I created a model of Saturn using a disc for the rings, but I kept running
into a strange problem that I'm wondering if it's a bug.

To depict the gas giant's atmosphere, I added type 4 scattering media to the
planet sphere to give a little pale yellow glow at the edge of the sphere.
But when I placed the camera either above or below the rings **inside** the
outer radius of the rings, the scattering media on the sphere disappeared on
the side of the disc that was the **opposite** of the disc's normal vector.

For instance, if I declared the rings disc like this:

#declare Saturn_Rings =  disc {0, y, 4500, 2250 texture {...}}

and I placed the camera at <0,-100,2500>, the atmospheric scattering on the
edge of the Saturn sphere would disappear below the plane of the disc. If I
reversed the vector like so:

#declare Saturn_Rings =  disc {0, -y, 4500, 2250 texture {...}}

and re-ran the render, the scattering edge would reappear below the plane of
the disc, but it would disappear **above** the disc plane.

I know I'm not declaring the media on the planet incorrectly, because the
problem went away when I substituted a very, very thin cylinder for the disc
to depict the rings (the media on the sphere was visible no matter where I
placed the camera).

So, is this a known problem with the disc object and I missed it in the
documentation and the bug reports forum, or what? Thanks...

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