Re: [ATM] Phase contrast and the 10 incher.

[Dominic-Luc Webb <dlwebb@canit.se>]

On Mon, 23 Apr 2007, Arjan te Marvelde wrote:

> Dale,
>
> > saw kurf.  One of the worm tracks crosses over into
> > the core.  So the worm tracks are probably either on
> > or close to the surface.
>
> How can something below the surface become visible?

Exactly by same means that something above the surface
becomes visible, but then, we would be getting into
my earlier suggestions. I am not used to comparing
fuocault images with phase contrast, but I would be a
bit surprised if the foucault and the phase contrast
system were both focused on the same point, but they are
clearly very close. On the other hand, we have no sense of
his depth of field. There is a bit of a problem actually
with phase contrast in that perception of the point of
focus is confusing since you are observing a field that
has contour. To sense where things are, you can defocus,
focusing a bit in front and behind where you think the
mirror surface is. Sooner or later the back side should
become apparent. Likewise, you can get a sensation as to
whether these patterns are inside or outside the glass. I
have swirls in glass that was not very homogeneous, and
have seen wax cause smears on the outside. I really would
not want to guess on this one. Since this poster can come
up with these excellent pics, I could suggest taking more
pics, especially one for comparison showing the back side.

BTW... how thick is this glass?

Dominic

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