Well, I tried a sphere in place of the plane and it does not affect the 
"invisible" sphere like the plane did (make a refractive appearance occur at 
that spheres surface).
If you tried this also, and got the same affect as the plane/sphere combo did, 
I must have a faulty pvengine or something here :)
Sorry you felt you had to go into that lengthy explanation of inverse, etc. 
(save for the others reading here), as I already know enough of that stuff, 
only get confused while actually making something. Check my gallery out and 
you'll see I've done plenty of csg, though I suppose I'm like many of us POV 
users and am constantly at odds with the script writing. Much trial and error 
always going on.

Well, before I posted this I decided to try a few other primitives in place of 
the plane and guess what I found...
Ones that *didn't* cause the refraction "jump" into the test sphere:
box, cylinder, cone, disc, sphere.
What *did* cause refraction at the test sphere surface:
blob (made up of 2 spheres, just so it was typical at least).
Come on now, try this stuff out for yourselves. There is something amiss.

Anyway, I say again, the other primitives I've tried do not do the same 
effect. So, suppose it is the infinite nature causing this? Or solely a plane 
thing? I'm not going to try every possible object to find out I think, I'll 
just wait to see what happens in the scenes I make and be surprised.

 ;|

Message <3667e9bf.0@news.povray.org>, Nieminen Mika  typed...
>
>  I think you haven't completely understood the plane object.
>  You say: "if other shapes in csg did this it would be a disaster". Actually
>all other shapes DO this. Just put a box or a sphere or whatever in the
>place of the plane and make it inverse. You will get the same effect.
>
>  Most of the objects have well defined inside and outside. Everything that
>is inside the surface of a sphere, is the inside of the sphere, and
>everything else is outside (this may sound very trivial, but it's important
>to understans). Everything that is below the surface of a plane is the
>inside of the plane, and everything else is outside.
>  Now, when you apply the 'inverse' keyword, this is changed: Everything
>that is inside the sphere surface becomes outside, and everything outside
>the sphere surface is now inside. When you make an 'inverse' plane,
>everything that is below the plane becomes outside, and everything else
>is inside.
---snip---
>  This means that a point that is, for example, at a distance of 3 from the
>origin is outside the intersection, and thus will not be affected by the
>interior of it.
>  If you don't believe this, you can try it with the "problematic" primivite,
>the plane. For example:
>
>intersection
>{ sphere { 0,2 }
>  plane { y,0 inverse }
>}
>
>  A point is inside this intersection if it's inside the sphere and above
>the plane (because the plane is inversed). This means that a point can
>be inside the intersection only if it's at a distance less than 2 from
>the origin and also in the positive y direction.
>  So a point that is, for example, 3 units above the plane is not inside the
>intersection and it will not be affected by its interior.
>
>  So there's nothing problematic about the plane.
>  There's absolutely no difference between a sphere, a box or a plane when
>we are talking about CSG or inside and outside. The only difference is
>that the plane is infinite, but that's just a mathematical detail, and
>we shouldn't get confused by such little peculiarity.
>

-- 
 omniVERSE: beyond the universe
  http://members.aol.com/inversez/POVring.html
=Bob

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