that's been my experience.

For what it's worth...

Denis V.