On 17/10/2012 3:16 PM, Thomas de Groot wrote:
> I am frankly at a loss for a solution, but some questions impose
> themselves maybe.
>
> 1) are the two ends overlapping?
>
I checked and they were. :-(
Even after correcting that. The gold ring was still there but much 
diminished.

> 2) the ends do not seem to be exactly parallel, (outer) edges flowing
> smoothly into one another.
>

Too true, I used a poor end point.

> 3) could the effect be due to refraction?
>

Yes, I think so. When I commented out the sand. The ring was still there 
but much less visible.
Ah! Well. Back to the drawing board.

-- 
Regards
     Stephen

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