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Trevor Kenchington's contributions are always perceptive; as when he said-

>.... I'm not convinced
>that Ken is not giving him, and George himself is not claiming, a little
>too much credit this time around.

>.... One mechanism is clearly George's
>suggestion of reflection from the surfaces of the glass. But that does
>not preclude Ken's mechanism of light gathered from the silvered half of
>the mirror being perceived as coming from the other half, due to the
>boundary between the two being grossly out of focus when seen through a
>telescope focused on infinity.

Trevor is quite right. The two effects must occur together, to some extent.
In an early sextant without a telescope, observed directly by eye through a
peep, then the greater depth-of-focus of the eye would vastly reduce any
such apparent "spreading" of Sun light-image from the silvered part to the
unsilvered part; wouldn't it?

A sextant may be fitted with a mechanism (called a "rising-piece, perhaps??
Joel would know.) that can shift its telescope bodily away from the frame,
keeping parallel with itself.  If it could be moved far enough to shift the
silvered part of the mirror completely out of its field of view, only
reflection in the unsilvered glass would then remain to give a dimmed image
of the Sun. I suspect that is precisely the purpose of such a mechanism; to
provide a degree of controlled dimming of the Sun. What else is it for? My
own cheap-and-cheerful plastic Ebbco has no such elaboration, of course.

In the case of a sextant with a truncated horizon-mirror (i.e. no
clear-glass portion to look through, just a short mirror to look past) then
presumably such a "rising-piece?" would be able to dim this "ghost-image"
of the Sun right down to nothing, if the telescope shifted far enough. You
could try it for yourself by grinding-away the unsilvered part of your
horizon mirror (though I doubt if many will...).

George

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