> But when the spheres are merged a ray on the x-axe coming from -x to +x
> would enter the merge at ior from the the first sphere. But the point
where
> it leaves the merge (<3,0,0>) the surface would have a different ior which
> leads to the same problem as described in chapter 6.6.1 when ior was a
> surface property.
>

See the image I posted in p.b.i.
The camera is orthographic and looking down on 6 sets of two stacked clear
spheres.
1st row: both spheres have an ior of 1.5
2nd row: bottom sphere has ior of 1.5, top sphere has an ior of 2.5
3rd row: bottom sphere has ior of 2.5, top sphere has an ior of 1.5

for each row, the left hand spheres are 'union'ed and the right hand ones
are 'merge'ed

as you can see, no two groups look the same.

-tgq

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